Fossil Assemblages And Environmental Implications Of The Ximatang Formation Of The Lower Triassic In Qiubei,yunnan Province (Southern China)

This study is focused on the Ximatang Formation of the Lower Triassic in Qiubei,Yunnan Province(southern China).The lithostratigraphy,the biostratigraphy,and the sedimentary environment of the lower Triassic in this area is extensively researched.The aim of the study is to understand the fossil assemblages and its sedimenmtary environment of the Ximatang formation,and its implications on the Early Triassic biotic recovery after the Late Permian Mass Extinction.This research mainly adopts the method of field investigation and laboratory experiments by using the theory of petrology,sedimentology,and paleontology.Several sections of the Ximatang Formation are measured and abundant fossils were collected.Many progresses have been made and improved our understanding of the Ximatang formation.The main progresses are listed as follows.(1)A thick layer of oolitic limestone and several layers of K-Bentonite wrere identified during the section measurement,which corrects the error of the former lithological description of the Ximatang Formation.(2)A large number of fossils were collected from the mudstone of the Ximatang Formation,including bivalves,gastropods,brachiopods,conchostracans,ophiuroids(Echinodermata),amonoids,conodonts,plants,and animals with uncertain affinities.The ophiuroids and Pleurmeia was recovered from the Ximatang Formation for the first time.The discovery of diverse and abundant fossils from the Ximatang Formation shed new light on the biotic recovery after the Late Permian Mass Extinction.(3)The index conodont Isarcicella staeschei are identified from the basal part of the Ximatang Formation.New conodont and bivalve zones were established.The age of the Ximatang Formation is confirmed as Indian substage of Gresbachian,Early Triassic period.It can be correlated with the Feixianguan formation in Sichuan and Guizhou provinces.An age date of 249±5Ma is obtained according to the detrital zircon U-Pb dating.(4)Both palaeontological and sedimentological evidences indicate that the palaeoenvironment of the Ximatang Formation was an oxygenated shallow marine,with land mass nearby.The unique environmental setting of the Ximatang Formation make it a refugia during the aftermath of the profound Late Permian Mass Extinction,providing an unique place for rapid recovery of marine ecosystems at that time.