Climate-ocean control on the depositional watermass conditions and organic matter enrichment in lower cambrian black shale in the upper Yangtze Platform

摘要

The lower Cambrian sediments including the Qiongzhusi and Maidiping formations in the Sichuan Basin are generally considered as the main source rocks of Precambrian-Cambrian petroleum system. A comprehensive study integrated with multiple geochemical methods was conducted on 119 lower Cambrian drillcore samples collected from the upper Yangtze Platform. The results show that the fluctuation of sea level and climate during the deposition of the Qiongzhusi and Maidiping formations act as the critical factors controlling the watermass conditions and thus OM accumulation. During the deposition of upper Maidiping Formation (epsilon(1)m), the paleo-climate was arid and sea level was relatively low in the study area. The depositional environment indexes (U-EF-Mo-EF, gammacerane index, beta-camtane and V-EF) suggested that the upper epsilon(1)m was deposited in a strong restricted marine setting with high water salinity and anoxic water conditions in Ziyang-Weiyuan area. However, during the early Qiongzhusi Formation period, the paleoclimate became humid and sea level rose in the study area. High paleoproductiviy caused by high concentrations of nutrients brought by seawater and freshwater plays an important role in OM accumulation in the basal part of the Qiongzhusi Formation (epsilon(1)q). Furthermore, the model of paleopmductivity promotion caused by the coordinated control of climate and extensive transgression in the lower part of Gig may also be applicable to interpret the origin of high organic carbon abundance in the strata deposited in the same period with the epsilon(1)q (such as Shiyantou, Shuijingtuo and Niutitang formations) in other parts of the upper Yangtze Platform.