Much of the unique character of the siliceous facies of the Miocene Monterey Formation stems from diagenesis. At localities in California, soft, porous diatomites and diatomaceous mudrocks give way vertically and laterally to hard, dense, and brittle cherts, porcelanites, and siliceous mudrocks. Vertical lithologic transformations typically occur through several tens of meters of section; lateral changes may span several kilometers or more. A well-documented mineralogic progression from highly disordered amorphous silica (opal-A) to microcrystalline quartz through an intermediate cristobalitic stage (opal-CT) commonly accompanies these changes.

X-ray diffraction analyses of surface and subsurface samples define present boundaries of silica zones. Within the cristobalitic silica zone the d-(101) spacing of opal-CT may vary between 4.12 A and 4.04 A. In the Taft and Chico Martinez areas of the Temblor Range, boundaries between silica zones and stratigraphic horizons are generally parallel. In the Santa Maria region and in the Santa Ynez Mountains, silica zones cut obliquely across stratigraphic horizons. Off central Baja California, the opal-A to opal-CT transition in Monterey-equivalent rocks corresponds to a prominent bottom-parallel seismic reflector.

Time, temperature, and sediment composition affect rates of silica transformations. Oxygen isotopes of opal-CT and quartz provide estimates of the temperatures at which these transformations occurred. In nature the thermal history of any sediment is largely a function of the thermal gradient and sedimentation rate. In the Santa Maria region, most silica conversions probably occurred during the last 3 to 4 m.y. in response to accelerated rates of sedimentation and, therefore, to burial heating during the Pliocene. In contrast, rates of silica transormations in the Monterey Shale in the Cholame area probably varied with proximity to hydrothermally altered intrusive serpentine.

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