USING ICHNOLOGY TO DETERMINE RELATIVE INFLUENCE OF WAVES, STORMS, TIDES, AND
RIVERS IN DELTAIC DEPOSITS: EXAMPLES FROM CRETACEOUS WESTERN INTERIOR
SEAWAY, U.S.A.
M. ROYHAN GANI1, JANOK P. BHATTACHARYA1, and JAMES A. MACEACHERN2
1
Geosciences, University of Texas at Dallas, P.O. Box 830688, FO 21, Richardson, TX 75083-0688, USA,
E-mail: [email protected] (Gani)
2
Earth Sciences, Simon Fraser University, Burnaby, B.C, Canada, V5A 1S6
Detailed logging of ichnological variations within parasequences of several Cretaceous (Upper Turonian) delta complexes from Wyoming and
Utah are correlated with inferred short and long-term changes in depositional processes. These changes reflect various proportions of river,
flood, wave, storm, and tide influences. Event beds, such as storm and river-flood deposits, tend to show low BI (Bioturbation Index) values
of 0-2, owing to high accumulation rates, although this also depends on event frequencies. Upper surfaces of individual storm/river-flood beds
may show BI values of 4-5, reflecting the transition to longer-lived fair-weather conditions. Fair-weather waves facilitate persistent agitation
near the bed, buffering environmental stresses. Therefore, wave-dominated deposits that are not affected by storms yield climax communities
with robust and diverse ichnofacies signatures reflecting “uniform and high” BI trend with values that average 4.
River-dominated intervals show the least uniform trends of BI, because of the highly variable conditions related to river jet and plume
behavior. BI values vary from 0 to 4, with generally low ichnogenera diversities. These alternations likely record seasonal to centennial
fluctuations in sedimentation rate (river discharge) and water turbidity, which influences substrate conditions near distributary mouths.
Tide-dominated intervals tend to show the most ‘stressed’ conditions, reflecting “non-uniform and low” trend of BI, with values of 0-2. These
reflect salinity fluctuations, heightened water turbidity, rapidly shifting substrates, and narrow colonization windows associated with daily and
monthly changes in tidal periodicity.
Individual parasequences are characterized by either upward-increasing and upward-decreasing trends of BI, indicating protection from storm
erosion and proximity to river input, respectively. Ichnological signatures change significantly across initial flooding surface, principally
showing a marked increase in BI as delta lobes are quickly abandoned and transgressed. In contrast, across the maximum flooding surface,
changes in ichnological signatures are subtle and rather uniform.
We suggest that different parts of a single delta may experience marked differences in river, wave, and tide influence over time, reflecting the
enormous complexity of operative processes at various temporal and spatial scales. Detailed intra-parasequence, bed-scale analyses of trace
fossils help to reveal this complex evolutionary history of a single delta.
INTRODUCTION
The widely used tripartite classification of deltas has stemmed from
analysis of the planform morphology of modern deltas (Galloway
1975). This classification assumes that there is a direct relationship
between the overall sand body and planform geometry and the
internal framework-facies of deltas, both reflecting the same relative
dominance of either river, tide, or wave energy. Consequently, deltas
are routinely identified as fluvial-, tide- and wave-dominated end
members. If the sediment source is not directly from a river, then the
shoreline deposits are identified as one of the categories of nondeltaic shoreline types. Recently, the assumption of tripartite delta
classification has been questioned, based on the fact that the planview morphologies of modern deltas reflect dominant surficial
process but may not necessarily reflect the short-lived constructional
processes, such as river floods, that dominate the internal facies
architecture (Gani and Bhattacharya in review). Moreover,
Bhattacharya and Giosan (2003) have pointed out that not only may
a single delta show river-, wave-, and tide-dominated facies at
various locations within the delta, but that a complete spectrum
exists from strandplain systems with minor deltaic promontories to
river-dominated deltas with minor wave reworking. We would make
the additional point that the modern examples of deltas are a static
snapshot of geologic time, and that it is possible for a single delta to
evolve from one type to another during a single progradational
history. For example, during the last 4 ka, the Mekong Delta has
evolved from a ‘tide-dominated’ delta to a ‘tide-wave-dominated’
delta (Ta et al. 2002). Therefore, it may be misleading to designate
an ancient deposit to any particular type of delta (or even non-delta),
based on the proportion of dominant facies types in a few vertical
logs, without first identifying the temporal and spatial scale of that
depositional element within the broader context of the regional
stratigraphy. We speculate that the mixed-energy deltas should be
the norm rather than the exception. This internal complexity of
various deltaic processes can be revealed by analyzing the bed-scale
variations of ichnological signatures in a novel way, as presented in
this paper.
Ichnology has emerged as a powerful tool in paleogeographic and
paleoecologic analysis and interpretation that integrates trace fossil
analysis with that of facies analysis and sequence stratigraphy.
Ichnological models of non-deltaic shoreline successions are
comparatively well established (e.g., Howard 1972; Pemberton and
Frey 1984; MacEachern and Pemberton 1992; MacEachern et al.
1999; among many others) compared to their deltaic counterparts.
Early studies of deltaic trace fossils (Turner et al. 1981; Ekdale and
Lewis 1991) concentrated on identification and paleoenvironmental
significance of individual ichnogenera. Recent studies (Gingras et al.
1998; Coates and MacEachern 1999, this volume) have made a
comparative analysis of ichnology in deltaic (wave- and riverdominated) and non-deltaic shoreline deposits from subsurface cores.
Another recent study (Gouramanis et al. 2003) of fluvio-deltaic
ichnology mainly focuses on the description and identification of
trace fossils from patchy outcrop data. Trace fossil analyses of tide-
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influenced deltaic deposits are largely lacking. MacEachern et al. (in
press) recently gave a thorough review on the ichnology of deltas.
As deltas are very complex and dynamic shoreline depositional
systems reflect a wide range of river and basinal processes (tide,
wave, storm, and oceanic currents), a deltaic ichnological model
should ideally be based on bed-scale variations of trace fossil
signatures throughout the successions. High-resolution integration of
sedimentology and ichnology can be very useful to identify the
relative influence of waves, storms, tides, and rivers influx in the
construction of a delta. This paper presents one of the first
ichnological studies of delta deposits that systematically integrates
bed-scale variations in sedimentology and ichnology through an
entire deltaic parasequence. Our objective is to show the relative
effects of fundamental shoreline processes (wave, storm, tide, and
river) in resultant paleoichnology of a given deposit. In this paper,
we use both core and outcrop data of Turonian deltaic strata from the
Wall Creek Member, Wyoming, and the Ferron Sandstone Member,
Utah (Fig. 1). An extensive database incorporating both outcrop and
core taken behind the cliffs, are available for both the Wall Creek
Member and the Ferron Sandstone Member. The paleogeographic
and sequence stratigraphic interpretations of these strata, roughly
equivalent in age, are well established (e.g. Chidsey 2001; Howell
and Bhattacharya 2004; Gani and Bhattacharya in review). The
Ferron was deposited in a high accommodation setting, whereas the
Wall Creek was deposited in a low accommodation setting.
GEOLOGY OF STUDY AREA
The Cretaceous Western Interior Seaway (WIS) stretched roughly
north-south in central North America, and developed on an eastward
migrating and asymmetric retro-arc foreland basin (Lawton 1994;
Fig. 1A), which contains one of the most studied sedimentary
successions in the world. The present study deals with upper
Turonian (Cobban, 1990; Gardner 1995a, 1995b) strata of Utah and
Wyoming (Figs.1, and 2) deposited at the western margin of the
Seaway. In Utah, these successions belong to an ‘intermediate-term
stratigraphic cycle’ of the fluvio-deltaic ‘Ferronensis Sequence’
(from 90.25 Ma to 89 Ma), which, in turn, consists of seven ‘shortterm stratigraphic cycles’ (Fig. 2B) each averaging a 0.2 ma duration
(Gardner 1995b). In Wyoming, the Wall Creek Member (the
uppermost member of Frontier Formation) is roughly timeequivalent to ‘Ferronensis Sequence’ or Upper Ferron Sandstone
Member, and also consists of seven transgressive/regressive cycles
of deltaic strata (Howell and Bhattacharya 2004; Fig. 2A), each
averaging 0.2 ma in duration. As much of the Cretaceous is thought
to represent ice-free green-house conditions, the origin of these
stratigraphic cycles has variously been assigned to Milankovitch
climatic cycles (Scott et al. 1998), or foreland basin tectonics related
to thrust loading and fore-bulge migration (Gardner 1995b;
Bhattacharya and Willis 2001). More recently, however, Miller et al.
