The Proto-Indian Ocean and a probable paleozoic/mesozoic triradial rift system in East Africa

A revised Paleozoic/Mesozoic stratigraphy of coastal Kenya (including, in particular, the Karroo) based on current geological mapping near Mombasa is briefly described. This stratigraphy provides the geological framework for proposals concerning the Proto-Indian Ocean and the tectonic setting of the Karroo depositional basins.Recent geophysical evidence suggests that, within Gondwanaland, Madagascar was situated off East Africa near Kenya/Tanzania. The southern limits of the marine Lower Jurassic and southern limits of the marine Middle and Upper Jurassic are in similar positions in mainland Africa and Madagascar using the latter reconstruction. These paleogeographic limits also define the position, during the Jurassic, of an embayment from an ocean to the north. Regional geological similarities also support this reconstruction and are reinforced by paleocurrent data from the Karroo of Kenya indicating drainage north-northeast during the Permian and Triassic and possibly the Lower Jurassic. Marine connections during Karroo times appear to be of different ages in Kenya, Tanzania, Somalia, and Madagascar, probably reflecting physical limitations to marine access in fault-separated basins.The above embayment encroached across the Karroo depositional basins from northeast Kenya to southern Tanzania during the Lower and Middle Jurassic, i.e. from the direction towards which the Karroo drainage had been previously directed. Marine conditions remain to the present day so this embayment can be considered the Proto-Indian Ocean for East Africa. The marine incursion took place before the breakup of Gondwanaland suggesting that during the Jurassic the Proto-Indian Ocean in East Africa was an epicontinental sea and not a true ocean (i.e. floored by simatic crust). The epicontinental nature of this sea is confirmed by the lithologies of the associated sediments. Paleontological data indicate that this sea was an arm of Tethys. True oceanic conditions could not have been established until the displacement of Madagascar away from Africa, probably in the Cretaceous.Accepting the above northern position of Madagascar, the writers also postulate that in East Africa the fault-bounded Karroo depositional basins (troughs) were located within a major triradial rift system extending from Lake Malawi at least as far as eastern Kenya (some 1600 km). This rift system, if valid, was established within Gondwanaland over a period ～100 m.y. in the Paleozoic/Mesozoic (pre-breakup) in marked contrast to the East African Rift System (classical rift valleys) which is mainly a Cainozoic phenomenon (post-breakup). It is, therefore, considered that there is a fundamental difference in origin between the two rift systems.