Sedimentary Responses to Vertical Movements in Afro-Arabia: Back to the Outcrops

Rob Crossley
Fugro Robertson Ltd, Llandudno, United Kingdom

Interpretation of SRTM data, combined with outcrop study, was used to help determine the geologic history of uplifted segments of the Afro-Arabian margin.

Sediments shed from the Red Sea margin are trapped along the Arabian Gulf as alluvial fans. The Pliocene fan sediments are much coarser than underlying Tertiary to Mesozoic deposits, suggesting that Arabia was low relief for much of its pre-Pliocene history.

On the Indian Ocean margin of South Africa, at elevations up to 300-400m, marine Palaeogene and Miocene sediments rest unconformably on Karoo or Cape Fold Belt rocks. These deposits are only preserved on the “lee” side of palaeobathymetric shoals formed from the resistant underlying rocks. The Tertiary marine deposits were much more extensive prior to uplift through the surf zone. The uplift that created much of this escarpment zone triggered incision of meandering fluvial systems, demonstrating that uplift is a Late Tertiary to Quaternary event.

Similar geomorphic relationships occur along the west coast of Africa at least as far north as the Cunene River. Interpretation of alluvial terrace deposits suggests that this river only captured its major Angolan catchment in the Plio-Pleistocene. SRTM data show that uplift of originally low gradient areas in southern Angola played a key role in this capture.

It is inferred that the south and east margins of the continent were at relatively low elevations and had low relief for much of the late Mesozoic and early Tertiary. These observations add force to the case for a major role for Pliocene to Quaternary tectonics in the clastic sediment fluxes to the offshore areas of Afro-Arabia.

The adjacent modern near-shore environments, charged by large fluxes of coarse fluvial sediment, may therefore not be appropriate exploration analogues for many early Tertiary and Mesozoic sections of the Afro-Arabian margin.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery