ABSTRACT: Lower to Middle Miocene Isotope (⁸⁷Sr/⁸⁶Sr, ^dgr¹⁸O, ^dgr¹³C) Standard Sections, DSDP Site 608

Kenneth G. Miller, Mark D. Feigenson, James D. Wright

Isotopes changes (⁸⁷Sr/⁸⁶Sr, ^dgr¹⁸O, ^dgr¹³C) have been correlated to the geologic time scale primarily by biostratigraphy. Biostratigraphic correlations suffer from problems of diachrony and taxonomy. Magnetostratigraphy provides a facies-independent correlation tool, but there are few Tertiary sections with unambiguous magnetostratigraphy. We previously developed an isotope standard for the Oligocene at the only location with a pristine magnetochronology, Site 522. We extend this approach to Site 608 in the northeastern North Atlantic, which contains a relatively straightforward Miocene magnetochronology. We establish Miocene oxygen isotope Chronozones MI1 through MI6 at Sites 522 and 608, which are directly tied to the geoma netic polarity time scale (GPTS). The integration of stable isotopes, Sr isotopes, biostratigraphy, and magnetostratigraphy at site 608 provides a standard section with

which other Sr isotope and oxygen isotope records can be correlated. For example, using oxygen isotopes to correlate, the Sr isotope record from Site 608 compares well with previously published records from Sites 516 and 590. The firm ties of the Oligocene to middle Miocene isotope records with the GPTS allows us to establish the nature of the change in Sr isotopes between 38 and 8 Ma.

There were moderately high rates of ⁸⁷Sr/⁸⁶Sr change during the Oligocene (^sim0.000030/m.y.), yielding stratigraphic resolution of ±1.0 m.y. The rate of change of ⁸⁷Sr/⁸⁶Sr increased during the early Miocene. We estimate that the rate of change between 23 and 15 Ma was greater than 0.000060/m.y. Given our ability to reproduce Sr isotope measurements (±0.000020 to ±0.000030), temporal resolution is better than +0.5 m.y. for the early to early middle Miocene. The rate of change was much lower from 15 to 8 Ma (on average, 0.000014/m.y.), and maximum stratigraphic resolution in this interval is ±2.0 m.y.

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