Sedimentary environment of the Upper Cretaceous Yogou Formation in Termit Basin and its significance for high-quality source rocks and Trans-Saharan Seaway

Abstract

The Yogou Formation of the Upper Cretaceous is a potential source rock in the Termit Basin of Niger, and the TOC content of the upper member is 1.8–9 times that of the lower member. However, the influence of the sedimentary environment evolution on its organic matter heterogeneity is still unclear. In this paper, the evolution of the sedimentary environment of the Yogou Formation profile was studied from the aspects of micropaleontology, element geochemistry and biomarkers. A total of three third-order sequences (YSQ1, YSQ2 and YSQ3, from bottom to top) were identified in the Yogou Formation. The distribution of micropaleontology in different sequences indicates that YSQ1 mainly deposited in the marine environment, while, the YSQ2 and YSQ3 mainly deposited in the marine-continental transitional environment. The elemental geochemical analysis shows that the water salinity of YSQ1 was higher than that of YSQ2 and YSQ3. From YSQ1 to YSQ3, the terrestrial input increased, the climate became warmer and wetter, and the reducibility of water generally weakened. The biomarker characteristics and organic geochemical parameters indicate that the YSQ1 generally deposited in a sub-anoxic to anoxic environment, while the YSQ3 deposited in the environment frequently alternating from reduction to oxidation, and the reducing environment may provide a good preservation condition for the enrichment organic matters. The abundant terrigenous materials input and good preservation conditions may be the basic reason to form the mudstone with high TOC content in the upper member of the Yogou Formation. The same micropaleontological species found in the Termit Basin, Benue Trough and Bornu Basin provide evidence for the hypothesis that there was a trans-Saharan seaway between the Tethys Sea and the South Atlantic during the Cenomanian and Turonian transgressions. This study not only helps to clarify the enrichment and distribution regularity of high-quality source rocks in the Yogou Formation, but also provides some evidence for the existence of trans-Saharan seaway.