STRATIGRAPHIC DIVISION AND CORRELATION OF THE PROTEROZOIC STRATA IN NORTHERN QAIDAM BASIN,NORTHWEST CHINA:A REVIEW
The Proterozoic strata developed in the Olongbuluke terrane,the northern Qaidam Basin,Northwest China are composed of siliciclastic and carbonate rocks that unconformably sit on the Paleoproterozoic metamorphic basement and unconformably underlie the Cambrian rocks.The Proterozoic strata crop out well in the Quanjishan,Shihuigou,and Olongbuluke areas,and are most complete in the Quanjishan section,with an increasingly missing of sediments eastwards.There have been different stratigraphic schemes for the subdivision and correlation of the Proterozoic successions,as well as different opinions about their depositional age in the Olongbuluke terrane.According to the Stratigraphic Chart of China(2014),the Quanji Group consists of the Mahuanggou,Kubaimu,Shiyingliang,Hongzaoshan,Heitupo,Hongtiegou,and Zhoujieshan formations;the Mahuanggou,Kubaimu,and Shiyingliang formations have been regarded as the Nanhua System(roughly equals to the Cryogenian System),whereas the Hongzaoshan,Heitupo,Hongtiegou,and Zhoujieshan formations have been assigned to the Sinian System(equals to the Ediacaran System).However,recent advances in biostratigraphy and radiometric geochronology indicate that the Hongzaoshan Formation and its underlying strata were deposited at～1.7-1.6 Ga,representing a set of rift basin sediments from late Paleoproterozoic to early Mesoproterozoic,whereas the overlying Heitupo,Hongtiegou,and Zhoujieshan formations may belong to middle to late Neoproterozoic.Considering that there is a sedimentary hiatus of 800-900 Myr between the Hongzaoshan and Heitupo formations,we propose that the Quanji Group shall be redefined to contain only the late Paleoproterozoic to early Mesoproterozoic Mahuanggou,Kubaimu,Shiyingliang,and Hongzaoshan formations,and the overlying Neoproterozoic strata,including the Heitupo,Hongtiegou,and Zhoujieshan formations,shall temporarily not be included in any group-level lithostratigraphic units.
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