摘要
在扬子地台东南缘,下寒武统龙王庙阶清虚洞组主要由浅水碳酸盐岩组成。在野外露头剖面实测和室内镜下薄片观察的基础上,大量风暴沉积被发现于不同剖面清虚洞组的不同层位中,同时大量风暴诱发形成的沉积构造被识别出来,包括侵蚀基底、粗粒滞留沉积、粒序层理、平行层理、丘状交错层理(少见并且值得怀疑)以及沙纹层理,组成了多种类型的风暴沉积序列。结合更靠扬子东南缘的深水剖面中重力流沉积的缺乏,可以推断早寒武世龙王庙期扬子地台的沉积模式为碳酸盐缓坡。结合风暴的形成机制以及清虚洞组风暴沉积的发育特征(尤其是粗粒滞留砾屑的定向排列和典型丘状交错层理的缺乏),可以推断研究区风暴沉积形成于强烈的冬季风暴作用,并且早寒武世龙王庙期华南的古地理位置应当位于中纬度地区,这一结论对一些著名的全球古地理重建方案提出了质疑。同时中纬度地区大规模发育蒸发岩和碳酸盐岩还佐证了寒武纪地球处于热室(Hothouse)时期。
The Lower Cambrian Lungwangmiaoan Qingxudong Formation in the southeastern Yangtze area of South China mainly consists of shallow sea carbonate.Based on measurement of field outcrops and detailed petrographic observations of thin sections,a large amount of storm deposits was found to occur in different layers of the Qingxudong Formation,and a lot of storm induced sedimentary structures were found,such as sharp erosional base,coarse lag deposits,graded bedding,parallel bedding,hummocky crossstratification(rare and questionable)and ripple bedding,and many types of storm deposit sequences.Due to scarcity of gravity flow deposits in deep section more close to the southeastern margin of the Yangtze platform,it can be inferred that the depositional model of Yangtze area during the early Cambrian Lungwangmiaoan should be carbonate ramp with a gentle gradient.Combined with the formation mechanism of storm deposits and the characteristics(especially the directional alignment of the coarse lag gravel-size intraclasts and the lack of typical hummocky cross-stratification)of the storm deposits of the Qingxudong Formation,this study suggests that the storm deposits in the study area should be formed in intense winter storm,and the palaeogeographic position of South China should be located at midlatitudes during the early Cambrian Lungwangmiaoan.This conclusion comes up with queries to some famous palaeogeographic reconstruction results.What’s more,the large-scale development of evaporates and carbonates at mid-latitude areas favors the view that the Earth was in the period of hothouse during the Cambrian.
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