Grain-size Characteristics And Sedimentary Environment Of Quaternary Sediments In Jiaocheng Area

The Jiaocheng area is located near the boundary(Jiaocheng fault)between the Taiyuan basin and the uplift of the Lvliang Mountain.The study area is the Jiaocheng section of Jiaocheng fault.Based on lithologic characteristics of borehole sediments,sedimentary cycles and chronology,combined with previous achievements,the Quaternary strata in the study area were divided into the upper part of the Pleistocene(Qp3)and the Holocene(Qh),and discordant contact with the underlying bedrock,among which the lack of the middle of the(Qp2)and the lower part of the Pleistocene(Qp1).Fifty samples of 76.1m cores in N-1 hole in Nailin Village,Jiaocheng County were selected for systematic grain size analysis,and the lithologic grain size characteristics,hydrodynamic conditions and environmental significance of drilling were analyzed from different angles of grain size.The results show that since the late Pleistocene,it can be divided into 4 types of hydrodynamic types and 5 intervals.The sedimentary hydrodynamic conditions experienced have been changes: moderate energy(Qp3-1)?moderate-high energy(Qp3-2)?moderate energy(Qp3-3)?moderate-low energy(Qp3-4)?moderate energy(Qh).According to the particle size analysis scatter plots and C-M plots,it is shown that the sedimentary facies were fluvial facies since the late Pleistocene.The comprehensive analysis shows that since the late Pleistocene,the area has experienced 5 evolutions of meandering river cycles,and it has obvious sedimentary characteristics of river binary phase.According to changes in the composition and thickness of channel and floodplain sediments,from the bottom up(from early to late),the sediment thickness of the floodplain,which is characterized by silty clay and clay,is thickening and increases,and the thickness of the gravel composition decreases.The evolution of the meandering river cycle from the higher energy channel facies to the lower energy floodplain facies is presented.Jiaocheng fault is a Holocene active fault and is a south dip normal fault.According to the analysis of the four cross-fault exploration sites,the distance of fault dislocation is decreasing from bottom to top.Through the analysis of stratum and sedimentary evolution in this paper,combined with the analysis of the four cross-fault exploration sites,the amplitude of the faults are gradually decreasing from the bottom up.The analysis of the above sedimentary cycles and environmental evolution also indicates that since the late Pleistocene in the study area,the tectonic activity has weakened.