湘黔桂地区新元古代“下江群”地层划分对比研究--重新启用下江系的探讨

摘要

近年来随北方青白口系层型剖面的解体，华南梵净山群及下江群时期地层的归属成为争议较大的问题。中国地层表（2011～2014）将武陵造山运动界面之下的梵净山群及相当地层（870～820Ma）与武陵造山运动界面之上，南华系之下的下江群及相当地层（820～780（720）Ma）一并划归为青白口系，给前寒武系地层划分对比与研究带来不便。在湘黔桂相邻区发育一套武陵造山运动界面之上至南华系（820～720Ma）间，分布广泛、出露完整、层序连续的裂谷盆地沉积建造———“下江群”，为研究解决该问题提供了天然而理想场所与物质基础。本文在对“下江群”时期的泥市群、板溪群、下江（芙蓉溪）群、丹州（高涧）群地层分区、岩石地层详细划分对比、充填序列及岩相古地理特征等研究基础上，认为下江群具有岩石建造组合特征典型，岩石层序序列完整、出露连续及划分标志清楚等优势。结合近年年代地层研究的新进展，提出重启“下江系”，以约束华南、全国新元古代地层划分对比，对完善我国及国际新元古代地层表具有深远意义。%With the disassembly of Qingbaikou system stratotype section of North China in recent years,there comes a controversial issue on the belonging of the Fangjingshan Group and Xiajiang Group in South China.Both the Fangjingshan Group and the Xiajiang Group were belonged to the Qingbaikou System in the chronostratigraphic chart of china(201 1 ~2014),the former (the same strata age of 870~820Ma)occurs below the surface of Wuling Movement,and the latter (the same strata age of 820 ~780 (720 )Ma)occurs just above the Wuling Movement and below the Nanhuan System, these bring inconvenience to the stratigraphical division and correlation in Precambrian.“Xiajiang Group”occurs just above the Wuling Movement and below the Nanhuan System (820 ~720Ma),as a formation of lift basin with wide distribution,complete exposion and well succession in Hunan, Guizhou and Guangxi province,which provides a natural ,ideal place and the material basis to solve this problem. On the basis of the stratigraphic study of the Nishi Group,Banxi Group,Xiajiang (Furongxi )Group,Danzhou (Gaojian)Group,detailed lithostratigraphic study ,division and correlation,and the filling sequence and changing lithofacies study of the “Xiajiang Group”in Hunan,Guizhou and Guangxi province,wo think that the Xiajiang Group has advantages of typical rock building combination,complete rock sequence,exposed continuously and clear division mark.Combining with the latest progress of chronostratigraphic study in this area,we reboot the“Xiajiang System”in this paper to restrain the study of the stratigraphical division and correlation in South China , and it also has a far~reaching significance in perfecting the table of Neoproterozoic strata in China.