Rift-related sedimentation and stratigraphy, southern Yemen (Gulf of Aden)

Abstract

Syn- and post-rift successions exposed on land in a 200 km stretch along the northern margin of the Gulf of Aden in southern Yemen can be divided into three depositional sequences. Sequence 1 is a patchily developed and highly variable alluvial plain and saline playa facies assemblage of probable Oligocene age. Sequence 2 comprises fine-grained fluvial sands grading into marginal marine, coastal sabkha-type deposits with some local reefal beds. It has a cumulative thickness of between one and two kilometres and displays evidence of upward gradation to isolated depocentres. Sequence 3 facies reflect the current physiography of the margin with alluvial sediments interfingering with coastal clastic and carbonate facies.

These depositional sequences and their relationship to the rift structure indicate that rift flank uplift occurred before any significant regional extension. The main control on sedimentation was a series of basement highs which compartmentalized the rift margin into numerous sub-basins. Thick sedimentary packages were deposited between the high regions while thin and patchy deposits characterize the highs. Three phases of tectonism are indicated: at 35 Ma the onset of rifting occurred followed by an acceleration in subsidence at around 30 Ma; the rifting climax occurred between 20 and 18 Ma. Sequences 1 and 2 were deposited during extension but do not show growth relationships to faults or any synsedimentary deformation. Sequence 3 represents a post-rift stage which began at about 18 Ma and was characterized by regional uplift, tectonic quiescence and rift shoulder erosion. The facies and geometry of the depositional units (sequence 1 and 2) suggest that sediments accumulated passively in a low relief marginal marine/coastal system dominated by regional subsidence. This is not typical of classical facies models of rift fill and is interpreted to be due to sediment passively infilling earlier formed, fault-generated, accommodation space.