Stratigraphic Framework And Sedimentary Evolution Of The Permian-Jurassic Strata In The Surrounding Area Of The Bogda Mountains

This paper is based on the project of the sedimentary facies and reservoir development mechanism of the Permian-Mesozoic in the northern margin of the Bogda Mountains.Combined with extensive published data,deposition characteristics and other relevant informations of the Permian-Jurassic in Bogda Mountains and its surrounding areas,based on the outcrop survey and data analysis,this thesis systematically studied sequence development,sedimentary facies and sedimentary system distribution of the Permian-Jurassic in the Bogda Mountains and its surrounding areas.The main contents of the dissertation are as follows:(1)Under the control of large-scale tectonic activity,there was a significant change in the regional tectonic system in the Bogda Mountains and its surrounding areas.The activity of the Bogda Mountains in pre-tectonic was intense and the thrusting system in the southern margin of the Junggar Basin was further active.There were three-tectonic elevation in the Bogda Mountains of the Permian-Jurassic.The time of the elevation was in the Late Middle Permian,the Late Triassic and the Late Jurassic.(2)Under the guidance of sequence stratigraphy theory,a regional sequence stratigraphic framework was established for the Permian-Jurassic strata in the Bogda Mountains and its surrounding areas.With the large-scale regional tectonic events and the Bogda Mountains elevation match to identify the four first-order sequence surface,we divided it into three first-order sequences.Under the control of the base change characteristics of tectonic activity and climate change controlled in the first-order sequence evolution,we identified eight secondary sequence interfaces and seven secondary sequences were identified.Based on the identification of the tectonic activity,palaeoclimate and sedimentary environment,identification of the first and second orders of sequences,the mutations of lithology and sedimentary facies are used as the basis for the identification of sixteen cascade sequences,we divided it into fifteen third-order sequences.(3)The Permian-Jurassic sequence in the Bogda Mountains and its surrounding areas were divided into marine facies,transitional facies and continental facies by combining the data of sedimentary facies,the lithology and palaeontological features of outcrop sedimentary strata.We discerned ten types of sedimentary facies,which include the littoral neritic facies,neritic shelf facies,turbidite fan,tidal flat subphase,alluvial fan facies,fan delta facies,braided delta facies,meandering delta facies,fluvial facies and the semi-deep lake subfacies.Among them,it mainly developed the marine sedimentary environment in the Lower Permian,transitional sedimentary environment in the lower part of the Middle Permian,and continental sedimentary environment in the upper part of the Middle Permian-Jurassic.(4)Based on the research of "point-line-plane-body" and the study of the elevation of the Bogda Mountains,basin structure,basin types and evolution,the deposition process of the Bogda Mountains and its surrounding area is clarified.Influenced by theclosure of the Bogda Trough,they were mainly marine sediments of the Bogda Mountains and its surrounding areas in Early Permian Epoch.The troughs were closed completely in the early of Middle Permian Epoch.Tidal flat facies were developed.Residual basin water further became shallow subsequently,and it developed the normal meandering delta facies-braided delta facies.In the late Middle Permian Epoch,the lake basin water increased with the development of a large area of the semi-deep lake-shore shallow lake deposition led to the extensive development of source rock.The first large-scale elevation of the Bogda Mountains,alluvial fan and alluvial plain sediments developed overall in the late of Middle Permian Epoch.The Bogda Mountains lowly uplift in the Triassic,and fan delta facies-braided delta facies-shore shallow lake facies developed.The second stage elevation of the Bogda Mountains was forwards the end of the Late Triassic.As the enhancement of the supply capacity,alluvial fan and alluvial plain developed in the early Jurassic.The elevation of the Bogda mountains was basically stopped latterly,and the braided river-shore shallow lake sediments developed.The Bogda Mountains began to rise again in the late of Middle Jurassic Epoch,which led to the development of braided delta.In the Late Jurassic Epoch,the Bogda Mountains reached its peak upturn,with the extensive development of alluvial fan phase and fan delta facies deposition.