Application of Strontium Isotope Geochronology to Monterey Formation, Offshore Santa Barbara, California

Nathan R. Miller

Dating paleo-oceanographic events requires a detailed geochronologic framework that cannot always be constructed by traditional techniques. The Miocene Monterey Formation provides an excellent opportunity to apply the newly developed strontium isotope method of dating where diagenesis limits preservation and access to biogenic components. The method takes advantage of the rapid increase in the ratio of ⁸⁷Sr/⁸⁶Sr in seawater, as recorded by marine precipitates during the Neogene subperiod. A 300-m continuous core of the Monterey Formation, recovered from offshore Santa Barbara, was examined for its strontium isotope stratigraphy. The core contains a unique record of Monterey deposition in an outboard basin. Calcareous foraminifera are abundant and wel preserved in the lower two-thirds of the core but sparse to absent in the upper third where primary lithologies have been partially dolomitized. Silica-phase diagenesis and the difficulty of extracting preserved diatoms from siliceous sediments limit the application of diatom biostratigraphy. Strontium isotope data indicate that the core spans more than 12 m.y. in its deposition, with an overall accumulation rate on the order of 2.4 cm/1,000 years. A lithology-based sampling scheme in concert with selective leaching techniques was used to sample abundant and well-preserved foraminifera throughout the core interval. Compared to conventional dating techniques, strontium isotope geochronology of Monterey lithologies offers the advantages of being unaffected by silica-phase transformations nd of being independent of temperature and species-specific isotopic fractionation factors. In addition, the technique is not limited by biofacies recovery, and stratigraphy indistinct calcareous microfossils can be used.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.