Lacustrine strata of the Wudaoliang Group demonstrate that two vast lake complexes covered the central Tibetan Plateau during early Miocene time. The Wudaoliang Group, which is characterized by dolostone, limestone, and marl as thick as 310–350 m, lies horizontally or with a slight dip above Oligocene reddish-brown sandstone, and is covered by upper Miocene and Pliocene mudstone and sandstone. The extensive distribution of this group outlines interconnected lake basins of 5000–15,000 km2 or larger in the north-central Tibetan Plateau, such as the Wudaoliang, Beiluhe, Tuotuohe, Tongtianhe, and East Wenquan Basins, and lake basins 2000 km2 or larger in the south-central plateau, such as the Ando, Naqu, Bangoin, Lunpola, and Shuanghu Basins. These lake basins, separated by mountain ranges and islands, were linked by water passages in early Miocene time, when they apparently formed a huge lake complex that covered as much as 100,000 km2 in the north-central Tibetan Plateau and another one >50,000 km2 in the south-central part of the plateau. Such immense lakes existed for several million years between ca. 23.5 and ca 13.5 Ma after the Tibetan Plateau rose, and their interconnected basins define the topography during early Miocene time. The fossils contained in these basins indicate a change from the warm, dry climate of the Oligocene to moderately cool and wet conditions at the beginning of the early Miocene, followed by progressive cooling and drying, which suggests continuous uplift during deposition of the Wudaoliang Group.

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