Abstract 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 HCO 3 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 CO 2 that occurred with depth. The isotopic analyses show that dolomite formation did not begin until the pore waters were free of dissolved sulfate. The Ca contents of the dolomites decrease, and both the Mg and Fe contents increase, with depth of formation. Manganese contents correlate with Fe contents. Sodium contents of the dolomites are relatively high, probably reflecting their poor ordering and nonstoichi-ometry. Strontium contents of the dolomites are typical of those from hemipelagic sediments with moderate sedimentation rates.