Abstract ........................................................ 1
Introduction .................................................... 3
Tephra layers of Blind Spring Valley at the Cowan Pumice
Mine ......................................................... 4
Tephra layers from other source areas in the western
United States ................................................ 6
Acknowledgments ................................................. 6
Previous work ................................................... 7
Analytical methods .............................................. 8
Field methods ................................................ 8
Laboratory preparation of samples for analysis ............... 8
Laser-fusion 40Ar/39Ar and conventional K-Ar
analyses ..................................................... 8
Chemical analysis of volcanic glass shards ................... 9
Electron-microprobe (EM) analysis ......................... 9
Energy-dispersive X-ray fluorescence (EDXRF) analysis ..... 9
Instrumental neutron activation (IIMA) analysis ........... 9
Methods of data evaluation .................................. 12
Stratigraphic section at Cowan Pumice Mine—results of age
analysis .................................................... 12
Stratigraphic section in and below Pit 6 .................... 12
Basal basalt flows ....................................... 12
Basal arkosic mudflows and the tuff of Benton Hot
Springs .................................................. 12
Tuffs of Blind Spring Valley ............................. 15
Lower tuffs of Glass Mountain ............................ 16
Tephra layers exposed in Pits 1 through 5 ................... 17
The Bishop ash ........................................... 17
Tuffs of Blind Spring Valley ............................. 17
Isotopic ages of the upper Glass Mountain tephra layers
at Chalk Cliffs, at the south end of the Volcanic
Tableland ................................................... 19
Interpretation of the isotopic ages at the Cowan Pumice
Mine ........................................................ 21
Correlation of the tuffs of Blind Spring Valley to sites
in the region ............................................... 28
Northeastern Fish Lake Valley ............................ 28
Willow Wash section ...................................... 30
Yellowjacket Canyon, Chalfant Valley, east-central
California ............................................... 32
Waucoba lake beds, near Big Pine, Owens Valley,
California ............................................... 33
Correlation of the Tuffs of Blind Spring Valley and
related tephra layers to distal sites in the western
United States ............................................ 35
Confidence Hills, Death Valley, southeastern
California ............................................... 35
Lake Tecopa, southeastern California ..................... 38
Manix Basin, central Mojave Desert, southern
California ............................................... 39
Peninsular Ranges, California ............................ 41
Ventura Basin, southwestern coastal California ........... 41
Beaver Basin, west-central Utah .......................... 41
Discussion and conclusions .................................. 41
References ..................................................... 49
Appendixes
Appendix 1. Analytical data for laser-fusion 40Ar/39Ar
age determinations of tephra layers ................ 54
Appendix 2. Analytical data for conventional K-Ar
analysis of obsidian clasts from the
lowertephra layers of Glass Mountain
at the Cowan Pumice Mine, Blind Spring
Valley, Calif ...................................... 61
Appendix 3. Analytical data for laser-fusion 40Ar/39Ar
analysis of samples that were contaminated with
detrital material .................................. 62
Figures
1. Map showing sample localities of tuffs of Blind
Spring Valley and related tephra layers, known
distribution of tuffs of Blind Spring Valley,
and locations of Glass Mountain and the Long
Valley Caldera ............................................... 3
2. Generalized geologic map of the vicinity of
Long Valley Caldera, Glass Mountain, Cowan
Pumice Mine, and the south end of the
Volcanic Tableland ........................................... 4
3. Map showing locations of pumice pits at Cowan
Pumice Mine and cross sections A-A'andB-B' ................... 5
4. Cross sections A-A' and B-B' in area of Cowan
Pumice Mine, Blind Spring Valley, showing
locations of samples in Pits 1-6 and 40Ar/39Ar
