MOUNT FLOYD VOLCANIC FIELD

OBSIDIAN SOURCES

NORTHERN ARIZONA

Partridge Creek (Round Mountain). Sections 28, 33, 34, R4W, T24N USGS Mount Floyd 7.5' Quad, southwest Coconino County, Arizona. There is some evidence that the source area may exhibit some chemical and megascopic variability, but the majority of the material appears to derive from the area described here (Jim Burton, Larry Lesko, and Dave Nealy: personal communication 1986). The primary source is located on the southeast flank of Round Mountain probably deriving from a number of rhyolite domes in the area (Bush 1985). The nodules are found in a rhyolite ash flow distributed mainly to the southeast of Round Mountain. The obsidian occurs as secondary deposits along Partridge Creek drainage for at least 15-20 km. Nodule sizes reach at least 16 cm in diameter, although most are near 10 cm. The highest nodule density can approach 10 per m2 (but see below). Cortex on the nodules varies from a thin gray-black to brown-black and tuff frequently covers many specimens. Red oxidized areas occur on the outside of some nodules. The aphyric interior approaches a very vitreous opaque black, but thin flakes exhibit a gray cloudy matrix. Cloudy banding infrequently occurs. The material is some of the best knapping obsidian of the large nodule varieties in Arizona, perhaps approached only by Government Mountain.

Prehistoric reduction of the material was common both at the source and along Partridge Creek. No high density reduction stations were noted in the 1980, but the material was probably 'test knapped' everywhere. A sandstone two-handed mano was the only other artifact type recorded.

updated August 2014: Surveyed around entire circumference of Round Mountain with Jeff Ferguson (U Missouri).  While Round Mountain is a rhyolite dome complex, the vast majority of the archaeological obsidian is derived from a small dome complex on the south side of Round Mountain, probably vents off the main dome that quenched more rapidly.  In August a very large obsidian/artifact pavement was discovered with 100s or cores and flakes per 5m2 within a geological pavement (see image below).  Additionally, at or near the top of a portion of the small dome complex upslope from the obsidian pavement, perlitic lava exhibited in-situ obsidian, and high density of nodules up to 10 cm in diameter, probably where the overlying obsidian zone has eroded southward (see image below).  Analysis of a sample of the perlitic lava (080614-4-1) exhibits identical elemental composition and can be seen as part of the same eruptive event (see table below).

The first archaeological mention of this source was by Cameron and Sappington (1984), Lesko (1986) and Shackley (2005). Bush's (1985) recent geological study of the Round Mountain area was quite thorough (see also Moore, Wilson, and O'Haire 1960).

 Eroded perlitic lava at top of small dome on the south slope of Round Mountain (August 2014, Jeff Ferguson for scale; samples 080614-4-n)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Geological/artifact pavement downslope from above image (080614-1)

 

 

 

Raw elemental concentrations for Partridge Creek (Round Mountain) source standards including collections in August 2014 (080614-n). All measurements in parts per million (ppm).

Locality/Sample Mn Fe Zn Rb Sr Y Zr Nb Ba Pb Th
RM-1  513 8998   268 5 38 102 54 49    
RM-2  551 9115   265 6 39 97 54 46    
RM-3  485 8444   252 6 42 97 55 45    
RM-4  479 8379   250 6 38 95 54 46    
RM-5  466 8595   254 5 40 98 54 48    
RM-7  413 8085   244 5 38 95 54 44    
RM-8  519 8894   271 7 40 103 56 44    
RM-9  485 8623   256 6 38 98 58 46    
RM-10  480 8524   254 0 41 95 58 42    
RM-11  429 7986   242 6 38 93 56 47    
RM-12  449 8091   251 6 40 101 55 44    
RM-13  449 8106   241 8 37 95 51 44    
RM-14  515 9027   265 4 40 102 55 52    
RM-15  533 9118   276 7 40 102 54 46    
RM-16  491 8460   253 5 41 96 53 44    
080614-1-1 482 9938 56 260 9 41 95 56 25 49 36
080614-1-2 467 9810 45 254 9 40 93 55 0 50 40
080614-1-3 424 9240 42 246 9 42 89 53 0 44 38
080614-1-4 497 9797 49 252 10 39 91 52 0 48 38
080614-1-5 471 9752 49 255 8 41 95 58 1 47 34
080614-2-1 404 9425 63 242 11 39 92 52 0 44 34
080614-2-2 451 9620 49 246 9 39 90 56 0 47 40
080414-2-3 385 8800 42 228 11 40 85 55 0 36 32
080614-2-4 444 9531 43 247 9 40 94 49 0 41 30
080614-4-1 422 8941 38 239 13 39 84 50 18 37 35
080614-4-2 429 9300 43 244 9 43 95 59 0 48 41
080614-4-3 432 9213 44 244 9 38 91 51 0 42 38
080614-4-4 397 8978 38 234 8 40 85 53 0 40 34
080614-4-5 447 9481 44 247 11 44 89 60 0 46 34

 

Mean and central tendency for the data in table above.

