Lower Eocene mixed carbonate siliciclastic ramp facies, Sohag Governorate, Upper Egypt

The study area (Egyptian Nile Valley, Upper Egypt, east Sohag Governorate) represents a minor slice of the El-Maaza Plateau, mainly composed of Lower Eocene carbonate succession. Three well-selected Lower Eocene sections were investigated (Wadi Bir Al-Ain, Awlaad El-Sheekh Village, and Hashem Al-Eseri Village). The studied Lower Eocene succession distinguished into distinct two formations: Thebes Formation (yellowish white laminated and bedded limestone with bands of chert and fine-sliciclastic beds) and conformably followed upward by Drunka Formation (snow white color, more or less horizontality and chert concretions). Field observations revealed numerous structural elements (faults, joints, folds, and shear zones associated with the Pan-African Orogeny) that greatly influenced the lithology and areal distribution of the Lower Eocene sequence. These structural elements were followed by a series of tectonic reactivations, especially during the Cretaceous and Oligocene periods. Particularly in the Wadi Bir Al-Ain, the carbonates of the Drunka Formation are often characterized by a distinct depositional cyclicity feature and more landward facies covering more basinward facies throughout each cycle. These cyclic carbonate deposits show shallowing upward, reflecting shallow epi-continental environmental conditions. The thickness of these cycles and lack of truncated cycles indicate that the amplitude of sea-level fluctuation and periodicity were generally consistent, pointing to a eustatic process for relative sea-level change rather than a tectonic one. Rockfalls, joints, tilting, and faulting are examples of deformation features seen within the Drunka Formation that could be post-depositional. Additionally, there is no argillaceous lithic succession within the Drunka Formation in the studied area. Consideration of the studied Lower Eocene rock units as carbonate ramp deposits was concluded based on thorough field investigations, sedimentological, and lithofacies of the studied Eocene succession, and careful reading of published works of Eocene rock units in Egypt and other international localities (Thebes Formation in inner-middle ramp environmental conditions and Drunka Formation in an outer-ramp environment). Based on standard classification schemes, five microfacies of the studied carbonate rock units (lime mudstone, lime wackestone, lime packstone, lime grainstone, and lime dolostones) were identified from the petrographic microfacies analysis conducted on 85 representative samples of the studied Lower Eocene succession. These microfacies were considered a main tool for better understanding the depositional environments of the studied Lower Eocene rock units.