Establishment of the Early Permian Hongliuhe Group in the Beishan area on the border region of Xinjiang and Gansu,China

The 1:50000 regional geological survey indicates that the original Hongliuhe Formation in the Beishan area is large in scale and shows marked differences in lithology and lithofacies. According to the principle of lithostratigraphic division, it should be subdivided into several formations. The original Hongliuhe Formation has four rock associations with notably different lithologies and lithofacies. They are marked by three conglomerate events and one subvolcanic event, representing four different stages of deposition, which correspond to four important unconformities. Based on the four unconformities, the Early Permian Hongliuhe Formation has been subdivided into the Lower Permian Hexi Formation (P1h), Hongliucun Formation (P1hl), Luweijing Formation (P1l) and Hongliujing Formation (P1hj). Plenty of marine animal fossils such as brachiopods and bryozoans have been collected in the Hexi Formation, and a Permian sporopollen assemblage has been obtained from carbonaceous siltstone in the Second Member of the Luweijing Formation. In fact, the four newly established formations are the result of subdivision of a larger-scale, older formation; so according to the requirements for the revision of lithostratigraphic units in the international stratigraphic guide, the authors suggest that the previously established Hongliuhe Formation should be raised to the group rank.