THE FIRST EXPLOSIVE DIVERSIFICATION OF SSFS DURING THE DEPOSITION OF THE LOWERMOST CAMBRIAN PHOSPHORITE IN CHENGJIANG, SOUTH CHINA

In the earliest Cambrian, a major diversification of small shelly fossils (SSFs) occurred as the prelude to the Cambrian Explosion during the large-scale phosphorite deposition in several regions in the world including South China. In order to clarify the cause of these events, we examined the detailed stratigraphy of the lowermost Cambrian bedded phosphorite in Chengjiang, Yunnan, South China. The 40 m-thick bedded phosphorites (the Zhongyicun Mb of the Zhujiaqing Fm) are composed of phosphatic limestone with abundant phosphate grains in calcite matrix. This texture suggests that phosphorite was primarily formed in an extremely shallow-water setting. Clastic grains of phosphorites were delivered into slightly deeper settings to deposit as bedded phosphorites or interbedded phosphorite-limestone. The SSFs in the bedded phosphorites occur in the same manner as the phosphate clastic grains. This indicates that the SSF animals lived in extremely shallow-water environments rather than in the depositional site of the bedded phosphorite.

By identifying 15 genera of SSFs from 10 horizons in the bedded phosphorites, we recognized 2 distinct SSF assemblages; i.e. the first assemblage with simple tubular fossils, such as Anabarites and Protohertzina, from the basal part of the bedded phosphorites, and the second assemblage with various mollusc shells, such as Paracarinachites and Ocruranus-Eohalobia group, from the middle and the upper parts. We newly identified the first occurrence horizon of the second assemblage at least 5 m below (ca. 2 m.y. earlier than) the previously reported level in Chengjiang. The assemblage turnover from the first to the second marks the major diversification of SSFs. We confirmed that the first and the biggest SSF diversification occurred during the earliest Cambrian deposition of the lower part of the bedded phosphorites.

Eastern Yunnan was located then in a restricted basin surrounded by land areas on the Yangtze platform. Smaller basins might become isolated from open-ocean particularly during the low sea-level intervals. The phosphorites might be primarily deposited in a uniquely phosphate-enriched seawater in very shallow restricted basins. The SSF diversification was triggered possibly in such extreme conditions with unusually phosphate-rich seawater.