Development of Sedimentary Cycles on the East Sahara Craton Since Silurian Time (Northwest Sudan/Southwest Egypt)

Peter Wycisk

The sedimentary succession of southwest Egypt and northwest Sudan, formerly called the "Nubia(n) Sandstone," been subdivided into a number of formations. The predominantly fluvial sediments which characterize Silurian to Upper Cretaceous strata of this region were repeatedly interrupted by marine transgressions that rapidly progressed toward the south since Ordovician time. Thin, shallow marine sequences of different ages can be traced for more than 1,000 km within the studied area. The development of the sedimentary cycles will be pointed out by surface and subsurface data along a cross section from the southern Dakhla basin in the north to the Misaha trough and Abyad basin in the south.

From Silurian time onward, sheet-like sandstone units of large lateral extent within the shallow basins have been built up by vertical and lateral stacking of fluvial sandstone bodies as well as by the amalgamation of fluvial sequences from mainly braided to low-sinuosity rivers and sandy nearshore deposits. Since sedimentological processes are closely connected with the structural history of the region and with the paleogeographic and paleoclimatic conditions, a classification of distinct sequence developments can be given due to the respective structural situation. (1) Fluvial/shallow marine and predominantly fluvial cratonic sheet sandstones are attributed to a mild crustal warping structural style, partly affected by reactivated fault systems. Strata of the Silurian to Lower Carbo iferous, Lower Cretaceous, and Upper Cretaceous characterize this depositional style. (2) Alluvial clastic wedges, attributed to vertical block movement, have been formed by fluvial sediments in intracratonic grabens and display various facies within the alluvial paleoenvironments of Upper Carboniferous to Lower Jurassic and to some extent also of Upper Jurassic sediments.

The relationship between distinct sequence developments related to their stratigraphic position will be discussed as well as their controlling factors: subsidence, facies stability, mineralogical maturity, sediment load, and soil-formation. Increasing soil-forming process affected the depositional style of the alluvial deposits from Carboniferous time onward and became especially dominant at the Late Cretaceous.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.