THE ORDOVICIAN: A WINDOW TOWARD UNDERSTANDING ABUNDANCE
AND MIGRATION PATTERNS OF BIOGENIC CHERT AND IMPLICATIONS FOR
PALEOCLIMATE
A thesis presented to
the faculty of
the College of Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Master of Science
Iulia Tomescu
August 2004
© 2004
Iulia Tomescu
All Rights Reserved
This thesis entitled
THE ORDOVICIAN: A WINDOW TOWARD UNDERSTANDING ABUNDANCE
AND MIGRATION PATTERNS OF BIOGENIC CHERT AND IMPLICATIONS FOR
PALEOCLIMATE
BY
IULIA TOMESCU
has been approved for
the Department of Geological Sciences
and the College of Arts and Sciences by
David L. Kidder
Associate Professor of Geological Sciences
Leslie A. Flemming
Dean, College of Arts and Sciences
TOMESCU, IULIA. M.S. August 2004. Geological Sciences
The Ordovician: a window toward understanding abundance and migration patterns of
biogenic chert and implications for paleoclimate (228 pp.)
Director of Thesis: David L. Kidder
The comprehensive global survey of Ordovician chert deposits undertaken in this
study reveals that most of them are biogenic, and documents significant
paleoceanographic relationships. Peritidal/lagoonal and shelf cherts hosted mainly by
carbonate facies are nodular, and formed predominantly by lithistid demosponges.
Slope/basinal cherts are bedded, hosted by siliciclastic sediments, and bear radiolarians.
Lower Ordovician cherts are abundant in all settings. The Middle Ordovician shows a
decrease in cherts characterized by a strong decline in silica burial within
peritidal/lagoonal and shelf environments. The distribution of silica secreting biotas
suggests an Arenigian- Llanvirnian onshore-offshore migration whereby siliceous
sponges migrate from peritidal/lagoonal to shelf-basinal environments, and radiolarians
move to basinal settings. The Caradocian marks the highest peak in the abundance of
Ordovician cherts, in both shelf and slope/basinal environments, that appears to correlate
with a warming interval. A decline of cherts takes place in the Ashgillian, when
glaciation was in effect.
Approved:
David L. Kidder
Professor of Geological Sciences
Acknowledgments
My sincere thanks go to Dr. David Kidder, my mentor. He introduced me to the
silica system and all its biotic and abiotic components. During our long discussions he
kindly guided me through the maze of the Ordovician Earth system and Paleozoic silicasecreting organisms. I am very grateful for having had the chance to work with him.
I would also like to thank Drs. Thomas Worsley and Gregory Nadon. Dr. Worsley
opened the world of Earth system evolution to me. His broad-picture integrative models
were a constant source of inspiration, and his feed-back greatly encouraged me in my
scholarly pursuits. Dr. Nadon was always available for discussions and interactions with
him sharpened my critical thinking.
6
Table of Contents
Page
Abstract ................................................................................................................................4
Acknowledgments................................................................................................................5
List of Tables .......................................................................................................................8
List of Figures ......................................................................................................................9
Chapter 1. Significance of the study ..................................................................................11
Chapter 2. Hypotheses of the study ...................................................................................13
Chapter 3. Assumptions of the study .................................................................................17
Chapter 4. Previous work...................................................................................................19
Chapter 5. Bedded versus nodular cherts...........................................................................27
5.1 What is chert? ..................................................................................................27
5.2 Chert formation................................................................................................30
5.3 Chert deposit types: characteristics and genesis theories ................................32
5.4 Time and temperature of certification and depth of formation........................40
Chapter 6. Methodology ....................................................................................................42
6.1 Data acquisition ...............................................................................................42
6.2 Data management.............................................................................................43
6.3 Data structure ...................................................................................................43
6.4 Data manipulation............................................................................................45
6.5 Pitfalls/Problems of a study compiling data from published reports...............54
6.6 The usefulness of a study compiling data from published reports...................57
Chapter 7. Results ..............................................................................................................58
7.1 Epoch - level temporal trend in the chert abundance.......................................58
7.2 Epoch - level temporal trend in chert types .....................................................60
7.3 The spatio-temporal distribution of chert occurrences ....................................63
7.4 Chert type - depositional environment relationship over time.........................77
7.5 The temporal and spatial distribution of the Ordovician silica secreting
organisms present in chert deposits .......................................................................78
7.6 Testing the significance of associations between the type of siliceous
skeletal remains, chert deposit styles, and depositional environments..................82
7.7 Evaluation of the efficiency of the method......................................................83
7
Table of Contents: continued.
Page
Chapter 8. The Lower Paleozoic silica secreting organisms: radiolarians
and siliceous sponges.............................................................................................91
8.1 Radiolarians .....................................................................................................91
8.2 Siliceous sponges ...........................................................................................101
Chapter 9. The origin of the Ordovician cherts - organic versus inorganic genesis........111
9.1 Previous theories on chert origin ...................................................................111
9.2 Evidence of the present study ........................................................................116
9.3 Silica burial: a balance between silica dissolution and preservation rates.....122
Chapter 10. Discussion: the Ordovician silica secreting biotas - living style,
distribution and onshore-offshore evolution patterns ....................................127
10.1 Lower Ordovician ........................................................................................127
10.2 Middle Ordovician .......................................................................................136
10.3 Upper Ordovician.........................................................................................141
Chapter 11. Interpreting the chert abundance patterns: why chert peaks or lows?..........143
11.1 Lower Ordovician: a greenhouse interval....................................................143
11.2 Middle Ordovician: a greenhouse interval...................................................155
11.3 Upper Ordovician: the Caradocian - a warm - cool transitional,
ice free mode (?) ..................................................................................................160
11.4 Upper Ordovician: the Ashgillian - an icehouse period...............................173
Chapter 12. Conclusions ..................................................................................................182
References........................................................................................................................187
Appendix .........................................................................................................................220
8
List of Tables
Table ..............................................................................................................................Page
7.1 Temporal distribution of the chert deposits at epoch and age/stage level. ..................59
7.2 Temporal distribution of the chert deposit types at epoch (A) and stage (B) level. ....61
7.3 Temporal (A epoch level; B stage level) and spatial (Onshore – Offshore)
distribution of the chert deposits....................................................................................... 64
7.4 Distribution of the chert deposit types by epoch and depositional environment.........72
7.5 Temporal trends in the paleolatitudinal distribution of Ordovician silica burial.........75
7.6 Temporal and geographical distribution of chert deposits...........................................76
7.7 Temporal distribution of Ordovician silicified oolites.................................................76
7.8 Epoch level distribution of chert deposits with siliceous skeletal remains..................79
7.9 Relationships between chert deposit types and types of siliceous skeletal
remains for the Lower, Middle, and Upper Ordovician.....................................................80
9
List of Figures
Figure .............................................................................................................................Page
4.1 Ordovician paleogeography, chert distribution, and oceanic circulation. ...................21
4.2 Phanerozoic distribution of bedded cherts compiled at the system level. ...................23
4.3 Phanerozoic distribution of bedded cherts compiled at the system level. ...................23
4.4 Phanerozoic distribution of bedded cherts compiled at the series (epoch) level. ........24
4.5 Sedimentary silica through time. .................................................................................25
4.6 (A) Temporal distribution of silicified oolites and bedded cherts at the
series level.(B) Onshore-offshore migration patterns of three faunas. ..............................26
6.1 FileMaker database entry sample.................................................................................44
6.2 Ordovician correlation chart used in this study. ..........................................................52
7.1 Ordovician distribution of chert deposits.....................................................................59
7.2A Temporal distribution of chert deposit types at the epoch level. ..............................62
7.2B Temporal variation of the relative abundance of chert deposit types at the epoch
level....................................................................................................................................62
7.3 Ordovician spatio-temporal distribution of chert occurrences.....................................65
7.4 Paleogeographic distribution of Lower Ordovician cherts. .........................................66
7.5 Paleogeographic distribution of Middle Ordovician cherts. ........................................68
7.6 Paleogeographic distribution of Upper Ordovician cherts...........................................70
7.7 Relationships between chert types and depositional environments over time.............73
7.8 Epoch-level variation of the relative abundance of chert formations that bear
siliceous skeletal remains...................................................................................................79
10
List of Figures: continued.
7.9 Chi-square test of the relationship between the type of siliceous organisms. .............84
7.10 Chi-square test of the relationship between the siliceous organisms.........................85
7.11 Chi-square test of the relationship between the depositional environment
and the chert deposit style..................................................................................................87
10.1 Ordovician dominant locus of silica burial and onshore-offshore migration
patterns of silica secreting organisms and associated facies............................................128
11.1A The Ordovician biotic system. ..............................................................................144
11.1B The Ordovician abiotic system..............................................................................146
11.2 Lower Ordovician - Chert abundance model...........................................................151
11.3 Middle Ordovician - Chert abundance model..........................................................156
11.4 Temperature vs. calcium carbonate shell δ18O (PDB) for various values
of seawater δ18O (SMOW) in an ice-cap free world........................................................164
11.5 Upper Ordovician: Caradocian - Chert abundance model.......................................168
11.6 Upper Ordovician: Ashgillian - Chert abundance model. .......................................177
11
Chapter 1
THE SIGNIFICANCE OF THE STUDY
The present study represents a unique comprehensive systematic survey of the
worldwide Ordovician early diagenetic in situ chert deposits, addressing not only bedded
cherts, but also less conspicuous cherty facies such as nodular cherts and associated
silicified oolites. The distinction of the chert deposit types (nodular cherts vs. bedded
cherts vs. silicified oolites), the depositional environment in which cherts formed
(peritidal/lagoonal vs. shelf vs. slope/basin), and a systematic analysis of the content in
siliceous skeletal remains (radiolarians and siliceous sponges) preserved in the compiled
chert deposits, were among the main targets in surveying the data. The compilation was
realized with a high temporal resolution at the epoch level, and where possible, even at
the stage level (e.g., the Caradocian).
This study is the first qualitative and quantitative test of two common
assumptions: 1) the biogenic origin hypothesis for most of the Ordovician cherts and a
biologically controlled marine silica cycle and burial and, 2) the relationship between the
chert deposit types (nodular vs. bedded chert) and the hosting facies (carbonate vs.
siliciclastic environments). This research also evaluates: 1) the relationship between the
chert deposit types (nodular vs. bedded) and the depositional environment in which they
formed (peritidal/lagoonal/shelf vs. slope/basinal); and 2) the relationship between the
chert deposit types (nodular vs. bedded) and the content in biogenic siliceous remains
12
(radiolarians vs. siliceous sponges). The results demonstrate that the Ordovician onshoreoffshore organism migration, a mechanism suggested to date only for the benthic
invertebrate calcareous macrofauna, included planktonic radiolarians and siliceous
sponges as well.
The study goals were: 1) to identify and interpret Ordovician temporal and spatial
patterns in the locus of marine silica accumulation in fluctuations of the marine biogenic
chert abundance, in possible migrations of the silica secreting organisms, 2) to correlate
these patterns with paleoclimate, paleoceanography, paleogeography, nutrient level
inputs, and the behavior of silica secreting organisms, and 3) to translate these
interconnections in terms of causal relationship. Through this, the study represents a
multi-approach analysis of the Ordovician Earth system, whose main goal is to explore
the connections between the biotic and abiotic worlds.
13
Chapter 2
THE HYPOTHESES OF THE STUDY
Based on previous work (discussed in Chapter 4) the following hypotheses have
been proposed to be tested:
I. Did the Early Paleozoic radiolarians and siliceous sponges behave similarly to
diatoms regarding their response to abrupt changes in oceanic circulation and climate
parameters?
Assessing the worldwide abundance and distribution of Early Paleozoic cherts in
relation to paleoclimatic events will help clarify whether radiolarians and siliceous
sponges were as responsive to oceanographic changes as are diatoms. This behavior was
tested for the Ordovician, a geological period characterized by a succession of
antagonistic climates: Lower and Middle Ordovician warm greenhouse periods, as
opposed to the late Upper Ordovician, an icehouse interval. A chert peak is expected to
be associated with an upwelling environment driven by the late Ordovician glaciation. In
contrast, a gap in chert abundance in the Early Silurian may reflect sluggish ocean
circulation and possible climate warming similar to the Middle Cretaceous and Early
Triassic, both of which are marked by lows in chert abundance.
The accumulation and formation of cherts depends on different factors –
upwelling is very important in offshore environments, while in nearshore facies other
14
factors are important, such as the amount of dissolved silica supplied to seawater by
riverine input and by the siliceous organisms that inhabit these areas (e.g., siliceous
sponges). Consequently, during glacial periods bedded chert accumulation should be
common in offshore settings, driven by an enhanced upwelling. On the contrary, in
nearshore facies the amount of siliceous deposits could decrease as a consequence of
changes in the ecological distribution of siliceous organisms due to sea level fluctuations.
Changes in chert distribution could be driven by a drop in sea level appreciated to have
amounted to at least 50m for the Late Ordovician glaciation. On the other hand, a drop in
sea level would increase the volume of terrestrial silicates available to weathering, and
possibly the dissolved silica input into the basinal facies, while the shelf would witness
decreased deposition being more or less emergent. The amount of dissolved silica that
enters the oceans via rivers (the exogenous silica input) is directly connected to the
continental weathering rates, whose intensity is a result of the interplay between the rock
volume exposed to weathering, atmospheric conditions (temperature, precipitations), and
the average paleolatitudinal distribution of the world landmasses. If the interaction
between the three parameters drives a higher weathering rate, the exogenous, dissolved
silica input in coastal seawaters will be higher resulting in more chert accumulation in
areas of low detrital influx.
15
II. The timing of the Late Ordovician glaciation - Did a brief glaciation result in a
short cherty interval? Did cherts respond quickly enough to reflect the duration of the
glaciation?
The Late Ordovician is characterized by a glaciation the timing of which is open
to controversy. Brenchley et al. (1994) suggest that the late Ordovician glaciation may
have lasted less than 1 m.y. during an otherwise long greenhouse interval. Conversely,
Pope and Steffen (2001) suggest that extensive chert in the Montoya Group represents a
glacial-related upwelling system that persisted for up to 15 m.y.
If the Paleozoic siliceous organisms can be demonstrated to be sensitive to
climatic and oceanographic variations, then we need to know the degree of this
sensitivity. A short-lived chert pulse, coeval with the glaciation, would support the short
glaciation hypothesis, and would argue that Ordovician biogenic cherts are very sensitive
to abrupt changes in ocean circulation. A longer interval of chert deposition would be
consistent with a glaciation of longer duration or in the case of a short glaciation, might
mean that siliceous sponges and radiolarian cherts were not as responsive to
oceanographic changes as are diatom cherts. If glaciation was short, an alternative
explanation for abundant, pre-glacial chert in the Late Ordovician is required.
III. The Early Paleozoic dominant locus of silica deposition migrated from shallow
water to deeper water environments within the Ordovician period.
Analysis of the Ordovician spatial and temporal distribution of chert sedimentary
facies, such as bedded, nodular, and silicified oolite deposits, can be used to: 1) estimate
16
the dominant locus of silica deposition, onshore vs. offshore; 2) infer onshore-offshore
migration patterns of silica secreting organisms perceived through changes in the
depositional environment of biogenic siliceous facies; 3) analyze the timing, the rate and
the nature of this retreat of chert to deeper water settings; and 4) identify and interpret
marine chert abundance fluctuations and patterns.
17
Chapter 3
THE ASSUMPTIONS OF THE STUDY
The theory according to which seawater was undersaturated with respect to
amorphous silica not only during the Cenozoic and Mesozoic, but also during the
Paleozoic, is widely accepted today (Siever, 1957; Grunau 1965; Hein and Parrish, 1987;
Maliva et al., 1989). Paleozoic marine burial of silica has been controlled for most of the
time by the Paleozoic silica secreting organisms represented by radiolarians and siliceous
sponges. In a marine silica cycle in which silica burial is dominated by the biogenic
component, the dissolution of the siliceous skeletons, which takes place during the living
stage of the organisms (Spencer, 1983; De la Rocha et al., 1997) as well as within the
water column while dead organisms are sinking and continues during burial (DeMaster,
1981), represents the main mechanism for remobilizing and recycling dissolved silica in
the sea water (Tréguer et al., 1995). The silica secreting biotas, due to their biochemical
efficiency of silica uptake, are responsible for keeping the seawater undersaturated with
respect to amorphous silica, and thus preventing the precipitation of silica and the
formation of extensive inorganic cherts.
The Ordovician chert deposits are considered in this study to be mainly biogenic
in origin, induced either organically (based on biological silica recycling), or
inorganically (based on inorganic silica and other nutrient inputs that drive biological
blooms of the silica secreting organisms). Deposits demonstrated to be inorganically
18
generated (e.g., chert units associated with tephra layers or bentonites, and possibly the
bedded cherts formed in peritidal/lagoonal environments characterized by restrictive
circulation) are exceptions. Biogenic chert deposits are considered those for which the
bulk silica input originates in the opal skeletons of silica secreting biotas, representing a
net output of Si from the biochemical cycle.
Taking into account that skeletons of the siliceous organisms are the main
supplier of reactive silica to sediments, the abundance of nodular and bedded cherts
would reflect environments rich in such organisms, in either deep or shallow waters. In
other words, “in situ” early diagenetic chert deposits are assumed to reflect the presence
of silica-secreting organisms in their proximal environment, and temporal changes in the
depositional environment of silica burial perceived through the rock record reflect
biological variations of the silica-secreting organisms.
19
Chapter 4
PREVIOUS WORK
Previous chert compilation-based studies surveyed worldwide records of bedded
chert deposits only, with the aim to identify and interpret Phanerozoic chert abundance
patterns (Grunau, 1965; Dietz and Holden, 1966; Ramsay, 1973; Ronov (1982); Hein and
Parrish, 1987; Kidder and Erwin, 2001). Grunau’s (1965) work led to the suggestion that
past chert abundance (such as radiolarites) may have been related to ophiolite genesis.
This theory was based on his finding that ophiolite thickness appears to correlate with the
thickness of approximately coeval chert deposits. This direct relationship between
ophiolite and chert formation, combined with the “eugeosynclinal” position of many of
these radiolarites (Grunau, 1965) implied a volcanic source of silica. Racki and Cordey
(2000) suggest that volcanic sources of silica may have been more significant in the
Paleozoic than in younger rocks. Given the relative lack of orogenic activity in the
Cambrian and early to middle Ordovician, it is reasonable to suspect a greater relative
role for mid ocean ridge silica sources. Silicate weathering associated with Late
Ordovician orogenic activity (e.g., Kump et al., 1999) probably increased the terrestrial
input of silica. The bedded chert compilations of Ramsay (1973) and Ronov (1982)
attempted to compare the volume of chert abundance through time along with volumes
for other sedimentary rocks. Hein and Parrish (1987) expanded the work of Grunau
(1965) and Ramsay (1973), and constructed a paleo-upwelling model of bedded chert
20
distribution (fig. 4.1). All these chert data ware assembled into period-level groupings.
The high sensitivity of radiolarians to water temperature and chemistry has rendered
radiolarians an excellent paleoceanographic and paleoclimatologic marker, mainly for the
Cenozoic era. Distribution patterns and abundance of distinct modern radiolarian
assemblages are tightly linked to the amount of nutrients dissolved in the seawater and to
water depth (Kruglikova, 1993; Abelmann and Gowing, 1997). Based on an actualistic
scenario, the abundance of radiolarian-bearing strata has been used as an indicator of
paleo-upwelling and high productivity areas for the Phanerozoic (Hein and Parrish,
1987). Also, radiolarian-bearing chert deposits preserved since the Ordovician have been
used successfully as an indicator of deep-water settings (distal basinal or oceanic
environments associated with orogenic belts), but have been also associated with cratonic
seas of moderate depths as well (Aubouin, 1965; Ormiston, 1993). Vishnevskaya (1997)
has shown that evolutionary changes undergone by radiolarians throughout the
Phanerozoic could be linked to sea level changes, paleoceanographic regime, and plate
tectonics events. Numerous Lower Paleozoic radiolarian assemblages are incorporated
into the accretionary prisms associated with ancient subduction zones (Aitchinson, 1998).
Because numerous Paleozoic radiolarian-bearing chert deposits are associated with
ophiolitic complexes, many authors emphasize the use of radiolarians as a
biostratigraphic tool in dating lower Paleozoic orogenic belts devoid of other
biostratigraphic markers and consequently lacking a good age control, (Grunau, 1965;
Aitchinson et al., 1998; Blome et al., 1995b; Nazarov and Ormiston 1993).
21
Figure 4.1: Ordovician paleogeography, chert distribution (after Hein and Parrish, 1987), and oceanic circulation (after Berry et al.,
1995).
22
Kidder and Erwin (2001) analyzed the Phanerozoic distribution of bedded chert in order
to determine the effects of extinctions and climate changes on siliceous facies. They
refined the Hein and Parrish (1987) data to the epoch level and distinguished basinal from
shelf cherts based on facies associations. The raw abundance data at system-level shows a
marked decrease in both basinal and shelf chert during the Silurian period but the
Ordovician period displays no peak or decrease in the number of chert occurrences (fig.
4.2.). On the other hand, the chert data normalized to outcrop area/time (after Raup,
1976) suggests a small Ordovician chert peak (fig. 4.3.). The distribution of chert data
compiled at the epoch level produces an Upper Ordovician chert peak (fig. 4.4.) that
might be associated with the Late Ordovician glaciation through enhanced oceanic
circulation that would favor stronger upwelling and implicitly higher organic productivity
and chert formation rate. The difficulty with the Kidder and Erwin (2001) compilation is
that the number of bedded cherts per epoch is commonly less than ten. This low sample
size provides only weak support for high versus low oceanic productivity. Those results
are sufficient however, to stimulate testable hypotheses.
Maliva at al. (1989), Kidder and Erwin (2001), and Kidder and Mumma (2003)
analyzed the dominant locus of biogenic silica burial and its migration patterns
throughout the Phanerzoic. The work of Maliva at al. (1989), based on the depositional
distribution of chert facies, showed that the Cambrian-Ordovician interval represents a
transitional period in the dominant style of silica burial from peritidal-dominated
environments of early diagenetic chert facies, to a pattern of bedded chert-dominated
basinal and nodular chert-dominated shelf/slope environments, that they suggest to have
23
Figure 4.2: Phanerozoic distribution of bedded cherts compiled at the system level (after
Kidder and Erwin, 2001). Raw abundance data. Arrows highlight Ordovician chert.
Figure 4.3: Phanerozoic distribution of bedded cherts compiled at the system level (after
Kidder and Erwin, 2001). Raw chert data presented in figure 4.2 is normalized to outcrop
area. Arrows highlight Ordovician chert.
24
Figure 4.4: Phanerozoic distribution of bedded cherts compiled at the series (epoch) level (after Kidder and Erwin, 2001).
25
persisted from the Silurian through the Cretaceous (fig. 4.5). During the Cenozoic the
main locus of silica burial is confined to bedded chert-dominated deep-sea settings
(Maliva et al., 1989).
The worldwide compilation of the Phanerozoic in situ silica-replaced oolites
silicified during early diagenesis (Kidder and Mumma, 2003), and the depositional
environment analysis of shelf bedded cherts vs. basinal bedded cherts (Kidder and Erwin,
2001), suggest that the Early Paleozoic migration of cherty facies from shallow-water to
deeper environments may have occurred within the Ordovician (fig. 4.6). Again,
however, low sample size restricts this interpretation to the status of a testable hypothesis.
Figure 4.5: Sedimentary silica through time (after Maliva et al., 1989).
26
Figure 4.6: (A) Temporal distribution of silicified oolites and bedded cherts at the series
level. Numbers at right are the percentages of shelf chert occurrences from total chert
occurrences in each series. (B) Onshore-offshore migration patterns of the three faunas
after Cowen’s (2000) reinterpretation of Sepkoski and Miller’s (1985) data (after Kidder
and Mumma, 2003).
27
Chapter 5
BEDDED CHERTS versus NODULAR CHERTS
5.1. What is chert?
5.1.1. Definition and terminology
After siliceous organisms die, their opaline skeletons either accumulate on the sea
bottom (in the case of the planktonic siliceous fauna), or dissolve and reprecipitate (in the
case of the benthic siliceous organisms) and are subsequently transformed into chert
through complex diagenetic processes. Chert refers to any siliceous sedimentary rock
whose origin can be inorganic (volcanic and/or hydrothermal) and organic (biochemical
or biogenic). “Siliceous sedimentary rocks are fine grained, dense, very hard rocks
composed mainly of the SiO2 minerals such as quartz, chalcedony, and opal, with minor
impurities such as siliciclastic grains and diagenetic minerals” (Boggs, 1987, p.239). In a
strict sense, the term chert designates a dense microcrystalline or cryptocrystalline
sedimentary rock, consisting of quartz crystals less than about 30 µm in diameter
(Calvert, 1974). Other names of chert are: flint, designating a dark gray or black variey of
chert; jasper, a red variety of chert containing disseminated hematite; novaculite,
defined as a very dense, fine-grained, even-textured chert that occurs mainly in midPaleozoic rocks of the Arkansas, Oklahoma, and Texas region of the south-central United
States; porcellanite, representing a fine-grained impure siliceous rock characterized by a
texture and fracture close to unglazed porcelain and composed mainly of cristobalite and
28
subordinately clay minerals; silex, representing early diagenetic siliceous nodules in
limestones of biochemical origin, with a large variety of colors, fine grained and very
hard, composed mainly of chalcedony and cryptocrystalline quartz.
5.1.2. Mineralogy, texture and chemical composition
The main minerals that constitute the chert deposits are: amorphous silica (opalA), cristobalite (opal-CT and opal-C), tridymite, chalcedonic quartz, microquartz and
various combinations of these phases. Cristobalite and tridymite are polymorph varieties
of quartz. According to Lancelot (1973b), the mineralogy of chert is influenced by the
lithology of the host rock. His hypothesis claims that chert in clay-rich sediments (zeolitic
clays, clayey radiolarian oozes, marls and marly limestones) is porcelanite (rock
composed mainly of disordered cristobalite – opal-CT), whereas in calcareous oozes
chert mineralogy is exclusively represented by quartz. Based on clay mineralogy of the
bed, Lancelot suggests that chert is more prone to form within a silicic environment
relatively rich in K-felspars, and palygorskite (a chain-lattice clay mineral with good
bleaching and adsorbent properties).
Opal-A or amorphous silica, with up to 10% water, represents the constituent
material of the skeleton in siliceous organisms. Von Rad and Rösch (1974) define opal-A
as highly disordered, nearly amorphous natural hydrous silica. It is metastable and
therefore it becomes less frequent back through time, being absent from Paleozoic cherts
(Tucker, 2001).
Opal-CT represents an intermediate phase of the opaline silica transformation
into chert (quartz). The 3-layered mineralogical structure is represented by disordered
29
stacking of a low temperature cristobalite layer and two low-temperature trydimite layers
(Wise and Weaver, 1974; Boggs, 1987). Opal-CT may occur as microcrystalline
aggregates 5-15 µm in diameter called lepispheres, in turn composed of fine blade-shaped
crystals 300-500 Å thick (Calvert 1974). Cristobalite represents an important constituent
of many deep-sea cherts. In the chert literature, low temperature cristobalite can be found
under various names such as: cristobalite, α-cristobalite, disordered cristobalite, lussatite,
opal-CT, opal-cristobalite, unidimensionally disordered cristobalite (Wise and Weaver,
1974). It is considered to be an early diagenetic product of low temperatures.
Quartz in chert is a late diagenetic product and based on grain size is divided into
megaquartz (> 20 µm) and microquartz (< 20 µm). Microquartz can be of two types:
microcrystalline (1-4 µm), and chalcedonic quartz with a fibrous habit (Carson, 1991).
According to von Rad and Rösch (1974), a quartz-chert lacks fossils and porosity.
Chalcedonic quartz is fibrous quartz of three types, depending on the optical
orientation of the fibers: two length-slow types (quartzine and lutecite), and one lengthfast type of chalcedony (chalcedonite). The latter is considered a late diagenetic product
(Heath, 1974).
From a chemical point of view, in addition to silica (SiO2), cherts contain minor
amounts of Al, Fe, Mn, Ca, Na, K, Mg, plus some impurities such as authigenic hematite
and pyrite, siliciclastic minerals, or pyroclastic particles. Aluminum represents commonly
the second most abundant element in cherts, followed by Fe, Mg, or K, Ca, and K. The
amount of silica in chert ranges from 99% for a pure chert (such as the Arkansas
Novaculite), to less than 65% in some chert nodules (Boggs, 1987).
30
5.2. Chert formation
Boggs (1987, p.92) maintained that chert formation is a dual process: 1) a
sedimentary process reflecting the accumulation and concentration of biogenic opaline
tests by burial and, 2) a diagenetic process reflecting crystallization and recrystallization
of the amorphous silica after burial.
The amount of dissolved silica in the pore water of marine sediments is higher
than that of adjacent bottom waters (Tréguer et al., 1995). Within the sediment, silica is a
mobile component: a part is recycled into the overlying water by dissolution, another part
is involved in the formation of aluminosilicate mineral phases, and the rest crystallizes
and is preserved as chert. From the total biogenic silica production, 97.5% is recycled and
only 2.5% is preserved in the sediment as chert (Tréguer et al., 1995). According to
Heath (1974), 4% of the dissolved silica is buried, out of which only 2% will be left after
the post-depositional dissolution, and preserved as chert. Within the sediment high
dissolved silica concentrations correlate with biogenic silica-rich sediments. Dissolved
silica increases rapidly in concentration in the uppermost layers of the sediment, and
reaches a constant value some tens of centimeters below the surface of the sediment
(Calvert, 1983).
Von Rad and Rösch (1974) contend that the first steps in chertification are the
remobilization of dissolved biogenous opal-A and precipitation of lussatite blades (low
temperature cristobalite), with formation of microspherules (an early diagenetic process),
followed by the conversion of the lussatite into chalcedony and/or micro- (>1 µm) to
cryptocrystalline (< 1 µm) quartz (a late diagenetic processes).
31
Rapid siliceous test accumulation results in a high opal concentration per surface
unit area. Burial and continuous dissolution of the opaline skeletal material increasingly
enriches the pore waters in silica favoring a slow precipitation of chert. The most widely
accepted diagenetic progression is the maturation theory (Kastner et al., 1977) in which
opal-A passes into opal-CT (disordered cristobalite) that in turn converts with time into
quartz. However, opal-A can pass directly into quartz (Kastner, 1981). According to
Calvert (1974), the transformation of opal-CT into quartz is a solid-solid inversion whose
rate depends solely on temperature.
The rate of this chert diagenetic evolution depends on different factors: in situ
temperature (Li, 1995; Carson, 1991; Calvert, 1974; Kastner et al., 1977), pore fluid
chemistry, pH, time, host rock mineralogy, organic matter abundance, clay minerals,
burial depth, host bed porosity and permeability, specific surface area (Williams et al.,
1985; Tribble et al., 1995). Von Rad and Rösch (1974) state, “under normal deep-sea
conditions, time appears to be the single most important parameter” (p. 344). The ideal
diagenetic sequence derived from the solubility-specific surface area graph of Williams et
al. (1985) would be represented by a sedimentary column in which opal-A overlies opalCT, that in turn overlies quartz, with transitional zones of mixed phases. Kastner et al.
(1977) demonstrated experimentally the strong influence of the host rock and the solution
composition, along with time and temperature, over the transformation rate of opal-A to
opal-CT. Their conclusion was that the transformation rate is greatly enhanced within a
carbonate host rock, as compared to clay-rich host sediments, by highly alkaline solutions
rich in magnesium. The idea is that magnesium (provided by the seawater) and hydroxide
32
(OH-, provided by calcite dissolution) act as magnesium hydroxide sites of opal-CT
nucleation and subsequent growth of opal-CT lepispheres. The affinity between
carbonate and chertification is explained in terms of alkalinity – by calcite dissolution the
pore water solutions become alkaline favoring the transformation of opal-A into opal-CT
at a higher rate. According to the experiments by Kastner et al. (1977), active clay
minerals, such as smectites, retard the opal-A to opal-CT transformation, by competing
with opal-CT for the available alkalinity in seawater, which results in an enrichment of
the clay minerals in Mg.
5.3. Chert deposit types: characteristics and genesis theories
Chert deposits in the geological record are preserved mainly as bedded and
nodular cherts. They can form in shallow and deep waters, but many shallow water
siliceous deposits do not become cherts because they are masked by a high terrigenous
input. Numerous Mesozoic and Cenozoic bedded chert deposits occur in orogenic belts
and within ocean floor deposits.
