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The thin fossiliferous and gypsiferous carbonate unit located in the base of evaporite sequence outcropping in an exploited gypsum quarry in Al-Barqan area, northern Western Desert of Egypt represents a transitional stage of sedimentation between the normal marine carbonates and the evaporite deposits. The variations of the rock fabrics of the carbonate unit from peloids-bioclasts grainstone to mudstone and the mineral composition from calcite to dolomite and the association of the carbonate rocks with some gypsum at the top of the unit followed by the precipitation of gypsum deposits, reflect changes in the chemical composition of the water (e.g. variation in HCO3 − and SO 4 −2 concentrations and water salinity), climatic variations (wet vs. dry), brine restriction and progressive shallowing of the basin. The highly negative δ18O values (up to −10.0 ‰) and the low Sr content of the fossiliferous limestones reflect that the early formed shallow marine calcareous sediments might be recrystallized by 18O-depleted freshwater. On the other hand, the very finely crystalline nature of the dolomite, its association with some gypsum and the relatively high Na content reflect that this dolomite is usually interpreted as a primary evaporative dolomite or is considered a very early diagenetic product. Meanwhile, the stoichiometric composition of the dolomite, the negative shift in oxygen isotope values (−1.2 to −0.77 ‰) and the low Sr content may reflect the freshwater-mediated recrystallization of early formed marine-evaporative dolomite. Although the δ13C values (−0.17 to 1.34 ‰) of the carbonates indicate that the major source of CO 3 −2 was derived from inorganic HCO3 −, the presence of small filaments of bacteria and biofilms coated calcite and dolomite crystals and the spheroidal fabrics of some carbonate grains may point to the presence of a relationship between the metabolic activities of bacteria and precipitation of these carbonates with some contribution of organic carbon. Keywords Carbonates Evaporites Paleoenvironment Miocene Al-Barqan Egypt
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