Palaeogeographic and stratigraphic significances of Fortunian small skeletal fossils from the southern Shennongjia Region (China)

Abstract

Situated on the northern margin of Yangtze Block, the Shennongjia Region is divided into southern and northern parts by the Yangri Fault and has attracted significant interest for understanding the collisional history of the Yangtze Craton. Traditionally, both regions were thought to lack the earliest Cambrian strata. Here, we report the discovery of Fortunian small skeletal fossils (SSFs) and phosphatized algal fossils from the lower Yanjiahe Formation (traditionally referred to as the Xihaoping Member) in the southern Shennongjia Region. These fossils are assigned to established SSF biozones of the Yangtze Platform, specifically the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone and the Paragloborilus subglobosus–Purella squamulosa Assemblage Zone, thereby confirming the presence of Fortunian strata in the southern Shennongjia Region. The composition of these assemblages shows strong similarity to those in other parts of the Yangtze Block, such as the Three Gorges area, northern Sichuan, and southern Shaanxi. In contrast, the absence of this Fortunian fauna in the Zhenba-Fangxian Block suggests the existence of a denudation zone during the early Cambrian. This denuded zone lies between the outer shelf facies of the South Qinling Orogenetic Belt and the onshore facies of traditional Yangtze Block, highlighting the distinct palaeogeographic position of this tectonically unique region.