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TABLE 1. Chemical composition of volcanic glass shards from tephra layers analyzed in this study, and comparative compositions of correlative tephra layers from other localities. Concentrations of major and minor oxides given below are in oxide weight percent, recalculated to a 100% fluid-free basis. Original totals on analysis, and numbers of shards analyzed for each sample, are given on the right side of the table. Note that several samples have multiple compositional modes. Tephra layers present in the Crowder Flat Road section west of Alturas are listed in stratigraphic order, from youngest (top of table) to oldest. The stratigraphic separation between the base of each tephra layer and the Devil's Garden basalt at the top of the section are also given. Analyses were made on a JEOL 8900 electron microprobe. Analyses were conducted during 1979 - 2009 by Charles Meyer, James Walker, David Wahl, and Elmira Wan, U.S. Geological Survey Tephrochronology Laboratory, Menlo Park, California.

Field Sample Number Date of analysis SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2O Total (recalculated) Total (on analysis) No. shards analyzed
                   
ALT-1(1), T42-3 11/23/1982 76.57 12.39 1.58 0.07 0.01 0.48 0.18 3.43 5.29 100.00 94.52 *
ALT-1(2), T7-2, pd 1/23/1979 76.35 12.49 1.55 0.06 0.03 0.53 0.20 3.34 5.45 100.00 94.05 *
ALT-16 T304-6 3/25/1994 76.38 12.45 1.55 0.07 0.04 0.55 0.19 2.94 5.83 100.00 95.50 19
 
ALT-2A-N T567-1(Pop1) 1/8/2009 76.20 12.48 1.62 0.08 0.03 0.52 0.18 2.63 6.28 100.02 94.66 11
ALT-2A-N T567-1(Pop2) 1/8/2009 74.24 12.90 2.68 0.08 0.07 0.66 0.22 2.92 6.23 100.00 94.69 2
 
ALT-21 T317-5 REDO 4/95 4/11/1995 76.36 12.58 1.58 0.07 0.03 0.51 0.19 3.43 5.26 100.01 93.62 17
 
ASW-61185-28 T109-2 9/24/1985 76.94 12.13 1.53 0.07 0.02 0.49 0.18 3.41 5.24 100.01 92.41 12
 
Kilgore Tuff (mean)   76.70 12.33 1.56 0.07 0.03 0.52 0.19 3.27 5.33 100.00   ~240
Standard deviation (1 sigma)   0.31 0.16 0.04 0.01 0.01 0.04 0.01 0.27 0.39 0.01    
                           
ALT-12 T299-2 4/14/1994 73.67 14.79 1.73 0.16 0.08 0.66 0.20 4.71 4.01 100.01 93.62 18
ALT-26 T574-1 8/4/2009 75.92 15.06 1.69 0.16 0.07 0.64 0.19 3.41 2.86 100.00 94.63 20
ALT-28A T574-3 8/4/2009 76.05 15.10 1.71 0.16 0.08 0.66 0.21 3.27 2.76 100.00 93.63 19
ALT-28B T574-4 8/4/2009 76.11 15.01 1.69 0.16 0.07 0.63 0.20 3.28 2.85 100.00 93.89 20
                           
ALT-11A T567-2 1/8/2009 69.02 15.32 3.85 0.88 0.09 2.38 0.72 4.55 3.19 100.00 94.82 15
ALT-11B T567-3(Pop2) 1/8/2009 65.21 15.57 5.79 1.51 0.12 3.67 0.97 4.54 2.62 100.00 95.17 7
ALT-11B T567-3(Pop1) 1/8/2009 71.92 15.64 2.34 0.27 0.10 0.94 0.34 5.14 3.32 100.01    

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