Use of organic carbon content as a chronostratigraphic tool

Abstract In the Mulhouse potash basin (France) the lithology and the organic carbon content reveal an extended succession of recurrent sequences. Variation in lithology (shales-anhydrite-halite) reflects variations in depositional environment. Organic matter, exclusively present in the shaly horizons, shows variations in content and origin. These changes highlight small variations of their depositional environment which are not detected in the lithology of the shales. On the other hand, when these variations increase and are large enough, they are recorded by the lithology. In homogeneous facies (i.e. shaly facies), analysis of the distribution of the organic content in sediments thus appears to be an interesting method for determining small variations of the depositional environment. The organic carbon signal showsvariations of sedimentation rates over time and can be interesting for small-scale geochronology. Quite a similar duration for each cycle lasting an average of 13,000±3400 yr has been calculated. This time span based on biostratigraphy is in agreement with results obtained by lamination counts. This duration can be criticized due to the lack of precision of the dating methods but is significantly different from the well-known periodicities of Milankovitch.