Cretaceous-Cenozoic History of the West Saharan Continental Margin (NW Africa): Development, Destruction and Gravitational Sedimentation

  • Ulrich von Rad
  • Gerd Wissmann

Abstract

The most importantconstructional phases of the mature passive West Saharan continental margin involved the buildup of a thick Jurassic carbonate platform, overwhelmed by up to 4 km of Early Cretaceous Wealden-type deltaic sediments. Rollover structures between antithetic growth faults produced the “lower slope anticline” and a convex Cretaceous paleoslope. We recognized no salt diapirs, but local piercement structures probably caused by mobilized Early Cretaceous prodeltaic shale. During late Cretaceous to Tertiary times the continental slope was hardly prograded. During Tertiary (especially Neogene) times the depocenter shifted to the upper rise.

Evidence from seismic profiles correlated with DSDP, commercial well, dredge, and land geological data allow the differentiation of the complex, multi-phase mid-Cretaceous to Cenozoic history ofslope destruction. At the following times slope canyons were excavated and regional unconformities formed: (1) mid-Cretaceous (upper slope; precursor valleys); (2) early Senonian; (3) Paleocene (pronounced unconformity); (4) late Eocene (local); (5) mid- to late Oligocene (major lower slope erosion; up to 100 M.Y. hiatus; major canyon cutting phase);(6) mid- to late Miocene (local); (7) Pliocene (shelf and upper slope; deeply incised Uad Craa Channel); (8) late Pleistocene (canyon reactivation, slope gullies, and incisions).

Some of these accentuated erosional phases (especially 5 and 8) can be correlated with intensified Atlantic bottom water circulation from climatic cooling during periods of exceptionally low sea level causing hiatuses on the shelf and increased mass wasting at the slope (Fig.10). A very large sediment volume (7,500 — 15,000 km3 off Cape Bojador) was removed during the mid-Tertiary sculpturing of the lower slope, probably by enhanced contour currents (pre-AABW?) and associated gravitative mass transport. In total 900 km3 of sediments were excavated by slope canyons, most of which do not extend to the shelfbreak. Both processes caused a gradual retreat of the base-of-slope line and accentuated the convex shape of the slope in this area.

During early to mid-Miocene times, massive slope failure led to thegravitative deposition of olistostromes, debris flows, turbidites and slumps on the oversteepened lowermost paleoslope, until an equilibrium gradient was achieved (Figs. 11,12). During the late Pleistocene the same processes caused giant translational slides with up to > 100 m high slide scars (Figs. 1, 8). The triggering of these massive slides at this very time and setting (passive margin, low gradient and sedimentation rates) is still enigmatic.

Keywords

Sedimentation Shale Pyrite Holocene Mudstone 

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Copyright information

© Springer-Verlag Berlin Heidelberg 1982

Authors and Affiliations

  • Ulrich von Rad
    • 1
  • Gerd Wissmann
    • 1
  1. 1.Bundesanstalt für Geowissenschaften and RohstoffeHannover 51Germany

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