(2004) claimed that continental ice sheets paced sea-level changes
during the Late Cretaceous. Accordingly, the stratigraphic cycles of
the Cretaceous seaway may represent a complex origin of in-phase
and/or out-of-phase interaction of glacio-eustatic and tectonic cycles.
A pronounced and widespread Cenomanian-Turonian transgression
led to the opening of the epeiric seaway towards the south, which
established a northerly surficial flow of warm Tethyan water,
countered by a southerly bottom flow of cooler Boreal waters along
western margin of the seaway (Leckie et al. 1998).
The upper Turonian clastic wedges of Wyoming and Utah show
great sedimentologic and stratigraphic variability. The Wall Creek
Member comprises seven major sandstone bodies (Howell and
Bhattacharya 2004; Fig. 2A) that prograded southeast. Each of these
sand bodies shows one or more upward-coarsening facies
successions, called parasequences, bounded by a marine flooding
surface commonly marked by a chert-pebble and shark-tooth lag.
These parasequences show a mixture of top-truncated deltaic and
shoreface deposits with varying amounts of wave, storm, tide, and
river influence. Subaerial deposits are notably absent in these
deposits because of the top truncation associated with marine
transgression. The present study deals with the Wall Creek Member
at the “Raptor Ridge”, located northeast of Natrona County, central
Wyoming (Fig. 1B), and emphasizes Sandstone-3 to Sandstone-6.
Following a detailed paleogeographic study, Sandstone-6 was
interpreted as a mixed-influence delta, named ‘Raptor Delta’ (Gani
and Bhattacharya, in review; Fig. 3A). River- and tide-dominated
facies of this delta are present only at Raptor Ridge, where the sand
body is also the thickest. The entire northeast flank of the delta
shows wave-dominated facies, and could comprise the updrift side of
the delta, according to the ‘asymmetric wave-influenced delta’ model
of Bhattacharya and Giosan (2003).
Regionally, the Ferron Sandstone forms a large lobate body that
prograded northwest, north, and northeast, forming a large western
embayment of the seaway, called the Sanpete Valley Embayment
(Chidsey 2001; Fig. 1C). The Upper Ferron Sandstone Member
consists of seven transgressive-regressive cycles (Fig. 2B) that, like
the Wall Creek, also show a mixture of deltaic and shoreface strata
with varying degrees of river, wave, and tide influences (e.g.
Gardner 1995b; Chidsey 2001; Garrison in press). For example,
Bhattacharya and Davies (2001) studied the first (basal) cycle of
Upper Ferron Sandstone at the outcrop along Muddy Creek, and
interpreted it as a river-dominated delta lobe that prograded
northwestward. The present study examines the core of MC-2 at
Muddy Creek Canyon (Chidsey 2001) (Fig. 2C). The entire core of
MC-2 belongs to the ‘second transgressive-regressive cycle’ of the
Upper Ferron Sandstone (Fig. 3A), which has been interpreted to
contain four parasequences (Chidsey 2001). The logged interval of
this article is the second parasequence from base (parasequence ‘Kf2-Mi-b’), that was interpreted to be deposited in a deltaic
environment (Chidsey 2001; Fig. 3B).
DATA AND METHODOLOGY
An integrated research project, focusing on deltaic, bed-scale facies
architecture and reservoir characterization of the Wall Creek
Member at Raptor Ridge, Wyoming (Fig. 1B) generated a superb
data base of sedimentological, ichnological, and petrophysical
characteristics (e.g. Gani et al. 2004; Lee et al. 2004; Tang et al.
2004; Gani and Bhattacharya in review). Along with the outcrop
study of several gully faces (both depositional pseudo dip and strike
oriented), ten cores were taken behind the outcrop within an area of
300 m by 100 m. In this study, we used two of these ten wells (RR-2
and RR-7). Well RR-7 is located 220 m depositionally down-dip of
Well RR-2. Well data were correlated and cross-checked with
outcrop data at the Raptor Ridge site. The lower six sand bodies (Fig.
2A) of the Wall Creek Member were observed at Raptor Ridge.
Sandstone-3 to Sandstone-6 were encountered at RR-2, whereas only
Sandstone-6 was encountered in RR-7. We used another cored
interval, from 93 m (303 feet) to 76 m (250 feet), of the MC-2 well at
Muddy Creek, Utah (Fig. 1C). This interval represents part of the
second transgressive-regressive cycle of the Upper Ferron Sandstone
Member (Fig. 2B).
Integrated sedimentological and ichnological analysis along with the
interpretation of each of these three cores are presented (Figs. 5, 6, 9,
and 11). The Seilacherian approach (e.g., Pemberton et al. 1992;
Pemberton et al., 2001) is taken in grouping the identified suites of
ichnotaxa into the temporally and geographically recurring
ichnofacies. Quantification of degrees of bioturbation in the
sediments is emphasized in this study. The quantification scheme of
bioturbation has been develop by Howard and Reineck (1972), and
modified later by Taylor and Goldring (1993). The Bioturbation
Index (BI) scheme of Taylor and Goldring (1993) is used in this
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Fig. 1.– A) Cretaceous paleogeography of the Western Interior Seaway (WIS) during the middle Turonian. During this time the Wall Creek
(in Wyoming) and Ferron (in Utah) were deposited into a retroarc foreland basin, sourced from topographic highs in the west developed
during the Sevier Orogeny (modified from Bhattacharya & Willis 2001; and others). B) The Wall Creek crops out on the southeastern flanks
of the Big Horn Mountains. This study focuses in the Wall Creek Member at Raptor Ridge. C) Ferron outcrops are located within the San
Rafael Swell, northeast Utah. This study focuses on Muddy Creek Canyon (modified from Bhattacharya and Davies 2001).
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distribution of the trace fossils are better understood. Moreover, the
changes of ichnological signature across bounding discontinuities are
investigated and integrated with the developing sequence
stratigraphic framework of the intervals.
The generalized paleogeography of the Upper Turonian shoreline
systems at the western margin of the Interior Seaway in Wyoming
and Utah has been presented as a series of juxtaposed delta lobes fed
from rivers generated at the emerging edges of thrust sheets to the
east (e.g., Gardner 1995; Chidsey 2001). In a situation like this, any
of these shoreline deposits can be considered as part of a large
‘deltaic complex’, and can show variable proportions of river-, wave, storm-, and tide-dominated/influenced facies, depending upon the
part of the delta to which they belong to (e.g. updrift vs. downdrift;
open marine vs. bay vs. embayment). The clastic successions
presented in this study are part of one of these deltaic complexes
(Fig. 3). However, when characterizing various sub-environments of
these deposits, we have used deltaic terminology (e.g., prodelta, delta
front) where evidence directly points to a delta, and general shoreline
terminology (offshore, shoreface) where this is not. This paper is not
aimed at differentiating ‘deltaic’ and ‘non-deltaic’ strata with the
help of ichnology, but rather at showing the control of fundamental
shoreline processes (wave, storm, tide, and river) in the resultant
paleoichnology of a given deposit.
