Sedimentary Environment Characteristics And Evolutionary Analysis Of The Late Permian-Early Triassic In Dejiang Area,Guizhou Province

Taking the late Permian-early Triassic strata in Dejiang area of Guizhou Province as the main research object,on the basis of detailed field investigation,based on the tectonic geology theory,sedimentology theory,sequence stratigraphy theory,Under the guidance of paleobiological theory,the sedimentary environment and evolution of the late Permian-early Triassic in Dejiang area,Guizhou Province,were studied in depth.1)In Permian-Triassic strata in the study area well developed,the sequence is complete,abundant fossils,the middle and upper Permian system is mainly in the series of strata,mainly for a film at the bottom of the carbonaceous shale,carbonaceous mudstone,quartz sandstone and flint layers,local area visible line coal formation,the upper lithology as the carbonate rocks,almost all of sediment particle size by fine--fine changes;In the Triassic strata,the lithology is still dominated by carbonate rocks,but the monolayer thickness of rocks is changed from thin to thick,and the grain size of sediments is characterized by coarse to fine to coarse.2)According to the test data of the zhongba section and the back section in the study area,the climate in dejiang area was relatively warm and humid in the Permian.Because it was difficult for oxygen from the air to get into the sea water,the oxygen content in the sea water was low.At that time,some microorganisms in the ocean gained rapid growth through photosynthesis and began to multiply rapidly in the ocean.At the end of the Permian period,the sea level began to fall in dejiang region of guizhou,and oxygen in the air began to enter seawater in large quantities,which caused the oxygen content in local seawater to rise rapidly.During the early-middle Triassic,the local ocean area began to shrink due to the joining of continental plates.At this time the climate of the whole continent changed greatly and became drier and hotter on the whole.At this time,oxygen in seawater evaporates into the atmosphere along with seawater,and Marine life dies and a large-scale extinction occurs,which may be the main reason for the low ¹³C in the ocean.The characteristics of carbon isotope fluctuations near the permian-triassic boundary in the study area can be compared well with those in south China and other places,reflecting that the southeastern margin of the upper Yangtze platform in the northeast of guizhou experienced the same complex extinction and recovery process.3)Based on the study of the permian-triassic strata,the petrographic strata,chronostratigraphic strata and biostratigraphic strata,and the field investigation of the study area,the structural types of the upper Yangtze platform and the evolution rules of the sedimentary basin have been further understood.The results show that the study area was in the development stage of the Yangtze platform cover during the Permian and Triassic,and the carbonate platform facies during the Permian,and the tidal plateau and platform facies during the Triassic.The sea water fluctuates in the Triassic and is deepest in the huangcun dam section of the middle yelang formation.At the same time,according to the variation characteristics of the species and number of the permian-triassic fossils in the study area,the different sedimentary environments at that time were also verified.That is to say,a large number of fossils,such as brachiopods,bivalves,corals and fusulinids,were found in the Permian strata in the study area,especially in the heshan period.In the Triassic period,due to the great extinction event at the end of the Permian period,few biological fossils were preserved in the early Triassic period,and some bivalves and ammonite fossils could be seen locally.Since different types of biological fossils are suitable for different living environments,the biological fossils also confirm the different sedimentary environments at that time.4)The mineral resources related to the Permian-Triassic sedimentary environment in the study area were studied.The results show that the mineral resources formed in the Permian and Triassic in dejiang area of guizhou province are mainly coal mines,including two coal-forming stages of liangshan and heheshan.