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ABSTRACT: Stratigraphic Correlation of Pleistocene California Borderland Marine Carbonate Using Strontium Isotopes

Rosemary C. Capo, Donald J. Depaolo

High-precision measurements on carbonate sediments have refined the history of the 87Sr/86Sr ratio in Pleistocene seawater and allowed us to construct a standard 87Sr/86Sr vs. age curve, which we have applied to stratigraphic correlations in the California Borderland basins.

Foraminifera-nannofossil ooze samples from DSDP (Deep Sea Drilling Project) Site 590 in the Tasman Sea (31°S) were analyzed for 87Sr/86Sr to determine the Sr isotopic ratio of ocean water over the past 2.5 m.y. Modeling suggests that changes in river input associated with large variations in global chemical weathering rates are responsible for the observed variations in the 87Sr/86Sr record during the Pliocene-Pleistocene. From 2.4 m.y. to 0.3 m.y., the 87Sr/86Sr ratio of seawater increased rapidly by 14 × 10-5, which makes this period ideal for high-resolution correlations using the Sr isotope method.

Based on our standard seawater curve, strontium isotope analyses of macrofossils and foraminifera from carbonate sections from the Santa Barbara-Ventura and Los Angeles basins indicate that the Bathhouse Beach section of the Santa Barbara Formation was deposited between 0.9 and 0.4 m.y. ago, and in part is syndepositional with western portions of the nearly Pliocene-Middle Pleistocene Pico Formation of the Ventura basin, and with the lithiogically similar Lomita Marl of the San Pedro Formation at Palos Verdes.

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