LITHO- AND CHEMO-STRATIGRAPHY OF THE LOWERMOST CAMBRIAN AT HONGJIACHONG, YUNNAN, SOUTH CHINA

The diversity of SSFs increased dramatically in the earliest Cambrian from the primitive assemblage composed of simple tubular forms (anabaritids, protoconodonts, etc.) to that dominated by more complex cap-shaped ones (molluscs, etc.). In order to constrain the horizon of this major diversification in high resolution, we analyzed the litho- and δ¹³C_carb stratigraphy of the lowermost Cambrian, i.e. Zhujiaqing Fm (Daibu Mb, Zhongyicun Mb, Dahai Mb) and Shiyantou Fm, at Hongjiachong in the Chengjiang area, utilizing a ~120 m-long drilled core sample. The diversified cap-shaped SSF asemblage first appeared at the middle horizon of the phosphatic Zhongyicun Mb (Jiang & Chen, 2008). The measurement of δ¹³C_carb for 55 samples from the studied interval identified two negative δ¹³C_carb shifts (N1 and N2) and one positive shift (P1); i.e. N1 from -1 ‰ to -5 ‰ in the upper Daibu Mb, N2 from -2 ‰ to -6 ‰ in the middle-upper Zhongyicun Mb, and P1 from -6 ‰ to -1 ‰ in the uppermost Zhongyicun Mb. The δ¹³C_carb profile at Hongjiachong confirmed that the major SSF diversification had occurred in the middle of the N2 interval. After recording a negative peak in the uppermost Zhongyicun phosphorite, the δ¹³C_carb values started to increase to record the P1 interval, that can be correlated with the global signal detected immediately below the sub-Tommotian unconformity. The major SSF diversification likely occurred in the Nemakit-Daldynian, particularly during the negative δ¹³C_carb excursion (N2), at least in Yunnan, rather than during the positive δ¹³C_carb shift as previously regarded. This diversification, probably with biomass enlargement, possibly increased the input of isotopically light carbon into the ocean by consuming the DOC pool.