Geological and geochemical modeling, an approach for understanding organic cyclic sedimentation in evaporitic sequences. Application to the Mulhouse Basin (France)

In the Upper Eocene to Lower Oligocene Mulhouse Basin (France, Upper Rhine graben) an organic-rich evaporitic series has been deposited. First and second order cyclicities can be defined at the scale of the study: a short period cyclicity corresponding to an alternation of marl and evaporitic layers; a long period cyclicity corresponding to an evolution of the evaporitic layers from an anhydritic to halitic (or sylvitic) composition. The characteristics and abundance of organic matter in the marl also show cyclic variations in amount and origin. A geological scenario, based on numerical modeling, served for testing relationships between the environmental evolution (including state of stratification of the basin, sea level change and water balance variations) and the organic sedimentation. Phytoplankton productivity as well as terrestrial organic matter input have been simulated and compared to the organic data. The model results account for the distribution of organic matter in the basin and are in agreement with the geological and geochemical data. They serve to place the paleo-environment elaborated by Hofmann et al. (1993) within a more dynamic sedimentological framework. This work demonstrates the benefit of numerical modeling in tentatively identifying the respective influence of eustasy and climate on the organic sedimentation in an evaporitic basin.