Foraminifera Biostratigraphy And Sedimentary Environment Of The Cretaceous Yuqiang Formation In Bangor,Tibet

The study area is located in the southern Qiangtang block of the Qinghai-Tibet Plateau,the northern foothills of the Gangdese-Nianqing Tanggula Mountains,and is located in the Bangongcuo-Nujiang suture zone.The Yuqiang Formation in the study interval has relatively complete strata,continuous outcrops,representative lithologic assemblages,and rich paleontological fossils and sedimentary phenomena.The selected section PM035 is located in a ravine 300 m southeast of the Chadigala.This group is mainly carbonate rock with a small amount of fine clastic rocks,producing foraminifera,brachiopods,bivalves and gastropods.Fossils such as algae and calcium algae,with a thickness of about 973.2m.The lower part is in integrated contact with the Kangqu Formation,but has not reached the top.It is the highest marine layer before the closure of the Ban-Nuyang.The Yuqiang group is extremely rich in foraminifera,and has been identified as 22 species in11 genera,including Orbitolina,Palorbitolinides,Eorbitolina,Columnorbitolina,Mesorbitolina 5genus of big benthic foraminifera,Endothyra,Textulariina,Nezzazata 3 genus of small benthic foraminifera,Herdbergella,Blefuscuiana,Favusella 3 genus of Planktonic foraminifera,and the big benthic foraminifera is divided into three combinations.Based on the comparison of foraminifera stratum distribution characteristics and evolution relationship,the Yuqiang Formation era was designated as Aptian in the early Early Cretaceous to Cenomanian in the early Late Cretaceous.The lithology of the Yuqiang Formation is dominated by bioclastic limestone,micrite bioclastic limestone,bioclastic micrite limestone,scaled limestone,dolomite limestone,and sandy mudstone,and presents multiple rhythmic cycles.Based on field observations and indoor rock thin section identification,combined with the characteristics of fossil-bearing assemblages,nine types of sedimentary microfacies were divided in the Yuqiang group and further classified into five types of sedimentary facies: open platform facies,limited platform facies,platform marginal shoal facies,Platform edge reef facies and platform edge front slope facies.According to the phase sequence structure of the profile,the carbonate deposition process of the Yuqiang Formation generally constitutes three large-scale sea level rise and fall cycles.Overall,although the Yuqiong Formation is the highest marine layer before the Ban-Nuyang closure.However,according to thesedimentary facies reflected by some rock assemblage features and biota assemblage features exposed by the Yuqiang Formation,the water body of the Yuqiang Formation was deepened again during the deposition period and its sea level rose.By comparing the sedimentary facies of the profile with the fossils produced,it is found that different types of foraminifer fossils and their degree of differentiation and richness are different in different sedimentary facies,that is,in the open terraces the foraminifera are more differentiated and richer;in the limited platform,it is easy to see the small benthic foraminifers;in the shoal of the edge of the platform the foraminifera is reduced due to the enhancement of water energy;The reef-building fossil coral and algae can be seen on the reefs at the edge of the platform;the planktonic foraminifera can be seen on the front slope of the platform edge.