Abstract: Through geological field investigation and indoor experiments in the northern Qaidam Basin, the Early Ordovician platformslopebasin facies sedimentary system was studied in detail, according to geological profiles of rock combination types and characteristics of sedimentary structures of the study area. The results show that the continuing sea level rise in the study area, subduction of the northern margin of the Qaidam Basin, and continental arc collision, were the important controlling factors for the basin mountain pattern development and sedimentary filling evolution during this period. In the northern margin of the Qaidam Basin, the Early Ordovician platform margin particle shoal facies developed in the lower part of the Duoquanshan Formation, and lithology included bioclastic, oolitic and micrite limestones. The carbonate rock materials on the deep water slope are mainly different slope sediments, including fine microcrystalline mud, slumps breccia, grain flow, nodular limestone, debris flow and turbidite, brought by gravity flow transportation and sedimentation from various locations. The study area basin lies in a transition area between the carbonate and arc clastic sedimentary systems. Affected by the North Qaidam oceancontinent subduction and continental arc collision, uplifted continental arc material and crust basement provided a large amount of detrital material to the basin. As a result, arc basin deposits developed in the areas close to the edge of the island arc, supported by the gravity flow depositional system of sandy debris flow and turbidity current types.

Key words: northern margin of Qaidam Basin, Early Ordovician, carbonate slope facies, basin facies