Microfacies and Paleoenvironmental Significance of Upper Ordovician Carbonate Rocks in Northern Margin of the Upper Yangtze Platform

The Late Sandbian to the Early Katian of the Late Ordovician, as a key period before the end-Ordovician mass extinction event, records paleo-marine and paleo-environments information related to the mass extinction. Based on section measurement, sample collection and thin-section preparation and microfacies analysis, a total of eight carbonate microfacies are identified through system analysis in the Xixiang Formation to Pagoda Formation of the Upper Ordovician(Sandbian-Kadian stage) in Qiaoting section(Nanjiang, Sichuan), which are purplish-red thin laminated dolomite, gray laminated dolomite, gray-black laminated dolomite with black shale, laminated dolomite without bioturbation, calcisilicarenite, peloids bond limestone, bioclastic packstone, and bioclastic wackestone. According to the characteristics of each microfacies and combining with the field observation, we classify these samples into three sedimentary facies types, namely, epicontinental sea carbonate tidal flat, clastic-carbonatite diamictite, and carbonate ramp. The results indicate that most of the Xixiang Formation are tidal flat deposits with shallow water, which began to be transgressive at the top and then rapidly changed to carbonate ramp deposits in the Pagoda Formation. The obvious sea level rise corresponds to the global transgression in the early Katian period, and the global GICE event may be related to this transgression.