On the Lower Cambrian biotic and geochemical record of the Hetang Formation (Yangtze Platform, south China): evidence for biogenic silica and possible presence of Radiolaria

Abstract. Research on Lower Cambrian siliceous sedimentary rocks is important for understanding the origin and early involvement of polycystine Radiolaria in the silica cycle. During our study, thin sections and HF acid processing of black cherts and shales from the Hetang Formation that crops out in the Xintangwu section (west Zhejiang Province, south China) were made. We report on the presence of siliceous spherical microfossils (possibly Radiolaria) associated with sponge spicules and acritarchs. Their size and the presence of residual spines on some spherical siliceous microfossils observed in both residues and thin sections of cherts from the top of Member ‘a’ of the Hetang Formation argue for the possible presence of radiolarians. Based on the Small Shelly Fossil assemblages reported in previous studies, this interval should be considered as Qiongzhusian (Atdabanian–Early Botomian) in age. Finally, the values of the Ge/Si ratio measured on black cherts of the Hetang Formation point to a biogenic origin of the silica.