Geochronology, especially U-Pb zircon geochronology, has made important contributions to our understanding of the Jurassic Coast Range ophiolite of California. However, much of the older work is primitive by modern standards, and even some recent U-Pb work is limited in its precision and accuracy by a range of factors. We apply a new zircon analysis method, chemical abrasion–thermal ionization mass spectrometry (CA-TIMS), to generate high-precision, high-accuracy multistep 206 Pb*/ 238 U plateau ages for zircons from plagiogranites from the Point Sal (Coast Range ophiolite) and San Simeon (Coast Range ophiolite) ophiolite remnants. These remnants have been postulated to have been part of a single, contiguous remnant prior to offset along the San Gregorio–San Simeon–Hosgri fault system. Two fractions of zircon from a Point Sal Coast Range ophiolite plagiogranite, and one fraction of zircon from a San Simeon Coast Range ophiolite plagiogranite yield 206 Pb*/ 238 U plateau ages that are indistinguishable from one another—a mean age for the three determinations is 165.580 ± 0.038 Ma (95% confidence, mean square of weighted deviates [MSWD] = 0.47). The error quoted is an internal precision, which is appropriate for comparison of the ages to one another. The fact that the San Simeon and Point Sal ages are indistinguishable, even with such very small internal precision errors, is a remarkably robust confirmation of the correlation between the San Simeon and Point Sal ophiolite remnants.

Abstract Extensive coastal dunes rise southward from the Santa Maria Valley to the Point Sal Ridge, south-central California. In Mussel Rock ravine and elsewhere, eolian sands occur in association with fluvial deposits. Radiocarbon ages and correlative deposits in nearby areas indicate a sequence of events postdating marine-terrace deposits of oxygen isotope stage 5. Dissected paleodunes ( Qe ,), probably related to stage 4, are overlain by mostly sandy fluvial deposits ( Qf ) whose higher units yield 14 C ages in the 30- to 23-ka range. Transverse paleodunes ( Qe 2 ) began to form after 26 ka, as sea level fell during the transition from stage 3 to stage 2, and probably continued to accumulate westward during stage 2. Parabolic dunes ( Qe 3 ) formed from new and reactivated sand masses during the Flandrian transgression and were stabilized before 3 ka. Lobate dunes ( Qe 4 ) subsequently formed and the presently active transverse dunes ( Qe 5 ) have developed within the past 200 years. The chronology revealed near Point Sal may provide a valuable time frame for coastal-dune development elsewhere in California. The investigation indicates, for example, that extensive transverse dunes form during periods of sediment abundance, ideally when sea level falls and large quantities of sand are exposed on emergent continental shelves, but also when stabilized dunes are reactivated by environmental changes, including human land use. Parabolic dunes develop during periods of sediment deficiency—for example, as sea level stabilizes following a marine transgression.