| Abstract: | Miocene marl is the most widespread Tertiary stratigraphic record in the northern Tibet Plateau, termed the Wudaoliang Group in the Hoh Xil region and the correlative Suonahu Formation in the Qiangtang region. The uniform marl overlies red beds of the Eocene‐Oligocene Fenghuoshan Group. The Wudaoliang 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) regions respectively. Based on observations from eight measured sections and outcrops, the thin‐bedded marl, which varies in colour from grey‐white to light 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 concordant summit levels, e.g. the Fenghuoshan and Bairizhajia Mountains. These mountains with a flat ridge are considered to be remains of the palaeo‐planation surface. However, the spatial distribution of the Wudaoliang Group is not confined by the current mountain‐basin landform configuration. We have observed the Miocene Wudaoliang Group marl exposed on a 5233 m‐high mountain peak. The largest difference in height between the current lake level and the mesa crest is 600 m; the maximum dip angle is 25°, but usually below 10°, which is obviously different from the Fenghuoshan Group red beds with moderate to strong structural deformation. The horizon of the Wudaoliang Group thin‐bedded marl and its widespread occurrence throughout the northern Tibetan Plateau can only be reasonably inferred to a sedimentary 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 sign of geomorphic rise and collapse after the erosion. |
