Nature 312, 356 - 359 (22 November 1984); doi:10.1038/312356a0

Enhanced preservation of marine-derived organic matter in Cenomanian black shales from the southern Angola Basin

P. A. Meyers^*, M. J. Leenheer^†, O. E. Kawka^*‡ & T. W. Trull^*‡

^*Department of Atmospheric and Oceanic Science, University of Michigan, Ann Arbor, Michigan 48109, USA
^†Cities Service Technology Center, PO Box 3908, Tulsa Oklahoma 74102, USA
^‡Present addresses: School of Oceanography, Oregon State University Corvallis, Oregon 93701, USA (O.E.K.); Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA (T.W.T.).

Black shales possessing high concentrations of organic carbon¹ were deposited in many parts of the proto South Atlantic Ocean during the Cretaceous period². The way such sediments accumulated is not fully understood, but is likely to have occurred through a combination of low oxygen availability and abundant supply of organic matter. Thin, centimetre-thick layers of black shales are commonly interbedded with thicker layers of organic carbon-deficient, green claystones, as found in strata of Aptian to Coniacian age, at Deep Sea Drilling Project (DSDP) Site 530, in the southern Angola Basin³ and elsewhere. These differences in carbon content and colour reflect the conditions of deposition, and possibly variations in the supply of organic matter^4,5. We have compared, using organic geochemical methods the compositions of organic matter in three pairs of closely-bedded black and green Cenomanian claystones obtained from Site 530. Kerogen analyses and distributions of biological markers show that the organic matter of the black shales is more marine and better preserved than that of the green claystones.

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