--> Depositional Environment and Distribution of Upper Cretaceous Source Rocks from Costa Rica to West Africa, by R. N. Erlich, Z. Sofer, A. J. Nederbragt, L. M. Pratt, and S. E. Palmer; #90986 (1994).

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Abstract: Depositional Environment and Distribution of Upper Cretaceous Source Rocks from Costa Rica to West Africa

R.N. Erlich, Z. Sofer, A. J. Nederbragt, L. M. Pratt, S. E. Palmer

Upper Cretaceous source rocks from Costa Rica, western and eastern Venezuela, and Trinidad were studied using organic and inorganic geochemistry, biostratigraphy, and sedimentology to determine their depositional environments. Bulk mineralogy and major-element geochemistry for 347 samples were combined with Rock Eval data and extract biomarker analyses to infer the types and distributions of the various Upper 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 Upper 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 late Campanian show that upwelling and excessive fluvial runoff were probably the dominant sources of nutrient supply to the coastal productivity systems.

The upper Santonian to Maastrichtian rocks examined in this study indicate that organic matter was poorly preserved after deposition, even though biological productivity remained constant or changed only slightly. One possible explanation for the change in preservation may be the rapid influx of cold, oxygenated bottom water from the Antarctic, following the establishment of a deep water connection through the South to the central North Atlantic.

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 #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994