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Abstract: Late Cretaceous Biostratigraphy of the La Luna Formation, Maracaibo Basin

Irene Truskowski, Francia Galea-Alvarez, William V. Sliter

Micropaleontological analysis, sedimentological studies, and geochemical data are presented for the Upper Cretaceous "black shales" of the La Luna Formation of Western Venezuela. The detailed planktonic foraminiferal studies allowed the establishment of the first biozonation, determination of sedimentation rates, and documentation of occurrences of benthic foraminifers for these unusually thick black shales that extend stratigraphically nearly 100 m.

Hedbergellids, whiteinellids and Heterohelix characterize the lower part of the La Luna Formation, dated from the late Cenomanian Rotalipora cushmani Zone to middle Turonian Helvetoglobotruncana helvetica Zone. The high productivity of these groups associated with phosphatized fish remains suggest upwelling and a poorly stratified water column. The presence of buliminids and Favreina sp. at some levels, imply disoxic conditions in this anoxic interval.

Planktonic foraminifers in the middle and upper parts of the formation range in age from the late Turonian Marginotruncana sigali- Dicarinella primitiva Zone to the early Campanian Globotruncanita elevata Zone. The increase in keeled planktonic foraminifers toward the top of the formation suggest more stratified, oligotrophic surface waters. Benthic foraminifers found at the top are indicative of dysaerobic conditions.

This study provides new opportunities for utilizing the petroleum system in the La Luna Formation, arguably the most prolific source rock in northern South America.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela