A well documented section across the continental margin at Cape Bojador is based on information of D.S.D.P. Sites 397 (uppermost continental rise) and 369 (slope), several on- and offshore commercial wells, and the interpretation of seismic profiles (figure 2). Within the Aaiun-Tarfaya coastal basin Mesozoic to Gainozoic sediments thicken from a hinge line to the present coast. Over a lateral distance of about 100 km under the present shelf, slope, and uppermost rise, a sedimentary sequence of nearly constant thickness (12-14 km) overlies continental to transitional basement. This uniformly subsiding part of the margin is termed 'Cape Bojador marginal basin’. Seaward of the upper rise the basement rises and the sediment cover thins. For time intervals with unambiguous interpretation of palaeo-water depth, we inferred the subsidence rates of 'marginal basin’ sites directly from sediment thickness and corrected them for compaction (figure 4). Since at the slope and rise sites the facies interpretation of the Late Cretaceous and Tertiary record does not allow an estimation of palaeo-water depth, this depth was found by ‘backtracking’ the present depth of stratigraphic boundaries below sealevel by using the subsidence rates from comparable shelf and coast sites. As a ‘continental approach’ to the margin this method infers the subsidence and palaeobathymetry of the poorly known continental slope and rise environments with the better established stratigraphic and facies information from the coastal basin and the shelf. During the Jurassic, the subsidence rates in the ‘Cape Bojador marginal basin’ (table 1) were high (80-100 m/Ma ); they increased slightly during the early Cretaceous (130-140 m/Ma). During the late Cretaceous and Cainozoic, subsidence slowed down more or less exponentially. During the (Triassic?-) Jurassic, we assume shallow-water conditions (possibly carbonate build-up) for the present slope and rise sites. The earliest Cretaceous is represented by a very thick Wealden-type deltaic sequence with shallow-marine deltaic sediments below Site 369 (?) and distal prodelta muds (about 500 m water depth) at Site 397. Subsequently, accumulation rates lagged behind the subsidence rates, causing a gradual deepening of the outer continental margin to its present depth. The continuous deepening of the sea floor has been accompanied by a landward facies migration until the Turonian. A 1-2 km deep erosional scarp was formed by deep geostrophic currents during the Oligocene at the upper rise (Site 397), which was rapidly filled by early Miocene mass flows, until equilibrium conditions were gradually established.