Fig. 2.– Regional stratigraphy of the study area. A) Cretaceous
stratigraphy of eastern Powder River Basin, Wyoming (modified
from Willis et al., 1999). B) Upper Cretaceous (Turonian)
stratigraphy of eastern Uinta Basin, Utah (at Emery) (modified from
Gardner 1995b).
article to semi-quantitatively measure the degree of bioturbation in
the sediments. However, rather than plotting it as a histogram (as
suggested by Taylor et al. 2003) a continuous line curve of BI
(comparable to a wireline log) is produced for each of the studied
cores. This “BI log” (Fig. 4) facilitates illustration of subtle bed-scale
variations, and, when presented alongside with other data, helps to
integrate the evolutionary history of the deposits by trend analysis of
log patterns throughout a vertical facies succession. The terminology
and interpretation used in this trend analysis of the BI log is
explained in Fig. 4, and is followed throughout this study. We also
analyzed the preserved interrelationships of the ichnotaxa and their
relationships with the sedimentary facies in which they occur. This
type of continuous documentation of ichnological signature through
the entire studied interval is best suited in subsurface core, because
of the preservational bias of the outcrop data. These high-resolution
data allowed us to make an ichnological comparison of the wave-,
storm-, river-, and tide-dominated shoreline facies deposited in the
Turonian Sea, so that the effects of these processes on the
Fig. 3.– A) Paleogeography of the Raptor Delta (Sandstone-6 of
the Wall Creek Member). For location, see Fig. 1B (from Gani and
Bhattacharya, in review). B) Paleogeography of the deltaic
parasequence ‘Kf-2-Mi-b’ of the Upper Ferron Sandstone (modified
from Chidsey 2001). This study uses core of well MC-2. For
location, see Fig. 1C.
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of 2. The lower muddy interval of Sandstone-4a contains uncommon,
2-4 cm thick, sandstone beds, interpreted as remnants of distal
tempestites (Fig. 8D). Bioturbation uniformly increases and then
decreases upward (from BI 5 to 6, then back to 5) through the upper
part of sandstone-4a, with a concomitant obliteration of primary
sedimentary structures. A turnaround of relative sea level is
interpreted at 18.3 m, marked by the development of a TSE,
highlighted by an abrupt decrease of BI values and the reappearance
of storm erosion. The interval 18.3-15 m lying above this TSE
comprises transgressive deposits, showing a “high and uniform”
trend of BI with values of 4 (Figs. 4 and 6). This interval gradually
culminates in an MxFS. It is notable that there is no trend break in BI
values across the MxFS. Sandstone-4b is only 2 m thick and shows
an upward transition from a distal expression of the Cruziana
Ichnofacies to the archetypal Cruziana Ichnofacies. Low-angle
parallel laminations and wave ripple laminations indicate wavegenerated facies. Overall, BI values show a uniform and upward
increasing trend, although there is a sudden increase in BI at 14.6 m.
However, there is no change in BI values across the MxFS capping
the top of Sandstone-4b.
Fig. 4.– Hypothetical BI (bioturbation index) log, with the
terminologies and interpretations used in this study to characterize
the log’s trend.
DESCRIPTION AND INTERPRETATION OF CORES
WALL CREEK MEMBER
Well RR-2– Sedimentological and ichnological data and their
interpretations of the RR-2 cores are presented in Fig. 6 (for legend
see Fig. 5). Core photos are presented in Fig. 7. Rather than
repeating the data presented in Fig. 6, only important observations
are described here. The entire core represents, from oldest to
youngest, Sandstone-3 to Sandstone-6, each bounded by a
transgressive surface of erosion (TSE), or maximum flooding surface
(MxFS). In Sandstone-3, the lowermost interval (35-33.2 m) shows
remnant parallel lamination, largely destroyed by bioturbation
ranging from BI 1-4. The BI log of this interval shows an overall
high (BI 3-4) but “non-uniform” trend (Figs. 4 and 6). Parallel
laminations are better preserved in the overlying 33.2-31.5 m
interval, reflected by suites of the archetypal Skolithos Ichnofacies.
Episodic sedimentation is indicated by the development of fugichnia,
and locally unburrowed sandstones, erosionally truncating an
underlying highly burrowed interval (Fig. 8A). The BI trend of this
interval is “low and non-uniform” (BI 1-2). The parallel laminations
of these two intervals are thought to represent tempestites deposited
in a shoreface environment. The remainder of this parasequence is
devoid of visible traces (BI 0; cryptobioturbation is not included in
this calculation). Indistinct development of parallel lamination (Fig.
8B) and a thin horizon of mud chips probably indicate high-energy
sedimentation. The lack of bioturbation could be either due to
removal of traces by erosion, or habitat unfavorable to infauna.
The overlying Sandstone-4 overlies a TSE, with a thin (8 cm) and
predominantly muddy interval, containing suites reflecting a low
diversity expression of the Cruziana Ichnofacies (Fig. 8C) and a BI
The Sandstone-5 is broadly similar to Sandstone-4a, although the
sedimentary structures are more obscured in Sandstone-5 (Fig. 7).
The interval 13.1-8.3 m shows a high BI (averaging 5), with a
uniform, and “upward-increasing” trend of the BI log. BI values
decline sharply across a transgressive surface (TS) at 8.3 m. The TS
is overlain by transgressive deposits hosting suites characterized by a
high diversity distal expression of the Skolithos Ichnofacies, passing
upward into a low diversity distal expression of the Cruziana
Ichnofacies. This transgressive deposit, characterized by a uniform
and “upward-decreasing” trend of BI (Figs. 4 and 6), culminates with
an interpreted MxFS at 6.8 m. Like the previous example, there is no
trend break in the BI log across the MxFS.
The Sandstone-6 differs from all of the older sandstones in several
respects. The most important is the dominance of tide-generated
sedimentary structures. In outcrop, older parasequences show a
relatively low inclination (roughly < 10) of clinoforms, whereas, the
clinoforms of Sandstone-6 reach 40 inclinations. The lower part (6.84 m) of Sandstone-6 shows remnant parallel laminations and suites
attributable to the Cruziana Ichnofacies with high BI values (3-4).
Higher BI values are consistent with wave-affected facies. This
interval shows a rather uniform trend of the BI log. The uppermost
2.6 m characteristically contains herringbone cross-bedding,
originating from alternating flood and ebb tidal condition, with a
non-uniform and low trend of the BI log (BI 0-1). Overall, the
sandstone shows an upward-decreasing trend of BI values.
Well RR-7– The core of RR-7 well (Fig. 9) only contains Sandstone6. The basal 11-9.2 m interval of this parasequence comprises
silty/sandy mudstone, mostly homogenized due to thorough
bioturbation (BI 4). Suites are attributable to the distal expression of
the Cruziana Ichnofacies (Fig. 10A); hence the interval is interpreted
to reflect wave-affected offshore deposits. This interval includes a
single, 25 cm-thick sandstone bed (Fig. 10B), showing remnant
parallel lamination interpreted to reflect either storm or river-flood
deposits. The BI log of this interval shows a high and uniform trend
(BI 4). BI values decrease sharply at 9.2 m. The interval 9.2-8.8 m
comprises heterolithic muddy sandstone, containing a low diversity
expression of the archetypal Cruziana Ichnofacies. This interval is
interpreted as prodelta deposits. A 1.6 m thick sandstone bed with
low-angle accretion lamination and an erosional lag at its base
directly overlies the prodelta deposits. In the outcrop, Sandstone-6
shows a number of channelized deposits encased within lower to
middle delta front deposits. These channels are interpreted as
subaqueous terminal distributary channels (Gani and Bhattacharya in
review) filled with gravites (deposits of sediment gravity flows,
sensu Gani 2004). The thick sandstone bed
5
Fig. 5.– Legend for lithologic and ichnologic columns presented in Figs. 6, 9, and 11 (ichnological symbols after Bann and Fielding 2004).
of RR-7 represents one of these channelized gravites, and shows a
low and uniform trend (Figs. 4 and 9) of the BI log (BI mostly 0,
rarely 1). The next interval (7-5.5 m) shows a dominance of
landward-directed, dune-scale cross-bedding, which locally includes
thin mm-scale mud drapes and couplets along foresets (Fig. 10E).
The interval reflects tidal deposits with a strong flood-tidal
component (Gani and Bhattacharya, in review). The BI trend of this
interval is low and non-uniform (BI 0-2). The next interval (5.5-5.3
m) shows parallel laminations, interpreted as delta-front gravites,
with an upward-increasing trend of BI values (0-2). The remainder of
Sandstone-6 reflects monotonous successions of cross-bedding, with
a low and uniform trend of the BI log (BI mostly 0, rarely 1),
probably generated in a tide-/river- dominated condition. A couple of
calcite-cemented horizons, burial diagenetic in origin (Nyman 2003),
are present. The overall BI values of Sandstone-6 show an upwarddecreasing trend.
