Tephra layers of blind Spring Valley and related upper pliocene and pleistocene tephra layers, California, Nevada, and Utah: isotopic ages, correlation, and magnetostratigraphy
Professional Paper 1701
By:
Andrei M. Sarna-Wojcicki,
Marith C. Reheis,
Malcolm S. Pringle,
Robert J. Fleck,
Doug Burbank,
Charles E. Meyer,
Janet L. Slate,
Elmira Wan,
James R. Budahn,
Bennie Troxel,
and James P. Walker
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
40Ar/39Ar method, mostly on sanidine feldspar; two were determined by conventional K-Ar
analysis on obsidian clasts. These tephra layers, all underlying the Bishop ash bed and listed in
order of concordant age and stratigraphic position, are:
Tephra Unit Method Material Age
Bishop Tuff (air-fall pumice) Ar/Ar sanidine 0.759?0.002 Ma*
Upper tuffs of Glass Mountain Ar/Ar sanidine 0.87?0.02 Ma
Upper tuffs of Glass Mountain Ar/Ar sanidine 1.13?0.19 Ma
Lower tuffs of Glass Mountain K-Ar obsidian 1.86?0.09 Ma (avg of 2 dates)
Ar/Ar sanidine 1.92?0.02 Ma (avg of 2 dates)
Tuffs of Blind Spring Valley Ar/Ar sanidine 2.135?0.02 to
sanidine 2.219?0.006 Ma (10 dates)
Tuffs of Benton Hot Springs Ar/Ar plagioclase 2.81?0.02 Ma
*Date published previously
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 40Ar/39Ar 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:
Tephra Unit Method Material Age
Tephra layers of Glass
Mountain (undiff.)
P-mag.*; correlation N/A <1.78 Ma
Tephra layers of Glass
Mountain (lower)
P-mag.*; correlation N/A >1.78 , <1.96Ma
Tephra layers of Emigrant
Pass
P-mag.*; correlation N/A >1.96, <2.06 Ma
Tephra layer of Confidence
Hills
Correlation, p-mag.* N/A >1.96, <2.06 Ma
Huckleberry Ridge ash bed Ar/Ar; correlation sanidine 2.063?0.007 Ma
Tephra layers of Blind
Spring Valley
Correlation N/A 2.135 to 2.219 Ma
Tephra layers of the
Badlands (upper)
Correlation; p-mag.* N/A >2.22, <2.57 Ma
Tephra layers of the
Badlands (lower)
P-mag.*; correlation N/A >2.57, <2.89 Ma
Tephra layers of Benton Hot
Springs
Ar/Ar; correlation plagioclase 2.89?0.03 Ma
*Magnetostratigraphic polarity determination
At the Cowan Pumice Mine, only a partial section of the eruptive record is preserved,
but the best materials for laser-fusion 40Ar/39Ar 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
Additional publication details
Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Tephra layers of blind Spring Valley and related upper pliocene and pleistocene tephra layers, California, Nevada, and Utah: isotopic ages, correlation, and magnetostratigraphy
