Characteristics Of The Neoproterozoic Liantuo Formation Seccession And Its Basin Dynamics In Northern Jiangxi

The Liantuo(Xiuning)Formation in the North Jiangxi Province and the Liantuo Formation in the Yichang and Tongshan areas,Hubei Province,is of one important stratigraphic units of the middle Neoproterozoic in South China.However,there have been controversies on the sedimentary geotectonic settings and depositional interval.On the basis of detailed regional sedimentological and regional comparative study,LA-ICP-MS zircon U-Pb dating has been carried out for gravel-bearing sandstone and sedimentary tuff from the bottom and middle of Liantuo formation in Shimentang section,Tongshan county,and the dacite from the top of Shangshu Formation and sedimentary tuff from the middle Xiuning formation in the Huatan Ganling section,Shangrao city,respectively.The maximum deposition age of Liantuo Formation is 784.0 ± 11 Ma,the middle Liantuo Formation is 727.3 ± 6.7 Ma in Shimentang section,and the dacite from the topmost Shangshu Formation is 779.0 ± 6.6 Ma,the middle Liantuo Formation is 733.6 ± 5.9 Ma in the Huatan Ganling section,respectively.The sedimentological investigation shows that the sedimentary structure of Xiuning formation in northeast Jiangxi is not developed.Instead,they are represented by deposited facies from alluvial fan-shallow-water shelf-deep-water shelf.The sedimentary structures of Liantuo formation in northwest Jiangxi are developed,such as diagonal bedding,cross bedding,sand cross lamination and ripples,which are indicative of coastal to shallow shelf depositional settings.Based on previous research results,therefore,the deposition interval of the Liantuo Formation or Xiuning Formation in North Jiangxi and its adjacent areas is constrained to be about 780 ? 720 Ma.The bottom boundary of the Liantuo Formation is an unequal interface,and the missing strata in the bottom of Liantuo Formation depends on different lithofacies and paleogeographic units.Combined with field sedimentary survey,the sedimentary seccession analysis of Liantuo Formation indicated that the northern Jiangxi area was generally “one uplift and four depressions” during the sedimentary period of the Liantuo Formation: one uplift was the central uplift spreading from southwest to northeast,and the four depressions were the Hubei and Jiangxi depression,Zhejiang and Jiangxi fault depression,southern Anhui depression,and Hunan and Jiangxi depression.The regional comparative study shows that in the Yangtze Block,the Liantuo formation(Xiuning formation)in the North Jiangxi Province and its adjacent areas,and the Chengjiang Formation in central Yunnan Province,and the upper Banxi Group in Hunan,Xiajiang Group in Guizhou and Danzhou Group in Guangxi areas are simultaneous heterogeneous deposition sequences,which is the first set of rifting cover deposits in the Nanhua Rift system under the settings of regional thermal subsidence before the onset of the first Neoproterozoic glaciation.