BRISTOL MOUNTAINS (BAGDAD)

SOUTHEASTERN CALIFORNIA

Brstlmts.jpg (37925 bytes)

Bristol Mountains Volcanic Field in background. Most of the features are rhyolite dome structures.

This source is located in the Bristol Mountains, San Bernardino County, California in Quaternary alluvium consisting predominately of rhyolite, tuff, and marekenites (Apache Tears). The primary context of this late Tertiary (probably Miocene) source appears to be located in rhyolite domes of the Bristol Mountains on the Siberia 7.5' Provisional Quad (1985) in the northeast quarter, particularly in Sections 7 and 18 T7N R11E, and Sections 12 and 13 T7N R10 E (see also Bagdad 15' 1956). The domes of this Tertiary source are so eroded that the obsidian zone has apparently completely moved into the alluvium, not unusual in the Southwest (Shackley 1988a, 1990). Secondary deposits continue down Siberia and unnamed washes to Lavic and Bristol dry lakes approximately 15 to 20 km south. This is an interior drainage basin and so further downstream deposition is impossible.

The density of nodules near the primary context is up to 10 per 2 m2 with nodule sizes up to 72 mm, although most are 40 to 50 mm. Ten km downstream the nodule density declines to less than 1 per 500 m2 and nodule sizes are generally near 20 mm or less. In the desert pavements the cortex is generally reddish-brown which erodes to black after entering the washes. The glass is typically translucent gray, sometimes banded, and an excellent medium for tool production. Few modified cores or bipolar flakes were noted in two days of survey, although these artifact forms have been recovered from archaeological contexts in the area (Jerry Schaefer: personal communication 1992).

This source was first located by Philip Wilke in the 1980s (Wilke:personal communication 1993). Wilke and others noticed the presence of nodules in the alluvium near Bagdad and the source was colloquially called Bagdad. It is more properly called Bristol Mountains obsidian because the glass is derived from the Miocene rhyolite in the mountains. There is no known geologic publication on this source other than Dibblee (1966) for the adjacent Broadwell Lake 15' Quad. This source was reported in the Journal of California and Great Basin Anthropology (Shackley 1994).

2010: In September 2010, Janet Harvey of the California Institute of Technology forwarded four marekanites found in-situ in perlite in the Bristol Mountains a few km north of the original study (samples with prefix 10 below).  These are well within the range of the collections from the 1990s discussed above.  Included is also a major oxide analysis of one sample and a revised data table.

Elemental concentrations for Bristol Mountain source standards. All measurements in parts per million.

Sample1

Mn

Fe

Zn

Rb

Sr

Y

Zr

Nb

Ba

Pb

Th

La

Ce

BM1-1

378

9390

 

194

148

19

133

20

901

27

 

66

112

BM1-2

366

8956

 

180

137

22

129

23

899

29

 

68

116

BM1-3

385

8734

 

181

139

21

128

20

1096

28

 

84

137

BM1-4

400

9142

 

189

143

20

130

22

837

26

 

66

108

BM1-5

381

8516

 

175

133

21

120

18

1054

25

 

82

134

BM2-2

423

9341

 

193

146

20

126

22

845

27

 

65

108

BM2-3

433

9863

 

196

147

20

128

26

889

23

 

64

113

BM2-4

381

8770

 

184

138

23

124

23

956

27

 

76

122

BM2-5

346

8813

 

189

142

17

132

17

979

25

 

66

133

BM2-6

421

9497

 

194

144

18

128

20

832

25

 

62

108

BM3-2

382

9326

 

194

145

20

130

24

848

25

 

69

106

BM3-3

399

9435

 

192

143

21

133

27

834

28

 

62

108

BM3-4

426

9476

 

198

144

21

135

26

923

29

 

73

108

BM3-5

528

8486

 

167

129

22

125

21

856

27

 

65

114

BM3-6

390

9298

 

194

147

21

129

21

846

27

 

64

104

BM3-7

386

9083

 

184

141

19

126

20

890

28

 

65

114

BM3-8

424

9208

 

189

145

21

129

24

890

29

 

64

118

BM3-9

362

9073

 

186

135

19

128

23

832

28

 

64

102

BM3-10

397

9744

 

199

149

22

138

22

873

30

 

58

110

10-591-1

386

8454

48

183

140

22

136

24

910

26

24

 

 

10-591-2

320

6980

40

162

122

20

128

25

1191

25

25

 

 

10-591-3

372

7471

49

174

134

21

133

22

1040

22

22

 

 

10-591-4

400

8603

49

188

145

25

136

24

1127

28

24

 

 

1 The BM prefix samples are from the Shackley (1994) analyses, and the 10 prefix samples from Harvey.  Empty cells indicate elements not analyzed in that run.

 

Table 2.  Oxide analysis of one Harvey source rock and RGM-1, USGS standard.

Sample

SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

MnO

Na2O

TiO2

 

%

%

%

%

%

%

%

%

%

10-591-1

76.288

11.939

1.037

1.163

5.927

0.001

0.073

3.238

0.133

RGM1-S5

74.509

12.239

1.502

2.214

5.228

<.001

0.056

3.766

0.295

 

Figure 3.  Silica-alkali plot of one Harvey sample.

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright 2013 M. Steven Shackley. All rights reserved.
Revised: 19 August 2015

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