Limestone-marl alternations in epeiric sea settings –
witnesses of environmental changes, or of rhythmic diagenesis?
Hildegard Westphal1, Axel Munnecke2, Florian Böhm3, Stefan Bornholdt4
1) Fachbereich Geowissenschaften, Universität Bremen, Marum Building, Leobener
Straße, D-28359 Bremen, Germany, [email protected]
2) Institut für Paläontologie, Universität Erlangen, Loewenichstraße 28, D-91054 Erlangen,
Germany, [email protected]
3) Leibniz-Institut für Meereswissenschaften, IfM-GEOMAR, Wischhofstraße 1-3, D24114 Kiel, Germany, [email protected]
4) Institut für Theoretische Physik, Universität Bremen, Otto-Hahn-Allee, D-28359
Bremen, Germany, [email protected]
For: Dynamics of Epeiric Seas: Sedimentological, Paleontological and Geochemical
Perspectives; edited by Chris Holmden and Brian R. Pratt; Geological Association of Canada
Special Volume
“[A test] appears to indicate that many limestones can be produced solely
by rhythmic unmixing of CaCO3 during diagenesis. Unless such limestones
can be clearly distinguished from those that record genuine environmental
signals, orbital cycle analysis based on such sequences will give
meaningless results“(Hallam, 1986)
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
Abstract
Limestone-marl alternations are widespread and
typical sediments of epeiric basins and are present in
variable abundance throughout the entire
Phanerozoic. In many cases, their rhythmic
appearance is interpreted as a direct response to
orbital forcing. However, it is a challenge to
unequivocally prove a sedimentary origin of the
rhythmic intercalation of the two lithologies. This
difficulty arises from differential diagenesis that
alters limestone beds in different ways than
interlayers (marls), causing a loss of comparability
between the lithologies. Differential diagenesis,
between other effects, causes passive enrichment of
2
absolute concentrations). Systematic differences in
diagenetically inert parameters can provide
unequivocal proof of primary differences. In the
studied limestone-marl alternations, however, such
parameters do not directly reflect the lithological
rhythm, shedding doubt on limestone-marl
alternations as direct archives of environmental
change.
Box model computer simulations visualize
possible effects of early diagenetic change acting on
limestone-marl alternations, independent of the
presence or absence of a primary rhythm. The
simulations demonstrate that diagenesis has the
potential to seriously distort any primary rhythm
Fig. 1: Occurrence of fine-grained calcareous rhythmites (limestone-marl alternations and nodular limestone successions) from literature
compilation based on databases including ISI Web of Science, GEOREF and GEOBASE (data compilation available upon request from the
authors). The reported occurrences are normalized for the time span of the interval they represent. Also shown: tropical shelf area (TSA) through
time (dashed line, after Walker et al., 2002), and ratio of Mg to Ca ions in ocean waters through time (solid line, after Stanley and Hardie, 1998).
the inert non-carbonate fraction in interlayers, where
calcium carbonate is being dissolved, as well as
passive dilution in limestone beds, that are cemented
by imported calcium carbonate. Therefore,
unequivocal information about systematic differences
in the precursor sediments of limestones and
interlayers therefore is preserved only in parameters
that are not modified during diagenesis. Such
diagenetically inert parameters include the spectra of
organic microfossils (but not their absolute
concentration in the bulk sediment) and the ratios of
diagenetically inert trace elements (again not the
present in the pristine sediment. In particular,
differential compaction acting mainly on the marl
interlayers induces distortions of the ratios of the
original frequencies. These simulations emphasize
the difficulties in conducting frequency analyses on
carbonate contents of real-world successions.
Keywords for index:
anactualism, aragonite, aragonite sea, calcite, calcite
sea, cellular automaton, cementation, compaction,
computer simulation, diagenesis, differential
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
diagenesis, dissolution, distortion, environmental
archive, environmental conditions, insolubles,
limestone-marl alternation, Milankovitch, model,
overprint, palynomorphs, sedimentary record, selforganization, stratigraphic record, rhythmites
1. Introduction – a brief review of limestonemarl alternations
Limestone-marl alternations are a widespread and
characteristic facies of epeiric sea basins with famous
examples in the Ordovician of N-America and the
western and central High Atlas (Morocco), the
Silurian of Northern Europe, the Triassic of the
Carpathians, the Mississippian of Montana, the
Jurassic and Cretaceous of Central and Western
Europe, and the Cretaceous of the Western Interior
Seaway and in Venezuela (e.g., Davaud and
Lombard, 1975; Courtinat, 1993; Holmden et al.,
1998; Samtleben et al., 2000; Westphal and
Munnecke, 2003; Chacrone et al., 2004; Rey et al.,
2004; Tomasovytch, 2004; see also several chapters
in Einsele et al., 1991). Limestone-marl alternations
are known from deposits of all Phanerozoic ages,
even though their abundance varies strongly for the
different geologic periods (Fig. 1; see also Westphal
and Munnecke, 2003). The abundance roughly
follows the oscillations between calcite and aragonite
seas (Sandberg, 1983; Stanley and Hardie 1999) with
high abundances during times of calcite seas and
lower abundances during times of aragonite seas. In
Jurassic and Cretaceous epeiric sea deposits, such
alternations are particularly widespread.
Limestone-marl
alternations
are
characterized by their conspicuous outcrop
appearance with a pronounced ABAB rhythm of
more weathering-resistant limestone beds and softer
interbeds (see overview in Einsele and Ricken, 1991).
They can be viewed as part of a continuum of
bimodal micritic alternations ranging from limestonechalk to limestone-shale alternations, and including
lithographic and nodular limestones, and well-bedded
limestones (Munnecke and Samtleben, 1996;
Westphal et al., 2000; Munnecke et al., 2001;
Munnecke and Westphal, 2004). This group of
sedimentary facies is referred to as "fine-grained
calcareous rhythmites" in the present manuscript.
