Stratigraphic Division And Comparison And Sedimentary Facies Analysis Of Middle-late Triassic In Dafang County Of Guizhou Province

Abundant Triassic fossils have been reported from Dafang region,Guizhou Province,nonetheless the stratigraphic system and the timing of the stratas were less constrained.Based on the data of ithology and biology obtained from the measurement of field stratigraphic section in the area,We refined the stratigraphy and sedimentary facies of the area,the results are illustrated in a Middle-Late Triassic sedimentary model.In this paper,The five field sections and one observation section are listed as: A-Stratigraphic section of Triassic Guanling formation-Erqaio formation in Gao Jianxiang Chahe of Dafang County,Guizhou province.B-Stratigraphic section of Triassic Yangliujing formation-Erqiao formation in Xiangshui Xiang Jingkou of Guizhou province.C-Stratigraphic section of Triassic Guanling formation in Yangchang Gaojiazhuang of Dafang County,Guizhou province.D-Stratigraphic section of TriassicGuanling formation in Qixi County of Guizhou Province.E-Stratigraphic section of Triassic Yangliujing formation-Gaicha formation in Zhiba of Wuli Township,Qianxi County,Guizhou Province.These sections yield out the plausible lithological reference for this study.The rock stratum of the study area is divided into Guanling Formation(Shizishan Section,Songzikan Section),Yangliujing Formation,Gaicha Formation,Erqiao Formation.According to the palaeontological species and their characteristics,exposed horizons and distribution laws of the palaeontology in the study area,two groups of fossil assemblages were established in Dafang area of Guizhou Province:Myophria(Costatoria)goldfussi-Halobia rugosoides and Indosinion-Permophorus.The types and contents of fossils in the study area are not particularly abundant.The middle Triassic of the Guanling Formation(Lower Anisian)is equal to Yangliujing Formation(Upper Anisian)with respect to the characteristics of the existing fossils and the stratigraphic division of the quarries in the study.The Gaicha Formation,of which few fossils were distributed in the study area.The top of the study area was the ancient weathering denudation surface,and the thickness of the residual strata was different.The Gaicha Formation was classified as Middle Triassic,roughly the same as the middle and lower part of the Ladinian.And the chronostratigraphy of the Erqiao Formation is attributed to Upper Triassic Rhaetian?The sedimentary facies analysis is done in the study area according to the rock strata and biostratigraphy in the study area.The Middle-Late Triassic in Dafang County of Guizhou are divided into carbonate platform facies and fluvial facies.The carbonate platform faceies are divided into limitation platform subphase ? open platform subphase?evaporate platform subphase and shoal subphase.The fluvial facies are divided into braided river subphase and meandering river subphase.Based on the study of the sedimentary facies of the Middle-Upper Triassic in the Dafang area,the sedimentary facies model of the Middle-Upper Triassic in Dafang area is established and the sedimentary facies pattern of the study area is summarized.The Triassic Guizhou sea basin was the center of the southwestern basin,inherited its palaeogeographic patter from the end of the Permian,continued to develop its semi-closed setting.The early Middle Triassic in study area is a rapid transgressive large shallow sea,to the middle is a marine regression semi-closed inland sea.The whole Middle Triassic in study area is marine sedimentary.To the late Middle Triassic,the water of the sea to pull out completely in study area,thus forming a continental deposition.