FERRON SANDSTONE MEMBER
Well MC-2– The core of MC-2 well shows several parasequences
that belongs to the second transgressive-regressive cycle of Gardner
(1995). The thickest parasequence (‘Kf-2-Mi-b’ of Chidsey 2001,
interpreted as deltaic deposit; Fig. 3B) was chosen for this study
(Fig. 11). The basal interval (92.5-91.5 m) consists of sandy
mudstones, with intercalated 1-2 cm thick horizons of current and
wave rippled cross-lamination. Small-scale failure structures (Fig.
12A) are observed in this interval. In the nearby Muddy Creek
outcrop, synsedimentary growth faults have been described (e.g.
Bhattacharya and Davies 2001). The above failure structure is
probably associated with growth faults commonly developed due to
the overloading of delta-front sand over under-compacted prodelta
mud. This interval shows a non-uniform and upward-increasing trend
(Figs. 4 and 12) of BI values (0-4). In the next interval (91.5-89.7
m), sandstone beds commonly contain parallel lamination and locally
distinct normal grading, thought to represent delta-front gravites.
Several discrete horizons (1-3 cm) of unburrowed, black mudstones
are also observed. The BI log of this interval shows a non-uniform
trend, where BI values vary from 0 to 3. The facies from 92.5 m to
89.7 m are interpreted as river-dominated deposits.
The next interval (89.7-87 m) is intensely bioturbated (BI 5) with
few remaining parallel laminations (Fig. 12C). The absence of riverand tide-generated structures, and overall high BI values probably
suggest that this interval is wave-affected. The BI log
characteristically shows a uniform and high trend. The interval 8784.5 m consists of repetitive, upward-fining cycles (averaging 25 cm
6
Fig. 6.– Sedimentological and ichnological graphic logs of RR-2 with interpretations. For legend, see Fig. 5. For location, see Fig. 1B.
7
Fig. 7.– Box photos of cores of RR-2. Boxes are in order from base (a) to top (c), and from right to left.
Fig. 8. – (Page Opposite) Ichnology of wave-dominated RR-2 and outcrop equivalents. See Fig. 6 for the litholog of RR-2. A) Parallel
laminated tempestite truncating burrowed fairweather interval [note Chondrites (Ch)]. Lower shoreface deposits, 34.29 m depth, Sandstone-3. B)
Monotonous, parallel lamination of unburrowed upper shoreface deposits (28.1 m depth, Sandstone-3). C) TSE at the top of Sandstone-S3, 27.88
m depth. Note that the mudstone above this surface is less intensely bioturbated than the silty mudstone at the top part of the photo. D) Offshore
deposits of Sandstone-4a, 26.92 m depth. Traces include Chondrites (Ch), Zoophycos (Zo), Phycosiphon (Phy), and Terebellina (Ter). Note thin
remnant tempestites and marcasite nodule (mar) filling Chondrites tube. E) Lower shoreface deposits (BI 5) of Sandstone-4a, 24.4 m depth.
Traces include Phycosiphon (Phy), Thalassinoides (Th), Chondrites (Ch), and Helminthopsis (He). Phycosiphon occupy ghosts of Thalassinoides.
F) Lower shoreface deposits of Sandstone-4a, 22 m depth. Homogenization is due to repetitive burrowing (BI 6). Only Chondrites (Ch) are
identifiable. G) Lower shoreface deposits of Sandstone-4a, 20.85 m depth. Traces include Thalassinoides (Th), Chondrites (Ch), and Phoebichnus
(Pho). Large diameter (5 cm) Thalassinoides are passively filled with parallel lamination. H) Transgressive deposits (BI 4) at the upper part of
Sandstone-4a, 16.68 m depth. Traces include Phoebichnus (Pho), Chondrites (Ch), Ophiomorpha irregulaire (Op), Phycosiphon (Phy) and
Skolithos (Sk). Note scattered shell fragments (sh) toward the top. I) Middle shoreface deposits (BI 5) of Sandstone-5, 9.43 m. Traces include
Ophiomorpha irregulaire (Op), Chondrites (Ch), and Phycosiphon (Phy). Burrow walls of Ophiomorpha are ramified only in the upper half. J-L)
Outcrop of middle shoreface deposits. J) Chondrites on the upper surface of a sandstone bed, Sandstone-1. K) Large (length >50 cm) Skolithos in
Sandstone-4. L) Large (6 cm in diameter) Skolithos in Sandstone-5.
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thick), in which parallel laminated sandstones locally pass upward
out of massive sandstone units, and grade into burrowed muddy
sandstones (Fig. 12D). These beds, interpreted as delta-front
gravites, probably originated during high discharge river-floods. This
interval shows a non-uniform (alternating low and high) trend of BI
values (1-4). The next interval (84.5-80 m) includes stacked, trough
cross-stratified, ripple cross-laminated, and parallel laminated
sandstones (Fig. 12E). A single muddy sandstone horizon with softsediment deformation structures and almost vertically aggraded
climbing ripples, are thought to represent rapid, river-born
sedimentation. The BI log of this interval shows a non-uniform
trend, where BI values range from 0 to 3. The next interval (80-78
m) yields no sedimentary structures, and gives a uniform and
upward-increasing trend of BI values (increases from 1 to 5). The
uppermost interval comprises trough cross-bedded sandstone
overlain by transgressive deposits. The BI log shows a low and more
or less uniform trend (BI 0-1), with a sudden increase in BI across
the transgressive surface.
9
Fig. 9.– Sedimentological and ichnological graphic logs of RR-7 with interpretations. For legend, see Fig. 5. For location, see Fig. 1B.
ICHNOLOGICAL COMPARISONS OF WAVE-, STORM-,
TIDE-, AND RIVER-DOMINATED SEDIMENTATION
In the deltaic successions described above, a distinct pattern of
ichnological signatures has been observed for each of the controlling
processes of waves, storms, tides, and river sediment influx. These
patterns are described below, according to the interpreted
subenvironments of deposition. In general, Sandstone-3 of RR-2 is
storm-dominated, Sandstone-4 and Sandstone-5 of RR-2 are wavedominated, Sandstone-6 of RR-2 and RR-7 is mixed-energy (river,
tide and wave), and the parasequence of MC-2 is river- and wavedominated successions (Figs. 6, 9, and 11).
OFFSHORE/PRODELTA DEPOSITS
Wave-dominated offshore to prodeltaic successions (Sandstone-4 to
Sandstone-6 at RR-2 and RR-7), consist mostly of sandy mudstones,
10
Fig. 10.– Ichnological signatures of RR-7. See Fig. 9 for the litholog of this well. A) Wave-dominated offshore/prodelta deposits consisting
of sandy mudstones with uniform bioturbation (BI 4), 10.9 m depth. Trace fossils include Helminthopsis (He), Phycosiphon (Phy), Terebellina
(Ter), Zoophycos (Zo), and Planolites (Pl). B) Event bed in wave/river-dominated prodelta deposits (BI 4), 10.53 m depth. Trace fossils
include Thalassinoides (Th), Rosselia (Ro), Asterosoma (As), and Helminthopsis (He). C) Tide-dominated middle delta-front deposit, showing
thin interval of weakly bioturbated (BI 2) sandstone with mud chips, 6.85 m depth. Trace fossils include Asterosoma (As), Palaeophycus
tubularis (Pa), and Chondrites (Ch). D) Tide-dominated middle delta-front deposit consisting of trough cross-bedded sandstones (BI 1), 6.68
m depth. Trace fossils include Ophiomorpha irregulaire (Op), and Skolithos (Sk). Note that Ophiomorpha walls are lined with pellets on all
sides. E) Tide/river-dominated upper delta-front deposit, consisting of trough cross-bedded sandstones without visible burrows, 5.16 m depth.
