Depositional Environment And Tectonic Significance Of Early Cretaceous In Jinling River Basin,Baoji,Shaanxi

The Jinling River Basin of the North Baoji,Shaanxi is situated in the combining site between Qilian-Qinling Orogenic Belt and Ordos block and belongs to the south-east section of Liupanshan Basin.Since the Late Mesozoic,the Jinling River Basin has experienced Early Cretaceous Epoch fault depression deposition,Upper Cretaceous lifting and swelling,Paleogene-Neogene lifting denudation,and Quaternary vibrating lifting to form the current construction-landform pattern.Based on the study of Lower Cretaceous Series profile in the basin,Lower Cretaceous Series of the Jinling River Basin only developed Sanqiao Formation and Heshangpu Formation.The center of sedimentation is located in the west of the basin.According to the sedimentation features of Lower Cretaceous Series,Jinling River Basin is the deposition of the alluvial fan-river-lake-fluvial facies,developing the cycle sequence from being lower coarse,upper fine,and then coarse.Through the gravel particle size and direction from field survey,statistical analysis of debris fabric is conducted.The research results indicate that the composition of conglomerate is relatively simple,mainly granitic and metamorphic rocks,and contains some sandstone.The gravel particle size ranges from 0.7cm to 8.5cm,especially granule conglomerates.The hydrodynamic conditions are unstable.Gravel sphericity F ranges from 0.91 to 1.65,the flattening value ?is 2.02-2.24,and the roundness P range is 52.9-71.67.The gravel coefficient ranges is varied from 1.32 to 2.20.By recording field gravel lithology,the maximum average content of granitic is 66.5%through gravel property analysis,followed by metamorphic rocks(24.33%)(gneiss and fewer metamorphic rocks).The average content of quartzite is 11.67%while sandstone and siliceous rocks have the less content.Through the measured field data,the rose diagram of gravel a-b side is plotted in the gravel-oriented analysis,gaining the multiple paleocurrent directions in the basin.The main paleocurrent directions include northwest and southeast directions.Through the comprehensive analysis,Heshangpu Formation,Jinling River Basin is based on the alluvial deposition with the flowing water cause.The provenance is derived from northwest of the basin and Qinling Mountain-Qilian Orogenic Belt in the south east.Jingling River Basin is affected by Pacific Plate with western ward diving.Early Cretaceous Epoch takes place stretching fault depression and accepts sedimentation under the east-west extrusion.In late Early Cretaceous Epoch,affected by the late Yanshan movement,under the role of NW-SE extrusion stress,it slowly lifts to result in the inclined deformation of Lower Cretaceous Series Sanqiao Formation(K₁s)and Heshangpu Formation(K₁hs)stratum,forming the uniclinal structure from North-East direction.Under the influence of Sichuan Movement in Upper Cretaceous,the basin is closed.Jinling River Basin is thin in east and thick in west as a whole because the west fault depression degree of the basin is stronger than the fault depression in the east.Moreover,the fault depression time in the west is earlier than that of the east.Entering Cenozoic,affected by uplifting of the Qinghai-Tibet Plateau,Jinling River Basin continuously remains the lifting and denudation state.In Miocene,the west section of Liupanshan Basin is affected by lifting of the Qinghai-Tibet Plateau to expand toward the east,forming arcuate tectonic belt.In this way,Jinling River Basin in the South-East section of Liupanshan is thus extrude.However,Jinling River Basin only develops uniclinal structure.Deformation gradually dies away from the west to the east,showing that the influence of Qinghai-Tibet construction fades away.In quaternary,Jinling River Basin is affected by B curtain of the Qinghai-Tibet Plateau to form the vibrating fault lifting,creating the five-level terrace and the current construction-landform pattern at both sides of the river.