Abstract

Rocks in the Monterey Formation have a wide range of bulk physical properties which can be attributed to differences in silica phase and in mineral abundance.

Rocks in the Monterey Formation along the Santa Barbara coast principally contain silica (5-90%), detrital minerals (5-70%), carbonates (0-80%), apatite (0-30%), and carbonaceous organic matter (1-25%). Silica may be either biogenous (diatomaceous opal-A) or diagenetic (opal-CT or quartz). In terms of silica phase alone, rocks containing abundant biogenous silica differ strikingly from rocks containing abundant diagenetic silica, whereas opal-CT rocks and quartz rocks are very similar. In terms of mineral abundance, field characteristics are related mainly to the silica/detrital ratio. As this ratio decreases among rocks with abundant diagenetic silica, nearly all field characteristics are affected–in particular, cohesiveness, overall hardness, and brittleness decrease. Carbonate content has comparatively little effect, although cohesiveness and resistance to erosion increase somewhat with the presence of dolomite. Differences related to mineral abundance are much less pronounced among diatomaceous rocks than among rocks bearing diagenetic silica.

Owing to strong dependence of lithologic characteristics on silica phase, the most pronounced lithologic boundary in the Monterey Formation along the Santa Barbara coast is the diagenetic “boundary” between opal-A and opal-CT. Because this boundary is oblique to lithostratigraphic units in the area, considerable care is needed when interpreting depositional facies trends.