Depositional model and sequence stratigraphy of the Paleocene-Lower Eocene succession in the Farafra Oasis, Western Desert, Egypt

摘要

This work is to reconstruct a depositional model and establish a sequence stratigraphic framework for the exposed Paleocene-Lower Eocene succession in the Farafra Oasis, Western Desert, Egypt. This has been performed by an integration of the microfacies analysis with aid of the nannofossils and P/B foraminiferal ratio in the studied rocks. The identified microfacies and their related palaeoenvironments suggest a deposition in a carbonate shelf (inner, middle and outer shelf) environment. The Paleocene deposits (Dakhla, Tarawan and lower part of Esna formations) were laid down in a mid-to outer-shelf setting, whereas the deposits of Early Eocene age (Farafra Formation and upper part of Esna Formation) were deposited in an inner shelf setting. An integration of the resultant microfacies, nannofossils and P/B foramineferal ratio in the studied four stratigraphic sections led to record five 3rd order depositional sequences (SQs) separated by six sequence boundaries (SBs). Each sequence consists of transgressive (TST) and highstand (HST) systems tracts. The TST consists of a retrogradational package of facies (planktonic foraminiferal wackstones/packstones, as well as shales and marls rich in planktonic foraminifera) ended by marine flooding surface (MFS). The HST is made up of aggradational package of fades (nummulitic bioclastic packstone, algal alveoline packstone, alveoline-miliolid packstone and shale with a relatively low P/B foraminiferal ratio) topped by sequence boundary (SB). The sequence boundaries are distinguished by lithologic and paleontologic criteria. The recognized lithologic criteria comprise calcretes, iron-stained surface, extensive bioturbation and erosional irregular surfaces. The paleontologic criteria are evidenced by the time gaps (hiatus). The maximum flooding surfaces are recognized where there are a relatively high P/B foraminiferal ratio in the studied fades. The integrated results suggest that sedimentation regime of the studied Paleocene-Lower Eocene rocks was mainly controlled by tectonic activities and subsequent sea-level changes that were a result of a reactivation of the Syrian Arc System during Early Paleogene time.