Large epeiric seas offer favourable conditions
for the generation of fine-grained calcareous
rhythmites for several reasons (cf. various chapters in
Einsele et al., 1991). (1) Tropical epeiric seas provide
marginal areas for the production of shallow-water
carbonate deposits that can be winnowed into the
3
more distal parts of the basin. (2) A shallow wave
base, as usually encountered in epeiric seas, and the
low relief prevent deposition of coarse-grained
sediments in the distal parts of the basin and favor the
fine-grained sedimentation typical for limestone-marl
altenations. (3) Furthermore, the surrounding land
provides the input of siliciclastics, of which, in a
sufficiently large epeiric basin, mostly clay-sized
material will reach the depositional site where it can
form marls.
Such an epeiric setting provides all ingredients
for forming potentially sensitive recorders of climate
cycles as fine-grained calcareous rhythmites.
Therefore, the cyclic patterns of fine-grained
calcareous rhythmites are often regarded as indicators
and recorders of orbitally forced climatic cycles (see
chapters in de Boer and Smith, 1994 and Einsele and
Ricken, 1991). In contrast to shallow-water carbonate
platfoms, the basinal setting allows for continuous
records even during sea-level lowstands.
Nonetheless, the basinal sediments record the
environmental changes affecting the shallow-water
platforms in their imported, shallow-water produced
portion. The clay portion also potentially reflects
climatic conditions that influence the weathering of
the hinterland. In addition, varying nutrient fluxes
related to variations in weathering patterns are
potentially recorded in paleontologic parameters such
as calcareous nannofossil ratios. Thus, fine-grained
calcareous rhythmites offer a wealth of parameters
that turn them into potential archives of climate
variations.
One difficulty, however, arises from the fact
that fine-grained calcareous rhythmites generally are
strongly affected by differential diagenesis. Their
bimodal lithologic character of limestone beds
intercalated in softer interlayers is largely a product
of differential diagenesis, regardless of the presence
or absence of primary sedimentary rhythms (see
below). This fact is still underestimated in many
studies that interpret rhythms of various lithological,
paleontological, or geochemical parameters in
limestone-marl alternations as direct expressions of
orbital climate forcing (cf. Fischer, 1980; Sprenger
and ten Kate, 1993; Schwarzacher, 2000; Cleaveland
et al., 2002; Strasser et al. 2005). In this paper we
will review and present published and new
geochemical data and modeling results to illustrate
the problems involved in interpreting fine-grained
calcareous rhythmites as one-to-one recorders of
climate cycles.
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
1.1.
What is so special about limestonemarl alternations?
Fine-grained calcareous rhythmites show a wide
range of morphological variations from limestoneshale alternations to “classical” limestone-marl
alternations, to lithographic limestone, and to nodular
limestone. They formed in both, well-oxygenated
waters, as indicated by strong bioturbation and
benthic fossils, and under dysoxic or anoxic
conditions, as indicated by internal lamination and
the absence of benthic fossils.
Despite of the wide range of morphological
variations, the two intercalated lithologies are clearly
distinguishable in the field because the limestone
beds are more weathering-resistant than the
interlayers. The boundaries between the two
lithologies usually are relatively sharp. Within an
alternation, the limestone beds generally have higher
carbonate contents than the adjacent softer
interlayers, even in successions where on a large
scale the carbonate content in each of the two
lithologies varies strongly along the vertical
succession. In contrast to other rhythmically
deposited facies such as glacial varves or tidalites,
fine-grained calcareous rhythmites show indications
of strong diagenetic alteration that are fundamentally
different for the two intercalated lithologies. The two
lithologies have followed two distinctly different
diagenetic pathways in spite of their common burial
history. This is called differential diagenesis
(Reinhardt et al., 2000; Westphal et al., 2000), and
results in distinctly different characteristics for the
two lithologies: Limestone beds in such rhythmites
are largely uncompacted, as indicated by the usually
undeformed trace fossils and organic-walled
microfossils (e.g., Kent, 1936; Hallam, 1964;
Henningsmoen, 1974; Jones et al., 1979; Möller and
Kvingan, 1988; Raiswell, 1988; Westphal and
Munnecke, 1997). This clearly indicates that
cementation occurred during early diagenesis, i.e.
prior to mechanical compaction. Interlayers, in
contrast, are uncemented, resulting in higher
vulnerability to weathering. They are always
intensely compacted as indicated by their deformed
trace fossils that in many cases form diagenetic
pseudolamination (Kent, 1936; Sujkowski, 1958;
Noble and Howells, 1974; Ricken, 1986, Munnecke
and Samtleben, 1996).
This differential diagenesis most conclusively
is explained by migration of calcium carbonate into
the limestone layers where it forms cement occluding
the initial porosity, whereas no calcium carbonate is
imported into the interlayers. It is widely accepted
today that the calcium carbonate cement in the
4
limestone beds was sourced from calcium carbonate
dissolution in the interlayers (“donor” and “receptor”
limestones of Bathurst [1971]; see also, e.g., Ricken
1986, 1987). Many researchers today favor a model
for the origin of fine-grained calcareous rhythmites
where subtle differences in primary carbonate content
steer the diagenetic reinforcement that finally leads to
strong lithologic differences of the diagenetically
mature rhythmite (e.g., Reboulet and Atrops, 1997;
Schwarzacher, 2000; Hilgen et al., 2003). Other
researchers propose a purely diagenetic origin at least
for certain limestone-marl alternations (e.g., Hallam,
1964; Munnecke and Samtleben, 1996).
Two models on differential diagenesis of
fine-grained calcareous rhythmites are currently
discussed, that differ fundamentally in their
mechanisms. The now classical model of Ricken
(1986, 1987) involves pressure dissolution (chemical
compaction) of calcite in the interlayers in the deepburial environment as source of the cement for the
limestone beds. In contrast, the model by Munnecke
and Samtleben (1996) is based on dissolution of
aragonite in the shallow marine burial environment
sensu Melim et al. (1995, 2002), where aragonite
dissolution and reprecipitation as calcite cement
results from biogeochemically induced gradients in
the pore-water (Raiswell, 1988; Munnecke and
Samtleben, 1996; Munnecke et al., 1997; Westphal,
1998; Melim et al., 2002).
