Multiproxy analysis of paleoredox conditions, paleoproductivity and organic matter enrichment in Cretaceous mudrocks of Bombe-Ediki and environs in the Douala sub-basin

Abstract

The formation and preservation conditions of organic matter-rich mudrocks in the Edki-Mabonji and Mbalangi-Bombe area of the Northwestern section of Douala sub-basin, Cameroon, can play an important role in the accumulation of hydrocarbon. This study integrated detailed geochemical techniques such as total organic carbon content (TOC), Total sulfur (TS), stable isotopes (δ¹³C_org. and δ³⁴S), elemental geochemistry, and Scanning electron microscope (SEM) to reconstruct marine paleoredox conditions, paleoweathering conditions and primary productivity thereby, promoting the understanding of the sedimentary model of organic matter accumulation and enrichment in mudrocks in the study areas. Elemental compositions and their ratios suggest that the mudrocks were dominantly derived from a source area mainly consisting of felsic igneous rock with more silica (granitic rock) than an average granodiorite. Paleoweathering proxies suggest that during sediment production, the source area experienced mostly moderate chemical weathering as a result of variation in climate and sea-level changes. The proxies to reconstruct paleoredox conditions demonstrates fluctuating redox variations from mildly suboxic-to dominantly anoxic, most probably sulphidic benthic conditions, which describes anoxic organicmudrocks sandwiched between the oxygenated sandstone/claystone strata. Paleoproductivity parameters like biogenic barium (Ba_bio), TOC, P_org/Al, Cu/Al and S/¹³C_org ratios, and δ³⁴S_py, δ³⁴S_OS, δ¹³C_org suggest that the deposition of the mudrocks was with a high primary productivity. Consequently, the conceptual model of organic matter accumulation and enrichment reveals that moderate-to high primary productivity, suboxic-to dominantly anoxic sulphidic bottom water, organo–mineral complexes, bacterial sulfate reduction (BSR) and deposition of claystone capping bed isolating the benthic sediments influenced the accumulation and preservation of organic matter at shortened exposure duration in dominantly anoxic deeper water setting.