Shallow-water aragonite recorded in bundles of limestone-marl alternations-the Upper Jurassic of SW Germany

摘要

Characteristically, limestone-marl alternations are conspicuous in outcrop because of differential weathering, the marls being predisposed to stronger deterioration. Superimposed on the bedding rhythms are in many cases lower-frequency meter-scale rhythms, so-called bundles. These bundles are eye-catching from a distance, and are typified by variations in thickness of limestone beds and marl interbeds. The palaeoenvironmental significance of the bundles received markedly less attention in the literature than the origin of the small-scale couplets. Here we focus on the palaeoenvironmental significance of the bundles by reconstructing the initial mineralogical composition. As an example, we present a classical limestone-marl alternation from the Upper Jurassic (Oxfordian-Kimmeridgian) of SW Germany that is typified by two orders of bundles superimposed on the limestone-marl couplets. Our method of reconstruction 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. Calculated initial clay, aragonite, and calcite contents show rhythmic variations on a scale of several meters that correspond to the bundles as seen in the outcrop. Limestone-dominated parts of the bundles correspond to intervals of elevated aragonite content in the precursor sediment, but not necessarily to elevated calcite contents. Additionally, an overlain fluctuation in initial mineralogy on the order of tens of meters (mega-bundles), reflecting the former local subdivision of the SW German Upper Jurassic into "Weissjura alpha", "beta", and "gamma", is observed. The limestone-dominated "Weissjura beta" (lower Kimmeridgian) is the interval with highest initial aragonite content. Our new 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. In the Jurassic example, the observed variations in reconstructed aragonite content are interpreted as variations in flooding events and related export of aragonitic material from marginal shallow-water platforms.