About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 64 (1980)

Issue: 3. (March)

First Page: 444

Last Page: 445

Title: Montery Rocks Along Santa Barbara Coast, California: ABSTRACT

Author(s): Caroline M. Isaacs

Article Type: Meeting 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

End_Page 444------------------------------

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.

End_of_Article - Last_Page 445------------

Copyright 1997 American Association of Petroleum Geologists