Limestone-marl alternation genesis on trial: testing an environmental proxy

In this doctoral thesis, the debated origin of limestone-marl (LMA) alternations is surveyed in order to improve their reliability for palaeoenvironmental interpretations. LMA are rhythmically alternating beds of relatively CaCO3-rich, solid limestones, and relatively less CaCO3-rich and less solid marls. The alternations vary in dominance of limestone or marl, manifestation of beds from well-bedded to nodular, and their depositional setting from lagoonal to deep marine. Their rhythmicity is often taken as evidence for cyclic sedimentation due to periodic climate change forced by e.g. Milankovitch cycles. LMA are thus commonly interpreted as a direct record of palaeoclimate and used to reconstruct palaeoenvironmental conditions. However, several studies suggest a diagenetic origin of LMA, implying possible misinterpretations of palaeoenvironmental conditions. Two models are discussed with respect to the origin of LMA. The first model assumes primary sedimentary differences caused by environmental change, which are enhanced by late diagenetic CaCO3 redistribution by pressure dissolution of calcite. In contrast, the model of differential diagenesis assumes early diagenetic CaCO3 redistribution by aragonite dissolution and reprecipitation as calcite cement, possibly also in homogenous precursor sediments. This CaCO3 redistribution enriches the CaCO3 content of the future limestones and lithifies them, while the marls are relatively depleted and less lithified. Despite of the recognition for both models in the scientific community, the origin of LMA could not be unequivocally solved yet. The aim of this doctoral thesis was to identify the origin of six selected LMA testing the two models, and to develop methods to untangle the process of LMA formation. For this purpose, a combination of methods from taphonomy, sedimentology, and geochemistry was used. In several cases, an early diagenetic overprint as well as a pure diagenetic origin could be demonstrated with newly introduced methods. The factor of enrichment and the orientation of calcitic fossils in marl relative to limestone allowed to differentiate between the diagenetic models in fossil rich successions. The comparison of the life span and growth interruptions of synsedimentary growing colonial calcitic organisms, with the change in lithology in a circumambient LMA allowed to reconstruct the diagenetic history and actual time span of LMA couplets, questioning the completeness of the geologic record. The vector lengths as measure for CaCO3 redistribution in combination with the CaCO3 concentrations in limestone and marl, turned out to be particularly suitable to identify a LMA origin, because they detect original variations in sedimentary input. The combination of X-ray diffraction analyses and thin section analyses of lithified and unlithified monospecific gastropod accumulations allowed to detectprimary changes in the depositional environment and their mismatch with the change in lithology. Despite the variety of LMA manifestations which require flexible methods to determine their origin, differential diagenesis seems to be the driving mechanism for LMA formation. Further late diagenetic overprint is possible, though the differentiation into limestone and marl happens earlier and often not in concordance with primarily depositional changes. The results of this doctoral thesis are a clear warning signal against the underestimation of diagenetic transformations from the precursor sediment to the mature LMA.