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ERLICH, R. N., Amoco Overseas Exploration Company, Houston, TX, L. M. PRATT, Indiana University, Bloomington, IN, Z. SOFER, Amoco Production Company, Houston, TX, and S. E. PALMER, Amoco Research Company, Tulsa, OK

ABSTRACT: Depositional Environment and Distribution of Late Cretaceous "Source Rocks" from Costa Rica to West Africa

Late Cretaceous "source rocks" from Costa Rica, western and eastern Venezuela, and Trinidad were studied using organic and inorganic geochemistry, biostratigraphy, and sedimentology in order to determine their depositional environments. Bulk mineralogy and major element geochemistry for 304 samples were combined with Rock Eval data and extract biomarker analysis to infer the types and distributions of the various Late Cretaceous productivity systems represented in the dataset.

When data from this study are combined with published and proprietary data from offshore West Africa, Guyana/Suriname, and the central Caribbean, they show that these Late Cretaceous units can be correlated by their biogeochemical characteristics to establish their temporal and spatial relationships. Paleogeographic maps constructed for the early to late Cenomanian, Turonian, Coniacian to middle Santonian, and late Santonian to latest Campanian show that upwelling and excessive fluvial runoff were probably the dominant sources of nutrient supply to the coastal productivity systems.

The late Santonian to Maastrichtian rocks examined in this study indicate that organic material was poorly preserved after deposition, even though biologic productivity remained constant or changed only slightly. A rapid influx of oxygenated bottom water may have occurred following the opening of a deep water connection between the North and South Atlantic oceans, and/or separation of India from Africa and the establishment of an Antarctic oceanic connection.

This study suggests that the most important factors that controlled source rock quality in northern South America were productivity, preservation, degree of clastic dilution, and subsurface diagenesis.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.