Note the double mud drapes along troughs. F) Tide/river-dominated upper delta front deposit, consisting of trough cross-bedded sandstones
with cm-scale mud drapes, 3 m depth. Note that mud drapes are devoid of trace fossils.
and contain suites that reflect a distal expression of the Cruziana
Ichnofacies passing upward into the archetypal Cruziana Ichnofacies
(Fig. 8D). The dominant ichnogenera include Chondrites,
Zoophycos, Helminthopsis, and Phycosiphon. The latter two
represent grazing/foraging structures. Chondrites (Figs. 8D and 8J),
the single most abundant ichnogenus in the prodeltaic and offshore
deposits of the studied successions, may represent deep-tier
chemosymbiont behavior (e.g., Seilacher 1990; Bromley 1996). The
sporadic occurrence of 1-4 mm diameter marcasite (FeS2) nodules in
these sediments (Fig. 8D) suggests that such structures may have
tapped a sulphide-rich substratal environment. Locally, some of
these nodules are found within the tubes of Chondrites.
Ophiomorpha, Trichichnus, and Skolithos are found very rarely in
the remnant thin sandstone beds, which show oscillation ripples and
HCS. This suite is diminutive in size, and is probably indicative of rselected opportunistic colonization (e.g., Saunders et al. 1994). The
subordinate ichnogenera include Asterosoma, Teichichnus,
Planolites, Terebellina (sensu lato), Thalassinoides, Palaeophycus
tubularis, Palaeophycus heberti, Rosselia, and Cylindrichnus, in
order of decreasing abundance. Among these, Terebellina (sensu
11
lato), Palaeophycus tubularis, and Palaeophycus heberti probably
indicate dwelling structures of passive carnivores; the remainder
record structures attributable to deposit-feeders. The uniform trend of
the BI log is very characteristic of these wave-dominated deposits
(Figs. 4, 6, and 9). A low value of BI (averaging 2) is consistent with
a prodelta environment, whereas higher values (averaging 4)
probably points to an offshore environment mostly unaffected by
river effluent (Figs. 6 and 9). High diversities of trace fossils
generally indicate open marine conditions (Pemberton et al., 1992;
MacEachern and Pemberton 1992; Gingras et al. 1998; MacEachern
et al. in press). Uniform bioturbation reflects homogeneous
distribution of food, normal salinity, and available oxygen in the
seawater, due to persistent wave agitation, and/or a wider
colonization window (the time available for colonization by tracemakers) reflecting slow deposition rates.
River-dominated prodelta deposits consist of interbedded mudstones
and sandstones, and are found in the lowermost part of MC-2 (Fig.
11). These deposits show the suites consistent with the diverse,
archetypal Cruziana Ichnofacies. Ichnogenera comprise Chondrites,
Palaeophycus tubularis, Thalassinoides, Planolites, Teichichnus,
Cylindrichnus, Zoophycos, Phycosiphon, Conichnus, Asterosoma,
fugichnia, Ophiomorpha nodosa, and Palaeophycus heberti, in order
of decreasing abundance (Fig. 12A). Of the above, the dwelling/
suspension-feeding (or omnivore?) structure of Ophiomorpha
nodosa, the dwelling structures of Palaeophycus tubularis and
Conichnus, and fugichnia are confined to the rippled sandstone beds,
and may represent doomed pioneers (cf. Föllmi and Grimm 1990).
Two different sizes of Chondrites are observed. Robust forms show
tube diameters of 1 mm, whereas diminutive forms are 0.5 mm in
diameter (Fig. 12A). BI trend is characteristically non-uniform, with
values ranging from 0 to 4. This may reflect a small colonization
window associated with high and episodic deposition rates, and is
the most characteristic of river-dominated prodelta deposits. Thin (13 cm) and unburrowed layers of black, fissile mudstones, thought to
be a characteristic of river-dominated prodelta environments
(Leithold 1989; MacEachern et al. in press) occur sporadically.
SHOREFACE/DELTA FRONT DEPOSITS
The deposits of this setting show a marked variation in ichnological
signatures, largely reflecting the dominant process of deposition and
local paleogeographic settings.
Wave- and storm-affected deposits –Wave- and storm-dominated
shoreface deposits are extensively developed in Sandstone-3, 4, and
5 of RR-2 (Figs. 6, and 7). Lower shoreface successions pass upward
out of units interpreted as the offshore transition, which yield suites
reflecting diverse and archetypal Cruziana Ichnofacies. Lower
shoreface successions contain suites consistent with a high diversity
proximal expression of the Cruziana Ichnofacies. The suites include
Thalassinoides,
Palaeophycus
tubularis,
Helminthopsis,
Phycosiphon, Ophiomorpha irregulaire, Phoebichnus, Chondrites,
Terebellina (sensu lato), Skolithos, Asterosoma, Palaeophycus
heberti, Teichichnus, Rosselia, Rhizocorallium, Macaronichnus,
Trichichnus, Zoophycos, fugichnia, and Siphonichnus, in order of
decreasing abundance (Fig. 8E). This comprises the largest diversity
suite in the studied successions. The suite displays robust burrows.
Thalassinoides, for example, may reach 5 cm in diameter, and are
passively filled with laminated muddy sandstone (Fig. 8G). These
are interpreted as ‘tubular tempestites’ (cf. Tedesco and Wanless
1991). In many cases, a close affinity between the semi-vagile to
vagile, shallow- to mid-tier deposit-feeding structure Phycosiphon
and dwelling structures Thalassinoides has been observed (Fig. 8E).
This may indicate that organic matter in abandoned Thalassinoides
dwellings yielded a harvest of microbes, that later attracted the
Phycosiphon trace makers (e.g., Bromley 1996).
Lower shoreface deposits are overlain by wave-dominated middle
shoreface units, displaying suites of the archetypal Skolithos
Ichnofacies (Fig. 8I). Dominant ichnogenera include the sessile,
deep-tier, suspension-feeding structures Ophiomorpha irregulaire,
Skolithos, and Arenicolites, as well as the dwelling/deposit-feeding
structures Thalassinoides, Phoebichnus, and Palaeophycus tubularis
(passive carnivore structures). Subordinate ichnogenera include
Chondrites,
Helminthopsis,
Macaronichnus,
Phycosiphon,
Terebellina (sensu lato), Diplocraterion habichi, Asterosoma,
Trichichnus, Siphonichnus, Taenidium, and Rhizocorallium, in order
of decreasing abundance. Locally, coal fragments contain
Teredolites, but these are probably not in situ. Middle shoreface
deposits yielded favorable substrates for suspension feeders. This is
indicated by the presence of robust Skolithos, some up to 50 cm in
length (Fig. 8K) and others up to 5 cm in diameter (Fig. 8L).
Typically, the horizontal branches of Ophiomorpha irregulaire are
ramified only along their upper margins (Fig. 8I), as the collapsing
pressure acts from top down.
The most characteristic signature of these lower and middle
shoreface units is the uniform, high (BI 4-6), and upward-increasing
trend of the BI log. This intense bioturbation gives these deposits a
mottled appearance with few preserved parallel laminations (Figs.
8E, 8G). This uniform and high BI trend probably indicates a wide
colonization window removed from river mouths. This is also
supported by the predominance structures of inferred suspension
feeders, the activities of which are hindered by heightened water
turbidity generated from river discharge (Moslow and Pemberton
1988; Gingras et al. 1998; Coates and MacEachern 1999;
MacEachern et al. in press).
Where storm activity is prominent in the shoreface deposits
(Sandstone-3 of RR-2; Fig. 6), the ichnological signature is different.
Increasing storm activity normally leads to a sporadic distribution of
ichnogenera and reduced bioturbation intensities (e.g., Coates and
MacEachern 1999). The lower shoreface units contain suits
attributable to a distal expression of the Skolithos Ichnofacies,
grading to suites reflecting a proximal expression of the Cruziana
Ichnofacies. Middle shoreface deposits display suites that correspond
to the archetypal Skolithos Ichnofacies (c.f. MacEachern et al. 1999).
However, the diversity of ichnotaxa is low with a clear
predominance of dwelling of presumed suspension feeding infauna
in both the lower and middle shoreface rocks (Fig. 6). Event
deposition is highlighted by discrete storm events truncating the fairweather suite, and generating frozen tiers (Fig. 8A). Upper shoreface
deposits do not show any trace fossils, with the exception of some
suspected cryptic bioturbation. As a result, sedimentary structures
(parallel to low-angle laminations) are well preserved in the deposits
(Fig. 8B). Possible scenarios are: 1) the trace fossils have been
removed completely by successive periods of storm erosion, and/or
2) burrowing activity was prohibited because of close proximity to a
river source creating a harsh environment for infaunal organisms.
