[关键词]
[摘要]
扬子台地北缘神农架群发育一套中元古代中期和晚期的海相碳酸盐岩与碎屑岩地层, 其中台子组是神农架群中部地层, 也是该群古生物化石丰度最高的组。本文利用化学浸泡法对中元古代中期台子组细粒碎屑岩进行微体化石研究, 结果显示: 台子组化石总的特征是组合简单、化石个体较小, 大部分标本直径介于20–50 μm。 本文共鉴定出微体化石7属11种, 包括光面球形类Leiosphaeridia crassa、L. minutissima、L. jacutica、L. bicrura 和Leiosphaeridia sp., 细胞聚合体Synsphaeridium sp.、Eomicrocystis malgica和相对复杂的疑源类Navifusa sp.、 Satka sp.、Germinosphaera sp.与Arctacellularia tetragonala。台子组微体化石组合中原核生物是主要的化石类型, 真核生物丰度很低。其化石多样性和膜壳平均直径远小于邻近时期的生物化石群落, 这可能与台子组沉积期的地表环境特征以及独特的生物群落特征有关。拉曼光谱分析表明台子组有机质处于较高的热演化阶段, 埋藏温度约280℃, 过高的埋藏温度也可能降低了台子组化石保存质量。
[Key word]
[Abstract]
The Shennongjia Group on the northern margin of the Yangtze Platform consists of thick marine carbonate and clastic rocks of the middle and late Mesoproterozoic Era. The middle part of the Shennongjia Group, i.e., the Taizi Formation, consists of sandstone and silty shale in the lower part and fine-crystalline limestone in the upper part. According to previous investigations, organic-walled microfossils of the Taizi Formation are the most abundant among the formations of the Shennongjia Group. To further improve the understanding of the biodiversity during the deposition of the Taizi Formation, we carry out a micropaleontological investigation on the fine-clastic rocks of the Taizi Formation from the Tiechanghe section, Shennongjia National Park, Hubei Province. Our investigation reveals a mode- rately diverse assemblage of organic-walled microfossils dominated by spherical vesicles and aggregates with a few complex vesicle forms. In addition, the fossil specimens of the Taizi assemblage are very small, generally ranging from 20 to 50 μm. A total of 11 microfossil taxa belonging to seven genera have been identified. These taxa include simple spherical form (Leiosphaeridia crassa, Leiosphaeridia minutissima, Leiosphaeridia jacutica, Leiosphaeridia bicrura, and Leiosphaeridia sp.), cell aggregate form (Synsphaeridium sp. and Eomicrocystis malgica), and complex vesicle form (Navifusa sp., Satka sp., Germinosphaera sp., and Arctacellularia tetragonala). As for biological affinity, prokaryotes are the predominant components, whereas eukaryotes are scarce. The low diversity and small individual size of the microfossils of the Taizi assemblage, which might be caused by a harsh living environment or insufficient essential nutrients, are significantly different from those of the earlier and later biological communities. Furthermore, the Taizi Formation has experienced low- to middle-grade metamorphism with peak metamorphic temperatures of ~280°C based on Raman geothermometers of individual organic-walled microfossils, which is higher than those of other well-preserved Proterozoic microfossils. The high burial temperature may affect the preservation quality of the microfossils of the Taizi assemblage.
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[基金项目]
中国科学院战略性先导科技专项(B类) (XDB26020102)和国家自然科学基金创新群体项目(41821001)联合资助
