Characteristics of Ordovician sedimentation and the discovery of storm deposition in the Wangcang area , northern Sichuan

To clarify the sedimentary pattern and environment of the Ordovician strata in the northern margin of the Upper Yangtze Block, the Dalianghui section was selected as the research profile. Combining the previous studies of the sedimentary facies of the Ordovician strata with detailed profile surveys and microscopic observations in the Wangcang area of the north Sichuan Basin, we conducted a fine division of these sedimentary facies of the Ordovician strata. Results show that the Dalianghui section can be divided into four formations, which are the Zhaojiaba, Xiliangsi, Baota, and Wufeng formations. The lithofacies of the Zhaojiaba and Xiliangsi formations feature tidal flat subfacies of coastal facies, while the Baota Formation is dominated by carbonate platform subfacies of neritic facies. The lithofacies of the Wufeng Formation feature shelf subfacies of neritic facies, which can be further divided into several suborder facies. In this study, storm sedimentary structures were first identified, which mainly consist of scouring-fill structures, storm-generated gravelstones, rip-up clasts, parallel bedding, graded bedding and hummocky cross-bedding. Four types of tempestite sequences were also identified. Therefore, it is suggested that the northern margin of the Upper Yangtze Block was located in the low-latitude hurricane zone of the coastal environment in the Early to Middle Ordovician, and changed into a shallow marine environment in the Late Ordovician.