MAGNETIC STRATIGRAPHY OF THE MIOCENE-PLIOCENE ETCHEGOIN GROUP, WESTERN SAN JOAQUIN BASIN, CALIFORNIA

One of the best Pliocene sequences on the Pacific Coast is the Etchegoin Group of the west-central San Joaquin Basin. The formations of the Etchegoin Group (from bottom to top: Jacalitos, Etchegoin, San Joaquin) were the original basis for the Pliocene Pacific Coast marine chronology of the Weaver Committee in 1944, and their mollusks and stratigraphy have been intensively studied for over 100 years. Yet the age control on these crucial strata has long been poor. We sampled 34 sites spanning 8000 m of section at nearly every exposure in the Big Tar Canyon-Baby King Canyon area of Reef Ridge. Most samples showed a single component of remanence held mainly in magnetite, with some overprinting on reversed samples. The samples passed a reversal test, showing that the remanence is primary and not an overprint. The entire Jacalitos Formation is reversed in polarity, and based on dates of 5.0 and 5.3-5.5 Ma on the upper and lower parts, probably correlates with Chron C3r (5.3-6.0 Ma, latest Miocene to earliest Pliocene). Most of the San Joaquin Formation is normal in polarity, and based on a date of 2.5 Ma at the top, probably correlates with Chron C2An (2.5-3.6 Ma). The upper Etchegoin Formation is reversed in polarity and appears to correlate with Chron C2Ar (3.6-4.2 Ma). The lower Etchegoin Formation appears to correlate with part of Chron C3n, but a large unconformity at the base has apparently removed the earlier parts of Chron C3n.