Naseem Waqas, Ullah Jan Irfan, Mustahpa Khairul Azlan, Hakimi Mohammed Hail, Ahmed Shabeer, Kahal Ali Y
National Centre of Excellence in Geology, University of Peshawar, Peshawar 25129, Pakistan.
Department of Geology, University of Poonch Rawalakot, Azad Jammu and Kashmir, Rawalakot 12350, Pakistan.
ACS Omega. 2024 Sep 10;9(38):39430-39451. doi: 10.1021/acsomega.4c02995. eCollection 2024 Sep 24.
The Early Cretaceous clay-rich facies of the Sembar Formation represent the most significantly occurring organic-rich sediments in the Southern Indus Basin of Pakistan. In this study, detailed geochemical research of total organic carbon, biomarker, mineralogy and trace elemental compositions, together with kerogen microscopic analysis, were carried out and used to understand the organic matter input and the dispositional environmental setting of the organic-rich Sembar shale. The Sembar shales have high organic matter, as indicated by the total organic carbon (TOC) content of up to 2.31 wt %. These shales contain a high abundance of liptinites (i.e., alginite and bituminite), with significant reactive vitrinite maceral, reflecting a mixed-organic matter with a high contribution of marine-derived input. The biomarker distributions evidence the finding of mainly marine organic matter. The biomarker characteristics such as Pr/Ph, Pr/n-C, Ph/n-C, and tricyclic terpane ratios together with C-C regular sterane distributions show that the source of organic matter was primarily marine-derived from mainly phytoplankton algae, along with amounts of land plant input, and accumulated under a low oxygenated environment. The Sembar shale facies are characterized by high Mo trace element content and high V/Ni and Mo/TOC ratios, suggesting anoxic and nonsulfidic environmental conditions during time deposition. The geochemical proxies such as the Sr/Ba ratio and gammacerane/C hopane (G/C) index suggest that the Sembar shale facies were deposited in a relatively low moderate stratified water column. The higher gallium (Ga) than rubidium (Rb), with high Ga/Rb ratios, indicates a high abundance of the kaolinite mineral, thereby deducing the subaerial weathering during the warm and humid climatic conditions. In this case, these warm and humid climatic conditions cause an influx of masses of nutrients, which could be attributed to increasing marine primary bioproductivity. These large masses of nutrients into the photic zone are also attributed to the presence of the upwelling system during the deposition time. Therefore, such conditions of bioproductivity and preservation of organic matter (OM) resulted in the accumulation of organic carbon in the shale facies of the Early Cretaceous Sembar Formation.
森巴尔组早白垩世富粘土相代表了巴基斯坦印度河盆地南部最显著的富含有机质沉积物。在本研究中，对总有机碳、生物标志物、矿物学和微量元素组成进行了详细的地球化学研究，并结合干酪根显微镜分析，以了解富含有机质的森巴尔页岩的有机质输入和沉积环境背景。森巴尔页岩有机质含量高，总有机碳（TOC）含量高达2.31 wt%。这些页岩含有大量的类脂体（即藻质体和沥青质体），具有显著的活性镜质体组分，反映了一种以海洋来源输入为主的混合有机质。生物标志物分布证明主要为海洋有机质。Pr/Ph、Pr/n-C、Ph/n-C和三环萜烷比率等生物标志物特征以及碳-碳规则甾烷分布表明，有机质来源主要是海洋浮游藻类，伴有一定量的陆地植物输入，并在低氧环境下积累。森巴尔页岩相的特征是钼微量元素含量高，V/Ni和Mo/TOC比率高，表明沉积时期为缺氧和非硫化环境条件。Sr/Ba比率和伽马蜡烷/C藿烷（G/C）指数等地化指标表明，森巴尔页岩相沉积于相对较低的中等分层水柱中。镓（Ga）含量高于铷（Rb），Ga/Rb比率高，表明高岭石矿物含量高，从而推断在温暖湿润气候条件下发生了陆表风化作用。在这种情况下，这些温暖湿润的气候条件导致大量营养物质涌入，这可能归因于海洋初级生物生产力的提高。这些大量营养物质进入光合带也归因于沉积时期上升流系统的存在。因此，这种生物生产力和有机质（OM）保存条件导致了早白垩世森巴尔组页岩相中有机碳的积累。
