Silica-replaced oolites, bedded shelf cherts and Paleozoic changes in the silica cycle

Oolites that were replaced by early diagenetic silica and were not reworked from their original depositional setting can serve as a useful tool to evaluate a previously suggested retreat of biosiliceous facies from peritidal environments to deeper water settings in the Early Paleozoic. The silica-replaced oolites suggest that the retreat was largely over by the Middle Ordovician, which is consistent with emerging compilations of the spatial and temporal distribution Ordovician cherts. Whether the retreat was a function of declining concentration of dissolved silica in shallow marine waters or of radiation of faunas from nearshore into offshore settings or some combination of both is an issue that may be clarified with improved resolution of the timing of the retreat. A small Upper Carboniferous peak in silica-replaced oolite abundance coincides with a minor rebound in shelf chert accumulation and the common presence of coastal flint deposits in Upper Carboniferous cyclothems, suggesting that cherty facies returned to shallow water environments at this time. [Copyright &y& Elsevier]