Prototypes and Evolution of the Mesozoic Basin in Western Qaidam

Based on an investigation of Mesozoic outcrops as well as paleocurrent and seismic interpretation data,the authors studied the prototypes and evolution of the Mesozoic Qaidam basin.The results show that the Mesozoic Qaidam basin started with Middle Triassic small intermontane depressions.There also developed an intermontane basin between the old Alabasitao Mountain and the Kunlun Mountains in Mesozoic,trending in E-W direction.The basin underwent three tectonic evolution stages,i.e.,Middle-Late Triassic,Early-Middle Jurassic and Late Jurassic-Cretaceous,corresponding to the development of three prototype basins.The Middle-Late Triassic basin was of the depression type,the Early-Middle Jurassic basin was of the fault basin type,and the Late Jurassic-Cretaceous basin was of the depression type.The Middle-Late Triassic depression was restricted in the Yueyashanbei region,and the sediments were red clastic rocks which deposited under an oxidizing environment with no hydrocarbon-generating potential.The Early-Middle Jurassic fault type basin was characterized by the coal-bearing formation,which was widespread in the present Altun Mountains and the neighboring areas in the Qaidam basin.The depocenter was in the present Altun Mountain area.During Late Jurassic-Cretaceous,the Altyn Mountainsa-Qingshuigou cross section;b-Meigou cross section;c-Quanyagou cross section;d-Quan nan 1 cross section;e-Caishiling cross section;f-Heishishan cross section;g-Sanjiaojia cross section;h-Yueyashanbei cross section;i-Meiyaogou cross section;1-Proterozoic;2-granite;3-Ordovician;4-Triassic;5-Upper Jurassic;6-Lower-Middle Jurassic;7-Cretaceous;8-Cenozoic;9-location of cross section;10-study area;11-location of well uplifted rapidly and,as the major provenance,began to separate the Tarim basin from the Qaidam basin.The Late Jurassic-Cretaceous sediments were coarse clastic formations.This study is of significance in the exact evaluation of hydrocarbon potential and the rational arrangement of the exploration work in the Qaidam basin.