Climate-driven paleoceanography change controls on petrology and organic matter accumulation in the upper Permian Dalong Formation, western Hubei Province, southern China

The influence of paleoclimate on shale sedimentary processes and organic matter accumulation is of great significance for reconstructing the “deep time” paleoclimate as well as for oil and gas exploration. Here, a detailed study on the Lopingian Dalong Formation organic-rich shales in the western Hubei area of the Middle Yangtze region is performed to understand how paleoclimate affects the depositional process and organic matter accumulation of shales. By combining a detailed core description and microscopic observations, along with mineralogical and geochemical analysis, two major lithological facies were identified from the H1 drilling core, including siliceous shale and clay/carbonate bearing mixed shale. The changes in depositional observations and geochemical indicators (including detritus index, Ln (Al₂O₃/Na₂O), CuEF, ZnEF, UEF, MoEF, Cd/Mo and Co × Mn) indicate that the siliceous shale was deposited in an anoxic basin with well-developed upwelling under warm and semihumid conditions. However, the clay/carbonate bearing mixed shale was deposited in a hypoxic basin with weakly developed upwelling, and the climate was arid and hot. These deposits suggest that paleoclimates developed into hot and arid climates over time and that such climate change trends may be a response to global warming at the end of the late Permian. High temperatures inhibit the development of upwelling but increase the input of terrigenous clastic materials by enhancing weathering, and controlling the shale deposition. The average total organic carbon (TOC) content of the basin varies significantly in different lithological facies. We infer that the organic matter accumulation of clay/carbonate bearing mixed shale is comprehensively controlled by the carbonate production rate and primary productivity, while the organic matter accumulation of siliceous shale is primarily controlled by the authigenic quartz production rate. When the detrital index in the study area reaches 80 %, TOC does not increase or even decrease. Therefore, in the presence of authigenic quartz silicon, the increased silicate content is not always conducive to the accumulation of organic matter.