The Carboniferous Biostratigraphy,Chronostratigraphy And Bioconstructions From The Jueluotage Orogenic Belt In The East Tianshan,Northwestern China

The East Tianshan is an important component of the Central Asian Orogenic Belt(CAOB),which located in the joint part of Siberian Craton,Tarim Block and North China Block.It separates the Tarim Block to the south from the Angaran Continent to the north.Generally,the East Tianshan is divided into three tectonic units from south to north including the Central Tianshan Microblock,the Jueluotage Orogenic Belt and the Bogda-Harlik Orogenic Belt.The Jueluotage Belt,between the Dacaotan Fault to the north and the Aqikebulake Fault to the south,is further subdivided into the Xiaorequanzi-Jingerquan Belt,the Kangguer-Huangshan Belt,and the Aqishan-Yamansu Belt,which are separated by the approximately east-west-trending Kangguertag and Yamansu faults,respectively.The Jueluotage Orogenic Belt mainly consists of the Carboniferous volcanic-sediment rocks,especially in the south Aqishan-Yamansu Belt.However,the ages of the Carboniferous lithostratigraphic units in the Jueluotage Orogenic Belt are controversial.A complete and precise stratigraphic framework is imperative for the mineral exploitation and the understanding of the tectonic evolution for the south CAOB.Here,we focus on the palaeontology,lithology,sedimentology and chronostratigraphy of several important lithostratigraphic units,including the lower Carboniferous Nanbeidagou Formation,the lower to upper Carboniferous Yamansu Formation and Kushui Formation,the upper Carboniferous Shaquanzi Formation,the upper Carboniferous Tugutubulake Formation in the Aqishan-Yamansu Belt,the upper Carboniferous Dikaner Formation in the west of the Xiaorequanzi-Jingerquan Belt and the upper Carboniferous Qi'eshan Formation in the east of this belt.The major results as follows:(1)A rich diversity of fusulinids and rugose corals were documented in the Carboniferous strata in the Jueluotage Orogenic Belt.Totally,35 species in 15 genera rugose corals in the Nanbeidagou Formation;88 species in 33 genera rugose corals and 8 species in 2 genera fusulinids in the Yamansu Formation;6 species in 2 genera rugose corals in the Kushui Formation;2 species in 2 genera rugose corals and 18 species in 5 genera fusulinids in the Shaquanzi Formation;5 species in 2 genera fusulinids in the Dikaner Formation;19 species in 18 genera rugose corals and 7 species in 4 genera fusulinids in the carbonate member of Qi'eshan Formation.In additional,1 species in 1 genus heterocoral was collected from the Nanbeidagou Formation;1 species in 1 genus chaetetes and 18 species in 11 genera foraminifers were found in the Yamansu Formation;1 species in 1 genus tabulate coral was obtained from the Qi'eshan Formation.The rugose coral fauna in the upper Carboniferous were few recorded before;and the fusulinid fauna in this study were the first report in the Carboniferous strata in the Jueluotage Orogenic Belt.(2)A few important age-diagnostic conodonts were obtained: Gnathodus postbilineatus Nigmadganov et Nemirovskaya,Hindeodus sp.,Neognathodus symmetricus Lane from the second member of Yamansu Formation;Hindeodus sp.from the third member of Yamansu Formation;Diplognathodus coloradoensis Murray et Chronic,Streptognathodus parvus Dunn,Mesogondolella clarki(Koike),Idiognathodus sp.from the Shaquanzi Formation;Declinognathodus noduliferus(Ellison et Graves),Neognathodus medexultimus Merrill from the Kushui Formation.These species are typical elements during the Serpukhovian stage to Moscovian stage in the Carboniferous.(3)7 rugose coral assemblages were recovered,including the Zaphrentites-Meniscophyllum Assemblage in the Nanbeidagou Formation,the Gangamophyllum-Palaeosmilia Assemblage,the Siphonodendron irregulare-Aulina rotiformis Assemblage and the Lithostrotionella Assemblage in the second member of the Yamansu Formation,the Koninckophyllum-Bothrophyllum Assemblage in the third member of the Yamansu Formation,the Psedozaphrentoides zhaoi Assemblage in the Tugutubulake Formation and the Cystophora-Vesiorionastraea Assemblage in the Qi'eshan Formation.4 fusulinid zone were established: the Profusulinella priscoidea-Profusulinella parva Zone in the third member of Yamansu Formation,the Paraeofusulina imporplana-Eofusulina trianguliformis Zone in the Shaquanzi Formation,the Fusulinella-Fusulina Zone in the Dikaner Formation,the Triticites Zone in the Qi'eshan Formation.