--> Abstract: Deposition of Lacustrine Source Rocks, Congo Basin, West Africa: Controls by Lake Hydrology, by N. B. Harris and K. H. Freeman; #90937 (1998)

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Abstract: Deposition of Lacustrine Source Rocks, Congo Basin, West Africa: Controls by Lake Hydrology

HARRIS, NICHOLAS B., and KATHERINE H. FREEMAN, Department of Geosciences, The Pennsylvania State University, University Park, PA

The Marnes Noires Formation of the Congo Basin is a source rock remarkably rich in organic carbon, averaging more than 6 weight % TOC (total organic carbon). We have begun an investigation of sedimentological, chemical and paleoclimatic factors governing deposition of organic carbon in this and other syn-rift formations in the Congo basin.

Sedimentological evidence indicates that the Congo basin lake(s) was saline-stratified during Marnes Noires deposition. Evaporative conditions promoted formation of halite cements in carbonates on the lake margins and abundant dolomitic sediment; saline conditions in deep parts of the lake preserved halite cement m redeposited shallow water clasts. Lakes were probably less saline during deposition of other syn-rift shales.

Core data indicate geochemical variability in the Marnes Noires, perhaps related to fluctuating lake level. Organic carbon richness is high in high-stand intervals and decreases toward the upper contact of these units. This suggests a response to lowering lake level, possibly due to a decrease in nutrient supply or less favorable conditions for organic matter preservation. Organic richness is relatively low in the basal part of the two low-stand intervals; %TOC values increase dramatically in the upper part of one interval suggesting a response to rising lake level.

Smaller scale cycles in %TOC approximately 5 meter thick are evident in the high-stand units; these equate to approximately 6000 year-long periods. These could record short-term cycles in lake level, possibly due to variations in rainfall and associated runoff and/or nutrient supply.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah