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

Abstract

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); Al 2 O 3 /TiO 2 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 TiO 2 plots indicate sediments derived from felsic rocks. The K 2 O/Na 2 O 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 * Corresponding author.