The general paucity of clear erosion surfaces suggests that the latter
scenario is more plausible. Overall, the bioturbated interval of this
shoreface deposit displays a non-uniform and upward-decreasing
trend of the BI log (BI 0-4) (Fig. 6).
In the case of Sandstone-6 of RR-2 and parasequence of MC-2,
isolated wave-affected deposits alternate with the river-, and tideaffected deposits. In RR-2, the interval from 5.6-2.7 m represents a
wave-influenced lower and middle delta front deposit (Fig. 6). The
lower delta front contains suites corresponding to a distal to
archetypal expression of the Cruziana Ichnofacies, with Chondrites,
Helminthopsis, Planolites, Terebellina (sensu lato), Phycosiphon,
Thalassinoides, Skolithos, Palaeophycus heberti, and Zoophycos, in
decreasing order of abundance. On the other hand, middle delta-front
deposits show suites recording a low diversity, distal to archetypal
expression of the Skolithos Ichnofacies, including Palaeophycus
12
Fig. 11.– Sedimentological and ichnological graphic logs of MC-2 with interpretations. For legend, see Fig. 5. For location, see Fig. 1C.
13
tubularis, Ophiomorpha irregulaire, and Thalassinoides. Overall,
these wave-affected delta-front deposits show a uniform and upwarddecreasing trend of BI values (BI 4-1). Intense (BI 4) and uniform
bioturbation, along with comparatively diverse suites of trace fossils
in the lower delta front deposits suggest a well-oxygenated seafloor
subjected to wave agitation. On the other hand, low-intensity
bioturbation consisting of a low diversity suite in the middle deltafront deposits reflects both the proximity to a river source and
increasing intensity of storm erosion associated with shallowing. In
MC-2, there are two intervals of wave-dominated units enclosed
between river-dominated intervals (Fig. 11). The interval from 89.787 m reflects a lower shoreface succession, which contains suites
that record a diverse expression of the mixed Skolithos-Cruziana
“ichnofacies”. Ophiomorpha irregulaire, Ophiomorpha nodosa,
Palaeophycus
tubularis,
Helminthopsis,
Thalassinoides,
Phycosiphon, Phoebichnus, Chondrites, Macaronichnus, Terebellina
(sensu lato), fugichnia, Rosselia, Asterosoma, and Palaeophycus
heberti occur, in order of decreasing abundance. The BI trend is
more or less uniform and values are constant (BI 5) throughout the
interval, giving rise to a mottled appearance (Fig. 12C). The second
interval (80-78 m) belongs to middle shoreface deposits,
characterized by suites corresponding to a low diversity expression
of the mixed Skolithos-Cruziana “ichnofacies”. Ophiomorpha
irregulaire, Palaeophycus tubularis, Helminthopsis, Asterosoma,
and Cylindrichnus are present, in order of decreasing abundance
(Fig. 12F). A uniform and upward-increasing trend displayed by the
BI log (BI 1-5) is observed. The ichnological signatures of these
wave-affected deposits are similar to those of the wave-dominated
deposits of Sandstone-3, 4, and 5 of RR-2.
River-affected deposits– River-dominated delta-front deposits are
observed in RR-7 and MC-2 (Figs. 9, 11). The interval from 8.8-7 m
in RR-7 mainly represents lower delta-front channelized gravites,
which contain low diversity suites attributable to the archetypal
Cruziana Ichnofacies. Assemblages include Chondrites, Planolites,
Macaronichnus, Arenicolites (1-2 mm in size), and allochthonous
(found within mud chips) Asterosoma. The BI trend is low (BI 0-2)
and uniform. This, coupled with the low diversity of ichnotaxa,
probably indicates a narrow colonization window and overall
stressful environment. This is consistent with channels, interpreted as
subaqueous terminal distributaries. These deposits reflect high rates
of sedimentation, increased water turbidity, and probably salinity
reduction as well (e.g., Gingras et al. 1998; Coates and MacEachern
1999).
River-dominated sedimentation is best developed in MC-2 (Fig. 11).
In this well, the lower delta-front deposits (interval 91.5-89.7 m)
suites that correspond to the archetypal to proximal expression of the
Cruziana Ichnofacies and a distal expression of the Skolithos
Ichnofacies. The Cruziana Ichnofacies developed in the muddy
sandstones is reflected by Chondrites, Thalassinoides, Planolites,
Ophiomorpha nodosa, Palaeophycus tubularis, Helminthopsis,
Phycosiphon, Teichichnus, fugichnia, and Macaronichnus, in order
of decreasing abundance (Fig. 12B). Fugichnia are associated with
the gravite beds. Two distinct horizons (1-3 cm thick) of unburrowed
black mudstone also occur. These types of organic-rich mudstones in
river-dominated systems are thought to generate anoxic bottom water
conditions through decay of organic matter, reducing biogenic
activity (e.g., Rice et al. 1986; Savrda and Bottjer 1987, 1989;
Leithold 1989; Raychaudhuri and Pemberton, 1992; Wignall and
Pickering 1993). Distal expressions of the Skolithos Ichnofacies are
developed in the upper, well-bedded sandstones, featuring normal
grading and climbing current ripple cross-lamination. Suites
comprise Palaeophycus tubularis, Ophiomorpha irregulaire,
Ophiomorpha
nodosa,
Palaeophycus
heberti,
fugichnia,
Thalassinoides, Planolites, Helminthopsis, and Chondrites, in order
of decreasing abundance. Fugichnia are associated with the climbing
ripples (Fig. 12B), thought to reflect high accumulation rates close to
a river mouth. The lower delta-front interval of the above succession
is characterized by a non-uniform trend with relatively low values
(BI 0-3) on the BI log. This may be due to the narrow colonization
window associated with the high deposition rates, and to intermittent
anoxic bottom water condition associated with the degradation of
organic-rich mudstones. Nevertheless, the suites are comparatively
diverse, possibly indicating salinity levels close to fully marine
conditions.
The interval from 87-84.5 m of MC-2 (Fig. 11) is also interpreted as
a lower delta front, characterized by mixed river/wave-affected
deposition. These successions of cyclic event beds (Fig. 12D) show
suites reflecting the mixed Skolithos-Cruziana “ichnofacies”, and
include Ophiomorpha irregulaire, Palaeophycus tubularis,
Thalassinoides, Skolithos, Arenicolites, Planolites, Helminthopsis,
Phycosiphon,
Cylindrichnus,
Chondrites,
fugichnia,
and
Palaeophycus heberti, in order of decreasing abundance. Each event
bed (average 25 cm thick) truncates the burrows of the preceding
bed, and produces frozen tiers. The deepest tier burrows of
individual bed comprised only Skolithos, fugichnia, Ophiomorpha,
and Palaeophycus tubularis (Fig. 12D). The overall pattern shown
by the BI log of this succession is cyclic, non-uniform, and upward
decreasing (BI 0-4). Large numbers of dwellings of suspension
feeders probably indicate that water turbidity was not elevated, and
concentrations were dissipated by subordinate wave activity. This is
consistent with the diverse assemblages of ichnotaxa present.
The overlying interval from 84.5-80 m (Fig. 11) represents
deposition in a river-dominated middle delta front. Overall, the
interval is characterized mostly by suites attributable to the
archetypal Skolithos Ichnofacies; subordinate suites correspond to
the proximal expression of the Cruziana Ichnofacies. Suites include
Ophiomorpha irregulaire, Skolithos, Thalassinoides, Palaeophycus
tubularis, Asterosoma, Phoebichnus, fugichnia, Cylindrichnus,
Helminthopsis, Phycosiphon, Planolites, and Bergaueria, in order of
decreasing abundance. There is a clear predominance of
Ophiomorpha irregulaire in this interval. Four types of
sedimentation patterns are recognized. The first type constitutes
trough cross-stratified sandstones, which show only Ophiomorpha
and Skolithos (BI 0-2). A second overlying type comprises a thin
muddy sandstone unit with penecontemporaneous deformation
structures (probably related to growth faults). It is notable that in
spite of the deformation, the unit shows a relatively high degree of
bioturbation (BI 0-3) with at least six ichnogenera identified that
correspond to inferred deposit feeders. This probably indicates that
the unit was affected by faulting at a later time, when more proximal
cross-bedded sandstones overloaded it. A similar observation of fault
initiation was made by Bhattacharya and Davies (2001) in the ‘first
cycle’ of the Upper Ferron Sandstone at the Muddy Creek outcrop.
