Paleoenvironmental significance and provenance of the Cretaceous calcareous deposits from the Djerem sub-basin (Adamawa, Cameroon) during the Gondwana evolution: sedimentary structures and geochemical constraints

The calcareous deposits were affected by syn-sedimentary (lamination, bedding, convolute, ripple marks, pseudo-nodule) and post-sedimentary structure (diaclase, fault, and desiccation crack). Lamination and bedding expressed by color variation and granulometry are linked to a calm period of sedimentation and seasonal fluctuations and are periodically affected by seismic shocks giving the convolute bedding and pseudo-nodules. The presence of symmetrical ripple marks and desiccation cracks indicate respectively back and forth water movements and periodic phases of emersion characteristics of lacustrine to swampy environments. According to geochemistry data, the major element composition classifies the studied samples as shales and wackes. High LREE/HREE ratios (9.07-13.08; average: 11.77); slight positive and negative Eu anomaly (0.98-1.11); Al2O3/TiO2 ratios (16.52-21.87; average: 19.73), Th/Co ratios (1.24-4.18; average: 2.26), La/Sc versus Th/Co and Zr versus TiO2 plots indicate sediments derived from felsic rocks. The K2O/Na2O ratios (0.24-1.90; average: 1.01); index of compositional variability (0.88-1.35); PIA (64.94-98.42) and the Zr/Sc versus Th/Sc diagram indicate that the source rocks have experienced globally a moderate recycling and sorting as well as moderate weathering. The chemical index of alteration values and the SiO2 versus Al2O3 + K2O + Na2O diagram could indicate relatively warm and arid to semi-arid climate during deposition of sediments. The Sr/Ba (0.22-2.01; average: 0.58) ratios indicate a variant salinity. The Y/Ho (<40) and U/Th (<0.75) suggest respectively nonmarine environment and oxic conditions. The Djerem continental sub-basin would not have undergone marine influences yet known in the Nigeria basins of Benue.