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The Drakes Bay Formation is an upper Miocene sequence of siliceous mudstones containing many small dolomite nodules. The nodules probably formed without a precursor biogenic calcite supplying Ca or HCO3 for dolomitization. Dolomite formation preferentially took place in sediment layers slightly richer in organic C than the surrounding sediments. More extensive sulfate reduction in these layers raised the porewater HCO, concentration and caused carbonate precipitation. The initial carbonate may have been dolomite or calcite, which was later converted to dolomite.

Carbon and oxygen stable isotope ratios vary systematically and clearly illustrate changes in the isotopic composition of dissolved...

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