Sedimentological, Petrographic, and Geochemical Characteristics of Eocene Limestones in the Minia District (Egypt): Implications for Alteration by Oligocene Basalt Intrusions

The Paleogene was marked by transient warming events and climatic instabilities linked to shifts in palaeoceanographic conditions and the development of widespread carbonate platforms. Along the southern Tethys margin, the Eocene Samalut Formation of the Minia district (central Egypt) represents a laterally persistent carbonate platform that developed under warm greenhouse conditions and a relatively stable shelf setting. This study integrates field-based sedimentology, carbonate microfacies characterization, X-ray diffraction (XRD), and major oxide geochemistry with petrographic analysis for Eocene limestone and intruded Oligocene basalts at three localities (Bahnasa, Bani Khaled, and Alawy Al-Zorq) to elucidate the depositional architecture, diagenetic evolution, variations in carbonate purity, and alteration processes at basalt–limestone contacts. The carbonate succession is dominated by nummulitic and bioclastic packstones/rudstones, bioclastic wackestones, and crystalline limestones, with calcite as the primary mineral. Geochemical results revealed elevated CaO concentrations (~55 wt.%) in Bani Khaled and Alawy Al-Zorq, whereas samples from Bahnasa display lower CaO and higher SiO2, indicating detrital input and/or localized alteration associated with basaltic intrusions. Petrographic observations of the Oligocene basalts document complex crystallization histories and heterogeneous cooling behavior, while alteration patterns at basalt–limestone contacts extend a few meters and display distinct characteristics among the studied localities. At the Bahnasa area, the alteration process is intense and produced different phases of clay minerals and oxidation products. At the Bani Khaled area, the basalt intrusion produced a zone of breccia and shales extending a few meters around the basalt body. At the Alawy Al-Zorg area, the basalt intrusion introduced chlorite veinlets along the fractures, evaporite minerals, and localized limestone recrystallization. Despite their geographical proximity, the studied localities display different alteration histories derived from the Oligocene basalt intrusion.