Source of silica and silicification of the lowermost Cambrian Yanjiahe Formation in the Three Gorges area, South China

摘要

The rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the Ediacaran-Cambrian (E-C) interval, even if the exact timing and biological triggers of this event are still debated. Although Ediacaran-Cambrian chert depositions are widely distributed, multiple evidences converge to support their hydrothermal origin. In this study, we provide petrographic study supplemented by elemental concentration, SEM and Raman spectral data on the siliceous rocks from the Yanjiahe Formation (the Three Gorges area, South China), in order to ascertain the origin of included silica and discuss the involved processes of silica deposition. Contrary to most previous studies, our analyses suggest that studied cherts mainly resulted from biogenic silica, and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input. The deposition model of silica includes direct deposition of siliceous bioclasts, secondary replacement of carbonate by dissolved biosilica, and microbial induced silicification from seawater. The sink of silica recorded in the Yanjiahe Formation was characterized of a transition from typical Precambrian to Phanerozoic process. In addition, micropalaeontological and taphonomical analyses of the Yanjiahe Formation revealed two pulses of microfossil abundance and diversity in the Fortunian and Stage 2, respectively. They can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux.