Aspects of cratonal sedimentation: facies distribution of fluvial and shallow marine sequences in NW Sudan / SW Egypt since Silurian time

Abstract

The predominantly fluvial sediments, which characterize Silurian to Upper Cretaceous strata of NW-Sudan and SW-Egypt, were repeatedly interrupted by marine transgressions, which rapidly progressed toward the south since Ordovician time. Most of the thin, shallow marine deposits of different ages can be traced for more than 1000 km within the studied area of the southern Dakhla Basin, Misaha Trough and the Abyad Basin.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 bythe amalgamation of fluvial sequences from mainly braided to low-sinuosity river sand sandy nearshore deposits.Since sedimentological processes are closely connected with the structural history of the region and with paleogeographic and paleoclimatic conditions, a classification of distinct sequence developments can be given, due to the respective structural situation: 1.a) Fluvial/shallow marine and predominantly fluvial cratonic sheet sandstones are attributed to a mild crustal warping structural type, partly affected by reactivated fault systems (Silurian to Lower Carboniferous, Lower Cretaceous and Upper Cretaceous strata).2.b) Alluvial clastic wedges, attributed to vertical block movement, have been formed by fluvial sediments in intracratonal grabens and display various facies within the alluvial paleoenvironments (predominantly Upper Carboniferous to Lower Jurassic and Upper Jurassic strata).Increasing soil-forming processes have been affecting the depositional style of the alluvial deposits from Carboniferous time onward, and became especially dominant at the Upper Cretaceous.