Stratigraphic sequence and tectonic-sedimentary evolution of Sinian-Cambrian boundary in Yangmu-Yandong area, Guangyuan, Sichuan Province

The formation mechanism and time of the Mianyang-Changniang sag are still controversial, which significantly limit the petroleum exploration of the Dengying Formation. In order to provide a better time constraint on the deep-water strata in the Mianyang-Changning intracratonic sag and deepen the understanding of its tectonic-sedimentary evolution process, this study carries out comprehensive stratigraphic correlation and sedimentary environment analysis of four sections in Guangyuan area, based on outcrop description, thin section observation and carbon isotope analysis. The characteristics of petrology and carbon isotopes show that the four outcrop profiles in the study area can be divided into four lithologic units from bottom to top (namelyⅠ,Ⅱ,Ⅲ and Ⅳ). Lithologic unit Ⅰ is composed of thin-bedded argillaceous limestone intercalated with argillaceous dolomite band, which is equivalent to the 3rd Member of the Doushantuo Formation with the Shuram-Wonoka negative δ¹³C excursion. Lithologic unit Ⅱ is represented by black shale, corresponding to the 4rd Member of the Doushantuo Formation. Lithologic unit Ⅲ showing the transition from massive dolomite to chert, can be correlated with the Dengying Formation. Lithologic unit Ⅳ is marked by platy chert, which is equivalent to the Maidiping Formation. Of these, during the early stage of the second member of Dengying to Maidiping period, massive dolomite shifted to thin-bedded chert with slump breccias and fold deposited in the study area, indicating rapid evolution from carbonate platform to slope-basin depositional environment. This deepening depositional trend is opposite to the shallowing to exposure of the whole Middle-Upper Yangtze platform, indicative of rapid subsidence induced by extensional faulting.