The difference between bedded and nodular chert deposits is regarded as either a
morphological difference (Boggs, 1987) or as a genetic difference. Bedded cherts are
considered primary accumulations of siliceous tests of planktonic silica-secreting
organisms, such as radiolarians for the Paleozoic (radiolarites), and radiolarians and
diatoms (diatomites) for the Mesozoic and Cenozoic (Grunau, 1965; Aubouin, 1965;
Ramsay, 1973; Hein and Parrish, 1987; Maliva and Siever, 1989), whereas nodular cherts
are regarded as diagenetic products resulting from silica remobilization and mineral
33
precipitation within a limestone host rock (Maliva and Siever, 1989; Maliva et al., 1989).
Some authors differentiate between the two types based on the host rock – the chert
deposits occurring in carbonate host rock being considered nodular cherts, and the chert
deposits occurring in siliceous sequences termed bedded cherts (Wise and Weaver,
1974).
5.3.1. Bedded Cherts
General characteristics
Bedded cherts, regarded as primary chert accumulations of either diatom or
radiolarian tests, can form in both deep and shallow water. Presently silica is
concentrated in deep oceans, below the CCD, forming siliceous oozes composed entirely
of biogenic silica of radiolarian tests and diatom frustules. Bedded cherts can form also
beneath shallower waters on the shelf, if surface waters are fertile, there is a paucity of
calcareous plankton, and there is a very low terrigenous sediment input. Therefore, the
formation of bedded cherts depends directly on the organic productivity of the planktonic
siliceous organisms (diatoms and radiolarians), that in turn depends on oceanic
circulation and nutrient supply, on the sediment input, the presence of calcareous
plankton, and water depth. Aubouin (1965), discussing radiolarites, defined them from
two points of view: 1) petrographically and tratonomically. The former form through test
accumulation, occurring in all sedimentary environments of an epicontinetal domain, and
considered of local significance. The latter are composed of a massive, homogenous
sequence of siliceous facies “uniquely characteristic of geosynclinal chains, and
34
particularly of the eugeosynclinal furrows” (p. 115). These can be associated with pelagic
limestones, limestone microbreccias, pelites, sandstones, and ophiolites.
Origin and genesis theories
The abiogenic versus biogenic 1origin of chert has long been debated. The main
factors supporting the inorganic origin theory are the association of cherts with volcanic
complexes in many cases, on the presence in certain bedded cherts of some minerals
regarded as volcanic alteration products, such as montmorillonite, palygorskite, sepiolite,
and clinoptilolite (Wise and Weaver, 1974), and on the lack of siliceous skeletal remains.
Bedded cherts associated with bentonites and devoid of any siliceous skeletal remains are
widely considered to be inorganic in origin.
At the beginning of the twentieth century Davis (1918, in Wise & Weaver, 1974),
studying some radiolarian bedded cherts of the Franciscan mélange in California,
proposed another theory known as the “gel theory”. This theory explains bedded cherts as
being formed by the solidification of layers of silica gel entrapping radiolarian tests as the
gel was solidifying. The main mechanism that induces a high accumulation rate of
siliceous tests of the planktonic silica-secreting organisms, and ultimately the
transformations into bedded cherts, has been debated for centuries: submarine volcanisminduced plankton blooms (the so-called volcanic-sedimentary hypothesis of Aubouin,
1965) based on the spatial association between many bedded cherts and igneous rocks in
eugeoclinal settings (Grunau, 1965; Aubouin 1965); abyssal accumulations below the
calcite compensation depth, unrelated to igneous activity (Aubouin 1965; Garrison, 1974;
35
Heath, 1974); and upwelling-induced high planktonic productivity (Calvert 1966;
Ramsay, 1973; Hein and Parrish, 1987).
The concept of maturation of siliceous oozes from an initial biogenic silica ooze
through porcelanite to chert, was introduced by Bramlette (1946, in Kastner et al., 1977).
In concordance with this theory, the transformation of biogenic opal-A is regarded and
interpreted as a process dependent on depth and temperature and ultimately is viewed as a
time-dependent process (Wise and Weaver, 1974; von Rad and Rösch, 1974).
5.3.2. Chert nodules
General characteristics
Chert nodules are highly variable in size and shape, ranging from spherical to
irregular. They may coalesce to form more continuous layers resembling bedded cherts.
Their internal structure can be laminated or not, and they may contain silicified fossils.
Chert nodules, regarded as diagenetic products, occur predominantly in carbonate rocks
such as shelf limestones, pelagic carbonates and chalks, in environments ranging from
tidal flats to deep ocean basins (Maliva and Siever, 1989). According to Maliva and
Siever (1989; p.425), “there is no intrinsic depositional environmental restriction on
nodular chert formation”. Nodular cherts form in hypersaline, marine, and mixed marinemeteoric pore waters. Also, nodular cherts can form in mudrocks, evaporites, burrow
fills, but to a much smaller extent, and can nucleate around fossils. In general, chert
nodule distribution is a non-random stratigraphic event tending to occur parallel with the
bedding planes. The great affinity between carbonate rocks and silica precipitation was in
part explained experimentally by Kastner et al. (1977).
36
Origin and genesis theories
Based on mineralogical transformation, Wise and Weaver (1974) discuss 2 basic
theories of nodular chert formation: 1) the maturation theory, and 2) the quartz
precipitation theory. According to the maturation theory proposed by Heath and Moberly
(1971, in Wise and Weander, 1974), chert nodule formation begins with precipitation of
disordered cristobalite that in time converts to chalcedonic quartz from the nodule center
toward the periphery.
The quartz precipitation theory was suggested by Lancelot (1973a). To this theory
the driving mechanism of chert nodule formation is primary quartz precipitation, with
disordered cristobalite representing a by-product of the process. The theory is applicable
only for carbonate ooze sequences in which the chert mineralogy is exclusively
represented by quartz. The steps are: precipitation of quartz, growth of the quartz grains
along a quartzification front assuming a high permeability of the sediment, and
precipitation of cristobalite at the periphery of the quartz nodule.
Analyzing the two theories concomitantly with their own analyses of the
Kerguelen chert through scanning electron microscopy, Wise and Weaver (1974)
concluded that chert nodule formation occurs as follows: 1) diffusion of supersaturated
fluid with respect to silica; 2) precipitation of cristobalite lepispheres; 3) development of
a dense nodule nucleus with concomitant dissolution and expulsion of host rock
carbonate; 4) growth of nodule; 5) inversion of the cristobalite to quartz from the center
toward the periphery; 6) pore space filling by fibrous chalcedony and cryptocrystalline
quartz; and 7) growth until the depletion of dissolved silica.
37
Maliva and Siever (1989) describe three earlier models proposed for the origin of
nodular chert, and propose themselves a fourth chert nodule genesis model: 1) the
organic-matter oxidation model, 2) the hydrogen-sulfide oxidation model, 3) the mixing
zone model, and their own model 4) force of crystallization-controlled replacement
model.
The organic-matter oxidation model claims that the decomposition of organic
matter may induce early diagenetic nodule formation. When organic matter is oxidized,
the pore water CO2 partial pressure increases driving an increase in the solubility of
carbonate minerals. On the other hand the solubility of amorphous silica decreases with
decreasing pressure. This model is applicable only for early diagenetic chert nodules,
occurring as long as there is organic matter available for oxidation (Siever, 1962).
The hydrogen-sulfide oxidation model was proposed by Clayton (1986, in
Maliva and Siever, 1989) to explain the flint nodule formation in the Upper Cretaceous
Chalk of Western Europe. According to Clayton, the flint nodules formed at the oxicanoxic boundaries within the sediment. Below the anoxic-oxic boundaries anaerobic
bacterial sulfate reactions produce hydrogen sulfide (H2S). If the sediment is clay-rich,
the H2S will combine with iron derived mainly from the clay minerals and will form iron
sulfide. But if the sediment is clay-poor, the excess of H2S will migrate toward the oxicanoxic interface where it will combine with free molecules of oxygen according to the
following reaction:
H2S + 2O2
SO42- + 2H+
38
The released hydrogen ions will react energetically with the calcite rock resulting in
intense calcite dissolution. At the site of this boundary the pH registers a significant
decrease allowing the nucleation of opal-CT and formation of flint nodules.
The mixing model was proposed by Knauth (1979) who suggested that many
nodular cherts in limestones have formed in mixed meteoric-marine coastal systems.
According to this model, the mixing zone is characterized by diagenetic solutions that are
undersaturated with respect to calcite and aragonite, and supersaturated with respect to
silica. This geochemical environment is one where carbonate can be replaced by silica
under equilibrium conditions. The assumptions of the model are: the meteoric water
passes through a carbonate stack rich in debris of biogenic silica (“proportional to the
amount of offshore silica production”, p. 274), a relatively high permeability of this zone,
and the system is closed with respect to CO2.
Assessing the three previous models, Maliva and Siever (1989) conclude that
none of them are consistent with their observations obtained by studying six limestone
sequences of different ages (Paleozoic, Mesozioc, and Cenozoic), nor with the different
carbonate diagenetic histories. Their model is that calcite dissolution during chertification
is restricted to areas of replacement and that the rate of silica precipitation has to be equal
to the calcite dissolution rate. None of the previous three models explains these two
necessary conditions in the formation of chert nodules and therefore they proposed the
force of crystallization-controlled replacement model. This model is an expression of
the force of crystallization combined with intergranular pressure solution. According to
the model, the undersaturation with respect to calcite occurs only at silica-calcite contacts
39
and therefore calcite dissolution is restricted to sites of silicification. The rate of silica
precipitation is then equal to the rate of calcite dissolution. This model explains the
formation of chert nodules in all environments and suggests that the only necessary
condition for chert nodule formation is the presence of pore water supersaturated with
respect to opal-CT or quartz. Maliva and Siever (1989) listed three properties of the host
sediment as possible controls of chert nodule nucleation and the inter- and intra-bed
distribution of chert nodules: 1) organic matter content, 2) porosity and permeability
influencing the silica transport and molecular diffusion, and 3) biogenic-opal
concentration.
The force of crystallization-controlled replacement model of Maliva and Siever
(1989) explains only how individual crystal grains of quartz or opal-CT can replace
carbonate grains. The formation of a chert nodule consists of silica nucleation and
growth. Nucleation is not enough to generate nodule formation but has to be
accompanied by nodule growth. Any crystal formation includes two steps: 1) the
formation of crystal embryo by nucleation, and 2) the crystal growth. It is known that
opal-CT nucleation during chertification is principally a heterogeneous nucleation on preexisting opal-CT crystals (isomineralic heterogenous nucleation; Maliva and Siever,
1989). If a nucleus forms in contact with some surface, commonly another solid, the
nucleation process is called heterogenous nucleation in contrast with the homogenous
nucleation in which nucleus form in the bulk of the solution (Drever, 1997). However,
chert nodule formation is a combined process of heterogenous and homogenous
nucleation of opal-CT lepispheres, followed by crystal growth (Maliva and Siever, 1989).
40
The growth of the silica phase generates a pressure at the silica-carbonate grain contact,
driving an increase in the Gibbs free energy and therefore in the solubility of the calcite.
5.4. Time and temperature of chertification and the depth of formation
According to von Rad and Rösch (1974), a period of 70-90 My is necessary for an
initial biogenic silica sediment to become a mature quartz chert. Kastner et al. (1977)
suggested that siliceous oozes can transform into cherts in 25-50 My. If the in situ
temperature is increased by different mechanisms, for example by burial, the period
necessary for the completion of the opal-A to quartz transition decreases. For a
temperature close to 30ºC, less than 30 My are necessary to form quartz (Heath, 1973, in
von Rad and Röch, 1974). However, these estimates are valid only if the silica diagenetic
history includes the whole maturation process: opal-A passes into cristobalite, that then
passes into quartz. If quartz precipitates directly form solution during early diagenesis as
suggested by Lancelot (1973a,b), the age estimates does not apply.
Chert nodules can represent early or late diagenetic events occurring at moderate
to deep burial depth (Maliva and Siever, 1989). According to Maliva and Siever, the
depth of chert nodule formation can be estimated by the presence of ghosts of
intergranular pressure solution contacts and deformed grains. However, they assert that
the minimum depth of burial and the time of chertification are uncertain. Studies have
showed that opal-CT forms before significant burial at temperatures close to the
temperature of the overlying bottom water, while granular microcrystalline quartz forms
during deeper burial at higher temperatures (Wise and Weaver, 1974).
41
Data from the Miocene Monterey Formation of California suggest an even lower
temperature for chert formation (Matheney and Knauth, 1993). Oxygen isotopic analysis
of opal-CT and quartz suggest that silica diagenesis can occur at temperatures as low as
17º - 21ºC, instead of 35º - 50ºC as previously supposed to be for opal-CT, and 44º –
77ºC for quartz, while δ18Opore water = 0‰ (Murata et al., 1977; Pisciotto 1981). Based on
the difference in formation temperatures of the opal-CT and quartz chert samples studied
(differences of 40º - 50ºC), Matheney and Knauth (1993) concluded that quartz chert
formed from opal-CT at approximately 900 – 1000 m beneath the opal-CT transition
zone, and that opal-CT formed from biogenic siliceous deposits at approximately 330 m
beneath the sea floor. The assumptions of the model are: a geothermal gradient of
45.2ºC/Km, a sea-floor temperature of 5ºC, and the precipitation temperature of opal-CT
of 20ºC.
42
Chapter 6
METHODOLOGY
6.1. Data acquisition
Compilation of the worldwide Ordovician early diagenetic in situ chert deposits,
such as bedded cherts, nodular cherts, and associated silicified oolites yielded a high
resolution record at the epoch/series level and, in some cases, at the stage/age level. The
compilation is based on several sources: a) online databases - GeoRef, Science Citation
Index Expanded, geology journals, General Science Full Text, and Journal Storage, the
scholarly journal archive (JSTOR), b) electronic journal archives such as the American
Association of Petroleum Geologists Bulletin since 1917, and the CD-ROM archive of
the Journal of Sedimentary Research between 1931 and 1997 and, c) printed articles. All
the electronic sources (GeoRef to a lesser extent) present a predilection toward the North
American literature, therefore a chert database built solely based on them would have
presented a strong bias toward the North American continent (Tomescu and Kidder
2002). The bias was highly diminished toward an equitable balance between the North
American continent and the rest of the world, by running Georef literature searches, and
especially by analyzing numerous printed reports. The electronic search, using “chert”
and “Ordovician” as keywords, yielded around 1900 articles, but only around 400 of
those articles were relevant to this study. Approximately 100 articles obtained in addition
to those found using electronic search engines produced useful results.
43
6.2. Data management
The chert database was designed, built and managed using FileMaker Pro software. The
wide range of information collected is shown in Figure 6.1. The chert occurrences were
plotted on Ordovician paleogeographic maps at the epoch level, using the
Paleogeographic Information System/MacTM program, version 5.0, designed by Ross
(1997) based on the data of C.R. Scotese.
6.3. Data structure
Each chert occurrence included in the FileMaker database is characterized by four
main types of information: 1) bibliographic references, 2) geographic localization, 3)
geologic information and 4) comments (fig. 6.1). The bibliographic references section
contains all the useful works analyzed and used in extracting any valuable data pertaining
to the chert deposit considered. The geographic localization section includes the name of
the area where the outcrop was described (country, region, town) and the geographic
coordinates (the latitude and the longitude). The geologic information section comprises
information about: a) the type of the chert deposit (nodular versus bedded versus oolitic
chert) and its fossil content in siliceous organic remains, if such data was available; b) the
name of the rock formation and, if applicable, that of higher rank stratigraphic units; c)
the geologic age at the epoch level (Lower/Early, Middle and Upper/Late Ordovician),
and even at higher resolution if data was available (e.g., Caradocian, Ashgillian); d) the
associated lithologies and the stratigraphic position of the chert unit within the
44
Figure 6.1: FileMaker database entry sample.
45
sedimentary sequence; e) the depositional environment in which the chert deposit was
formed, with the aim to differentiate lagoonal/peritidal from shelf and from slope/basin
settings; f) tectonic and paleogeographic settings; and g) diagenetic features of the chert
deposit. The workable database built for the present study and that has been used in
quantifying the Ordovician chert distribution, includes only those Ordovician chert
deposits of well defined age at the epoch level, or as strata bearing siliceous organisms
but not defined as chert deposits.
All these various types of data linked to a chert deposit represent important
information in assessing the determining factors that could lead to the accumulation and
preservation of a certain type of chert deposit, in differentiating late from early diagenetic
cherts, in evaluating the origin of a chert deposit, and etc. For example, closed, semiopen, epeiric, and foreland basins should be regarded differently from the point of view
of chert formation because they are characterized by differences in specific features such
as tectonic activity, volcanism, local sources of dissolved silica, and local oceanic
circulation patterns.
6.4. Data manipulation
The main goals followed in assessing each chert deposit were: a) to establish the
age at the epoch/series level (Lower versus Middle versus Upper Ordovician) and, where
data were available data, at higher resolution, respectively at the stage/age level,
especially for the Upper Ordovician (Caradocian versus Ashgillian); b) to identify the
type of the chert deposit (nodular versus bedded cherts and subsequently silicified
46
oolites); c) to determine if a chert unit contains siliceous organic remains (siliceous
sponge spicules and/or radiolarians); d) to recognize the depositional environment in
which the chert deposit formed (peritidal/lagoonal versus shelf versus slope/basinal
settings); and e) to infer the genesis of the chert units.
Some of the recorded chert occurrences were obtained from general descriptive
sedimentologic and stratigraphic studies of particular areas and sedimentary sequences of
interest. Many of theses cases bore useful and reliable information pertaining to the cherts
(age, origin, fossil content, and depositional environment). The remaining occurrences
were collected from different geological studies focusing on problems other than the
cherty formations. Most of these merely mentioned the chert occurrences and in these
cases, various interpretations related to the goals of the study were made insofar as the
available data allowed.
6.4.1. Interpreting the type of a chert deposit – nodular chert versus bedded
chert
The types of in situ chert deposits considered and quantified herein are: nodular
cherts, bedded cherts, deposits with both nodular and bedded cherts, and “type
undetermined”. A priori, while compiling the data, the difference between bedded and
nodular chert deposits was regarded mainly as a morphological difference, and only
subordinately as a genetic difference. Laminated and stringer chert units have been
considered bedded chert deposits, even though a layer/bed of rock usually refers to a
thickness from a few inches to a few feet. The reason for this decision is that they still
represent a layered sedimentary structure that occurs continuously on a certain surface. In
47
contrast, nodular morphology is a spatially-restricted chert occurence within a host rock.
Many terms are used in the literature to designate a chert deposit. These include: chert,
flint, jasper, ooze, ribbon chert, radiolarian chert, spiculitic chert, chert with radiolarians
and sponges, solid chert, chert in shales, chert lime, chert facies, cherty limestone, cherty
dolomite, cherty carbonate, silicified limestone or dolomite, cherty argillite, cherty or
chert shale, cherty mudstone, siliceous shales, siliceous bed, siliceous argillites, siliceous
mudstones, and siliceous slate-like shale. The term “siliceous” is ambiguous and
inconsistently used in the literature. For example, theoretically, siliceous shales refer to
shales with very high silica content, as much as 85%, compared to the average shale that
has around 58% silica (Pettijohn, 1975). By extrapolating the definition of the siliceous
shales to any siliceous deposit type we should expect to have a sedimentary rock very
rich in silica (much above 50%). On the other hand, Shepard (1973) and Duxbury et al.
(2002), in analyzing the deep-sea sediments, define a siliceous ooze as a pelagic sediment
that contains more than 30% biogenous material. Usually, the term “siliceous” is used as
a descriptive word for any rock containing noticeable chert components. Hence, in some
cases it was hard to discern a compact and well-individualized chert-bearing unit from a
siliceous deposit that could bear siliceous particles disseminated within a host rock. Still,
“siliceous” deposits containing siliceous skeletal remains could give valuable information
about the silica secreting organisms living in the proximity of that depositional
environment. Therefore, if a chert deposit was mentioned as a “siliceous” deposit with no
further information regarding any siliceous faunal remains or the chert itself, the deposit
was not taken into account as a “true” chert unit, but was still recorded as a putative chert
48
deposit in the expanded chert database. Numerous examples of such deposits were
recorded from China and Kazakhstan.
Many “siliceous” deposits are mentioned in the literature as radiolarian-bearing
strata devoid of further details regarding the type of the chert deposit, depositional
settings, and associated lithologies. Even though they are presumably bedded cherts,
these chert facies were not classified as well-individualized chert deposits such as nodular
and bedded cherts, but as an undetermined type, in order to prevent a suprarepresentation of one type of chert deposits over the others.
6.4.2. Interpreting the depositional environment of a chert deposit
Environmental interpretations of the chert occurrences belong to the original
authors, and rely on paleogeography, lithologic associations, sedimentary structures and
other paleobathymetric and environmental reconstruction criteria. Based on this
information chert-forming environments were classified into three categories: 1)
lagoonal/peritidal 2) shelf and, 3) slope/basin. Difficulty in determining the environment
of some deposits is an ongoing problem (Murray, 1994), leaving some of the analyzed
cherts in an undecided category (see Appendix).
The general paleogeography of the area in which sedimentation occurred provided
useful information with respect to the depositional environment of the chert deposits.
Within this context, clastic and carbonate platforms point to shallow water – shelf
environments in miogeoclinal settings. Tectonically active settings in which volcanism is
associated with clastic sediments and located away from the craton, suggest deeper water
settings.
49
Lithologic associations, sedimentary structures and other depositional
environment indicators have represented some of the best tools in evaluating the
depositional setting. A peritidal/lagoonal depositional setting was inferred based on the
presence of paleobathymetric indices represented in many cases by silicified oolites
and/or, to a lesser extent, stromatolite associations. Cruiziana ichnofacies and hummocky
cross-stratification indicate a shelf depositional setting (Pemberton et al., 1992; Pratt et
al., 1992). Graptolite-rich shales, turbidite facies, flysh sequences, ophiolitic, pillowbasalt and volcaniclastic chert associations are ascribed to basin/slope environments
(Aubouin, 1965; Grunau, 1965; Murchey et al., 1983). Bentonites, the chemical alteration
product of volcanic ash, might represent significant silica sources due to their high
content of clay minerals, covering vast areas. Consequently, some bedded chert deposits
have been discovered associated in the geologic record with volcanic ash layers. Owing
to the fact that bentonites do not reflect local environmental conditions, bedded cherts
associated with them were not interpreted in terms of depositional environment in which
they have been found.
6.4.3. Biases in designating a chert deposit type and the corresponding
depositional environment in which it formed
It is tempting to classify any radiolarian-bearing chert as bedded and any siliceous
sponge-bearing chert as nodular, but it has been demonstrated (Coniglio, 1987, for the
Cow Head Group) that both types of siliceous skeletal remains can occur in both bedded
chert and nodular chert deposits. Therefore no such assumptions were made in the
absence of other pertinent information.
50
Using the type of the host rock as an indicator for the depositional environment in
which cherts occur, introduces another strong bias. In the case of limestone host rocks,
chert deposits tend to be classified as nodular, shallow water, or shelf deposits. At the
other end of the spectrum, if the host rock is shale, especially black shale with graptolitic
fauna, the tendency is to designate the chert deposit as bedded, deep water, or basinal
(e.g., the geosynclinal cherts of Pettijohn, 1975). Biogenic components and pelagic clays
dominate many deep-sea sediments (Kennett, 1982). However, even though these
associations between host rocks and depositional environments are in most of the cases
consistent, some exceptions to the rule are inevitable.
Pettijohn (1975) described bedded cherts, termed cratonic cherts, from shallow
water limestone sequences, which he considered stable shelf associations (p. 401). Also, a
siliceous argillite deposit can accumulate in various settings extending from shallow
water shelves to deep-water environments (Murray 1994; Girty et al., 1996). Murray
(1994) asserts, “general criteria by which depositional environments of cherts can be
determined are not yet available” (p. 213). Coniglio (1987) in describing the chert units
of the Cow Head Group of western Newfoundland, pointed to the presence of chert
nodules in shale sequences. Berry and Wilde (1978) in analyzing lower Paleozoic black
shales concluded that they can form on shelves, slopes and in deep-sea settings. Their
differentiation is possible by integrating paleotectonic and associated lithologic
information. For example, a eugeoclinal tectonic setting correlated with volcanic rocks
indicates deep-sea environments. This tectonic setting could also be a back-arc basin (e.g.
the Sea of Japan) which is of shelf depth. I am not aware of any chert in back-arc basins,
51
perhaps because of siliciclastic dilution. In the absence of other types of evidence, no
depositional environment or chert deposit type was assigned.
6.4.4. The fossil content analysis
No assumptions were made regarding the content of siliceous organic remains.
Therefore, only explicit information from the analyzed works has been taken into
consideration in assessing the siliceous fossil content of the rocks.
6.4.5. The geologic age adjustment
A crucial requirement for compiling worldwide chert occurrences with epochlevel resolution is the integration of different Ordovician stratigraphic scales – British,
Scandinavian, Russian, North American, Chinese, Australian (fig. 6.2.). In order to
compare and interpret worldwide chert deposits within a consistent temporal framework
at the epoch/series level, and even at a higher temporal resolution, a unifying correlative
Ordovician stratigraphic scale was built and used (based on Nikitin et al., 1986; Harland
et al., 1990; Webby, 1995; Harris et al., 1995; Palmer and Geissman, 1999; Veevers,
2000; Ferguson and Fanning, 2002). The Ordovician time scale used herein as a template
is the “1999 Geologic Time Scale” of the Geological Society of America (Palmer and
Geissman, 1999) that in turn relies on the British stratigraphic nomenclature.
Potential for confusion arises at the stage level in the Upper Ordovician. This
epoch includes the Caradocian and Ashgillian stages. Unfortunately, the epoch placement
of the Caradocian stage has not been consistent through time. Presently it is widely
52
Figure 6.2: Ordovician correlation chart used in this study (compiled from Nikitin et al.,
1986, 1991; Harland et al., 1990; Harris et al., 1995; Webby, 1995; Palmer and
Geissman, 1999; Veevers, 2000; Ferguson and Fanning, 2002).
53
accepted that the Caradocian represents the basal stage of the Upper Ordovician epoch.
However, until less than one decade ago, the American geological literature considered
the Caradocian stage as the uppermost stage of the Middle Ordovician. Even some of the
most recent works that deal with the Ordovician use this stratigraphic system. One of the
main problems encountered during this study in terms of age, was facing published
Ordovician reports that did not define the stratigraphic system used. Hence, a late Middle
Ordovician chert deposit in the old stratigraphic system is an early Upper Ordovician
deposit according to the new stratigraphic system. In concordance with the newest
geochronological scale for the Ordovician period (Palmer and Geissman, 1999), the
duration of the Caradocian stage was 9 My (458 Ma – 449 Ma). Including these 9 million
years into the Middle or the Upper Ordovician changes significantly the duration of the
two epochs. Furthermore, analysis and interpretation of the abundance of the same rock
type at a worldwide scale for periods of time with coincident names but different age
ranges can yield considerable errors.
6.4.6. The chert occurrence quantification guidelines – over-representation
versus under-representation of the Ordovician chert abundance at the epoch
level
The basic geological stratigraphic unit, that is, the formation, represents the
quantifying unit used in this study: one chert-bearing formation is counted as one chert
occurrence. At the epoch level, the same one chert-bearing formation spanning over two
Ordovician epochs or even the whole Ordovician, can result theoretically in two and
respectively three chert occurrences. Considering any deposit of suspicious age as a
54
deposit that spans multiple epochs, and counting it as such, could eliminate the errors, but
at the same time it would generate a over-representation of chert. The risk of such a chert
over-representation is even greater if we consider that one of the most problematic
Ordovician age limits at epoch level is that between the Middle and the Upper
Ordovician, a time demonstrated herein as a transition from a period poor in chert
formation to a very rich chert deposition period. In view of these facts, the chert deposits
of uncertain or widely spread age were eliminated, and only those with well defined
epoch or stage ages were considered in the quantitative results, even though this decision
results in an under-estimation of the worldwide Ordovician chert abundance.
Each formation was considered as only one chert occurrence, even for formations
with more than one chert members. If a group had more than one chert formation, as
many chert occurrences as defined formations were taken into account. Additionally, the
same formation can correspond to two different adjacent depositional environments
(lagoonal/peritidal and shelf or shelf and slope/basinal settings), in which case it resulted
in two chert occurrences, each corresponding to one of the two depositional settings. The
chert deposits associated with bentonites were not counted in the quantification phase of
the depositional environment analysis, and are not included in chert abundance plots.
They were nevertheless recorded as bedded chert deposits.
6.5. Pitfalls/Problems of a study compiling data from published reports:
In any compilation of data from multiple sources, inconsistencies arise. Some of
the problems encountered in this study are listed below.
55
1) Older rocks are less represented in the rock record and therefore an
extrapolation of the Ordovician chert deposits with the aim to compare their abundance
with those of other geological periods could generate errors. Normalization to outcrop
area can lessen the impact of this problem at the system level, but to date outcrop area has
not been calculated at the epoch and stage level for the Ordovician.
2) Some chert deposits are less well documented than others, resulting in sparse
information regarding the age, stratigraphy, sedimentology, faunal content, depositional
environments, and etc.
3) The lack of information of the published data relative to the definition of
certain geological notions whose connotations may largely vary, with strong implications
on interpretation (e.g., the Ordovician time scale used considering that it has been
changed through time, the common shallow- vs. deep-water setting ambiguity with
respect to the basin morphology and implicitly the basin type).
4) Some stratigraphic intervals are easier to date than others depending mainly on
the type of faunal content.
5) Some paleoenvironmental settings are characterized by particular factors (e.g.,
starved, closed basins) and therefore certain inter-basinal correlations are pointless.
Moreover, the formation of chert deposits within such localized-conditions may not
follow a general chert genesis mechanism, but they are useful because they evidence
endemic conditions and the amplitude (at the basin and even global level) of a peculiar
mechanism responsible for their formation.
56
6) Stratigraphic inconsistencies regarding definitions of formations and groups
lead to some artificial variations in chert abundance. For example, an over-estimation of
the chert occurrences is possible when a sedimentary unit with multiple chert horizons
was considered a group and not a formation (e.g., the Plattin Group with three chert
formations resulting in three chert occurrences; before it was considered a formation and
not a group, and would have been recorded as only one chert occurrence).
7) The paleogeographic position of some micro-continents is still debated and
thus the data interpretation with respect to the paleolatitudinal distribution of the
occurrences encounters errors. An eloquent example is that represented by the Argentine
Precordillera which is considered either as part a of Gondwana that collided with
Laurentia during the Ordovician (Dalla Salda et al., 1992), or as a microcontinent that
rifted from the Ouachita embayment of North American Laurentia during the Cambrian
and collided with Gondwana during the Ordovician (Thomas and Astini, 1996).
8) The worldwide rock sequences are unequally studied due to objective
impediments (relief, scientific potential, political regimes, and etc.), and thus the absence
of a geological character of certain geographical areas might reflect a lack of knowledge
and not a physical absence. A prime and very strong bias of a worldwide compilation
study is represented by the geographic component.
9) The published information easily accessible is strongly biased toward the North
American continent, and thus geological studies achieved in regions with a weak
international communication system are highly underrepresented in databases (e.g., South
America, Western Europe, Asia, Eastern Europe, and etc.). This bias is even larger if we
57
consider that these modern regions were representing different paleocontinents,
paleolatitudes, and very likely different physical and biologic ecosystems.
6.6. The usefulness of a study compiling data from published reports:
1) This study integrates the biotic component with the abiotic physico-chemical
factors as interconnected parts of the same Earth system.
2) Lithologic compilations offer a general image of the worldwide Ordovician
early diagenetic chert occurrences such as bedded, nodular cherts, and silicified oolites.
3) Paleogeographic chert distribution can be used to infer paleo-upwelling areas
and/or high endogenous and exogenous dissolved silica and other nutrient inputs that
could lead to the organic, inorganic or organic but induced inorganically chert formation.
The study can also offer a good image of the Ordovician shelf areas suggesting the spatial
position of the paleo-shorelines.
4) Based on depositional settings, siliceous organism migration patterns, as well
as potential factors responsible for the formation, accumulation, and preservation of chert
units can be inferred.
