HOFMANN, PETER M., Research Center Julich (KFA), Julich, Germany; and DETLEV LEYTHAEUSER, University of Cologne, Cologne, Germany

ABSTRACT: Timing of Cyclic Organic Carbon accumulation in a Lacustrine Evaporite Depositional Environment: The Lower Oligocene of the Mulhouse Basin, France

The sediments of the lower Oligocene of the Mulhouse Basin (Upper Rhine Graben) display lithofacies characteristic for the deposition in a perennial evaporitic lake that received frequent marine influx. The sediments consist of marls, anhydrites (deposited as gypsum), halite and sylvite.

The organic matter content of these sediments stems dominantly from algal and most likely bacterial sources. Terrigenous, plant-derived organic matter comprises between 10 and 40 percent of the preserved kerogen. The bulk of the organic matter (up to 4.5% TOC) is concentrated in the marl lithofacies. The stratigraphic distribution of the organic matter content of the marls follows a cyclic depositional pattern. A "TOC-cycle" starts with marls that display a low organic carbon content (usually in the order of 0.5% TOC), and increases to a "TOC maximum" (up to 4.5% TOC) and then drops back to organic carbon contents below 1%. The marl sediments display a varve-like lamination, which is interpreted to result from an annual depositional cycle. Sedimentation rates determined by varve count ng allow the establishment of a detailed time-stratigraphic framework. The duration of one TOC cycle lies in the order of 35,000-40,000 years and may be a result of climatic variation. The accumulation of organic matter-rich sediments in the Mulhouse Basin is to a first order determined by the sedimentation rate. Organic matter-rich sediments are associated with low sedimentation rates (7 cm/ky), whereas sediments with low organic matter content display high sedimentation rates (up to 46 cm/ky). Elevated paleoproductivity approximated by measured accumulation rates of aquatic organic matter further aids the formation of organic matter rich potential source rocks in this lacustrine evaporite depositional setting.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.