The effect of early Priabonian sea-level change on the depositional architecture of the lower Qasr El-Sagha Formation, Kom Aushim, NE Fayum, Egypt

Abstract

The vertebrate fossil-bearing upper Eocene strata in the Fayum area, Egypt exhibit complex lateral facies variations making their stratigraphy and depositional setting a matter of debate. They are exceptionally well exposed in the Kom Aushim Quarry region, NE Fayum, where they have hitherto been rarely studied. A recent study in a nearby locality had imposed a tide-dominated delta for these strata. The present study provides insight into this sedimentary succession and introduces an updated sedimentological interpretation of the Umm Rigl Member; basal Qasr El-Sagha Formation relying on outcrop facies analysis and sequence stratigraphic data. Eight short stratigraphic sections were measured and fully described in terms of lithology, grain size, stratal boundaries and sedimentary structures. The mud-dominated lower unit of the Umm Rigl Member consists of four vertically stacked shallowing/coarsening upward bay-margin/lagoonal deltas. The strata in the upper unit of the Umm Rigl Member are interpreted as incised valley back infilled with fluvial, upper estuary, tidal flat passing upwards into shallow and open marine deposits. The strata represent two incomplete depositional sequences, separated by a sequence boundary defined by an abrupt basin-ward shift of facies and equates with the base of incised valley previously recognized in Wadi Al-Hitan area. This boundary may equate with the early Priabonian Pr-2 global sea-level fall. Based on transgressive stacking of facies, the strata of the upper unit are interpreted as estuarine rather than deltaic system. This updated interpretation enhances understanding of depositional history and the paleogeographic evolution of NE Fayum during the late Eocene.