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At Site 397, nearly continuous coring and a total penetration of 1453 meters revealed a thick but geologically complex and intriguing Neogene sediment wedge (1300 m thick) directly overlying Lower Cretaceous (Hauterivian) strata on the upper continental rise off Cape Bojador. A major erosional event stripped sediment from the upper continental rise and lower slope, probably during Oligocene time and produced a hiatus of approximately 100 m.y. This event may have been related to enhanced geostrophic currents scouring the base of the continental slope, and to critical overloading of sediment on the slope during a concurrent regression. Slumping of slope sediment and cutting of submarine canyons continued from early through middle Miocene. A thick sequence (554 m) of slump units, debris flow units, intercalated mud turbidites, and turbiditic sands accumulated during this time on the upper continental rise. Slope gradients gradually decreased, and during early-late to middle Miocene about 200 meters of slumped hemipelagic marls and turbiditic sands intercalated with undisturbed hemipelagic marls were deposited. By early-late Miocene time, redeposition of sediment had ceased at the site. Most sediment deposited in the region of Site 397 during early through middle Miocene time was derived from nearby slope environments with some contribution of calcareous biogenic detritus and coarse quartz, glauconite, and phosphate from outer shelf settings. Volcaniclastic debris flow units or turbidites arrived at the site during middle Miocene time (17.5 to 15.5 m.y.B.P.), probably originating from near Fuerteventura. These may record an early shield-building phase on the eastern Canary Islands.

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Early Neogene Base-of-Slope Sedimentation at Site 397, DSDP Leg 47A: Sequential Evolution of Gravitative Mass Transport Processes and Redeposition along the Northwest African Passive Margin