The The Jiuxi Basin Jiuquan Gravel Cosmogenic Burial Dating

The relationship among Cenozoic tectonic movement, surface process and climate change is an important subject on the modern geoscience research. The Tibetan Platea is the key area of revealing this scientific issue. During the research on the Tibetan Platea uplift, an important theoretical question is the time and cause of late Cenozoic conglomerate widely distributed around the Platea. However, it is difficult to measure the Cenozoic conglomerate age. Commonly Paleomagnetic dating methods are used, which dependents on the contrast between measured polarity and the standard polarity. so it is a little subjective. Cosmogenic burial dating may be a new method to measure thick conglomerate age.Jiuxi basin lies in the northern of the Tibetan Plateau. Its formation and evolution are controlled by the Altyn Tagh Fault and the Qilian Shan marginal Fault. It is the typical area that should begin uplift at 5MaB.P. according the viewpoint of stepwise rise and growth. Laojumniao Section which has six unconformable relationship and clear gyrations sediment is the ideal area for researching the relationship between se dimentary, structure and environment change. This article is on the basis of the former research on the high-resolution paleomagnetism age and ESR age. eight cosmogenic burial dating samples were collected along the Jiuquan Gravel(227m). Burial dating were preprocessed at nanjing normal university and tested in the PRIME Laboratory,Puedue University,USA.Sample depth are 0m;138m;156m;180m;195m;207m;224m, and the tested age are 0-0.156+0.165;1.040-0.172+0.182;1.097-0.125+0.129;1.068-0.345+0.401;10218-0.543+0.439;1.985-0.513+0.664;1.291-0.156+0.163 respectively. There is a good linear relationship between sample location and burial age (correlation coefficient= 0.8), It shows that the measured burial age is reliable. Compared the burial age with paleomagnetic age, most of the buried age is about 0.50 million years older than paleomagnetic age. Jiuquan Gravel is a growth strata. Before its deposition, there was a strong tectonic movement in Jiuxi basin and Laojunmiao Anticline was uplifted rapidly, so that a piggyback basin was formed between the Laojunmiao anticline and the northern marginal fault in Qilian Shan. During that interval, Shiyouhe River was not able to cut through the anticline, and the clastics from the Qilian Shan were deposited within the piggy back basin. When shiyouhe River re-cut the anticline,, the compositon of the Jiuquan gravel layer includes both the clastics directly from the mountain and the the gravel layer reworked from the pippy back basin, leading to lower 26Al/10Be ratios, and hence the cosmogenic nuclide buring dating results are older than paleomag results.