Analysis Of Sedimentary Environment In Ordovician-silurian Transition Layer In Xainza Area, Tibet

The Tibetan Plateau is the youngest one on the earth. Located in the middle of the Tibetan Plateau, the Gangdese Belt has different geological history compared to the other plates in our country. Xainza Area belongs to the Gangdese Belt. This area exposed severals of continuous sections contains the Ordovician and Silurian, which has complete biological sequence and can make comparison with classic sections in southern China. It is more useful to understand the Gangdese Belt in early Paleozoic through the analysis of the carbonate microfacies in upper and lower boundary, because it can be reflected sedimentary environment, and also response of biological extinction event in this area. As we all know, biological extinction event of late Ordovician was the first time in Phanerozoic five major mass extinctions.We collected the successive limestone samples in upper and lower boundary at 5118 section, and typical samples in every formation in the section at the same time. According to the characteristics of particles type, biological assemblages and their paleoecology by using polarized light microscope, 9 kinds of carbonate rocks microfacies may be recognized. i.e., micrite contained bioclasts(Microfacies-1, or MF-?1 for short) bioclastic micrite(MF-?2) micrite(MF-?3) fine powder crystal limestone contained bioclastic(MF-?4) fine powder crystal bioclastic limestone(MF-?5) powder crystal limestone(MF-?6) coarse powder crystal limestone contained bioclasts(MF-?7) bioclastic limestone contained sponge spicules(MF-?8) bioclastic limestone contained echinoderms(MF-9). With thickness graptolite shale(MF-10) between the limestone and dolomite(MF-11) upper the limestone, the total section in 5118 were explained to form in three facies belts, namely the deep water basin(MF-10), restricted platform(MF-?1 MF-?2 MF-?3 MF-?5 MF-8 and MF-9), open platform(MF-?4 MF-?6 MF-7 and MF-11). And biological extinction events in late Ordovician haven't changed ecological model in neritic setting, but only show the biological taxa, in Xainza Area. This is the same with the universal phenomenon.