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ABSTRACT: Paleoenvironmental Interpretation of Carbon, Sulfur, and Iron Contents in Miocene Monterey Formation, Santa Maria Basin, California

D. A. Zaback, L. M. Pratt

Within the onshore part of the Santa Maria basin, the Miocene Monterey Formation of California can be broadly subdivided into (1) lower calcareous shale, (2) middle phosphatic shale, and (3) upper siliceous shale facies. Organic and inorganic geochemical data on 15 outcrop and 43 core samples (13 wells) provide insight regarding environmental changes during deposition of these major lithofacies.

Plots of total organic carbon vs. total sulfur indicate that redox conditions in the upper sediment layer varied markedly. The calcareous facies displays a wide range in values and absence of correspondence among total organic carbon (0.5-6.0 wt. %), total sulfur (0.05-2.7 wt. %), and total iron (0.5-3.5 wt. %), reflecting alternation between biogenic and clastic sedimentation. The phosphatic zone is characterized by relatively constant total sulfur (0.8-2.4 wt. %) compared to total organic carbon values (2.5-10.1 wt. %). Normalized to total iron content, total sulfur is greater than values stoichiometric for pyrite, suggesting iron-limited euxinic conditions during deposition of this unit. Organic geochemical data indicate excess sulfur was incorporated into organic compounds. For th upper siliceous unit, there is a positive correlation between total organic carbon and total sulfur content, with average ratios close to modern normal marine values. Higher Al2O3 and total iron contents in the siliceous zone relative to the phosphatic zone reflect increased detrital input.

Geochemical data corroborate previous sedimentological evidence for the onset of oxygen-depleted benthic conditions during deposition of the lower calcareous facies and intensification of these conditions during deposition of the phosphatic middle facies. Benthic ventilation improved during deposition of the upper siliceous facies. Preliminary data indicate that stable sulfur isotope values for coexisting pyrite and organosulfur compounds are sensitive to changing redox environments and record the reactivity of sulfur during early diagenesis.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990