FACIES SEQUENCES OF THE MIDDLE-UPPER JURASSIC CARBONATE PLATFORM (AMRAN GROUP) IN THE SANAA REGION, REPUBLIC-OF-YEMEN

Rocks of the Amran Group in the Sana'a region of Yemen disconformably overlie the Kohlan Formation and unconformably underlie the Tawilah Gr oup. Sea-level rises have produced landward migration of facies belts and the development of deeper water facies over shallow ones. The geom etry of the depositional environments identified has enabled the passa ge of relatively short-lived transgressive-regressive cycles of sedime ntation to be recognized. Three facies associations are introduced: (1 ) Carbonate platform facies, (2) Carbonate-marl alternation facies, an d (3) Shallow water coral and stromatoporoid build-up facies. These fa cies are widely distributed and the whole sequence reflects deposition on a broad platform upon which shoals separated platform carbonates f rom basin sedimentation and an open marine environment. The repetition and interfingering of both fining-and shallowing-upward cycles within the study areas suggest that deposition occurred within the same basi n with slightly different conditions in different places. The main fac tors controlling their deposition are sea-level changes and tectonics. (C) 1997 Elsevier Science Ltd.