Biostratigraphy And Sedimentary Environment Study In Kangqu Formation,Lower Cretaceous In The Area Of Bange,Tibet

The study area is located within the Bangonghu-Nujiang Suture and is located in Ri'a area,near Bange County,Tibet.The Lower Cretaceous carbonate deposits in the study area are part of the evolution of the remnant sea in the late Banu Ocean closure period,which is of great significance to the evolution of the Tethys Ocean.However we have found Early Cretaceous sedimentary thickness of carbonate rocks-Langshan Formation,as a typical shallow sea sedimentary environment.The mixing deposits in the carbonate with terrigenous clastic rock,belongs to the production of mixed shelf deposit,so the original Langshan formation can be redefined as Langshan Formation,Kangqu Formation and Yuqiong Formation.This paper studies the biostratigraphy and sedimentary facies of the Kangqu Formation.A large number of foraminifera,gastropods and other fossils were found in the field,among which foraminifera fossils were not only abundant but also various in total species.According to layers different fossil was found and characteristic combination,a total of 14 species from 10 genus of foraminifera fossils were identified in this work,including Mesorbitolina texana Roemer,Mesorbitolina subconcava Leymerie,Mesorbitolina birmanica Sahni,Mesorbitolina aperta Erman,Palorbitolinoides hedini Cherchi & Schroeder,Praeorbitolina wienandsi Schroeder,Pseudochoffatella cuvillieri Deloffre,Pseudocyclammina lituus Yokoyama,Dictyoconus cuvillieri Foury,Palorbitolina lenticularis Blumenbach,Pleodicytoconus sp.,Palorbitolinoides orbiculata Zhang,Daxia sp.and Cuneolina pavonia d'Orbigny.Two foraminifera fossil assemblages are recognized in this paper which including Praeorbitolina wienandsi-Mesorbitolina texana and Cuneolina pavonia-Mesorbitolina birmanica.Based on study of the foraminiferal assemblages,it is suggested in this paper that the age of the Kangqu Formation is Early Cretaceous at Late Aptian – Early-Middle Albian by comparison in domestic and abroad research.A dominant species of Lower Cretaceous-Ampullina xainzaensis Yu was found as gastropod fossils.It is a predominant Early Cretaceous species in Tibet,which further supports the research of foraminifera biostratigraphy.According to geological survey and carbonate microfacies analysis in Kangqu Formation in this paper,we have identified seven kinds of carbonate microfacies.Through rock types and microfacies research,we can identify four sedimentary facies: mixed clastic bioclastic shoal,bioclastic shoal,depression and open marine shelf.They represent a whole transgression sedimentary sequence,the overall sea level were more turmoil at that time.