Both models have in common that
diagenesis takes place outside the reach of fresh
water influence, and that the soft precursor sediments
of limestones and interlayers were much more alike
in terms of carbonate contents than the diagenetically
mature limestone beds and marls. A profound
difference between both models is the availability of
carbonate cement. In the model of Munnecke and
Samtleben (1996) the amount of cement carbonate is
limited by the amount of aragonite in the precursor
sediments. When the entire aragonite in the interlayer
has been dissolved, early diagenesis comes to a halt.
In aragonite-poor sediments, for example, this
necessarily results in a low limestone/interlayer
thickness ratio of the diagenetically mature
succession (Munnecke, 1997; Munnecke et al., 2001).
In Ricken’s (1986) model, theoretically the entire
calcium carbonate present in the precursor sediment
of the interlayer layer can be dissolved by chemical
compaction.
The widely applied model of Ricken (1986,
1987) faces the problem that limestone beds are
usually uncompacted, which contradicts cementation
in the deep burial diagenetic environment (Kent,
1936; Hallam, 1964; Henningsmoen, 1974; Jones et
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
al., 1979; Möller and Kvingan, 1988; Raiswell, 1988;
Westphal and Munnecke, 1997). Petrographic
observations that have caught differential diagenesis
“in act” in Neogene sediments from the Bahama
slopes have revealed that cementation in the
limestone beds occurs very early and prior to
compaction, and at the same time, the interlayers are
strongly compacted and show signs of dissolution in
aragonitic components (Westphal, 1998).
Additionally, studies of numerous successions have
demonstrated that while all other biotic associations
in limestone beds and interlayers might be identical
in limestones and interlayers, aragonitic remains only
are preserved in limestones as molds or
neomorphoses, whereas usually no traces of
aragonitic skeletons are preserved in the interlayers
(Wepfer, 1926; Kent, 1936; Seibold and Seibold,
1953; Walther, 1982; Munnecke and Samtleben,
1996; Reboulet and Atrops, 1997), implying that the
absence of originally aragonitic material is caused by
postdepositional processes. In contrast, primary
calcitic fossils down to the size of nanofossils are
present and usually well preserved both in limestones
beds as well as in the interlayers, often with even
higher abundances in the interlayers compared to
limestones (Hemleben, 1977; Munnecke and
Samtleben, 1996; Pittet and Mattioli, 2002). These
findings argue against differential diagenesis by
calcite pressure dissolution in the deep-burial
environment and for aragonite dissolution in the early
marine burial diagenesis environment as source of the
cement in the limestone beds.
On this basis, the question arises if there is
sufficient aragonitic sediment during times of “calcite
seas” for aragonite-driven differential diagenesis.
More strikingly, why are fine-grained calcareous
rhythmites more abundant in times of “calcite seas”
compared to “aragonite seas” sensu Sandberg (1983)
(Fig. 1)? In contrast to the modern interval of an
“aragonite sea”, in times of “calcite seas” nonskeletal precipitates are thought to be predominantly
calcitic. Nevertheless, aragonite-producing organisms
flourish in “calcite times”, especially in tropical
settings, although probably less abundant or less
strongly calcified (Stanley and Hardie, 1998, 1999;
Moñtanez, 2002; Ries, 2005). Therefore, the
relatively low amounts of about 10% initial aragonite
(Munnecke et al.; 2001) required for driving
differential diagenesis in the shallow marine burial
environment have potentially been present at all
times in such tropical settings. The fact that finegrained calcareous rhythmites are much more
abundant in the rock record of times of “calcite seas”
than in that of “aragonite seas” nevertheless appears
contradictory (Fig. 1). However, “calcite times” in
5
several respects provide more favorable sedimentary
settings for the deposition of aragonite-bearing
sediment than “aragonite times”, even though seawater chemistry favors calcite precipitation. “Calcite
times” coincide with times of warmer global climate
and extensive epeiric seas – both favorable conditions
for carbonate-secreting communities including
aragonitic organisms and thus for the formation of
limestone-marl alternations. These conditions appear
to have overrun the effect of sea-water chemistry (cf.
Westphal and Munnecke, 2003; Munnecke and
Westphal, 2005).
1.2.
Environmental signals recorded in
fine-grained calcareous rhythmites
Limestone-marl alternations are usually interpreted to
directly reflect cyclic palaeoenvironmental signals.
The steering mechanisms for differential diagenesis,
however, still are a matter of debate. It is tempting to
regard primary differences in sediment composition
(e.g., carbonate content, concentration of organic
matter) as trigger. However, field observations show
that in some cases primary differences in sediment
composition do not match lithology. For example, in
Figure 2a, a shell layer passes laterally from
cemented limestone beds into uncemented
interlayers. This indicates that – at least in
exceptional cases – the same sediment (here the shell
layer) can be transformed to both a limestone bed as
well as an interlayer. On the other hand, some
cemented limestone layers consist of different
lithologies, e.g. mudstone and grainstone within a
single bed (Fig. 2b). Here, a specific part of the
primary sediment is diagenetically “united” within a
single limestone bed by cementation despite of the
strong primary differences in grain size, carbonate
content, permeability etc. between the carbonate sand
(now the grainstone) and the carbonate mud (now the
mudstone). This cemented limestone is now
underlain and overlain by carbonate-depleted
interlayers (not on the photograph). This indicates,
that differential diagenesis in this case did not strictly
follow the strong primary differences in sediment
composition but has “overrun” these differences.
Differential diagenesis has profound effects on
the interpretation of calcareous rhythmites: It not
only results in an increase in carbonate content in the
limestone beds, whereas at the same time the
carbonate content in the interlayers are impoverished.
