It is proposed that many nodular cherts in limestone have formed in the ground water of mixed meteoric-marine coastal systems where dissolution of biogenic opal and mixing of marine and fresh waters can produce waters highly supersaturated with respect to quartz and undersaturated with respect to calcite and aragonite. Aspects of cherts readily explained by this model include the observed isotopic ratios in cherts, typical field relationships, the relative resistance of dolomite to silicification, the source-of-silica problem, the preservation of siliceous fossils in cherts, and aspects of chert morphology and mineralogy.

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