The Comparative Study On Upper Devonian Standard Stratotype-section In The Eastern And Western Junggar,Xinjiang,NW China

The Upper Devonian standard stratotype-section in the eastern and western Junggar of Xinjiang are selected in this paper,following the principles of stratigraphic division and contrast in China Stratigraphic Guidelines,research on stratotype-section about rock assemblages,biofossils,top-bottom boundaries,sedimentary environments,and horizontal and longitudinal contrast in the region.The results are as follows.Redefining the Top-Bottom Boundary of the Kaxiweng Formation,it is clear that both the overlying Nanmingshui Formation and the underlying Tuorangekuduke Formation are angular unconformity contacts;It is confirmed that the Keankuduke Formation and the underlying Wulusubasitao Formation and the overlying Heishantou Formation are continuous deposition,clear integrated contact relationship;It is affirmed that there is an integrated contact relationship between the Tielieketi Formation and the Heishantou Formation,there is no diachronism in both groups of strata.The measured profiles of Kaxiweng Formation and Keankuduke Formation compensate for the lack of unknown contact relationship between the top and bottom of the original stratotype profile.It is recommended to be the sub-stratotype profile of the original stratotype profile.The dividing line between the top and bottom of the Upper Devonian in the study area is clearly defined.The boundary between Upper Devonian and Lower Carboniferous is between Keankuduke Formation and Heishantou Formation in Zhifang of eastern Junggar,and the same boundary is between Tielieketi Formation and Heishantou Formation in Daertasi of western Junggar;The boundary between Upper Devonian and Middle Devonian is between Wulusubasitao Formation and Keankuduke Formation in Zhifang of eastern Junggar,it is difficult to find the exact boundary between Upper Devonian and Middle Devonian in western Junggar.It should be emphasized that the top and bottom boundary of Upper Devonian are“lithology + biologic fossil" interfaces,and thus it is difficult to define accurately the top and bottom of the Upper Devonian in Junggar only by a single factor.Accordingly to the zircon U-Pb Age of 375.6+5.7Ma from Anshan Porphyrite of the Kaxiweng Formation in eastern Junggar,it is evidenced that the age of this group is the early Late Devonian(Frasnian);The U-Pb age of zircons from the clastic tuff of the Tielieketi Formation in western Junggar is 364.4+4.2Ma,which confirms that the age of this formation is the late Late Devonian(Famennian).This study yielded abundant leptodermis oblonga,brachiopods,corals and gastropods fossils from the Upper Devonian Formations,these fossils are very similar in biology.Forexample,Leptophloeum rhombicum,a standard plant fossil molecule from Late Devonian,is distributed to different extents in the Upper Devonian of each stratigraphic area.From the aspect of biologic fossil assemblage,it is proved that the stratigraphic bio-assemblage of the Upper Devonian in east and west Junggar is comparable.By longitudinal comparison,the Upper Devonian rock assemblages and sedimentary environments are very similar in their stratigraphic divisions,having strong comparability;But horizontal comparison,at the same time,the continental sedimentary formations in west Junggar and the marine sedimentary formations in east Junggar reflect to a greater extent a large-scale “ocean-land” transformation process,having poor comparability.In the Late Devonian Junggar region,the pattern of “West Land and East Ocean” was presented as a whole.During the Early-Middle period of the Late Devonian,there was a large retreat from west to east,that is,when east Junggar was still a “ocean” in the same period,west Junggar had evolved into a “land”.This palaeogeographic pattern lasted until the Famennian of the Late Devonian,when extensive transgression occurred in the western Junggar area and then turned to marine facies.