Sedimentary geochemistry of Late Cretaceous-Paleocene deposits at the southwestern margin of the Anambra Basin (Nigeria) : implications for paleoenvironmental reconstructions

Abstract

The Campano-Maastrichtian deposits of the Nkporo and Mamu formations of the Anambra Basin represent prospective hydrocarbon resources in southern Nigeria. In order to reveal the paleoclimate, sediment recycling and paleoenvironment during the deposition of these formations in the southwestern portion of the Anambra Basin, a detailed geochemical analysis was performed using major elements, trace elements and rare earth element data, as well as the mineralogical characterization of 20 samples from two wells. The combination of geochemical ratios (Rb/Sr, Th/U, and C-value) and weathering indices (CIA, CIW, and PIA) demonstrates that the region experienced a warm, humid tropical paleoclimate throughout the Late Cretaceous. Furthermore, West Africa experienced strong precipitation during the Cretaceous, leading to intense chemical weathering in the region, as documented by the geochemically matured sediments.

The paleosalinity (Sr/Ba) and paleoproductivity (Ba/Al and P/Ti) proxies show that the studied mudrocks were deposited primarily in brackish to shallow-marine settings, with poor primary productivity due to terrestrial clastic influx and hydrodynamic influence. Using multiple paleoredox indicators, deposition under oxic conditions is detected. The low authigenic U enrichment and depleted V and Cr values normalized to UCC standards further support oxic bottom water conditions. Wavy laminations observed in the mudrocks indicate retreating seawater and high hydrodynamic conditions. Ultimately, the southwestern Anambra Basin reveals Late Cretaceous-Paleocene deposits of a shallow sea controlled by a strong hydrodynamic regime.