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 R hine graben) an organic-rich evaporitic series has been deposited. Fir st and second order cyclicities can be defined at the scale of the stu dy: a short period cyclicity corresponding to an alternation of marl a nd evaporitic layers; a long period cyclicity corresponding to an evol ution of the evaporitic layers from an anhydritic to halitic (or sylvi tic) composition. The characteristics and abundance of organic matter in the marl also show cyclic variations in amount and origin. A geolog ical scenario, based on numerical modeling, served for testing relatio nships between the environmental evolution (including state of stratif ication of the basin, sea level change and water balance variations) a nd the organic sedimentation. Phytoplankton productivity as well as te rrestrial organic matter input have been simulated and compared to the organic data. The model results account for the distribution of organ ic matter in the basin and are in agreement with the geological and ge ochemical data. They serve to place the paleo-environment elaborated b y Hofmann et al. (1993) within a more dynamic sedimentological framewo rk. This work demonstrates the benefit of numerical modeling in tentat ively identifying the respective influence of eustasy and climate on t he organic sedimentation in an evaporitic basin.