The mineralogical composition of precursor sediments of calcareous rhythmites: a new approach

Previous studies in Silurian carbonates from Gotland (Sweden) have led to a model for the development of limestone-marl alternations. This model postu lates that early diagenesis of precursor sediments without strong primary d ifferences can result in a differentiation by selective dissolution of arag onite in marl beds and reprecipitation of calcite cement in limestone beds. This model is described as a set of mathematical equations that quantify t he diagenetic processes (aragonite dissolution and calcite reprecipi- tatio n) that occur during the formation of limestone-marl interbeds from a hypot hetical homogeneous precursor sediment. The calculations demonstrate that r esulting hypothetical limestone-marl alternations show characteristic mathe matical relationships between the ratios of the bed thicknesses of limeston es and marls on one side, and the carbonate contents, on the other. By reve rsing this model, the original mineralogical composition of the precursor s ediment of real-world rhythmic successions can be determined. In this study , alternations from the Silurian of Gotland, the Cambrian, Devonian, and Mi ssissippian of North America, the Jurassic of France and Germany, and the C retaceous of France are shown to exhibit mathematical relationships similar to those calculated for hypothetical precursor sediments without primary d ifferences. Therefore, the mineralogical composition of their precursor sed iments can be estimated. In contrast, the clear mismatch shown by the Lower Jurassic Belemnite Marls from Dorset indicates that these rhythms did not suffer an early diagenetic overprint. Our model helps to differentiate betw een rhythmites with strong, depositional variations and those without; howe ver, it cannot indicate whether a given alternation is the product of rhyth mic diagenesis of a homogeneous precursor sediment or the result of diagene tic enhancement of subtle underlying sedimentary rhythms. For horizontally correlated patterns, such as laterally extensive beds and layers of nodules , an a priori unknown external signal has to be assumed.