CHERT AND THE PROTEROZOIC OCEANIC SILICA CYCLE

Throughout the Phanerozoic Eon oceanic silica chemistry and chert formation have been strongly influenced by the activities of silica-secreting organisms. Precambrian oceanic silica concentrations should have been significantly higher than during Phanerozoic time due to the absence of a significant skeletal biota. Comparative study of Phanerozoic and Proterozoic cherts enables us to examine what insights chert petrography and sedimentology provide on Proterozoic oceanic silica chemistry.

Proterozoic oceans were in all likelihood supersaturated with respect to quartz and likely also opal-CT. Despite higher silica concentrations, there is little evidence that Neo- and Mesoproterozoic cherts formed primarily by direct silica precipitation. Neo- and Mesoproterozoic cherts have microtextures similar to those of Phanerozoic cherts that formed by carbonate replacement and are typically found as nodules or discontinuous beds within carbonate deposits that have similar depositional textures. Neo- and Mesoproterozoic cherts differ from Phanerozoic cherts in that they contain evidence of a peritidal locus of early diagenetic chert formation, which strongly contributed to the preservation of paleontologically important microbiotas. Subtidal cherts formed during later diagenesis also occur in Proterozoic successions.

Some Paleoproterozoic iron formation cherts are distinctly different from later cherts, suggesting that a secular change in oceanic silica chemistry may have occurred towards the end of the Paleoproterozoic Era. Granular iron formations (e.g., Gunflint and Sokoman Formations) contain chert that is microtexturally distinct from carbonate replacement cherts and appears to have formed by direct silica precipitation. The primary cherts lack ghosts or inclusions of carbonate precursors, have fine-scale grain fracturing (possibly from syneresis), low grain-packing densities, and are not associated with unsilicified carbonate deposits of similar depositional composition. Not all chert in iron formations is primary. Carbonate sediment and grains, where present, were also silicified and can be identified by carbonate ghosts and inclusions. Primary chert may be evidence of higher oceanic silica concentrations during Paleoproterozoic than later times, possibly due to greater hydrothermal circulation.