5) Local and general oceanic circulation patterns can be distinguished pointing
toward local and global events.
6) Possible source rocks and hydrocarbon reservoirs could be identified.
7) The study offers the possibility of not only intra-basinal correlations, but also
of inter-basinal stratigraphic correlations.
58
Chapter 7
RESULTS
The comprehensive compilation of Ordovician cherts yielded 191 deposits
covering 5 continents: North America, South America, Europe, Asia, and Australia. The
appendix includes all chert formations that have been found, regardless of the degree of
certainty of their age. However, the analysis of chert abundance includes only the 161
chert deposits with ages well defined at the epoch level or higher resolution. The
descriptive information included in the appendix represents a concise statement of the
information in the FileMaker database that is available upon request.
7.1. Epoch - level temporal trend in the chert abundance
Figure 7.1A illustrates the epoch-level chert distribution expressed as both raw
data (counts) and time-normalized data (Table 7.1). Both representations show the same
pattern throughout the three Ordovician epochs: abundance peaks for the Lower and
Upper Ordovician, contrasting with a sharp decline during the Middle Ordovician. More
than half of the Upper Ordovician chert deposits (approximately 51%) are Caradocian, 14
deposits are attested as Ashgillian (19%), and the rest of 22 deposits (30%), have
undifferentiated Upper Ordovician age (fig. 7.1B). Time normalized data (Table 7.1)
shows that the Caradocian chert peak stands as the highest Ordovician chert abundance
59
Table 7.1: Temporal distribution of the chert deposits at epoch and age/stage level. Raw
and time-normalized data.
Epoch
No.
Upper
74*
No.
Ashgillian
14
Caradocian
38
Normalized data to Duration
time N/Ma
Ma
2.3
6
4.2
9
4.9
Middle
24
2
12
Lower
63
3.2
20
Total = 161
* the number represents 14 (Ashillian) + 38 (Caradocian) + 22 (undifferentiated Upper
Ordovician)
Figure 7.1: Ordovician distribution of chert deposits. Numbers next to dots represent
number of cherts per million years. (A) Epoch level distribution: raw (N) and timenormalized data (N/Ma). (B) Stage level distribution expressed as percentages from a
total of 74 chert deposits.
60
interval, with 4.2 chert deposits per 1 million years. If all of the undifferentiated Upper
Ordovician cherts would prove to be Ashgillian, then the maximum chert abundance
would shift toward the Ashgillian. However, this is not likely, because stratigraphic
information on these cherts suggests that some of them may have extended over both the
Caradocian and Ashgillian.
In concordance with the Late Ordovician short duration glaciation theory
(Brenchley et al., 1994), the two stages of the Upper Ordovician represent geological
periods of contrasting climates: the Caradocian corresponds to an ice free world whereas
the Ashgillian was an icehouse world. Hence, the differentiation of the Upper Ordovician
cherts at the stage level proves is significant.
7.2. Epoch - level temporal trend in chert types
The chert-bearing formations were differentiated into: i) nodular cherts, ii) bedded
cherts, and iii) formations with both nodular and bedded cherts (Table 7.2). Figure 7.2A
illustrates the epoch-level distribution in both raw and time-normalized absolute counts
for the three types of chert deposits, and shows a trend similar to that of the chert
abundance at the epoch-level: Lower and Upper Ordovician peaks – the latter steeper
than the former – separated by a Middle Ordovician drop. Nodular cherts decline in
relative abundance from the Lower to the Upper Ordovician while the number of bedded
cherts increases (fig. 7.2B), revealing a significant facies change in chert type through the
Ordovician.
61
Table 7.2: Temporal distribution of the chert deposit types at epoch (A) and stage (B) level. Raw, time-normalized, and relative
abundance data.. 17* = 3 + 10 + 4 (undifferentiated Upper Ordovician); 28* = 2 + 5 + 11 (undifferentiated Upper Ordovician); 16* =
6 + 7 + 3 (undifferentiated Upper Ordovician).
Nodular cherts
A
Bedded cherts
Nodular & bedded cherts
Undetermined
Total N
Epoch
N
%
N/Ma
Upper
17
23
1.1
Middle
4
16.7
Lower
21 33.3
N
%
N/Ma
N
%
N/Ma
28 37.8
1.9
16
21.6
1.1
13 17.6
0.9
74
0.3
9
0.8
6
25
0.5
5 20.8
0.4
24
1.1
20 31.7
1
12
19
0.6
10 15.9
0.5
63
37.5
N
%
N/Ma
161
B
Epoch
Upper
Nodular cherts
Stage
N
Ashgillian
3
Caradocian
10
Bedded cherts
N/Ma
17*
0.5
1.1
1.1
N
2
N/Ma
28*
15
Nodular & bedded
0.3
1.9
1.6
N
6
N/Ma
16*
7
1
1
0.8
Middle
2
0.2
5
0.4
12
1
Lower
21
1.1
21
1.1
17
0.9
62
Figure 7.2A: Temporal distribution of chert deposit types at the epoch level. Raw (N)
and time-normalized (N/Ma) data. Numbers next to dots represent number of cherts per
million years.
Figure 7.2B: Temporal variation of the relative abundance of chert deposit types at the
epoch level (light gray - nodular cherts; medium gray - bedded cherts; dark gray deposits with nodular and bedded cherts; white - undefined chert deposits).
63
7.3. The spatio-temporal distribution of chert occurrences
Table 7.3 and Figure 7.3 display the variation in time, space and depositional
environment of both raw and normalized count data through the Ordovician. The
variation of the normalized data parallels that of the raw data. The main Ordovician chert
deposition sites are plotted on paleogeographic maps (fig. 7.4, 7.5, and 7.6). The epochlevel distribution of these sites along with their depositional environments reveal
temporal, spatial, and evolutionary patterns in silica accumulation throughout the
Ordovician.
7.3.1. Lower Ordovician
Chert depositional environments during the Lower Ordovician are dominantly
shallow water carbonate settings in which most silica burial occurred as peritidal
silicified oolites, and as nodular and bedded cherts that are evenly dispersed between
peritidal/lagoonal and shelf environments. All peritidal/lagoonal chert occurrences are
silicified oolite-bearing formations. On the other hand, the slope/basin chert-bearing
sequences (a quarter of the Lower Ordovician total) indicate that deep-water silica
deposition represented a quite significant component of silica burial during this time as
well. The deep water chert facies are dominated by bedded cherts supplemented by
nodular cherts (Table 7.4, fig. 7.7A).
The paleolatitudinal range of the Lower Ordovician cherts (fig. 7.4) extends from
the equator up to 45 – 50˚ north and south. The deep-water cherts, confined principally to
lower paleolatitudes (0 - 30˚), cluster on the southeastern margin of Laurentia (in modern
Canada and Great Britain), plus a few on the Kazakhstan block and Gondwana
64
Table 7.3: Temporal (A epoch level; B stage level) and spatial (Onshore – Offshore) distribution of the chert deposits. Raw and time-normalized
data. 33* = 17 + 9 + 7 (undifferentiated Upper Ordovician); 38* = 19 + 4 + 15 (undifferentiated Upper Ordovician); * the total 171 is different
from the total 161 (Tab. 7.1) because a couple of chert deposits have been assigned to multiple depositional environments. Raw, relative
abundance, and time-normalized data.
A
Peritidal/Lagoonal
Shelf
Slope/Basin
Undecided
Total N*
Epoch
N
%
N/Ma
N
%
N/Ma
N
%
N/Ma
N
%
N/Ma
N
%
Upper
3
3.8
0.2
33
42.3
2.2
38
48.7
2.5
4
5.1
0.3
78*
100
Middle
2
7.7
0.2
5
19.2
0.4
12
46.2
1.0
7
26.9
0.6
26*
100
Lower
21
31.3
1.1
21
31.3
1.1
17
25.4
0.9
8
11.9
0.4
67*
100
171*
B
Peritidal/Lagoonal
Epoch
Upper
N
Ashgillian
0
Caradocian
3
Shelf
N/Ma
3
0
0.2
0.2
N
9
N/Ma
33*
17
Slope/Basinal
1.5
2.2
1.9
N
4
N/Ma
38*
19
0.7
2.5
2.1
Middle
2
0.2
5
0.4
12
1
Lower
21
1.1
21
1.1
17
0.9
65
Figure 7.3: Ordovician spatio-temporal distribution of chert occurrences (percentages and time-normalized data). The dots represent
number of cherts per million years.
66
Figure 7.4: Paleogeographic distribution of Lower Ordovician cherts
67
Figure 7.4.: Paleogeographic distribution of Lower Ordovician cherts. Red bullets
represent peritidal/lagoonal cherts; black bullets represent shelf cherts; stars represent
slope/basinal cherts; dotted bullets are cherts bearing siliceous sponge spicules from
Argentina, not included in the Appendix; diamonds represent cherts whose depositional
environment is unknown. Solid arrows represent cold currents and dashed arrows
represent warm currents (after Berry et al., 1995). The paleoegeography was obtained
using the PGIS/MacTM software, version 5.0 (Ross, 1997)
68
Figure7.5: Paleogeographic distribution of Middle Ordovician cherts
69
Figure 7.5.: Paleogeographic distribution of Middle Ordovician cherts. Red bullets
represent peritidal/lagoonal cherts; black bullets represent shelf cherts; stars represent
slope/basinal cherts; diamonds represent cherts whose depositional environment is
unknown. Solid arrows represent cold currents and dashed arrows represent warm
currents (after Berry et al., 1995). The paleoegeography was obtained using the
PGIS/MacTM software, version 5.0 (Ross, 1997).
70
Figure 7.6: Paleogeographic distribution of Upper Ordovician cherts
71
Figure 7.6.: Paleogeographic distribution of Upper Ordovician cherts. Red bullets
represent peritidal/lagoonal cherts; black bullets represent shelf cherts; stars represent
slope/basinal cherts; diamonds represent cherts whose depositional environment is
unknown. Solid arrows represent cold currents and dashed arrows represent warm
currents (after Berry et al., 1995). The paleoegeography was obtained using the
PGIS/MacTM software, version 5.0 (Ross, 1997).
72
Table 7.4: Distribution of the chert deposit types by epoch and depositional environment (TL – TM – TU = total Lower – total Middle
– total Upper; e.g. 21 - 2 - 3). Raw data.
Peritidal / Lagoonal
Epoch
Shelf
Slope/Basin
Undefined
Total
N
B
N&B
U
N
B
N&B
U
N
B
N&B
U
N
B N&B U
Upper
0
0
2
1
16
1
14
2
1
24
4
9
0
2
0
2
78
Middle
1
0
1
0
2
0
2
1
0
7
3
2
2
2
1
2
26
Lower
13
0
7
1
6
5
4
6
0
12
3
2
3
3
1
1
67
21-2-3
21-5-33
17-12-38
8-7-4
171
73
Figure 7.7: Relationships between chert types and depositional environments over time.
Relative abundance data. (A) Lower; (B) Middle; (C) Upper Ordovician.
74
supercontinent (in Australia and New Zealand). Only one slope/basin chert deposit occurs
beyond 30˚ south on Gondwana (Bolivia). The mid-latitudinal cherts (30˚ - 50˚), which
are represented mainly by shelf deposits, are located north of the equator on Gondwana
(China and Korea). All the peritidal/lagoonal cherts are restricted to Laurentia (the southsouthwestern margin) at low paleolatitudes, all occur in the USA, except for one in Great
Britain.
Overall, the trend of the silica burial during the Lower Ordovician is dominated
by low paleolatitudinal cherts (90.5%), followed by mid-latitudinal cherts (9.5%;Table
7.5). Geographically, approximately 75% of the total Lower Ordovician cherts occur on
Laurentia, averaging 2.4 deposits per My (Table 7.6).
7.3.2. Middle Ordovician
During the Middle Ordovician, a pronounced decline in silica burial occurred,
coeval with a significant migration of chert deposits from shallow settings,
peritidal/lagoonal and respectively shelf, toward slope/basin sites (fig. 7.3). Middle
Ordovician deep-water environments represent the dominant locus in silica accumulation,
mainly as bedded cherts (fig. 7.7B, Table 7.4). The formation of silicified oolites almost
ceases (Table. 7.7), and that of the shelf cherts strongly diminishes. A slight decrease is
recorded in the abundance of mid-latitudinal cherts concomitant with an increase of the
low-latitudinal ones (Table 7.5). Paleogeographically, the Laurentia data reveal a
decrease in the chert abundance by approximately 20%, or by more than a half
considering the time-normalized data (Table 7.6), most of the occurrences again on the
southern margin (fig. 7.5). Only the Kazakhstan block registers a slight increase in the
75
Table 7.5: Temporal trends in the paleolatitudinal distribution of Ordovician silica
burial.. Raw and relative abundance data.
Epoch
0 - 300
30 - 600
> 600
Total N
N
%
N
%
N
%
N
%
Upper
71
96
1
1.3
2
2.7
74
100
Middle
22
91.7
2
8.3
0
0
24
100
Lower
57
90.5
6
9.5
0
0
63
100
161
chert abundance. The geographic distribution of deep-water cherts follows the same
patterns as that characteristic for the Lower Ordovician ones, that is, the southeastern
margin of Laurentia, the Kazakhstan microcontinent, and Australia.
7.3.3. Upper Ordovician
The Upper Ordovician silica burial as recorded by numbers of chert deposits
surpasses by far the two older Ordovician epochs (fig. 7.3). Both shelf and slope/basinal
deposits record a significant increase compared to both previous epochs, Lower and
Middle Ordovician. A balance between shelf and deep-water cherts, that was tipped
heavily toward deep-water cherts in the mid-Ordovician, is restored. The few
peritidal/lagoonal chert facies hosting silicified oolites all occur in the Caradocian (Table
7.6). The first high latitude and north margin Laurentian cherts appeared in this time
76
Table 7.6: The temporal and geographical distribution of the chert deposits. Raw, relative abundance, and time-normalized data.
Gondwana & close
blocks
Laurentia
Epoch
Kazakhstan block
Others
Total
Duration Ma
N
N/Ma
%
N
N/Ma
%
N
N/Ma
%
N
N/Ma
%
9
1.5
64.3
3
0.2
21.4
2
0.1
14.3
*
*
*
14
Ashillian: 6
28
3.1
73.7
9
1
23.7
1
0.1
2.6
*
*
*
38
Caradocian: 9
Middle
13
1.1
54.2
5
0.4
20.8
4
0.3
16.7
2
0.2
8.3
24
12
Lower
47
2.4
74.6
13
0.7
20.6
3
0.2
4.8
*
*
*
63
20
Upper*
* The Upper Ordovician cherts do not include the deposits whose age is either undifferentiated Upper Ordovician (12 deposits) or span the whole
Upper Ordovician (10 deposits). The numbers were obtained from Appendix.
Table 7.7: Temporal distribution of Ordovician silicified oolites. Raw and time-normalized data.
Epoch
Stage
N
N/Ma
Ma
Ashgillian
0
0
6
Caradocian
3
0.3
9
Middle
2
0.16
12
Lower
12
0.6
20
Upper
77
interval (fig. 7.6; Table 7.5). The low latitude deposits continued to dominate the system.
Deep-basin deposits center around Laurentia and numerous Gondwanan deep chert facies
are identified in Australia. Most of the shelf deposits are limited to the southern and
western margins of Laurasia. The examination of the spatial distribution of the cherts by
continents shows that Caradocian cherts increased by almost a factor of three in
Laurentia, and doubled in Gondwana and the proximal microcontinents (Table 7.6).
7.4. Chert type - depositional environment relationship over time
Figure 7.7 illustrates the relationship between the chert deposit types and the
depositional environment in which they formed (Table 7.4). Nodular cherts dominate the
peritidal/lagoonal and shelf settings, in contrast to the bedded cherts that dominate deepwater habitats. This relationship between chert type and depositional environment
persists throughout the Ordovician: shelf settings are characterized by evenly dispersed
nodular and bedded cherts within a carbonate environment, whereas deep-water settings
are clearly dominated by bedded cherts within a siliciclastic environment. Nodular cherts
can nonetheless occur in slope/basinal settings, and bedded cherts in shelf and even
shallower environments. By the Middle Ordovician the silicified oolites almost disappear
from the chert system.
78
7.5. The temporal and spatial distribution of the Ordovician silica secreting
organisms present in chert deposits
More than a third of the chert occurrences (37.2%) contain siliceous skeletal
remains (Table 7.8). The majority host radiolarians, others siliceous sponge fragments,
and a few contain both types (fig. 7.8).
A look at the depositional environments of cherts bearing siliceous organisms
reveals that during the Early Ordovician both radiolarians and siliceous sponges inhabited
deep as well as shallow water niches (Table 7.9Ab). During the Lower Ordovician most
of the radiolarian remains are hosted by slope/basin bedded cherts. The siliceous sponge
remains have been found only in nodular and bedded chert type. Most sponge-bearing
cherts are found in shallow-water sedimentary sequences (peritidal/lagoonal and shelf),
and only two, or possibly three, slope/basin cherts associated with ophiolitic successions
preserve siliceous sponges (e.g., the Burubaital Formation in Kazakhstan, the lower part
of the Cow Head Group of Newfoundland, and possibly the Durness Limestone of
Scotland). Overall, siliceous sponges are associated with carbonate rocks, and
radiolarians with clastic sediments such as shales, graptolitic black shales, and ophiolitic
complexes.
During the Middle Ordovician, more than half of the chert occurrences contain
siliceous skeletal remains (Table 7.8), the majority represented by radiolarians.
Radiolarian-bearing formations are exclusively bedded chert from deep-water sequences
(Table 7.9 Ba and Bb). The few siliceous sponge-bearing formations are both shallow
and deep water cherts of bedded and nodular types.
79
Table 7.8: Epoch level distribution of chert deposits with siliceous skeletal remains. *SSR = siliceous skeletal remains. Raw and
relative abundance data.
Radiolarians
Sponges
Radiolarians & Sponges
Epoch
N
%
19
25.7
Middle
10
Lower
Total
Upper
Total fm. with
SSR*
%
Total
%
N
%
5
6.8
4
5.4
28
37.8
74
41.7
2
8.3
2
8.3
14
58.3
24
8
12.7
6
9.5
4
6.3
18
28.6
63
37
23
13
8
10
6.2
60
37.2
161
N
Figure 7.8: Epoch-level variation of the relative abundance of chert formations that bear siliceous skeletal remains (deposits with
radiolarians, deposits with siliceous sponges, and deposits with both radiolarians and siliceous sponges).
80
Table 7.9: Relationships between chert deposit types and types of siliceous skeletal
remains for the Lower (Aa), Middle (Ba), and Upper Ordovician (Ca). Relationships
between depositional environments and types of siliceous skeletal remain for the Lower
(Ab), Middle (Bb), and Upper Ordovician (Cb). Absolute counts.
Lower Ordovician
Aa
N
Radiolarians
Sponges
Both
B
N&B
U
Total
2
2
2
2
1
8
6
4
Total
18
U
Total
6
2
1
Ab
P/L
Radiolarians
Sponges
Both
3
1
Shelf
S/B
2
3
7
9
6
3
2
Middle Ordovician
Ba
N
Radiolarians
Sponges
Both
B
N&B
U
Total
6
1
2
1
3
10
2
2
Total
14
1
Bb
P/L
Radiolarians
Sponges
Both
1/2
Shelf
S/B
U
Total
2
1
8
1/2
2
10
2
2
81
Table 7.9: continued.
Upper Ordovician
Ca
N
Radiolarians
Sponges
Both
B
N&B
U
Total
7
19
5
4
Total
28
S/B
U
Total
16
3
19
5
4
12
3
2
2
2
Cb
P/L
Radiolarians
Sponges
Both
Shelf
5
1
3
Siliceous skeletal remains (SSR), mostly radiolarians, occur in 28 Upper
Ordovician chert facies. Here too, radiolarians characterize deep-water sedimentary
successions, whereas siliceous sponges are found in nodular and bedded cherts in either
shelf or slope/basin settings.
The Ordovician formations with siliceous skeletal remains reveals a clear affinity
between radiolarians and deep-water bedded cherts, in contrast to the siliceous sponge
remains that can occur in any type of chert deposit (bedded or nodular) and depositional
environment (deep or shallow water) (Table 7.9). From the Lower to the Upper
Ordovician, formations bearing siliceous organisms change from mainly
82
peritidal/lagoonal settings to shelf/basin settings, so that by the Upper Ordovician there is
no evidence of peritidal/lagoonal chert formations bearing siliceous sponge skeletal
remains. Radiolarian-bearing formations also exhibit trend from shelf to slope/basin
settings by the Middle Ordovician, so that in the Middle and Upper Ordovician
radiolarians are almost exclusively preserved in deep-water sedimentary sequences.
Most nodular cherts are hosted by carbonate rocks such as limestones and
dolomites, and most bedded cherts by shales. This finding supports the view according to
which some authors make a difference between the two types based solely on the host
rock – the chert deposits occurring in carbonate host rocks being considered nodular
cherts, and the chert deposits occurring in shale sequences being considered bedded
cherts (Wise and Weaver, 1974).
7.6. Testing the significance of associations between the type of siliceous
skeletal remains, chert deposit styles, and depositional environments
The chi-square test is a statistic used for testing the significance of certain
distributions (the distribution between the columns and the rows of a table) or in other
words for comparing observed data with data expected to be obtained according to a
specific hypothesis. The null hypothesis (Ho) states that there is no significant difference
between the expected and observed data. If any expected value of any category is less
than 5, a correction has to be applied. In this respect, Yates’ correction represents an
arbitrary adjustment to the chi-square test when applied to tables with one or more cells
83
with frequencies less than 5 (e.g., the expected values of the R-P/L and SS-P cells in fig.
7.10). However it can be applied only to 2x2 tables.
The Chi-square test was applied to the following distributions: 1) chert deposit
type (nodular vs. bedded cherts) and silica secreting organisms (radiolarians vs. siliceous
sponges) (fig. 7.9); 2) the silica secreting organisms (radiolarians vs. siliceous sponges)
and the depositional environment (shallow represented by peritidal/lagoonal together
with shelf vs. slope/basinal) (fig. 7.11); and 3) the chert deposit type (nodular vs. bedded
cherts) and the depositional environment (peritidal/lagoonal vs. shelf vs. slope/basinal)
(fig. 7.10). All of the distributions proved to characterize statistically significant
associations between the tested pairs of variables.
7.7. Evaluation of the efficiency of the method
The global compilation of Ordovician chert occurrences undertaken in this study
has several characteristics that single it out among other such attempts, both in its
approach, and the results generated. The approach which targeted only one geological
period, the Ordovician, allowed extensive data gathering and detailed processing of these
data with respect to multiple aspects: the chert deposit type, depositional environment,
fossil content with respect to silica secreting organisms, and age. The results thus obtain
offer a solid empirical base for tackling several questions related to chert formation and
preservation, and allow outlining and discussion of patterns in the spatial and temporal
biogenic chert distribution.
84
Figure 7.9: Chi square test of the relationship between the type of siliceous organisms
(R = radiolarians; SS = siliceous sponges) and the chert deposit style (B = bedded
cherts; N = nodular cherts).
B
N
Upper
Middle
Lower
R-B
R-N
SS-B
SS-N
R
SS
Total
34
6
15
16
49
22
40
31
71
R-B
R-N
SS - B
SS - N
16
9
9
2
2
2
6
4
5
7
3
6
34
6
15
16
O
E
O-E
(O - E)2
(O - E)2/E
34
6
15
16
27.61
12.39
21.39
9.61
6.39
-6.39
-6.39
6.39
40.89
40.89
40.89
40.89
1.48
3.30
1.91
4.26
10.95
χ calc = 10.94; χ 0.05,1 = 3.84
H0: there is no relationship between the chert deposit type and the organisms
H1: there is a relationship between the chert deposit type and the organisms
calculated χ > theoretic χ → the null hypothesis is rejected
85
Figure 7.10: Chi square test of the relationship between the siliceous organisms (R =
radiolarians; SS = siliceous sponges) and the depositional environment (P/L =
peritidal/lagoonal; Sh = shelf; S/B = slope/basinal).
I. The depositional environment is divided in three different depth-related settings
P/L
Sh
S/B
R
SS
Total
1
3
38
5
11
8
6
14
46
42
24
66
R-P/L
R-Sh
R-S/B
SS-P/L
SS-Sh
SS-S/B
0
0
1
1
0
2
19
10
9
0
1
4
6
1
4
3
3
2
1
3
38
5
11
8
O
E
O-E
(O-E)2
(O-E)2/E
R-P/L
1
3.82
-2.82
7.94
2.08
R-Sh
R-S/B
SS-P
SS-Sh
SS-S/B
3
38
5
11
8
8.91
29.27
2.18
5.09
16.73
-5.91
8.73
2.82
5.91
-8.73
34.92
76.17
7.94
34.92
76.17
3.92
2.60
3.64
6.86
4.55
Upper
Middle
Lower
23.65
Because three cells have values equal or less than five, and because the Yates’s
correction can be applied to only 2x2 table, the significance has been tested between
the silica secreting organisms and shallow waters represented by peritidal/lagoonal
together with shelf environments vs. slope/basinal settings.
86
Figure 7.10: continued.
II. The depositional environment is divided in two different depth-related settings:
shallow (peritidal/lagoonal and shelf) vs. deep (slope/basinal)
P/L & Sh
S/B
P/L&Sh-R
S/B-R
P/L&Sh-SS
S/B-SS
R
SS
Total
4
38
16
8
20
46
42
24
66
O
E
(O-E)-0.5
(O-E-0.5)2
(O-E-0.5)2/E
4
38
16
8
12.73
29.27
7.27
16.73
-9.23
8.23
8.23
-9.23
85.14
67.69
67.69
85.14
6.69
2.31
9.31
5.09
23.40
χ calc = 23.40
χ 0.05,1 = 3.84
H0: there is no relationship between the depositional environment type and the
siliceous organisms
H1: there is a relationship between the chert deposit type and the siliceous organisms
calculated χ > theoretic χ → the null hypothesis is rejected
87
Figure 7.11: Chi square test of the relationship between the depositional environment
(shallow waters represented by P/L = peritidal/lagoonal together with Sh = shelf vs.
deep waters represented by S/B = slope/basinal) and the chert deposit style (B = bedded;
N = nodular).
P/L&Sh
S/B
P/L&Sh - N
S/B - N
P/L&Sh - B
S/B - B
N
B
Total
68
11
36
53
104
64
79
89
168
O
E
O-E
(O - E)2
(O - E)2/E
68
11
36
53
48.90
30.10
55.10
33.90
19.10
-19.10
-19.10
19.10
364.63
364.63
364.63
364.63
7.46
12.12
6.62
10.75
36.94
χ calc = 36.94
χ 0.05,1 = 3.84
H0: there is no relationship between the depositional environment and the type of the
chert deposit
H1: there is a relationship between the depositional environment and the type of the
chert deposit
calculated χ > theoretical χ → the null hypothesis is rejected
88
This global compilation resulted in the extensive database of 191 worldwide
Ordovician chert occurrences. Another important contribution of the study resides in the
fact that, in contrast to previous surveys that have focused only on bedded cherts or
silicified oolites, the present study compiles chert occurrences including three types of
chert deposits - bedded cherts, nodular cherts, and silicified oolites. This differentiation
provides a strong quantitative foundation for reliable inferences that can led to much
deeper, broader, and more detailed insights into various aspects of the silica cycle.
The epoch-level age resolution of this study is the highest achieved for an analysis
of chert occurrences. A total of 161 are dated at the epoch level or even higher resolution,
at the stage level. Work at this resolution demonstrates that perturbations in the silica
cycle exist, they can be perceived at a level of detail higher than the system level, and
they can be interpreted in terms of physico-chemical oscillations in the Earth system.
The temporal resolution of the study highlighted problems with stratigraphic
nomenclature in previous surveys. Resolution of these problems and compilation of data
for the Caradocian and Ashgillian separately was particularly important as these two
Upper Ordovician stages were very different climatically. Their separate treatment
reveals important changes in chert abundance between the two stages, changes that would
have been assigned to the Middle – Upper Ordovician transition otherwise, and
particularly linked to the Late Ordovician glaciation. These observations allows detailed
insights into the climate fluctuations during the Upper Ordovician, fluctuations that a
gross epoch-level treatment would have overlooked because of lack of resolution.
89
The structure of the dataset provided a comprehensive, and at the same time
detailed, framework for systematic surveys undertaken within this compilation on the
depositional environment of formation corresponding to each chert occurrence
(peritidal/lagoonal vs. shelf vs. slope/basin), and on the type of siliceous skeletal remains
(radiolarians vs. siliceous sponges) preserved in each chert deposit. The volume and
resolution of data thus developed allow quantitative analyses that demonstrat for the first
time, with statistically significant results, relationships between chert deposit types,
depositional environments, and silica secreting organisms. These tests support the organic
origin interpretation of most Ordovician chert deposits.
The quantitative results of this study show environmental and geographic patterns
in chert abundance, as well as temporal and spatial trends in chert distribution.
Considered in conjunction with various types of data of different nature developed by
previous studies for the Ordovician, the patterns and relationships revealed by this study
help to better understand the paleoecology of the Paleozoic silica secreting organisms,
their needs in terms silica and nutrients, and affinity for other physical and chemical
parameters, as well as their behavior regarding their response to changes in oceanic
circulation and climate parameters. They allow documentation and timing of an onshoreoffshore migration of silica secreting organisms in response to evolutionary patterns and
system perturbations.
The results also shed light onto the various factors that controlled the production,
accumulation and preservation of opal, such as organism productivity, sediment influx,
water temperature, salinity, host rock, or depositional environment. Interpreted in light of
90
actualistic and non-actualistic scenarios, the Ordovician data reveal functioning of a chert
preservation model associated or not with volcanically-induced high organism
productivity, within well aerated or anoxic bottom environments. Within this context,
another important result is the documentation of chert accumulations on the shelf, a
depositional environment not known to accumulate cherts in modern oceans.
91
Chapter 8
THE LOWER PALEOZOIC SILICA SECRETING ORGANISMS:
RADIOLARIANS AND SILICEOUS SPONGES
8.1. RADIOLARIANS
8.1.1. Systematics
Radiolarians are single-celled, siliceous-skeletonized planktonic marine protozoa,
classified within the Superclass Actinopoda, Subphylum Sarcodina, Phylum
Sarcomastigophora, Kingdom Protista (De Wever et al., 2001). As fossil remains, only
polycystine radiolarians (Order Polycystina) have been well preserved due to their
opaline silica skeleton. Polycystines are represented by spumellar radiolarians (Suborder
Spumellaria) exhibiting a spherical symmetry, and nasellar radiolarians (Suborder
Nasellaria) characterized by bilateral symmetry.
Radiolaria studies greatly intensified with the initiation of the Deep Sea Drilling
Project in the 1960s, and a well-developed radiolarian systematics has emerged. Even
though radiolarians have extant representatives, the systematics of the modern
assemblages is based on skeleton morphology and not on cell anatomy and morphology
as in classic taxonomy, due to the difficulty in growing and studying radiolarians in the
laboratory. Radiolarian biozonation became a powerful biostratigraphic tool not only for
the Cenozoic and Mezozoic Eras (Aitchinson, 1998), but also for the upper Paleozoic
Eon. However, Jones and Murchy (1986) asserted that good taxonomic radiolarian
92
descriptions were achieved only for Cenozoic and upper Mesozoic faunas, and that 95%
of the Paleozoic radiolarian assemblages were undescribed.
It is worth mentioning that the impossibility of extracting radiolarian skeletal
remains from chert samples in view of a taxonomic study has represented a major
impediment in the development of radiolarian taxonomy and biostratigraphy. Only after
designing of a technique for extracting radiolarians from siliceous rocks, the hydrofluoric
acid etching method (Pessagno and Newport, 1972; Dumitrica, 1970), did the
biostratigraphy and taxonomy of radiolarian assemblages greatly improve.
8.1.2. Ecology
Modern radiolarians are among the marine zooplanktonic microorganisms that are
represented by various species adapted to a wide ecological spectrum, inhabiting all of
Earth’s oceans from shallow water settings to deep waters of the abyssal plain. They
range from 30 microns to 2 mm in diameter, and have various test shapes with attached
spines. Radiolarians are filter feeders, predators, or live as symbionts with unicellular
algae such as dinoflagellates
(http://www.ucmp.berkeley.edu/protista/radiolaria/rads.html). Radiolarians can be
bacterivores, herbivores, symbiotrophes, detritivores, and etc (Casey, 1993). Although
most radiolarians are solitary, some species are prone to form thread colonies up to a few
meters in length (Casey 1993).
