THE RETROGRESSIVE EROSION RATE AT HUKOU WATERFALL,YELLOW RIVER AND THE RELATION TO THE HANCHENG FAULT

On the basis of consulting historical records about the positions of Hukou waterfall at different times,we conduct a field geological survey along the Yellow River and ultimately determine the specific locations of the Hukou waterfall in the different periods.Based on this,the retrogressive erosion rates in different periods are calculated as about 1.66m/year during the Xia Dynasty to the Tang Dynasty period,about 1.01m/year in the Tang Dynasty to the Yuan Dynasty,about 0.97m/year in the Yuan Dynasty to the Ming Dynasty,about 1.28m/year in the Ming Dynasty to the Republican period,and 0.6m/year from the Republican period to the present.Considering the complex geological conditions along the Yellow River,the average retrogressive erosion rate of Hukou waterfall on the Yellow River is obtained to be 1.51m/year since the historical records (early Qin Dynasty to the present).Lithology surrounding the Hukou waterfall includes mainly the Triassic gray,gray-green thick-layered mid-grained feldspar sandstone and dark purple,yellow-green mudstone,this hardness and softness combination feature is the unique geological condition of the Yellow River.After abrasing the softer shale driven by water cyclotron at this position,water washes off the debris,causing the overlying feldspar sandstone suspended for a long period.Feldspar greywacke block collapses under accumulative water erosion in long years,and then retrogressive erosion occurs in Hukou waterfall.In the process of 1 ︰ 50 000 active fault mapping of Hancheng Fault,we excavated a trench at Shaojialing,and the trench profile shows that:in the early and middle period of late Pleistocene,there are obvious surface ruptures produced by the fault.Cumulative offset near the trench is more than 20 meters in height difference.Yellow River terraces survey at Yumenkou also confirms that a fault slip of about 20 meters occurred during the early and middle period of the late Pleistocene.Assuming the retrogressive erosion rate is constant,the author thinks the Hancheng Fault was activated at early and middle age of the Late Pleistocene,forming a 20~30m high scarp (knick point),and today's position of Hukou waterfall may be the position of this knick point after the retrogressive erosion of about 40 to 50ka.