Applications of Strontium Isotopes in Correlating Monterey Formation, California

Donald J. Depaolo, Kenneth L. Finger

Recent improvements in the accuracy with which the strontium-isotope evolution of seawater can be determined allows strontium isotopes to be used for correlating marine sediments at resolutions comparable to biostratigraphic methods. In this study, we geochemically and biostratigraphically analyzed outcrop sections along Naples Beach and Graves Creek, in order to correlate the Monterey Formation with concurrent global paleo-oceanographic events. By correlating the ⁸⁷SR/⁸⁶SR ratios in these sediments with the strontium isotope vs. time curve for seawater derived from calcareous oozes in DSDP Corehole 590 (southwestern Pacific) and DSDP Corehole 575 (central Pacific), we determined ages at resolutions ranging from < 0.1 to 2.5 Ma.

With an average resolution in the early Miocene of 200,000 years, strontium-isotope dating has enabled us to determine the onset of Monterey deposition with exceptional accuracy. Our results date the basal Monterey Formation at approximately 17.8 Ma in both the Salinas basin and the Santa Barbara-Ventura basin. This date corresponds to a major shift in the ^dgr¹³C seawater curve, supporting the inference that Monterey-type deposition was responsible for the sequestering of light carbon. Although the ratio increases monotonically in the lower Miocene at Graves Creek, this is not the situation in the coeval part of the Naples Beach section, where it reveals heretofore unidentified structure. The isotopic and biostratigraphic data indicate that the major hiatus in the Naples B ach section ranges from approximately 14 to 11 Ma, which is about 2 m.y. shorter than previously interpreted.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.