Preliminary study of alluvial pebbles on high terraces of the Yellow River from Baode to Kehu in Shanxi-Shaanxi Gorge

Seven river terraces are recognized based on detailed field investigation in the Heiyukou area, Shanxi-Shaanxi Gorge of the Yellow River, of which the highest T7 and T6 terraces are covered by red clay. The lithology and grain size of pebbles on different terraces along the river are studied. It is found that the lithology of pebbles on the sixth and seventh terrace is similar to that of lower Yellow River terraces and floodplain in the Heiyukou area, but different from that of Weifen River, a tributary of the Yellow River running from east to west. The limestone pebbles, which are dominant pebbles in Baode area, not only become less in amount, but become smaller in grain size from the upstream to the downstream. This indicates that the pebbles of Late Tertiary are the relic of an ancient river running from north to south. Magnetostratigraphy study shows that the bottom age of the strata, 70 m fine sands in lower part and red clay in the upper part, covering on the pebbles of the T7 terrace is about 6.8 Ma, and the bottom age of the red clay covering on the T6 terrace is about 3.3 Ma. This indicates that the ancient river formed at least before 6.8 Ma in this area, and the age of T6 (3.3 Ma) matches well with Phase A of Qinghai-Xizang Movement. There is about 160 m incision in this area after Phase A of Qinghai-Xizang Movement. The average incision rate is more than five times faster than that before Phase A of Qinghai-Xizang Movement.