Abstract Sea floor spreading between Africa and the Madagascar-India-Seychelles block began during the Jurassic Magnetic Quiet Zone and was preceded by earlier rifting of Gondwanaland and deposition of Karroo sediments. Basal clastic deposition was terminated in the Early Jurassic by a regional transgression, largely a result of continental separation, followed by a general depositional regression on shelves. A major transgression occurred over most of East Africa, from late Callovian to Oxfordian, which was related to the final breakup of this area and subsequent phase of regional subsidence. Two distinct deformational episodes, documented by erosional unconformities and siliciclastic sedimentation, occurred during the pre-Aptian and late Maastrichtian. The older event was probably the distal intraplate effect of the separation of South America and Africa; whereas the Maastrichtian tilting of northern Somalia was possibly related to a rebound effect when the Oman subduction failed at about 70 Ma. The Cretaceous-Tertiary history of the Indian Ocean continental margins is the result of a complex depositional regression that covered the underlying Early and Middle Jurassic rifted margin. To the north, Oligocene-Miocene sediments were deposited during the opening of the Gulf of Aden and accumulated in disconnected structural depressions formed by downfaulted rotating blocks bordering the rising Somali plateau.

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.

Times of origin of vertebrate classes are known to the epoch for the higher fishes and for amphibians, and to within about a period for reptiles, birds, and mammals. Origin of the phylum in the Ordovician rather than earlier is doubtful because of the sparsity of early Paleozoic fresh-water deposits. An interval of one or two periods elapses between the first appearance and maximum diversification of a vertebrate class. Times of rapid expansion of successive classes and subordinate groups are distributed rather evenly throughout geologic time since the Ordovician and do not cluster around era boundaries or coincide with local orogenic episodes. Local conditions in northwestern Europe following the Caledonian orogeny may have favored the evolution of land-dwelling vertebrates. No relationship of this type can be traced for the origin of other vertebrate classes. Reptiles, more fully adapted to land life than amphibians, appear in Pennsylvanian coal swamps, though the unique occurrence of the advanced reptile Petrolacosaurus associated with a Permian type flora in late Missourian (upper Pennsylvanian) deposits at Garnett, Kansas, suggests a possible upland origin of reptiles. The origin of mammals is known from a rather imperfect evolutionary sequence from the Permo-Triassic Karroo deposits of South Africa. Similar faunas inhabited Asia. Although local environmental effects of the Cape orogeny may have influenced evolution in the direction of mammals, the trend toward mammalian structure was initiated early in the Permian, well before conditions became favorable for reptilian life in South Africa, and continued until the end of the Triassic. Close approximation in time of the pre-Molteno compressive movements and technical origin of mammals is purely fortuitous. Nor can late Paleozoic glaciation of the southern hemisphere have been a critical factor, for it preceded marked development toward the mammalian condition by a full Period. If birds, as is probable, originated from arboreal reptiles, widespread forest conditions must have been the chief environmental factor favoring their development. Forests flourish in both quiescent and tectonically active times, so no correlation should be expected between orogeny and the origin of birds. Scarcity of deposits containing fossil vertebrates makes the localization of areas of origin of classes or orders almost impossible. Evidence concerning the temporal duration of land connections between continents is afforded by the distribution of terrestrial and fresh-water vertebrates. The Bering land bridge is documented by successive faunal interchanges between North America and Eurasia during the Cenozoic. More spectacular is the profound modification of the South American Tertiary fauna when connection with North America in the Pliocene permitted intermigration. Precise location of land bridges must be established on evidence (mainly geologic) other than the distribution of fossil vertebrates, and faunal distributions must be interpreted in the light of geologic data.