Modern radiolarians are most abundant in eutrophic, nutrient-rich warm surface
waters principally of the equatorial zone, but are also present in subtropical and tropical
environments. Their spatial distribution is controlled mainly by salinity, temperature, and
93
nutrient richness among other factors
(http://www.ucmp.berkeley.edu/protista/radiolaria/rads.html). However, as pointed out
by Abelman and Gowing (1997; p.4) “the knowledge of the vertical distribution of
polycistine radiolarian taxa in the water column is limited” so that the latitudinal and
basinal distribution patterns of radiolarians and a consistent relationship between global
distribution patterns and physico-chemical parameters of the seawater are lacking (Chang
et al., 2003). Most controlled-environmental laboratory studies have analyzed the extent
to which the seawater temperature, salinity and light intensity influence the growth,
longevity, abundance, and opal productivity of modern radiolarian species (Anderson et
al., 1989a,b,c; Anderson et al., 1990; Matsuoko and Anderson, 1992), and a few looked at
the effect of dissolved silica levels (Sugiyama and Anderson, 1997). In this respect,
studies of two modern radiolarian species demonstrate that polycystine radiolarians can
be very effective in silica uptake, occurring abundantly in surface waters that exhibit low
concentrations of dissolved silica (ca. 1 µM), and yet mineralizing robust siliceous
skeletons (Sugiyama and Anderson, 1997). The four cultures of the study characterized
by different dissolved silica level (0 µM, 50 µM, 100 µM, and 150 µM) showed that: 1)
increased silica levels do not increase the mean weight of silicate deposited per shell (the
skeletal size), 2) nor the longevity, 3) the test morphology is not enhanced with
increasing dissolved silica, 4) silica levels as high as 150 µM strongly influence
negatively the longevity, growth, and weight gain of the two species of radiolarians,
probably having a poisoning effect that inhibits their metabolic activity and kills them.
However Sugiyama and Anderson (1997) pointed out that other environmental factors
94
such as temperature, salinity, and nutrients may represent more significant limiting
factors on radiolarian growth and longevity than a low silica concentration level.
Anderson et al. (1989c) show that the optimum growth and longevity of Spongaster
tetras tetras occurred at moderately warmer temperature and higher salinities (27.5ºC and
40‰ salinity); conversely, too high (above 33ºC) or too low a temperature (15ºC) has
strong negative effect on longevity.
8.1.3. Evolution
Radiolarians go back in time as far as the Middle Cambrian (Knoll and Lipps,
1993; Won and Below, 1999) and represent the oldest planktonic microorganisms
capable of secreting a mineral skeleton (Tolmacheva et el., 2001). Their ancestor is
unknown.
The earliest-known reliably identified fossil radiolarians are known from Middle
Cambrian phosphorite-rich sediments that have accumulated in relatively shallow water
facies of a siliciclastic platform in the Georgina basin, Australia (Won and Below, 1999).
Many radiolarian siliceous spicules were extracted from calcareous concretions of three
different formations. Many other radiolarian-like fossils have been reported for the early
and middle Cambrian times (Nazarov and Ormiston, 1986; Nazarov and Ormiston, 1993;
Rozanov and Zhuravlev, 1992; Iams and Stevens, 1988; White, 1986), but they have been
regarded with skepticism by some authors (Aitchinson et al. 1998; Lipps, 1992).
The validity of the oldest reports of radiolarian skeletal remains is a debated
problem because the siliceous skeletal components most frequently preserved within
early Paleozoic facies are spicular skeleton elements, the morphology of which resembles
95
that of both siliceous sponge and radiolarian spicules. The hollow tube structure that
seems to be characteristic only for early Paleozoic radiolarians is a morphological style
common to sponge spicules and Silicoflagellata skeletons (Won and Below, 1999). As a
consequence, many Early and Middle Cambrian radiolarian-like spicules that have been
reported are regarded as hexactinellid or demosponge siliceous spicules.
The depositional environment where radiolarians emerged is a disputed subject
too – deep water versus shallow water. A few radiolarian-bearing strata from relatively
shallow water sedimentary facies of Middle and Upper Cambrian age, argue for a shallow
water radiation (Won and Below, 1999; Xiping et al., 1997). Xiping et al. (1997) in their
study of Late Cambrian Radiolaria of the Bitiao Formation (part of the Yangzte platform
margin in China), describe radiolarian species classified in the family Entactiniidae,
Suborder Spumellaria. The polycystine remains were found in shallow-water facies,
supporting the shallow water origin of the radiolarians. However, the depositional
environment of the oldest radiolarian fauna (Middle Cambrian of the Georgina Basin)
described by Won and Below (1999) is puzzling considering that during the Cambrian,
certain southern regions of the Georgina basin subsided at a much higher rate than the
rest, exhibiting rapid changes in facies (shales, limestones, dolomites and sandstones) and
thickness (Brown et al., 1968). Other deep-sea radiolarian-bearing strata often associated
with remnants of oceanic crust in accretionary prisms of ancient subduction zones,
suggest a deep water (basinal) origin. According to Tolmacheva et al. (2001), the oldest
possible deep-sea radiolarian bearing strata could be those occurring in the basaltsiliceous-terrigenous Zasuriya Formation of Upper Cambrian age, in the Altai Mountains
96
of central Asia. Aitchinson et al. (1998) describe lower Ordovician radiolarian-rich
sediments (spherical – spumellarian, and bi-polar forms) associated with ophiolitic
complexes, from the Little Port Complex in western Newfoundland (Tremadocian) and
from the Ballantrea Complex in Scotland (middle Arenigian). To date, the oldest wellattested abyssal deep-sea biogenic chert was reported recently from south-central
Kazakhstan by Tolmacheva et al. (2001). The ribbon-banded radiolarian chert fomation is
part of an ophiolitic sequence. The ages based on conodont biozonation showed that the
sequence represents 15 m.y. of continous deep-sea biogenic siliceous sedimentation
across the Cambrian-Ordovician boundary. In addition to radiolarians, megasclerites of
hexactinellid sponges have been identified in the chert, illustrating the fact that siliceous
sponges were already present in deep-sea environments by the Early Ordovician.
Another issue still open to controversy is the style of life that characterized the
first radiolarians. The fact that the oldest radiolarian-like fossils have been found
associated with and attached to sponge spicules (Bengston, 1986; Won and Below, 1999),
along with laboratory cultures that show the ability of radiolarians to attach to surfaces,
lend some support to the hypothesis that radiolarians originated from benthic forms
(Petrushevakaya, 1977; Rigby and Milsom, 2000). With regard to this, Nazarov and
Ormiston (1986) assert that Radiolaria stand for that group of organisms that might
reflect the transition from a benthic to a planktic life habit. The fauna including definitive
Cambrian radiolarians is represented by speciemens of only one family, the Echidninidae,
which are supposed to have lived as equatorial plankton (De Wever et al., 2001).
97
Regardless of the depositional environment in which radiolarians originated in the
Middle Cambrian, by the Lower Ordovician radiolarians inhabited shelf (Blome et al.,
1995a; Renz, 1990; Kozur et al., 1996), as well as deep-water settings (Aitchinson et al.,
1998; Tolmacheva et al., 2001). However, the present study shows that most Lower
Ordovician radiolarian remains are predominantly associated with bedded cherts of deep
water sedimentary sequences, a style that probably represents a continuation from the
Upper Cambrian, and possibly the Middle Cambrian. Twenty million years later, all
Middle Ordovician radiolarian-bearing formations are exclusively bedded, chertdominated deep water facies, suggesting a shelf to slope/basin migration from the Lower
to Middle Ordovician. This status is preserved in the Upper Ordovician and apparently in
the Silurian too (Tomescu et al., 2003).
During the Ordovician, radiolarians became the dominant component of the
microzooplanktonic fauna (Tappan and Loeblich, 1973). Radiolarian diversity increased
during the Ordovician when several groups were present, dominated by spherical porous
polycystines (Nazarov and Ormiston, 1986; De Wever et al., 1994). The Ordovician
radiolarians were diverse and widespread enough to allow interbasinal biostratigraphic
correlations between Eurasia and North America (Nazarov and Ormiston 1993), and a
radiolarian biozonation (Noble and Aitchinson, 2000). Nonetheless, the OrdovicianSilurian radiolarian biota records a slow evolution of spherical Spumellarians since their
emergence, compared with Middle and Late Paleozoic biotas (Nazarov and Ormiston,
1986; Vishnevskaya, 1997). If the Ordovician and Silurian radiolarian assemblages are
characteristic at the series level, during the Middle and Upper Paleozoic they developed a
98
higher diversity, allowing biostratigraphic subdivision at the stage level, and even at a
finer scale for Permian assemblages (Nazarov and Ormiston, 1988; Nazarov and
Ormiston 1993). Vishnevskaya (1997) studying Paleozoic and Mesozoic radiolarian
assemblages of the Northwestern Pacific Rim (including eastern and northeastern
Russia), distinguished four Paleozoic megacycles in the development of radiolarian
fauna, of which the first, represented by the Ordovician-Silurian interval, is characterized
by a low diversity compared to the other Upper Paleozoic megacycles. Vishnevskaya’s
worldwide compilation table (Fig. 11.1) based on radiolarian literature published until
1995, points out that world-wide species diversity of radiolarians was much higher during
the Ordovician (10 species in the Tremadocian and up to 100 species in the Caradocian)
than during the previous and subsequent periods - Cambrian with around 20 species, and
Silurian with approximately 30-40 species. According to her data, the Middle Ordovician
species diversity is surpassed only by Late Devonian (up to 125 species) and Lower
Carboniferous (around 115 species) faunas in the Paleozoic.
Contrary to the view of a low Ordovician diversity, Renz (1990), analyzing the
Ordovician radiolarian assemblages from Nevada and Newfoundland at the family level,
reported a significant radiolarian diversification as a component part of Ordovician
evolutionary changes. His study showed that by the Upper Ordovician, the dominance of
spherical polycystines (family Inaniguttidae of Nazarov and Ormiston, 1986) is replaced
by a group characterized by bilateral symmetry and similar to family Palaeoscenidiidae, a
family considered by Nazarov and Ormiston (1986) and Jones and Murchey (1986) to
have emerged in the Middle Silurian. The Upper Ordovician radiolarian assemblages
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inhabiting Gondwana (Australia) and Laurentia (Nevada, Newfoundland), but situated in
the same paleolatitudinal range (20º from the equator), have distinct characteristics
(Renz, 1990), suggesting at least a continental-level endemism.
Whether or not the Late Ordovician glaciation affected radiolarian faunas is
controversial. In contrast to the lack of major radiolarian extinction suggested by Noble et
al. (1997), Vishnevskaya (1997) remarks a noteworthy radiolarian extinction across the
Ordovician-Silurian boundary (Ashgillian – Llandoverian), the first one among the five
identified Paleozoic extinctions. Across this boundary the radiolarian diversity drops to
20 species. Following this drop, the rest of the Silurian witnesses a slight increase in
diversity upwards of 60 species (Vishnevskaya, 1997; Vishnevskaya Kostyuchenko and
2000).
8.1.4. Geologic importance
Radiolarians exemplify the oldest and the only skeletonized planktonic organisms
preserved in sedimentary sequences of all Phanerozoic systems, inhabiting the oceanic
realm since at least the middle Cambrian and until Recent times. Polycystine radiolarians
also represent the only skeletonized protists that have been found in Cambrian rocks,
albeit in low abundance (Horodyski et al., 1992). Numerous Lower Paleozoic radiolarian
assemblages are incorporated into the accretionary prisms associated with subduction
zones (Aitchinson, 1998). Because Paleozoic radiolarian-bearing chert deposits are
commonly associated with ophiolitic complexes, many authors emphasize the use of
radiolarians as a biostratigraphic tool in dating lower Paleozoic orogenic belts devoid of
other biostratigraphic potential and consequently lacking a good age control (Grunau,
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1965, Aitchinson et al., 1998; Blome et al. 1995a,b; Nazarov and Ormiston 1993).
According to Noble and Aitchinson (2000), early Paleozoic radiolarian faunas include
several test morphotypes. A Lower Paleozoic radiolarian biostratigraphy and biozonation
was set up by Nazarov and Ormiston (1993) for Eurasia, and extended subsequently
toward the North American continent, most recently by Noble and Aitchinson (2000).
The high sensitivity of radiolarians to water temperature and chemistry has
rendered them an excellent paleoceanographic and paleoclimatologic marker, mainly for
the Cenozoic era. Distribution patterns and abundance of distinct modern radiolarian
assemblages are tightly linked to the amount of nutrients dissolved in the seawater and to
water depth (Abelmann and Gowing, 1997; Kruglikova, 1993; Chang et al., 2003), in
addition to warm temperature (around 27ºC) and salinity (35-40‰) (Anderson et
al.,1989a,b,c; 1990; Matsuoko and Anderson, 1992; Sugiyama and Anderson, 1997).
Based on an actualistic scenario, the abundance of radiolarian-bearing strata has been
used as an indicator of paleo-upwelling and high productivity areas for the Phanerozoic
(Hein and Parrish, 1987). Also, radiolarian-bearing chert deposits preserved since the
Ordovician have been used successfully as an indicator of deep-water settings (distal
basinal or oceanic environments associated with orogenic belts), as well as of cratonic
seas of moderate depths (Aubouin, 1965; Ormiston, 1993). Vishnevskaya (1997) has
shown that evolutionary changes undergone by radiolarians throughout the Phanerozoic
can be linked to sea level changes, paleoceanographic regime, and plate tectonics events.
Modern Spumellarian species inhabit mainly shallow waters, as opposed to
Nasellarian species that inhabit deep waters. This ecological separation has been used to
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deduce nearshore versus offshore environments and to propose sea level fluctuations by
calculating the Spumellarian/Nasellarian ratio (Casey, 1993). However, this is possible
only for sediments younger than the Jurassic because the oldest unquestioned
Nasselarians date from the Middle Jurassic (Vishnevskaya, 1997).
8.2. SILICEOUS SPONGES
8.2.1. Systematics
Sponges, the simplest metazoan organisms (Wainright et al. 1993; Conway,
1994), feature specialized cells, but these are not grouped into distinct tissues. Sponges
are grouped into Phylum Porifera, Kingdom Animalia. The sponge classification system
is based, as in the case of radiolarians, mainly on skeleton morphology. Presently,
phylum Porifera comprises 4 classes: Demospongea, Hexactinellida, Calcarea, and
Sclerospongea (Rigby 1983). Fossil siliceous sponges have siliceous spicules as
structural skeletal elements and are represented by Class Hexactinellidae or
Hyalospongea (glass sponges), and Class Demospongea.
8.2.2. Ecology
Sponges are sessile, benthic filter-feeding metazoans. Presently, Demosponges represent
approximately 95% of all sponges (Rigby 1983), with over 600 living genera that inhabit
almost any aquatic environment from shallow to deep settings (Prothero, 2004). In the
past, including the Ordovician and the Silurian, Demosponges were important reefbuilding organisms or major components of the reef communities (Finks, 1970; Rigby,
1983). Even though most of the modern species of sponges prefer clear, shallow marine
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waters, siliceous sponges prefer deep quiet waters up to 5000 m deep (Prothero, 2004). If
we consider the hexactinellids versus lithistid demosponges, there is a differentiation in
their most favorable living habitat. Most living Demosponges prefer relatively warm,
shallow, tropical seas. However, they are known to inhabit environmental settings as
deep as 1500 m (Rigby, 1983; van Soest, 1994), and some modern Demospongea species
(e.g., Halichondria panicea) live in shallow-waters of high latitude environments (55˚ N)
as cold as 10˚C, such as those offered by the Baltic Sea (Reincke and Barthel, 1997). This
siliceous sponge genus with a cosmopolitan distribution (van Soest, 1994) represents an
important epifaunal component of the red algae zone at depths ranging from 4 to 11 m.
As such, it could compete with diatoms for the available dissolved silica in the seawater,
but this is a topic still open to controversy. On the other hand, Hexactinellids inhabit
mainly the upper bathyal zone (between 200 and 2000 m in depth, Noble, 1997;
Tabachnick, 1994), and are found also toward the deepest habitats such as abyssal and
hadal niches (below 6000 m) (Hartman 1983; Tabachnick, 1994).
The differentiation between Demosponges and Hexactinellids in terms of their
most favorable living habitat is seen as a result of their different requirements for
dissolved silica levels in the seawater: hexactinellid sponges need higher Si(OH)4 levels
than demosponges. Consequently, the deep water habitats of modern hexactinellids are
known to be the silica-rich environments (Tréguer et al., 1995; Nelson et al., 1995), or
shallow water settings that are rich in dissolved silica, such as those of higher latitudes, or
microhabitats with enhanced Si supply via rivers (Maldonado et al., 1999; Boury-Esnalut
and Vacelet, 1994).
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Siliceous sponges generally need higher dissolved silica levels than the diatoms
and radiolarians and consequently the silica limitation characteristic for neritic
environments could explain the Cenozoic migration of most siliceous sponges toward
deeper silica-rich settings (Maldonado et al., 1999). Under very low silica concentration
environments, as low as 0.741 ± 0.133 µM (the mean ± s.e. of a silica concentration range
from 0.03 to 4.5 µM), siliceous sponges are capable of secreting only very small and thin
needle-like spicules (Maldonado et al., 1999). Experiments demonstrated that there is an
asymptotic relationship between the silica uptake rates in Demospongea and the amount
of dissolved silica in the seawater (Reincke and Barthel, 1997; Frøhlich and Barthel,
1997; Maldonado et al., 1999) – the silica uptake rate increases asymptotically with
increasing silica concentration. The higher the available dissolved silica level, the more
complex and varied the spicules that are secreted by siliceous sponges (Maldonado et al.,
1999). In contrast to diatoms, the saturation level of the Demosponge genus Halichondria
panicea is much higher (above 150 µM Si-concentration) and silica uptake rates are
much lower (17 µmol h-1g-1 ash-free dry weight) (Frøhlich and Barthel, 1997). By
comparison, the saturation point of some diatom species is reached at 10µM, with an
uptake rate of 0.001 µmol Si h-1cell-1.
Is silica or nutrient limitation of greater importance? This a question raised by
many authors trying to explain the growth and evolution of siliceous sponges. However,
modern studies still argue about which parameter dominates. The silicon limitation
growth theory is supported experimentally by the work of Reincke and Barthel (1997)
and Maldonado et al. (1999). However, Maldonado et al. (1999) studying cultures of
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Crambe crambe, a demosponge species characteristic of relatively low latitude coastal
waters in tropical/subtropical enviroments (e.g., the Mediterranean Sea, between
approximately 30˚ and 45˚ north), notice that overly high seawater silica concentrations
(100 µM) may inhibit silica uptake. This observation does not fit the results obtained by
Reincke and Barthel (1997) in their experiments, probably because they used a different
demosponge taxon also characteristic for shallow waters, but populating much higher
geographical latitudes, respectively around 55˚ north.
The nutrition limitation growth theory is supported by experiments of Frøhlich
and Barthel (1997). They showed experimentally that silica uptake is an energyconsuming process and therefore in a nutrient-starved environment the biogenic secretion
of dissolved silica decreased by 15%. However, their experiment does not show what
happens to siliceous sponges if the low nutrient level goes below a certain threshold (do
siliceous sponges cease silica uptake and die?).
Normally, at higher temperatures higher silica uptake rates should occur. With
increasing temperature the silica cycling is increased by higher dissolution rates of the
skeletons that generate higher dissolved silica levels, and therefore trigger higher silica
uptake rates. However, it was shown that an increase from 10˚C to 15˚C, did not produce
a measurable increase in silica uptake rate for modern demosponges (Frøhlich and
Barthel, 1997). This could be explained by differences in the magnitude of the effects of
temperature changes on the rates of silica dissolution and silica uptake. A higher increase
in temperature would perhaps trigger perceivably higher silica uptake rates.
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The role of the siliceous sponges in the modern silica cycle is viewed as
insignificant because of the silica cycle domination by diatoms. Maliva et al. (1989)
claim that siliceous sponges once exerted the main control on the Si cycle in neritic
environments, and that throughout the Phanerozoic siliceous sponges have undergone
changes in their environment.
8.2.3. Evolution
Phylogenetic theories propose that metazoans evolved from a protist by acquiring
specialized multicellularity (Cowen, 2000), but their phylogeny is still an issue open to
controversy. Molecular work based on comparisons of small subunit ribosomal RNA
(rRNA) sequences (Wainright et al. 1993) shows that within the Metazoa sponges were
the first to radiate. Different molecular studies of sponge phylogeny since the late 1980s
support this interpretation and suggest that sponges have paraphyletic rather than
monophyletic origins (Borchiellini et al., 2000; Borchiellini et al., 2001).
All the major classes of sponges (Calcarea, Hexactinellida and Demospongea) are
known from at least the Cambrian. Porifera has been considered a conservative group of
organisms because sponge taxa display long geologic ranges (Carrera and Rigby, 1999).
Siliceous sponges occur as far back as the Neoproterozoic, as a part of the Ediacaran or
Vendian faunas. According to Wood et al. (2002), calcareous sponges could be as old as
549 Ma (Neoproterozoic). They describe a calcite-biomineralized large, modular
metazoan from a shallow marine carbonate - siliciclastic sequence (the Nama Group) in
southern Namibia. The fossil, dated at 548.8 ± 8 Ma B.P. (Grotzinger et al. 1995),
presents some morphological structures of both poriferan and cnidarian affinity.
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The oldest well-preserved siliceous axial spicules discovered to date were
identified as monaxonid Demosponges, and dated at approximately of 580 Ma (Li et al.
1998), a time of expansion of the Ediacaran fauna. They are preserved within the
Vendian Doushantuo phosphatic sequence in south China, central Guizhou. The shape of
the siliceous spicules is mostly globular, but a few of them are tubular. The identification
of the oldest well-documented siliceous sponge spicules as Demosponges suggests that a
demosponge was the ancestor of the sponges.
Younger Vendian spicules that are unequivocally from sponges, classified as
hexactinellids and not as demosponges, have been described by Brasier et al. (1997).
These spicules exhibit various morphologies demonstrating that skeletal diversity had
been achieved by Hexactinellids by the latest Precambrian. They were discovered in
Mongolia within the upper part of the Tsagaan Formation in a limestone unit overlying a
phoshorite-chert-black shale succession. The unit was dated as late Ediacaran age, 543–
549 Ma (Grotzinger et al., 1995). This finding refuted the earlier theory according to
which hexactinellids appeared during the early Cambrian (Finks, 1970; Finks, 1983) or
Middle Cambrian (Webby, 1984a). Ediacaran-age siliceous sponge spicules attributed to
Hexactinellids, have also been reported from South Australia, in the Pound Subgroup
(Gehling and Rigby, 1996), but their origin is disputed (Li et al., 1998).
The issue of the ancestry of either Demospongea or Hexactinellida within Porifera
is still debated. However, the emergence of siliceous biomineralization preceding the
calcareous biomineralization in sponges is widely accepted (Li et al., 1998; Brasier et al.,
1997).
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Based on associated lithologies, it was shown that the Late Proterozoic siliceous
sponges inhabited eutrophic environments within shallow seas, in contrast to early
Cambrian calcareous sponges that inhabited “more interior” oligotrophic settings (Li et
al., 1998; Brasier, 1992). Speculations are made on a eutrophic outer shelf origin of the
Neoproterozic mineral skeleton faunas, closely related to high Si, P and Ca
concentrations in the water column, and on a shoreward migration of these faunas during
the Cambrian transgression (Brasier, 1992; Brasier et al., 1997). The shallow water
radiation of sponges is supported by the widely accepted theory of the shallow water
origin of life (Crimes, 1992). In contrast, Webby (1984a) proposes that sponges emerged
in deeper slope environments during Late Precambrian – early Cambrian times, and
migrated and dispersed subsequently onto carbonate platform habitats “after the demise
of archaeocyaths”. Since the Neoproterozoic sponge faunas have been characterized by
high tiering levels (Yuan et al. 2002), demonstrating their evolved ability in extracting
nutrients from the water column. By the Neoproterozoic, shallow waters were dominated
by calcareous sponge associations and subordinate siliceous sponges, in contrast to deep
waters characterized by basin siliceous assemblages (Finks, 1960).
In the Early Cambrian, both monaxonic Demosponges and Hexactinellids already
represented a relatively diversified component of the benthic fauna (Finks, 1970),
inhabiting both shallow and deep-water settings. Numerous siliceous sponge fossils have
been discovered in China (early Cambrian Hexactinellids and Lithistids; Zhang and Pratt,
1994), South Australia (early Late Cambrian hexactinellid type spicules ; Bengston,
1986), and the Siberian platform (Early Cambrian Hexactinellids and Demosponges;
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Rozanov and Zhuravlev, 1992). In the Lower Cambrian Chengjian Formation of China
15 genera and 30 species of Demosponges were identified by Li et al. (1998). Yuan et al.
(2002) in their study of early Cambrian epifaunal tiers from China, identified giant
hexactinellids and smaller demosponges. Zhang and Pratt (1994), studying Early
Cambrian siliceous spicules (Hexactinellidae and Lithistidae) from argillaceous
limestones interpreted as a slope deposit in Shaanxi (south China), asserted that siliceous
sponges became moderately widespread in low-energy, offshore environments in China,
Australia, and Siberia by the Atdabanian (Lower Cambrian). Their theory supports the
proposal that siliceous sponges emerged in deep water setting (Webby, 1984a). Won and
Below (1999) mention carbonate concretions rich in siliceous sponge spicules from
Middle Cambrian sediments that accumulated in the relatively shallow water facies of a
siliciclastic platform in the Georgina basin, Australia. Kruse (1983) identified siliceous
sponges from the Georgina basin as lithistid demosponge forms. Deep water
Hexactinellid siliceous sponges are known from a Late Cambrian biogenic chert deposit,
part of an ophiolitic sequence, in Kazakhstan (Tolmacheva et al. 2001).
During the Ordovician the siliceous sponge faunas, mainly Lithistid demosponges
(Finks, 1970; Rigby 1983), became even more complex, diversified, and widespread
being represented not only by endemic but also by cosmopolitan genera. According to
Finks (1970), by the Middle Ordovician lithistids reached a relatively high diversity (17
genera), representing an important reef-building component. Overall, Ordovician sponge
records point to a higher diversity of demosponges compared to hexactinellids (Carrera
and Rigby, 1999). By the end of the Ordovician, several orders of sponges with multiple
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suborders had already evolved, demonstrating a high degree of diversification. According
to Carrera and Rigby (1999), the most abundant Ordovician endemic sponge fauna occurs
in Australia, especially in the New South Wales area, with endemism levels being lower
in the Laurentian Appalachians and China. Calcareous sponges, especially sphinctozoan
sponges, almost equaled the siliceous sponge diversity in the Middle and Upper
Ordovician.
The paleogeographic distribution patterns of Ordovician sponges illustrates a
change in sponge faunas during the Ordovician period (Carrera and Rigby, 1999). Early
Ordovician sponge faunas have relatively low diversity and are dominated by
demosponges. Middle Ordovician faunas, also dominated by demosponges, display a
higher diversity characterized by two main associations – Appalachian faunas (including
South China and the Argentine Precordillera), and Great Basin faunas. The Late
Ordovician exhibits pronounced increases in diversity of both siliceous and calcareous
sponges and provincialism with two biogeographic associations: the Pacific association
(western North America and New South Wales of Australia), and the Atlantic association
(Midcontinent Laurentia and Baltica).
Rigby and Webby (1988) analyzed Late Ordovician sponge associations from the
Malongulli Formation of Central New South Wales, Australia, and described 34 genera
divided into 44 species of siliceous sponges (both demosponges and hexcatinellids, with
the lithistid type dominating) and calcareous sponges. According to the authors, this
sponge assemblage exemplifies a unique record of deeper-water carbonate environments
bordered by an island-arc shelf sequence. This Late Ordovician New South Wales sponge
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association also suggests a clear-cut differentiation between calcareous sponges that
dominated shallow waters, and siliceous sponges (both demosponges and hexactinellids)
that dominated slope-basin environments (Rigby and Webby, 1988).
Within a shallowing interval of the lower Hanson Creek Formation (late Upper
Ordovician) Finney et al. (1999) recognized a faunal turnover in siliceous sponges. The
change from deep water, hexactinellid-dominated assemblages, to shallow, subtidal
lithistid-dominated assemblages provides an example of the association between sponge
types and depositional environment and illustrates the responsiveness of sponges to
environmental changes.
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Chapter 9
DISCUSSION: THE ORIGIN OF THE ORDOVICIAN CHERTS - ORGANIC VS.
INORGANIC GENESIS
9.1. Previous theories on chert origin
The inorganic formation of chert through direct precipitation from seawater
supersaturated with respect to silica seems the most appropriate mechanism to explain the
formation of Precambrian cherts. Conversely, the genesis of most chert deposits from the
mid-Paleozoic on is viewed and widely accepted as a biologically induced process with
the common inference on association of bedded cherts with radiolarians (Grunau, 1965;
Ramsay, 1973; Hein and Parrish, 1987), and nodular cherts with siliceous sponges
(Maliva and Siever, 1989; Maliva et al., 1989) are genetic associations. However,
controversy still exists regarding the dominant mechanism responsible for the formation
of chert deposits during the Early Paleozoic, principally the Cambrian and to a lesser
extent the Ordovician.
The Proterozoic cherts are unanimously thought to be inorganically generated
through the precipitation of silica from a silica supersaturated sea, based on dearth of
skeletal structures within cherts owing to the absence of major silica secreting biotas that
could balance the excess of silicic acid in the seawater (Siever, 1957). Several workers
have proposed that the silica cycle has changed through time, from an ocean supersatured
with respect to silica during the Precambrian, toward a silica-undersaturated ocean during
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the Phanerozoic, as siliceous organisms evolved (Siever, 1957, 1962; 1992; Maliva et al.,
1989).
The organic theory proposed as the main mechanism responsible for the
formation of most chert deposits, principally of bedded cherts, from the mid-Paleozoic
on, is widely accepted because the Paleozoic silica secreting organisms had reached an
evolutionary level that could explain their ability to keep the silica concentrations
undersaturated at all times. This conclusion is based on the high biological diversity
(Nazarov and Ormiston, 1986; Vishnevskaya, 1997), the abundance of skeletal remains
preserved in cherts during the Late Silurian, Devonian, Carboniferous and Permian, and
the inference that dissolved silica concentrations might have been highest where
sediments contained more biogenic silica. The latter is based on a modern scenario
(Calvert, 1983). The lack of any consistent inorganic process of sufficiently high
magnitude to account for the genesis of areally extensive cherts and the lack of any
coherent association patterns between sedimentary sequences supposed to be responsible
for high silica inputs and chert deposits further support a biogenic origin of Mid- to Late
Paleozoic chert.
The inorganic genesis as a dominant mechanism responsible for the formation of
Early Paleozoic cherts is supported mainly by a higher magnitude of the volcanic activity
(Ronov et al., 1980) and of silicate rock weathering (François et al., 1993). This theory is
consistent with the presence in certain bedded cherts of minerals regarded as volcanic
alteration products, such as montmorillonite, palygorskite, sepiolite, and clinoptilolite
(Wise and Weaver, 1974) and the hypothesis that there is a direct relationship between
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the abundance of clay minerals in sediments and silica concentrations (Lancelot, 1973a,b;
La Porta, 1998a). The association of certain siliceous deposits such as the bedded cherts
with volcanogenic massive sulfide (VMS), (Grenne and Slack, 2003) and the dearth or
low preservation of skeletal remains in many cherts (Von Rad and Rösch, 1974),
principally for the Cambrian, also suggests an inorganic mechanism for chert formation.
This is consistent with the calculation that seawater concentration with respect to
dissolved silica was high enough to allow the precipitation of opal-CT as chert nodules
(Siever, 1991). The Early Paleozoic oceans had high endogenous and exogenous silica
input levels driven by high continental weathering rates during the Cambrian (as inferred
from the isotopic strontium curve; Qing et al., 1998) and energetic marine volcanic
activity mainly during the Ordovician. The general assumption is that silica-secreting
organisms during the Cambrian and to a lesser extent during the Ordovician, were at the
beginning of their development, and had not reached the level of evolution and
abundance beyond which they could regulate the marine silica cycle and control the
biogenic burial of silica.
