Abstract
Ash Shumaysi Formation represents Oligo-Miocene fluvio-lacustrine succession consisting mainly of conglomerates, sandstones, siltstones, muddy, sandy and glauconitic ironstones, fresh water carbonates, tuffaceous mudstone and basalts in descending order. It shows the transition from proximal to distal parts of the depositional environments of braided streams into floodplain and finally into lacustrine facies. This succession comprises four main facies associations arranged according to their predominance from the lower to the upper parts of the succession of Ash Shumaysi Formation as follows: (1) proximal (conglomerates facies association I) braided streams which includes four facies of sandstones and conglomerates (F1, F2, F3 and F4), (2) distal (sandstones facies association II) braided streams which comprises three facies (F5, F6 and F7), (3) meandering streams (facies association III) which include the thinly bedded muddy, glauconitic and sandy ironstones-ferruginous sandstone facies (F8), (4) lacustrine delta (facies association IV) which includes three facies (F9, 10 and 11). The Oligo-Miocene siliciclastic sections of Ash Shumaysi represent deposition in mid to distal parts of alluvial fans formed as a result of interplay between climatic conditions, tectonic and sedimentation. In the extreme distal parts of these alluvial fans, river lakes were filled rapidly with conglomerates, sandstones, mudstones and finally fresh water carbonated were formed during arid periods of low siliciclastic input.