Preliminary Study On River Terraces Of Hukou Section In The Middle Yellow River

River terraces were ancient channels in geologic history, and as one of the basic form of fluvial geomorphology, research them not only can recover the evolutionary history of fluvial geomorphology but also extract the tectonic movement and climate change-related information. Study on chronology of river terraces is a significant subject in the river geomorphology. However the formation and development of the highest river terrace indicated the latest time of river formed in general. The article selected river terraces of Hukou region in the Middle Yellow River as research object, and trying to make a preliminary exploration on the age and evolutionary history of the Yellow River. The conclusions mainly are as follows:(1) We made a detailed field survey in Hukou region, and discovered it develops5levels of terraces which named T1、T2、T3、T4、T5、T6and T7. Base tug-of-war height is20.40.60.80.110.140and180meters respectively.(2) Based on palaeomagnetic data and loess-paleosol sequence, the ages of seven terraces (T1-T7) along in Hukou region successively is0.01MaBP.0.06Ma BP.0.128MaBP.0.245Ma BP.0.4Ma BP、0.78Ma BP and1.24Ma BP. There is paleosol on top of every terrace except the Tl (there have no sediment on the top of T1), is Sm, S1, S2, S4, S7and S14respectively. The paleosol overlying all terraces around Hukou region along Yellow River indicate that climate change played an important role in terrace formation. By the linear analysis of the terrace above river and age we find that tectonic uplift and climate change may each have played variable roles in the terraces around Hukou zone, tectonic uplift provide a vertical space of cut for the terrace formation, river incision should occur in the glacial to interglacial transition period.(3) The incised rate of the Yellow River in Hukou section is0.133mm/a since1.24Ma BP on average, it can be divided into three periods, and presents an increase trend gradually. During 1.24-0.4Ma BP, the Yellow River area incised0.083mm/a in Hukou region, by during0.4-0.128Ma BP, incised rate rise to0.184mm/a, by during0.128-0.01Ma BP, it is0.339mm/a.(4) Compared the river terraces sequences in Hukou region with Lanzhou and Wubu, we discovered that Wubu-Hukou section of the Yellow River should had been come from as early as1.24Ma BP, however, the Yellow River in Lanzhou section should had been formed in an earlier age. The ages of river terraces in Hukou and Wubu section indicate that downcutting of the Yellow River might happened in the same time.The formation of the Yellow River in Hukou section indicates that the Ordos Plateau incised by the Yellow River succeed in1.24Ma BP before and after, and then through the Jinshan Gorge.(5) We made a study on the anisotropy of magnetic susceptibility (AMS) of T7loess profile. The longest axis deflection angle direction of anisotropy of magnetic susceptibility indicate the paleowind direction near ground mainly to NE-SW in Jinshan Gorge and the average wind direction mainly is217.2°.