The Study On Silurian Sedimentary In Huji'ertai Of Nileke County, Xinjiang

In this paper, we are studing the Silurian sedimentary strata on the southern slope of Borohoroshan of western Tianshan Mountains, from the petrography, rock composition, appearance fossils, stratigraphic contact relations, sedimentary structures and other aspects of environmental analysis, we did somesysmetical research for the sedimentary strata in the surveyed area. In recent years, through field investigation and comprehensive study, we get some achievements as follows:1. Silurian strata, widely distrituting in the southern slope of Borohoroshan of Nileke country, is a nonnal land-based source of marine clastics and carbonate sedimentary rocks. Conjunction with the stratic character of rock composition, sedimentary facies and fossils face,it will be divided into the Nileke River Formation of Early Silurian epoch (S₁n), The Jifuke Formation of Middle Silurian epoch (S₂j), The Kuru'er Formation of Late Silurian epoch (S₃k) and The Borohoroshan Formation of Late Silurian epoch (S₃b).2. It is first found that The Borohoroshan Formation of Upper Silurian Formation (S₃b) and The Yuzan Formation of Upper Devonian Formation(D₃y) contact with a foult near to Nileke's cement plant. This fault is the northern margin of the Yili Basin,and separated the Yining area and Borohoro area.3. The rock composition of Nileke River Formation is a set of light gray rock-black light gray very thin-thin-layer-like (including soil) calcareous siltstone with carbon, and it can see carbonaceous laminae by microscope;The Jifuke Formation is a thin layer-thick layered light-light green carbonate terrigenous clastic construction, this study collected a large number of corals, brachiopods and Fomes Insects fossil. Corals: Amplexoides cf. borohoroshaensis Cai,? Halysites sp.,Taxopora sp.,Pycnostylus sp.,Favosites cf. forbisi Edwards et Haime,? Amplexoides sp.,Pycnostylus minor He,Mesofavosifes orientalis yu,Taxopora cf. sa-rairica Mironova.Brachiopods: Schellwienella sp.,Isorthis ? sp.,Spirigerina sp.,Atrypa sp..Fomes Insects: Paramphipora sp.;The rock composition of The Kuru'er Formation is mainly light green thin -thin-layer-like (including soil) calcareous siltstone with banded interlayer of (including calcium) fine-grained lithic sandstone, calcareous silty mudstone;The Borohoroshan Formation has a obvious of characteristics in rock combination,which is a set of thin-in layered silty in-fine-grained lithic sandstone, calcareous siltstone of mainly mulberry with light purple-light green interlayer, and sometimes it can be seen interlayer of silty mudstone and marl. Corals fossil collected in Marl: Mesofavosifes Simils (Sokolov),Alveolites sp..4. This area of the Silurian strata in the lithological composition, or biological outlook, sedimentary faceies can be compared with the adjacent areas, there is full envidences for strata's separation.5. Based on rock combinations, fossils, and basic biological sequence analysis, we known:The Nileke River Formation lays beneath the wave base, it is weakly subjected to the impact of tides and waves, water hydrodynamic is wake and retentional water produced reducing conditions of silt and soil hearing carbon which belongs to the enviranment of shallow shelf depositional.The Jifuke Formation's biologies mainly consist of small individual corals, brachiopods and algae, crinoids, echinoderms and so on.With localy enrichment of organism; carbonate rocks mainly contain marlite hearing continental clastics, with much interlayer of fine-grained clastic rocks, so the depositional environment should be the shallow marine,layering beneath of tidal flat with moist and worm climate, clean water, small turbulence.The Kuru'er Formation's rock combinations are unitary, sedimentary structures are rarely seen wheareas the pyrite crystals are common, which belongs to the typical reducing conditions of steady shallow marine.The water is much shallower than the Nileke River Formation, weakly sujected to wind and waves. The water has long-period changes and the small magnitude changes.The Borohoroshan Formation should be the sedimentary environment of coastal (marine)-shallow sea sedimentary, the main color of rocks show light mulberry with light gray-light celadon. The water is unstable and changes frequently, and allways affected by the tides and waves. Thin interlayers of carbonate rocks, coral fossils can be seen in this Formation, which belongs to carbonate doposition in seafloor.6. To sum up, the area of the Silurian sedimentary structures is obvious in the context of the passive (stable) continental margin sedimentary characteristics, the existence of much thick terrigenous clastic rocks and carbonate rocks in the construction, and no previous studies of the "composite Borohoro Paleozoic island-arc zone" active continental margin volcanic rocks in the construction of more deposits, but also not found on behalf of the basic characteristics of active continental margin volcanic arc (island arc or continental margin arc) - trench and Benioff zone (B-type subduction) of the sedimentary sequence of the three signs coexist.Combination of the regional stratigraphy of the and Cambrian sedimentary characteristics, as well as the newly discovered Late Devonian volcanic rocks and intrusive rocks, we should re-understand the tectonic setting of Borohoro 's Early Paleozoic ,the at least during the end of Ordovician to the end of Silurian Period this area is in steady passive continental marine and neritic basin,and it does not have the composite island arc or active continental margin tectonic setting of deposition, but the region continue to uplift or subsidence, which lead to transgression and regression .Its continental material mainly come from the south side of the "oldland Yining." The upper Silurian - lower Devonian, the late Caledonian orogeny, the area changed significantly in tectonic setting, and then entered the active continental margin depositional model. So it is too blurry to name "the Paleozoic compossite island-arc of Borohoro",wheara it should limite in middle Ordovician .in all, after the middle Ordovician, the tectonics setting of the region is in steady passive continental marine and neritic basin.