The third type of sedimentation is similar to the cyclic sedimentation
of the interval 87-84.5 m, and shows a mixture of inferred
suspension-feeding and deposit-feeding structures (BI 0-2). The
fourth and final type records rapid sedimentation of aggrading to
high-angle climbing ripples, and contains only Ophiomorpha,
Skolithos, and Thalassinoides (BI 0-1). The entire interval from 84.580 m reflects a non-uniform and low trend of the BI log (BI 0-3),
which indicate a narrow colonization window related to episodic and
rapid deposition during major river floods. Nonetheless, the overall
ichnotaxa diversity is moderate to high. Suspension-feeding/dwelling
ethologies of Ophiomorpha and Skolithos predominate in this
interval, which may indicate that the trace-makers avoided the turbid
water column, and managed to filter feed by staying inside their
burrows and circulating water through them (e.g., Bromley, 1996). It
is also likely that Ophiomorpha, thought to be produced by shrimp,
may not represent suspension feeding structures, as modern shrimps
are passive carnivores, detritus feeders, and scavengers. The last
14
Fig. 12.– Ichnological signatures of MC-2. See Fig. 11 for the litholog of this well. A) River-dominated prodelta deposits consisting of
interbedded mudstones and sandstones with non-uniform bioturbation (BI 1-3), 91.9 m depth. Sandstones show ripple cross-lamination with
small-scale collapse structures (related to growth faults). Trace fossils include Chondrites (Ch), Thalassinoides (Th), Palaeophycus tubularis
[Pa(tb)], Planolites (Pl), Zoophycos (Zo), Asterosoma (As), Phycosiphon (Phy), fugichnia (fu), and Palaeophycus heberti [Pa(hb)]. Note two
size groupings of Chondrites, one with tube diameters of 1 mm, and the other, 0.5 mm. B) River-dominated lower delta-front sandstones (BI
2) with climbing ripple cross-lamination (lower part) and parallel lamination (upper part), 90.2 m depth. Trace fossils include Ophiomorpha
irregulaire [Op(ir)], Ophiomorpha nodosa [Op(no)], fugichnia, and Planolites. C) Wave-dominated lower shoreface deposits showing
uniform and extensive burrowing (BI 5), 89 m depth. Trace fossils include Ophiomorpha irregulaire [Op(ir)], Ophiomorpha nodosa [Op(no)],
Palaeophycus tubularis [Pa(tb)], Helminthopsis (He), Phoebichnus (Pho), Chondrites (Ch), Macaronichnus (Ma), Terebellina (Ter), fugichnia
(fu), Asterosoma (As), and Rosselia (Ro). D) River/wave-dominated lower delta-front event bed showing upward-increasing BI, 85.3 m depth.
Trace fossils include Ophiomorpha irregulaire [Op(ir)], Thalassinoides (Th), Skolithos (Sk), Palaeophycus tubularis [Pa(tb)], Helminthopsis
(He), Phycosiphon (Phy), fugichnia (fu), and Planolites (Pl). E) River-dominated middle delta-front deposits of parallel laminated sandstone,
showing diverse but low intensity bioturbation (BI 2), 81.1 m depth. Trace fossils include Asterosoma (As), Cylindrichnus (Cy), fugichnia
(fu), Ophiomorpha irregulaire [Op(ir)], Skolithos (Sk), Helminthopsis (He), Phoebichnus (Pho), and Bergaueria (Be). F) Wave-dominated
middle shoreface deposits showing uniform burrowing (BI 5), 78.4 m depth. Trace fossils include Palaeophycus tubularis [Pa(tb)],
Ophiomorpha irregulaire [Op(ir)], Helminthopsis (He), Asterosoma (As), Cylindrichnus (Cy), and Planolites (Pl).
15
river-dominated interval (78.1-76.6 m) in MC-2 comprises upper
delta-front deposits. This interval shows a non-uniform trend and
very low values (BI 0-1) of BI on the log, including only
Ophiomorpha irregulaire. Like before, this BI trend is characteristic
of river-dominated processes.
Tidally affected deposits– Tidally affected sedimentation is observed
only in Sandstone-6 of the Wall Creek Member in RR-2 and RR-7
(Figs. 6, 9). In RR-7, the interval from 7-5.3 m is interpreted as a
tide-dominated middle delta-front deposit. The interval shows a low
diversity trace fossil suite, reflecting a proximal expression of the
Cruziana Ichnofacies to a distal expression of the Skolithos
Ichnofacies (Figs. 10C, 10D). Suites contain Asterosoma,
Ophiomorpha irregulaire, Palaeophycus tubularis, fugichnia,
Chondrites, Helminthopsis, Planolites, Skolithos, and possible
cryptic bioturbation, in order of decreasing abundance. The margins
of Ophiomorpha irregulaire are more or less uniformly lined with
pellets in all sides (Fig. 10D), indicating a highly shifting substrate.
Deposit-feeding structures are mainly found within intercalated
muddy horizons (Fig. 10C). Some mud chips contain Chondrites, but
these are probably not in situ. Overall, the interval is characterized
by non-uniform and low BI trend (BI 0-2). The topmost interval of
RR-7 and RR-2 reflects mixed tide/river-affected, upper delta-front
deposits. The interval shows suites corresponding to a low diversity
expression of the archetypal Skolithos Ichnofacies, with
Palaeophycus tubularis, Skolithos, fugichnia, Macaronichnus,
Ophiomorpha irregulaire, Planolites, and possible cryptobioturbation, in order of decreasing abundance. Suites show
impoverished numbers. Visibility of Macaronichnus is enhanced in
the cemented horizons of rippled sandstones. Notably, mud drapes
and mud clasts are almost devoid of burrows (Fig. 10F). In general,
the interval is characterized by non-uniform and very low (BI 0-1)
trend of the BI log. This BI trend probably indicates a narrow
colonization window resulting from comparatively high rates of tidal
sedimentation. The low diversity of ichnotaxa may reflect high
magnitudes of salinity fluctuations associated with tidal periodicities.
ICHNOLOGICAL
SIGNATURES:
PARASEQUENCE
DEPOSITION AND THEIR BOUNDING DISCONTINUITIES
Overall ichnological signatures, particularly the BI trend, of
individual parasequences (sandstones) is distinctive, depending to
what extent they are affected by wave, storm, river, and tidal
processes. Ichnological signatures also change characteristically
across initial marine flooding surface.
PARASEQUENCES
In general, wave-dominated deposits host the most diverse and
climax signatures of trace fossils. When protected from storm
erosion, tidal modification, and direct river input, these deposits
(Sandstone-3, 4, and 5 of RR-2) could show maximum activity of
trace makers (Figs. 6, and 7). All of these parasequences show
characteristic uniform and upward-increasing trends of BI with
overall high values (BI 2-6). This is probably due to a wide
colonization window (e.g., Bromley 1996) where the slow
sedimentation rate facilitates a homogeneous and thorough
modification of the substrate. This condition was likely coupled with
high oxygen availability and normal salinities, aided by wave
agitation. Overall ichnotaxa diversities are high, with a mixture of
grazing/foraging, deposit-feeding, and suspension-feeding structures.
Each of these ethologies is dominant in offshore, lower shoreface,
and middle shoreface, respectively. It is notable that when BI values
reach six, only a few traces (deep tier) are readily identifiable (Fig.
8F). This situation should not be confused with low diversity suites.
In these parasequences, some robust ichnogenera are observed (Figs.
