Abstract

Detailed biostratigraphic and Sr-isotope studies on two outcrop sections of the Miocene Monterey Formation of California demonstrate the applicability of the Sr-isotope method for detailed chronostratigraphic correlation. Multidisciplinary biostratigraphic ages generally agree with Sr isotopic ages determined by correlating 87Sr/86Sr ratios with a standard 87Sr/86Sr versus age curve constructed of data from Deep Sea Drilling Project Sites 575 and 590. Diagenetic modification of Sr isotopic ratios of the Monterey Formation was negligible. Strontium isotopes provide age resolutions of 0.1 to 0.2 Ma in the lower Miocene. Small faults and slump structures are identified as reversals within a general trend of increasing 87Sr/86Sr. Biostratigraphy yields the more definitive age assignments in the younger siliceous rocks where diatoms in particular have more utility than the corresponding flat and polytonal portion of the isotopic curve. Strontium isotopic correlation of the basal Monterey Formation with Deep Sea Drilling Project Site 575 indicates that Monterey deposition commenced at 17.85 ± 0.10 Ma (within planktic foraminiferal zone N6 and calcareous nannofossil zone CN2), concomitant with an increase in slope of the sea water 87Sr/86Sr versus age curve. This event dates 0.35 ± 0.10 m.y. before the beginning of the pronounced shift of δ13C, and 1.4 m.y. before the shift of δ18O, observed at Site 575. It is evident that Sr-isotope stratigraphy is a valuable supplement to biostratigraphy; the techniques are complementary in refining chronostratigraphic interpretations.

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