LOS VIDRIOS

NORTHERN SONORA

pinacate2.GIF (111336 bytes)

 

 

 

The mafic Sierra Pinacate Volcanic Field with the rhyolite structures of Los Vidrios to the east with the Rio Sonoyta flowing to the south. The Tertiary rhyolite is substantially older than the Quaternary mafic eruptive events in the Sierra Pinacate. Landsat photo.

 

 

 

 

 

 

 

Losvidr.JPG (43820 bytes)

Coalesced rhyolite domes at Los Vidrios, east of the Sierra Pinacate Volcanic Field, northern Sonora. Rio Sonoyta wash in foreground

Located between UTM 352 and 354 East and 29 and 30 North of the Puerto Peñasco H12-1 Mexican Topographic Quad 1:250,000 northwest Sonora, Mexico. This is another probable middle Tertiary glass source located about 8 to 12 km south of the US/Mexican border along the Rio Sonoyta and on the eastern border of the predominately mafic Quaternary Sierra Pinacate Volcanic Field. The source is comprised of a northwest-southeast arcing series of rhyolite and rhyolite/obsidian domes. Nodules up to 10 cm in diameter are found in a rhyolite/tuff/obsidian conglomerate eroding into Rio Sonoita and embedded in vitrophyre within the rhyolite domes. Rhyolite/obsidian units, some up to 30 m by 40 m, are common on many domes. One small dome between UTM 29-30 North and 352-353 East (on the 353 line), consists of an alternating series of rhyolite and obsidian strata with nodules embedded within the glass, similar to Slate Mountain. Densities within the devitrified glass matrix are up to 50 per m2, perhaps five times that if pea size nodules are counted. Within the conglomerate densities reach 10 to 20 per 5 m2.

Secondary deposition consists of erosion through the Rio Sonoita river system south at least as far as the Sierra Blanca and perhaps 65 km to the Gulf of California. The thin cortex on the nodules varies from brown to gray and velvet-like in the conglomerate above the Rio Sonoita. The aphyric glass is very consistently black and very opaque even in the thinnest flakes. No banding or lighter shades were noticed in hundreds of nodules collected.

The knapping quality of this obsidian is good, but the glass is quite brittle and bipolar reduction sometimes resulted in 'exploding' nodules. Pressure flaking, however was quite successful when a flake was successfully removed and flakes were easily removed with minimum platform collapse. There are no known geological or archaeological references to this source other than a short mention in Lumholtz (1912:286).

In October 2008 while on the way to the Los Sitios del Agua obsidian source along the Rio Sonoyta (former AZ Unknown A), I visited the bajada below the above collection locality and collected about 50 samples.  The analysis of a sample of these are included below with the date prefix 102508. The 1986 samples were analyzed on the old Spectrace 440 instrument.  The analysis here on the new ThermoScientific Quant'X indicates that the elemental concentrations are essentially the same.

Mean and central tendency for 20 samples collected in 1986 and 2008

Raw elemental concentrations for Los Vidrios source standards. All measurements in parts per million (ppm).

Sample

Ti

Mn

Fe

Zn

Rb

Sr

Y

Zr

Nb

Ba

Pb

Th

102508-1-11

784

230

10456

83

231

12

66

216

30

32

28

25

102508-1-12

816

251

10974

85

235

11

65

221

25

32

27

29

102508-1-13

743

244

10274

77

226

10

69

215

31

41

27

28

102508-1-14

728

246

10157

79

227

13

64

214

30

42

26

27

102508-1-15

765

244

11118

86

239

12

68

218

30

35

26

34

102508-1-16

848

265

12009

94

253

12

68

223

29

24

32

34

102508-1-17

795

231

10567

80

232

15

66

221

31

34

28

32

102508-1-18

815

252

11845

93

248

13

72

229

32

39

29

33

102508-1-19

760

253

11395

88

244

13

61

222

35

41

28

27

102508-1-20

830

265

12077

92

254

12

73

231

31

41

31

28

1986-1

725

216

13174

 

262

13

73

238

32

 

 

 

1986-2

709

198

11970

 

238

13

68

217

35

 

 

 

1986-3

833

203

13277

 

266

14

76

239

35

 

 

 

1986-4

893

232

13804

 

271

15

77

240

31

 

 

 

1986-5

837

223

13521

 

271

14

79

241

32

 

 

 

1986-6

798

198

13402

 

270

15

80

236

31

 

 

 

1986-A1

836

208

13104

 

258

13

73

236

29

 

 

 

1986-A2

675

179

11743

 

236

12

68

223

30

 

 

 

1986-A3

879

224

13666

 

266

16

76

242

34

 

 

 

1986-A3

658

201

13195

 

264

13

75

241

35

 

 

 

RGM1-S5

1503

314

12862

39

144

105

27

222

10

748

26

19

 

Major oxides for one Los Vidrios source standard collected in October 2008 and analysis of RGM-1 

Sample

SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

MnO

Na2O

TiO2

Los Vidrios

 

 

 

 

 

 

 

 

 

102508-1-16

74.542

12.304

1.474

2.214

5.215

0.047

0.054

3.68

0.255

RGM1-S4

75.680

12.477

1.3024

1.806

4.550

<.001

0.0379

3.77

0.196

 

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright © 2010 M. Steven Shackley. All rights reserved.
Revised: 19 August 2015

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