8G, 8K, and 8L), which also support an unstressed condition during
colonization. Although thin storm beds with hummocky cross
stratification (HCS) have been observed sporadically in these wave-
dominated successions, the BI trends are not obliterated, probably
due to low frequency and reduced intensity of storms.
In storm-dominated intervals (e.g., Sandstone-3 of RR-2), the
ichnological signatures are different and probably taphonomically
biased. Rapidly deposited tempestites predominate in the lower
shoreface deposits, whereas middle and upper shoreface units show a
strong signature of storm erosion and amalgamation (e.g.,
MacEachern and Pemberton 1992; Pemberton et al. 2002; Coates
and MacEachern 1999). This is the case for Sandstone-3. As a result,
this sandstone shows a non-uniform and upward-decreasing BI trend
(Fig. 6). The upper shoreface is almost entirely devoid of burrows
(Fig. 8B), which probably indicates either a taphonomic bias
resulting from storm erosion, or a narrow colonization window
related to heightened sedimentation rates.
Ichnological signatures are also distinctive when in close proximity
to distributary channels (e.g., Sandstone-6 of RR-2 and RR-7, and
the parasequence of MC-2). In general, these two parasequences are
characterized by an upward-decreasing and non-uniform trend of BI
log. Most importantly, each of these parasequences shows discrete
intervals of river-, wave-, storm-, and tide-dominated sedimentation,
giving rise to different ichnological signatures. Wave-dominated
intervals of these parasequences are characterized by uniform trends
and relatively high values of BI (mostly 3-5) with a diverse suite (up
to 13 ichnotaxa) (Figs. 12C, 12F) – signatures similar to those of
wave-dominated deposits described earlier. River-dominated
intervals show the most non-uniform trend of BI (BI 0-4), varying
from low diversity (1) to high diversity (up to 11) of ichnotaxa.
Seasonal fluctuations of sedimentation rate (i.e., fluctuating
colonization window) and suspended sediment concentration (water
turbidity) near distributary mouths probably generate an alternating
high-stress and low-stress environment for the trace makers. As a
result, infauna shows highly variable distributions and abundances in
the deposits. In the studied successions, tide-dominated intervals
show the most ‘stressed’ ichnological conditions, with non-uniform
trends and generally low values of BI (0-2). Salinity fluctuations,
continuously shifting substrates, narrow colonization windows
(related to diurnal-scale tidal periodicities), and the development of
fluid-mud layers are probably the dominant factors in controlling the
trace-maker distributions, abundances, and ethologies.
In RR-2 and MC-2, retrogradational transgressive deposits that are
associated with a relative rise in sea level, commonly lie above the
progradational parasequences. In general, when relative sea level
begins to rise, waves become the dominant process during deposition
as distributary channels are being abandoned. Therefore, all of the
transgressive deposits in this study display ichnological signatures
(Fig. 8H) comparable to those of unstressed (e.g., wave-dominated)
deposits. The resulting signatures are characterized by diverse suites
of ichnotaxa, coupled with uniform and high trend of the BI log (BI
3-4). In RR-2, these deposits also show an upward-decreasing trend
of BI values.
BOUNDING DISCONTINUITIES
A marked change of ichnological signatures has been observed
across the bounding discontinuities reflecting the initial (first) marine
flooding surfaces related to relative rise of sea level. In the study
intervals, TS (transgressive surface) or TSE (transgressive surface of
erosion) cap the progradational parasequences. A sharp change in BI
values occurs at these discontinuities. The change is generally
towards higher BI values if the underlying parasequence shows
upward-decreasing BI, but is towards lower BI if the underlying
parasequence shows a trend of upward-increasing BI. TS generally
indicate comparatively gradual deepening of the sea, and mark the
turnaround from progradation to retrogradation. In the studied
successions, this gradual deepening of relative sea level is
characterized by a gradual upward shift of ichnofacies from the
16
archetypal Skolithos Ichnofacies below the TS to distal Skolithos /
proximal expressions of the Cruziana Ichnofacies above the TS (e.g.,
parasequence of MC-2 and Sandstone-5 of RR-2). On the other hand,
the TSE demarcates rapid or abrupt changes of relative sea level
(Fig. 8C), accompanied by sharp upward shifts of ichnofacies from
the archetypal Skolithos Ichnofacies to the archetypal Cruziana
Ichnofacies across this surface (e.g. Sandstone-3, and 4 of RR-2).
When marine transgression reaches its highest position, an MxFS
(maximum flooding surface) develops, after which, the ensuing
depositional system starts to prograde. Therefore, this surface
normally demarcates the boundary between transgressive deposits
below and prograding parasequences above. Ichnological signatures
do not show any sudden changes across the MxFS. Notably, the BI
trend (BI 3-4) also does not break across MxFS.
CONCLUSIONS
1. Deltaic deposits of the Turonian Western Interior Seaway, show
marked variations in ichnological signatures, dependent on the
relative dominance of wave, storm, river, and tidal processes.
2. BI (bioturbation index) logs, when plotted beside lithologs and
integrated with ichnological suites interpreted within the
ichnofacies paradigm, can significantly assist in understanding the
bed-scale as well as parasequence-scale variations of trace
maker’s behaviors in the deposits. Four basic morphologies have
been identified that describe the trend of the BI log – uniform,
non-uniform, upward-increasing, and upward-decreasing (Fig. 4).
3. Wave-dominated deposits, when protected from frequent storm
erosion, show the most diverse, climax trace fossil signatures, with
“uniform and high” trends of BI (averaging to 4). This is probably
due to broad colonization window, coupled with persistent wave
agitation, which helps to minimize water turbidity, and optimize
the distribution of oxygen, salinity, temperature, and organic
matter in the water column, as well as in the substrate.
4. High storm frequency may preclude or obliterate trace fossil
distributions in the deposits, by reducing the colonization window
and/or eroding the fair-weather suites. The resulting ichnological
signatures reflect “non-uniform” and “upward-decreasing” trends
of the BI log.
5. River-dominated intervals show the most “non-uniform” trends
and highly variable values of BI (ranges from 0 to 4), displaying a
low-diversity to moderately high diversity of ichnotaxa. These
probably reflect seasonal to centennial fluctuations of
sedimentation rate and water turbidity in the vicinity of the
distributary mouth. Such setting favors alternations from highstress to low-stress conditions for trace makers.
6. Tide-dominated deposits show the most ichnologically ‘stressed’
conditions, with “non-uniform and low” trend of BI (0-2),
resulting from salinity fluctuations, continuously shifting
substrates, narrow colonization window associated with tidal
periodicities, and development of fluid-mud layers.
7. Individual parasequences are characterized either by upwardincreasing or upward-decreasing trends of BI. BI values and trace
fossils suites change significantly across marine flooding surfaces
that cap progradational parasequences,. However, there is no trend
break on the BI log across maximum flooding surfaces (MxFS).
8. Individual parasequences can show various proportions of river-,
wave-, storm-, and tide-affected facies, each characterized by
distinct ichnological signatures. We suggest here that different
parts of a delta complex may experience different degrees of
wave, storm, tide, and river influence during a single phase of
progradation, and indeed, may evolve from one type to another
over time. Consequently, many deltaic successions may not be
simply classified as end-member types (i.e., ‘wave-dominated’,
river-dominated, or ‘tide-dominated’).
9. Ichnological analyses, particularly trend analysis of BI log, of
ancient successions can help in identifying the intervals deposited
under specific dominant process, in a depositional system. This
would facilitate in the delineation of its evolutionary history, as
well as in the reconstruction of its paleogeography.
ACKNOWLEDGEMENTS
Funding for this research was provided by the U.S. Department of
Energy DOE Grant # DE-FG0301ER15166. Additional support was
provided by the University of Texas Quantitative Sedimentology
Research Consortium, of which BP and Chevron/Texaco are
members. Nahid Gani helped greatly in the field as well as with the
cores. Thomas Chidsey kindly allowed access to the Utah Core Lab
to study the cores of MC-2. JAM received funding for this work
through a Natural Sciences and Engineering Research Council
(NSERC) Operating Grant 184293. This is contribution number
1051 of the Geosciences Department, University of Texas at Dallas.
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