The Environment of Deposition of Black Shales in the Early Cretaceous: An Ongoing Controversy
Helmut Weissert, 1981. "The Environment of Deposition of Black Shales in the Early Cretaceous: An Ongoing Controversy", The Deep Sea Drilling Project: A Decade of Progress, John E. Warme, Robert G. Douglas, Edward L. Winterer
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During the last 12 years of the Deep Sea Drilling Project, numerous Early Cretaceous black shale horizons have been discovered in all the major ocean basins. Similar organic-carbon rich sediments had been described earlier from various land outcrops in the Alps, in the Carribbean region and in the southern Andes. New and refined methods in sedimen- tology, geochemistry and geophysics have been applied in the analysis of these sediments, which can be classified into four regional groups: (a) central North Atlantic, (b) northern North Atlantic, (c) South Atlantic and Indian Oceans, (d) Pacific Ocean. The black shales of Barremian-Albian age in the central North Atlantic and Tethys were deposited in a deep basinal environment under low oxygen or anoxic conditions. The average content of organic matter in the often- laminated black shales is 2%. Terrigenous organic matter dominates in the organic spectrum of the western North Atlantic, while marine-derived material dominates in the eastern North Atlantic. The black shales in the northern North Atlantic, deposited on a passive continental margin in a depth of a few hundred to a few thousand meters, are parts of turbiditic sequences. The organic matter is of terrigenous origin. These deposits were not necessarily connected with anoxic conditions. The Aptian-Albian black shales in the restricted basins of the South Atlantic and the Indian Ocean were deposited under anoxic conditions in a water depth of a few hundred to a few thousand meters. The organic matter has a mixed terrigenous and marine origin. The rare occurrences of black shales in the Pacific are restricted to submarine rises. The finding of black shales in all major oceans encouraged authors to construct models explaining these deposits as a result of global oceanic events. Detailed geochemical and sedimentological investigations revealed shortcomings in the early global scenarios, and a series of alternative spatially restricted models were proposed. Models explaining the formation of black shales in pelagic environments, such as the central North Atlantic, the Tethys and the South Atlantic-Indian Ocean, emphasize the importance of the regional paleotectonic and paleoceanographic situation controlling the black shale facies patterns. Anoxic bottom-water conditions were established during periods of stratified watermasses. Models explaining the black shales in the northern North Atlantic underline the importance of the continental climate controlling the input rate of terrestrial organic matter into marine environments. In the future, global models have to integrate these regional models; the exciting problem of possible feedback mechanisms between paleo- climate and paleoceanography has to be solved.
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At the present the Glomar Challenger has drilled over 500 holes over the world ocean, involving hundreds of scientists from dozens of countries. This volume is intended as a review of some of theimportant results from the most comprehensive, ambitious and successful earth-bound geologic project ever undertaken. The symposium upon which this volume originated was held April 4, 1979 at the SEPM/AAPG Annual Meeting in Houston. No comprehensive synthesis of all aspects of the DSDP has appeared, and the topic coverage in this volume is biased towards the sediments and fossils, and their significance for certain aspects of earth history – paleogeography, bathymetry, climatology, oceanography, ecology, environments – all in keeping with the audience of sedimentary geologists.