Evolution of the conjugate East African - Madagascan margins and the western Somali Basin

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Abstract
The geologic evolution of the conjugate sedimentary basins and margins produced during the early breakup of Gondwanaland by the relative motion between Madagascar and Africa is reconstructed utilizing interpretations drawn from outcrop, industrial onshore drilling, Deep Sea Drilling Project (DSDP) offshore drilling, Lamont-Doherty multichannel seismic (MCS) data, and single-channel seismic data. Herein we present (1) maps displaying lithological columnar sections for Karroo (Permo-Carboniferous through Early Jurassic) to Quaternary time slices, (2) depth-to-basement and sediment isopach maps, and (3) acoustic stratigraphy studies based on MCS data. Formation of the conjugate sedimentary basins began in Permo-Carboniferous time, and extension recurred intermittently over a 150-m.y. span until the initiation of sea-floor spreading between Madagascar and Africa in Middle Jurassic time. Occasional marine incursions and the resulting deposition of salt in isolated Tanzanian grabens, and in the conjugate Somali Coastal and Majunga basins, highlight the pre-breakup stratigraphy. At the initiation of sea-floor spreading, facies changed throughout the basins from dominantly continental to overwhelmingly marine, and volcanic activity and faulting occurred. The mid-Cretaceous was marked by the beginning of vigorous abyssal circulation in the Western Somali Basin, and the Late Cretaceous was a time of widespread regional volcanism. During the Paleogene, rifting was renewed in the Tanzanian Coastal Basins, extending to the Davie Fracture Zone, and all of the basins record numerous hiatuses in the Paleocene and Oligocene sections. A vast sediment slide offshore Somalia and Kenya occurred in mid-Tertiary time, demonstrating that the formation of olistostromes characterized by significant internal deformation (including thrust faults) may occur in passive margin settings. An intense erosional event in the Western Somali Basin marked the end of Paleogene time. Frequent volcanism affected the Diego Basin throughout the Cenozoic Era and the Comoros Islands during Neogene and Quaternary time. Folding and faulting of onshore and offshore strata of the Tanzanian margin continued through Neogene and Quaternary time to the present. We observed a major network of late Cenozoic canyons and channels on both the East African and Madagascan margins and in the Western Somali Basin. Accumulations of sediment on the Madagascan and East African margins total 5+ and 8+ km, respectively, for Middle Jurassic to Holocene time.

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