ORDOVICIAN TURNING POINT IN THE PHANEROZOIC SILICA CYCLE

Basinward withdrawl of most sedimentary chert from peritidal environments to deeper-water settings took place during the Early-Middle Ordovician. Analysis of more than 160 Ordovician formations bearing nodular cherts, bedded cherts, and silica-replaced oolites suggests that this reduction in shallow-water chert accumulation was faster than the more prolonged Cambro-Ordovician transitional retreat of biogenic siliceous deposits from shallow-water settings proposed by Maliva et al. (1989).

Radiolarians may have been a major stimulus for the basinward shift. This hypothesis is supported by a conspicuous increase in the proportion of Middle Ordovician radiolarian chert relative to siliceous sponge chert, even as numbers of chert deposits fell after the Early Ordovician. Increased abundance of shelf and basinal chert in the Late Ordovician coincides with a resurgence in chert abundance. If radiolarian drawdown of oceanic dissolved silica concentration depleted shallow waters to the point that sponges could not produce enough spiculitic silica to generate chert, then siliceous sponges may have been driven into deep waters by radiolarian monopolization of marine dissolved silica. Alternatively, Ordovician radiation of carbonate biotas could have displaced siliceous sponges from peritidal settings, but this carbonate biotic expansion does not explain the radiolarian radiation.

If the retreat of biosiliceous facies from shallow-water, nearshore environments reflects a stepdown in oceanic dissolved silica concentration to the point that siliceous sponges cannot make silica in shallow water, the mid-late Ordovician may mark a change in the marine silica cycle such that spiculitic chert could only return to peritidal depths at times of elevated input of dissolved silica to the oceans.

Greenhouse vs. icehouse modes of biogenic silica accumulation appear to be distinguishable in Ordovician rocks. These two modes may have application to cherty deposits in younger strata until diatom dominance of the marine silica cycle commenced in the Cenozoic.