扬子克拉通北缘神农架-崆岭地区中-新元古代地层厘定——兼论“神农架群底界”

Redefinition of Meso-Neoproterozoic stratigraphy in the Shennongjia-Kongling area on the northern margin of Yangtze Craton--Discussion on the bottom of the Shennongjia Group

综合近几年来扬子克拉通北缘中-新元古代最新研究成果,厘定扬子克拉通北缘神农架-崆岭地区中-新元古代地层序列,自下而上依次为:吴家台组、神农架群、马槽园组、南沱组、陡山沱组和灯影组。吴家台组仅出露于距神农架不到20 km的北崆岭西汊河地区,为一套浅变质砾岩-含砾砂岩-细砂岩-白云岩组合,与下伏古元古界黄凉河岩组、上覆南华系南沱组均呈角度不整合接触,详细的年代学工作限定其形成时代为~1800-1500 Ma,代表扬子克拉通核部中元古代最早期沉积记录,可视作与神农架群底部-下部相当的层位(吴家台组与神农架群未见直接的接触关系露头)。神农架群主要岩性为藻礁白云岩、泥粉晶白云岩、粉砂岩等,夹砾岩、细砂岩、泥岩、火山岩层,与上覆马槽园组角度不整合接触,不见底,形成于~1.4-1.0 Ga。马槽园组为一套白云质或砂质胶结的杂砾岩、白云质砾岩,角度不整合于神农架群和南沱组之间。南沱组为杂砾岩-含砾粉砂质泥岩-碳质泥岩组合,形成时代为~657-635 Ma,属Marinoan冰期产物。陡山沱组、灯影组为碳酸盐岩建造,是扬子克拉通基底之上第一套稳定盖层,沉积时限为~635-541 Ma,也是早期生命演化的关键时期。

The Meso-Neoproterozoic stratigraphic sequences in the Shennongjia-Kongling area on the northern margin of the Yangtze Craton are redefined based on the recent research,which are Wujiatai Formation, Shennongjia Group, Macaoyuan Formation, Nantuo Formation, Doushantuo Formation and Dengying Formation in ascending order. The Wujiatai Formation rocks are only exposed in the Xicha River,Northern Kongling area, less than 20 km from the Shennongjia Group, which are a set of epimetamorphic conglomerate-gravel sandstone-fine sandstone-dolomite assemblages. They are both in angular unconformity contact with the underlying Paleoproterozoic Huanglianghe Formation rocks and the overlying Neoproterozoic Nantuo Formation rocks, with detailed chronological deposited age of ~ 1800-1500 Ma, the earliest Mesoproterozoic sedimentary records in the Yangtze Craton nucleus, which can be considered similar with the bottom to lower part of the Shennongjia Group(there is no outcrop in direct contact between the two layers). The Shennongjia Group rocks include algal reef dolomite, micritic dolomite, siltstone, conglomerate,fine sandstone, mudstone and volcanic rock, which are in angular unconformity contact with overlying the Macaoyuan Formation with invisible bottom, and deposited between ~1.4-1.0 Ga. The Macaoyuan Formation rocks, angular unconformable between Shennongjia Group and Nantuo Formation rocks, are a set of dolomitic or sandy cemented diamictite and dolomitic conglomerate. The Nantuo Formation rocks, combination of diamictite and gravel silty mudstone and carbonaceous mudstone, are formed between ~657-635 Ma and by Marinoan Glaciation. The Doushantuo and Dengying Formations are the first stable carbonate rocks on the basement of the Yangtze craton, with sedimentary time of ~635-541 Ma, which are also the crucial evolution period for early life.