Mineralogy and geochemistry of elongated calcite-cemented concretions of the Upper Eocene Birket Qarun Formation (Fayum depression, Egypt): insights into concretions’ origin and paleoenvironmental implications

The upper Eocene fluvial and shallow-marine sandstone of the Birket Qarun Formation is marked by numerous occurrences of elongated calcite-cemented concretions. Using concretions’ mineralogy and phase- and stable-isotope geochemistry, this contribution provides new insights into their origin and paleoenvironmental significance. The concretions display perfectly flattened, elongated and ellipsoid morphologies, which are commonly fully preserved. The concretions are made of well-sorted, rounded calcite-cemented sandstone which features a uniform mineral composition. Analogue pattern is shown by stable isotopes of C and O with very light δ¹³C and δ¹⁸O values pointing to a freshwater source of carbonate cementation. Loose packing and high values of intra-granular volume (> 20%) reflect pre-compactional, syn-depositional origin of calcite cementation. The fluid flow and wind effectively shaped the geometries of the concretions. The directional permeability of the Birket Qarun sandstone favoured the formation of the studied elliptical concretions. The depleted δ¹³C (−6.79‰ to −13.58‰) values, as well as the incorporation of plant-derived light-root-respired carbon, support the freshwater origin of the calcite cement. Findings of fossilized remains of freshwater fish at sampling sites corroborate the continental, freshwater conditions in Fayum region with a flourishing vegetation cover. This is in line with a school of thought advocating an early initiation of the Nile drainage system and formation of a vegetated fluvio-deltaic environment in Fayum region during the Late Eocene contemporaneous to the East African uplift.