Siliceous sponges originated in the Neoproterozoic but were probably not able to
influence the marine silica cycle by lowering the silica concentration under the
precipitation level and produce biogenic cherts at a global scale until at least the Middle
Cambrian, when another group of organisms capable of extracting silica from the
seawater, the radiolarians, originated (Won and Below, 1999). The Cambrian is
moderately rich in bedded chert deposits, most of them formed in shelf environments
(Hein and Parrish, 1987; Kidder and Erwin, 2001). There is no survey regarding the
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faunal content of these cherts and the question remains: Did radiolarians and siliceous
sponges induce their formation?
The earliest reliably identified fossil occurrence of radiolarians is known from the
Middle Cambrian (Won and Below, 1999), but hypotheses were put forward for their
emergence earlier, in the Early Cambrian (Ormiston, 1993). Regarding the depositional
environment where radiolarians originated, evidence of Middle Cambrian radiolarianbearing sedimentary sequences argues for a shallow water radiation (Won and Below,
1999), a hypothesis supported by Late Cambrian radiolarian-bearing facies (Xiping et al.,
1997), and also in harmony with the shallow water origination of most major groups of
organisms (Webby, 1984a; Sepkoski and Miller, 1985). On the other hand, the deep-sea
origination hypothesis is supported by a Late Cambrian basalt-siliceous-terrigenous
formation bearing well identified radiolarians (Tolmacheva et al., 2001), the skepticism
regarding the identification of the earlier radiolarian-like fossils for the Early and Middle
Cambrian times (Aitchinson et al., 1998), and the consideration that during the Cambrian
certain southern regions of the Georgina basin, which is the area of the earliest-known
reliably identified radiolarians (Won and Below, 1999), subsided at a much higher rate
than the rest of the basin, exhibiting rapid changes in facies (shales, limestones, dolomites
and sandstones) and thickness (Brown et al., 1968), and thus casting doubt over the
hypothesized shallow environment.
The shallow water origination of radiolarians during the Cambrian would be
consistent with the more abundant shelf bedded cherts during the same period (Kidder
and Mumma, 2003), only if the association of bedded cherts and radiolarians was an
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interconnected genetic process as thought by many authors, and demonstrated herein at
least from the Ordovician on. It could also be that siliceous sponges were chiefly
responsible for chert formation in both the Early and Middle Cambrian, and that some
radiolarians accumulated in sponge cherts. No taxonomic evidence of radiolarian fossils
exists for the Lower Cambrian, when bedded cherts were forming as well (Hein and
Parrish, 1987). Yet, by the Early Cambrian, both monaxonic Demosponges and
Hexactinellids already represented a relatively diversified component of the benthic
fauna, inhabiting both shallow and deep-water environments (Finks, 1970). Thus it is
possible that by the Early Cambrian the siliceous sponge fauna might had came to the
point where it could reduce silica concentrations under the saturation levels, driving the
formation of the first biogenic cherts of both types – bedded and nodular. It is very likely
that the Cambrian witnessed a shift from a dominantly inorganic silica precipitation to a
globally biogenic induced chert formation, and from a siliceous sponge dominated silica
burial to a radiolarian-sponge dominated silica deposition, so that during the Ordovician
the silica burial was mainly biogenically controlled. The Early-Middle Cambrian shift
from a dominated inorganic silica precipitation to a biologically induced process is
supported by the deleterious effect of high silica concentration levels on the behavior of
siliceous sponges and radiolarians, finding discovered through experiments on modern
faunas (Sugyiama and Anderson, 1997; Maldonado et al., 1999). Some Cambrian nodular
chert bearing formations have been suggested of inorganic origin based on lithological
associations such as the presence of clay-rich seams at chert contacts and of clay
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inclusions within the silica matrix (La Porta, 1998a,b), but these deposits are not well
studied.
9.2. Evidence from this study
The data that support the organic origin of Paleozoic chert deposits, and the
association patterns between the type of skeletal remains (radiolarians vs. siliceous
sponges), the chert style (bedded vs. nodular), and the depostional environment in which
they form (peritidal/lagoonal vs. shelf vs. slope/basin) is mainly quanlitative. The few
compilations of worldwide Phanerozoic bedded cherts (Grunau, 1965; Ramsay, 1973,
Hein and Parrish, 1987) show the affinity and a possibly genetic relationship between
radiolarians and bedded cherts. As regards the possible association between nodular
cherts and siliceous sponges, to date there is no systematic survey, for any geological
period, of the nodular cherts and their corresponding content in biogenic siliceous
remains. The present study brings the first quantitative evidence in support of the
biogenic origin of cherts and genetically-dependent association patterns.
The organic origin of most Ordovician cherts are organic in origin is supported by
two aspects of this study. First, approximately 38% of the Ordovician chert formations
contain siliceous skeletal remains, mainly radiolarians and subsequently siliceous
sponges. Second, the onshore-offshore migration of the silica secreting organisms
perceived through the rock record of the siliceous organism bearing chert formations, is
accompanied by an onshore-offshore change in the locus of silica burial, perceived in the
stratigraphic record of cherts that lack preserved siliceous skeletal remains. The
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systematic analysis of chert content in terms of biogenic remains demonstrates that Early
Paleozoic silica secreting organisms have been underestimated with respect to their role
within the marine silica cycle and silica burial. It is worth mentioning that the
impossibility of extracting radiolarian and sponge skeletal remains from chert samples
has undoubtedly represented an impediment in the development of radiolarian and
siliceous sponge taxonomy and biostratigraphy, and hence in discovering which cherts
bear fragments of silica secreting organisms, until the 1970s. Many of the Ordovician
chert deposits were identified and described prior to the discovery of a technique capable
of extracting siliceous organisms from siliceous rocks, the hydrofluoric acid etching
method (Pessagno and Newport, 1972; Dumitrica, 1970). Based on this, it seems
probable that the number of Ordovician cherts bearing siliceous skeletal remains might
actually be higher than found by this study.
The various relationships examined in this study (Chapter 7) reveals four
statistically significant associations: first, a strong affinity between radiolarians and deepwater (slope/basinal) bedded cherts; second, siliceous sponge spicules can be found in
either bedded or nodular cherts formed preferentially in shallow environments of
peritidal/lagoonal and shelf settings; third, bedded cherts form preferentially in
slope/basinal environments, and subordinately in shallower settings; and fourth, nodular
cherts form preferentially in shallow waters of shelf and peritidal/lagoonal environments.
Even though this study reveals no single example of bedded chert deposits
containing only sponge spicules, the statistical analysis reveals an equal tendency of
siliceous sponges occuring in either type of chert deposit but mainly with shallow water
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deposits (peritidal/lagoonal and shelf). This finding supports the common conception that
bedded cherts formed principally in slope/basinal environments within a siliciclastic
regime represent primary accumulations of siliceous tests of silica secreting planktonic
organisms (Grunau, 1965; Aubouin, 1965; Hein and Parrish, 1987), and that nodular
cherts represent diagenetic products whose silica source is represented principally by
siliceous sponges (Siever, 1962; Maliva and Siever, 1989). It also emphasizes that the
formation of some shelf bedded cherts might by formed by siliceous sponges. None of
the chert occurrences from this study identified as bedded cherts that formed on the shelf
are associated with any volcanic elements or other potentially high silica generating
factors that could explain their inorganic formation. However, for a couple of Lower
Ordovician bedded cherts associated with peritidal/lagoonal environments, the biogenic
origin seems less probable (Pettijohn, 1975).
The present study shows that chert nodule formation occurred in the shallow
water settings of the shelf and peritidal/lagoonal environments, and not in slope/basinal
environments. This suggests there was a genetic relationship between siliceous sponges
and nodular cherts. The question is: why is the formation of nodular cherts mainly
confined to these shallow settings largely populated by siliceous sponges, when the
required inorganic precipitation level of opal-CT, whcih is much lower than that of
biogenic opal (40-60 mg/L SiO2 at 25°C) and which was achieved during the Ordovician
(Siever, 1991), indicates that the deeper, much richer silica environments would be more
suited for their inorganic chert formation regardless of the presence of benthic silica
secreting organisms? Moreover, the fact that many shallow water cherts have been found
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containing siliceous sponge spicules, in contrast with the scarcity of sponge spicules in
deep chert deposits, indicates an intrinsic relationship between the formation of nodular
cherts and siliceous sponges inhabiting the proximal shallow environments.
The transition from the Lower to the Middle Ordovician witnesses a migration of
the formations bearing biogenic siliceous remains toward deeper environments,
concomitant with a sharp decline in the peritidal/lagoonal and shelf cherty facies. Why
would the general pattern of the marine silica burial follow the migration of the silica
secreting organisms? The conjunction of the two events suggests that there is a direct
relationship between chert formation and the silica secreting organisms inhabiting the
most proximal environments. One could argue that during the early Middle Ordovician
the sharp sea level drop (Finney, 1997) exposed the shelf area, so that the change in the
locus of silica burial could be an artifact of a sea level lowering, and not a consequence of
the migration of the silica secreting organisms from the peritidal/lagoonal and shelf
environments. An argument against this view would be given by the total retreat of
radiolarians from shelf to basinal settings (indicated by the absence of Middle Ordovician
shelf radiolarian-bearing cherts) and the sharp drop in the abundance of the silicified
oolites (Table 7.7) characteristic of peritidal/lagoonal settings. The exposure of an
important part of the shelf area, as the one that has occurred at the Lower-Middle
Ordovician boundary, would diminish the area extent of the optimal environment for the
formation of oolites, and consequently fewer silicified oolites would be generated. Did
this surface reduction generate the drop in silicified oolites, while siliceous sponges were
still populating such very shallow environments? An answer to this question is provided
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by the depositional distribution of the chert facies in the Caradocian. During the
Caradocian, when the Ordovician sea level was at its highest (Ross and Ross, 1992), the
locus of silica burial is mainly confined to shelf and basinal settings, whereas the
peritidal/lagoonal environments are characterized by just two silicified oolite deposits.
It is true that the presence of skeletal remains does not guarantee the organic
genesis of a deposit, but neither does the lack of such remains demonstrate inorganic
precipitation of silica. While the preservation of biogenic siliceous fragments in a chert
can introduce a bias toward the idea that the chert might be of biogenic origin, the lack of
any skeletal structures gives equal chances to both organic and inorganic origins, as valid
mechanisms. Multiple types of evidence are needed in order to decide between these two
origins, and here I present the lines of evidence in support of biogenic origin for most
chert deposits:
1) The siliceous skeletal remains are found associated with cherty rocks only.
2) The onshore-offshore silica secreting organism migration is accompanied by a
corresponding change in the locus of silica burial, or in other words the depositional
patterns in which cherts occur are in concert with evolutionary events, such as onshoreoffshore faunal migrations.
3) The Early Paleozoic, silica-secreting organisms represented by radiolarians and
siliceous sponges were relatively well diversified, abundant, and globally dispersed with
efficient capabilities of silica uptake (as demonstrated by modern studies), with a high
ability to balance globally the excess of silicic acid in seawater. Additionally, studies of
modern species of radiolarians and siliceous sponges show that while they have the
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capacity to extract dissolved silica from the seawater at very low concentrations (ca.
1µM), high dissolved silica levels in the seawater (100 - 150 µM) might be toxic
(Sugiyama and Anderson, 1997; Maldonado et al., 1999) (Chapter 8). Under these
circumstances, it is im probable that the dissolved silica concentration reached saturated
or supersaturated levels at the basin level, favoring in this way the inorganic formation of
areally extensive cherts. Even though radiolarians and siliceous sponges are not as
efficient as diatoms in processing silica uptake, they still secrete silica and serve as an
effective “micro site” of silica bio-precipitation. Through the aggregation of their skeletal
parts silica-rich sites develop that, in turn, act as a silica source much higher than the
amount of the dissolved silica from the seawater. Moreover, thermodynamically it is
easier to export enough silica to generate the formation of a chert deposit via siliceous
organisms that are so efficient in silica uptake at the very low concentrations, than to
raise the dissolved silica level in the seawater at concentrations so high that a direct
precipitation of a chert deposit could occur.
4) Limestone is a very common rock in the Phanerozoic record. However only
some of them contain cherts even though, there is a great affinity between carbonate
rocks and certification (Kastner et al., 1977) (Chapter 5). The selective occurrence of
chert in only some limestones supports the biogenic origin as does the loss of chert
nodules from most Cenozoic limestones when sponges moved to deeper waters.
5) The spatial distribution of chert deposits does not not correlate with volcanic
sites or other high silica input sites. Instead it matches the distribution of the silicasecreting organisms.
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6) The weathering of continental silicate rocks is a continuous process
maintaining a permanent flux of oscillating magnitudes of silica into the basins, and thus
a pervasive formation of early diagenetic in situ silicified oolites should occur throughout
the Phanerozoic where calcareous oolites formed. However, few of the calcareous oolites
became silicified (Tomescu et al., 2003), and Phanerozoic onshore-offshore patterns of
cherts, as well as the decline of silicified oolites, seems to correlate with distribution
patterns of the silica secreting organisms (Kidder and Mumma, 2003). None of the
argumenst alone proves the biogenic origin, but integration of all of them is consistent
with it.
9.3. Silica burial: a balance between silica dissolution and preservation rates
The geological record of chert deposits is an issue of the preservation rate of
biogenic opal that might not correlate with either high biosiliceous productivity areas and
high silica-nutrient inputs, or with eutrophic water masses. The net silica output into chert
expresses the balance between biogenic silica production, dissolution, dilution by host
sediment, and the degree of preservation. A change in the amount of chert burial could be
the result of variations in the above factors, and reflect an end product of the conjugated
action of the parameters that play a role in the silica cycle and burial as chert. An analysis
of the factors that influence the marine biogenic silica production, dissolution and burial
is required in order to understand chert abundance patterns through time and their
underlying mechanisms. Some key factors include: 1) the productivity of the silicasecreting organisms, 2) the magnitude of the terrigenous sediment input, 3) surface water
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temperature, 4) water column depth, 5) opal rain rate, 6) the presence of trace metals (Al,
Fe, Ga, Gd, Y), and especially of Al, 7) the presence of organic matter, 8) test
aggregation through organism blooms due to high silica-nutrient influxes, 9) the specific
surface area of the siliceous skeleton, and burying sediment composition (Nelson et al.,
1995; Maliva and Siever, 1989; Bidle et al., 2002).
The evidence that most modern biogenic silica accumulates in areas with cold
surface waters (DeMaster, 1981), suggests that the surface water temperature is a
dominant factor for the opal preservation, due to the strong dependence of silica
dissolution on temperature (Kamatami, 1982). Based on an actualistic upwelling
scenario, biogenic silica accumulations discovered in the stratigraphic record are viewed
as tracers of eutrophic water masses induced by the upwelling of cold nutrient-rich deep
waters (Ramsey, 1973; Miskell et al., 1985; Hein and Parrish, 1987; Barron and Baldauf,
1995; Murray et al., 1994; Kidder and Erwin, 2001). However, studies showed that the
accumulation and burial of modern biogenic silica is governed by different mechanisms
in coastal settings and deep-abyssal environments (Nelson et al.1995; DeMaster, 1981;
Tréguer et al., 1995).
• Preservation of silica in the deep-sea: the temperature-dependent preservation model
The deep-sea domain is generally characterized by low terrigenous influx, so that,
the sediment input will play a minimal role in the dilution of silica accumulation. Deepsea sediments are dominated by biogenic components and pelagic clays (Kennett, 1982).
Sediment thickness in the ocean basins depends on the biological productivity, proximity
of the basin to areas characterized by high river discharge (hence the amount of sediment
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influx), rates of dissolution versus rates of preservation of the skeletons, the age of the
ocean floor, and the distribution of bottom currents (Kennett, 1982; Duxbury et al.,
2002).
Silica dissolution is strongly temperature dependent, so that the specific
dissolution rate (dissolution rate per unit mass of biogenic silica) increases by
approximately an order of magnitude with each 15ºC increase in temperature (Kamatani,
1982). Silica dissolution also increases with increasing pressure. Heath (1974) found that
a temperature decrease of 1/2ºC per kilometer cancels out the solubility increase due to
rising pressure, which accounts for the high silica concentrations are found in deep waters
where temperature is low and pressure is high.
In the deep sea, modern biogenic silica accumulations occur in high latitude
regions, representing the largest present-day opal accumulation sites (Tréguer et al.,
1995; DeMaster, 1981). The assessment of the abyssal siliceous sediments points out that
“the largest and most important area of modern opal accumulation in the ocean is one
where annual rates of opal accumulation in surface waters are not particularly high nor is
the annual carbon-based primary productivity” (Nelson et al.,1995; p.366). According to
this model, the dominant mechanism for enhancing deep-sea (abyssal) opal preservation
is the low dissolution rate of biogenic opal at low surface water temperature (the upper
100 m), irrespective of high or low surface productivity (Nelson and Gordon, 1982 in
Nelson et al., 1995; DeMaster, 1981). By contrast, the oligotrophic areas where the
production of biogenic silica in surface waters represents about 75-90% of the global
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production, is characterized by very little preservation; the dissolution of silica within the
water column and at the sediment/water interface is almost total (Nelson et al., 1995).
• Preservation of biogenic silica in coastal margins: the high productivity model
Although the preservation of biogenic opal in abyssal areas is strongly influenced
by surface water temperature and latitude, in the shallower waters of continental margins,
it is tightly linked to upwelling (Nelson et al., 1995). The upwelling of nutrient-rich deep
waters to the photic zone favors high organism productivity that increases the opal rain,
decreases the test dissolution rate through test aggregation, and leads to enhanced burial
rates. The formation of ice caps at the poles triggers low and middle latitudinal deep
water upwelling, a mechanical motion that introduces into the nutrient cycling system
important quantities of nutrients buried upon deep sedimentation. As Nelson et al. (1995)
remark, opal sediment formation in coastal areas is associated with high primary
productivity regardless of the latitude.
In view of all of these facts, the analysis of the stratigraphic chert abundance
record and fluctuation patterns through time proves to be more complicated than the
hypothesized relationships for radiolarian and sponge cherts of high chert abundance =
high organic productivity = coastal upwelling of cold nutrient-rich waters, regardless of
the silica burial site within the basin, the type of the silica secreting organisms, or the
various physico-chemical factors which directly influence silica dissolution and burial
rates. Examination of two systems – continental margins vs. deep-water abundance –
should reveal distinct differences between these two depth-defined systems that function
differently with respect to silica accumulation and preservation (Nelson et al., 1995).
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However, in the stratigraphic record it is difficult to differentiate between slope and
basinal settings. For this reason, the data of the present study have been divided into very
shallow (peritidal/lagoonal), shallow (shelf), and deep settings (slope/basinal). If biogenic
chert formation in peritidal/lagoonal sites and the shallower parts of the shelf is directly
influenced by continental weathering fluctuations via river runoff, that of deeper
environments (the deepest area on the shelf, slope and basin) is tightly linked to the
intrabasinal ambient (water depth, oceanic circulation, surface water temperature,
salinity, seawater chemical composition, volcanic, hydrothermal, and spreading activities,
sediment composition) and latitude, in addition to variations of the continental inputs of
silica and other nutrients, as well as clastic sediment fluxes.
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Chapter 10
DISCUSSION: THE ORDOVICIAN SILICA SECRETING BIOTAS - LIVING
STYLE, DISTRIBUTION AND ONSHORE-OFFSHORE EVOLUTION
PATTERNS
The results of Chapter 7 show that at the beginning of the Ordovician the
peritidal/lagoonal and shelf waters were inhabited by both siliceous sponges and
radiolarians, with the supremacy held by sponges (fig. 10.1). Conversely, the biosiliceous
component of slope/basinal sediments was provided mainly by radiolarians, and
subordinately by siliceous sponges. During the Middle Ordovician, radiolarians were
confined to waters above slope/basin environments, and siliceous sponges retreated from
peritidal/lagoonal to shelf and basinal environments, suggesting an ArenigianLlanvirnian retreat with a migration time of less than 15 Ma. By the Ashgillian, the chert
record suggests that siliceous sponges had retreated completely from peritidal/lagoonal
settings, inhabiting mainly shelf environments and subordinately basinal areas which
were clearly dominated by radiolarians in terms of biosiliceous accumulation (Fig. 10.1).
10.1. Lower Ordovician
The Lower Ordovician distribution pattern of the dominantly biogenic chert
deposits is characterized by shallow water siliceous sponge-dominated cherts contrasting
with a predominance of radiolarian chert in slope/basinal settings. This is superimposed
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Figure 10.1: The Ordovician dominant locus of silica burial and onshore-offshore
migration patterns of the silica secreting organisms and the associated facies. The relative
size of the radiolarian and sponge symbols reflects the dominance of the corresponding
type of organisms in cherts in a particular environment. The relative abundance of the
nodular and bedded chert symbols reflects also the dominance of the chert type in a
particular environment. Ovals represent nodular chert and horizontal long lines represent
bedded cherts.
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on another distribution pattern whereby shallow water nodular-bedded chert dominates
within carbonate environments, whereas deep water bedded cherts dominate within finegrained siliciclastic environments (Fig. 10.1). These patterns may represent carryovers
from the Late Cambrian and possibly the Middle Cambrian, considering the MiddleUpper Cambrian bedded chert distribution (Kidder and Mumma, 2003), and the
distribution of siliceous sponge and radiolarian faunas (Won and Below, 1999;
Tolmacheva et al., 2001).
Siliceous sponges were able to populate depositional environments of varying
depths, from shelf to basinal environments, since the beginning of the Cambrian, when
both lithistid demosponges and hexactinellids were already representing a relatively
diversified component of the benthic fauna (Finks 1970; Yuan et al., 2002; Li et al.,
1998; Zhang and Pratt, 1994). The fact that the two silica secreting faunas, radiolarians
and siliceous sponges, were prevalent in distinct depositional environments emphasizes
their ecological separation, probably based on different needs in terms of dissolved silica
and other nutrients, water temperature, chemistry, salinity, and sediment substrate.
Considering that siliceous sponges generally need higher dissolved silica levels than the
radiolarians (Maldonado et al., 1999), I can hypothesize that during the Lower
Ordovician the shallower siliceous sponge-rich environments, i.e., peritidal/lagoonal and
shelf carbonate platforms, were characterized by higher silica levels compared to the
surface waters of the deeper settings, a status quite possibly preserved since the
Cambrian. The strontium estimates for the same geological period indeed reveal a
relatively high continental radiogenic 87Sr/86Sr input compared to the ensuing Ordovician
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epochs (Shield et al., 2003) (Fig. 11.1B). Additionally, siliceous sponges are filterfeeding organisms and the very stable, nutrient rich environments offered by the
widespread Lower Ordovician carbonate platforms would explain their preference for
these shallow habitats.
Studies of Lower Ordovician siliceous sponge assemblages show a differentiation
within the siliceous sponge biota itself, namely between demosponges and hexactinellids,
in their distributional patterns within a given basin: the lithistid demosponge fauna, quite
diversified at that time, prefered the shallow, warm environments offered by the
carbonate platform factories, while hexactinellid sponges, much less diversified, occupied
deeper environments of slope and basinal sequences within siliciclastic environments
(Mehl and Lehnert, 1997, in Beresi and Esteban, 2004; Beresi and Rigby, 1993, in Beresi
and Esteban, 2004; Carrera and Rigby, 1999; Beresi and Esteban, 2004; Tolmacheva et
al., 2001). This environmental segregation of the Early Paleozoic siliceous sponge fauna
is supported by modern siliceous sponge assemblages. The deep-water Hexactinellids
need higher silica concentrations than the shallower water Demosponges (Maldonado et
al., 1999). This evidence seems to contradict the hypothesis that during the Lower
Ordovician the shallower marine habitats might have been richer in silica and other
nutrients than the deeper areas. However, it suggests that other environmental factors,
such as the nature of the sediment substrate and seawater temperature, in conjunction
with a high silica level might have been responsible for a dominant species-based
distribution and it does not rule out the possibility that there was even more dissolved
silica in deeper waters like today. It also suggests that the Early Paleozoic siliceous
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sponge fauna may have been represented by species different from the modern ones
and/or that the distribution of the Early Paleozoic siliceous sponge fauna was governed
by different factors not present in the modern ecosystems. Overall, during the Lower
Ordovician the Hexactinellid sponges (the deep water population) were a less important
component of sponge faunas compared to the lithistid demosponges (the shallow water
population) that were predominant in the sponge fauna (Finks, 1970; Rigby, 1983;
Carrera and Rigby, 1999). The dominance of Lithistids may explain the abundance of
shallow-warm-water siliceous sponge assemblages within a carbonate environment,
relative to the deep-water siliceous sponge species within a siliciclastic setting, and a
possible biogenic origin via siliceous sponges of the peritidal-shelf nodular cherts and the
shallow water bedded cherts (Chapter 9).
More recent findings bring new data relative to the Ordovician siliceous sponge
fauna. The shallow-water carbonate sequences found in the Argentine Precordillera show
a clear dominance of lithistid demosponges in the siliceous sponge fauna (Beresi and
Esteban, 2004). That study, representing the broadest South American record of
Ordovician siliceous sponge fauna, supports the affinity of lithistid demosponges to warm
shallow carbonate environments, for a fauna included in the “eastern North American
Laurentian faunas” (Carrera and Rigby, 1999; Beresi and Esteban, 2004).
Is silica or nutrient limitation of greater importance? This is a question raised by
many authors trying to explain the growth and evolution of the siliceous sponges and
radiolarians (Maldonado et al., 1999; Maliva et al., 1989). Modern studies on siliceous
sponges still disagree about which parameter exerts the main control, and consequently
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two antagonistic theories have been proposed: the silicon limitation growth theory
(Reincke and Barthel, 1997; Maldonado et al., 1999; Racki and Cordey, 2000) and the
nutrient limitation growth theory (Frøhlich and Barthel, 1997; Pettijohn, 1975; Siever,
1962) (see Chapter 8).
The silicon limitation growth theory is based on experimental demonstration that
the silica uptake rate of siliceous sponges increases asymptotically with increasing silica
concentration (Reincke and Barthel, 1997; Maldonado et al., 1999). Insufficient silica
concentration weakens the ability of the siliceous sponges to develop complex spicule
morphology, and very simplified, small and thin skeletons are secreted instead, but the
organisms are still capable to survive. Alternatively, seawater with silica concentrations
in excess of 100 µM may inhibit silica uptake (Maldonado et al., 1999). The same
relationship was observed in the radiolarians’ behavior: while they are very effective in
silica uptake, occurring abundantly in surface waters that exhibit low silica
concentrations (ca. 1 µM) and yet mineralizing robust siliceous skeletons, silica levels as
high as 100-150 µM have a deleterious effect on their longevity, growth, and weight gain,
acting as a toxin that inhibits their metabolic activity and kills them (Sugiyama and
Anderson, 1997).
Conversely, the nutrition limitation growth theory is based on experimental data
showing that silica uptake is an energy-consuming process and therefore in a nutrientstarved environment (as that proposed by Pettijohn, 1975, to explain the inorganic
formation of some cherts within a lagoonal setting with restricted circulation and
starvation of nutrients other than silica), the biogenic extraction of dissolved silica by
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siliceous sponges will decrease by 15% (and presumably ceases?) (Frøhlich and Barthel,
1997). Regarding the modern radiolarian fauna, a direct relationship was observed
between their abundance and eutrophic, nutrient-rich, highly saline (40‰) warm (28ºC)
surface waters principally of the equatorial zone or associated to subtropical and tropical
environments (Anderson et al., 1989c). Numerous studies point out that the spatial
distribution of radiolarians might be controlled mainly by water salinity and temperature
(Anderson et al., 1989a,b,c; Anderson et al., 1990; Matsuoko and Anderson, 1992; Chang
et al., 2003), showing a negative correlation with nutrients and primary productivity of
the surface water (Chang et al., 2003). However, regardless of which parameter prevails,
during the Lower Ordovician the strontium isotopic ratios are consistent with the shallow
environments being continuously enriched in silica and other nutrients through riverine
runoff, which may account for the high abundance of the more silica-dependent siliceous
sponges (Fig. 10.1).
The dominance of radiolarians in deeper-water environments may be explained
and interpreted in three ways: first, a vestige from the Middle or Upper Cambrian, and
possibly of their deep water origin (Tolmacheva et al., 2001); second, while the deep
water environments are always characterized by an adequate nutrient supply for their
biological needs, other parameters must have prevailed in stabilizing the dominant
basinal distribution of radiolarians; and third, they thrive principally in upwelling settings
where silica and other nutrients are readily available.
The paleolatitudinal distribution of Lower Ordovician cherts reveals a low to mid
latitudinal increase in silica burial (Fig. 7.4 and Table 7.5) with the radiolarian dominated
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deep-water and peritidal/lagoonal deposits confined principally to low paleolatitudes (0° 30°), and the mid-latitudinal cherts (30° – 60°) occurring mainly in shelf deposits (North
China and Korea). However, relative to the Lower Ordovician paleogeography of Korea
and North China, more recent paleogeographic studies (Xiaofeng and Xiaohong, 1995)
place the benthic faunas at low latitudes and not at mid latitudes as shown in Fig. 4.
The distribution analysis of the chert formations bearing biogenic siliceous
remains shows their low latitudinal expansion (0° - 30°) was principally in the southern
hemisphere, in Kazakhstan, South China, and the southern margin of Laurentia. The
reports from Bolivia, Poland, Korea, and North China do not include any mid-latitudinal
cherts that contain sponge or radiolarian fragments, but more recent findings of studies
from northern Argentina which was situated at middle paleolatitudes (Fig. 7.4)
demonstrate the presence of hexactinellids in siliciclastic sedimentary sequences during
the Lower Ordovician (Beresi and Esteban, 2004). However, it is worth mentioning that
the paleogeography of the Argentine Precordillera (western Argentina) is a debated
problem: some workers consider that it was a part of Gondwana during the Cambrian,
and that it collided with Laurentia during the Ordovician (Dalla Salda et al., 1992), while
others argue that it was a microcontinent that rafted from the Ouachita embayment of
Laurentia during the Cambrian, and collided with Gondwana during the Middle
Ordovician (Astini, 1995; Thomas and Astini, 1996). The latter view seems to be
supported by more pertinent evidence and is more widely accepted.
Assessing this global distribution pattern and interconnecting multiple factors
related to it, a paleolatitudinal based partition is evident: 1) deep-water, radiolarian
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dominated cherts are present in low latitudes; 2) lithistid demosponge dominated chert
facies dominate peritidal/lagoonal and carbonate shelf; and 3) the mid latitudes are
characterized by deep shelf and basinal environments dominated by Hexactinellids.
The confinement of radiolarians to low latitudes offers insights into some
ecological aspects of the Early Paleozoic radiolarian fauna. Their apparent preference for
warmer waters, was a consequence of since their emergence in the greenhouse Middle
Cambrian climate (De Wever et al., 2001). The mid and high latitude Lower Ordovician
waters were probably too cool for their survival, suggesting that during that time there
was a latitudinal thermal gradient high enough to constrain radiolarian latitudinal
distribution. The distribution of Early Paleozoic radiolarians is tightly linked to water
temperature, a hypothesis suggested by Sugiyama and Anderson (1997) for modern
radiolarian communities, and in agreement with the general line of thought that associates
plankton distribution with abiotic factors such as temperature and latitudinal zonation
(Lipps, 1970; Tappan and Loeblich, 1973).
Siliceous sponge faunas are able to inhabit environments with a wider latitudinal
distribution probably because of: 1) either the specialization of the two different sponge
groups, Lithistid Demosponges and Hexactinellids, to life in different environments,
shallow warm carbonate settings for the former, and respectively cooler, deeper,
siliciclastic habitats for the latter, perhaps an evolutionary ability gained due to their
participation in and survival from the Latest Precambrian Icehouse; or 2) they evolved as
primitive generalists that could adapt readily to variable environments as long as they
have clear waters and sufficient nutrients and dissolved silica.
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10.2 Middle Ordovician
The distribution of Middle Ordovician cherts within siliceous skeletal remains
reveals that radiolarians are exclusively confined to slope/basinal environments whereas
siliceous sponges seem to characterize both shelf and slope/basinal settings (Fig. 10.1).
