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Field Classification and Oxygen Isotope Composition of Quartz Cherts from the Monterey Formation, Santa Maria Basin, California

BEHL, RICHARD J., University of California, Santa Cruz, CA, BRIAN M. SMITH, University of California, Berkeley, CA, and ROBERT E. GARRISON, University of California, Santa Cruz, CA

Field, petrographic, and isotopic studies indicate that chertification occurred at different times during the burial history of the Miocene Monterey Formation sediments. Early, intermediate, late, and dolomite-replacement quartz cherts can be identified by differential compaction, cross-cutting relationships, and by precursor lithology. Early cherts form nodules or lumpy beds that display differential compaction, distorted laminations, or greater tectonic deformation than neighboring cherts. Intermediate quartz cherts formed from replacement of opal-CT chert precursors, or are assigned to this category because they show no clear evidence of unusual precursor or timing of chertification. Late cherts show cross-cutting silicification fronts without subsequent differential compaction of rimary stratification. Dolomite-replacement cherts form by the patchy replacement of thick dolostone beds and nodules, or by the replacement of siliceous, dolomitic laminations. Cherts that are interbedded with predominantly shaly strata make up an additional field classification.

Our oxygen isotope analyses largely substantiate our field and petrographic classification of Monterey cherts. Early quartz cherts contain oxygen isotope ratios of d18O (SMOW) = 24.5 to 29.4(sigma), intermediate cherts = 26.5 to 29.4(sigma), late cherts = 24.8 to 25.9(sigma), dolomite-replacement cherts = 27.9 to 30.2(sigma), and shale-associated cherts = 29.4 to 31.2(sigma). Assuming a prograde sequence of chertification, and that d18Opore water remained relatively constant at about 0(sigma) (SMOW), shale-associated cherts were formed the earliest at 40 to 49 degrees C, dolomite-replacement cherts followed at 45 to 56 degrees C, normal cherts formed between 48 and 64 degrees C, and late cherts formed at 67 to 73 degrees C (temperature expression of Matsuhisa et al., 1979). Early quar z cherts apparently inherited their differential compaction features from early-formed opal-CT cherts; their isotopic composition indicates replacement by quartz at a wide range of temperatures, from 48 to 75 degrees C. Further synthesis of field and isotopic data will also help us constrain the oxygen isotopic evolution of the Monterey Formation pore waters.


AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)