THE ORIGINAL PLACE OF THE CAMBRIAN ANIMAL DIVERSIFICATION: PHOSPHORITE DEPOSITIONAL BASIN IN YUNNAN, SOUTH CHINA

The Cambrian animal diversification was likely originated in South China based on the fossil richness; small shelly fossils (SSFs) first diversified in South China, and the Chengjiang Fauna is the earliest group among the Burgess shale-type fossils. South China is also characterized by the largest phosphorite deposits in history during the Ediacaran-Cambrian period. In order to clarify the geological background of these events, we have conducted fieldworks in 8 major sections in the phosphorite depositional basin in Yunnan; Meishucun, Chengjiang, Wangjiawan, Laolin, Zhujiaqing, Xianfeng, Baideng, and Huote. We examined stratigraphy, sedimentary textures, and fossil occurrence in each section and integrated paleogeographic features.

The lowermost Cambrian phosphorite, the Zhongyicun Member of the Zhujiaqing Formation, is basically composed of clastic phosphate grains with calcite matrix. SSFs themselves are phosphatized and occur in same manner as the phosphate clasts. We speculate that phosphorites might primarily deposited in very-shallow restricted embayments where P-rich seawater was concentrated, and that the SSF diversification possibly occurred in such extreme conditions.

In the N-S extended rift basin of Yunnan, the southern area is much more P-rich than north. In Huote section, the southern end of the basin, phosphorite consists of coarser grained phosphate clasts than other sections. It suggests that the primary phosphorite factory was near the Huote section. While in Baideng section, the southwestern end of the Yunnan basin, we found various trace fossils, mainly the surface tracks, in the phosphorite bed. Moreover, we found large worm-like body fossils (ca. 1 cm wide and ca. 10-20 cm long) in the lower phosphorite bed with SSFs, mainly Anabarites sp.. These facts show that ecosystem with both SSFs and large soft-bodied ones already existed just after the Ediacaran-Cambrian boundary. After all, the southern end of the Yunnan phosphorite basin was the best candidate for the origination place of the Cambrian diversification. The trigger may be geographical condensation of phosphorite, possibly together with the P-rich high-alkaline magma along the rift margin.