The Middle Ordovician distribution pattern of silica secreting biotas reflects a change in
of siliceous sponge faunas from peritidal/lagoonal to shelf-basinal environments, and of
radiolarians to basinal settings, suggesting an Arenigian- Llanvirnian onshore-offshore
migration of the silica secreting organisms in less than 15 My. This shift of the cherts
containing preserved skeletal remains is accompanied by a general onshore-offshore drift
in the locus of marine silica deposition, so that during the Middle Ordovician the
slope/basinal environments represent the dominant domain of silica burial. The faunal
retreat is also coeval with a turnover in the chert style from shallow-water carbonate
environments dominated by nodular-bedded cherts characteristic for the Lower
Ordovician, toward a slope/basin bedded chert-dominated silica system within
siliciclastic environments in the Middle Ordovician (Fig. 10.1).
10.2.1. Migration mechanisms
Speculations can be made about the style of the organism migration. If
radiolarians moved offshore because of various causes, if the linkage between
radiolarians, a phytoplankton food source, and siliceous sponges, a sessile benthic
component of the food web, was as strong as in modern marine communities, and if silica
was a limiting factor for the Ordovician siliceous sponge fauna, siliceous sponges may
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have followed the offshore migration of radiolarians (Kidder, 2000). Today, radiolarians
and sponges are both consumers but at different trophic levels (De Wever et al., 2001).
The higher trophic levels characteristic for sponges depend on the lower levels including
radiolarians. Within the food web as we know it today, benthic faunas, especially sessile
organisms such as sponges, depend on the more flexible planktonic organisms. Siliceous
sponges may have followed radiolarians because the marine environments proximal to
those inhabited by radiolarians were richer in dissolved silica owing to the fact that
radiolarians dissolve at faster rates than siliceous sponges (Scholl et al1983).
Potential mechanisms for the shallow to deeper water migration of the silica
secreting biotas include: i) onshore-offshore evolutionary patterns, and/or ii) changes in
availability of dissolved silica and other nutrients used by siliceous sponges and
radiolarians (Tomescu and Kidder, 2002).
i) Onshore-offshore evolutionary patterns of silica secreting organisms
The shift of cherty facies from nearshore settings toward deeper environments
might have been driven by onshore-offshore patterns of evolution (Kidder, 2000), an
Ordovician mechanism suggested for benthic invertebrate calcareous macrofaunas
(Sepkoski and Sheehan, 1983; Jablonski et al., 1983). The Ordovician contains evidence
of an onshore-offshore migration of the benthic communities of the three Phanerozoic
faunas: the declining Cambrian fauna became restricted more or less to deep water
environments moving offshore perhaps by the Paleozoic fauna diversifying across the
shelf, and the Modern fauna developing in nearshore environments. The displacement of
the Cambrian fauna by the Paleozoic fauna occurred over the Lower and Middle
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Ordovician, and the rise of the Modern fauna in nearshore settings occurred in the mid to
late Ordovician (Jablonski et al., 1983; Sepkoski and Miller, 1985). However, these
analyses of Early Paleozoic marine faunas do not include any planktonic group of
organisms, and it is unclear whether they apply to the siliceous sponges. Camparable
timing of migration would be consistent with siliceous biotas moving in concert with
calcareous ones.
ii) Changes in availability of dissolved silica and other nutrients
The onshore-offshore migration of the silica secreting biotas can be explained in
terms of a shift in the dominant source of silica and other nutrients (P, N, Fe), from an
allochtonous-dominated (exogenous input through riverine runoff) to an autochtonousdominated (endogenous inputs through volcano-hydrothermal and seafloor weathering
activities) system.
In modern oceans, among the four silica sources that replenish the marine silicic
acid, river runoff accounts for 82%, volcano-hydrothermal together with seafloor
weathering inputs only 10% of the total silica, and the remaining of 8% is attributed to
eolian supply (Tréguer et al., 1995). The nutrient load of the rivers is strongly influenced
by the silicate weathering rates and the volume of rocks exposed to weathering.
Consequently, a change in weathering will affect the river flux, that in turn induces
changes in the marine realm. The volume of silicate rocks exposed to weathering, and the
weathering rates are directly related to changes in uplift of mountain belts, climate, sea
level fluctuations, the interplay between the latitudinal disposition of the landmasses and
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the emerged land area, and indirectly related to tectonic rifting and oceanic spreading
rates.
The Ordovician was a geological period characterized by energetic mid-oceanic
ridge activity (Mackenzie and Piggot, 1981), intensive volcanism (Ronov, 1982; Huff and
Kolata, 1990; Huff et al., 1992; Huff et al., 1996) common in submarine islands, coastal
and cordilleran sites (Stillman, 1984), numerous volcanogenic massive sulfide deposits
compared to the Cambrian and the Silurian (Eastoe and Gustin, 1996), a low area of
continental deformation and orogenic activity during the Lower and Middle Ordovician
(Richter et al., 1992; Miller and Mao, 1995), and intensive reorganization of the major
tectonic plates (Ford and Golonka, 2003; Worsley et al., 1994).
The 87Sr/86Sr ratio in marine sediments is tightly linked to the balance between
continental weathering (radiogenic flux) and oceanic spreading rates (non-radiogenic
flux). The minimal change in the ration through the Lower and Middle Ordovician is
consistent with a relatively unchanging chemical dissolved runoff from the landmasses.
10.2.3. Considerations regarding the Middle Ordovician sea level fluctuations
The Middle Ordovician is a geological time characterized by a generally much
lower sea level compared to the Lower Ordovician and the ensuing Caradocian (Ross and
Ross, 1992). The sedimentary sequences of most of the Whiterockian series (the lower
part of the second-order Tippecanoe sequence of the North American stratigraphy) reveal
a sharp Middle Ordovician sea level drop that exposed almost the entire craton of
Laurentia (Finney, 1997; Fortey, 1984). The evidence is based on both deep
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(slope/basinal) (Finney, 1997) and cratonic shelves sedimentary successions (Ross and
Ross, 1992; 1995). The Whiterockian – Ibexian boundary in North America coincides
approximately with the Lower – Middle Ordovician boundary (late Arenigian, Fig. 6.2).
According to Finney (1997), this regression boundary is a second-order eustatic sea level
drop that represents a conspicuous horizon recognizable across the North American
continent, and which lasted 3 to 5 million years. The Sauk-Tippecanoe sequence
boundary, known as a global paleokarst unconformity (Sloss, 1963), has been also
recognized in South Korea, North China, the Canadian Appalachians, and the Southern
Appalachians (Ryu, 2002), in southern Wales, on the Australian platform, as well as in
Sweden (Fortey, 1984), and northern Europe (Webby, 1984a,b).
A sea level drop results in a reduced habitable space on the shelf and thus shallow
water communities would be forced to track the drop of their habitat (Fortey, 1984).
However, shallow waters would still exist even though these environments would suffer
the greatest loss in space, and coastal settings might not change much in areal extent. The
degree to which sea level changes and glaciations affect marine life is controversial (e.g.,
Wilde and Berry, 1984; Fortey, 1984).
Unfortunately the potential mechanisms responsible for the global early Middle
Ordovician sea level drop are not well documented. According to Ryu (2002) regional
and tectonic events generated local and regional variations of the Sauk-Tippecanoe
sequence boundary. Considering that the Sauk-Tippecanoe sea level drop is seen as a
major eustatic event, driving mechanisms might be represented by:
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1) fluctuations in the rate of sea-floor spreading that would directly disturb
significantly the volume of ocean water (Donovan and Jones, 1981; Hays and Pitman,
1973). This mechanism that would be in agreement with a decrease in the rate of seafloor
spreading suggested by François et al. (1993, after Gaffin, 1987), and implicitly a
decreasing area of continental deformation (Richter et al., 1992; Qing et al., 1998). A
decrease in the volume of oceanic ridges would affect directly the basin volume process
depending on oceanic spreading rates (see paragraph 1) – but seen as a long-term
mechanism, lasting 10-30 m.y. (Donovan and Jones, 1981).
2) changes in mean oceanic temperature, through thermal seawater contraction;
these would have an effect of smaller magnitude susceptible to be masked by the effects
of other processes (Donovan and Jones, 1981).
3) increase in the volume of ice sheets (Fortey, 1984; Lindström, 1984) based on
the geographic position of Gondwana over the South pole. However, no glacial deposits
support this hypothesis.
10.3. Upper Ordovician
The Upper Ordovician distribution of cherts illustrates a depositional environment
distribution pattern similar to that of the Middle Ordovician and consistent with the
proposed onshore-offshore patterns inferred previously. Radiolarian bedded cherts were
confined to slope/basinal settings, and shelf deposits are dominated by nodular and
bedded cherts with siliceous sponge spicules preserved. The three Caradocian silicified
oolites on the southern margin of Laurentia suggest that siliceous sponges might have
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still been present sporadically in some peritidal/lagoonal areas characterized by peculiar
local conditions. During the Ashgillian, the peritidal/lagoonal cherts disappear
completely. If the chert depositional migration occurred within the same range of low and
middle paleolatitudes in the Lower and Middle Ordovician, during the Upper Ordovician
it expanded to the high paleolatitudinal habitats, (the nodular cherts from Italy and
France; Loi and Dabard, 2002).
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Chapter 11
INTERPRETING THE CHERT ABUNDANCE PATTERNS: WHY CHERT
PEAKS OR LOWS?
11.1. Lower Ordovician: a greenhouse interval
The present study demonstrates that the Lower Ordovician was characterized by
high chert abundance principally in shallow, peritidal/lagoonal, and shelf environments
and secondarily in slope/basinal settings (fig. 7.1A, 7.3, 10.1, and 11.1A). The Lower
Ordovician was a period characterized by: 1) high sea level, including the climax of the
Precambrian – Lower Ordovician Sauk transgressive sequence that occurred during the
late Ibexian (early Arenigian; fig. 6.2), when the sea level reached one of the highest
levels in the entire Phanerozoic (Finney, 1997; Ford and Golonka, 2003); 2) maximum
dispersion (disassembly) of continents by the northward drifting of Laurentia and Baltica
concomitant with widening of the Iapetus Ocean and Tornquist Sea (Worsley et al.,
1994; Ford and Golonka, 2003); 3) intensive volcanic activity since the late Cambrian
(Stillman, 1984); 4) widespread carbonate platforms (Overstreet et al., 2003); and 5)
organism radiation (fig. 11.1A). The strontium curve (fig. 11.1B), during the early
Lower Ordovician, that is the Tremadocian, has higher radiogenic values than during the
ensuing Ordovician epochs, suggesting higher continental weathering rates of the silicate
rocks exposed by the Pan-African orogeny (Ebneth et al., 2001), but still low
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Figure 11.1A: The Ordovician biotic system
145
Figure 11.1A – The Ordovician biotic system: continued.
a. Chert abundance patterns evidenced by the present study. The data is normalized to
time.
b. Chert abundance by depositional environments, as evidenced by the present study.
Gray bullets represent peritidal/lagoonal cherts, black bullets represent shelf chert
deposits, and stars represent slope/basinal chert formations. The data is normalized to
time.
c. Radiolarian diversity; after Vishnevskaya, 1997.
d. Black shale abundance patterns in the British Isles; after Leggett et al., 1981.
e. Euxinification episodes based on black shale abundance; after Leggett et al., 1981.
f. Graptoloid diversity; after Leggett et al., 1981.
g. Brachiopod diversity; after Leggett et al., 1981.
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Figure 11.1B: The Ordovician abiotic system
147
Figure 11.1B – The Ordovician abiotic system: continued.
a. Strontium and oxygen isotope curves based on calcitic brachiopods; after Shields et al.
(2003).
b. Extent of continental deformation areas; after Reichter et al. (1992) and Qing et al.
(1998).
c. Sea level fluctuations; after Ross and Ross (1992; 1995); Qing et al. (1998); Finney,
(1997).
d. Tropical seawater temperature based on oxygen isotope values; after Shields et al.
(2003) and Tobin et al. (1996).
e. Tectonic data: orogenies and continental migration patterns; duration of Taconic
orogeny and continental migration patterns after Ford and Golonka (2003), Qing et al.
(1998). A Ringworld is dominated by tropical continents, while a Capworld is
characterized by polar continents (Worsley and Kidder, 1991; Worsley et al., 1994).
Question marks for uncertain timing.
f. Average latitude of the world landmass for the “2-10” model (Worsley and Kidder,
1991; Worsley et al., 1994).
g. Volcanogenic massive sulfide deposits (Eastoe and Gustin, 1996).
h. Oolitic ironstone accumulations (Van Houten and Arthur, 1989).
148
enough to allow the development of widespread carbonate platforms, that were
overwhelming the shelf system, up to relatively high latitudes (50º in Asia; Frakes et al.,
1992). Shelf areas were very extensively developed in North America, Australia, Asia,
and South America, and were centered close to the equator and tropics. The widespread
marine carbonates deposited in shallow epeiric seas (Overstreet et al., 2003), the oxygen
isotopic data (Tobin et al., 1996; Shields et al., 2003), and the relatively abundant oolitic
ironstone deposits (Van Houten and Arthur, 1989) provide support for a warm and
humid greenhouse mode.
11.1.1. Peritidal/lagoonal and shelf environments
The peritidal-lagoonal and shelf environments during the Lower Ordovician were
inhabited mainly by siliceous sponges and subordinately by radiolarians (fig. 10.1),
resulting in abundant nodular and less frequent bedded cherts (fig. 7.3 & 7.7).
Geographically, approximately 75% of the worldwide Lower Ordovician shallow water
cherts have been identified on the south-southwestern margin of Laurentia, such as on
the Appalachian and Cordilleran passive margins, the south Oklahoma aulacogen, and
the Ozark Uplift (Overstreet et al., 2003) (Table 7.6), as well as in the Kazakhstan block,
China, Korea, Australia, and Argentina (fig. 7.4).
• Why abundant chert burial?
The modern silica cycle does not exhibit any shallow biogenic opal accumulation
and burial, and therefore we can only speculate about the driving mechanisms that
generated biogenically-induced chert formation in the past. First of all, both
peritidal/lagoonal and shelf depositional environments were inhabited by silica secreting
149
organisms, principally siliceous sponges and secondarily radiolarians, which provide the
necessary silica for the chert formation. Also, these shallow water settings were
characterized by a generally low terrigenous input that could not mask the biogenic silica
accumulation, or inhibit the development of large carbonate platform factories, and the
expansion of filter feeding siliceous sponges and other benthic organisms. Jones and
Desrochers (1992) emphasize that shallow platform carbonates can form in areas isolated
from extensive siliciclastic flux. The strontium curve (fig. 11.1B) is consistent with the
fact that Lower Ordovician riverine runoff must have been still high enough to account
for the needs in terms of dissolved silica and other nutrients required by siliceous
sponges and radiolarians. Moreover, the host rock represented by carbonates probably
played an important role in the chert formation and burial, based on the affinity between
carbonates and chertification (Kastner et al., 1977; Maliva and Siever, 1989). Another
factor in chert formation could be the presence of abundant organic matter whose
decomposition through oxidation may induce early diagenetic nodule formation (Siever,
1962; Maliva and Siever, 1989). According to the force of crystallization-controlled
replacement model of Maliva and Siever (1989), the ideal host sediment has three
properties: organic matter content, porosity and permeability, and biogenic-opal
concentration.
Taking into account that seawater was warm (as shown by oxygen isotopic ratios;
fig. 11.1B), the opal dissolution rate is should have been high and thus driving silica
recycling and preventing opal accumulation. However, the loss of opal through
dissolution would have been counterbalanced and surpassed by: 1) the dominance of
150
siliceous sponges that dissolve at lower rates than radiolarians (Scholle et al., 1983); 2)
the shallow depth of the water column that would reduce the time available for sinking
and dissolution of opal radiolarian skeletons; 3) low terrigenous input that could mask
silica concentrations; 4) the carbonate composition of the host rock; 5) the presence of
abundant organic matter, and 6) a possibly rapid burial of the benthic silicisponges
(before their silica can get into ocean water) through a relatively high vertical
accumulation of the carbonate production; instead, the dissolution of the opaline skeleton
makes chert nodules.
In conclusion, while we can only speculate about a good organism productivity in
these shallow settings, due to stable and favorable environmental conditions, the chert
abundance could be accounted for in a low sediment dilution - enhanced chert formation
model (fig. 11.2).
11.1.2. Slope/Basinal environments
The slope/basinal settings were inhabited principally by the planktonic
radiolarians, and subordinately by siliceous sponges. It was demonstrated that at least in
certain basinal environments the bottom waters were well aerated (Wilde, 1989) and
could sustain the activity of some benthic siliceous sponges (Blome et al., 1995a;
Tolmacheva et al., 2001). Also, Leggett et al. (1981) discussed the presence of burrows
and primary iron oxide in cherts of the British Isles (Iapetus Ocean) associated with
pillow lava, that indicate well oxygenated bottom waters, as well as high silica and other
nutrients inputs. The well oxygenated deep water theory is also supported by the presence
of deep burrowing trace fossils (up to 79 cm deep), discovered in an early Ordovicia
151
Figure 11.2: Chert abundance Model. LOWER ORDOVICIAN [490 – 470 Ma]
152
deep-water flysch sedimentary sequence of Nova Scotia (the Meguma Group; Pickerill
and Williams, 1989). That study represents the first evidence of infaunal behavior in deep
water environments, in contrast to the previous much shallower estimates that were based
on the analysis of shelf settings (Pickerill and Williams, 1989). Also, according to Crimes
(1974; 1992) a clear increase in the deep-water Nereites ichnofacies can be seen during
the Ordovician.
On the other hand, a warm world with warm surface waters would lead to higher
metabolic activity, higher silica dissolution and thus higher silica recycling in the
uppermost water layers. This recycling would lead to a decreased potential for biogenic
opal preservation. The significant silica accumulation and burial in the deep-sea may
account for high preservation rates at the bottom part of the basin. If under these
circumstances additional processes generating silica and other nutrients inputs (e.g.,
volcanic, hydrothermal, and oceanic spreading activities) were active, these could create
an environment with a higher potential for blooms of silica secreting organisms, higher
opal rain rates, and a higher potential to increase the silica net output through the
formation of biogenic chert.
A relatively high radiolarian productivity within a warm water world is consistent
with: 1) the ecology of some modern radiolarians shown to be tightly linked to warm,
highly saline surface waters (Anderson et al., 1989c); 2) the origin of radiolarians as part
of the Cambrian explosion that occurred in a warm climate; 3) the theory that submarine
volcanism and hydrothermal activity, coupled with a warm climate, and a sea level rise,
provide the right conditions for increased productivity (Vermeij, 1995); and 4) major
153
tropical continental margins upwelling sites along the equator in western America and
Arctic Islands in Laurentia, North China, New Zealand, and Newfoundland (Wilde,
1989).
In concordance with Vermeij’s theory (1995), a high temperature constitutes the
main stimulus for high productivity through enhanced metabolico-chemical reactions
that are greatly accelerated with increased temperature. The model emphasizes the fact
that increased nutrient supply only is not enough to generate significant high
productivity, and that organisms revolutions are mainly generated by increased energy
(such as temperature) and subordinately by nutrient supply. The survival of radiolarians
during this interval, along with observations showing that for modern taxa high
temperatures above 33º C have deleterious effects on radiolarian longevity (Anderson et
al., 1987c), indicate the presence of favorable marine environments. The presence of
abundant radiolarians appears to rule out the possibility of overly high surface seawater
temperatures at low latitudes, which are suggested by the very negative oxygen values
obtained for this epoch (fig. 11.1B; Shields et al., 2003; p. 2016).
Good preservation rates within a well-aerated deep bottom environment may be
the result of the interplay of multiple factors such as: 1) a fairly high rate of opal rain
induced by a good productivity in the surface waters, 2) the presence of organic matter
that intensifies silica preservation (Spencer, 1983; Siever, 1957); 3) a water column
shorter than that of the modern abyssal sites directly dependent on the basin morphology,
and 4) possibly higher concentrations of cations, such as Al, Fe. These cations that were
shown to combine with diatom silica to decrease dissolution rates (Lewin, 1961). Nelson
154
et al., (1995) assert the dissolution rate and the equilibrium solubility of diatom silica
decreases as the Al/Si ratio increases.
The clustering of Lower Ordovician basinal bedded cherts on the southeastern
margin of Laurentia, might be because of least two of the factors that would enhance
silica preservations: 1) high cation concentrations of intrabasinal origin, through
submarine volcanism driven by the widening of the Iapetus Ocean (Ford and Golonka,
2003) and 2) favorable organic matter concentrations, such as the presence of
zooplanktonic graptoloids whose diversity reaches the highest peak for the Ordovician,
at least in the Iapetus Ocean (fig. 11.1A; Leggett et al., 1981). Even though black shale
studies do not indicate any significant organic-rich sediments formed during this
interval, a finding otherwise consistent with well aerated bottom demonstrated for the
Iapetus Ocean (fig. 11.1A; Leggett et al., 1981), the levels of organic matter were
seemingly high enough to enhance silica burial in concert with other factors. Though it is
difficult to prove or test, a preservation model is consistent with abundant chert in a
green house model.
However, there is no study showing a relationship between radiolarians and Al
intake similar to the one documented for diatoms. Also, the role of organic matter in
silica preservation is arguable. It may be enhancing by its adsorption on the silica surface
or, on the contrary it may be deleterious by the formation of organic-silica complexes, so
that no conclusive relationship has been supported to date (Siever, 1962). Bidle and
Azam (1999) and Bidle et al., (2002) showed that bacterial activity is important in silica
155
dissolution by degrading the organic film that protects the opal diatomaceous frustules
from dissolution.
In conclusion, the dominance of bedded cherts in basinal settings (fig. 7.7A)
might reflect relatively high biosiliceous productivity, associated with good taphonomic
conditions (high preservation rates) induced by the conjunction of various opal enhancing
mechanisms, under an oxic regime within the bottom areas (fig. 11.2).
11.2. Middle Ordovician: a greenhouse interval (fig. 11.3)
The general chert abundance is strongly diminished during the Middle
Ordovician, exclusively due to a 75% decrease in silica burial in peritidal/lagoonal and
shelf environments (Table 7.3 & fig. 7.3). The data show also that the formation of
nodular cherts has been intensely affected in comparison with that of the bedded cherts,
during this interval (fig. 7.2A). All these facts, along with the genetic relationship that
exists between siliceous sponges and peritidal/lagoonal-shelf chert formation (Chapter 7
and 9) suggest that the benthic siliceous sponge biota was more involved in this general
chert abundance low. The slope/basinal bedded cherts, during the same interval on the
contrary, record a slight augmentation suggesting that radiolarian faunas and implicitly
radiolarian-induced chert formation were not responsible for the Middle Ordovician
global silica shortage. Geographically, the most affected area by the silica burial low was
Laurentia, which suffered a 50% reduction in chert formation (Table 7.6).
The Middle Ordovician shallow water chert low may reflect either an apparent, or
a real silica burial slowdown. The apparent curtailment may be due to some common
156
Figure 11.3: Chert abundance Model. MIDDLE ORDOVICIAN [470 – 458 Ma]
157
errors and biases inherent to any compilation study, such as sampling issues, afe
assignment problems, and/or to the fact that the worldwide sedimentary sequences are
unequally studied so that the absence of chert deposits in certain geographical areas
might reflect just a lack of knowledge and not a physical absence.
On the other hand, an actual slowdown of silica burial may be linked to changes
in the biotic and abiotic Middle Ordovician systems. The Lower-Middle Ordovician
onshore-offshore organism migration, especially that of siliceous sponges, that have
been demonstrated herein to be responsible for the formation of shallow chert deposits
and which proved to be most affected by this chert waning. Physico-chemical changes of
the depositional environments (e.g., a sea level transgression) that would have influence
the balance between opal dissolution and opal preservation and the balance between
shallow (peritidal/lagoonal and shelf) and deep water (slope/basinal) environments.
The Middle Ordovician represents a time of (fig. 11.1): 1) onshore-offshore
migration of both benthic and planktonic organisms that try occupy new ecological
niches (Sepkoski and Sheehan, 1983; Jablonski et al., 1983); 2) major plate tectonic
reorganization as the Iapetus Ocean and Tornquist Sea begin to narrow down, and the
Rheic Ocean is forming (Ford and Golonka, 2003); 3) low relative sea level (Ross and
Ross, 1995; Finney, 1997); 4) warm surface water temperatures (27ºC - 32ºC; Shields et
al., 2003); 5) decreased emergent land area coeval with lower average latitude of the
world landmasses (Worsley et al., 1994); 6) higher radiolarian diversity (Vishnevskaya,
1997); and 7) more abundant back shale deposits (Leggett et al., 1981). However, the
present study reveals a decrease in the mid latitudinal cherts concomitant with an
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increase of the low latitudinal deposits (Table 7.5). That suggests either a less warm
climate compared with that of the Lower Ordovician or less available shelf area in which
chert could form.
11.2.1. Peritidal/lagoonal and shelf environments
The relationship between the formation of shallow water cherts and siliceous
sponges (Chapter 9), and the fact that Middle Ordovician proves to be a geological
interval of biological adaptation to new, deeper-water ecological niches by a onshoreoffshore organism migration (see Chapter 10), suggest that the Middle Ordovician chert
low was driven by a decrease in the shallow water silica burial due to unstable conditions
in the evolutionary scenario of siliceous sponges and subsequently to a lesser extent of
radiolarians.
Another mechanism that could impact the Middle Ordovician chert preservation is
represented by a reduction in the shelf area in which cherts could form. In the early
Middle Ordovician, the main Lower Ordovician chert bearing continent, Laurentia, was
entirely emergent due to a short-lived major regression (Finney, 1997) recognizable
worldwide (Ryu, 2002; Fortey, 1984; Webby, 1984a,b; Sloss 1963) (Chapter 10). Thus,
it can be argued that the slowdown in chert deposition may be strictly a consequence of
the physical displacement of shelf environments by the retreat of the sea level, and not
linked at all to the organism migration. Finney (1997) estimated that the regression had a
short duration (3-5 Ma), and that the rest of the Middle Ordovician and up to the early
Caradocian witnessed a gradual transgression and implicitly a re-expansion of the shelf
area. However, at this point in time, the siliceous sponges inhabited deeper sites of the
159
carbonate shelf environment and radiolarians were populating exclusively the
slope/basinal waters, so that siliceous sponges could not compensate for the drop in
overall shallow water chert production.
11.2.2. Slope/basinal environments
The planktonic organisms (e.g., radiolarians and graptoloids) that accumulated in
deep waters would have greatly benefited from a higher intrabasinal input of silica and
other nutrients (P, N, Fe). Such an increased input is consistent with the strontium curve
(a change that has occurred before the Lower-Middle Ordovician boundary, sometime in
the Ashgillian), and is due to an energetic midocean activity and a higher infaunal
activity resulting from the invasion of deeper nutrient-rich areas as a consequence of the
onshore-offshore Ashgillian-Llanvirnian organism migration. Innovations in burrowing
organisms such as the population of new deeper-water ecological niches would constitute
a stimulus for a higher productivity of the planktonic organisms, by making available
new amounts of nutrients entrapped into sediments during precedent periods. The release
of phosphorous entrapped into the deep-sea sediments triggered by a migration of the
benthic organisms was proposed as one mechanism responsible for the Cambrian
explosion (Bengston, 1994). The increased deposition of black shales (fig. 11.1A) during
this period, coeval with warm surface waters, and a slight increase in the deep basinal
silica burial (fig. 7.3), provide support for good organism productivity (Vermeij, 1995)
concurrent with enhanced silica preservation mechanisms (i.e., a higher influx of organic
matter due to a higher productivity). The intense deep water activity of the benthic faunas
160
indicates the presence of oxic bottom deep waters at least in some regions (Leggett et al.,
1981). However, the higher abundance of organic-rich sediments intercepted through
more abundant black shales compared to that of the precedent period (fig. 11.1A),
suggests a higher organic productivity and/or somewhat less oxygenated bottom
environment than that of the Lower Ordovician. Consequently, the high basinal chert
abundance during this interval seems to conform to a high organic productivity and
enhanced preservation rates model (fig. 11.3).
11.3. Upper Ordovician: the Caradocian - a warm - cool transitional, ice free
mode (?)
During the Caradocian, opal burial reached the highest Ordovician peak in both
shelf and slope/basinal environments. A balance between shelf and deep-water cherts
that was tipped heavily toward deep-water cherts during the Middle Ordovician is
restored. While radiolarians are confined to deep basinal cherts, siliceous sponges
populate principally carbonate shelf areas and secondarily deep water niches (fig. 10.1).
The three Caradocian silicified oolite occurrences on the southern margin of Laurentia
suggest that siliceous sponges may have still been present sporadically in some
peritidal/lagoonal settings that were characterized by peculiar local conditions.
Geographically, approximately 74% of the worldwide Caradocian cherts have been
found around Laurentia, mainly on the southwestern, southern, and southeastern
margins. The remaining occurs in Australia and China (23%), and on the Kazakhstan
block (3%). Overall, the high chert abundance suggests a biotic-abiotic earth system
161
highly favorable for chert burial. The changes in the rate of biogenic silica burial may be
due to fluctuations in biogenic silica production, dissolution, and/or preservation rates
that could be linked to physico-chemical modifications of the Earth system.
11.3.1. The abiotic system
The Cardocian epoch witnessed major changes in abiotic components of the Earth
system. Major physical changes that occurred over the Middle – Upper Ordovician
transition and during the Caradocian includ (fig. 11.1B): 1) a volcanic climax (Stillman,
1984); 2) West Gondwana crossed the South Pole (Ford and Golonka, 2003); 3) the
Taconic orogeny (Ford and Golonka, 2003; Qing et al., 1998); 4) a positive shifts in δ18O
(Shields et al., 2003; Tobin et al., 1996) and δ13C (Patzkowsky et al., 1997) values; 5) a
highly non-radiogenic strontium ratio indicating strong mid-oceanic ridge activity (Qing
et al., 1998; Shields et al., 2003) and large ridge volume (Mackenzie and Piggott, 1981) ;
6) a minimum in continental deformation (Richter et al., 1992); 7) a peak in global sea
level (Ross and Ross, 1992; Qing et al., 1998); 8) abundant volcanogenic massive sulfide
deposits in the Iapetus Ocean (Eastoe and Gustin, 1996); 9) abundant oolitic ironstone
accumulations (Van Houten and Arthur, 1989); and 10) reduced emergent area coeval
with lower average latitude of the world landmass (Worsley et al., 1994) (fig. 11.1B). All
these activities induced changes in the global climate and oceanic seawater that would
affect the biotic system and the biogenically-controlled silica cycle.
One of the most important changes affecting the Upper Ordovician world is the
initiation of climatic cooling and the development of ice sheets over both the South and
the North Pole (Poussart et al., 1999), but the timing of these processes, Caradocian
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versus Ashgillian is still open to controversy (Frakes et al., 1992; Brenchley et al, 1994;
Pope and Steffen, 2003). Although the initiation of permanent ice cap sheets in the
Caradocian is viewed skeptically, it is accepted that the cooling mode that heralded the
late Upper Ordovician glaciation originated in the Caradocian (Patzkowsky and Holland,
1993; Worsley et al., 1994; Tobin et al, 1996; Patzkowsky et al., 1997; Pope and Steffen,
2003; Shields et al., 2003). Recent modeling studies relative to the Late Ordovician
glaciation show the necessary conditions for the formation of ice sheets (Herrmann et al.,
2004). Based on the relationship between atmospheric pCO2 values, sea level, poleward
ocean heat transport, and paleogeography, the simulations show that the high sea level
and atmospheric pCO2 ( 8 x PAL) during the Caradocian could have induced the
formation of small ice sheets if the poleward ocean heat transport was reduced to half of
the modern value. However, Caradocian simulations evidence ice volumes that are
strongly inferior to those obtained for Asghillian simulations.
A cooling trend was initiated sometime in the Caradocian based on paleontologic,
tectonic and isotopic evidence (Frakes et al., 1992; Patzkowsky and Holland, 1993;
Worsley et al., 1994; Tobin et al, 1996; Patzkowsky et al., 1997; Pancost et al., 1999;
Ainsaar et al., 1999; Pope and Steffen, 2003; Shields et al., 2003). However, the time of
origination of the Caradocian cooling mode is disputed and ranges between the beginning
of the Caradocian, based on oxygen isotopic data (e.g., Shields et al., 2003), and middlelate Caradocian, based on high planktonic poductivity and carbon isotopic data
(Patzkowsky et al., 1997) and an offset feedback induced by the Taconic orogeny that
started earlier (Kump et al., 1999).
