Middle Ordovician sediments locally occurred at the western and southern margins of Ordos platform and are represented by sand-shale facies and carbonates respectively.They are formed in Qilian-Qinling marine trough and Weibei carbonate platform is line with comprehensive analysts of simple-factor based diagrams

facies sequence and charaetus of paleobiota of individual atratigraphical column. Weibei carbonatc platform has correspondingly been subdivided into backbeach Iagoonal zone

shallow-water beach zone and forebeach seaward-sloping Zone.