Geological mapping in northeastern Somalia (Midjiurtinia region): Field evidence of the structural and paleogeographic evolution of the northern margin of the Somalian plate

Abstract

A detailed geological investigation of the continental margins of northeastern Somalia, in land areas contiguous to major oceanic structures (e.g. the Alula–Fartaq fracture zone), was carried out from 1988 to 1991, and the results are given in the enclosed 1:200,000 geological map, derived from photointerpretation and fieldwork at the scale 1:50,000. With respect to the rift of the Gulf of Aden, the stratigraphic sequences illustrated in the map have been distinguished as pre- and syn- and post-rift sediments. The pre-rift sediments rest on the low-grade ?pre-Palaeozoic phyllites and pan-African granitic intrusions and consist of sedimentary cover of continental, lagoon and marine facies ranging in age from Dogger to Eocene. Cretaceous–Eocene sediments pass eastward into clastic and marly deposits of the pelagic domain of the Indian Ocean. The syn- and post-rift sequences ranging in age from Oligocene to Miocene crop out only in narrow “en-echelon” basins striking WNW–ESE along the coast of the Gulf of Aden. The deposits belonging to the syn- and post-rift sequences are discordant and transgressive over the Meso–Cenozoic substratum and are composed by organogenic neritic marine, lagoonal and continental deposits. The tectonic setting of the area is characterized by half-graben bordered by faults with displacements of several kilometres striking WNW–ESE. Faults bordering contiguous half-graben systems often dip in opposite directions, so that in the transition area between different ones a “transfer” or “accommodation” zone develops; the zones are on the landward projection of the oceanic fracture zones, whilst the syn-rift basins are on the landward projection of the oceanic ridge. Based on our fieldwork the following reconstruction is put forward: 1. During the Early Oligocene the Afro-Arabian plate underwent a phase of intense faulting which led to the formation of small syn-rift basins elongated in the WNW–ESE direction, separated by transfer zones and structural highs. 2. Progressive crustal extension led to the formation of spreading centres of oceanic crust corresponding to earlier syn-rift basins. The fracture zones linking spreading centres formed in alignment with continental transfer zones.