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Oxygen isotopic data and seawater temperature
Numerous studies have looked at the Ordovician seawater oxygen isotopic
composition in brachiopod shells and marine cements such as fibrous calcite. The values
obtained are controversial but all of them show generally a steady rise of the calcite δ18O
levels throughout the Ordovician (Tobin et al., 1996; Shields et al., 2003), interpreted as a
general cooling trend. Upper Ordovician seawater δ18O values obtained from
geographically widespread stratigraphic sequences (Shields et al., 2003) show an oxygen
depletion trend, by approximately 3‰ PDB that occurred over a period of less than 10
My around the Middle-Upper Ordovician boundary, (fig. 11.1B; Shields et al., 2003).
Two driving mechanisms have been hypothesized with the aim to explain this positive
shift in δ 18O (fig. 11.4).
1) The first mechanism is a decrease in minimal tropical seawater temperature
from a 27°-32°C to 16°-25° C, within an ice-cap free world, for a δ18O SMOW (standard
mean oceanic water) of -3‰ (Frakes et al., 1992; Tobin et al., 1996; Shields et al., 2003)
(fig. 11.1B and 11.4). The estimate for the temperature drop is in agreement with that of
Worsley et al. (1994) for equatorial temperatures as low as 25ºC, a cooling considered as
having occurred prior to the late Ordovician glaciation and within a world that lacked ice
sheets.
2) The second mechanism is given by a decrease of the δ18O SMOW by 2-3‰,
along with a decrease of lower magnitude in the tropical sea surface temperature.
Although this mechanism is view as a long term process (Gregory, 1991 in Shields et al.,
2003) and thus failing to elucidate the short term of the δ18O SMOW change, more
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Figure 11.4: Temperature vs. calcium carbonate shell δ18O (PDB) for various values of
seawater δ18O (SMOW) assuming an ice-cap free world. Even maximum δ18O values for
the Early Ordovician and early Middle Ordovician (-5.4‰and -6.3‰, respectively) yield
unrealistic minimum seawater temperatures (37°C – 43°C) if δ18O seawater was -1‰
SMOW, while δ18O seawater of -3‰ SMOW yields more plausible minimum tropical
sea-surface temperatures of 27°C – 32°C (Shields et al., 2003).
recent modeling studies indicate that such δ18O SMOW changes can occur over much
shorter periods of time (5 to 50 My), particularly during periods of high oceanic
spreading rates (Lécuyer and Allemand, 1999). The short-term δ18O SMOW shift theory
is consistent with the inferred high oceanic spreading rates supposed to characterize the
Caradocian based on the 87Sr/86Sr isotopic ratio (Qing et al., 1998; Shields et al., 2003).
In a world characterized by energetic intra-basinal activities, that would generate not
only higher input levels of nutrients (favoring high organism productivity), but also
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abundant CO2, a widespread anoxia event can occur. The more anoxic the bottom waters
become, the more the vertical partitioning of the oxygen isotopes takes place, generating
positive shifts in the δ18O. Consequently, a sharp positive shift in the δ18O documented at
the beginning of the Caradocian, could also reflect a strongly stratified water column
(Tobin et al., 1996), with the heavier oxygen entrapped in the bottom-most water layers,
and not necessarily a climatic cooling. The energetic volcanic activity in the southern
margin of Laurentia is also supported by two regionally widespread Middle Caradocian
air-fall ash beds, the Deicke and Millbrig k-bentonites (Huff and Kolata, 1990).
11.3.2. The biotic system
Caradocian biotas witnessed high taxonomic diversity for some groups of benthic
and planktonic organisms (e.g., radiolarians, graptoloids, brachiopods; fig. 11.1A), and a
decreased diversity and extinction for other groups (Sloan, 1988a; 1988b; Patzkowsky
and Holland, 1993). High organism productivity is shown by a middle Caradocian
positive excursion in the carbon isotopic values (Patzkowsky et al., 1997; Pancost et al.,
1999).
Sea level during this epoch was high (Ross and Ross, 1992; Qing et al., 1998), so
that the Caradocian chert peak and organism radiation coincide with an important
transgression (fig. 11.1B). The Caradocian transgression is believed to be the largest in
the Phanerozoic (Ross and Ross, 1992) due to fast oceanic floor spreading resulting in
high ridge volume (Qing et al., 1998). Although extinction events have been linked to
transgressions through the invasion of the shelf area by anoxic waters (Wilde and Berry,
1984), in this instance there was a proliferation of a variety of benthic and planktonic
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groups: radiolarians (Vishnevskaya, 1997), graptolites, brachiopds (Leggett et al., 1981)
(fig. 11.1A), and siliceous sponges (Rigby and Webby, 1988). Higher diversities and
revolutions in the marine realm have been correlated on one hand with cooling episodes
and low nutrient supply (Lipps, 1970), and on the other hand with high submarine
volcanism and hydrothermal activity providing a thermal stimulus (Hays and Pittman,
1973; Vermeij, 1995).
The Caradocian radiation was concentrated around regions of active mountain
building and volcanisms (e.g., the Iapetus Ocean), which suggests a direct relationship
between organism diversity and the proximity of tectonic activity (Vermeij, 1995; Miller
and Mao, 1995; Miller, 1997). In addition, many Caradocian deep-sea chert deposits
formed in active areas such as the southeastern part of Laurentia characterized by arccontinent collision tectonism, and Kazakhstan, which was a massive island-arc volcanic
region (Nikitin et al., 1991), supporting the high diversity - high productivity - high
volcanism inter-relationship model.
11.3.3. Considerations on the chert abundance
Slope/basinal environments. Why a chert peak?
The bottom water environment: anoxia?
At the bottom of basins, it appears that anoxic waters develop as a consequence of
the converging action of two factors: high CO2 input due to high volcanic – mid-oceanic
activity, and high input of organic matter from the surface layers due to an increased
productivity volcanically induced (fig. 11.5). Oxygen-depleted bottom water model
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would be consistent with the observed abundance of graptolite-rich black shale, (Berry
and Wilde, 1978; Leggett et al., 1981), abundant volcanogenic massive sulfide deposits
(VMS) identified in the Iapetus Ocean and marginal basins (Eastoe and Gustin, 1996),
and abundant oolitic ironstone accumulations (Van Houten and Arthur, 1989) (fig.
11.1A&B). However, whereas the Caradocian age is well documented for the black shale
peak (Leggett et al., 1981), the timing of the VMS and oolitic ironstones abundance
peaks is less well established.
The analyses of Phanerozoic massive sulfide deposits (Eastoe and Gustin, 1996)
and oolitic ironstones (van Houten and Arthur, 1989) and their associated lithologies,
emphasized their frequent association with black shales. Sulfide minerals, as well as
black shales, may be preserved either in oxygen depleted environments (the preservation
model), or due to high organic productivity accompanied by rapid burial under well
aerated conditions (the productivity model) (Calvert, 1987; Morris, 1987; Arthur and
Sageman, 1994; Eastoe and Gustin, 1996). However, considering that modern VMS
deposits that form in oxygenated water on the mid-oceanic ridges are preserved to a small
extent, it has been inferred that large accumulations of VMS are tightly linked to anoxic
bottom waters (Eastoe and Gustin, 1996).
Based on the common association between massive sufide deposits, oolitic
ironstones, and black shales, along with the Caradocian age of the Ordovician black shale
peak (Leggett et al., 1981), it can be speculated that the VMS peak along with that of the
oolitic ironstones may be of Caradocian age as well. According to Eastoe and Gustin
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Figure 11.5: Chert abundance Model. CARADOCIAN [458 – 449 Ma]
169
(1996), the abundance of VMS in the Iapetus Ocean and proximal basins during the
Ordovician, is the result of the interplay between anoxic waters and tectonic factors.
Caradocian VMS and oolitic ironstone abundances would be consistent with the
Caradocian climax of the mid-oceanic ridge activity inferred from the strontium isotopic
ratio (Shields et al., 2003) that, in turn, generated a high sea level, conditions that
represent the most favorable ambient for widespread anoxia (Mackenzie and Piggott,
1981). Documenting extensive black shale deposits for the Ordovician and Silurian,
Fischer and Arthur (1977) remarked on their association with transgression phases and
predicted the Caradocian chert peak based on the inference that during a polytaxic
interval (a greenhouse oxygen-depleted state; Fischer and Arthur, 1977), deep water
sediments should be characterized by more widespread cherts than those that form during
olygotaxic times.
The sole presence of black shales, or massive sulfide deposits, does not indicate
anoxic waters, however their coexistence along with oolitic ironstones in a world
characterized by energetic intra-basinal activity, such as high mid-oceanic spreading
rates, marine volcanism and hydrothermal activity, point toward an euxinic environment.
Within such an environment opal accumulation and burial would be greatly enhanced
(e.g., De Wever et al., 1994).
The surface waters: low or high organism productivity?
A variety of evidence points toward an anoxic deep water environment during the
Caradocian, but whether or not it was accompanied by a high productivity is unclear. It is
known that radiolarians, graptoloids, and brachiopods record high taxonomic diversities
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during the Caradocian (fig. 11.1A). Even though a high diversity and abundance
commonly show a reverse correlation (Tappan and Loeblich, 1973), studies of modern
radiolarian faunas from the East China Sea show that high productivity and high diversity
can co-occur (Chang et al., 2003).
The high productivity and organic carbon burial model proposed for the surface
water planktonic organism activity is in agreement with a middle Caradocian (P. tenuis –
P. undatus conodont zones) positive excursion in the carbon isotope composition of
carbonate (~ 3‰) and of organic carbon (~ 4‰-6‰) (Patzkowsky et al., 1997; Pancost et
al., 1999). Huff and Kolata (1990) identified two regionally widespread air-fall ash beds,
the Deicke and Millbrig k-bentonites, of the exact same age (P. tenuis – P. undatus
interval), which is viewed as chronostratigraphic marker horizons throughout much of the
eastern Mid-Continent. The question is did this volcanic pulse induce indirectly the
carbon spike by killing organisms? The carbon spike has been also correlated with
Caradocian sequences in Estonia (Ainsaar et al., 1999, in Shields et al, 2003), suggesting
an event of larger magnitude than one confined to the eastern margin of Laurentia. If this
volcanic burst affected marine biotas to such a wide extent as to imprint a strong carbon
shift in the rock record, we can ask if it was not also responsible for a short-lived
warming driven by the high amount of CO2 exhaled into the atmosphere. The oxygen
data of Shields et al. (2003) show a sharp, short-lived negative shift during the same age
interval (fig.11.1B) also coeval with a sea level rise, that could be the result of the same
large-scale volcanic event. All these converging phenomena provide evidence in support
of the greenhouse hypothesis, at least for the early and middle Caradocian. Further, a
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warm Caradocian model implies that the cooling mode did not start until the late
Caradocian, which means that the middle Caradocian carbon shift does not reflect a high
organism productivity. However, it is possible that the cooling mode could have been
triggered by rapid CO2 drawdown resulting from high biotic productivity. More data are
needed in order to test a Caradocian greenhouse mode based on the above evidence.
The Caradocian chert peak is coeval with high oceanic spreading rates, energetic
volcanism, and hydrothermal activities that are consistent with a high sea level (Qing et
al., 1998). Vigorous intra-basinal volcanic activity would stimulate higher silica and other
nutrient fluxes so that the productivity of radiolarians and other planktonic organisms
would be boosted inasmuch as the surface water temperatures are still favorable.
Energetic intra-basinal activities may initiate energetic vertical water advection, including
coastal upwelling (volcanic-induced upwelling of Vogt, 1989). Thus, increased nutrient
input has two sources: mid-oceanic activity, and upwelling of rich-nutrient sediments.
Numerous studies have stressed the linkage between high biogenic chert abundance, high
silica secreting organism productivity, and intensive volcanic/tectonic/magmatic activity
(Grunau, 1965; Steinberg, 1981; Aubouin, 1965; Racki, 1999; Racki and Cordey, 2000).
If we accept the initiation of a climatic cooling at the beginning of the
Caradocian, as suggested by the oxygen isotopic ratios, and taking into account that the
solubility of silicic acid in water decreases with temperature, in a cooler environment less
silica is dissolved in the water column and thus more opal would reach the seafloor to be
preserved as chert. A cooling would affect principally the surface waters in contrast to
much more stable deep waters. Modern studies show that 50% of the total biogenic silica
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production is remobilized in the upper layers of the ocean (the upper 100 m) and
therefore a coling would increase the potential of opal accumulation and burial through
an increased opal rain rate. Also, silica dissolution depends directly on water temperature,
most of the opal accumulation occurring in areas where the surface water is cold
(DeMaster, 1981). Because large temperature variations occur within the surface layers
of water (100-200 m) and the deep-sea is confined to a relatively narrow range of low
temperatures (Bidle et al., 2002), a very important factor in preservation of opal in
basinal settings is ultimately the temperature of surface layers of water (Nelson et al.,
1995). However, a decrease in the opal dissolution rate in the surface waters would be
triggered by lower water temperature. The magnitude of a potential Caradocian seawater
cooling is constrained by the high diversity of radiolarians whose metabolism in modern
settings is negatively affected by low temperatures (Anderson et al., 1989c).
In conclusion, higher opal accumulation and burial rates would be generated by:
1) a global productivity acme in coastal and abyssal settings triggered by high
endogenous inputs of silica and other nutrients; and 2) excellent taphonomic conditions
within an anoxic environment driven by the high fluxes of organic matter, CO2 and other
oxygen-consuming cations. This model suggests that preservation and productivity do not
exclude each other as mechanisms that induce the formation of cherts, and that their
concurring action only generates a higher chert peak, under the incidence of tectonovolcanically induced taphonomic, oceanographic, and biosiliceous productivity
conditions (fig. 11.5). A surface water cooling could add even more to the potential for
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opal accumulation due to lower dissolution rates under the circumstances the surface
water temperature still represents a favorable environment for radiolarians.
Shelf settings
During the Caradocian, widespread carbonate platforms, representing a good
environment for the development of siliceous sponge faunas, were again dominating the
shelf system. Siliceous sponges that at this point were inhabiting the deeper sites of the
shelf area, could have taken advantage of a volcanically-induced coastal upwelling of the
nutrient rich waters. Strontium data show strong non-radiogenic values indicating a
nutrient input cycle dominated by endogenous fluxes and a possible seawater cooling
would not affect too strongly this opportunistic group capable of inhabiting shallow to
deep environments since the Early Cambrian (Finks, 1960), and low to high latitudes at
least since the Lower Ordovician (Beresi and Esteban, 2004). After a period of ecological
adaptation to new ecological niches after their migration from onshore toward more
offshore settings during the Middle Ordovician, siliceous sponges would proliferate in a
stable, nutrient rich carbonate environment. Chert burial would be increased due to higher
silica levels via siliceous sponges, and possibly due to a lower opal dissolution rate in
cooler seawater.
11.4 Upper Ordovician: the Ashgillian - an icehouse period
During the Ashgillian, the chert abundance decreases in all three depositional
environments (Table 7.2.B and 7.3.B), but it is still higher than the Middle Ordovician
chert low. While the formation of peritidal/lagoonal chert deposits ceases, shelf cherts
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dominate by a factor of two over the slope/basinal deposits. The slope/basinal chert
deposits exhibit the first drop in abundance for the Ordovician. The shelf cherts that
dominate the Ashgillian biosiliceous burial surpass both the Lower and the Middle
Ordovician shelf chert occurrences. The distribution of silica-secreting organisms within
basins show no changes - planktonic radiolarians are still confined to slope/basinal
settings, and siliceous sponges to shallow and deep water environments (shelf and
slope/basin). Taking into account the genetic relationship that exists between radiolarians
and slope/basinal bedded chert formation (Chapter 7 and 9), it seems that radiolarian
fauna played an important part in this general chert abundance decrease. From the point
of view of the geographical distribution of cherts, both Laurentia and Gondwana witness
significant decreases in silica burial based on the decrease in the number of chert deposits
(Table 7.6).
11.4.1. The abiotic system
The Ashgillian was characterized by the presence of ice sheets (over both poles,
Poussart et al., 1999), low sea levels, and the ongoing Taconic orogeny. Considerable
land area was exposed by the glacially-induced sea level drop whose magnitude has been
estimated between 50m and 100m (Brenchley and Newall, 1980; Finney, 1997), and by
the ongoing Taconic orogeny, two physical processes that induce the exposure of fresh
silicate rocks. However, the strontium data still exhibit strong non-radiogenic values and
no differences between the Caradocian and the Asghillian (fig. 11.1B; Shields et al.,
2003). These values have been explained by a delay in the continental strontium influx
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caused by the time necessary for erosion to remove the emerged widespread carbonate
shelves before exposing the more radiogenic basement rocks underneath (Qing et al.,
1998). The oceanic circulation underwent an overturn through the formation of ice sheets
that cause the sinking of denser oxygenated waters at the poles, thus inducing the
formation of active coastal upwelling and an increase in wind, wave and storm action as a
consequence of larger thermal gradients between the poles and the tropics (Berry and
Wilde., 1978).
Short-lived oxygen (3‰-4‰) and carbon (as much as 7‰) excursions have been
used as arguments to confine the timing of the glaciation to the late Ashgillian, the
Hirnantian stage, suggesting a short-lived glaciation (Brenchley et al., 1994, 2003; Gibbs
et al. 1997). Muehlenbachs (1998) asserts that during glacial periods the seawater δ18O
can increase with up to 2‰ - 3‰ assuming an originally ice-free world. Mechanisms that
have been proposed to explain such a short-lived glaciation include high organism
productivity (Brenchley et al., 1994) and the Taconic orogeny (Kump et al., 1999), which
may have initiated cooling followed by a more dramatic shift into full icehouse
conditions. Global biochemical models suggest that the Late Ordovician glaciation has
occurred under high pCO2, levels between 8 and 16 times higher than today’s level
(Berner, 1994; Kump et al., 1999; Yapp and Poths, 1992; Gibbs et al., 1997; Herrman et
al., 2003).
11.4.2. The biotic system
The Late Ordovician extinction, considered one of the five big Phanerozoic
extinctions (Sepkoski, 1995), took place in two phases (two major faunal turnovers)
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related to two climatic and environmental changes: first, a cooling period ending with the
onset of the glaciation, secondly, a deglaciation and warming event (Brenchley, 1989;
Brenchley et al., 2003 the second phase characterized by deglaciation and warming
witnessed most part of the late Ordovician animal extinction (Brenchley et al., 2003).
Whether or not the late Ordovician glaciation affected radiolarian faunas is
controversial. Noble et al. (1997) reported a lack of major radiolarian extinction while
Vishnevskaya (2000) noted a major radiolarian extinction across the Ordovician-Silurian
boundary (Ashgillian – Llandoverian) with radiolarian diversity droping from 100 to 20
species (fig. 11.1A). Following this drop, the rest of the Silurian witnesses a slight
increase in diversity upwards to 60 species (Vishnevskaya, 2000). However, to date there
are no known individual rock sequences in which extinction in the siliceous radiolarian
fauna is clearly recorded.
Relative to the Ordovician silicisponges, within a shallowing interval of the lower
Hanson Creek Formation (late Upper Ordovician) Finney et al. (1999) recognize a faunal
turnover in siliceous sponges, defined by a change from deep water, hexactinelliddominated assemblages, to shallow, subtidal lithistid-dominated assemblages as a
consequence of the sea level drop.
11.4.3. Considerations on the chert abundance (fig. 11.6)
During glacial periods abundant chert accumulation should occur at upwelling
sites due to a high organism productivity. Within the shallower facies the amount of
siliceous deposits could decrease as a consequence of changes in the ecological
distribution of siliceous organisms driven by the glacial-eustatic sea level regression.
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Figure 11.6: Chert abundance Model. ASHGILLIAN [449 – 443 Ma
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Moreover, a drop in sea level would increase the volume of silicates available to
weathering, as well as the dissolved silica input into the basinal facies, while the shelf is
more or less emerged. The amount of dissolved silica in seawater could be higher and
more offshore chert may occur.
Shelf environments
Shelf water environments were populated exclusively by the silicisponges during
the Ashgillian. Even though the shelf chert abundance registers a decrease as compared to
the Caradocian, the normalized data show that Ashgillian shelf occurrences rank second
in abundance even under the circumstances of a severe sea level drop (50-100m) and
exposure of shelves. This finding suggests two main posibilities:
1) The siliceous sponge faunas were not affected significantly by seawater
cooling induced by the Late Ordovician glaciation. This view is supported by the ecology
of modern silicisponges which demonstrates that they tolerate large thermal ranges,
paleolatitudinally as well as vertically within the basin (Reincke and Barthel, 1997). The
low to mid-paleolatitudinal distribution of silicisponges shown by the present study
(Chapter 10), also supports the interpretation that siliceous sponge faunas were not
significantly affected by the cooling.
2) The glacial-eustatic sea level drop was of short duration so that the induced
loss of living habitats did not affect the opal production and burial to any great extent.
The normalized data demonstrates a high shelf chert abundance during the Ashigillian,
outnumbered only by the Caradocian siliciponge-induced shelf cherts. Moreover,
silicisponges are able to inhabit varied ecological niches, carbonate or not, as long as they
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have clear waters and sufficient nutrients. Under these circumstances, the fact that they
were populating the shelf environment during the Ashgillian demonstrates low
terrigenous inputs, contrary to the high input that we would expect as a consequence of
the continental weathering of fresh silicate rocks exposed by the Taconic orogeny. This
hypothesis is consistent with the strontium ratio values that still show non-radiogenic
characteristics. At the other and of the spectrum, a long-term sea level drop would
drastically affect siliceous sponge habitats and implicitly the silica burial. An analysis
limited just to Ashgillian siliceous sponge shelf cherts would not offer insight into their
opal production potential and the factors that influence it. However, such an analysis is
clearly put in perspective when performed in the context of the whole Ordovician.
Slope/basinal environments
Because this survey does not differentiate between the slope and deep sea
(basinal) environments of chert formation, two oceanographic systems that function
differently with respect to opal accumulation and preservation (Nelson et al., 1995), I can
only speculate about the implications of the glacially-induced effects on biosiliceous
productivity, opal accumulation and burial. The impact of a vigorous oceanic circulation
through vertical advection of cold, nutrient-rich, deep waters, might generate
volumetrically important chert deposits spatially concentrated at upwelling sites rather
than widespread abundant smaller cherts (Kidder, oral communication). In view of this
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fact, estimation of chert abundance based on frequency of occurrences becomes futile in
interpreting any change in silica burial caused by glacially induced oceanic turnovers.
High chert abundances, on the slope as a result of high organism productivity
induced by upwelling, would imply that the silica secreting biota responsible for chert
formation behave as well in cooler environments as in warm waters. During the
Ashgillian, radiolarians were very likely inhabiting the whole transect of the deep part of
a basin, from slope to deep-sea, and thus they were under direct influence of the coastal
upwelling. The oxygen isotopic ratios show an important cooling during this interval (11
- 17°C; Shields et al., 2003). To date both modern data (Sugyiama and Anderson, 1997;
Anderson et al., 1989, a,b,c) and paleo-ecological results (the present study) show the
affinity of radiolarians with warm environments, demonstrating that environmental
factors such as temperature and salinity are more significant limiting factors in
radiolarian growth and longevity.
The slope/basinal bedded chert drop therefore is interpreted as a negative
perturbation of radiolarian productivity due to cold surface seawater. In deep-sea settings,
within a scenario of cooler oceanic surface waters, the lower silica dissolution rate would
increase the potential for higher preservation and chert formation. However, such a high
preservation is not substantiated in the rock record, which supports the hypothesis that the
slope/basinal bedded chert drop was a result of productivity slowdown due to low water
temperature. Moreover, a general high organism productivity, induced by the oceanic
overturn, associated with the whole Ashgillian interval has not been demonstrated either
by abundant black shale deposits (Leggett et al., 1981), or by positive excursions in δ13C.
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The first important Ashgillian change in the amount of organic carbon burial has been
documented as occurring in the Hirnantian, the youngest time-defined level of the
Ashgillian (Brenchley et al., 1994).
It is worth mentioning that the Ashgillian shelf and slope/basinal cherts surpass
and almost equal those of the Lower Ordovician in spite of the sharp decline in silica
burial recorded between the Caradocian and the Ashgillian. Silica output into cherts
expresses ultimately the balance between biogenic silica production, dissolution, dilution
by host sediment, and preservation rates. Therefore two different mechanisms can be
considered responsible for triggering the chert formation corresponding to the two
antagonistic climatic modes that characterize the Lower Ordovician and the Ashgillian:
high biosiliceous productivity, associated with good taphonomic bottom water conditions
within a greenhouse mode (Lower Ordovician) vs. low biosiliceous productivity
associated with low dissolution rate within an icehouse mode (Ashgillian).
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Chapter 12
CONCLUSIONS
A global survey of the spatial and temporal distribution of Ordovician nodular and
bedded cherts, and subordinately of silicified oolites, was undertaken to address three
questions: 1) Did the Early Paleozoic radiolarians and siliceous sponges behave similarly
to diatoms regarding their response to abrupt changes in oceanic circulation and climate
parameters? 2) Did cherty facies retreat from peritidal settings during the Ordovician? 3)
Can chert deposits be of use in evaluating whether the late Ordovician glaciation spanned
most of the Caradocian and Ashgillian (Pope and Steffen, 2003) or was the glaciation
more tightly constrained to within the Ashigillian (Brenchley et al., 1994)? Further on,
the volume and resolution of the data allowed quantitative and qualitative analyses of the
relationships between chert deposit types, depositional environments, and silica secreting
organisms.
The results of this study can be summarized as follows:
1) Most Ordovician chert deposits are of biogenic origin due to: a) approximately
38% of the Ordovician chert formations contain siliceous skeletal remains; and b) the
onshore-offshore migration of the silica secreting organisms, perceived through the rock
record of chert bearing siliceous organisms, is accompanied by an onshore-offshore
change in the locus of silica burial, reflected in the stratigraphic record of cherts that lack
preserved siliceous skeletal remains.
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2) The analysis of chert types and reported skeletal remains within cherts reveals
a segregation of chert types due to a genetic relationship between the silica secreting
organisms and the types of the chert deposits. Peritidal/lagoonal and shelf cherts are
mainly nodular cherts hosted by carbonate facies. These cherts appear to have formed
predominantly via silica secretion by lithistid demosponges. Most of these cherts are low
latitude occurrences (< 30ºN and S). Slope/basinal cherts are predominantly bedded
cherts, mainly hosted by fine-grained siliciclastic sediments, and bear radiolarians. Most
of these cherts are also confined to low latitudes. At mid latitudes, the skeletal
components of these cherts are mainly hyalosponges. Also, the analysis showed that the
formation of some shelf bedded cherts might by induced by silicisponges.
3) The Middle Ordovician distribution pattern of silica secreting biotas reflects a
migration of siliceous sponge faunas from peritidal/lagoonal to shelf-basinal
environments, and of radiolarians to basinal settings, suggesting an ArenigianLlanvirnian onshore-offshore migration of the silica secreting organisms. This refines the
timing of an earlier suggested retreat in the locus of silica burial during this period. It also
indicates that Early Paleozoic planktonic faunas accompanied the benthic communities.
Whether the timing of the retreat matches the roughly coeval onshore-offshore shift of
the benthic Cambrian and Paleozoic faunas (Jablonski et al., 1983) is unclear. The Middle
Ordovician minimum of peritidal/lagoonal and shelf cherts may be a result of shelf area
reduction during a relatively low sea level. However, the reappearance of shelf cherts in
the Caradocian indicates that, although silicisponges did retreat significantly from
peritidal/lagoonal settings in the early Middle Ordovician, they still largely populated the
184
shelf settings. The few Middle Ordovician and Caradocian silicified oolites suggest that
silicisponges may have still been present sporadically in some peritidal/lagoonal settings
that were characterized by peculiar local conditions.
4) The Ordovician spatial and temporal distribution patterns of siliciponges and
radiolarians suggest that silica limitation governed the distribution of silicisponge faunas,
and that physical factors such as temperature played a major role in the distribution of
radiolarians.
5) Lower Ordovician cherts are abundant and widespread, occurring in equal
proportions in peritidal/lagoonal, shelf, and slope/basinal settings. Within a warm world
characterized by stable and favorable environmental conditions, high sea level, and
widespread carbonate platforms, the chert abundance in peritidal/lagoonal and shelf
environments seems to be the result of the interplay between good organism productivity
and enhanced chert preservation. The dominance of bedded cherts in slope/basinal
settings may reflect relatively high biosiliceous productivity, probably associated with
favorable taphonomic conditions under an oxic regime within the bottom waters.
6) A significant decrease in the number of chert deposits occurs in the Middle
Ordovician. While the peritidal/lagoonal and shelf environments record a strong decline
in silica burial, the slope/basinal bedded cherts record a slight augmentation. This finding
suggests that the redistribution of benthic shallow water siliciponges played an important
role in this general chert abundance low. The integration of data on biotic and abiotic
changes that occurred during the Middle Ordovician indicates that the Middle Ordovician
chert low was driven by a shortage in the shallow water silica burial due to redistribution
185
of silicisponges and their habitat and subsequently to a lesser extent of radiolarians,
possibly accentuated by a general Middle Ordovician sea level low. The slightly
increased slope/basinal chert abundance during this interval seems to conform to a good
organism productivity, enhanced preservation rates and aerated bottom waters model.
7) A resurgence in the number of chert deposits occurs in the Upper Ordovician,
more precisely during the Caradocian. This occurs in both shelf and slope/basinal
environments, where chert abundance surpasses that of the Lower Ordovician by 50%.
However, Upper Ordovician chert abundance in peritidal/lagoonal settings remained at
essentially the same low levels as in the Middle Ordovician.
8) The Caradocian marks the highest point in the abundance of Ordovician cherts,
that appear to have accumulated during a warming interval. This chert peak is coeval
with a transgression, high abundance of black shales, oolitic ironstones, and volcanogenic
massive sulfides, as well as the Ordovician peaks in diversity of radiolarians and other
organisms, are all consistent with a warm stable world and / or oceanic anoxia. Changes
in δ18O values at the Middle-Upper Ordovician boundary are consistent with warming
late in the Caradocian. Values of 87Sr/86Sr shift toward non-radiogenic values and the sea
level toward transgression. These suggests that continental input of dissolved silica
declined instead of rising in the Caradocian, despite the coeval Taconic orogeny. The
high chert abundance may have been the result of a global productivity acme triggered by
high endogenous inputs of silica and other nutrients, along with excellent taphonomic
conditions within an anoxic environment driven by the high fluxes of organic matter,
CO2, and other oxygen-consuming cations.
186
9) The decline of cherts during the Ashgillian, when the glaciation was clearly in
effect, was not expected based on assumptions of modern silica accumulation in an
icehouse with vigorous ocean circulation. While the slope/basinal cherts record a strong
decline, shelf cherts exhibit only a fairly low reduction. This finding suggests that
radiolarians played an important role in this relatively low general chert abundance. The
decline may reflect: a) a negative perturbation of radiolarian activity due to cold surface
seawater; b) accumulation of fewer but volumetrically more important basinal cherts
reflecting upwelling settings in the Ashgillian, under the circumstances where
radiolarians behave as well as in cold waters as in warm waters; c) depletion of oceanic
dissolved silica content via silica burial during glacially induced intensive upwelling
again under the circumstances where low temperatures are not a limiting factor for
radiolarian growth. The relatively high silicisponge-induced shelf chert abundance within
an icehouse world outnumbered only by the Caradocian shelf cherts could be linked to a
glacially-induced sea level drop of short duration. An analysis limited just to Ashgillian
silica secreting organisms would not offer insight into their behavior and opal production
potential and the factors that influence them. However, such an analysis is clearly put in
perspective when performed in the context of the whole Ordovician, by epoch. Chert
abundance alone probably will not resolve the debate about the duration of the
Ordovician glaciation. However, consideration of their abundance patterns in conjunction
with other aspects of the Earth system is consistent with Caradocian warming followed
by a brief Ashgillian cooling and glaciation, the causes of which remain enigmatic.
187
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Yapp, C.J. and Poths, H., 1992. Ancient atmospheric CO2 pressure inferred from natural
goethites. Nature, 355: 342-344.
Yiren, L. and Hanying, F., 1989. A candidate stratotype section of Ordovician
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220
LOWER ORDOVICIAN
APPENDIX
North America
USA
No
Chert
type
Age
1
2
Alsate Shale
Blakely Fm.
l, Ar
l
3
Chepultepec Fm.
l
4
Cool Creek Fm.
l, Tr
5
Cotter Fm.
l
E Kansas, W Missouri, Arkansas, 36047'N/94 027'W
peritidal/lagoonal
N, O
?
