(Axel Munnecke & Hildegard Westphal) Phanerozoic limestone-marl alternations are known from shallow-water to deep-marine settings, and are in most cases interpreted as archives of short-term (Milankovitch-range to millennial-scale) palaeoclimatic fluctuations. The eye-catching appearance of limestone-marl alternations in outcrop is caused by differential weathering as a result of differential diagenesis, where usually tightly cemented limestone layers are interbedded in less resistant, little to uncemented marl layers. This differential diagenesis of limestone-marl alternations makes a fundamental difference with other sedimentary deposits. Differential diagenesis of limestone-marl alternations is a process of redistribution of calcium carbonate from marl layers to limestone beds by dissolution, migration of ions, and reprecipitation. Reconstructing of the original mineralogical composition of the precursor sediment of limestone-marl alternations is based on a diagenesis model that assumes dissolution of aragonite and reprecipitation as calcite cement as the major mechanism of early diagenesis. The method allows for quantification of initial mineralogical composition. As aragonite and calcite usually have different sources (shallow-water benthic versus planktic), this approach potentially adds significant new possibilities for palaeoceanographic interpretation of calcareous rhythmites.
The reconstruction is based on a model for differential diagenesis of limestone-marl alternations as a process of aragonite dissolution and calcite reprecipitation (Munnecke and Samtleben, 1996). The basic assumption of the model is that the source of carbonate cement precipitating in limestone layers is derived from dissolution of aragonite in the marl layers (Munnecke and Samtleben, 1996; Munnecke, 1997; Munnecke et al. 1997, 2001; Westphal et al., 2000, in press). Consequently, the amount of aragonite present in the precursor sediment limits the amount of calcium carbonate available for redistribution and cementation. Differential diagenesis comes to a halt when the aragonite has been completely dissolved. This relationship opens the possibility for mass balances and, thus, for calculating initial contents of aragonite, calcite, and insolubles. In the approach presented here, we modify the mass balance approach in order to gain vertical resolution of fluctuations in the precursor mineralogy. For the mass balance calculations, several simplifications
are made. For example, it is assumed that the diagenetic system is completely
closed, aragonite is the only carbonate phase that is dissolved, and primary
differences between limestones and marls are only minor or absent. The
assumption of a nearly closed system is justified by the low permeability
of fine-grained sediment typical of most limestone-marl alternations. Limestone
layers are assumed to be largely uncompacted. Due to the differential diagenesis
that has overprinted most limestone-marl alternations, primary differences
between limestones and marls are difficult to prove. The composition of
calcareous fossil assemblages might be of limited use to determine primary
differences where, in order to extract a microfossil assemblage, hard limestone
layers have to be treated in a different way than the much softer marl
layers. A prerequisite for the validity of the diagenetic model is that
primary calcitic skeletal grains, such as coccoliths, are more abundant
in marl layers than in limestone layers. This is the expected result of
diagenetic calcium carbonate redistribution by aragonite dissolution in
marl layers and calcite cementation in limestone beds (regardless of possible
primary differences in the assemblages). The early diagenetic processes
of aragonite dissolution and calcite precipitation result in mathematical
relationships between thickness of limestone and marl layers and their
carbonate content. These relationships can be visualised as ternary ACT
plots (Fig. 1, 3; Aragonite, Calcite, Terrigenous
material). These mathematical relationships provide a basis for the numerical
determination of initial portions of aragonite, calcite, and terrigenous
material of a diagenetically mature limestone-marl alternation based on
thickness and carbonate content of the individual layers. In these ternary
plots the parameters measured in the field and laboratory (thickness of
individual beds and interbeds, and carbonate contents of limestones and
marls) are plotted as isolines (Fog. 1). If these isolines meet in or close
to a common point, the mineralogical composition of the precursor sediment
can be graphically determined. For successions where the isolines fail
to intersect close to a common point, processes other than those underlying
our model have taken place and the model cannot be applied.
In the present study, this graphical approach has been extended by implementing the mathematical relationships between precursor sediment and diagenetically mature limestone-marl alternation in a simple computer program. This program iteratively determines the initial mineralogical composition (Fig. 2). Input parameters for the calculation are: (1) thickness as measured in the field or core, (2) average carbonate content, and (3) average porosity for each individual limestone and each individual marl layer. Where porosity measurements are not available, porosity can be estimated with the empirical equation of Ricken (1986, p. 149). .
The initial aragonite content and the pre-cementation porosity are unknown and are determined by iteration. Because the marls are a compacted residual sediment depleted in aragonite, the calcite-clay ratio of the diagenetically mature marls is identical to the ratio in the precursor sediment, and thus is given by the carbonate content of the marl. Therefore, the initial calcite content can be calculated for each run. When the calculated carbonate content of the limestone and the calculated marl-limestone thickness ratio match the measured values of the succession studied, the iteration stops. Then the values of the other primary parameters (initial contents of aragonite, calcite, and terrigenous material, pre-cementation porosity, and compaction rate of the marls) are unequivocally determined and listed. In many ancient successions, the calculated initial porosity is lower than typical porosities of comparable modern sediments in the shallow burial realm (few dm to m below the sea floor). As discussed by Munnecke et al. (2001), this offset is probably due to a limited loss of calcium carbonate in the early diagenetic system that is not, as taken for the calculation, completely closed. .
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