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

Abstract

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. The authors' oxygen isotope analyses largely substantiate their field and petrographic classification of Monterey cherts. Early quartz cherts contain oxygen isotope ratios of {delta}{sup 18}O (SMOW) = 24.5 to 29.4{per thousand}, intermediate cherts = 26.5 to 29.4{per thousand}, late cherts = 24.8 to 25.9{per thousand}, dolomite-replacement cherts = 27.9 to 30.2{per thousand}, and shale-associated cherts = 29.4 to 31.2{per thousand}. Assuming a prograde sequence of chertification, and that {delta}{sup 18}O{sub pore water} remained relatively constant at about 0{per thousand} (SMOW), shale-associated cherts were formed the earliest at 40 to 49C, dolomite-replacement cherts followed at 45 to 56C, normal cherts formed between 48 and 64C, and late cherts formed at 67 to 73C (temperature expression of Matsuhisa et al., 1979). Early quartz 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 75C. Further synthesis of field and isotopic data will also helpmore » constrain the oxygen isotopic evolution of the Monterey Formation pore waters.« less