The continental rise, within the 200-mi (320 km) economic zone, represents both the largest accumulation of terrigenous sediment on this planet and perhaps the largest unexploited hydrocarbon repository.

Several models have been presented for the origin of the rise; in the 1950s and 1960s, R. Dietz, M. Kay, C. Drake, M. Ewing, and others suggested a model of pyroclastics and turbidity-current deposition which produced a thick accumulation (eugeosyncline) of alternating coarse- and fine-grained sediment.

This view of continental-rise sedimentation was proposed prior to the discovery that deep thermohaline circulation could play a significant geologic role at abyssal depths. This latter concept was first hypothesized by the writer, who presented a model for along-slope fine-sediment dispersal driven by thermohaline circulation. This view held that turbidity currents inject sediment into contour-following, near-bottom currents; only fine-grained material is wafted parallel with contours, whereas the coarser turbidite material largely bypasses the continental rise and is deposited on the abyssal plains. This concept is supported by the results of Leg 11 of the DSDP.

The present sedimentary environment of the lower continental rise (3.5 to 5 km deep) in the western North Atlantic is now known to be dominated by vigorous near-bottom, contour-following currents (i.e., "contour currents"). Dense water originating in the Labrador, Norwegian, and Irminger Seas flows south and west through various fracture zones until it reaches the North American continental margin, where it is constrained to flow against the massive sedimentary apron of the continental rise.

The obvious implication of the contour-current concept for petroleum interest is that there would be little or no deposition on the continental rise of sediments

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suitable for forming reservoir rock during the time that deep circulation has been an important process in the western North Atlantic. This process is thought to have begun about 50 m.y.B.P. There would be deposition only of fine-grained, relatively impermeable, potential source beds.

It is concluded that Cenozoic sediments of the continental rise, at least off the east coast of North America, may not be a likely source for future hydrocarbon recovery. A few deep holes into the continental rise (preceded by complete seismic surveys) are needed to assess the potential of the underlying, deeply buried Paleozoic section.

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