It also accounts for differences in fossil associations
and quantities, in the quantities of non-carbonate
constituents, and in petrophysical properties
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
6
Fig. 2: Sketch showing the transformation of soft sediments to lithified rocks by differential diagenesis. (A) Detail of the Lower Visby
Formation (lowermost Wenlock, north of Högklint, Gotland, Sweden) showing that a specific sedimentary layer can be transformed to both
limestone and interlayer. (B) Detail of the Upper Visby Formation (lower Wenlock, Hallshuk, Gotland, Sweden). Despite the pronounced
primary differences both sediments have been united in one cemented limestone bed.
including porosity, permeability, and sonic velocity
(Bathurst 1987, 1991; Munnecke, 1997; Westphal,
1998; Kenter et al., 2002). Carbonate redistribution
results in a passive enrichment of insolubles (mainly
clay minerals and primary calcitic constituents) in
interlayers where aragonite is dissolved, and a
passive dilution of the insolubles in the limestone
beds by cementation. The spectrum of calcareous
macro- and microfossils in limestone beds and
interlayers therefore in many cases are not directly
comparable in terms of primary differences between
the two lithologies for two reasons. Firstly, the
preservation potential in both lithologies is markedly
different, and delicate fossils are commonly
destroyed by mechanical compaction in interlayers
(e.g., Munnecke and Samtleben, 1996). Secondly,
because of the different physical behavior of the two
lithologies, in many cases they are studied using
different methods (e.g., thin sections for the
limestone beds and sieving for the interlayers) that
are well suited for each of the two lithologies, but can
introduce artificial differences in the observed faunal
spectra (e.g., foraminifers, see Seibold and Seibold,
1953). Thus, the absolute values of many measurable
parameters are not directly comparable for the two
lithologies, and cannot be directly interpreted in
terms of primary, pre-diagenetic, sediment properties.
These diagenetic peculiarities result in the
highly controversial discussion on their origin and
7
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
interpretation that has a long tradition and is still
ongoing. Are such calcareous ABAB rhythmites the
direct result of fluctuating environmental parameters,
or are they created by diagenesis, or by a combination
of both (e.g., Semper, 1917; Seibold, 1952;
Sujkowski, 1958; Hallam, 1964, 1986; Schwarzacher
and Fischer, 1982; Ricken, 1986; Munnecke and
Samtleben, 1996; Westphal et al., 2004b)? It appears
straightforward to interpret such successions as
manifestation of changes in input of siliciclastic
sediment or primary carbonate productivity or both,
in many cases driven by orbital forcing. However, the
diagenetic changes related to redistribution of
calcium carbonate introduce a severe problem in
comparability between the lithologies. Study of
diagenetically inert parameters offers the possibility
to look behind the veil of diagenesis.
2. Diagenetically inert parameters as tool for
assessing the record in a calcareous
rhythmite
As a result of differential diagenesis, the
interpretation of many sediment parameters is far
from straight-forward. Only parameters that are
robust against differential diagenetic change are truly
reliable as indicator of primary signals being
represented in the rhythm of the alternation. Study of
diagenetically inert parameters is a robust way to
assess the environmental fluctuations recorded in the
alternating lithologies of fine-grained calcareous
rhythmites, and to avoid uncertainties imposed by
diagenetic alterations (Westphal et al., 2000, 2004b).
Two approaches for proving unequivocal primary
signals appear promising: a paleontological and a
geochemical one. We here first shortly discuss the
paleontological approach before introducing the
geochemical approach. We then present new results
of geochemical studies.
As mentioned above, there is a long-lasting
discussion in the literature whether or not a
diagenetic “unmixing” of a rather homogenous
sediment is possible. Why is it so difficult to answer
this question? To prove an environmental cause of a
sedimentary rhythm is straight-forward: If limestones
and interlayers show systematic differences in the
composition (not the quantities) of diagenetically
stable constituents, a sedimentary origin of the
rhythm is evident (Fig. 3 left arrow). However, if
such a systematic difference is not observed (right
arrow) it is, in contrast, no proof for a diagenetic
origin. In the latter case systematic differences could
have existed in sediment parameters that have not
been analyzed. They may also have been present in
primary sediment parameters that were completely
altered by the diagenetic processes, e.g., differences
in primary porosity or in the content of organic
material. In this case, absence of proof is no proof of
absence. In any case, interpretations with respect to
environmental changes such as orbital forcing should
not be undertaken until a sedimentary origin of the
rhythm is unequivocally proven (Hallam, 1986). This
holds also for frequency analyses, which are no proof
for a primary, environmental cyclicity (see below).
2.1.
Paleontological approach
Diagenetically inert parameters such as palynomorph
assemblages (organic-walled microfossils, e.g.
acritarchs, dinoflagellate cysts, etc.) potentially
survive alterations imposed by typical early carbonate
diagenesis, except for strong oxidation (Lind and
Schiøler, 1994). Systematic differences in the
qualitative composition of the assemblages in
limestone beds and interlayers would clearly prove
rhythmic changes of environmental conditions.
Whereas many calcareous microfossils can be
destroyed by compaction in the interlayers,
simulating primary differences, palynomorphs are
generally flattened in the interlayers, but usually are
not destroyed, allowing for comparability of the
assemblages in both limestone beds and interlayers
(Westphal and Munnecke, 1997). Strong oxidation
that could result in modification or even destruction
of the palynomorph content can be determined by the
presence/absence of amorphous organic matter
(AOM) because this part of the organic residue is
most vulnerable to oxidation. Palynological bed-bybed studies in fine-grained calcareous alternations are
rare to date because (a) palynologists usually are
more interested in long-term trends and therefore
sample at lower resolution, and (b) palynologists
prefer sampling interlayers because the abundance of
palynomorphs normally is higher in interlayers
compared to limestones (less acid required for
preparation). In the few existing studies, however,
changes in the marine palynofacies are not related to
the cyclical lithology (Bergman, 1987; Brenner,
1988; Courtinat, 1993; Westphal et al., 2000, 2004b;
Holstein, 2004).
2.2.
Geochemical approach
Early marine burial diagenesis usually affects certain
chemical components of sediment much less than
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
8
Figure 3: Schematic drawing illustrating the “diagenetic dilemma”: Whereas a primary origin of a rhythmic deposit can be proven by
systematic differences in the ratio of diagenetically stable components (e.g. differences in the palynomorph communities), it is
impossible to prove a diagenetic origin. Even if no differences are observed in such ratios a primary origin of the rhythm is still possible
because differences could have existed in either parameters that simply have not been measured, or in parameters that are completely
altered by diagenetic processes (e.g. porosity, or content of organic matter).
calcium carbonate components. In particular the
components that are part of the siliciclastic portion of
the sediment are less mobile than those of calcareous
constituents. Many clay minerals and heavy minerals,
such as rutile, usually are left unaltered by the early
diagenetic processes that lead to the dissolution of
aragonite and reprecipitation of calcite (Bausch,
1994; Bausch et al. 1994; Deconinck et al., 2003).
