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ABSTRACT: Development of Mesozoic Organic-Rich Sedimentary Facies Across Southwestern Gondwanaland Margins and Basins

Suzanne O'Connell, Sherwood W. Wise

Lower Cretaceous, organic-rich sediments recovered by several Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) legs in the circum-Weddell Sea/South Atlantic region and on land from the Antarctic Peninsula to South Georgia Island (Nordenskjoll Formation) help define the limits of highly restricted, apparently anoxic basins that formed significant hydrocarbon sinks during the early breakup history of southwestern Gondwana. The existence of major physical barriers between the incipient basins, coupled with a generally sluggish, haline-driven global oceanic circulation system, set the stage for basin-wide accumulation of black shales in these areas at a time when the earth's depositional system were especially vulnerable to the accumulation of organic carbon.

Particularly instructive are recently drilled ODP Leg 113 sites off Dronning Maud Land of East Antarctic which, along with DSDP Sites 330/511 (Falkland Plateau), 361 (off South Africa), and 249 (Mozambique Plateau) and the outcrops of the Nordenskjoll Formation, delimit the incipient Weddell basin. At Site 692, 45 m of Valanginian, organic-rich sediments (up to 18% TOC; mostly algal-derived kerogens) at the base of a 450-m-thick Lower Cretaceous package are dominated by strongly laminated nannofossil claystones and mudstones with pelloids, calcispheres, and radiolarians. Macrofossils, thin lenses and nodules containing carbonate-fluorapatite (<1 cm thick), and water escape structures are abundant. A 68-m interval at the top of the sequence (Site 693) yielded lower organic contents up to 4% TOC) in dark Aptian claystones that grade upward through thin limestone beds and glauconitic sediments to a carbonate-free, lower Albian diatomite that suggests deposition under highly productive, open marine conditions.

The early Albian ventilation of the Weddell basin, recorded here and on the Falkland Plateau, resulted from the sequential destruction of physical barriers to deep ocean circulation that accompanied the dispersal of the Gondwanaland continents. The existence of silled oceanic basins in the Southern Hemisphere at a time when basins and margins were already prone to anoxia produced along continental margins organic-rich sedimentary sequences that spanned exceptionally long periods of time, unlike those along incipient Mesozoic/Cenozoic ocean basins that opened before or since.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990