Planktonic Foraminifera of the Middle and Upper Eocene Successions at the Northwestern and Northeastern Sides of the Nile Valley, Egypt: Stratigraphic and Paleoenvironmental Implications

This study deals with the biostratigraphic determination and paleoenvironmental reconstruction of the middle–upper Eocene sediments along the northwestern and northeastern banks of the Nile Valley, Egypt. The studied successions are classified into four rock units as follow: The Qarara (Lutetian–Bartonian), the El Fashn (Bartonian), the Gehannam, and the Beni Suef (Bartonian–Priabonian) formations. A total of eighty planktonic foraminifera species belonging to twenty-two genera and eight families are identified, and their vertical distribution enabled us to recognize four planktonic biozones, namely the Morozovelloides lehneri Zone (late Lutetian–early Bartonian), Orbulinoides beckmanni Zone (early Bartonian), Morozovelloides crassatus Zone (late Bartonian), and Globigerinatheka semiinvoluta Zone (late Bartonian–early Priabonian). The faunal assemblages characterizing these zones showed a great similarity with those recorded in the lower latitudes (tropical and sub-tropical) regions and correlated with the planktonic zones in the northern and southern Tethyan provinces. The appearance of Orbulinoides beckmanni distinguishes the early Bartonian period, its lowest occurrence defines the upper boundary of the Morozovelloides lehneri Zone, and its highest occurrence marks the lower boundary of the Morozovelloides crassatus Zone. The disappearance of the spinose forms of morozovellids and the large acarininids, besides the highest occurrence of Morozovelloides crassatus, defines the lower boundary of the Globigerinatheka semiinvoluta Zone. The middle/upper Eocene boundary is traced based on the last and first appearance of the marker planktonic species and located herein within the Globigerinatheka semiinvoluta Zone. The paleontological data, including the planktonic to benthic foraminiferal ratio (P/B), statistical analyses of different foraminiferal groups, and ternary plot diagrams in conjunction with the sedimentological features, indicate changes in the depositional settings, fluctuating between the inner to middle and outer neritic environment and the uppermost bathyal environment at some levels.