Sedimentary record of the planation surface in the Hoh Xil region of the northern Tibet Plateau

Miocene marl is the most widespread Tertiary stratigraphic record in the no rthern Tibet Plateau, termed the Wudaoliang Group in the Hoh Xil region and the correlative Suonahu Formation in the Qiangtang region. The uniform mar l overlies red beds of the Eocene-Oligocene Fenghuoshan Group. The Wudaolia ng Group is generally 100-400 m thick, but the thickest strata are 700-1300 m, located in the Haidinghu (Haiding Lake) and Tuotuohe (Tuotuo River) reg ions respectively. Based on observations from eight measured sections and o utcrops, the thin-bedded marl, which varies in colour from grey-white to li ght brown-grey, is explained as a large-scale or serial lacustrine deposit stretching throughout northern Tibet. The Wudaoliang Group commonly crops out on geographic lowland at an average elevation of 4600 m above sea level within the mountain chains, showing co ncordant summit levels, e.g. the Fenghuoshan and Bairizhajia Mountains. The se mountains with a flat ridge are considered to be remains of the palaeo-p lanation surface. However, the spatial distribution of the Wudaoliang Group is not confined by the current mountain-basin landform configuration. We h ave observed the Miocene Wudaoliang Group marl exposed on a 5233 m-high mou ntain peak. The largest difference in height between the current lake level and the mesa crest is 600 m; the maximum dip angle is 25 degrees, but usua lly below 10 degrees, which is obviously different from the Fenghuoshan Gro up red beds with moderate to strong structural deformation. The horizon of the Wudaoliang Group thin-bedded marl and its widespread occurrence through out the northern Tibetan Plateau can only be reasonably inferred to a sedim entary record relevant to the palaeo-planation. Its deposition occurred on primary land floor by erosion at 20 Ma and its bed surface is a typical sig n of geomorphic rise and collapse after the erosion.