Geochemical and organic petrographic analyses on the Late Eocene bentonitic shale bed in Kom Aushim vicinity, Fayum, Egypt; Paleoenvironmental implications

Abstract

Bentonitic shale bed of the Qasr El Sagha Formation (Late Eocene), located in the Kom Aushim area, Fayum, composes of 5 m thickness of dark massive stratum intercalated with veins of iron bands. In this study, twelve fresh shale samples from this area were investigated concerning their geochemical and organic petrography analyses. A total organic carbon (TOC) of the shale samples is very low (0.07–0.39 wt%). Petrographic analysis of samples shows a dominance of the vitrinite maceral followed by either inertinite and/or liptinite macerals. Theoretically derived vitrinite reflectance (Rocal) reveals that the bentonitic shale bed was subjected to an abnormal increase in maturity toward the invasion of Fe, Zn, Cu rich hydrothermal sulfates and oxides veins. The composition and origin of these veins were investigated by XRD and XRF analyses. The results suggested that the Fe brought from these veins affected the main composition of the shale stratum to exhibit Fe-shale facies. Moreover, rich iron in the depositional medium leads to form siderite minerals (≈10%) and the replacement of lime associated clay to Ferron dolomite (≈25%). The dominant paleoclimate synchronized shale deposition was semi-arid and arid climate, as confirmed by the domination of evaporite (≈44% gypsum). The source area of shale components was found to be sedimentary and mafic igneous (basaltic trend). CIA-ICV and A–CN–K ternary diagrams showed that the shale components were subjected to a variety of intensive to moderate weathering, and they were probably coming from gabbro and tonalite igneous rock. An active sedimentary margin and island arc are the main provinces, as the shale was deposited at the marine anoxic condition of a clay-lime platform of a pro-delta sedimentary setting. The organic matters (OM) are of type III and IIIC that refer to terrestrial OM of marginal aquatic environments. Organic and inorganic studies confirm a prevailing depositional environment of a shallow shelf, shoreline mudflats and evaporitic lagoon that reflects the general trend of sea-level regression in the Fayum area during the Late Eocene.