In the past decades, lower Toarcian strata have gained attention on a global paleogeographical
scale but these strata have been poorly studied along the East African Margin. This
study presents a comprehensive review of the Toarcian geological evolution of the
East African Coastal Margin focusing mainly on the Mandawa Basin in Tanzania, the
Morondava and Majunga Basins in Madagascar, the Lamu and Mandera Basins in Kenya,
and the Luuq Mandera and Ahl-Mado Basins in Somalia. We discuss controversies in published
reports regarding facies, stratigraphy, and pathways of marine ingressions during
the Toarcian transgressive episode that led to the extension of a Neotethys sea arm
(the Somali Ocean) into the East African Margin prior to the main phase of Gondwana
drifting and the opening of the Indian Ocean.The earliest Toarcian transgressive phase
was accompanied by rifting between East Africa and Madagascar, which most likely rejuvenated
Karoo rifting, drove marine influxes and inundated existing depressional troughs in
the Ahl-Mado, Luuq Mandera, SW Majunga, and Mandawa Basins. This resulted in the formation
of the Arab-Indo-Madagascar embayment, a NNE-SSW striking epicontinental sea (Somali
Ocean), which was linked to the Tethys Sea southward via the horn of Africa route,
as well as in widespread deposition of Toarcian marine to coastal-lagoonal to continental
sediments. The occurrence of major facies changes from predominantly conti-nental
to shallow/marginal marine facies and the widespread distribution of an Arab-Indo-Madagascan
Bou-leiceras ammonite association confirms the connection between the East African
coastal sedimentary terrain and the Tethys Sea during the Toarcian. Influxes of marine
waters from Tethys Sea were limited in extent to the rift -graben regions and reactivated
Karoo faults system. Restricted marine deposition within a limited epicontinental
sea likely ended during Late Aalenian-Bajocian times, when establishment of fully
marine conditions began due to opening of the Indian Ocean.