LATE PLEISTOCENE GLACIATION IN THE WESTERN U.S. GREAT BASIN AS RECORDED IN MONO BASIN, CALIFORNIA

Beginning nearly a decade ago, research in the Mono Basin, CA, has been impetus for reinterpretation of late Quaternary glaciations in the western U.S. Great Basin (Zimmerman et al., 2006). The renewed interest followed publication of Ar/Ar dates for several volcanic ash layers (Kent et al., 2002) in the Wilson Creek Fm that indicate the ashes near the base of the section were erupted from the Mono Craters perhaps 67,000 years ago (Zimmerman et al., 2006). That age led to a suggestion that the Mono Lake Excursion (MLE) is older by nearly 8,000 years than previously proposed (Benson et al., 2003), and that the MLE might not be a separate one of the paleomagnetic field but instead is the Laschamp Excursion (LE) that is dated at about 40,000 years B.P. (Guillou et al., 2004)(Kent et al., 2002). However, the recent discovery of the MLE and LE in cored sediment from Pyramid Lake, NV (Benson et al., 2008), and subsequent Ar/Ar dating of several of the volcanic ashes in the Wilson Creek Fm (Cassata et al., 2010) indicate that the two excursions indeed are separate events in the paleomagnetic record. On the assumption that the base of the Wilson Creek Fm at its type locality along Wilson Creek is about 67,000 years old and not about 38,000 years old that previously was the age assignment based on Carbon-14 dating of ostracodes, a match of the relative paleointensity in the formation to the Global PaleoIntensity Stack (GLOPIS)(Laj et al., 2004) has resulted in a reinterpretation of the paleoclimate record in the Mono Basin (Zimmerman et al., 2006). Possible evidence that the Wilson Creek Fm might not be 67,000 years old at its base comes from a comparison of the stratigraphic intervals between volcanic ash layers that are exposed on opposite sides of the Mono Basin—at the South Shore Cliffs and at Wilson Creek. The relative spacing is very similar for all the ash layers except between two near the bottom of the sections where there is more than a fivefold difference (decrease) at the type section; at Wilson Creek the interval between ash layers 16 and 17 is 20 cm but at the South Shore Cliffs the interval is 108 cm (Zimmerman et al., 2006). For the 20 cm of lacustrine silt at Wilson Creek to represent nearly 30,000 years of geologic time is not consistent with deposition rates for other large Pleistocene pluvial lakes (Bonnevillle, Lahontan, Searles) in the Great Basin.