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); 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 Al 2 O 3 +K 2 O+Na 2 O 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.
Highlights
Sedimentary structures and geochemistry analyses play a key role in the reconstruction of depositional conditions
The various structures observed in sedimentary rocks reflect the environmental conditions as well as the evolution of the depositional settings. These geological elements had been widely used to reconstruct the depositional environment of several sedimentary basins, including the Cretaceous sedimentary basins in Cameroon, for example, the Babouri-Figuil sedimentary basin [Ndjeng, 1992], and the Mamfe sedimentary basin [Eyong, 2003]
Many sedimentary structures are observed on this outcrop including pseudo-nodules convoluted bedding, parallel laminae, and lenticular structures sometimes bearing coal
Summary
Sedimentary structures and geochemistry analyses play a key role in the reconstruction of depositional conditions. The various structures observed in sedimentary rocks reflect the environmental conditions as well as the evolution of the depositional settings These geological elements had been widely used to reconstruct the depositional environment of several sedimentary basins, including the Cretaceous sedimentary basins in Cameroon, for example, the Babouri-Figuil sedimentary basin [Ndjeng, 1992], and the Mamfe sedimentary basin [Eyong, 2003]. The Djerem continental sub-basin (Figure 1) is located along the Precambrian fault (Central Cameroon Shear Zone, CCSZ) and belongs to the basins which are linked to the South Atlantic Ocean opening occurred between the Late Jurassic and the Early Cretaceous [Ngangom, 1983]. The Djerem sub-basin shows some peculiarities including the presence of calcareous deposits of Cretaceous age
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