Holocene loess sedimentation along the Qilian Shan (China):: significance for understanding the processes and timing of loess deposition

We present optical ages for well preserved loess deposits along the Qilian Shan, a 500-km-long mountain belt forming the tectonically active northeastern margin of the Tibetan Plateau. Loess on tectonically controlled fill terraces along the Shiyou and Baiyong rivers was dated by applying the single-aliquot regeneration (SAR) technique to coarse-grained (90-160 mu m) quartz samples. Additional measurements on polymineralic fine-grained (5-10 mu m) samples were carried out to allow a comparison between various optical dating techniques currently in use. The results demonstrate that (i) loess accumulation started between 13 and 11 ka B.P., continuing to the present, (ii) mean loess accumulation rates range between similar to 9 and similar to 16 cm ka(-1), and (iii) Holocene loess is not restricted to certain sites only, but is a prevalent phenomenon along the Qilian Shan mountain front. The absence of loess deposits of Late Pleistocene age is a result of a combination of cold and dry climate conditions, strong winds, and the absence of a vegetation cover during the last glacial period. Only with the change to a warmer and moister climate at the onset of the Holocene did a plant cover develop, which acted as a dust trap, thus initiating the formation of loess. (c) 2005 Elsevier Ltd. All rights reserved.