中国西北干旱环境的形成时代与成因探讨*

西北干旱环境的形成时代是我国古环境研究的重要问题。干旱环境的存在可据地层中风成沉积、风化沉积、水成沉积、古生物等信息综合确定。对塔里木、准噶尔、柴达木、河西走廊、陕甘宁等盆地的中、新生界干旱环境沉积记录分析表明,我国西北干旱沉积及其时代可上溯到晚侏罗世初期(约160Ma以来),其间可分为三大阶段:1)晚侏罗世至古近纪(约160~24Ma),以红色干旱沉积与亚热带干旱环境为主; 2)晚第三纪(约24.0~2.5Ma),以红色干旱沉积与亚热带干旱环境向灰、黄色干旱沉积与温带干旱环境逐渐过渡为特征;3)第四纪(约2.5Ma以来),以灰、黄色干旱沉积与温带干旱环境为主。干旱环境在晚侏罗世的大范围出现主要是侏罗纪全球气温回升、北亚热带北界北推引起的。相比可见,新近纪温带干旱环境的形成主要与古特提斯海关闭、青藏高原及盆地外围山脉的隆升等构造运动引起内陆盆地干旱效应的加强有关。进而推知,我国西北在新近纪出现的温带内陆干旱环境既是盆地外围高原、山脉隆升的重要直接结果,也是亚洲季风系统的重要直接形成因素。

DISCUSSION ON THE FORMATION AGE OF ARID ENVIRONMENT IN WEST CHINA

When did the arid environment in West China begin has been a long debating question.Many researchers thought that it should be from the Neogene,and some insisted in much earlier origination such as the Paleogene or the Cretaceous.The actual answer should be come from geological sedimentary records.Arid environment can be identified by evidence from aeolian deposits,weathering deposits,aqueous deposits and paleontological records based on the comparison with geomorphology and sedimentology in modern arid environment.Based on the analysis of Mesozoic and Cenozoic arid sediment sequences from basins including Tarim,Dzungaria,Tsaidam,Hexi corridor and Ordos,it is concluded that arid sediments have existed continuously in these big basins since the Later Jurassic(about 160Ma),and the formation age of arid environment in West China should thus be considered as Later Jurassic.Concretely,it started at a time corresponding to the starting of the Qigu Formation((J_(3)q)) in the Tarim Basin and the Dzungaria Basin,the Hongshuigou Formation(J_(3)h)in the Tsaidam Basin,the Chijinqiao Formation(J_(3)c) and the Kushuixia Formation(J_(3)k)in the Hexi Corridor Basin,and the Fenfanghe Formation(J_(3)f) in the Ordos Basin.Although Cretaceous strata is absent in many places in the basins of Ordos,Hexi Corridor,and Dzungaria,it exists extensively in the Tarim Basin.The sediment sequence from Jurassic to Quaternary is thus complete,supplying a good basis for this study.Arid environment in West China has experienced 3 obvious evolutionary periods : (1) during Late Jurassic to Paleogene,inland reddish sediments standing for arid environment were deposited in subtropical zone;(2) in Late Tertiary,a transition from reddish deposits in subtropical zone to grayish or yellowish deposits in temperate zone was existed;(3) in Quaternary,gray or yellow arid deposits of temperate zone was dominated.The starting of arid environment at Late Jurassic in West China coincides with the warm-up event during the period from Jurassic to Middle Cretaceous,which drove the boundary of the subtropical zone to north in the Northern Hemisphere within which West China was included.By comparison with the cases in the geologic history,it can be concluded that arid environment of temperate zone in West China occurring in Neogene was mainly caused by tectonic movements,e.g.,the disappearing of the Tethys,the uplifts of the Qinghai-Tibetan Plateau,and the formation of mountains surrounding the study basins.While global climate became colder in Cenozoic,the north boundary of subtropical zone was withdrew southward and the vast area of Northwest China should have had humid weather of temperate zone formed by the planetic westerly which transferred much water vapour form west to east.However,tectonic movements including the formation of the Qinghai-Tibetan Plateau and a series of mountains around the study basins made climate developed in a different style,to be arid.Arid environment of West China is a outcome of tectonic movements and a factor to the formation of the Asian monsoon system.