Gansu Linxia Pendi hong se di ceng ci zu gou te zheng yu cheng ji huan jing fen xi

Linxia is a district in Gansu Province, locates in the north-east edge of the Tibetan Plateau. Huge thick of Cenozoic red stratum over last 4 -29Ma was reported there, providing valuable geological record to recognize the Plateau uplifting process and climatic evolution during Cenozoic. Of which deposits, red-gray interbeds with paleomagnetic age of 13.07 -7.78Ma are especially very eye-catching. These beds were vividly named as ＂Zebra Layer＂. These interbed layers are nearly horizontal. Because of the gray layers,which are absolutely different from aeolian loess,it is generally accepted as lacustrine sediment： during Late Tertiary,this region was a lake,gray and white layers were formed during deeper water, and the red beds were formed during shallow water, repeating several times formed the above-mentioned across-layers. Through large area inspection in the field, we found that there are still many paleosols with calcium nodules/or layers in the red stratum, which are similar to the later Tertiary Red Clay; These Zebra Layers tilt with the terrain in side-slope of the modern valley. This common geological phenomenon is difficult to be explained by lacustrine sediments. In order to explore the process of its＇ formation, one section from Maomao Ditch （35°40＇11＂N, 103°21 ＇56.8＂E） ,where Zebra Layers follow the terrain tilting in side-slope of the modern valley, was selected for paleotopographic study. Two groups of orientated samples were collected from the Zebra Layers： 1）horizontal layers,and 2）the side-slope layers （50 samples in total）to measure their anisotropy of magnetic susceptibility （AMS）. Meanwhile Quaternary aeolian loess samples from both positions： 1 ）horizontal layers （18 samples）and 2）the side-slope （15 samples）, were also collected for comparison. The overlaying Quaternary loess, no mater samples from horizontal or from side-slope, their minimum axis K3 always focus around 90°, indicating aeolian almost ＂ horizontal bedding＂. The horizontal Zebra Layers display similar K3 characteristics to aeolian loess. However,the AMS result from side-slope samples from Zebra Layers is different. The K3 directions of side-slope Zebra Layers from both red and white samples does not focus, but distribution in general follow the SEE-NWW direction of slope. The results show that： both horizontal and titled ＂Zebra Layers＂ are all likely to be the original deposition, no evidence to support the titled bedding was due to the tectonic movement made the horizontal layers to be titled. These tilted red and white Zebra Layer following modern terrain were unlikely formed in underwater environment,but more likely similar to the overlying loess and paleosols, forming on the ancient terrain which accepted aeolian loess and soil sequence at that time, and as a result of the later water overflowing,the side-slope got gradually thinner from the top to the bottom. That water overflowing downwards slop might also made magnetic particles stirred. So that AMS of these samples do not show the same AMS characteristics as the overlying loess. These gray colors should not be original,but probably due to the beds had ever been soaked （or moisture to much） and deoxidized by water after loess and soils formation. Finally when iron materials were removed away and their colors became gray due to gleying process. The modern gullies or valleys are apparently cut down following the ＂Zebra Layer＂ tilted terrain, suggesting that water system had already been formed and working at least before the ＂ Zebra Layer＂ formation. Today Maomao Ditch, Jiaozi Ditch and Mao Ditch, are all first level of branch of Xia River system, while Xia River is the first level of branch of Yellow River, indicating very likely that the Yellow River for this certain part may also have formed at least 13Ma ago according to paleomagnetic dating.