Abstract

The diatomaceous deposits of the Monterey Formation along the Santa Barbara coast were subjected to temperatures and burial depths that increased westward after deposition. As a result, the silica phase in originally similar, laterally equivalent strata generally changed westward from biogenous opal-A to diagenetic opal-CT to diagenetic quartz. In individual beds at the same stratigraphic level, however, silica phases changed at various temperatures, producing a transformation—or kinetic—sequence related to variations in the ratio of silica to detrital minerals. Compared to silica in rocks with minor detrital minerals, in rocks with progressively more abundant detrital minerals: (1) opal-CT formed progressively later, (2) the initial d-spacing of opal-CT was progressively smaller, and (3) quartz formed progressively earlier.

Comparison of associated calcareous, dolomitic, and carbonate-free rocks with equal silica/detrital ratios shows that carbonates did not significantly affect rates of silica diagenesis in the vast majority of rocks. In carbonate-bearing rocks with silica/detrital ratios ≥8, however, some (2% to 10%) quartz formed early in diagenesis, probably prior to opal-CT formation. In addition, atypical quartz cherts formed locally in carbonate-bearing strata by pore filling and replacement early in diagenesis, possibly before widespread opal-CT formation.

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