This diagenetic inertia offers a tool for assessing
environmental conditions such as subaerial
weathering in the provenance area of the siliciclastic
portion of mixed successions such as limestone-marl
alternations. Therefore, diagenetically inert elements
bound to these minerals (e.g. Ti, Al) potentially
preserve information on environmental conditions.
However, the absolute concentrations of these
elements are shifted during diagenesis by dissolution
of calcium carbonate in the interlayers and
cementation in the limestones. The values therefore
need to be normalized independently of calcium
carbonate content; and the ratio of the concentrations
of such elements has to be studied (e.g.,Ti/Al).
Clay minerals represent the end product of
continental weathering, and are ultimately transported
into sedimentary basins. The climatic conditions and
the composition of the rock being weathered have a
profound influence on the type of clay being formed
(Visser, 1991; Chamley, 1998; Thiry, 2000; Net et
al., 2002; Ruffell et al., 2002; John et al., 2003). In a
setting where alternating environmental conditions
generated a rhythmic succession of limestone beds
and marl interlayers, these changing conditions may
be reflected in a unique chemical composition of each
lithology. Subsequent early marine burial diagenesis
may alter the bulk elemental percentages by adding
or removing CaCO3, but will not affect the
characteristic element ratios. Plotting two such
element percentages (e.g. Al2O3 and TiO2) in a X/Y
chart will show two separate populations of data
points corresponding to the two lithologies, each
following a different trend line. In particular the ratio
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
between Al2O3/ TiO2 appears suitable for detecting
primary differences in the sediment, because the
titanium concentration shows a wide range for
different clay minerals. Other elements are less
suitable for this approach, e.g., elements such
potassium that can adhere to clay minerals, show a
less stable behavior during early diagenesis. In the
literature, this rather simple test for a primary, nondiagenetic signal in a fine-grained calcareous
rhythmite, however, is rarely applied, and systematic
geochemical studies focusing on the ratios of
diagenetically stable constituents are rare (e.g.
Bausch, 1992, 2001, 2004; Bausch et al., 1994;
Biernacka et al., 2005; Niebuhr, 2005).
3. Geochemical analyses
Several successions of fine-grained calcareous
rhythmites were studied with this geochemical
approach, based on own data or on published data
sets, by analyzing the ratios between Al2O3 and TiO2
concentration. In order to investigate the reliability of
this method we compare “classical” calcareous
rhythmites with a section that clearly reflects an
environmental rhythm (“Trubi” Formation):
(1) The “Trubi” Formation; lower Pliocene; Punta di
Maiata, Sicily. In outcrop, the “Trubi” Formation
shows grey-white-beige-white quadripartite colour
cycles. The grey layers consist of bioturbated marl
with an average carbonate content of 67%, whereas
the beige coloured layers represent indistinctly
laminated marls with an average carbonate content of
61%. The whitish layers, separating the two coloured
marl types, have slightly elevated carbonate contents
(average 72%; own unpublished data; cf. Nijenhuis et
al., 1999). A primary, environmental origin of the
alternating lithologies is evident from the differences
in bioturbation and lamination between the grey and
beige marls. Additionally, the two different marl
types are clearly distinguished in the geochemical
data that show a bimodal distribution in their Al/Ti
ratios (Fig. 4A). The white layers are less well
defined by Al/Ti ratios. They may represent a variety
of the marls that was diagenetically enriched in
calcium carbonate. Both, sedimentological and
geochemical data unequivocally prove a primary,
non-diagenetic origin of the cyclicity in the “Trubi”
Formation.
(2) “Classical” limestone-marl alternations of
Permian, Jurassic, and Cretaceous age. Without the
development of two different populations, the
Al2O3/TiO2 ratios of five "classical" limestone-marl
alternations and one Plattenkalk succession (Fig. 4B
9
“Weltenburg”) do not reflect a primary sedimentary
cyclicity (Fig. 4B). The data in each of the
successions plot on a single regression line with high
linear correlation coefficients (average r2 = 0.98).
This means that even though limestones are
characterized by lower contents of Al2O3 and TiO2
compared to the interlayers, both lithologies are
indistinguishable in terms of the ratio of these
diagenetically stable constituents. This uniformity of
Al/Ti ratios in limestones and interlayers was
confirmed by an additional statistical test. Assuming
that Al/Ti ratios are equal in limestones and
interlayers and that the observed variability is mainly
due to random variations, the Al/Ti ratios are
expected to be normally distributed. As shown in the
histogram insets in Figure 4B this appears to be the
case; in contrast to the clearly bimodal distribution of
the "Trubi" Formation (Fig. 4A). The probabilities
that the measured Al/Ti frequency distributions
conform to a normal distribution are high in all cases,
except for the "Trubi" Formation (Fig. 4). Only in
this latter case we can reject the hypothesis of a
normal Al/Ti distribution (goodness-of-fit ! 2 test;
Davis, 2002). Clearly, this is no proof that the
rhythmic successions summarized in Figure 4B
represent entirely diagenetic rhythms. It only shows
that a primary origin of these rhythms is not proven
by the measured parameters (according to Fig. 3,
right arrow).
4. Computer simulations
4.1.
Simulations as a tool to assess postdepositional distortions
To gain a better understanding of the dynamics and
complex effects of differential diagenesis, we created
a computer model that implements in a schematic
way the diagenesis model of aragonite dissolution
and calcite reprecipitation described above. The
crucial questions to be answered by the model
approach are: What had diagenetically overprinted
successions looked like prior to diagenesis? Is a
rhythmic variation required in the original
composition of the sedimentary succession, or can
differential diagenesis generate a cyclic succession
from homogeneous sediment? Can differential
diagenesis transform random variations in the pristine
sedimentary composition into a cyclic succession?
