Facies and Depositional Environments of Cretaceous Siliciclastic Sediments in Al Bauga Area , Northern Sudan

The outcropping Cretaceous siliciclastic sediments in the Al Bauga area in Northern Sudan reach a maximum thickness of about 30 meter. They rest with angular unconformity on tilted Precambrian basement rocks and are capped by Tertiary-Quaternary basalts forming a type of nonconformity. The sedimentary sequence was subdivided into three major units separated by two disconformities. The lower unit of the Al Bauga sediments consists of siltstone interlayer by sandstone and mudstones that were deposited in a fluvio-lacustrine environment. The siltstone characterized by massive structure, fine to coarse silt grain sizeand contains well developed crystals of gypsum. The sandstone is laminated, very fined grained and moderately sorted while the mudstone is rich in organic matter and show a high concentration of strontium element. This facies usually occurs in subsurface in most of the area overlain by trough cross-bedded sandstone. The middle unit consists of aeolian sediments. In addition to their high content of medium grained, rounded and moderately well sorted of quartz the aeolian sediments are characterized by large-scale trough cross-bedded and large-scale planar cross-bedded sandstones with foresets that dip up to 30degrees. The upper unit include sets of stacked tabularcross-bedded sandstones which recognizable a deposits of transverse bar complex. This facies characterized by medium grained size, poorly sorted, subangular to subrounded and usually resets on the large scale cross bedding facies. These three sedimentary units are overlain by lacustrine Hudi chert formation and alluvial fan deposits. The Hudi chert of Tertiary age occurring as well rounded boulders and contains gastropods fossilswhereas, the alluvial fan deposits consist of horizontally stratified gravel showing binary rythmatic layering and matrix supported gravel characterized by massive structure, subrounded to subangular and unordered fabric.