A unique condition for early diversification of small shelly fossils in the lowermost Cambrian in Chengjiang, South China: Enrichment of phosphorus in restricted embayments

Abstract

In the earliest Cambrian the major diversification of small shelly fossils (SSFs) was the first episode of the so-called Cambrian Explosion. In order to clarify the background environmental conditions of this event, we examined the lowermost Cambrian strata with bedded phosphorites in the Chengjiang area, South China. The lowermost Cambrian (the Zhongyicun Mb of the Zhujiaqing Fm) in eastern Yunnan is composed of bedded phosphorites and phosphatic limestones with diverse SSFs. We investigated 3 sections within the Chengjiang area at Hongjiachong, Maotianshan, and Xiaolantian. Detailed lithostratigraphic analysis of outcrops and drill cores at Hongjiachong indicates that the Zhongyicun Mb consists of 5 distinct units, A to E in ascending order. The presence of 15 genera of SSFs in 20 horizons shows that the Zhongyicun Mb yields two distinct SSF assemblages of Fortunian age (earliest Cambrian; 541–529Ma); i.e. the first assemblage with simple-shape SSFs (Anabarites, Protohertzina) from basal Unit A, and the second assemblage with various molluscan shells (Paracarinachites, Ocruranus–Eohalobia) from Units C–E. As well as abundant phosphate grains, all SSFs occur as clastic grains, suggesting that phosphorite was primarily formed in extreme shallow-water settings, as were the small shelly animals. We established that the first occurrence of the second SSF assemblage is at least 5m lower (ca. 1–2myr earlier in age) than previously reported in Chengjiang, and we speculate that the major diversification in SSF assemblage likely occurred during the Fortunian, at least before ca. 534Ma. Judging from the rift-related tectonic setting and relevant paleogeography of western South China, we further speculate that the Zhongyicun Mb was primarily deposited in restricted embayments in the Kangdian rift basin, and that the rapid SSF diversification in the Fortunian occurred in a unique setting in highly phosphorus-rich seawater.