Keywords:Cambrain, SSF, Yunnan
The earliest Cambrian evolution of animals is marked by the appearance and rapid diversification of small shelly fossils (SSFs). We examined the litho- and bio stratigraphy of the lowermost Cambrian at the Xiaolantian and Hongjiachong sections in the Chengjiang area, eastern Yunnan, South China. By analyzing both outcrop and drilled core samples, we picked up many SSFs from the ca. 30 m-thick phosphorites (Zhongyicun Mb), and obtained the following results.1) the Zhongyicun Mb comprises 6 lithostratigraphic units, which include Unit 3 with characteristic black mudstone beds for a significant key for the mid-Zhongyicun Mb.2)SSFs were found from 26 horizons out of 83 in the Zhongyicun Mb and the overlying Dahai Mb. Five zones were discriminated. Interval 1 withAcanthocasis orthocanthus, Spirellussp., Protohertzina unguloformis andProtohertzina anabarica(from uppermost of Unit 1 to lower part of Unit 2); Interval 2 with Ocruranus finial, Purellasp.andYunnanopleura biformis(from upper part of Unit 2). This marks the first occurrence of cap-shaped SSFs. Interval 3 with Helcionellasp.andOelandiella korobkovi(from Unit 3). Interval 4 with Obtusoconussp.,Conopoconissp. andErongia acculatus(from lower part of Unit 6). Interval 5 withIgorella sp., Bemella simplelxandParagloborilus subglobosus(from upper part of Unit 6). 3) In addition, at the Hongjiachong section, a unique interval with Halkieria sp.,Pteromus sp.andSinosachites delicatusoccurs between Interval 1 and 2.
These results from the Xiaolantian and Hongjiachong sections, the following 6 SSF zones are proposed here. Namely, 1)Acanthocasis orthocanthus- Protohertzina anabaricaZone, 2) Halkieria- PteromusZone, 3) Ocruranus finial-Yunnanopleura biformisZone, 4)Paracarinachites sinensis-Oelandiella korobkoviZone, 5) Obtusoconus-ConopoconisZone, and 6) Igorella -Bemella simplelxZone.Previously the Zhongyicun Mb was divided into two SSF assemblage zones (Steiner et al., 2007); however, the present study offers 6 zones. It is noteworthy that the key bed in the mid-Zhongyicun Mb, restricted within theParacarinachites sinensis-Oelandiella korobkoviZone,appears useful for regional correlation.