Late pleistocene evolution of salt lakes in the Qaidam basin, Qinghai province, China

Abstract The Qaidam basin is a tectonically controlled depression at 2700 m altitude on the northern margin of the Qinghai—Xizang (Tibet) Plateau. Rimmed by the high Aljun-Qilian ranges to the north and Kunlun peaks to the south rising above 5000 m, the basin floor experiences hyper-arid conditions with evaporation/precipitation ratios exceeding 100:1. Periglacial and glacial conditions prevail on montane peaks with permafrost above 3800 m. On the basin floor, small waterbodies lie on extensive halite salt flats. On the largest, the Qarhan salt flat, 5800 km2 in extent, the small lakes Dabuxun and Taijinier represent contracted remnants of a once expansive water-body. Throughout Neogene time, a succession of clastic sediments indicates lake depo-centre migration from NW to SE. Towards late Pliocene, evaporitic conditions developed with formation of gypsum, halite, celestite, mirabilite and bloedite. Continued deformation through Pleistocene time has developed a series of NW—SE trending domes and troughs. This setting has continued to exert a controlling influence on basin and brine evolution with progressive development through sulfate to chloride brines as dry conditions intensified in late Pleistocene. Expanded lake conditions (Qarhan mega-lake), corresponding to near the 2800 m contour, indicate the existence of past climates much more humid than those of today. A drilling program has defined the lateral and vertical extent of evaporitic halite deposits. Freshwater clastics and shell deposits which predate the halite phase indicate humid lacustral conditions until after 40,000---30,000 yr B.P. Desiccation from about 25,000 yr until about 9000 yr B.P. resulted in deposition of halite to 60 m thick with thin-parting muds representing brief returns to more humid environments. High lake shorelines with aragonitic ooids surrounding the dry playa, Kunteyi, and high-level ostracod-rich beach and near-shore deposits on Xiao Qaidam attest to the presence of expanded, late Pleistocene lacustral conditions on lakes other than Qarhan preceding the final evaporite stage. Clastic sediments associated with the halite are dominated by loessial silts equivalent to the late Pleistocene Malan deposits of central China. Simultaneous halite and loess deposition reflect the dry conditions that developed in the Qaidam Basin during the latter part of the last glaciation and which have persisted ever since. The final evolution of brines has resulted in development of extensive potassium-rich brines and salts with the mineral carnallite providing the basis for a large potash extraction industry near Golmud. The hydrologic history documented by the analyses of lake sediments reflects the interaction of tectonic deformation and climatic change in the development of brines to the potash stage.