TABLE 1. Comparison of the chemical composition of glass of the Marker Tuff Bed in the Horned Toad Hills with that of the 4.83-Ma Lawlor Tuff (type locality) in the San Francisco Bay area. Analysis by elecron microprobe (JEOL 8600), at the U.S. Geological Survey, Menlo Park, Calif. Charles Meyer, James Walker, David Wahl, and Elmira Wan, analysts (1982-2006). Analyses recalculated to a 100% fluid-free base to compensate for differences in hydration of glass. Sample HTH-5, obtained from Member 3, playa sediments, may be partly leached of Fe2O3. Composition of the tephra layer underlying the Marker Tuff Bed, HTH-3, is shown for comparison.
SiO2
Al2O3
Fe2O3
MgO
MnO
CaO
TiO2
Na2O
K2O
Total,R
Marker Tuff Bed at Horned Toad Hills, Mojave Desert (HTH-1, western part of study area; HTH-4, central part; HTH-5, eastern part)
HTH-1, T43-2
73.41
14.46
2.15
0.08
0.05
0.84
0.17
5.05
3.78
99.99
HTH-4, T546-2
74.02
14.19
2.11
0.10
0.04
0.85
0.19
4.62
3.88
100.00
HTH-5, T546-3
74.08
14.20
1.99
0.09
0.04
0.88
0.17
4.93
3.63
100.01
Lawlor Tuff, type locality in Lawlor Ravine, Los Medanos Hills, eastern San Francisco Bay area. LAWLOR-1, fall pumice; LAWLOR-2, ash-flow tuff
LAWLOR-1 T325-9
73.20
14.29
2.15
0.09
0.04
0.84
0.18
4.83
4.37
99.99
LAWLOR-1 T530-7
73.78
14.07
2.13
0.10
0.05
0.85
0.18
4.66
4.19
100.01
LAWLOR-2 T325-10
73.10
14.32
2.21
0.10
0.04
0.86
0.17
4.79
4.41
100.00
Statistics for the Lawlor Tuff, ash/pumice fall and ash-flow, from 43 localities in California, Nevada, and Arizona
Lawlor Tuff, Mean, 43 samples
73.79
14.23
2.16
0.10
0.04
0.88
0.17
4.44
4.18
100.00
Standard Deviation (±1
0.61
0.27
0.05
0.01
0.01
0.03
0.01
0.46
0.53
0.01
Range
2.83
1.32
0.24
0.04
0.05
0.12
0.06
2.38
3.01
0.04
Minimum
72.95
13.49
2.03
0.08
0.02
0.83
0.14
2.98
2.29
99.98
Maximum
75.78
14.81
2.27
0.12
0.07
0.95
0.20
5.36
5.30
100.02
Tephra Layer HTH-3, underlying the Marker tuff Bed with ~14 m separation, at the Horned Toad Hills
HTH-3, T546-1 (all shards)
64.75
15.42
5.68
1.65
0.09
3.94
1.00
4.15
3.32
100.00
HTH-3, T546-1 (high Ca, Fe)
61.65
16.05
6.95
2.30
0.10
5.00
1.07
4.15
2.74
100.01
HTH-3, T546-1 (med, Ca, Fe)
65.94
15.08
5.21
1.37
0.08
3.55
1.00
4.22
3.56
100.01
HTH-3, T546-1 (low Ca, Fe)
69.19
14.77
3.82
0.82
0.07
2.38
0.87
3.96
4.12
100.00
MSE 202 (1) T75-1
65.53
15.17
5.34
1.57
0.13
3.83
0.99
3.86
3.59
100.01
TWN-L-2.235B
66.00
15.48
5.05
1.49
ND
4.02
1.04
4.19
2.72
99.99
T2023M.12MTB;2, T187-2
65.15
16.06
5.11
1.70
0.09
3.93
0.93
4.40
2.63
100.00
Samples of the Pinole Tuff, eastern San Francisco Bay area
758-190B, T1-7, P
63.55
16.15
6.98
1.07
0.15
3.27
0.92
6.13
1.97
100.01
758-181A. T1-9. P
61.57
16.36
5.32
2.27
0.09
4.60
0.96
4.11
4.71
99.99
758-178B, T1-13. P
71.27
14.39
3.56
0.09
0.08
1.09
0.23
5.79
3.49
99.99
)