Gravel deposits on the margins of the Qinghai-Xizang Plateau, and their environmental significance

The thick deposits of Quaternary gravels found along the margins of the Qinghai-Xizang (Tibet) Plateau have been studied for decades. Here, we concentrate on two sediment series widely developed along the Kunlun, Tian Shan and Qilian mountains: the Xiyu Formation, a molassic series of Lower Pleistocene age, and the Middle and Upper Pleistocene Gobi Gravels. These two series, with their contained fossil assemblages, throw light on the question of the mean altitudes and uplift rates of the Tibetan Plateau during the Quaternary and on the related problem of the vigour and penetration of advected summer monsoonal moisture into central Asia. The sedimentary series on the northern piedmonts of the Tibetan Plateau considered here challege the view that Plateau uplift accelerated in the Upper Pleistocene. They indicate that the Plateau had reached mean altitudes in the range 4000–5000 m perhaps by the end of the Lower Pleistocene, and certainly by the Middle Pleistocene. This diminished the incursion of southerly monsoonal moisture, and so imposed arid and semi-arid conditions across much of a vast region north of the Plateau which, among other effects, precluded widespread mountain glaciation in the Lower Pleistocene and general (ice sheet) glaciation of the Tibetan Plateau as a whole.