Fossil assemblages and sedimentary environment of the Ordovician Pingliang Formation on the southern margin of the Ordos Basin: A case of the Xilinggou section in Jingyang, Shaanxi Province

The Middle to Late Ordovician deep-water deposits along the western margin of the Ordos Basin have been to the target horizons for hydrocarbon exploration. The Pingliang Formation, occurring along the southern margin of the Ordos Basin, is lithologically comparable to the Wulalike and Kelimoli formations of the western margin. Meanwhile, the Pingliang Formation has been considered one of the most extensively developed deep-water successions of the Early Paleozoic in the Ordos Basin. However, the lithology assemblages of the Pingliang Formation vary significantly from west to east along the southern margin of Ordos Basin. Therefore, detailed investigation about the depositional characteristics and environments is crucial not only for understanding the evolution of depositional settings during the Pingliang Period, but also for evaluating its potential in hydrocarbon exploration. This study conducted systematic paleontological and carbonate microfacies analyses on the Pingliang Formation at the Xilinggou section, Jingyang, Shaanxi Province. Six microfacies types have been identified within the Pingliang Formation, in ascending order: silicalite with radiolarian, gray bioclastic micrite, limestone with shell concentrations, wackestone with a diverse and well-preserved infaunal or epibenthic sessile fauna, fine-crystalline dolomite and densely packed reef rudstone. The sedimentary evolution of the Pingliang Formation varies from the bottom to the top, as evidenced by microfacies and evolution of fossil assemblages. At the bottom, deep-water and shallow-water faunas, including brachiopods, trilobites, bryozoans, gastropods, silicate sponge spicules and radiolarians have been found in same horizon. Meanwhile, the lower part of the Pingliang Formation, is dominated by shallow-water organisms, with representatives of lingulate brachiopods, dendroid graptolites, crinoid stems, and trilobites. Fossils are absent in fine-crystalline dolomite of the lower part. In the middle to upper part, there are abundant shallow-water fossils, including pentamerid shell beds, as well as numerous bryozoans and gastropods. Based on the analysis of microfacies and the evolution of fossil assemblages, slope, platform-margin shoal, intertidal, and platform-margin reef-shoal facies have been identified within the Pingliang Formation, in ascending order. The sedimentary evolution recorded in the Xilinggou section indicates a rapid transgression during the initial stage of the Pingliang Formation deposited, led to the co-occurrence of shallow-water and deep-water faunas within the bottom of the Pingliang Formation, characterized by several chert beds with radiolarians and sponges, co-occurred with shallow water epifaunal or infaunal organisms. In the lower to middle part of the Pingliang Formation, the transgression receded, leading to the deposition of fine-grained dolomites. From the middle to upper part of the Pingliang Formation, depositional environments were stable, characterized by a high content of shallow-water bioclasts and algea, indicating platform-margin reef-shoal environments.