Sedimentary facies in the first member of the Upper Triassic Xujiahe Formation in Hexidong, Beibei, Chongqing

The present paper deals, in detail, with the stratigraphy, lithology, sedimentary facies and evolution in the first member of the Upper Triassic Xujiahe Formation in Hexidong, Beibei, Chongqing in the light of field outcrop observation, typical profile measurement, sample collection and laboratory analysis. The lithology in the first member of the Xujiahe Formation consists of grey and brownish grey fine-grained sandstone and dark grey, grayish black shale and silty mudstone intercalated with thin coal seams in which carbonized plant remains and siderite nodules are observed. The sandstones are composed of feldspathic lithic quartz sandstone and feldspathic litharenite with large-scale trough cross-bedding, parallel bedding, horizontal bedding, and ripple lamination. The first member of the Xujiahe Formation is interpreted to be deposited in the shallow-water delta environment including the sedimentary subfacies such as the delta plain, delta front and prodelta subfacies, and the sedimentary microfacies such as the distributary channel, swamp, interdistributary bay and channel mouth bar microfacies. The vertical sequences indicate the evolution of sedimentary environments from the delta plain through the delta front to the prodelta and finally to the delta plain.