Tephra layers of Blind Spring Valley and related upper Pliocene and Pleistocene tephra layers, California, Nevada, and Utah: Isotopic ages, correlation, and magnetostratigraphy

Abstract

Numerical ages have been determined for a stratigraphic sequence of silicic tephra layers exposed at the Cowan Pumice Mine in Blind Spring Valley, near Benton Hot Springs, east-central California, as well as at Chalk Cliffs, north of Bishop, Calif. The tephra layers at these sites were deposited after eruptions from nearby sources, most of them from near Glass Mountain, and some from unknown sources. The ages were determined primarily by the laser-fusion 40 Ar/ 39 Ar method, mostly on sanidine feldspar; two were determined by conventional K-Ar TAB. analysis on obsidian clasts. These tephra layers, all underlying the Bishop ash bed and listed in order of concordant age and stratigraphic position, are: tab. The above tephra layers were also petrographically examined and the volcanic glass shards of the layers were chemically analyzed using the electron microprobe and, for some samples, instrumental neutron activation analysis and X-ray fluorescence. The same types of chemical and petrographic analyses were conducted on stratigraphic sequences of tephra layers of suspected upper Pliocene and Pleistocene age in several past and present depositional basins within the region outside of Blind Spring Valley. Chemical characterization, combined with additional dates and with magnetostratigraphy of thick sections at two of the distal sites, allow correlation of the tephra layers at the Cowan Pumice Mine with layers present at the distal sites and provide age constraints for other intercalated tephra layers and sediments for which age data were previously lacking. The identification at several sections of the widespread Huckleberry Ridge ash bed, derived from the Yellowstone eruptive source area in Wyoming, as well as a new 40 Ar/ 39 Ar age on this ash bed from a proximal locality, provide additional age constraints to several of the distal sections. The dated or temporally bracketed distal units, in order of concordant age and stratigraphic position, are: At the Cowan Pumice Mine, only a partial section of the eruptive record is preserved, but the best materials for laser-fusion 40 Ar/ 39 Ar and other isotopic dating methods were obtained. In the more distal Willow Wash and Confidence Hills sections, both persistent depositional basins for most of late Pliocene time, more complete sections of upper Pliocene tephra layers were preserved. In the region of Glass Mountain, the tephra layers that make up each of the mapped and dated pyroclastic units are multiple and complex, but a progressive simplification of the stratigraphy away from the source area was observed for more distal sites in southern and southwestern California and in Utah. This progressive simplification is attributed to both variable explosiveness and magnitude of individual tephra eruptions, as well as to variable dispersal of the tephra by winds during an eruptive episode. Lake beds present at several localities in the western Great Basin (Fish Lake Valley, Nev.; the Waucoba Road area near Big Pine, Calif.; and Confidence Hills, Death Valley, Calif.) are dated between -2.15 and ∼2.04 Ma and indicate that wetter or colder conditions than present existed at these sites. Age and correlation data presented here provide a chronostratigraphic framework for studies of late Neogene stratigraphy, tectonics, and environmental change in the southwestern United States.