Insights into deposition of Lower Cretaceous black shales from meager accumulation of organic matter in Albian sediments from ODP site 763, Exmouth Plateau, Northwest Australia

Abstraet The amount and type of organic matter present in an exceptionally complete upper Aptian to lower Cenomanian sequence of sediments from ODP site 763 on the Exmouth Plateau has been determined. Organic carbon concentrations average 0.2~o. Organic matter is marine in origin, and its production and preservation was low over the ca. 20-million-year interval recorded by this sequence. Because this section was tectonically isolated from mainland Australia in the early Aptian, it better represents global oceanic conditions than the many basin-edge locations in which Albian-age blaek shales have been found. Formation of the basin-edge black shales evidently resulted from rapid, turbiditic burial of organic matter rather than from enhaneed oceanic production or from basin-wide anoxia during the Albian. other sedimentary components, and the degree of postdepositional degradation. The nature of sedimentary organic matter is determined by its biotic sources, its transport routes, and also its degree of preservation. Information about many paleoceanographic parameters can therefore offen be inferred from the organic matter contents of ancient sediments. A nearly complete Albian nannofossil biostratigraphic sequence was recovered at ODP site 763 northwest of Australia (Bralower 1992), indicating that sediments at this location provide an exceptional record of late Early Cretaceous paleoceanographic conditions on the southern Neo-Tethyan margin. Implications of elemental and isotopic characterizations of the meager organic matter accumulations in these sediments are discussed in this report.