Quaternary Sedimentary Characteristics And Evolution Of Yuncheng Basin,Shanxi Province

Yuncheng basin is located in the southwestern end of Shanxi graben system,southeastern margin of the Loess Plateau,and in the transitional area between the Loess Plateau and sedimentary basin.Within the basin,there are two tectonic units,Emei platform and Zouhorizon prototype.Since Quaternary,the sedimentary model of Emei Platform is obviously different from that of Yuncheng Basin Depression.Because of the special geographical location and geomorphological characteristics of Yuncheng Basin,the Loess depositional characteristics on the platform are different from those in typical loess areas.Quaternary sedimentary cover in the basin is serious and outcrop is few.In recent years,due to the development of sand mining industry in the basin,more artificial profiles of strata have emerged in the basin since the Late Pleistocene,which provides a good condition for the study of Quaternary in the basin.Based on the previous research results and the data of this work,this paper redefines the stratigraphic units of the Upper Pleistocene in the study area,and compares and divides the Quaternary in the study area by using the data of predecessors,adjacent basins and newly obtained boreholes.In the early Pleistocene,most of the basin was fluvial-lacustrine sediments,and lacustrine sediments dominated.In the early Middle Pleistocene,influenced by the Kunlun-Yellow River tectonic movement,the Sanmen ancient lake began to disappear,and fluvial sediments dominated the basin.The ancient lake further shrank to the sedimentary depression in front of Zhongtiaoshan Mountain,and loess began to deposit in the high-lying areas.From late Middle Pleistocene to early Early Early Pleistocene,Emei platform uplifted relatively,the Gufenhe River flowing into the basin was cut off and diverted,the fluvial deposits decreased,the Yuncheng ancient lake shrank further,and the loess was deposited in the high terrain area;in the middle of Late Pleistocene,the lake transgression occurred in the basin,the lacustrine strata in the basin were widespread,and the loess was deposited in the high terrain area.The deposits in Holocene basin are mainly palaeogeomorphological,the loess deposits in high terrain and the lacustrine,swampy and Alluvial-diluvial deposits in low-lying areas.Based on the data of more than 300 water wells,hydrological monitoring wells and boreholes collected and collated,a three-dimensional geological model is established,which roughly restores the distribution of shallow stratum space within 300 meters of the study area,and focuses on the distribution of loose sand bodies in the Quaternary system of the study area.The study shows that the tectonic uplift since Quaternary forced the diversion of the ancient Fenhe River,making the Yuncheng Basin a relatively closed basin.Due to the absence of external water systems,the remnants of Sanmen ancient lake gradually shranks in the Yuncheng basin.The tectonic movement in the middle and late Late Pleistocene led to the further uplift of Mingtaogang,which made the Zoushui River move westward in the basin,further aggravated the shrinkage of Yuncheng ancient lake,thus forming the present salt lake.In addition,the Holocene floods in Yuncheng Basin may be closely related to the global climate change in Megalayan.Many tectonic activities in Yuncheng Basin since Quaternary have exerted an important influence on the sedimentary evolution of Yuncheng Basin.