A 1.2-km-thick section of the Miocene Monterey and overlying Pliocene Sisquoc formations in the Santa Maria basin area of California contains highly variable amounts of biogenic silica, detrital clay and silt, organic matter, carbonate, pyrite, and francolite. Organic-matter diagenesis resulted in the early precipitation of dolomite, pyrite, and francolite, and the concentration of trace metals. Dolostone horizons occur 1 to 10 m apart and consist of 50 to 95 weight percent pore-filling dolomite. The dolomite is low in Fe and Mn and contains an average of 0.8 to 5.3 mole percent excess Ca. Dolomite composition is related to texture in some samples and suggests several different episodes of dolomitization. There is a positive correlation between organic matter, pyrite, and the trace metals V, Cr, Ni, Cu, and Zn. Pyrite formation probably occurred below the sediment/seawater interface (noneuxinic basin) in the microbial-sulfate reduction zone, and was limited by Fe in sediment having a high organic matter-to-clay ratio and by reduced sulfur in sediment having a low organic matter-to-clay ratio.