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TRUSKOWSKI, IRENE I., Intevep, S.A., Caracas, Venezuela

ABSTRACT: Late Cretaceous Source Rocks of a Section in the Eastern Venezuelan Basin

Micropalentological and geochemical studies of Querecual and San Antonio formations were carried on a section (Cerro Negro) of the Serrania del Interior, in order to establish the imprint of the late Cretaceous "Oceanic Anoxic Events" (OAE), mentioned by numerous authors.

For this purpose, the distribution patterns of microfossil assemblages, variations of organic matter content and, V/Ni ratios have been taken into account to recognize anoxic conditions.

This stratigraphic section is characterized by the following:

1. "Anaerobic" bottom conditions with oxigenated surface waters, indicated by the presence of a well-developed planktonic forms and virtually no benthic foraminifera. These conditions fluctuated along the section, prevailing at its base.

2. "Dysaerobic" bottom conditions indicated by a low-diversity benthic foraminifera and, a well oxigenated surface waters suggested by high concentration of planktonic biota (foraminifera, radiolaria and nannoplankton).

3. An increase of organic matter content (TOC: 5-6%) and V/Ni ratios (3.7 to 4.2) are related to the "anaerobic" levels.

Marine sediments of Brazilian continental margin display similar characteristics, which indicate that deposition of anoxic sediments was extended to the North of South America.

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