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The Pliocene was a crucial episode in global climatic history, with an early Pliocene warm period followed by a late Pliocene cooling as the Arctic ice cap was established. One of the best Pliocene sequences on the Pacific Coast is the Etchegoin Group of the west-central San Joaquin Basin. The Etchegoin Group consists of (from bottom to top), three units: the uppermost Miocene-?Pliocene Jacalitos Formation, the lower Pliocene Etchegoin Formation, and the ?middle-upper Pliocene San Joaquin Formation. The marine rocks of the Etchegoin Group are capped by the upper Pliocene non-marine Tulare Formation. Paleontological studies showed that the molluscan succession in the western San Joaquin Basin was one of the best Pliocene sequences on the Pacific Coast.  As a result, the Weaver Committee used the names of the formations within the Etchegoin Group (Jacalitos, Etchegoin, and San Joaquin) as the basis for their three Pliocene provincial Pacific Coast molluscan stages. These highly fossiliferous strata have been of great interest, not only for their important paleontological and paleoclimatic data, but also because they occur in one of the richest oil-producing regions of California, the Coalinga and Kettleman Hills oil fields

Studies of these fossils and strata are very important, because the late Miocene-Pliocene is one of the most interesting intervals in climatic history. After the global Messinian event (marked by a pulse of glaciation and the repeated desiccation of the Mediterranean) that terminated the Miocene, the early Pliocene was a period of relative warmth and milder climates. But by the mid-Pliocene, the present-day Arctic ice cap was in place, and global climates became markedly cooler and drier. This mid-Pliocene glaciation is apparent even in the chronology of the Sierran glaciations, since the 3.2-Ma Deadman Pass glaciation marks the onset of glaciation in these mountains. In the San Joaquin Basin, there is evidence that the steep cooling trend of the late Miocene changed in the early Pliocene, with a slight warming trend evident in Pliocene molluscan faunas of the Etchegoin and lower San Joaquin formations.Fossil mollusks from these formations lived in waters with a mean annual temperature of 13-14°C (55-57°F), with a minimum of 11°C (52°F) and a maximum of 18°C (65°F), so conditions were not highly seasonal. These mollusks are most similar to those living in waters off southern California and Baja California. Diversity of the mollusks increased to almost mid-Miocene levels, as a number of subtropical taxa migrated northward. Most authors suggest that the late Pliocene faunas of the Etchegoin Group show a progressive cooling trend, consistent with the global climatic trend and the evidence of glaciation in the Sierras in the late Pliocene. Yet Stanton and Dodd found that the isotopic records indicated only slight cooling, despite the faunal changes noted by earlier authors. Loomis suggested that the San Joaquin marine embayment might have been partially insulated from global climate change due to its restricted flow, analogous to the way the modern warm-climate Gulf of California biota is less sensitive to changes occurring in the Pacific Ocean.

Yet the age control on these crucial strata has long been poor. To improve this situation, 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. Samples were taken as oriented blocks of rock with simple hand tools, then subsampled into cylinders for use in the automatic sample changer of the Occidental College paleomagnetics lab, and measured on a 2G cryogenic magnetometer. Samples were first measured at NRM (natural remanent magnetization), then demagnetized in alternating fields (AF) from 2.5 to 1.0 mT (millitesla), followed by thermal demagnetization from 100-630°C in 50°C steps.

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. Additional field work at Zapato Chino Canyon to the north should recover this missing interval and complete the sampling.