On the other hand, if there were a cyclic succession
of pristine sediments, how would differential
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
10
Fig. 4: Al2O3/TiO2 data from different sections. (A) Data from “Trubi” Formation (S-Sicily) plot on two separate trend lines, indicating a bimodal
chemical composition of the non-carbonate fraction, and, thereby varying environmental conditions.. (B) Data from several calcareous
rhythmic successions (own and published data) with a linear correlation. Frequency distributions of the Al/Ti ratios are shown in a histogram
for each section. A normal distribution is expected if the Al/Ti ratios vary randomly about a single mean value. More complex variations,
producing a multimodal distribution, are expected for non-random (e.g. rhythmic) fluctuations. The probability (p) that a measured Al/Ti
frequency distribution conforms to a normal distribution was calculated with a goodness-of-fit !2 test (Davis, 2002). The p values allow only
for the “Trubi” Marls to reject the hypothesis of a normal Al/Ti distribution.
diagenesis alter the original cyclic signal? We here
employ a two-dimensional computer simulation of
the relevant aspects of differential diagenesis
(aragonite dissolution, calcite reprecipitation;
compaction) to answer these questions. This
approach allows us to study lateral as well as vertical
processes.
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
4.2.
Approach and method
The computer simulation is based on a cellular
automaton. Space is divided into cells on a
rectangular lattice, where each cell models a
macroscopic volume of the sediment such that the
whole field can represent a geologic succession with
vertical and lateral dimensions. The interaction
between the cells implements the aragonite
dissolution and calcite reprecipitation dynamics of
the rhythmic diagenesis model by Munnecke and
Samtleben (1996) described above. It thus simulates
the diagenetic redistribution of calcium carbonate
during early diagenesis in the shallow marine burial
environment. The computer model originally has
been designed to produce output in the time domain,
i.e., for constant sedimentation rates and without
compaction (Böhm et al., 2003; Westphal et al.,
2004a). Here we present a modified version where
compaction has been implemented. A brief
description of the computer model is given below; for
a detailed description see Böhm et al. (2003) and
Westphal et al. (2004a).
Similar to standard stratigraphic simulations,
the model is a cellular automaton model setup
consisting of a rectangular matrix of cells. Here a
version is employed, where each cell can assume
only discrete states. In our case the model requires
three possible states of a cell (Fig. 5): (1) The first
11
state, “aragonitic”, represents a pristine sediment
consisting of aragonite, calcite and clay minerals; (2)
The second state, “non-aragonitic”, represents either
a pristine, originally aragonite-free sediment, or a
diagenetically altered, aragonite-depleted sediment
consisting of calcite and clay minerals; and (3) the
third state, “cemented”, represents sediment in which
calcite cement is added to the pristine (aragonitic or
non-aragonitic) sediment. In the pristine sediment,
“aragonitic” and “non-aragonitic” cells are randomly
distributed in each new layer. For implementing a
time-series signal, the percentage of “aragonitic” cells
in the pristine layers is varied randomly or
systematically. E.g., to illustrate the effects of
rhythmic diagenesis on a cyclic sedimentary
succession we use the 65°N summer insolation curve
of Berger and Loutre (1991) to which we couple the
percentage of “aragonite” cells in a layer. The
insolation curve includes the influences of
precession, obliquity, and eccentricity, i.e., the basic
“Milankovitch parameters”. It represents a complex
cyclic signal that has variance in a range of different
frequency bands of geologically meaningful
wavelengths.
The model diagenetic environment is layered
into several zones (Fig. 5): (i) The uppermost layer is
a non-reactive zone below the sediment-water
interface. (ii) The zone underneath is a cementation
zone, where calcite cement is precipitated. (iii)
Further below, a non-reactive zone may be present
Fig. 5: (A) Simplified diagenesis model for the computer implementation. Sediment is progressively buried and moves through stable diagenetic
zones where aragonite dissolution and calcite cementation take place. Time step I: layer B is cemented by calcite cement derived from
dissolution of aragonite in layer A. Time step II: The cemented layer B enters the aragonite dissolution zone, but the aragonite present in
layer B is “sealed” by cement, so little to no dissolution takes place; therefore layer C is not being cemented. Time step III: The
uncemented layer C enters the aragonite dissolution zone, aragonite is dissolved and reprecipitated as calcite cement in layer D. This model
demonstrates the possibility of self-organized diagenetic patterns. (B) Cellular automata model for diagenesis simulations. With each time
step, cells representing the sediment move one layer downward. Three possible states of cells are: 1 “aragonitic”: consisting of aragonite,
calcite, and terrigenous material; 2 “cemented”: calcite cement is added; 3 “non-aragonitic”: consisting of calcite and terrigenous material,
either originally aragonite-free, or diagenetically aragonite-depleted. As a layer passes through the aragonite dissolution zone, aragonite is
removed from state 1 cells. In this simulation run, for each cell where aragonite is dissolved, two cells in the cementation zone are
cemented. An insolation signal is implemented by varying the percentage of “aragonitic” cells (after Böhm et al., 2003 and Westphal et al.,
2004).
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
12
that is situated between the cementation zone and (iv)
the underlying aragonite dissolution zone. (v) Finally,
a non-reactive “historical zone” follows where no
further shallow marine-burial diagenesis takes place.
In the model, the thickness of the individual zones
can be varied. In the dissolution zone, “aragonitic”
cells are transformed to “non-aragonitic” cells. In the
cementation zone, “aragonitic” and “non-aragonitic”
cells are transformed to “cemented” cells. The
number of the latter is limited by the number of cells
that are dissolved in the dissolution zone. Layers of
dominantly “cemented” cells form diagenetically
mature limestone beds, whereas layers of dominantly
“non-aragonitic” cells form diagenetically mature
marl interlayers. For the simulations shown here, two
cells are cemented per dissolved aragonite cell. In all
sequential operations, cells are accessed in random
order to avoid artificial anisotropies. The simulation
output is a sediment column with cemented and
aragonite-free cells (Figure 6 gives an example of a
simulated succession).
