<p>The Um Bogma Formation exposed in Talet Seleim area consists of three unconforamable members; the lower two are made up of argillaceous rocks interbedded with carbonates that constitute the entire upper member. The argillaceous rocks are siltstones and mudstones consisting of kaolinite and minor to subordinate illite and montmorillonite-illite mixed layer, in addition to a large number of non-clays minerals. The carbonates are dolostones containing a few non-carbonate minerals. The Um Bogma Formation, especially the argillaceous rocks in its middle member, hosts mineralizations of heavy metals, rare earth elements (REEs) and uranium. The minerals associations in this formation are of mixed allogenic-authigenic and primary-secondary origins. As compared to the Upper Continental Crust, the Um Bogma rocks are enriched in MnO, REEs and uranium (U) and have higher LREEs / HREEs ratios. There are positive correlations between ∑REEs and U as well as between both the LREEs and HREEs and thorium. The radioactivity values for the argillaceous rocks, especially those in the middle member, are higher than those for carbonates and indicate a state of disequilibrium. These rocks were subjected to continuous and contrasting U and REEs mobilization processes that led to the marked increase in their original concentrations. These processes were related to the diagenetic actions of circulating subsurface and meteoric waters and, potentially, chemical weathering at the unconformable contacts. The detrital components of the Um Bogma Formation were inherited from felsic igneous rocks and, occasionally, metamorphic rocks. The products of their intensive chemical weathering were transported by rivers and streams having low energy. Deposition of sediments of the lower and middle members occurred in fluvial environments and, in places, littoral lakes and lagoons. Eastward sea transgression resulted in the development of subtidal and intertidal environments in which the carbonates of the upper member were deposited.</p>