Abstract
Along the Santa Barbara coast, field characteristics of individual Monterey rock vary greatly, relating to two principal factors: silica phase and bulk chemical composition.
In terms of silica phase, field characteristics are most affected by whether silica is dominantly biogenous (opal-A) or diagenetic (either opal-CT or quartz). Diatomaceous rocks differ from compositionally equivalent rocks bearing abundant diagenetic silica in hardness, density, cohesiveness, surface texture, and resistance to erosion. By contrast, opal-CT rocks differ from compositionally equivalent quartz rocks mainly in bulk density, and differentiation between the two groups is usually impractical in the field.
Monterey rocks contain: biogenous or diagenetic silica (5-90%), detrital minerals (5-70%), carbonate rocks (0-80%), apatite (0-30%), and (carbonaceous) organic matter (1-25%). Field characteristics are affected mainly by the silica/detrital ratio. As this ratio decreases among diatomaceous rocks, bulk density and color saturation (darkness) tend to increase. As the ratio decreases among rocks with abundant diagenetic silica, effects are more pronounced: bulk density systematically varies; cohesiveness, hardness, and brittleness all decrease. Carbonate rocks have comparatively little effect, although cohesiveness and resistance to erosion increase somewhat with the presence of dolomite.
Rocks most likely to be misinterpreted are: (1) quartz porcellanites (due to matte surface, viewed as opal-CT rocks); (2) opal-CT cherts (due to vitreous surface, viewed as quartz rocks); (3) carbonate-bearing mudstones with low (10-15%) detrital contents and high (60-80%) diagenetic silica contents (due to resistance to erosion, viewed as dolomites or limestones); (4) organic shales with low (10-25%) diagenetic silica contents (due to friability of weathered rock, viewed as diatomaceous—even where silica is quartz).
Overall lithologic characteristics suggest that diagenetically-produced boundaries may be more prominent, geophysically, than stratigraphic boundaries within—or formational contacts of—the Monterey Formation. Facies trends interpreted from diagenetic boundaries, however, may be highly misleading.
