Abstract: Strontium Isotope Geochronology of the Vaqueros Formation in the Western Transverse Ranges, California

Catherine A. Rigsby

Strontium isotope analyses of late Oligocene and early Miocene megafossils in the western Transverse Ranges yields ages that are consistent with lithostratigraphic observations and with foraminiferal data from adjacent units. Analysis of 20 samples from the Vaqueros and the adjacent Rincon and Alegria formations reveals that oyster fragments with well preserved, pearly, lamellar layers yield the most consistent ⁸⁷Sr/⁸⁶Sr ratios. Robust, unidentified bivalve fragments also yield usable data, as do barnacles. Other sample types were not useful.

Oyster fragments from two locations in the upper Rincon Formation yielded ⁸⁷Sr/⁸⁶Sr ratios of 0.708445 ± 25 and 0.70848 ± 2, giving an apparent equivalent age of 19 ± 1 Ma. Bivalve fragments from near the top of the Alegria Formation yielded ⁸⁷Sr/⁸⁶Sr ratios of 0.707772 ± 30 and 0.707936 ± 20, giving apparent equivalent ages of 34 ± 1 and 29 ± 1 Ma, respectively. These samples bracket the age of the Vaqueros in the study area.

The Vaqueros samples with the lowest ⁸⁷Sr/⁸⁶Sr ratios, 0.708165 ± 28 and 0.708178 ± 22, and the oldest ages (24 ± 1 Ma) are oyster fragments from Bulito Canyon and Little Sespe Creek, respectively. And, although other workers have reported that the Vaqueros decreases in age to the north and the east, data from this study do not confirm that trend. In fact, these data give average ages of 20 ± 1 Ma for the westernmost outcrops and 24 ± 1 Ma for the northern outcrops. Within the Santa Barbara coastal area, the data suggest that the Vaqueros ranges in age from 24 ± 1 Ma near its base to 20 ± 1 Ma (⁸⁷Sr/⁸⁶Sr ratio of 0.708404 ± 8) near its top.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994