Abstract
Geochemical data of fifteen Cretaceous sediment samples from Kumba area in the Douala sub-basin are presented to determine the provenance, source rock weathering, tectonic setting and paleo-oxidation conditions of the depositional setting of these rocks. For this purpose, the whole-rocks were analyzed for their major and trace element, including rare earth elements (REEs), contents by ICP-AES and ICP-MS methods respectively. On the basis of their major element composition, the rocks have been classified mainly as Fe-shale, shale, arkose and Fe-sandstone. For the provenance, the plot of Zr vs. TiO2, Y/Ni vs. Cr/V, TiO2 vs. Al2O3 diagrams, high LREE/HREE ratios (5.84 to 20.91) and negative and positive Eu anomalies (Eu/Eu*= 0.87 to 1.62) suggest that the studied rocks were mainly derived from felsic igneous rocks with lesser contribution of mafic components. The higher values of paleo-weathering indices such as Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW), ranging from 79.63 to 99.90 and 87.57 to 99.92 respectively, suggest that the sediments and their potential source rocks experienced intense weathering. Whereas the variable Chemical Index of Alteration (CIA) values (63.01 to 99.50) coupled with high content of K2O (up to 7.5) in most samples could be indicative of K-addition diagenetic processes. The A-CN-K plot also suggests a possible K-addition. The redox conditions during the sedimentation were suboxic to oxic as evidenced by the Ce anomalies of 0.88 to 1.03. This is also confirmed by the values of Ni/Co (less than 7), U/Th (less than 1.25) and Cu/Zn (mostly less than 1). Tectonic discrimination diagrams (e.g., SiO2-K2O/Na2O and Th-Sc-Zr/10) show that these rocks were deposited mainly in an active continental margin setting, and in various tectonic environments. This reflects probably the recycling effect experienced by the samples studied.
Highlights
The Douala/Kribi-Campo basin is one of a series of continental shelf basins extending in West Africa from the edge of the Niger delta in Cameroon to the Walvis ridge near the Angola-Namibia border
Fifteen Cretaceous sediment samples from Kumba area in the Douala sub-basin were analyzed in order to determine the provenance, source rock weathering, paleo-oxidation conditions of the depositional environment and tectonic setting of these rocks
2) These rocks were mainly derived from felsic igneous rocks with lesser contribution of mafic component as suggested by the plot of Zr vs. TiO2, Y/Ni vs. Cr/V, TiO2 vs. Al2O3 diagram, rare earth elements (REEs) characteristics and Eu anomalies
Summary
The Douala/Kribi-Campo basin is one of a series of continental shelf basins extending in West Africa from the edge of the Niger delta in Cameroon to the Walvis ridge near the Angola-Namibia border. It is divided into two sub-basins: the Kribi-Campo sub-basin to the south and the Douala sub-basin to the north. The onshore outcrops of these clastic sediments are not visible due to the dense landscape and the thick lateritic cover in the rainforest Few areas such as Kumba exhibit these Lower to Mid Cretaceous outcrops which have been less described in the framework of a regional geology [8]. No geochemical studies have been carried out on the outcrops of ancient sedimentary rocks of Douala sub-basin exposed in the Kumba area
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