Elemental geochemistry of Cretaceous deposits in the Dahomey Basin (Nigeria): Implications for paleoclimatic and paleoenvironmental reconstructions

Abstract

The Dahomey Basin, located in southwestern Nigeria, is an energy-rich frontier basin with sedimentary successions consisting of both (onshore) terrestrial and (offshore) marine deposits. Since the discovery of several bitumen seeps, tar sands, and oil and gas shows offshore the basin and the successful production of the Aje Field, the basin has gained immense interest from academia and industry. Nevertheless, surface and subsurface research focusing on paleoclimatic conditions, primary productivity, anoxic conditions, paleowater depth, and hydrodynamic influence in the Dahomey Basin is less documented. To reconstruct paleoenvironmental conditions, subsurface data from the Araromi-Obu (A1) and Idiobiolayo (A15) cores were used. The average chemical index of alteration (CIA) values ranging from 63.01 to 94.88 reflect varying degrees of weathering intensity in the sediment source area of the Cretaceous Araromi Formation. These values show a transition from relatively low to considerably high degrees of weathering in the source region of the Araromi Formation. The analysis of CIA values and Ga/Rb, Sr/Cu, Sr/Ba, Ca/Al, and 100*Mg/Al ratios suggests that the Araromi Formation was deposited in a brackish to marine environment.

Furthermore, chemical alteration-sensitive geochemical indicators revealed prevailing warm, humid tropical climates during the late Cretaceous period, with likely minor arid intervals. In addition, the geochemical markers used (Ba/Al, U/Th, Ni/Co, authigenic uranium (Uau), P/Ti, and [(Fe₂O₃ + CaO + MgO)/(SiO₂ + Al₂O₃)] indicate poor paleoproductivity for the Araromi Formation deposited in an oxygen-rich marginal to shallow marine environment. The upper Cretaceous period was characterised by fluctuating hydrodynamic circulation, with a weak hydrodynamic regime in core A1 and transitioning to strong hydrodynamic conditions in core A15. The inferred paleowater depth corresponds to the salinity measurements, with the A1 core displaying elevated salinity, indicating a greater depth compared to the A15 core, which was deposited in a shallower environment and displays lower salinity.