Universitaet Bremen, Federal Republic of Germany

The Demerara Rise is a deep water extension of the continental margin north of Suriname and French Guyana, South America; conjugate to the Guinea Plateau of West Africa. It is in an ideal location to investigate late-stage Atlantic rifting and opening of the Atlantic Gateway in the Mid to Late Cretaceous and post-rift paleoceanography of the equatorial Atlantic. The northern extension of the Demerara Rise was surveyed with industry exploration and high resolution multichannel seismic reflection data. Five sites forming a depth transect were drilled during Ocean Drilling Program Leg 207. Seismic data demonstrate evidence of Jurassic to Cretaceous trans-tensional extension with synrift clastic sedimentation. By mid-Cretaceous, rifting had succeeded in providing a passage between South and North Atlantic Oceans resulting in a regional unconformity. During subsequent thermal subsidence, a consistently thick unit of approximately 90 m of black shale deposited between Cenomanian and Santonian times. Younger sediments are mostly calcareous chalks and oozes and include a distinct K/T impact interval distinguishable on seismic data, and the Paleocene-Eocene Thermal Maximum. Miocene erosion removed much of the Oligocene succession and normal faulting and mass-wasting continued throughout the rise's history. Total organic carbon contents in the black shales are up to 30 wt% during extreme ocean anoxic events. Rocks of similar age and lithology represent source rocks for an estimated 29% of the World's hydrocarbon supplies and equivalent formations source reserves in the nearby basins of Surinam, Guyana and Brazil.