Organic geochemical characteristics and depositional models of Upper Cretaceous marine source rocks in the Termit Basin, Niger

Upper Cretaceous marine mudstones are widely known as the most important source rocks in the Western African rift basins. However, geochemical studies on their occurrence and formation mechanism are scarce. In this study, a detailed geochemical investigation within a sequence stratigraphic framework was carried out to reconstruct the palaeoenvironmental conditions and to establish models on the depositional history of Upper Cretaceous marine mudstones in the Termit Basin (Niger), western African. A total of six third-order sequences (DSQ1, DSQ2, YSQ1, YSQ2, YSQ3 and MSQ1, from bottom to top) were identified in the Upper Cretaceous. The distribution of biomarkers from 33 mudstone samples in different sequences indicate that mudstones from YSQ3 were mainly deposited in a suboxic to oxic environment, with significant contributions from mixed terrigenous higher plants and lower aquatic organisms in fresh-brackish water column. In contrast, the other sequences (YSQ2, YSQ1, DSQ2 and DSQ1) mainly represent a suboxic to anoxic marine environment with a stratified water column (0.59 < pristane/phytane < 1.71, 20.78% < gammacerane/C30 hopane < 52.23%), dominated by marine aquatic organisms such as algae and bacteria. Accumulation models of organic matter (OM) for different sequences were established to delineate the combined effects of a variety of geological and environmental controlling factors, such as global sea-level fluctuations, seawater circulation patterns, palaeoclimatic conditions, input of terrigenous OM and inherited rift palaeotopography. Most notably, the relative changes of global sea level and the patterns of seawater circulation were the critical factors controlling the sources, depositional conditions and preservation of organic matter in the Termit Basin. Organic matter rich marine mudstones in Termit Basin are closely related to high supply of terrigenous OM. This study not only provides practical depositional models for marine mudstones in the Termit Basin, but also improves the general understanding of marine organic matter accumulation in a relatively restricted inland rift environment.