Climatic and geomorphic controls on rift sedimentation: Oligo-Miocene syn-rift facies in the Gulf of Aden, Yemen

Abstract

Existing models for sedimentation in extensional basins provide a basis for the analysis of their facies distribution and stratigraphy but do not include all the variables which control rift basin sedimentation. Tectonic controls on basin geometry and drainage systems are well established, as are the effects of relative base level change, but climatic controls have been considered only for continental basins and the ways in which the relief and geology of the hinterland influence sedimentation have received little attention. A study of Oligo-Miocene sediments in the Gulf of Aden illustrates the need to take into account the geology and geomorphology of the rift flanks when considering the supply of detritus to the rift basin. It is also shown that climate is an important factor in controlling the facies distributions in marginal marine environments in rift settings. Rifting in the Gulf of Aden commenced in the Oligocene and continued until a passive margin developed in the late Miocene. Syn-rift deposits exposed in a coastal strip east of Mukalla in eastern Yemen can be divided into two sequences separated by an angular unconformity. Sequence 1 occurs unconformably on pre-rift limestones and includes localised alluvial fan deposits, algal limestones and gypsiferous sediments formed in evaporitic mudflats. Sequence 2 is more widespread and up to 1500 m thick: it comprises sandy to pebbly braided river deposits, lagoonal marls, localised patch reef limestones and extensive coastal sabkha deposits. Towards the end of Sequence 2, lateral facies variations were controlled by individual fault blocks which compartmentalised sedimentation. The predominance of fine-grained coastal plain and shallow marine facies suggests that there was little coarse clastic supply from the rift flank. This was partly a consequence of limited uplift of the rift shoulder and also the hinterland bedrock which, being largely Palaeogene carbonate rocks, supplied a low proportion of coarse bedload to the basin. A semi-arid to arid palaeoclimate resulted in widespread, syn-rift coastal evaporate deposits. This case study from the Gulf of Aden shows that a comprehensive framework for the analysis of sedimentation in extensional basins should include a consideration of the palaeoclimate, hinterland bedrock and rift flank relief as well as tectonic and base level controls.