The Coastal Basins of Somalia, Kenya and Tanzania
The western Indian Ocean seaboard is an Atlantic type of continental margin, hence the sedimentary basins involved are typical pull-apart basins of Klemme (1980). The margin has, however been subjected to transform movements through Late Jurassic to Late Cretaceous times when Madagascar was moving southwards relative to Africa. It is thus referred to as a transform continental margin by, among others, Bosellin (1986) and Mascle et al. (1987). The margin seems to have been an emergent and stable block during the whole of Palaeozoic time. Sedimentation starts on top of a peneplaned Precambrian basement surface made up of highly metamorphosed rocks. These crop out to the west of coastal sediments in what is called the Mozambiquan belt in Tanzania and Kenya (Figs. 6 and 9). Further north, in Somalia, basement rocks crop out as oval shaped areas in southern Somalia and along the northern Somali main escarpment (Fig. 2). Here they contain low grade (Inda series) and medium to high grade (Old Formation) metamorphic rocks related to the Pan-African tectonothermal event that terminated with the intrusion of granites, granodiorites, synites and abundant dikes (D'Amico et al., 1982). Basement exposures along the northern Somali coast, and along the escarpment are attributed to the separation of Arabia from Africa during the Miocene. Whereas exposures in the Bur-Acaba and Nogal areas are considered to be reactivated Mesozoic structures, similar to other structural uplifts recorded further south. At least four sedimentary cycles, each of which was probably associated with a major tectonic event, have been recorded on the east African margin during Phanerozoic time. First is the precursory tectonism of the opening of Indian Ocean and the fragmentation of East Gondwanaland (Late Carboniferous to Early Permian). This lead to the formation of continental rifts which subsequently hosted continental deposits. Marine connections during this time appear at different ages in Tanzania, Kenya and Somalia. These reflect physical limitations to marine incursion in the isolated fault separated restricted troughs of the proto "Malgash Gulf". Salt diapirs in Kenya and Somalia, and the Mandawa salt basin of Tanzania, are considered to be of this cycle. The second phase started with the deposition of basal arenaceous sandstone unit overlain by marine beds. This phase is related to the major faulting phase during the Early Jurassic, which indicates reactivation of basement before the deposition of the arkosic Ngerengere Beds in Tanzania and their equivalents, the Manzeras Sandstones in Kenya, and the Adigrat Formation in Somalia and Ethiopia. A strong marine transgression, which probably culminated during the Middle Jurassic (Bathonian), affected the whole of East Africa. Wholly marine conditions were already established in N.E. Kenya and Somalia by the Early Jurassic, whereas further south, in SE Kenya and Tanzania, this did not take place until the Middle Jurassic. This indicates that the sea was slowly encroaching the margin from the north and east, where open marine conditions already existed (Fig. 13). The southward drift of Madagascar relative to Africa occurred along a short spreading centre and along the Davie fracture zone, a transform fault believed to have been active from Late Jurassic (156 m.y.) to Early Cretaceous (130 m.y.) time. A Late Jurassic transgression was associated with tectonism related to this event. Another major transgression flooded most of East Africa in Aptian and Late Palaeocene to Early Eocene times. These two event could be related to the development of the Owen fracture zone and to the widening of the Indian Ocean at the close of Early Cretaceous and Early Tertiary to Late Eocene times. The fourth cycle includes tectonism related to the drift of southern Arabia away from Africa (northern Somalia) and the development of the East African rift system during Oligocene and Miocene times. This resulted in the total withdrawal of the sea from the present areas of northern Somalia (pre-drift doming), and in the reactivation of sedimentation in the areas bordering the Indian Ocean coastal belt. It also caused the formation of the present Gulf of Aden and the main physiographic features of northern Somalia. A Late Eocene to Oligocene regression occurs along the east African margin. This paper attempts to synthesize the geology and stratigraphic evolution of the coastal basins of the east Africa margin (Fig. 1). The principal data used include well logs, geophysical, geochemical data and literature available. The geology of each country is discussed separately so as to give emphasis to the local stratigraphic variations within the region, even though the margin evolved as one unit. The structural evolution is then looked at as one unit, and finally there is a section on economic considerations. Besides gas discovered in Tanzania, no commercial petroleum reserves have been reported so far in the region. Oil and gas shows have been reported everywhere, and the basin is considered to be a low potential gas prone province (Chatellier and Slevin, 1988), but we think that the conclusion is too premature with the present intensity of data available. Other geological resources, including gypsum, common salt, kaolinite, limestones and other building materials, are exploited at the moment, while heavy mineral beach sands, as well as palaeo-placers are reported to be abundant. The basins discussed in this paper include only those bordering the present Indian Ocean. They include the Somali Embayment, the Somali Coastal Basin, the Luug-Mandera Basin, the Kenya Coastal Basin, the Selous-Ruvu-Tanga Basin and Lindi Rift Basin to the south. These basins are separated by basement highs, either cropping out, or concealed beneath a thin sedimentary cover.