Analysis Of Sedimentary Facies And Tectonic Significance Of Upper Permian Linxi Formation In Linxi Area,Eastern Inner Mongolia

The study area is located in the eastern part of the southern margin of the Central Asian tectonic belt.The Late Paleozoic sedimentary environment and tectonic background are one of the hot issues in geosciences.The unconformity of Linxi Formation in the study area is over the Middle Permian Zhesi Formation.For a set of clastic sedimentary formations,the study of sedimentary facies is carried out with Linxi Formation as the research object,which is of great significance to the tectonic background of the Late Permian and the closure time limit of the eastern segment of the Paleo-Asian Ocean.Based on the field investigation of the outcrop section of Linxi Formation exposed in Linxi area and the systematic analysis of its sedimentary characteristics,the area from Shuiquan village in the southeastern Linxi County to Xiaobojigou in the northeastern Linxi County was selected to carry out the field section measurement of the upper Permian Linxi Formation in Linxi area.On this basis,under the guidance of sedimentology theory,combined with a large number of previous research results and based on the methods of petrology,paleontology,geochemistry,grain size analysis,detrital light minerals and debris analysis,the stratigraphic division,sedimentary environment,sedimentary facies model,sedimentary facies distribution,provenance direction and tectonic background of Linxi formation in the study area are systematically studied,and the following understandings are obtained :The lithology of Linxi Formation in the study area is mainly composed of polymictic conglomerate,lithic sandstone,feldspar lithic sandstone,lithic quartz sandstone and mudstone.The bottom is mainly composed of thick-bedded to massive conglomerate layers.The middlelower and upper parts are mainly composed of medium-thin fine sandstone,siltstone and argillaceous siltstone.The middle part is mainly composed of thick-bedded to massive mudstone and argillaceous slate.The upper part is mainly composed of medium-fine grained sandstone and fine sandstone in medium-thick layer.The characteristics of grain size parameters,probability cumulative curve,the Sahu discriminant function of sedimentary environment,the characteristics of C-M diagram and the discriminant scatter plot of sedimentary environment altogether show that the hydrodynamic conditions during the sedimentary period of Linxi Formation is dominated by drag flow,showing a weakening trend,and the sedimentary environment is fan delta-lacustrine environment.According to field section investigation and indoor comprehensive analysis,Linxi Formation in Linxi area can be divided into fan delta facies and lacustrine facies,which can be further refined to fan delta plain subfacies,fan delta front subfacies,shore-shallow lacustrine subfacies and semi-deep to deep lacustrine subfacies.Seven sedimentary microfacies are identified,including braided river distributary channel,backswamp,subaqueous distributary channels,estuary dam,subaqueous distributary interchannel,shore lacustrine sand beach,shallow lacustrine sand dam.The determination of sedimentary facies model of Linxi Formation indicates that the sedimentary evolution of Linxi Formation is accompanied by the generation,expansion,atrophy and extinction of lakes.By compiling the sedimentary facies distribution map of the early,middle and late periods of Linxi Formation in Linxi area,it shows that in the whole sedimentary period of Linxi Formation,the lake center was in Guandi,northeast of Linxi County.The variation characteristics of light mineral components in the study area indicate that the provenance of Linxi Formation is mainly from the south of Linxi County,and a small amount may be from the northwest of Linxi County.According to the analysis of debris composition characteristics and the Dickinson's diagrams Qt-F-L and Qm-F-Lt of the source tectonic background,the provenance of Linxi Formation is mainly from the recycling orogeny.Based on the analysis of Qp-Lv-Ls and LmLv-Ls diagrams,it is considered that a large number of provenance in Linxi Formation has the characteristic of collision suture belt.In conclusion,combined with the continental environment of Linxi area in Late Permian,the eastern part of the Paleo-Asian Ocean was closed before Late Permian.