AAPG Annual Convention and Exhibition

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Re-Evaluation of Organic Matter Deposition of Central Atlantic Black Shales

Abstract

It has been nearly two decades since scientific drilling has been conducted in the Atlantic basins. During this period a number of significant deep-water discoveries have been made along the continental margins. These discoveries and the planning for the JOIDES Resolution's return to the Atlantic, have spurred renewed interest in the geochemistry and sedimentology of the basins' black shales. Although a significant amount of data have been gathered for the Central Atlantic black shales, the data remains relatively sparse. To supplement the previously published data, core photographs and descriptions were examined from 18 DSDP and ODP sites and 725 samples were selected for TOC and Rock-Eval analyses. These data, along with the data from previously studied samples, permitted the re-evaluation of the spatial and temporal distribution patterns of the organic matter (OM) contained in black shales. Reinterpretation of the data has allowed us to understand the main controls on OM deposition and to predict organic-rich zones in data sparse areas. Comparison and contrast between the eastern and western margins of the paleo-Atlantic ocean reveals that more organic-rich sediments were deposited along the eastern margin (West Africa coast) during Neocomian and Albian-Aptian times, probably corresponding to more vigorous coastal upwelling and elevated productivity along the West Africa coast. In contrast, coals are present along the western Atlantic margin, indicating more terrigenous OM contribution from nearby continents. Strikingly, Cenomanian-Turonian (C-T) black shales contain abundant OM, but display no evidence for a strong gradient of organic richness between the eastern and western margins, consistent with a widespread oceanic anoxic event during C-T time. Therefore, the presence of oxygen-poor waters, and thus OM preservation, may be the main cause for development of the C-T black shales, whereas higher organic productivity likely controls the formation of Neocomian and Albian-Aptian organic-rich black shales. Organic-rich C-T black shales are more likely to be widespread along the Atlantic margins, while organic-rich Neocomian and Albian-Aptian black shales appear more localized, especially in the western Atlantic margin. The alternation of organic-rich and organic-poor shales even during the C-T suggests that the conditions favoring preservation were quasi-stable and that they are key in determining the volume of these wide-spread organic-rich sediments.