Origin of carbonate cements in Cretaceous sandstones from lower Benue Trough, Nigeria: Evidence from petrography and stable isotope composition

Abstract

Beds of authigenic carbonates were identified from three Cretaceous lithostratigraphic units of the Lower Benue Trough, Nigeria. Three carbonate lithologies were recognized by petrographic analysis in the study area. Carbonate-cemented sandstones are dominated by ferroan calcite cements with subordinate amount of siderite and dolomite/ankerite cements formed throughout the diagenetic history of the Asu River Group, Eze-Aku Group and Campano-Maastrichtian proto-Niger Delta sequences. δ18O for the cements range from -4.22 to -6.91‰(PDB) in Asu River Group; -5.23 to 12.66‰(PDB) in Eze-Aku and -4.45 to 6.89‰(PDB) in Campano- Maastrichtian proto-Niger Delta sandstones. δ13C values averaged -5.23‰ PDB for Asu River Group; -11.03‰ PDB and -5.88‰ PDB for Eze-Aku and Campano-Maastrichtian proto-Niger Delta sandstones. Petrography and geochemical data suggest that the mixing of meteoric and marine waters in the sediments caused dolomitization in Asu River group and Eze-Aku Group sediments while deep burial diagenesis under marine depositional environment precipitated calcite cements. Both authigenic and biogenic carbonate cements with low and high δ13C values exits in the sandstones analyzed. The low δ13C values of Asu River Group, Eze-Aku and Campano-Maastrichtian cements that range from 0 to 2.98 ‰ suggest that they are shallow water carbonates. Biogenic carbonate cements were also identified in some of the sandstones. The carbonate cements were formed under the influence of both meteoric- and marine diagenetic conditions which changed in time and space. The dolomitized rocks occur just below erosion surfaces on the continental sediments and on which shallow marine facies are developed.