HIGH-RESOLUTION FORTUNIAN (LOWER CAMBRIAN) SSF STRATIGRAPHY IN EASTERN YUNNAN

Litho- and small shelly fossil (SSF) stratigraphy of the lowermost Cambrian (Fortunian) was analyzed at the Xiaolantian and Hongjiachong sections in the Chengjiang area, Yunnan, South China, which consist of the Daibu Member (dolomite), Zhongyicun Mb (phosphorite) and Dahai Mb (dolomite), in ascending order. Six distinct SSF assemblages were discriminated within the 30-40 m-thick phosphorite of the Zhongyicun Mb; i.e., Assemblage-A with Acanthocasis orthocanthus, Protohertzina unguliformis, Protohertzina anabarica, Spirellus sp., As-B with Halkieria sp. and Sinosachites sp., As-C with Ocruranus sp., Protoconus sp., As-D with Purella sp., Oelandiella sp. , Hellcionella sp., As-E with Obtusoconus sp., Maikhanella sp., Conopoconus sp., and As-F with Igorella sp., Securiconus simus, Bemella simplex. Assemblage A corresponds to the Assemblage I of Steiner et al. (2007), whereas the Assemblages B-F as a whole to the Assemblage II. The mid-phosphorite marker bed (sandy/muddy dolomite of short-time gravity-flow nature) occurs in both sections and also in others in eastern Yunnan, suggesting its potential usage as a reliable key bed for time correlation. The post-535 Ma marker bed defines the top of Assemblage C and the base of Assemblage zone D, suggesting that the major diversification of SSF started clearly before the marker deposition. Further application of the Fortunian SSF zonation with 6 assemblages (< 2 myr long in average), together with the marker bed, is promising for correlation of other sections of shallow to deep-water facies.