Influence of rock composition on kinetics of silica phase changes in the Monterey Formation, Santa Barbara area, California

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.