Earlier simulations with the computer program
used here (Böhm et al., 2003; Westphal et al., 2004a)
did not consider further distortion of the primary
signal by differential compaction. Here we extend the
model with the ability to simulate differential
compaction of the sediment column. Differential
compaction generates uncertainties in the depth-totime transformation (Hinnov, 2000). In fine-grained
calcareous rhythmites cemented limestone beds
usually are not compacted whereas interbeds always
are compacted (see above). This early compaction is
a combination of loss of primary porosity and volume
loss by aragonite dissolution. A section measured in a
rock succession, i.e., in the depth domain, therefore
cannot be directly fed into frequency analysis, i.e.,
into the time domain. Differential compaction in a
fine-grained calcareous rhythmite distorts any
primary signal. Decompaction calculations that
distinguish between the two intercalated lithologies
are required to original cellular automaton model is a
succession of layers that, as sedimentation rates are
held constant, represent equal time of sediment
accumulation. The compaction algorithm uses as
input the average aragonite content of each individual
layer, measured before diagenetic alteration, and the
average cement content of the same layer, measured
after diagenetic alteration. The thickness of each
layer is then reduced according to these two variables
as follows. Layers with cement content above a
specified threshold value, Ct (40% in the standard
calculation, 70% in the enhanced-compaction
experiment), are not compacted. The thickness of all
layers with less cement is reduced by 20% to account
for mechanical compaction of primary pores.
Fig. 6: Example of simulated diagenetically mature fine-grained
calcareous rhythmites. The modeled sequence includes a
Milankovitch signal (65°N summer insolation spectrum;
Berger and Loutre, 1991) that is distorted by the diagenetic
overprint. Left: Milankovitch signal fed into the simulation.
Right: sedimentary column of cemented “limestone beds”
and uncemented “interbeds”. Conspicuous intervals with
decoupling of neighboring cell domains and subsequent
reorganization of well-defined layering are indicated on the
right. (Parameters used for simulation: diagenetic cycle
period: 50 ka; distance between cementation and dissolution
layer: 25 layers; simulation time step: 1 ka (1 layer per ka);
simulated time: 3 Ma (3003 layers.)).
13
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
Additional compaction of these layers results from
volume loss due to the dissolution of aragonite. In our
experiments we define that aragonite dissolution
leads to a compaction of 75% of the aragonitic
portion of each layer in addition to the 20%
mechanical compaction. The remaining 5% account
for non-dissolvable residuals including primary
calcite and non-carbonate materials. As a result, the
maximal compaction of an uncemented aragonitic
layer is 95%, which would correspond to a parting
between limestone beds. Slight early cementation of
such layers reduces compaction and is accounted for
by subtracting the cement portion from the aragonite
portion. The compacted layer thickness (CLT) is
therefore calculated as
CLT = 1 - k * (0.2 + 0.75 * (ara - cem))
where the initial layer thickness is 1, k is a
scaling factor, ara and cem are the aragonite and
Fig. 7: Power spectra of simulated rhythmic sequences (bold lines) overprinted by different diagenetic cycle frequencies compared to primary
signal (65°N summer insolation spectrum; Berger and Loutre, 1991) (fine lines). The original frequency spectra are modified in four
different ways: (A) Amplification: Minor peak in the insolation spectrum (11 ka) is amplified by resonance with the diagenetic cyclicity.
(Note: diagenetic cement spectrum shows harmonics due to the rectangular shape of the diagenetic cycles.) (B) Emergence: spectral peaks
(114 ka and 12 ka) not present in the input signal emerge. (C) Suppression: precession frequencies are suppressed. (D) Displacement:
obliquity cycle period (41 ka) displaced towards shorter wavelength. MEM=maximum entropy method. (From Westphal et al., 2004a)
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
cement content in vol%, respectively, and ara > cem.
For ara < cem the last term is set to zero. The scaling
factor k was varied between 0 (no compaction) and 1
(full compaction) to study the impact of increasing
compaction intensity on the recorded signal.
The cement content of a compacted layer has
to be recalculated, because this value represents a
volume-based percentage in our model and therefore
changes with compaction:
cemc = cemi / CLT
where subscripts c and i refer to compacted and
initial layers, respectively.
The compaction procedure results in a
sequence of layers of non-uniform thickness and
varying cement content. To generate an equal-spaced
14
series of cement content values for frequency
analysis (which corresponds to equal sampling
distance in real-world successions), the sequences
need to be resampled. Frequency spectra of the
resulting sequences were analyzed with the maximum
entropy method (MEM) using the SSA-MTM toolkit
of Ghil et al. (2002).
4.3.
Results of earlier simulations without
compaction
In simulations with the diagenetic cellular automaton
model, differential diagenesis generates laterally
extended layers of fine-grained calcareous rhythmites
only where a primary sedimentary signal
synchronizes the oscillating cells to form continuous
beds. Otherwise, randomly distributed nodules form
due to the limited horizontal coupling of the
Fig. 8: Effects of varying compaction on the frequency spectra of modeled fine-grained calcareous rhythmites. Simulated sediment column is
given in Figure 6, uncompacted frequency spectrum is shown in Figure 7C. Compaction further disturbs the spectrum. Ten spectra are
plotted in each panel with compaction intensity (k) increasing from top to bottom. Corresponding peaks are connected with grey bars. With
increasing compaction spectral peaks are shifted towards higher frequencies. (A) Standard compaction scenario with 40% specific
threshold value Ct (for explanation of this value see text). (B): In the enhanced compaction scenario (Ct = 70%) an additional peak occurs at
a periodicity of 19-20 ka at high compaction intensities (k>0.4). High-frequency peaks (periods <11 ka) show no consistent trends with
increasing compaction. Spectra are calculated in the linear depth domain. Peaks are labeled with the corresponding time domain
periodicities.
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
diagenetic behavior of neighboring cells (Böhm et al.,
2003). With typical diffusion coefficients and
compactional flow velocities for fine-grained
sediments, diffusive transport prevails only at
distances of less than a few meters (Berner 1980). At
larger distances transport will be dominated by the
vertical compactional flow. This limits the horizontal
range of chemical interactions to scales that are much
smaller than those typical for the laterally extensive
beds of fine-grained calcareous rhythmites.
An external signal, implemented in the
model by variations in the aragonite portion,
synchronizes the diagenetic bedding; even a random
or low frequency external signal imposes sufficient
control to synchronize the high-frequency couplets.
Temporal decoupling of neighboring cell domains is
shown in Figure 6 (middle and upper part of the
column). Phase offsets between neighboring cell
domains are caused by small random disturbances,
either when the primary signal becomes too weak or
when the diagenetic cycle and the primary signal
have different frequencies and get out of phase.
