ABSTRACT: High-Resolution Strontium Isotope Stratigraphy and Biostratigraphy of the Miocene Monterey Formation, Central California

Kenneth L. Finger, Donald J. Depaolo

Detailed biostratigraphic and Sr isotope studies on two outcrop sections of the Miocene Monterey Formations of California demonstrate the applicability of the Sr isotope method for detailed biostratigraphic ages. These ages generally agree with Sr-isotope ages determined by correlating ⁸⁷Sr/⁸⁶Sr ratios with a standard ⁸⁷Sr/⁸⁶Sr vs. age curve constructed of data from Deep Sea Drilling Project (DSDP) Sites 575 and 590. Diagenetic modification of Sr-isotope ratios of the Monterey Formation was negligible. In the lower Miocene, strontium isotopes provide age resolutions of 0.1 to 0.2 Ma and reversals within a general trend of increasing ⁸⁷Sr/⁸⁶Sr coincide with minor faults and slump structures. Biostratigraphy ields the more definitive age assignments in the younger siliceous rocks where diatoms in particular have more utility than the corresponding flat and polytonic portion of the isotopic curve.

Strontium isotope correlation of the basal Monterey Formation with DSDP Site 575 indicates that Monterey deposition commenced 17.85 ± 0.10 Ma (within planktic foraminiferal zone N6 and calcareous nannofossil zone CN2). This event dates 0.35 ± 0.10 Ma before the beginning of the pronounced shift of ^dgr¹³C observed at Site 575, which argues against a causal link between the two. It is evident that Sr-isotope stratigraphy is a valuable supplement to biostratigraphy; the techniques are complementary in refining chronostratigraphic interpretations.

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