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

Taking the upper Carboniferous standard stratotype-section in the eastern and western Junggar,Xinjiang,NW China as the research object,a comparative studies on the rock assemblages,biological fossil assemblages,contact relations and sediment characteristics were carried out in this paper.Support the abolition of the Tailegula Formation and new establishment of Chengjisihanshan Formation.The more recently geological survey conducted that the fine clastic rocks within the previously-established“Tailegula Formation”should be categorized into lower Carboniferous Baogutu Formation,while rock assemblages comprising the basic-intermediate volcanic rocks,peridotite,gabbro and silicalites should be attached to the Karamay ophiolitic mélanges.There still exsit"homonyms"confusions on the division and comparison of the“Tailegula Formation”,as well as the differences in the attributes and ages of geologic bodies due to the improper establishment of“Tailegula Formation”stratotypes.Therefore,it is imperative to abandon the original"Tailegula Formation"and build a new and reasonable stratigraphic unit according to the principle of stratigraphic division and the lithologic assemblage,hence,the Chengjisihanshan Formation is new established,which lies with an unconformity upon the underlying Early Carboniferous Baogutu and Xibeikulasi Formations.Redefined and revised the biostratigraphic succession and stratigraphic structure section of the Hala?alate Formation.The southern and northern strata of the original holostratotype profile of the Hala?alate Formation were separated by regional faults,instead of a successive deposit.The northern part should be classified as early Carboniferous Baogutu Formation,while the southern part should be attributed to the middle and upper layers of the Hala?alate Formation,and strata south of the stratigraphic profile?previous studies classified it into the early Permian Jiamuhe Formation?may be included in the lower layer of Hala?alate formation,and thus,the complete stratigraphic sequence and its sedimentary assemblages of the Hala?alate formation were reconstructed and restored.Rhyolite and dacite were newly identified in the Jimunai Formation,which yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 302.8±3.6 Ma,indicating the Jimunai Formation was formed in the Late Carboniferous.Abundant fossils,such as brachiopods,corals,bryozoans,gastropods and plant fossils,were found in the stratomicroregions of eastern and western Junggar,and they are relatively similar in biological categories,which,coupled with the Late Carboniferous eruption ages of the volcanic rocks within the investigated strata,indicating a comparability in biotic assemblages between the stratomicroregions of eastern and western Junggar.Combined with the previously geological memoirs and other achievements,therefore,we suggest that the rock assemblage within the Huxingliang Formation?Shiqiantan Formation?Liukeshu Formation in the Karamaili stratomicroregion in eastern Junggar shows that the volcanic rocksolcaniclastic rocks and the normal sedimentary coarse clastic rocks are produced rhythmically,which resembles to the rock assemblage within the Jimunai Formation?Qiaqihai Formation in the Sawuershan stratomicroregion in western Junggar.Comparing the Kelamayi stratomicroregion with the Baogutu stratomicroregion,the sedimentary environmentin the eastern Junggar was closer to the continental side and first entered the continental sedimentary environment in the late Carboniferous.In the early Permian,western Junggar basin gradually transformed into a continental environment.It indicates that seawater may gradually withdraw from the east.