McCracken (1955)
6
Crystal Mountain Fm.
l
Arkansas, 34031'N/94 004'W
undecided
B
sh, SST
Pitt et al. (1961)
7
Deepkill&Normanskill beds
E New York belt, 42 017'N/73 048'W
slope/basin
B
sh with grpt
8
El Paso Fm.
l
New Mexico, Texas, Arizona 31058'N/105 028'W
peritidal/lagoonal
N, B
LMS, Dol
9
Ellenburger Fm.
l
W Texas, Oklahoma 30025'N/103 044'W
peritidal/lagoonal
N,O
LMS, Dol
l
E New York, SW Vermont, 43 012'N/73 019'W
Central New York
peritidal/lagoonal
N, B
LMS
Mazzullo and Friedman (1975)
shallow shelf,
subtidal-tidal
N
LMS, Dol
Philips and Friedman (1995)
S Missouri, W Tennessee, Kentucky, N Arkansas,
36 045'N/89 050'W
peritidal/lagoonal
N, O
Dol
Woody et al., (1996)
Ross, (1964); Langenheim et al., (1962)
10 Fort Ann Fm.
11 Gailor Fm.
12 Gasconade Dolomite
l, Ar
l, Tr
l
Location Lat/Long
Depositional
environment
Formation
Se Oklahoma, SW Texas, 34008'N/96 056'W
Eastern Ouachitas of Arkansas, 34 031'N/94 033'W
Tennessee, Kentucky, W North Carolina, West Virginia,
Virginia, Maryland, Alabama, 36037'N/84 046' W
SW Oklahoma, 34025'N/97 059'W
Associated facies
SSR*
References
shelf
undecided
B
B
sh, LMS, SST
sh, SST, slts
Harlton (1966); Berry (1958)
Stone and Sterling (1962)
peritidal/lagoonal
N, O
Dol
H. B. Renfro, Dan E. Feray (1989)
peritidal/lagoonal
shallow shelf
N, B
LMS, evap
Ragland and Donovan (1991); Gao and Land (1991)
Rad
Dietz and Holden (1966)
Epis and Gilbert (1957), Lee and Friedman (1987)
SS
Udden (1921); Cole, (1941)
13 Goodwin Limestone
l
CS Nevada, California, Nevada, 37030'N/115 048'W
shelf
N
LMS
14 Great Meadows Fm.
l
E New York, SW Vermont, 43 012'N/73 019'W
peritidal/lagoonal
N, B
LMS
Mazzullo and Friedman (1975)
15 Gunter-Jordan Fm.
l
Illinois, 40 0N/88 017'W
peritidal/lagoonal
N, O
SST, Dol, sh
Workman and Bell (1948); Buschbach (1964)
16 Jefferson City Fm
l
Kentucky , Oklahoma, Kansas, Missouri, 94033'N/36 055'W
peritidal/lagoonal
N, O
Dol, Sh
SS
Renfro and Feray (1989)
SW Oklahoma, 34025'N/98 005'W
peritidal/lagoonal
N, B
LMS
SS
Ragland and Donovan (1991); Latham (1968); Harlton
(1963)
peritidal/lagoonal
N, O
LMS, Dol
17 Kindblade Fm.
l, Ar
18 Kingsport Fm.
l
W North Carolina, E Tennessee, E Kentucky, West Virginia,
W Virginia, Maryland, 36037'N/84 046'W
19 Lambs Chapel Dol
l
20 Longview dolomite or Nittany
dolomite
l
S Ohio, 39 024'N/83 047'W
Alabama, SW Virginia, Georgia, E Kentucky , Tennessee
34 044'N/85 018'W
Renfro and Feray (1989), Perry et al. (1979)
peritidal/lagoonal
N, O
Dol
Dolly and Busch (1972)
peritidal/lagoonal
shallow shelf
N, B, O
LMS, Dol
Bennison (1975); Folk and Pittman (1971)
221
No
Formation
Age
Location Lat/Long
Depositional
environment
Chert
type
Associated facies
SSR*
References
SS
Gerhard (1974)
21 Manitou Fm
l
central Colorado, 38 044'N/105 027'W
shelf
N, B
Dol
22 Mascot Fm
l
W Tennessee, Kentucky 36 034'N/88 052' W
peritidal/lagoonal
N
LMS, Dol
Renfro and Feray (1989)
23 Mazarn shale
l
Arkansas, 34031'N/94 004'W
slope/basin
B
grpt black sh
Pitt et al. (1961); Finney (1997)
24 McKenzie Hill Fm.
l
N
LMS, Dol
Latham (1968)
l
W Oklahoma, 35013'N/95000'W
Wisconsin, SE Minnesota, N Iowa, S Wisconsin,
43 014'N/91 010'W
undecided
25 Oneota dolomite
peritidal/lagoonal
N
Dol
Fisher and Barratt (1985); Prokopovich (1953)
26 Ontelaunee Fm.
l
W Pennsylvania, 40024'N/75 038'W
undecided
N
Dol, LMS
Hobson (1957)
27 Phi Kappa Fm.
l
central Idaho, 43 053'N/114 034'W
slope/basin
B
sh with grpt; Q
Churkin (1963)
28 Powell-Smithville Fm.
29 Rickenbach Fm.
l
l
E Kansas, W Missouri, Arkansas, 36047'N/94 027'W
W Pennsylvania, 40024'N/75 038'W
peritidal/lagoonal
undecided
N, O
N, B
Q, sh
Dol
McCracken (1955)
Hobson (1957)
30 Roubidoux Fm.
l
Missouri, S Kansas, Oklahoma, 37018'N/94 033'W
peritidal/lagoonal
N,O
LMS, SST
McCracken (1955); Overstreet et al. (2003)
31 Shakopee Fm.
l
Wisconsin, SE Minnesota, NE Iowa, 42 058'N/90 043'W
peritidal/lagoonal
O, undet
LMS, Dol
Fisher and Barratt (1985); Smith et al., (1993)
32 Siliceous Lime
l
N
LMS, Dol
Edson (1927); Gallardo and Blackwell (1999)
l
Oklahoma, Spavinaw, 34044'N/97 016'W
North Carolina, Tennessee, Kentucky, West Virginia,
Virginia, Maryland, Pennsylvania 36016'N/82 012' W
shelf
33 Stonehenge Fm.
peritidal/lagoonal
shallow shelf
N
LMS, Dol
Renfro and E. Feray (1989), Moshier (1986)
34 Tribes Hill Fm.
l
Central New York 42 050'N/75 036' W
shallow shelf
N
LMS, Dol
Philips and Friedman (1995)
35 Turkey Mountain lime
l
NE Oklahoma, Tulsa 36 017'N/95 045'W
undecided
N
LMS
Ruedemann and Redmon (1927)
36 West-Spring Creek Fm
l, Ar
SW Oklahoma, 34020'N/97 043'W
shelf
N,B
LMS
Ragland and Donovan (1990); Smith et al. (1993)
37 Windfall Fm.
l, Tr
Nevada, 39031'N/115 058'W
shelf
undet
lime mudst, sh
SS, Rad Kozur, et al (1996); Bird (1989)
Canada
l
NE Newfoundland, 49 052'N/55 060'W
slope/basin
B
argillite, volc
Kay (1967)
39 ?
l
Gaspe Peninsula, 48 046'N/65 017'W
slope/basin
B
sh, cgl, SST
Hein and Parrish (1989)
40 ?Little Port Complex
l
W Newfoundland, the Bay of Islands, 46 031'N/67 041'W
slope/basin
B
ophiolitic complex
Rad
41 lower part of Cow Head group
l
W Newfoundland, 50 0N/58 0W
slope/basin
N, B
LMS, sh with grpt
SS, Rad Suchecki and Hubert (1984)
42 Neruokpuk Fm.
l
Yukon Territory, Barn Mountains, 68 054'N/139 033'W
undecided
B
LMS, volc
Churkin, Jr. (1975)
43 Snook's Arm group, Wild Bight
Gps.
l
NE Newfoundland , W Notre Dame Bay, 49 054'N/57 010'W
slope/basin
N, B
LMS, sh, volc., SST
Hein and Parrish (1989)
38
Dildo sequence
Blome et al. (1995b); Aitchinson et al. (1998)
222
No
Formation
Location Lat/Long
Depositional
environment
Tarija region,South Bolivia,21030'S/64 040'W
slope/basin
B
black sh
Erdtmann et al. (1995)
l
l
E Victoria, 37 0S/147 0E
N territory, Daly River Basin, 150S/132 0E
slope/basin
shelf
undet
undet
LMS
Webby (1981)
Webby (1981)
l
NW South Island, 41 051'S/172 001'E
slope/basin
B
black sh with grpt,
SST
Rad
l
l
S Scotland, 57 004'N/4013'W
Ballantrae, Scotland, 58 015'N/4020'W
slope/basin
slope/basin;
peritidal/lagoonal
B
N, B
volc, black sh
ophiolitic complex
Rad
Grunau (1965); Hubert (1966)
SS, Rad Grunau (1965); Aitchison (1998); Armstrong et al. (1999)
l, Ar
Southern Uplands, Scotland, 55 0N/50W
slope/basin
B
oph. complex
Rad
Floyd (1996)
l, Ar
Spitzbergen = Svalbard, 78 045'N/15 032'E
shelf
undet
LMS
Rad
Blome et al. (1995); Renz (1990)
l
Holy Cross Mountains, 50 040'N/21 0E
undecided
undet
?
l
Urals, 57 039'N/65 059'E
slope/basin
B
sh
Rad
l
46 053'N/66 018'E
slope/basin
B
oph. complex
SS, Rad Blome et al. (1995b)
l
Kyzylkum desert, 42030'N/65 0E
slope/basin
undet
turbiditic seq.
Rad
Age
Chert
type
Associated facies
SSR*
References
South America
Bolivia
44 Cieneguillas Argillites
l, Tr
Australia and Oceania
Australia
45 Howqua shale
46 Oolloo Limestone
New Zealand
47 Anthill Black shale, Aorangi
Mine Fm.
Hein and Parrish (1989)
Europe
England
48 ?
49 Durness Limestone, Ballantrae
Complex
50 Raven Gill Fm.
Norway
51 Valhallfonna Fm.
Poland
52 Holy Cross Mount.
Fischer (1984)
Asia
Ural Area
53 ?
Hein and Parrish (1989)
Kazakhstan
54 Burubaital Fm.
Uzbeksiatn
55 Besopan Suite
Drew et al. (1996)
223
No
Formation
Depositional
environment
Chert
type
Age
Location Lat/Long
Associated facies
SSR*
References
l, Tr
Yongwol, Taebaeksan basin, 37 011'N/128 028'E
shelf, middle and
outer ramp
B
LMS, sh
Kim and Choi (2000)
l, Tr
Xichuan, Yangtze platform, SC China, 32 030'N/110 0E
shelf
B
LMS, Dol
Webby (1995)
l
Yangtze George area, western Hubei, 30030N/109 030'E
shelf
undet
LMS
SS
Liu et al. (1997)
59 Guanyintai Fm.
60 Honghuayuan Fm.
l, Tr
l
central China, 32 0N/120 0E
Yangtze George area, western Hubei, 30030N/109 030'E
shelf
shelf
B
undet
Dol
LMS
SS
Webby (1995)
Liu et al. (1997)
61 Liangchiashan Fm.
62 Woboer Fm.
l
l, Ar
Liangchiashan near Shimengzhai, 400N/120 0E
Hangwula Mountains, Alxa, Nei Monggol, 41 030'N/104 0E
shelf
shelf
N
B
LMS, Dol
dark argill, slate
Webby (1995)
Webby (1995)
63 Zhifangzhuang Fm.
l
Shangdong, North China Platf., 36 030'N/117 0E
shelf
undet
Dol
Webby (1995)
Korea
56 Mungok Fm.
China
57
Beilongmiao Fm.
58 Fenxiang Fm.
Others
64 ?
l,m
Sonora, Mexico; 110 052'W/28 036'N
slope/basin
B
black sh, SST, LMS
Finney and Bartolini (1993)
65 Garden City Fm.
l,m
SE Idaho, SW Wyoming, USA, 110003'/42 031'N
peritidal/lagoonal,
shelf
N, B
LMS
Berry (1962)
66 Heituao Fm.
l,m
Queerqueke area, Tarim block, NW China, 40 054N/88 020E
slope/basin
B
black sh
67 Karmberg Limestone
l,m
Australia, Tasmania, 146028'E/42 040'S
shelf
N
LMS
Webby et al. (1985)
Langenheim et al. (1956); Stokstad (1995)
Kay (1948)
Dolly and Busch (1972); Conley (1977)
Rad
Zhiyi et al. (1995)
MIDDLE ORDOVICIAN
North America
USA
1
2
3
Barrel Spring Fm.
Blackford Fm.
Chazy limestone
m
m
m
Mazourka Canyon, California, 36 052'N/117 037'W
Virginia, 36059'N/82 049'W
Ohio, C Pennsylvania, 40 039'N/78 014'W
middle-outer shelf
undecided
peritidal/lagoonal
shelf
N
N, B
N
SST, sh, LMS
LMS, sh
LMS, sh
4
Fort Pena Fm.
m
SW Texas, 30024'N/103 023'W
peritidal/lagoonal,
slope/basin
N, B, O
sh, clcr, cgl
5
Hardy Creek limestone
m
SW Virginia, 36050'N/81 048'W
undecided
N
LMS, argill. LMS
SS
Wilson (1954); McBride (1969)
Harris (1958)
224
No
Formation
Age
Location Lat/Long
Depositional
environment
Chert
type
Associated facies
SSR*
References
6
horizon no.2 & 3 of the VininiValmy Fm.
m
Nevada, E California, 38 013'N/118 035'W
slope/basin
B
LMS, sh, volc
Rad
D.E. Feray et al. (1968); Murchey et al. (1983)
7
8
9
Lincolnshire limestone
Mazourka formation
Pinesburg Strata
m
m
m
West Virginia, Tennessee, 36050'N/82 049'W
California, Badger Flat, 36 052'N/117 037'W
E Kentucky, W Virginia 36040'N/82 042'W
shelf, subtidal
shelf, subtidal
undecided
N, B
N, B
N
LMS
LMS
Dol, LMS
SS
Kay (1948); Bauert (1994)
Langenheim et al. (1956); Sloan (1976)
Renfro and Feray (1989); Bennison (1989)
10 ?Bay of Islands Complex
m
W Newfoundland, the Bay of Islands; 46 031'N/67 041'W
slope/basin
B
ophiolitic complex
Rad
Blome et al (1995b)
11 Cobbs Arm sequence
m
NE Newfoundland, 46 031N/67 041W
slope/basin
B
Volc, LMS, argill sh
with grpt
12 Red shale, Cow Head Gr.
m
W Newfoundland, 50 0N/58 0W
slope/basin
N, B
LMS, sh with grpt
SS, Rad Suchecki and Hubert (1984); Coniglio (1987)
13 Numeralla formation,
Adaminaby beds
m
S New South Wales, 38 004'S/144 043'E
slope/basin
B
sh with grpt,
turbiditic seq.
Rad
Dietz and Holden (1966); Fergusson and Fanning (2002)
14 Temperance Fm.
15 Wahringa LMS
m
m
SC New South Wales, 36 030'S/149 030'E
New South Wales, 38 006'S/147 020E
slope/basin
undecided
undet
B
Volc., SST, sh
LMS
Rad
Webby (1981)
Lyons and Percival (2002)
Southern Uplands,Scotland, 55 0N/50W
slope/basin
N, B
mudst
Rad
Floyd (1996); Danelianand Clarkson (1998)
m
Sylt, N. Germany, 54 054'N/8022'E
undecided
undet
?
m
Trondheim, 63 052'N/11 042'E
slope/basin
B
volcanics
Rad
Hein and Parrish (1989)
m
Ural area, 57 039'N/65 059'E
slope/basin
B
?
Rad
Blome et al (1995b);
m
Kazakhstan, 46 053'N/66 018'E
slope/basin
B
ophiolitic complex
SS, Rad Blome et al (1995b); Tolmacheva et al. (2001)
Canada
Kay (1967)
Australia and Oceania
Australia
Europe
England
16 Kirkton Fm., Crawfrod Gr.
m
Germany
17
Sylt ??
Schallreuter (1989)
Norway
18 ?
Asia
Ural Area
19 ?
Kazakhstan
20 Burubaital Fm.
225
Location Lat/Long
Depositional
environment
m
m
eastern Kazakhstan, 45 0N/75 0E
Dzhungaria-Balkhash zone, Tekturmas Mt. 46 015'N/66 0E
undecided
undecided
23 Klimuli Fm.
m
Tongxin, Ningxia, Zhuozishan, 390N/106 050'E
slope/basin
24 Lanmutan Fm.
m
Sandu, Yangzte platf, 26 0N/108 0E
shelf
Australia, CW New South Wales, 35 0S/147 0E
New South Wales, Australia, 33 052S/148 0E
NW and NE Jiangxi, China,
slope/basin
slope/basin
slope/basin
undecided
shallow shelf
slope/basin
shelf
No
Formation
Age
21 Bestamaksky LMS
22 Tekturmas Fm, Urtynzhal Gr
Chert
type
Associated facies
SSR*
References
sh with grpt, LMS
slts, breccia, volc
Rad
Rad
Renz (1990)
Nikitin et al. (1986)
undet
LMS, sh with grpt
Rad
Yu-jing (1991)
undet
argill LMS
Webby (1995)
undet
B
undet
B
N
undet
N
slate, cgl, Q
?
black sh, LMS
Rad
?
LMS
LMS, grpt sh
Rad
mudst, or clayey slts
Webby (1981)
Lyons and Percival (2002)
Xu et al. (1995)
Lyons and Percival (2002)
Mellen (1974)
Yu-Jing (1988)
Loi and Dabard (2002)
Jackson and Lenz (1962)
undet
B
China
Others
25
26
27
28
29
30
31
Clements Fm.
Hoskins Chert, Jindalee gr
Hulo Fm.
Mugincoble chert
Ottawa rocks
Pingliang Fm.
Postolonnec Fm.
m,u
m,u
m,
m,C
m,u
m,C
m, u
New South Wales, Australia, 38 006S/147 020E
N Mississippi, USA, 89015'W/34 035'N
Gansu, China, 35 035'N/106 031'E
the Armorican Massif (W France), Crozon Peninsula, SW
Sardinia, 48 015N/4 030W
32 Road River Fm.
m,u
N Yukon & Alaska, Canada, 137 020'W/66 054'N
slope/basin
N, B
grpt. sh, argill LMS,
Dol, SST
33
34
35
36
m,u
m, C
m, C
m,u
NC Newfoundland, Canada
Australia, CW New South Wales, 33 0S/150 0E
Arkansas, USA, 94033'W/34 031'N
Central Japan
slope/basin
slope/basin
slope/basin
?
B
undet
B
B
argil, black sh, flysh Rad
Bruchert et al. (1994)
volc
SS, Rad Webby et al. (1981)
black sh, LMS
Stone and Sterling (1962)
volc
Rad
Igo et al. (1980)
Ash
New Mexico, W Texas, SC Mexico, 31058'N/105 025'W
subtidal, shelf
N, B
LMS
Shoal Arm Fm.
Sofala volcanics
Womble Shale
Yoshiki Fm.
UPPER ORDOVICIAN
North America
USA
1
Aleman Fm.
2
3
4
Beckett Fm.
Bigfork Fm.
Black River limestone
C
C
C
SE Missouri, 37 012'N/90 033'W
Oklahoma, 34 039'N/94 026'W
Virginia, Tennessee, 43002'N/84 032'W
shelf
slope/basin
peritidal/lagoonal
shelf
N
B
O, undet
LMS, cgl, calc
sh
LMS, Dol
5
6
Brannon limestone
Carters limestone
C
C
SC Kentucky, 36 058'N/82 025'W
W Tennessee- Kentucky, 36 034'N/88 052'W
shelf
subtidal, shelf
N
N
LMS
LMS
SS
Pope and Steffen (2002); Brimberry (1984)
Larson(1951); Craig (1993)
Campbell and Suneson (1989)
Cohee (1948); Textoris (1968)
SS
McFarlan and White (1948); Fisher (1968)
Renfro and Feray (1989); Holland and Patzkowsky (1997)
226
No
Formation
7
8
9
Curdsville limestone
Deceit Fm.
Deicke K-bentonite beds***
10
11
12
13
14
15
16
17
18
Duzel Formation
Ely Springs Dol
Fernvale Fm.
Fish Haven Fm.
Fremont Fm.
Galena Fm.
Guttenberg limestone
Hager Fm.
Hanson Creek Fm.
19 Joachim dolomite
20 Keel Fm.
21 Kimmswick Fm.
22 Livengood Dome Fm.
23 Macy Fm.
24
25
26
27
28
Maquoketa Fm.
Maravillas chert
Martinsburg Fm.
McGregor limestone
Millbrig K-bentonite bed***
29 Munsungun Fm.
30 Murfreesboro-Pondspring Fm.
Age
Location Lat/Long
Depositional
environment
Chert
type
Associated facies
References
shelf
SC Kentucky, 36 058'N/82 025'W
slope/basin
Alaska, Kotzebue Sound, 65 030'N/166 0W
NW Georgia, S Minnesota, NW Iowa, E Missouri, Kentucky,
C Tennessee
N
B
B
LMS
LMS, grpt sh
ash beds
N California, Sierra Nevada, 42055'N/122 013'W
SC Nevada, 37011'N/115 033'W
SC Oklahoma, 34 044'N/97 009'W
Idaho-Utah; 42 002'N/112 046'W
W Colorado, E Utah, 38 049'N/109 002'W
Iowa, 41056'N/93 027'W
Iowa, Missouri, 40031'N/93 023'W
SE Missouri, 37 012'N/90 033'W
C Nevada, 38054'N/116 046'W
slope/basin
shelf
subtidal, shelf
shelf
shelf
shelf
shelf
shelf
shelf, slope/basin
B
N, B
N
N
N, B
N, B
N
N, B
N, B
LMS
LMS, Dol
LMS
Dol
LMS
LMS, Dol
LMS, sh
LMS
LMS
SE Missouri, 37 012'N/90 033'W
Oklahoma, Texas, 35045'N/100 038'W
shelf
shelf
N, B
N
Dol
LMS, Dol
Larson(1951); Craig (1993)
Kopaska-Merkel and Friedman (1989); Qing Sun (1995)
C
u
SE Missouri, 37 012'N/90 033'W
EC Alaska, 64 042'N/143 026'W
shelf
slope/basin
N
B
LMS
sh, tuff, LMS,
siltstone
Larson(1951); Craig (1993)
Hein and Parrish (1989); Harris et al. (1995)
C
shelf
SE Missouri, 37 012'N/90 033'W
outer shelf
S Kansas, Illinois, EC Iowa, 34 0N/97 016'W
slope/basin
W Texas, 30027'N/103 023'W
slope/basin
Virginia, New York, 390N/79 0W
shelf
E Iowa, 41 037'N/91 001'W
NW Georgia, S Minnesota, NW Iowa, E Missouri, Kentucky,
C Tennessee
N, B
LMS
N, B
N, B
B
N
B
Dol, argill rock
LMS, sh
sh, turbidites
LMS, Dol
ash beds
SS
Wiebe (1948); Grammer (1983)
SS, Rad King (1926)
Rad
Mcbride (1962); Carroll (1959)
Byers and Stasko (1978); Agnew (1955)
Huff and Kolata (1990)
NW Maine, 46030'N/68 0W
W Tennessee- Kentucky, 36 044'N/88 052'W
shelf, slope/basin
subtidal, shelf
B
N
sh, turbidites
LMS
SS, Rad Pollock (1987)
Renfro and Feray (1989); Holland and Patzkowsky (1997)
Rad
C
u
C
u
Ash
C,Ash
u
u
C
C
C
Ash
C
Ash
Ash
C,Ash
Ash
C
C
C
C
31 Phi Kappa Fm.
32 Polk Creek Shale
33 Red River Fm.
C,Ash
u
Ash
NE Nevada, 41059'N/115 035'W
Arkansas, 34031'N/94 033'W
North Dakota, Montana, 49 0'N/101031'W
slope/basin
slope/basin
shelf
B
undet
N, B
grpt sh, LMS, Dol
sh
LMS, dol
34
35
36
37
C,Ash
C
C
C
Idaho, 43 044'N/113 023'W
SC Oklahoma, 34 044'N/97 016'W
NW Ohio, 41010'N/81 018'W
SC Kentucky, 36 058'N/82 025'W
shelf
peritidal/lagoonal
offshore, shelf
peritidal/lagoonal
shelf
undet
N, B
N
O, N, B
grpt sh, LMS
sh, LMS, SST
LMS, Dol
LMS, Bentonite
Saturday Mountain Fm.
Sympson shale
Trenton Fm.
Tyrone limestone
SSR*
McFarlan and White (1948); Fisher (1968)
Harris et al.(1995); Ryherd and Paris (1987)
Huff and Kolata (1990)
Feray et al.(1968); Condie and Snansieng (1971)
Burchfiel (1964); Miller (1977)
Wengerd (1948); Denison (1997)
SS
Beus (1968); Churkin (1962)
Sweet (1954); Gerhard (1967)
Agnew (1955); Finney (1997)
Agnew (1955); Finney (1997)
Larson (1951); Craig (1993)
SS, Rad Blome et al. (1965b); Finney et al (1999)
Larson(1951); Craig (1993)
Paull and Paull (1981); Cone et al. (2002)
Stone and Sterling (1962)
Tanguay and Friedman (2001); Andrichuk (1959)
Churkin (1962)
Edson (1927); Suhm (1995)
Stieglitz (1975); Gregg and Sibley (1984)
McFarlan and White (1948); Bauert (1994)
227
No
Formation
38 Valmont dolomite of the
Fusselman Fm.
Age
Location Lat/Long
Depositional
environment
Chert
type
Ash
Texas, SC New Mexico, 31060'N/105 006'W
shelf
N, B
LMS, dol
Associated facies
SSR*
References
P'An (1982)
C,Ash
Nevada, E California, 38 013'N/118 035'W
slope/basin
B
LMS, sh, volc.
C
S Kansas, SC Oklahoma, 98 005'/37 014'N
shelf, slope/basin
N, B
Dol
41 ?Bay of Islands Complex
C
W Newfoundland, the Bay of Islands, 46 031'N/67 041'W
slope/basin
B
ophiolitic complex
42 Advance Fm.
43 Allen Bay carbonate rocks
C
u
British Columbia, Advance Mt, 55 0N/125 0W
Cornwallis and Griffith Islands, Queen Elizabeth Islands,
75 065'N/94 059'W
slope/basin
shelf, slope/basin
N
N, B
LMS, Dol, sh
Dol, LMS
Norford et al. (1995)
Sodero and Hobson (1979)
NC Newfoundland, 49 0N/55 030'W
Alberta, British Columbia, 51 039'N/116 040'W
Arctic Canada in northeastern Ellesmere, 79 037'N/81 052'W
slope/basin
undecided
slope/basin
B
B
B
volc-sed rocks
Dol
grpt sh
Kusky and Kidd (1996)
Severson (1950)
Gentzis et al., (1996); Trettin (1970)
Ash
SW Manitoba, Gretna, 49 050'N/100 014'W
shelf
N
sh, LMS
u
C, Ash
C
C
SC New South Wales, 32 045'S/145 048'E
SC New South Wales, 35 027'S/149 049'E
SC New South Wales, 36 0S/148 0E
Rockley - Taralga area, 34 030S/149 040'E
slope/basin
slope/basin
slope/basin
slope/basin
undet
undet
undet
undet
slts, slates,grpt
slts, slates,grpt
slts, slates,grpt
grpt black sh
Rad
Rad
Webby (1981); Iwata et al. (1995)
Webby (1981)
Webby (1981)
Webby (1981); Lyons and Percival(2002)
SE Australia, Cooma-Mandurama, 36.5 004'S/148 043'E
slope/basin
B
grpt sh
Rad
Dietz and Holden (1966);Fergusson and Fanning (2002)
Gunning - Blandarea, 33 0S/148 020'E
slope/basin
undet
LMS, grpt sh
slope/basin
slope/basin
B
B
grpt sh
LMS, tuff, grpt sh
slope/basin
undecided
slope/basin
slope/basin
B
undet
B
B
basalt, tuff, sh
mudst, LMS
volc, grpt black sh
grpt sh
Rad
Webby (1981)
Webby (1981)
Webby (1981)
Dietz and Holden (1966); Webby (1981)
slope/basin
B
black sh, mudst,
Rad
Grunau (1965); Hubert (1966)
39 Vinini-Valmy Fm.
40 VIOLA zone III
Rad
Feray et al. (1968); Noble and Finney (1999)
Wiebe (1948); Grammer (1983)
Canada
44 Blue Star Fm.
45 Halysites-Bearing Beds
46 Hazen Formation
47 Stony Mountain Fm.
u
u
C,Ash
Rad
Rad
Blome et al. (1965b)
Andrichuk (1959); Porter and Fuller (1959)
Australia and Oceania
Australia
48
49
50
51
Ballast beds
Birkenburg beds
Bogong Creek beds
Bubalahla Fm.
52 Bumballa Fm.
C
53 Cotton beds
Ash
54 Jerrawa beds
55 Malongulli Fm.
56
57
58
59
Nine Mile Volcanics
Uralba beds
Wagonga Fm.
Warbisco shales, Mallacoota
beds
C, Ash capital territory, Yass, 35 0S/149 0E
C, Ash Cliefden caves, 33050'S/148 025'E
C
Kiandra, 36 0S/149 0E
C
Attunga, 30 055'S/150 050'E
C
Narooma, 36 014'S/150 004'E
C
SE Australia,E Victoria, -38 004'S/144 043'E
Webby (1981); Lyons and Percival(2002)
Webby (1981)
SS, Rad Rigby and Webby (1988)
Rad
Europe
England
60 ? - a flysh seq.
C
S Scotland, 57 004'N/4013'W
228
No
61
62
63
64
Formation
Currarie Fm.
Glenkiln Fm.
Hartfell Shale Fm
Marchburn Fm.
Age
Location Lat/Long
Depositional
environment
C
C
C
C
Scotland, 55 0N/50W
Scotland, 55 0N/50W
Scotland, 55 0N/50W
Scotland, 55 0N/50W
slope/basin
slope/basin
slope/basin
slope/basin
u
SW Sardinia, 39 030'N/8030'E
u
C
Chert
type
Associated facies
SSR*
References
B
undet
undet
B
basalt, grpt
mudst, grpt sh
mudst, grpt sh
basalt, grpt
Rad
Rad
Rad
Armstrong et al.(1998); Floyd (1996)
Armstrong et al.(1998)
Floyd (1996)
Armstrong et al.(1998); Floyd (1996)
shelf
N
muds, slts
SS
Loi and Dabard (2002)
Armorican Massif, 48015'N/4030'W
inne-outer shelf
N
muds, slts
Urals, 57 039'N/65 059'E
slope/basin
B
?
Rad
Blome et al. (1995b)
Chuillisk Mountains, 46 053'N/66 018'E
slope/basin
B
LMS
Rad
Blome et al. (1995b); Nazarov and Ormiston (1993)
Italy
65 Portixeddu Fm.
France
66 Kermeur Fm.
Loi and Dabard (2002)
Asia
Ural Area
67
?
Kazakhstan
68 Ul'Kuntassk Fm.
C,Ash
69 ?Chu-Illisk Mt & rocks
ofPredchingiz
Ash
Chu-Illisk Mounatins, 44 0N/75 0E
undecided
undet
LMS
Rad
Renz (1990)
70 Kuvsky Fm.
Ash
Predchingiz, 46015'N/66 0E
undecided
B
volc, LMS, silts
Rad
Nazarov and Ormiston (1993)
C
Ash
C
Ash
Shuangjiakou, 26 049'N/112 007'E
Xainza, Gangmusang, 310N/88 030'E
Shuangjiakou, 26 049'N/112 007'E
Yangzte platf, 30 0N/107 030'E
slope/basin
shelf
slope/basin
slope/basin
B
N
B
undet
silic black sh
LMS, Dol
silic black sh
silic grpt sh, LMS
Rad
Yiren and Hanying (1989)
Webby (1995)
Yiren and Hanying (1989)
Yu-jing (1991)
China
71
72
73
74
Chengbu Fm.
Gangmusang Fm
Shuangjiakou Fm.
Wufeng Fm.