The Formation Of The Late Miocene Stratum In Longzhong Basin

The uplift of the Qinghai-Tibet Plateau and the role played in the Cenozoic climate evolution of eastern Asia are still hot areas about paleoclimate research, but also the most controversial. in the past, scholars, by analyzing sedimentary and stratigraphic cycles of Linxia basin in northeastern margin of the Tibet plateau, which are unusually sensitive to the process of uplift of the plateau, got the conclusion that the climate of basin was that represented by lake in the Miocene period, and made a theory of phased uplift of the Qinghai-Tibet Plateau.There are large tracts of Cenozoic red stratum distributed in linxia basin in Gansu province, of which red-gray interbeds 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 that the stratum are the lacustrine sediment:during late Tertiary, this region was a lake, gray and white layers were formed during the water was deep in the lake, and the red beds were formed in 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 layer 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, we collected orientated samples for two groups:1) horizontal layer, and2) the side-slope in these zebra layer to measure their anisotropy of magnetic susceptibility (AMS). Meanwhile aeolian loess samples from both positions of the side-slope and horizontal layer were also collected for comparison. Through a preliminary analysis, the results show that:both horizontal and titled layers of "zebra layer" are all likely to be the original stratum, 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 to the overlying loess, forming on the ancient terrain which accepted aeolian deposition at that time, and as a result of the latter 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 characteristics of magnetic anisotropy as the overlying loess. These gray colors might not original, but it is probably due to the red beds had ever been soaked and deoxidized by water after loess and soils formation. finally when iron materials were removed away and their colors became gray. That the "zebra layer" tilted with the modern terrain and apparently suffered water effect, approve that Jiaozi Ditch and Mao Ditch belonged to the Xia River system had been formed before the "zebra layer" formation, it maybe that the Xia River have developed13Ma ago according to paleomagnetic dating. Xia River is one of first branches of Yellow River, so it may suggest that the Yellow River may also have formed13Ma ago.