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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 545

Last Page: 545

Title: Stratigraphic and Paleo-Oceanographic Setting of Organic Carbon-Rich Strata Deposited During Cenomanian-Turonian "Oceanic Anoxic Event": ABSTRACT

Author(s): S. O. Schlanger, M. A. Arthur, H. C. Jenkyns, P. A. Scholle

Article Type: Meeting abstract


At, or very close to, the Cenomanian-Turonian boundary, strata from several basins bear the imprint of a global, short-lived "oceanic anoxic event" during which large amounts of organic carbon were sequestered in marine sediments. These strata are characterized by one or more of the following features. (1) The presence of a layer, up to 1 m (3.3 ft) thick, of black, laminated shale with total organic carbon contents of up to 23%. The general lack of bioturbation in these shales indicates an absence of benthic metazoan in fauna; the organic carbon is largely of marine planktonic origin. (2) The limestones, with or without an associated black shale horizon, at the Cenomanian-Turonian boundary level, have ^dgr13C values of +4.0 to +4.3 ^pmil as contrasted to ^dgr13C values of +2.0 to +3.0 ^pmil exhibited by limestones immediately above and below the boundary horizon. (3) Benthic foraminiferal faunas are lacking or consist of depauperate agglutinate faunas whereas radiolarians are locally very abundant as are diverse planktonic foraminiferal faunas.

These features are interpreted as indicating deposition in many areas within a water mass that was essentially depleted of oxygen. The high ^dgr13C values are taken to indicate enrichment of the global ocean in ^dgr13C as a result of the preferential extraction of 12C by marine plankton whose organic components were not recycled into the oceanic waters.

The basal and upper contacts between the black shales and the enclosing limestones are generally sharp or gradational over a distance of several centimeters indicating a rapid onset and equally rapid disappearance of deoxygenated waters. Sedimentation rate arguments lead to the conclusion that the Cenomanian-Turonian "oceanic anoxic event" occurred over a time span of approximately 350,000 to 700,000 years.

Paleobathymetric interpretation of strata from European and African shelf sequences and sections in the U.S. Western Interior basin show that shallow embayments, flooded by the rapid Cenomanian-Turonian transgression were particularly hospitable to deposition of anoxic sediments as were the neighboring shelves and cratonic shallow seaways. The distribution of the black shale unit indicates that the upper surface of the Cenomanian-Turonian oceanic oxygen-minimum zone was 200 to 300 m (650 to 985 ft) below the sea surface analogous to that of today.

The widespread distribution of anoxic sediments deposited synchronously during such a short-lived event indicates that such sediments were not the product of local climatic or local basinal water mass characteristics but were the product of a global expansion and intensification of the Cenomanian-Turonian oxygen-minimum zone. In some regions this was accompanied by increased biological productivity in surface waters.

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Copyright 1997 American Association of Petroleum Geologists