Synchronization by a strong primary signal, in phase
with the diagenetic oscillator, can reorganize the cell
domains to form laterally extended homogenous
layers (Fig. 6).
Thus, according to our simulations and
general considerations, a primary sedimentary signal
is a necessary prerequisite for generating laterally
extensive fine-grained calcareous rhythmites. A
purely self-organized origin of laterally extensive
beds is not possible with the currently available
diagenetic concepts. On the other hand, our
simulations show that differential diagenesis is able
to distort the primary sedimentary signal in a number
of ways. A primary (regular, random, or
Milankovitch) signal undergoes dramatic changes in
character during diagenesis. The frequency of the
signal not only may be shifted (Böhm et al., 2003),
but in addition new frequencies may emerge, other
frequencies may be suppressed, and some frequencies
may be amplified while others are reduced (Fig. 7;
Westphal et al., 2004a). These distortions of the
original frequencies have serious implications for the
interpretation of diagenetically mature fine-grained
calcareous rhythmites. Carbonate contents and
lithology (limestone beds versus interlayers) cannot
directly be interpreted in terms of frequencies of
original environmental signals without independent
proof (paleontological, geochemical) of the primary
origin of the intercalated lithologies.
15
4.4.
New simulations: the effect of
differential compaction
In addition to the distortions of the primary
sedimentary signals by dissolution and cementation,
differential compaction further distorts the spectral
characteristics of fine-grained calcareous rhythmites.
This compaction affects individual limestone and
interlayers with different intensities depending on the
primary aragonite content and the degree of
precompactional cementation. Therefore the
distortions are not removed by simple linear
decompaction algorithms. In addition, bio- or
magnetostratigraphic resolution is usually not
sufficient to resolve differential compaction effects,
so they cannot be accounted for in time-depth
models.
A straightforward effect of the decreased thickness of
interlayers due to compaction is that the spectral
peaks are shifted towards shorter periodicities with
increasing compaction intensity (Fig. 8). Higher
frequencies are generally more affected than lower
ones, leading to an increasing distortion of the
frequency ratios with increasing compaction. This is
particularly relevant as these ratios are often used by
sedimentologists to identify Milankovitch cyclicities
in sedimentary sequences. The most prominent
example for such frequency ratios is the 1:5 bundling
of the eccentricity/precession relationship that is
often used as a spectral Milankovitch fingerprint
(Schwarzacher, 2000). Finally, with very strong
compaction, spurious spectral peaks may emerge,
further obscuring the signal (Fig. 8). In summary, our
simulations show that a mature limestone-marl
alternation may constitute a very poor representation
of its original sedimentary precursor sequence,
especially with respect to its frequency
characteristics.
5. Conclusions
In this article we reviewed calcareous
rhythmites (including the “classical” limestone-marl
alternations) and their diagenesis, both, from the
perspective of geological models as well as with the
aid of a computer simulation that makes visible the
spatio-temporal dynamics of the diagenesis model.
Fine-grained calcareous rhythmites have undergone
differential diagenesis. Cemented, uncompacted
limestone beds are intercalated in uncemented,
compacted interlayers. Despite the distinct cyclicity
Westphal et al. “Limestone-marl alternations in epeiric sea settings”
16
seen in outcrop or core, however, it is surprisingly
difficult to determine the presence of an
environmental trigger forcing or underlying the
lithologic intercalation. Differential diagenesis
modifies many sediment parameters in different ways
in limestone beds and interlayers, destroying
comparability. The investigation of diagenetically
inert parameters including ratios of trace elements
proves an important tool for approaching this
question, because these ratios are not altered by
differential diagenesis. Clearly, searching for
unequivocal fingerprints of primary signals is
essential for interpreting such successions, e.g., in
terms of orbital forcing and chronostratigraphy.
Frequency analyses are only reliable for successions
where a sedimentary origin is clearly established, and
where the differential compaction of limestone beds
and interlayers can be accounted for. Surprisingly,
none of the investigated alternations shows
unequivocal evidence for environmental changes
between limestones and interlayers.
understood for reliably interpreting fine-grained
rhythmic carbonate successions.
The simulation results emphasize the possible
dramatic effects of differential diagenesis, i.e.,
aragonite dissolution in interlayers and cementation
in limestone beds, in overprinting and even
generating lithologic alternations. Diagenetic
redistribution of calcium carbonate and differential
compaction has the potential to severely distort any
primary signals recorded in the sedimentary column.
The processes of differential diagenesis potentially
result in amplification, displacement, suppression,
and even in emergence of new cycle frequencies.
Cyclic external input is not needed for the formation
of cyclically alternating limestone beds and
interlayers. Stochastic fluctuations in environmental
parameters appear sufficient for the production of
laterally extensive beds over distances much larger
than nodule size. If external periodic signals are
recorded in the pristine sediment, these signals may
be severely distorted in the diagenetically mature
succession. The simulations imply that even though
primary, external fluctuations are required for the
formation of cyclically intercalated limestone beds
and interlayers, the original frequency spectrum of
such primary signals may not survive differential
diagenesis. Compaction additionally modifies the
frequency spectrum of cycles. The thoroughly
dynamic processes of diagenesis thus strongly
influence the distribution of the interlayered
lithologies in fine-grained calcareous rhythmites in a
non-trivial way where new spectral peaks may
emerge that do not carry any information on primary,
environmental, frequencies. Diagenetic effects are far
more than a “bother” that can be neglected – they
have to be critically examined and ideally should be
Bathurst, R.G.C., 1971, Carbonate Sediments and
Their Diagenesis: Amsterdam, Elsevier,
Developments in Sedimentology, v. 12, 620 p.
Acknowledgements
This study was supported by the German Science
Foundation DFG (Fr-1134/4 und We-2492/1) and by
a HWP grant of the University of Erlangen/State of
Bavaria to HW. We gratefully acknowledge critical
comments by Paul Myrow and Mario Coniglio. The
geochemical data of the “Trubi” Formation samples
were produced as part of Martin Wolf’s Master’s
thesis.
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