A 100-m thick sequence of dolomite occurs within reef carbonates of the Permian Nabeyama Formation, central Japan. Analyses of sediments, fossils, minerals, and geochemical ingredients of a dolomitized sequence reveal that dolomitization was the consequence of active seawater circulation through the carbonate column. Stable carbon and oxygen isotopic data, precluding dolomitizing fluids of freshwater or hybridized seawater origin, provide arguments for atoll dolomitization by seawater. Thermal gradients between the atoll and the underlying volcanic structure caused the convection flow of interstitial water. Upwelling oceanic water, supersaturated with dolomite but undersaturated with respect to calcite, was subjected to thermal convection through the porous and permeable sediment column, leading to massive dolomitization of the oceanic atoll.
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