The Yihe River’s Paleo-valley Sedimentary Characteristics In Chuanliujie-Baitajie Since The Last Glacial Maximum

In this study, the characteristic of paleochannel in Chuanliu street-Baita street geological section, Yihe River since Last Glacial Maximum was selected as object. By the methods of collecting historical documents and materials, geophysical exploration, geological drilling, age samples and drawing geological profile, paleochannel morphology of Chuanliu street-Baita street geological section was restored and its depositional environment was also analyzed. The results are as follows:(1)By the combination of geological drilling and geophysical exploration methods, the paleochannel morphology of Yihe river in Chuanliu street-Baita street geological section was restored, which proved geological drilling and geophysical exploration were good methods in exploring the paleochannel morphology.(2)It was inferred by the OSL dating(16520±3600BP) of river sediments that Chuanliu street-Baitajie street geological section at the elevation of 70 meters formed in Last Glacial Maximum. The dating of sediments in borehole ZKX2 was(12650±40BP), at the elevation of 72 meters around borehole ZKX2 the river sediments formed in the early stage of the late glacial period, and the sand gravel sediments at the elevation of 72 meters formed in the Holocene period,and since the Last Glacial Maximum, the paleochannel morphology of Yihe river was coincide with the modern channel, and the channel reached to the Binhe western road in the west and the east bank of modern channel in the east.(3)The paleochannel in Yihe river had developed a lot since Last Glacial Maximum. The deepest channel near Binhe west road was about 54 meters deep, the depth of the paleochannel was 9.30 meters, and the width of top paleochannel was 249.53 meters, the ratio of width and depth was 1.70.Coarse sand bed, gravel stratum dominated the whole stratum with little clay, silty clay existence.The highest thickness of sediments was 21 meters and the lowest place was 3 meters. The totalwidth of channel exceeds 1043.46 meters, which overlaped with modern bed position and could be divided into four little paleochannels.(4)Being controlled by the little regional precipitation and small rivers flow, the paleochannel in Yihe river was little wide and shallow. In the bottom of Yihe river small rivers were numerous, but which unevenly distributed in the bottom of Yihe river. From west to east, channel geometry became lighter gradually and the thickness of sediments reduced gradually, and the erosion in right Yihe river bank was predominant, which possessed the characteristic of fork- braided stream.(5)The classification of sediments particle size in Chuanliu street-Baita street geological section was obvious. Dynamic of water flows changed obvious, the grain size of particle was coarse, coarse sand was dominant, and the mineral composition was mainly quartz and feldspar.(6)The terrain around Chuanliu street-Baita street geological section changed significantly and numerous low mountains distributed both in the east and west sides of river, which produced an binding effect on the swing of the Yihe river. In the light of geological drilling in downstream areas,the bedrock lied in the 60 cm below surface. It can be conferred Yihe river has hardly shifted that since the Last Glacial Maximum periods, and which can be confirmed by the fact that paleochannel location was coincidence with modern bed.