--> Carbon-Isotopic Stratigraphy of Cretaceous Marine Strata in Venezuela, by L. M. Pratt, C. Davis, L. Mompart, and B. Murat; #90986 (1994).

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Abstract: Carbon-Isotopic Stratigraphy of Cretaceous Marine Strata in Venezuela

Lisa M. Pratt, Cara Davis, Luis Mompart, Bruno Murat

Detailed profiles of carbon isotopic ratios in organic matter and contents of organic carbon, carbonate carbon, and total sulfur have been established through a 400-meter interval in Maraven core ALP-6, Maracaibo basin, western Venezuela. This stratigraphic interval spans Aptian to Campanian time, including petroliferous black shales in the Lower Cretaceous Machiques Member of the Apon Formation and the Upper Cretaceous La Luna Formation. Geochemical and isotopic data were also collected through a thick sequence of carbonates in the upper Apon, Lisure and Maraca Formations which are present between the two black shales. Within the stratigraphic interval encompassed by ALP-6, three sedimentary megasequences are identified and can be subdivided into periods of infilling, aggradation, an backstepping. Deposition of black shale is the last stage of backstepping and represents a drowning event that can be regionally traced. Smaller-order sedimentary sequences are recognized and are used for detailed intrabasinal correlation. The presence of multiple carbon-isotopic events with demonstrated global correlation provides a framework for testing the local, regional, or global extent of sedimentary sequences during the Cretaceous on the passive margin of northern South America.

The carbon isotopic profile shows brief positive shifts at three positions within the black shale units, as well as a gradual negative trend from -23 to -26 per mil upwards through the intervening carbonate section. Excursion I is about 2 per mil in magnitude and peaks at about -21 per mil in the upper Machiques. Excursion II is 6 per mil in magnitude, peaks at -20.5 per mil, and is coincident stratigraphically with a bed of volcanic ash approximately 8 cm in thickness within the lower La Luna Formation. Excursion III is 3 per mil in magnitude, peaks at about -25 per mil, and is overlain by an interval of unusually uniform isotopic ratios. The large magnitude of Excursion II suggests correlation with the global shift in carbon isotopic ratios at the Cenomanian-Turonian boundary, an is topic event that is well documented for the Villeta Formation in Colombia.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994