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