Lateral differences in Monterey strata along the Santa Barbara coast indicate increased diagenesis toward the west. Westward, silica alters from biogenic amorphous opal (in diatom frustules) to diagenetic opal-CT, and then to diagenetic quartz. Continuous exposure for 50 km, simple homoclinal structure, paleogeographic setting, and detailed analysis of 14 stratigraphic sections together show that sediments were laterally equivalent, as originally deposited. Distinctive stratigraphic differences in sediment composition, informally divided into five members, are also laterally age-constant.

Because silica phases differ in rocks of the same age, same depositional environment, and with identical bulk chemical composition, the differences must reflect post-depositional conditions. Overburden thicknesses and thermal changes in organic matter indicate that diagenesis increased westward owing to greater burial temperature.

Study of sample sets taken laterally shows that both silica phase changes occurred by rapid solution-precipitation accompanied by significant compaction, and by little movement of silica between beds. Distinctive field characteristics (hardness, brittleness, bulk density, and luster) changed mainly during opal-CT formation.

Although these changes affected rocks of nearly all compositions, details varied--even in carbonate-rich rocks--with the proportion of detrital material to biogenic or diagenetic silica. As this proportion increased, opal-CT formation was retarded, accompanying compaction decreased, "ordering" of opal-CT increased, and quartz formation was promoted. Nearly identical timing relation in associated calcareous, dolomitic, and carbonate-free rocks show that silica diagenesis was unaffected by carbonate except in rocks containing at least 10 times more silica than detrital material.

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