Basement Structure,sedimentary Filling And Tectonic Evolution Of Qianhe Basin,Baoji,Shaanxi Province

Large area of loess is continuously distributed in Baoji Qianhe area of Shaanxi province,the outcrop of bedrock is rare,the neotectonic movement is active in the area,and the basement faults and newborn faults are well developed.this dissertation is object on valley terrace feature of Cenozoic Era faulted basins and depositional sequence of Mesozoic in the area which is situated in Baoji Weihe-Wianhe basin of Shaanxi province.By means of drilling,geophysical exploration,remote sensing and other working methods,combined with the data of rock assemblage,Paleontology,Quaternary geology and Geomorphology,this dissertation summarizes the characteristics of valley terrace,basin basement and sedimentary sequence,sedimentary formation and sedimentary facies change of Baoji Weihe-Qianhe basin in Shaanxi province,we gained the achievements as follows:1.According to the differences in geology,geophysical combined detection section arranged in a different location and including 1:25000 high-precision gravity,1:10000high-precision gravity,controllable source audio frequency magnetotelluric sounding(CSAMT),shallow seismic reflection and multichannel seismic surface wave method,and based on engineering drilling of multi-parameter comprehensive logging,loess stratification and loess engineering mechanics characteristics research,find out the basic fault basin of Wei river-Qian river bedrock surface or deep coverage area of the base basin structure characteristics of the Cenozoic thickness,bedrock surface and basement.2.The lithology,lithofacies and time sequence of the Cenozoic strata in weihe basin(loess plateau in Weibei)and Wianhe river valley were identified through field observation and measured geological profiles and the exposure of standard drilling engineering,providing information for the Cenozoic tectonic evolution and climate change of Weihe basin.3.The Weihe-Qianhe fault depression basin deposited an aeolian accumulation represented by red clay formed in the Miocene under the warm and humid climate of Gansu Group(N₁G),and deposited the fluvial and lacustrine debris and clay of the Sanmen Formation(N₂s)in the pliocene.From the beginning of the quaternary period,with the uplifting amplitude of the qinghai-tibet plateau increasing,the paleoclimate changed from warm and humid to arid and cold,and the climate in the northwest of China was arid and cold.The range of desertification expanded,and the range of aeolian loess also expanded.4.The formation and evolution of Weihe-Qianhe fault depression basin went through four stages:extensional fault depression occurred in the early Cretaceous Qinling and the southwest margin of ordos,and the Qianhe fault depression basin formed.During the paleogenic-early Miocene,fault depression began on the south side of the uplift of the southern margin of ordos,and Weihe basin developed from east to west.The Early Pliocene,the Indian plate subduction northward,began to uplift of the Qinghai-Tibet Plateau,the effect of Qin-Qi Orogenic belts and The ordos basin is further uplifted,and between the two fault depression,thousands of Weihe basin began to appear for the western river-3.4 Ma Late Pliocene epoch from the Qinghai-Tibet plateau of the first act of the(A)rapid uplift,the southern margin of the ordos uplift again rapid uplift,ordos basin hinterland also appears as the overall uplift,Qian river basin-the Wei river basin fault sag.The quaternary tectonic movement was frequent.Curtain B of the qinghai-tibet plateau at the beginning of 2.5 Ma not only caused the overall uplift of the ordos block in the northeast margin of the qinghai-tibet plateau,but also formed 6 grade terraces on both sides of the Weihe and Qianhe rivers.