Abstract

The fine-grained rocks of 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 is 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-bearing rocks and quartz-bearing rocks are very similar. In terms of mineral abundance, field characteristics are related mainly to the ratio of silica to detrital minerals. 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 few effects–cohesiveness decreases where calcite is abundant, and cohesiveness and resistance to erosion increase somewhat where dolomite is present. 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 cuts obliquely across lithostratigraphic units in the area, considerable care is needed in interpreting depositional facies trends.