ages ......................................................... 6
5. Photograph showing exposure of tuffs of Blind
Spring Valley and lowertuffs of Glass Mountain
in Pit 6, Cowan Pumice Mine ................................. 13
6. Photograph of exposures in Pit 6 at Cowan Pumice
Mine ........................................................ 13
7. Stratigraphic section of exposure in Pit 6, Cowan
Pumice Mine ................................................. 14
8. Photograph showing upper part of section in Pit 6,
Cowan Pumice Mine ........................................... 16
9. Photograph showing two thick, coarse pumice-fall
tuffs of Blind Spring Valley at south wall of
Pit 5, Cowan Pumice Mine .................................... 17
10.Stratigraphic section of tuffs of Blind
Spring Valley at Pit 5, Cowan Pumice Mine ................... 18
11.Photograph of tuff of Blind Spring Valley
in Pit 2, Cowan Pumice Mine ................................. 19
12.Composite stratigraphic section at Chalk Cliffs,
south end of Volcanic Tableland ............................. 25
13.Stratigraphy of section exposed near Emigrant
Pass, northeast Fish Lake Valley, Nevada .................... 29
14.Photograph showing tuff of Benton Hot Springs,
near base of Rimrock section at Willow Wash ................. 30
15.Stratigraphy of sections at Willow Wash, south
of Fish Lake Valley, California-Nevada ...................... 31
16.Photograph of Bishop Tuff near head of
Yellowjacket Canyon ......................................... 32
17.Location map and stratigraphic section of
Waucoba Lake Beds near Big Pine, California ................. 33
18.Magnetostratigraphy, stratigraphy, and
tephrostratigraphy of the Confidence Hills
section, Death Valley National Park, California ............. 36
19.Correlation diagram of the tuffs of Blind
Spring Valley and related tephra units ...................... 39
20.Photograph showing exposure of tuff of Blind
Spring Valley in Beaver Basin, Utah ......................... 44
21.Correlation diagram showing magnetostratigraphy
and chronology of the tuffs of Blind Spring
Valley and related tephra units at selected
sites in the southwestern United States ..................... 45
22.Diagram showing chronology of tuffs of Blind
Spring Valley and related tephra layers
compared to chronology of near-source
Glass Mountain rhyolites .................................... 47
Tables
1. Chemical composition of volcanic glass shards,
determined by electron-microprobe analysis,
and ages of tephra layers sampled from quarry
pits at the Cowan Pumice Mine, near Benton
Hot Springs, California ..................................... 10
2. Electron-microprobe analysis of glass of
the Bishop Tuff and Bishop airfall pumice,
compared with the younger set of tephra
layers of Glass Mountain .................................... 20
3. Chemical composition of volcanic glass shards,
determined by electron-microprobe analysis,
from middle Pleistocene and upper Pliocene
tephra layers of northeastern Fish Lake Valley.
Calif.-Nev. (Emigrant Pass area and vicinity) ............... 22
4. Electron-microprobe analyses of volcanic glass
shards from middle Pleistocene and upper Pliocene
tephra layers of the Willow Wash area, south of
Fish Lake Valley, Calif.-Nev ................................ 23
5. Energy-dispersive X-ray fluorescence analysis
of volcanic glass from tephra layers ........................ 26
6. Results of electron-microprobe analysis of glass
from tephra layers in exposures of the Waucoba
lake beds, east side of Owens Valley ........................ 34
7. Glass chemical composition of tephra layers
exposed in deformed alluvial and lacustrine
sediments of the Confidence Hills, southern
Death Valley, Calif ......................................... 37
8. Glass chemical composition of tephra layers
from several sites in southern and southwestern
California and in Beaver Basin, Utah ........................ 40
9. Glass chemical composition of the Huckleberry
Ridge ash bed from sites in the western and
central United States, as determined by
electron-microprobe analyses ................................ 42
10.Instrumental neutron-activation analysis of
selected samples of the tuff of Blind Spring
Valley, from the Cowan Pumice Mine (TTC-),
and correlative or chemically related tephra
layers erupted from the Glass mountain volcanic
source area ................................................. 43
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