Stratigraphic framework, redox history, and organic matter accumulation of an Early Cambrian intraplatfrom basin on the Yangtze Platform, South China

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.