DISCOVERY OF THE EARLY AND MIDDLE ORDOVICIAN STRATA ON THE NORTHERN SLOPE OF THE KARAKORUM MOUNTAINS AND THEIR SIGNIFICANCE

The northern slope of the Karakorum Mountains is a geological blank area. When the authors carried out a regional geological survey on a scale of 1: 1,000,000 in 1982--1983, the Early and Middle Ordovician strata as well as a large amount of fossils of trilobites, cephalopods, brachiopods, bryozoans, crinoid stems, etc. were first found in Tianshendaban, Dongguashan, Keshir Village and Tianshuihai of the area. The sequence of the strata is relatively complete and overlie unconformably the stromatolite-bearing Tianshuihai Group of the Changcheng System. So the Lower Ordovician is named the Dongguashan Formation, but the Middle Ordovician continues to be called the Nakeshiqijiang Group of the western neighbouring area. Their biota and lith- ology possess the characteristics of the South China type. The discovery of the fossils represented by Szechuanella in the area and the establishment of the section of the Early and Middle Ordovician strata have provided evidence for studying the assignment of the biostratigraphic subregion on the northern slope of the Karakorum Mountains. More signifi- cantly, it has been confirmed that the strata the the area were deposits of the South China platform-type during the early Paleozoic, so the area should be- long to the western extension of the Jiangtang massif. Thus the evidence for the study of the old plates has been provided.