Differential Evolution Of The Western Slop Of The Qaidam Basin And Its Depositional Response

The Qaidam Basin is located in the northern Tibetan Plateau basin ridge tectonic domain,as the largest intermountain sedimentary basin,its Cenozoic evolution is influenced by the surrounding mountain system.Therefore,tectonic deformation and sedimentary record in the basin will reflect the tectonic activity history of surrounding mountain system and basin.According to the idea of "coupling between orogen and basin",this paper conducts a detailed study on sedimentary structure characteristics of Qaidam Basin based on sedimentology and tectonic geology.This is of great scientific significance and reference value for the exploration of oil and gas in the basin and the study of the uplift and evolution of the northern margin of the Tibetan Plateau.Based on the comprehensive analysis of seismic data,select basin NW-SE seismic profiles,using the balanced cross section restoration method to restore the tectonic evolution process of the west of Qaidam Basin.The lulehe formation and lower segment of the xiaganchaigou formation sedimentary period,the overall tectonic activity in the western Qaidam Basin was relatively weak.The deposition of upper segment of the xiaganchaigou formation,the west margin of the Qaidam Basin began to tilt-lift,the faults in the basin gradually development.During the deposition period from the end of the Lower Youshashan Formation to the Upper Youshashan Formation,a significant tectonic uplift occurred in front of the uplifted mountain and near the Altyn Tagh,after which the uplift of the Shizigou Formation weakened.During the sedimentary period of the qigequan formation in the Late Cenozoic,the tectonic activity was strong,the formation fold deformation was obvious and the strong denudation was suffered.On the basis of sorting and analyzing the drilling,well logging,mud logging data,the influence of the Cenozoic uplift of the Altyn Tagh Mountains on the sedimentary evolution of the west Qaidam Basin is analyzed by comparing the stratigraphic thickness of each period and vertical and horizontal lithologic changes.Research shows that since the Cenozoic,the Altyn Tagh Mountains(palaeohigh)begins to undergo multi-stage structural uplift during the deposition period of the upper member of the Lower Ganchaigou Formation.The more intense structural uplift occurred at the end of the Lower Youshashan Formation to the Upper Youshashan Formation,the Shiogou Formation and the Qigequan Formation,the strong erosion occurred in the front of the mountains.Each period of formation thickness changes and the layers of a conglomerate,sandstone,mudstone thickness statistics show that mountain uplift has an important effect on the sedimentary evolution of the west Qaidam Basin.The main performance is that there are large conglomerate deposits in the submountain region,the coarse clastic rocks are relatively high,the thickness of the strata is relatively thin and denuded seriously.The thickness of layer to the basin is gradually increasing,and the lithology gradually changed from more coarse clastic rocks in the submountain region to more fine mudstone deposits in basin.However,the uplift of the mountain has little influence on the sedimentation between the two palaeohigh and the area far away from Altyn Tagh Mountain.The strata thickness varies little at all stages and lithology shows that the fine grained mudstone deposit more,coarse clastic rocks are rare.According to the above analysis of the sedimentary and structure aspect,combined with the fission track data statistics and the study of growth strata and angular unconformity in the Altyn slope,this paper argues that in ca.53.5-40.5Ma,the Western Qaidam Basin in tectonic quiescence period,tectonic activity is relatively weak.At ca.40-14.9Ma,due to the activities of the Altyn Tagh Fault,palaeohigh in the south of the Altyn Tagh fault began to uplift.It makes the west margin of the Qaidam Basin lifted up to the basin as a whole and east-west fault development near the Altyn Tagh Fault.After ca.14.9Ma,the Altyn Tagh Fault began to largescale strike slip activity,the tectonic stress in this area released,the Qaidam Basin occurred NESW trending basal contraction.Affected by this strong deformation,NW trending faults and folds anticline are widely developed in the Qaidam Basin and the overall sedimentary framework of Western Qaidam Basin is changed.