Evolution and Sedimentary History of the Cape Bojador Continental Margin, Northwestern Africa

Abstract

The Cape Bojador region of the northwestern African continental margin is apparently an unusual passive margin setting. Climatic fluctuations, local volcanism, and possibly tectonics have played important roles in the sedimentary record in this region. A transect of drilling sites connected by seismic profiles is available from the coastal Aaiun Basin (CORC 15-1), across the shelf (Spansah 51A-1), on the continental slope (DSDP Site 369), the upper continental rise (DSDP Site 397), farther out on the rise (e.g., DSDP Sites 139 and 140), and the abyssal plain (DSDP Sites 137 and 138). These data allow construction of a profile perpendicular to the margin, and reconstruction of the sedimentary evolution there.Lower Cretaceous sediment (Hauterivian) is the oldest recovered in a DSDP site along this part of the margin. Equivalent sediment was recovered from hydrocarbon exploration wells in the Aaiun Basin. Subsidence rates under the present shelf, slope, and upper rise were rapid during the Jurassic and Early Cretaceous (110 to 140 m/m.y.), and diminished since the Late Cretaceous (average 13 m/m.y.). A sequence of Lower Cretaceous Wealden-type deltaic and prodeltaic environments has been delineated from well and outcrop data: a thick sequence of sandstone and siltstone (Spansah 51A-1) grades seaward to laminated prodelta muds containing thin siderite layers (DSDP Site 397). Deltaic sedimentation gradually wanes in the Middle Cretaceous in the coastal basin. This interval is represented by a hiatus at Site 397. A Middle to Late Cretaceous transgression resulted in deposition of more pelagic black shale, nannofossil marl, chert, and limestone on the Cape Bojador slope (DSDP Site 369) and in the Aaiun Basin.

Surficial Geology of the Continental Margin offshore New Jersey in the Vicinity of Deep Sea Drilling Project Sites 612 and 613

“…The geologic setting of the upper rise in the study area, with Neogene sediment unconformably overlying much older strata, is similar to the continental rise off Northwest Africa, where lower to middle Miocene massflow deposits overlie lower Cretaceous strata (Arthur et al, 1979).…”

Provenance and sedimentologic variations of turbidite and slump deposits at Sites 955 and 956

“…Abundant turbidites associated with this sea-level fall were noted at DSDP Sites 397 and 140 (Arthur et al, 1979;Sarnthein et al, 1982). Excavation of canyons on the continental shelf, near Cape Bojador (Fig.…”

“…Eolian-sand turbidites with ages between ~12 and 11 Ma are noted at DSDP Sites 397 and 140 (Sarnthein, 1978;Sarnthein and Diester-Haass, 1977;Arthur et al, 1979;Sarnthein et al, 1982). At Sites 955 and 956, turbidite depositional rates are low approximately during this same time period, with most of the turbidites being mixtures of foraminifers, quartz, and volcanic lithic fragments.…”

“…Although extensive current erosion is responsible for a hiatus at Sites 366, 368, 369, and 397 in the upper part of the middle Miocene sediments (Arthur et al, 1979;Sarnthein et al, 1982), no apparent hiatus is present at Sites 955 and 956 during this time.…”

“…Sedimentologic and provenance variations of coarse-grained units in the deep marine environment therefore provide an important tool in interpreting the depositional processes and their possible relationship to sea-level changes on the adjacent continental margin (e.g., Berger and von Rad, 1972;Sarnthein and Diester-Haas, 1977;Sarnthein, 1978;Arthur et al, 1979;Chamley and Diester-Haas, 1979;Sarnthein et al, 1982;Auffret et al, 1984;Faugeres et al, 1989;Tiedemann et al, 1989;Tiedemann et al, 1994). …”