Diagenetic trends in the siliceous facies of the Monterey Shale in the Santa Maria region, California

Abstract

ABSTRACTX‐ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the Miocene Monterey Shale near Santa Maria, California. Within many stratigraphic sections soft, porous diatomaceous rocks change gradationally to underlying hard and brittle chert, porcellanite, and siliceous shale; the accompanying silica mineral zones are, in descending stratigraphic order: (1) biogenic silica (opal‐A), (2) cristobalitic silica (opal‐CT), and (3) microcrystalline quartz.Boundaries between silica mineral zones and stratigraphic horizons are often discordant. Within the opal‐CT zone, the d(101)‐spacing of opal‐CT decreases in a smooth non‐linear fashion from about 4 10 Å to 4‐04 Å. In the Santa Maria Valley and Bradley oil field areas the thicknesses of the opal‐CT zones are greater and the present thermal gradients less than in the adjacent Orcutt oil field. Thin opal‐CT zones at shallow maximum burial depths apparently correlate with higher thermal gradients.Using present thermal gradients and reconstructed maximum burial depths from well data in the Santa Maria region, the ranges in temperatures for the top and base of the opal‐CT zone are 38–54 °C and 55–110 °C, respectively. The temperature difference between these two boundaries ranges from 17 to 60 °C. In comparison, temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal‐CT and quartz, extrapolated experimental quartz‐water fractionations, and assuming δO18= 0%o for the isotopic composition of the equilibrating fluid are 18–56 °C and 31–80 °C for the top and base of the opal‐CT zone, respectively. The temperature difference between these boundaries is 11–36 °C using this method.Thermal gradients and sedimentation rates strongly influence rates of silica transformations. Reconstructed thermal and diagenetic histories of siliceous rocks of the Monterey Shale at four well sites in the Santa Maria region demonstrate that most silica conversions probably occurred during the last 3–4 Myr in response to accelerated rates of sedimentation (and therefore burial heating) during the Pliocene.