Study Of Stratigraphic Classification And The Sedimentary System Of Early-middle Jurassic In Bijie Northwest Area Of Guizhou Province

Jurassic strata exposures are relatively limited in Guizhou,research area is located in Qiabei where there are larger exposed area.This paper found a variety of information in research area,including rocks,biology,etc.from ground geology observation.It also conducted the strata division and the comparison for research area and found the early-middle Jurassic sedimentary pattern in the research area according to sedimentary facies analysis on the basis of the combination of collected data and predecessor research results.This paper selected five representative sections from research area,including four measured sections and an observed section(the pearl period of formation measured section in Lijiawan,Gaoshi village,Dafang county,Guizhou province;the artesian well formation measured section behinds the Shuixiang mountain,Dafang county,Guizhou province;the Shaximiao formation measured section in the middle jurassic series Xintian formation of Xiangshui beach,Xiangshui village,Dafang county,Guizhou province;the Xintiangou formation measured section in the artesian well formation of Jurassic system,Mazongling,Xinchang village,Dafang county,Guizhou province;and the observed section in the artesian well formation of Bijie tai chi field,Guizhou province).The stratigraphic lithology and rock assemblage characteristics have been found from research area based on the research results of sections.According to the predecessor research results of research area,this paper provided a reasonable scheme of rock stratigraphic classification by comprehensive analysis that divided research area into Artesian Well formation,Xintiangou formation,Shaximiao formation and Suining formation from the bottom top.Based on the fossils discovered from research area,their regularities of distribu-tion,fossil species and combination characteristics have been found in layers.According to previous studies,four bivalve combination zone and a conchostracans combination zone have been established in accordance to the distribution and the combination characteristics of fossils,including bivalve Pseudocardinia combin-ation zone,Lamprotula(Eolamprotula)cremeri combination zone,Lam-protula(Eolamprotula)-Psilunio combination zone,Lamprotula(Eolamprotula)guangyuanensis-Undulatula combination zone,and conchostracans Euestheria ziliujingensis combination zone.The trace fossils of dinosaur activities first discovered in research area that eight footprint fossils of sauropod dinosaurs arranged in pairs in the sandstone layer which is in the middle part of the layer in Ma'an Mountain of artesian well formation,which has great research implications on the recovery and the reestablishment of the local palaeoenvironment,paleogeography and paleoclimate and the evolutionary history of the local palaeovertebrates community.There are relatively abundant ancient fossils in research area,the chronostrati-graphy could be further divided based on the exposure strata and the characteristics and the combination of fossils.The regional transverse comparison have been conducted to the layer and the regularities of distribution of each biological comb-ination zone based on the methods of biostratigraphy.It could be concluded that research area artesian well formation corresponds with Sinemurian-Toarcian,the upper part of Hettangian Stage;Xintiangou formation is attributed to aalenian stage;Shaximiao formation corresponds with callovian stage-bajocian stage;Suining formation corresponds with oxfordian.It could be concluded that early-middle Jurassic could be divided into two types of continental deposit system formation,and lacustrine facies and lake delta facies in the bijie prefecture in Qianbei by the comprehensive analysis of lithostratigraphy,biostratigraphy,sedimentary structure and other features.The lacustrine facies are divided into two subfacies,including lakeshore subfacies and shallow lake subfacies.Based on the sedimentary facies analysis of early-middle Jurassic in the bijie prefecture,sedimentary facies model in Qianbei has been established and sedimentary facies model pattern has been provided in this study.