THE EFFECTS OF DIFFERENTIAL DIAGENESIS ON DATA EXTRACTED FROM LIMESTONE-MARL ALTERNATIONS – EXAMPLES FROM THE ORDOVICIAN OF ANTICOSTI, CANADA (Invited Presentation)

Limestone-marl alternations (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 marginal to deep marine. Their rhythmicity in lithology 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 palaeoenvironmental conditions. However, several studies suggest a diagenetic origin of LMA, caused by an autogenic oscillating process in the shallow subsurface of aragonite dissolution and reprecipitation as calcite in adjacent layers called differential diagenesis.

Here, the effects of differential diagenesis on lithological rhythmicity, fossil preservation and geochemical proxies will be presented including examples from the Ordovician of Anticosti (Canada): (1) The loss of aragonitic fossils in marls causes both an enrichment of calcitic fossils and a stronger alignment due to compaction in marls. Comparing the factor of enrichment and the orientation of calcitic fossils in marls and limestones allows to differentiate the origin of LMA in fossil rich successions. (2) Growth interruptions of synsedimentary growing colonial calcitic organisms cross-cutting their circumambient LMA do not match the change in lithology and reveal time spans for limestone-marl couplets of only few tens of years. (3) The redistribution of CaCO₃ enriches or dilutes the concentration of diagenetically inert trace elements in marls and limestones, respectively. Calculating the difference in the concentration of diagenetically inert trace element ratios allows to identify variations of calcite and/or aragonite input and to identify phases of non-variation in the precursor sediment of LMA, i.e. a pure diagenetic origin.

The results are a warning signal against the underestimation of early diagenetic CaCO₃ redistribution. Nevertheless, the approaches provide tools to test the diagenetic origin of LMA, to assess bias in fossil preservation and geochemical proxies, and to identify original variations in aragonite and calcite in the precursor sediment.