Stratigraphic Characteristics And Development Patterns Of The Lower Permian In The Western Part Of The Southern Junggar Basin

The southern margin of Junggar Basin is close to the piedmont of Tianshan Mountains.Lower Permian strata are widely exposed,and most of the strata are mainly composed of volcanic rocks and sedimentary rocks.Although the predecessors have carried out a lot of research work on the tectonic sedimentation and the relationship between hydrocarbon accumulation in this area,and achieved relevant results.However,due to the complex tectonic and sedimentary environment caused.by the multi-stage tectonic movement,the understanding of the characteristics and formation mechanism of volcanic and sedimentary rock interbedded sedimentary development is still unclear,which restricts the understanding of the original basin in this area.In order to solve the above problems,the sedimentary environment of the Lower Permian strata exposed in the western section of this area was studied.Through field investigation and sample collection,the lithology,chronology and geochemistry of volcanic and sedimentary rocks in Lower Permian are studied.The results show that the Lower Permian strata in the study area are generally characterized by the interbedded occurrence of volcanic rocks and sedimentary rocks.Most of the lower part of the strata are volcanic rocks,and the lithologic assemblages are bimodal volcanic rocks interbedded with volcanic breccia,tuffaceous sandstone or sandstone and mudstone.The lithology is dominated by frequent interbedding of conglomerate,glutenite,tuffaceous sandstone and silty mudstone,with upward grain size gradually thinning,color changing from oxidation color to reduction color,and most of the gravels are volcanic rocks.There are five lithofacies types: volcanic eruption facies,subvolcanic facies,volcanic eruption facies,fan delta plain facies and fan delta front facies.On the basis of the above studies and the analysis of rare earth elements,trace elements and isotopes,the deep processes of the continental lithosphere and the shallow sedimentary responses of the Early Permian are systematically combined to clarify the mixing of the basaltic magma formed by the upwelling of the asthenosphere mantle and the acidic magma formed by the melting of the underlying crustal materials.Stratigraphic characteristics of dacite,dacite porphyry,rhyolite,basalt,and sedimentary rocks formed during intermittence of volcanic activities overlay each other,thus establishing the development model of "three stages,two sources,early eruption,medium denudation,late intrusion and interbedding",laying the foundation for the restoration of the prototype basin in this area,and for oil and gas in this area.It provides a theoretical basis for exploration decision.