Characteristics of olistostromes from the Ordovician Zhongbao Group in Shihuigou area, North Qilian orogenic belt and their palaeogeographic implications

As the first named location of the Ordovician Zhongbao Group, Shihuigou area in Yongdeng County of North Qilian Mountain is an ideal place for the study of the Ordovician tectonic evolution of North Qilian orogenic belt. On the basis of the measured stratigraphic section, the Zhongbao Group in Shihuigou area can be divided into upper and lower part respectively: the lower part mainly consists of mafic-intermediate volcanic rocks and pyroclastic rocks, whereas the characteristics of the upper part is the existence of carbonate, cherts and siltstone interbedded with intermediate-basic pyroclastic rocks. Field investigation shows that there are many layers of cherts and strong deformation in the upper part of the Ordovician Zhongbao Group. According to the study of the rock assemblage, stratigraphic sequence, sedimentary facies, volcanic eruption facies, deformation features and spatial combination relationship, this set of special sedimentary strata are considered to be the typical olistostromes. Considering its internal structure and volcanic-sedimentary succession, the authors hold that the olistostromes should be deposited in the deep-water basin near the island arc, which belonged to a tectonic background of archipelagic ocean. The discovery of olistostromes in the Zhongbao Group provides reliable evidence for recovering and establishing the Ordovician sedimentary environment and paleogeographic evolution of the North Qilian orogenic belt. It also offers important sedimentologic evidence for further studying the trench-arcbasin system spatial pattern of the North Qilian orogenic belt.