Stratigraphy, facies, and depositional environments of the Paleogene sediments in Cairo–Suez district, Egypt

Abstract

The study of eight stratigraphic sections along Cairo–Suez district, between Gebel Ataqa and Gebel Mokattam, reveals a thick Lower Eocene Ypresian carbonate section, approximately 210 m thick, at Gebel Abu Treifiya just to the west of Gebel Ataqa. It is represented by the Minia Formation rich in Nummulites praecursor, Orbitolites pharaonum Schwager, Alveolina frumentiformis Schwager, and one of the precursors of Nummulites gizehensis group. The Minia Formation is a fairly clear, warm, and shallow marine facies. The Middle Eocene Lutetian sediments are totally missing in all studied sections probably reflecting instability in deposition echoed in the active block movements the area witnessed since the Paleozoic. The Bartonian sea transgressed over the area depositing Gebel Hof Formation at the base, Observatory Formation in the middle, and Qurn Formation at the top. The Gebel Hof Formation was deposited in an open-marine environment, passed upward into shallow marine, neritic to reefal facies; for the Observatory and Qurn formations, the former changed laterally into a sheltered lagoon facies, Sannor Formation, rich in Somalina stefaninii Silvestri, Dictyoconus aegyptiensis Chapman, and Idalina cuvillieri Bignot and Strougo. The Upper Eocene sediments, Maadi Formation, are mainly represented by a carbonate–clastic section showing shallowing-upward cycles resulted by the retreating of the sea shoreline northward during the Late Eocene, with high supply of terrigenous sediments. The lower carbonate cycle of the Maadi Formation was deposited in a restricted platform and tidal flats, whereas the upper siliciclastic cycle containing Carolia placunoides banks was probably deposited in a winnowed platform edge. These depositional environments were developed in grabens, ramps, and footslopes of the down-faulted Middle Eocene blocks, i.e., syn-tectonic deposition. The fluviatile Oligocene sediments were highly controlled by the structural and topographic lows, where a substantial thickness was deposited, Gebel Ahmer Formation, occupying several grabens and gently sloping areas between many synthetic faults.