Sedimentology of the Cretaceous Outcropping Strata at the NE Margin of the Muglad Rift Basin, Western Kordofan State, Sudan

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Abu Obeida Aamir, Babiker
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This study investigates the facies, depositional environment, source area and paleogeography of the Upper Cretaceous outcropping sediments at the NE margin of the Muglad Rift Basin of the south western Sudan. These sediments are bounded from the West and Southwest by the Muglad Rift Basin and from the east and southeast by the Basement Complex roks of the Nuba Mountains. Methods of investigation included lithofacies analysis, grain size, petrography, heavy minerals and clay minerls analysis. The lithofacies analysis reveals the presence of massive to crudely trough cross-bedded pebble conglomerate facies (Gm, Gt) large and small scale trough cross-bedded sandstone facies (Stl. Sts), planar cross-bedded sandstone facies (Fl/Fm). These facies belong to four major facies association, namely alluvial fan, major channel/bars, minor channel and overbank/ flood plain facies. The basin and facies analysis indicates that the basin-fill model for the study area can be represented by a proximal transverse alluvial fan and braid plain system occupied the eastern and north eastern margins of the Muglad rift basin. This transverse system may pass distally into an axial drainage system or terminated into a lake. The sedimentation, basin development and proximal to distal facies distribution were controlled by both autogenic and allogenic processes such as tectonism and climate. The facies associations show tabular to sheet geometries made of braided channels and bar sequences. The sandstone bodies have generally good vertical and lateral continuity and interconnectedness, however, locally mud dapes and channel plugs may develop. Facies heterogeneities at the various scales from micro-to mega-scale reflect basin-fill model developed and post depositional changes. The laboratory analysis shows that these sediments are generally poorly sorted, coarse to very coarse grained sandstones. The sandstones are mainly quartz arenites with minor arkosic arenites. These sandstones were cemented by ferruginous and/ or siliceous materials and kaolinitic clays, and all appear to have reduced sandatone porosity significantly. The heavy minerals analysis revealed the dominance of the ultras stable assemblage zircon, tourmaline and rutile over the metastable assemblage staurolite and kyanite. Kaolinite dominated over the other clay minerals smectite illite and chlorite. The sandstone facies and composition indicate that the sediments of the study area mainly have carton interior and recycled origin. They were derived from a source area composed of igneous and/or low grade metamorphic rocks. The conditions were favorable for intensive weathering, leaching under hot humid climates and tectonic quiescence. The facies, heavy and clay minerals contents of sediments in the study area show similsrity to those of the Albian- Cenomanian Bentiu Formation.
Sedimentology of the Cretaceous Outcropping Strata at the NE Margin of the Muglad Rift Basin, Western Kordofan State, Sudan