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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 917

Last Page: 917

Title: Cretaceous Black-Shale Deposition Within an Oxidized Red Clay, Turbidite Environment, Southern Angola Basin, South Atlantic Ocean: ABSTRACT

Author(s): W. E. Dean, E. J. Barron, R. E. Boyce

Article Type: Meeting abstract


Beds of black shale, intercalated with red and green claystone of Albian to Coniacian age, were recovered at DSDP site 530 in the southern Angola Basin. The 260 beds of black shale have an average thickness of 4.3 cm (range of 1 to 62 cm) and an average organic-carbon content of 5.7% (range of 1.4 to 16%). The green claystone beds resulted from reduction of iron in red claystone beds around black shale beds. A green-black-green reduced sequence may occur alone within predominant oxidized red claystone, or several, closely spaced reduced sequences may merge to form interbedding of black and green lithologies. The predominant red claystone beds were deposited as distal turbidites. Many of the black shale beds contain graded silt laminae, very low amplitude ripple cross-lami ation, and fine, indistinct, discontinuous laminae, suggesting that the material in the black shale beds also may have been transported by turbidity currents. All lithologies are commonly bioturbated. The sequence, including the black shales, at Site 530 suggests that deposition of the distal turbidites, low in organic matter, in an oxidized bottom-water environment was interrupted periodically by the deposition of organic-carbon-rich clay. We conclude that the cyclic interbeds of more- and less-reduced strata, with frequencies and durations measured in thousands or even hundreds of years, resulted from variable supply of organic matter, most of which is of marine origin, and not from bottom-water stagnation. We favor periodic regional increases in organic productivity that resulted in i creases in production of organic matter, and an expanded and intensified mid-water oxygen minimum that impinged on the continental margin, as a cause of periodic increases in accumulation of organic matter. These conditions would produce variations in the amount of organic matter in both time and space, and result in interbedding of organic-carbon-rich, reduced sediments and organic-carbon-poor, oxidized sediments, a characteristic of all so-called anoxic sequences in the Atlantic Ocean.

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