THE DISCOVERY OF TRIASSIC UNCONFORMITIES IN THE MIDDLE AND EASTERN SECTORS OF THE JISHISHAN MOUNTAINS, QINGHAI PROVINCE, AND OTHER RELATED PROBLEMS

There has been much controversy over the definition of the Permian-Triassic boundary in the middle and eastern sectors of the Jishishan Mountains of Qinghai Province and no conclusion has been drawn as yet. When the authors took part in the surveying of the Sheet of the Tibetan Autonomous Prefecture of Golog （Ⅰ-47-17） and the Sheet of the Youyun Commune （Ⅰ-47-16） on a scale of 1:200, 000 in the area, they collected abundant geological data, and thus affirm the existence of an unconformity between the Triassic and the underlying strata and put forwards new ideas about related geological problems. The conglomerates at the base of the Triassic have a relatively complex composition. In the fissures of the underlying strata there are "wedge-shaped dikes", whose colour grades from purple at the base upwards to gray or grayish brown. At the base the iron oxide content in the cements is relatively high and the conglomerates are poorly sorted. The conglomerate bed extends continuously in the podiform form, and strikes at angles of a few degrees to 30 degrees to the strike of the underlying Lower Permian strata, while the reverse is found for its dip. The Triassic strata contain an abundance of fossils of cephalopods, lamellibranches and gastropods. According to the fossil assemblages. the Xiayan Formation (T_1by^a) of the Lower Bayan Har Group. the Shangyan Formation (T_1by^b) of the Lower Bayan Har Group, and the Middle Bayan Har Group （T₂ by） are distinguished from below upwards. The first is assigned to the Indian stage of the Early Triassic, while the last two are of Olenekian and Anisian ages respectively. As divergent views exist on the understanding of the melanges of this area, different opinions arise on the stratigraphic division. The authors hold that the exotic blocks in the area are boulders from neighbouring terrigenous areas. They are unstable interbeds occurring in ordinary sedimentary rocks, so it is appropriate to call them "olistostrome".