Nature of Cretaceous Sedimentation in Western Desert, Egypt

Abstract

The Cretaceous System in the Western Desert of Egypt has several sedimentary and tectonic features that are not observed in the underlying formations. Throughout Cretaceous time, changes in the tectonic pattern, depositional environment, and sediment types indicate different magnitudes of tectonic instability. From the available surface and subsurface data, 22 stratigraphic maps were constructed to show thickness variations, facies changes, and paleogeology of Early Cretaceous, Cenomanian, Turonian, and Senonian times. The Lower Cretaceous sequence is a thick fluviatile-marine to marine sequence of carbonaceous sandstone with intercalations of carbonaceous shale and a small amount of limestone. Two maximum thickness trends are present. Maximum thickness is about 3,100 m. A deeply weathered, high Basement Complex massif east of the desert, extended to the present Nile Valley. The configuration of this massif affected the nature and type of sediments that were deposited. As the sea transgressed southward, the sites of maximum Cretaceous deposition were changed. During Cenomanian time, the principal basin was at the center of the Western Desert; maximum thickness is about 900 m. The Cenomanian is marine and continental, and consists of sandstone and shale in the south, and limestone both at the rim of the basin and in the north. A partly restricted environment probably dominated; in the Baharia Oasis area, a prodelta environment is inferred to have existed. At least two stages of tectonic uplift took place in some localities (e.g., the Baharia Oasis area) during the Cenomanian. At the end of Cenomanian time, the basin was broken up, and Turonian sediments are present mainly in two basins. Open-marine conditions prevailed in both basins where a maximum thickness of 800 m was deposited. The section contains abundant carbonate, which is extensive laterally and vertically. Conspicuous uplift took place in some areas during and at the end of the Turonian. During Senonian time, the Western Desert area was the site of another open-marine transgression. A semi-restricted environment probably existed in the south, where the maximum thickness was deposited. Lithologically, the Senonian shows normal facies changes; sandstone and claystone are present on the south and limestone on the north. Subaerial and submarine highs were present, particularly near the middle of the Western Desert area. The high Basement Complex massif that existed during the Early Cretaceous was reduced in size during the Cenomanian-Senonian period. After Senonian time, the massif was not exposed, except for some islands. The Cretaceous rocks are good petroleum reservoirs. Cenomanian and Lower Cretaceous strata possibly were favorable source rocks. Economic deposits of oolitic iron ore and phosphorite accumulated in semirestricted depositional environments near the Senonian shoreline.