CLIMATIC AND GEOMORPHIC CONTROLS ON RIFT SEDIMENTATION - OLIGOMIOCENESYN-RIFT FACIES IN THE GULF-OF-ADEN, YEMEN

Existing models for sedimentation in extensional basins provide a basi s for the analysis of their facies distribution and stratigraphy but d o not include all the variables which control rift basin sedimentation . Tectonic controls on basin geometry and drainage systems are well es tablished, as are the effects of relative base level change, but clima tic controls have been considered only for continental basins and the ways in which the relief and geology of the hinterland influence sedim entation have received little attention. A study of Oligo-Miocene sedi ments in the Gulf of Aden illustrates the need to take into account th e geology and geomorphology of the rift flanks when considering the su pply of detritus to the rift basin. It is also shown that climate is a n important factor in controlling the facies distributions in marginal marine environments in rift settings. Rifting in the Gulf of Aden com menced in the Oligocene and continued until a passive margin developed in the late Miocene. Syn-rift deposits exposed in a coastal strip eas t of Mukalla in eastern Yemen can be divided into two sequences separa ted by an angular unconformity. Sequence 1 occurs unconformably on pre -rift limestones and includes localised alluvial fan deposits, algal l imestones and gypsiferous sediments formed in evaporitic mudflats. Seq uence 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 fa ult blocks which compartmentalised sedimentation. The predominance of fine-grained coastal plain and shallow marine facies suggests that the re was little coarse elastic supply from the rift flank. This was part ly a consequence of limited uplift of the rift shoulder and also the h interland bedrock which, being largely Palaeogene carbonate rocks, sup plied 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 comprehen sive framework for the analysis of sedimentation in extensional basins should include a consideration of the palaeoclimate, hinterland bedro ck and rift flank relief as well as tectonic and base level controls. (C) 1998 Elsevier Science Ltd. All rights reserved.