扬子区早寒武世早期磷质小壳化石的再沉积和地层对比问题

扬子区早寒武世早期的磷酸盐化小壳化石的发现,为早期动物演化及震旦系-寒武系界线地层的划分与对比研究做出了重要贡献。但碳酸盐岩地层中的这类小壳化石易于被风化剥离出来并成为磷质颗粒再沉积,这种现象在地层中常能遇到,它会给对某些生物分布时限的确定和地层对比造成混乱。考虑早寒武世一些主要小壳化石的再沉积和地层对比时要注意以下问题:1)带和带的磷质小壳化石产于Paokannia或Hunanocephalus的层位,它们无疑是再沉积的;2)组合带标志性化石与组合带标准化石共生,应判定其沉积时期为组合带时;3)扬子台地区的梅树村阶地层大都不全或缺失,保存的梅树村阶地层普遍与震旦系灯影组不同层位假整合接触,其岩性及生物特征均与下伏地层不同,另建组级地层单位是合理的;4)“杨家沟段”、“西蒿坪段”、“宝石坡组”不能作为灯影组顶部地层,它们是水井沱组沉积旋回的一部分,更不能因它们是筇竹寺期的沉积而把“灯影组”顶界上升到筇竹寺阶;5)滇东北地区大海段中产有丰富的Heraultipegmayunnanensis,它与Siphogonuchitestriangularis和Paragloborilussubglobosus共生,分布局限,其他地区仅在组合带中零星分布,所以在S.triangularis-P.subglo-bosus组合带()的上部建立一个H.yunnanensis组合亚带似乎更合适些。

RESEDIMENTATION OF THE EARLY CAMBRIAN PHOSPHATIZED SMALL SHELL FOSSILS AND CORRELATION OF THE SINIAN-CAMBRIAN BOUNDARY STRATA IN THE YANGTZE REGION, SOUTHERN CHINA

The discovery of the Early Cambrian phosphatized small shell fossils (SSFs) in the Yangtze Region is of importance to the studies on the early evolution of animals and the division and correlation of the Sinian-Cambrian boundary strata. However, the phosphatized SSFs, especially those preserved originally in carbonate rocks, were easily desquamated from mother rocks during weathering and redeposited in later rocks. If these reworked fossils were not distinguished from those preserved in situ, it might create confusion on the durations of these fossils and stratigraphic correlation. The Meishucunian strata are usually incomplete, or even absent, on the Yangtze Platform, and commonly overlie disconformably the different horizons of the Dengying Formation dolostones, these Meishucunian strata should be separated from the Dengying Formation and established as new stratigraphic units. The "Yangjiagou Member", "Xihaoping Member" and "Baoshipo Formation" were deposited in the Chiunqchussuian (Qiongzhusian) age and belong to the lowermost part of the sedimentary cycle of the Shuijingtuo Formation. Therefore it is inappropriate to put them in the uppermost part of the Dengying Formation. Heraultipegma yunnanensis is especially abundant in the Dahai Member of the Zhujiaqing Formation in northeastern Yunnan, but it coexists with Siphogonuchites triangularis and Paragloborilus subglobosus, both are index fossils of Assemblage Zone II, so it would be appropriate to establish a H. yunnanensis Assemblage Subzone in the upper part of the Assemblage Zone II, rather than a new Assemblage Zone Ⅲ.