The Definition Of Chengjisihanshan Formation And Its Significances In Baijiantan Region,West Junggar

The Carboniferous stratum is widely exposed in West Junggar, Xinjiang，which has longbeen the focus of many scholars. The detailed research based on1:50000regional geologicalsurvey mapping found out the "Tailegula Formation"stratotype is consist of BaogutuFormation and Karamay ophiolite. The rock asemblage, deformation characteristic andsequence relationship in each unit show obvious difference and the two units are in faultcontact. So we suggest discarding using "Tailegula Formation". There exists a rock asemblagemainly consists of volcanic-sedimentary which the predecessors also divided into "TailegulaFormation" is much different compared with Carboniferous Formations.The field1:5000section survey and route geology survey found out the distribution,the rock asemblage, thecontact relationship and the biological fossil features of this volcanic-sedimentary stratum.The conclusions are as follows:1.The marine fine clastic rocks in the "upper" of "Tailegula Formation"stratotype in faultcontact with Karamay ophiolite tectonic melange belt consists of tuffite, lithic debris tuff,tuffaceous silty mudstone and calcareous siltstone coagulation, with bits of sandwich feldsparlithic sandstone,tuff and limestone lens. The sequence relationship is clear and the plantfossils diacovered in the siltstone are Calamites sp.and Paracalamites sp and the coral fossilsinclude Lithostrotionella cf. Shimeri and Multithecopora.The fossil assemblage combinedwith zircon U-Pb age in tuff indicated the formed age was Early Carboniferous. The rockasemblage, deformation characteristics and the characteristics of fossils are accordance withBaogutu Formation.The volcanic rock in the "lower" part of "Tailegula Formation" stratotype undergoingmultiphase tectonic reformation, formated the disordered geological body, strongly brokenand be classified as part of karamay ophiolite tectonic melange belt is obvious different inrock asemblage and deformation characteristics compared with the "upper" of "TailegulaFormation" stratotype.2.Based on the1:5000section survey and geological route survey, we found out the rockasemblage, contact relationships and the distribution of this volcanic-sedimentary which iswidely exposed in Daerbut river-Kalaxiuka region. The coral and wrist fossils discovered inthe limestone layers indicates that the strata formation age lower limit is the Moscow stage inLate Carboniferous, and the zircons U-Pb isotopic dating for312Ma,which is the furtherevidence for the strata formation age. 3.Baijiantan area is the typical outcrope of this volcanic-sedimentary strata. So wespecify the Chengjisihan mountain section which in the north of Baijiantan region as thestratotype (E85°05’,N45°47’) and difined as Chengjisihan Formation and definited asfollows:It is a stratigraphic sequence weakly deformated and the zircon U-Pb dating and fossilassemblage indicates the age is Upper Carboniferous, which is wilde distribute in Daerbutriver-Saertuohai region,West Junggar and show angle unconformity contact overlaying onBaogutu Formation and Xibeikulasi Formation. Chengjisihanshan Formation is consists ofconglomerate, conglomeratic sandstone, sandstone, siltstone, tuff, basalt and siliceous rocklayer, a few clips of sandstone. The middle fossils Choristites sp. and Pseudotimania sp.warediscovered in limestone lens.4.The Chengjisihanshan Formation was divided into3lithologic members based on therock asemblage. The first member consists of coarse clastic rocks; the second member isconsists of lava, siliceous rocks and volcanic clastic rocks and the third member consists ofthe more volcanic clastic rocks and sandstone. The sedimentary environment analysis resultsindicates the rock asemblage of first member in Chengjisihanshan Formation formed incoastal environment. The second member for sea with shallow deepth with strong volcanismand can be roughly separated into two eruption cycles. The third member mainly consists oftuff, more interbedded clips sand,explain as coastal environment with intense volcanism.5.Compared the rock assemblage of Chengjisihanshan Formation and other units consistsof volcanic-sedimentary of Lower Carboniferous, West Junggar. The results are as follows:The rock assemblage of Chengjisihanshan Formation is similar to Halaalate Formation ofUpper Carboniferous in Halaalate mountian, but the fossil asemblage indicates the formationage of Chengjisihanshan Formation is significantly later than Halaalate Formation. Theforming age of Chengjisihanshan Formation is accordance with Aladeyikesai Formation,butthe rock asemblages of the two Formations are much differrnet. So we established thestratigraphic framework of Carboniferous in noth of Deerbut river-Saertuohai region from theold in the new order as:The Baogutu Formation, Xibeikulasi Formation of LowerCarboniferous and the Chengjisihanshan Formation of Upper Carboniferous.