ABSTRACT: Strontium Isotope Chronostratigraphy of Primary and Secondary Carbonates in the Monterey Formation, Santa Barbara Basin, California

Nathan R. Miller

Traditional approaches for constructing chronostratigraphic frameworks in profile hydrocarbon reservoirs of the Monterey Formation are often limited by a lack of preserved calcareous microfossils as a result of dissolution below the calcite compensation depth, paucity of preserved siliceous microfossil tests owing to seawater dissolution and/or diagenetic transformation, and exclusion of temporally useful benthonic foraminifera by low oxygen environments and/or presence of long-ranging planktonic foraminifera. Strontium isotope analysis of primary and secondary carbonates offers an alternative means for establishing Monterey chronostratigraphies. The method utilizes the rapid increase in the ratio of ⁸⁷Sr/⁸⁶Sr in seawater, as recorded by marine preci itates, during the Neogene subperiod. Strontium isotope chronostratigraphy was applied to a 300 m (984 ft) continuous Monterey Formation core, recovered from offshore Santa Barbara. The core contains a unique record of Monterey deposition in an outboard basin. Calcareous foraminifera are abundant and well-preserved in the lower two-thirds of the core but sparse to absent in the upper third where primary lithologies have been rhythmically cemented by dolomite. Calcareous and dolomitic samples were analyzed for their ⁸⁷Sr/⁸⁶Sr values and temporal significance. Strontium isotope data indicate that the core spans the time range of 5.5 to 18.0 Ma corresponding to late-early to latest Miocene time. Comparison of ⁸⁷Sr/⁸⁶Sr values obtained from stratig aphically proximal calcareous and dolomitic samples are in good agreement suggesting that the dolomite precipitated in equilibrium with original interstitial water and/or that the lag-time for dolomite precipitation by seawater diffusion is short compared to the resolution of the technique. Dolomite carbon isotope values range from -17 to +10 ^pmil (PDB) suggesting that they precipitated in microbial zones of sulfate reduction (depleted) and methanogenesis (enriched). Similarity of ⁸⁷Sr/⁸⁶Sr values between calcareous and dolomitic samples indicates that Monterey dolomites may retain temporal data.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990