Omdurman Fm

Khartoum Basin margins, central Sudan. The Omdurman Fm has surface exposures, therefore has been published by other studies; but is now essentially replaced by the Mansur Fm in the official Sudan geological usage (??). But we include it here; although not in stratigraphic column – it is coeval with the Mansur Fm – or in the paleogeography reconstructions. From Awad (1994): ""There is inconsistency in the stratigraphic nomenclature used to describe the sedimentary exposures in the Khartoum province. The term Omdurman Formation, formerly named Merkhiyat Sandstone Member of the "Nubian Sandstone Formation” (KHEIRALLA 1966), was first introduced by WHITEMAN (1970) to describe the outcropping, chiefly medium to coarse, and pebbly siliciclastic sediments west of the White Nile and River Nile in the Omdurman area. The formation was named after Omdurman City, and its type section is located at Jebel Merkhiyat, few kilometers west of Omdurma. The term Omdurman Formation is herein [Awad, 1994] extended downwards to include the subsurface sediments that

underlie the outcropping Omdurman Formation-type sediments mentioned above. New nomenclature is followed here to describe both the exposures, that is the traditional Omdurman Formation, and the subsurface sediments in the Khartoum area and vicinity. Two lithologically discrete stratigraphic sequences occur in these areas. Herein they are formerly designated as: Umm Badda Member and Merkhiyat Member. The former is mainly known from the subsurface and the latter form the sedimentary outcrops (traditional Omdurman Formation). In contrast to the Merkhiyat Sandstone Member of the former "Nubian Sandstone" formation , the present Merkhiyat Member is a subdivision of the Omdurman Formation as described herein."

Reference: Awad (1994), A. Gibreel (2015)

[Fig. 1: Khartoum Basin is #8 on map (Fig.3 in Awad, 1994)]

[Fig. 2: Khartoum Basin stratigraphy and depositional sequences (pg. 17 in A. Gibreel, 2015)]

The lower Umm Badda Member "consists mainly of moderately sorted fine-grained sandstone, siltstone and subordinate claystone and shales. The observed sedimentary structures include tabular sandstone bodies that consist internally of lenticular to planar complexes (cf. ALLEN 1983). Individual units show horizontal to low-angle bedded sandstones alternating with finely laminated siltstones and claystones and rarely massive claystone. This unit is mainly known from the subsurface. Only small parts of it are exposed at limited localities along Khor Shambat in the northern part of Omdurman City and in scattered excavations in the city of Omdurman."

The upper Merkhiyat Member " is a lithostratigraphic unit similar to the classical Omdurman Formation … The member consists predominantly of moderately to poorly sorted, medium- to coarse-grained pebbly sandstone; claystone, siltstone and conglomerate are subordinate. The major lithofacies are trough cross-stratified, tabular cross-stratified and massive sandstones; occasional ripple drift laminated and horizontally laminated sandstones are also present. It is also quite common to observe cut and fill structures, channel structures with abundant mud clasts and intraformational conglomeratic layers. In some instances load cast structure can be observed. The trough cross-stratification and the channel structures become more frequent to the west and northwest, and the mean grain size decreases in the same direction. Apart from thin lag deposits, the easterly located sections show, on top, coarsening upward sequences. Paleosol beds and bioturbation occur, in some places, in the transitional zone to the underlying Umm Badda Member. The trough cross-stratification exhibits a dominantly northwestward palaeoflow, with little variation in the northeast and the west." (Awad, 1994)