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Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO 2 –Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO 2 –Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The 87Sr/86Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region. • The tectonic background of the Late Ordovician-Early Silurian was studied. • The northern margin of the Upper Yangtze Platform is an extensional background. • The extensional tectonics caused the cold seepage events. • The extensional fault controlled the distribution of volcanic clastic sediments. • Organic matter accumulation was influenced by the extensional fault depression. [ABSTRACT FROM AUTHOR]

Details of the Lower Cambrian succession of the newly-discovered intraplatfrom basin of the Yangtze Platform (South China), source rocks of the giant Ediacaran-Cambrian gasfields of the Sichuan Basin, including its stratigraphic framework, redox and organic matter accumulation histories, remain unresolved. This paper describes results of a petrological and geochemical investigation of Lower Cambrian strata near the center of the Sichuan Basin with the goal of elucidating those processes that controlled the sedimentary history of the succession. We recognize two intervals of organic-rich source rocks deposited in the intraplatfrom basin, including the Maidiping Formation and overlying Qiongzhusi Formation. The Maidiping accumulated in a carbonate platform setting during Fortunian-Early Stage 2 time (~541–526.5 Ma) and comprises a variety of rock types, including carbonate, chert, phosphorite, shale, and Mn-ore layers. Total organic carbon (TOC) value ranges from 0.47 to 5.06 wt.%; the highest quality source rocks are found in its lower member. The Qiongzhusi Formation was deposited as part of a clastic shelf during Middle Stage 2-Middle Stage 3 (~526.5–515 Ma) time. This unit is dominated by shale and argillaceous siltstone and displays TOC values varying from 0.30 to 4.28 wt.%. The most organic-rich deposits are found in its lower and upper members. Redox and hydrographic proxies suggest that the intraplatfrom basin was dominated by a strongly restricted environment and anoxic bottom water conditions that experienced intermittent episodes of euxinic or dysoxic-oxic conditions during the Fortunian-Middle Stage 3 time. However, bottom water conditions appear to have been oxic during Late Stage 2 time. Accumulation of Lower Cambrian source rocks across the Yangtze Platform appears to have been closely linked to depositional setting. Our findings suggest that organic matter accumulation in the intraplatfrom basin was fundamentally redox-controlled whereas productivity exerted the greatest control on deposition of organic matter near the slope-deep basin of the Yangtze Platform. In summary, Early Cambrian high-quality source rocks preferentially accumulated in topographic lows, including the intraplatfrom basin and slope-deep basin, an association useful to targeting conventional and unconventional hydrocarbons in the Sichuan Basin. • Stratigraphic framework of the Lower Cambrian deposits in an intraplatfrom basin on the Yangtze Platform were established. • The intraplatfrom basin was dominated by anoxic bottom water with intermittent episodes of euxinic/dysoxic-oxic conditions. • A model depicting controls on OM accumulation across the Yangtze Block during the Early Cambrian was proposed. [ABSTRACT FROM AUTHOR]