(4)4 zircon U-Pb ages were obtained: 336.3±3.0 Ma from the first member of Yamansu Formation,331.3±2.1 Ma and 322.6±1.5 Ma from the second member of Yamansu Formation,311.0±1.7 Ma from the Tugutubulake Formation.Combined with palaeontological datum,the age of the Nanbeidagou Formation was late Tournaisian to early Viséan;the age of the Yamansu Formation is late Viséan to Bashkirian;the age of the Shaquanzi Formation is early Moscovian;the age of the Tugutubulake Formation is middle to late Moscovian;the age of the Kushui Formation is late Viséan to middle Moscovian;the age of the Dikaner Formation is middle Moscovian;the age of the carbonate member of Qi'eshan Formation is Kasimovian to early Gzhelian.In this study,the age of the Yamansu Formation was updated which was considered as the Mississippian time on the basis of the rugose corals before;the age of the Qi'eshan Formation extended to early Gzhelian which constrained by the fusulinids.(5)The fauna,especially the rugose corals fauna,in the Jueluotage Orogenic Belt shows mixed features during the Carboniferous period,which is characterized by mixture of the Boreal Realm(such as Gangamophyllum)and Tethyan Realm(Kueichouphyllum),and a diversity of cosmopolitan species(such as Palaeosmilia,Lithostrotion,Siphonodendron,Dibunophyllum)in the Mississippian period.However,during the Pennsylvanian,compound rugose corals dominant the coral fauna,which is character of more Boreal species(such as Cystophora,Vesiorionastraea)mixed with ‘South China type'(such as Huananophyllum,Nephelophyllum),associated with the decrease in the abundance of the cosmopolitan species(such as Caninia,Koninckophyllum and Timania).This feature shows more biota exchanges between the study area and the Boreal Realm,such as the Moscow Basin and Donets Basin,during the Pennsylvanian period.(6)Combining with the compilation of data about the Carboniferous strata in the Bogda-Harlik Orogenic Belt in the north,we established the stratigraphic correlation among the different tectonic units in the East Tianshan.The results show that different sediments sequences distributed in these two belts.For the Jueluotage Orogenic Belt,the marine sediments were widely distributed during the late Tournaisian to Gzhelian interval(the Nanbeidagou Formation,the Yamansu Formation,the Shaquanzi Formation,the Tugutubulake Formation,the Dikaner Formation,the Wutongwozi Formation and the Qi'eshan Formation).Partly,marine carbonate and clastic rock developed during the early time of the Kushui Formation;continental deposits occurred during the time of the Qishan Formation.In contrast,the marine sediments were restricted in the Bogda-Harlik Orogenic Belt.The littoral-shallow marine facies widely spread in this belt during the Mississippian time;but continental deposits occurred in the east Bogda-Harlik Orogenic Belt(the upper Qijiaojing Formation)in the Serpukhovian age.In the Pennsylvanian stage,marine sediments constrained mainly around Urumqi area in the west of this belt(the Liushugou Formation,the Qijiagou Formation and the Aoertu Formation).(7)Two different bioconstructions developed in the Jueluotage Orogenic Belt,including the fasciculate coral Siphonodendron biostrome in the Yamansu Formation in the late Viséan age;and the phylloid algal reef in the Qi'eshan Formation in the Kasimovian to early Gzhelian.The distribution of the same type buildups show these bioconstructions mainly developed in a shallow-water carbonate platform or margin of the carbonate platform settings around the world,which are quite different from what we found in the Jueluotage Orogenic Belt.Compared to the Siphonodendron biostrome in the eastern and western Paleotethys Ocean,the Siphonodendron biostrome in Yamansu shows a relatively low abundance and diversity of associated non-coral faunas;but relatively high content of bioconstructors.This biostrome was mainly controlled by regionally tectonic-volcanic activities.The phylloid algal reef was widely distributed in the world during the late Pennsylvanian interval.However,in our case,the regionally tectonic-volcanic activities are the key control factor as well as the late Viséan Siphonodendron biostrome in the Yamansu Formation.The bioconstructors and buildups are episodes of the global reef evolution during the Carboniferous time,charactering of coral-bryozoan-crinoid community in the Mississippian time,but calcareous algal community dominated in the Pennsylvanian time.Additionally,the evolution of these communities and buildups probably related to the glacial activities in the Gondwana during the Carboniferous time.