干旱区生态环境知识资源中心

It has long been suggested that the Eocene climate in East Asia was characterized by three latitudinally distributed zones, one middle arid zone and two humid zones to its north and south. The middle arid zone was firstly proposed based on the widespread red beds and evaporites in a NWW-trending zone of China. However, this long-standing model has recently been challenged by increasing doubts about the effectiveness of these arid climate indicators. The Qaidam Basin, as one of the Eocene evaporites-bearing basins in China, can be studied to test whether the bedded evaporites can validly indicate arid climate. Over 300-m halite beds have been found in the late Eocene Upper Xiaganchaigou (UXG) Formation in a local saline center (Shizigou area) of the Qaidam Basin. Its coexistence with source rocks and limited distribution, together with the absence of red beds, make it difficult to explain the halite genesis with the conventional climatic-controlled playa model. Based on strati graphic correlation, lithological and mineralogical observation, and oxygen, carbon and chlorine isotopic analysis, we identify several characteristics about the Shizigou saline center: (1) undulated lake floor; (2) shallow water; (3) limited hydrothermal impact; (4) evaporation-induced precipitation; (5) high eolian influence; and (6) fluctuated climate background. According to these investigations, we suggest a new model for the genesis of the bedded halite in the Qaidam Basin, the tectonic-climatic model. The key point for this model is that the undulated topography caused by tectonic activity and the fluctuated climate together leaded to the hydrological isolation of the Shizigou area. The Shizigou area was isolated from the larger brackish Qaidam paleolake for independent evaporation during arid climate stage while connected with it during humid climate stage, causing alternatively water influx and evaporation and inducing the occurrence of interbedded halite and source rocks. Thus, the widespread bedded halite in the Eocene strata over China may indicate fluctuated rather than arid climate in East Asia.