 

 

Oxide values for one sample of Round Mountain obsidian 

Sample

SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

MnO

Na2O

TiO2

Partridge Cr (Round Mountain)

 

 

 

 

 

 

 

 

 

080614-4-5

77.15

12.288

0.4312

0.9034

4.715

<.001

0.0601

4.33

0.021

RGM1-S4

73.96

13.103

1.3888

2.181

4.823

<.001

0.0437

3.94

0.319

 

updated 1995: Two newly discovered sources of glass have been reported in the Mount Floyd Volcanic Field in association with the Partridge Creek locality west of the San Francisco Volcanic Field (Lesko 1989).  Two new sources reported by Lesko (1989:389-390), have not been recognized in archaeological contexts outside northern Arizona, but were used prehistorically in that region (Lesko 1989).

Presley Wash.This is a gray and black glassy and sub-vitreous rhyolite most often opaque gray to gray-green to black. Sanidine  phenocrysts (not quartz as stated by Lesko 1989:389) are uncommon to common in the matrix hampering control during knapping. The black variety is generally sub-vitreous, rarely with phenocrysts and not well suited to the production of small bifaces. There are no chemical differences between these two varieties and they occur together in Presley Wash east of Round Mountain. After the juncture of Partridge Creek running east from Round Mountain and Presley Wash, all varieties of obsidian are found in the alluvium (see Lesko 1989:389).

There has been some question as to whether the Presley Wash glasses are actually rhyolites.  The EDXRF analysis of the major oxides however as shown in the table below indicates that it would be considered a high silica rhyolite.  The Cox et al. alkali/silica plot graphically indicated the high silica rhyolite of Presley Wash.  Evidently, the cooling rate and/or degassing was somewhat longer and slower than the Round Mountain glass.

updated August 2014: An additional collection along Presley Wash to the east of Round Mountain in August 2014 essentially exhibited the same results as the 1980s collections (samples 080614-5-n).  Again, there is no statistically significant differences between the glass gray-green samples and sub-vitreous black samples.

Raw elemental concentrations for Presley Wash source standards. All measurements in parts per million (ppm) B=black; GG-gray-green samples.

SAMPLE Ti Mn Fe Zn Rb Sr Y Zr Nb Ba Pb Th
PWB  1543 362 15342   92 194 15 141 23 1319    
PCD  2557 421 19224   83 205 18 140 18 1262    
PCE  1527 339 14257   93 187 16 138 22 1310    
PCF  2234 382 18328   84 204 17 135 17 1324    
PW1  1562 354 15391   93 189 13 144 24 1400    
PW2  1681 351 15477   95 196 13 138 21 1294    
PW3  1886 398 17226   89 211 15 136 21 1401    
PW4  2856 432 21818   83 224 16 140 22 1250    
PW5  4672 557 30743   74 273 19 146 20 1107    
080614-5-1B 1702 314 14471 39 84 188 13 132 17 1497 24 16
080614-5-2B 2411 364 18582 49 79 209 13 130 18 1339 17 16
080614-5-3GG 1959 339 14692 65 89 191 17 136 23 1519 28 17
080614-5-4GG 1831 322 14439 43 83 185 13 133 21 1486 21 20
080614-5-5B 2849 413 21819 52 74 229 18 132 21 1372 17 19
080614-5-6GG 1826 350 15025 44 88 189 11 133 22 1443 25 21
080614-5-7B 3333 462 25322 62 76 257 15 144 22 1347 18 14
080614-5-8GG 2210 441 17245 54 101 203 14 147 23 1701 29 24

Mean and central tendency for the data above.

 

THE MAJOR OXIDE ANALYSIS OF ONE SAMPLE OF THE GRAY-GREEN PRESLEY WASH GLASS AND THE RGM-1 USGS RHYOLITE STANDARD

Sample

SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

MnO

Na2O

TiO2

Presley Wash

 

 

 

 

 

 

 

 

 

PW-2-1

75.100

12.485

1.6251

1.7508

4.606

0.391

0.0402

3.62

0.228

RGM1-S4

75.680

12.477

1.3024

1.806

4.550

<.001

0.0379

3.77

0.196

The Cox et. al. alkali/silica plot of the alkali and silica oxides for the Presley Wash sample

Black Tank. I have not visited this source, but received samples from David Nealy (USGS, Flagstaff) in 1987. Lesko describes the source, also known as Rose Well obsidian, as located in the Black Tank area north of Round Mountain (1989:389-390). It occurs as small (<5cm diameter) black or black and mahogany nodules in an area disturbed by cattle tank construction. The black material is nearly identical megascopically to Partridge Creek glass, but exhibits a very different chemistry. Lesko describes the source as “small and the material not abundant” (1989:390). It rarely occurs in archaeological contexts even in northern Arizona.

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

SAMPLE Ti Mn Fe Rb Sr Y Zr Nb Ba
BT1  1345 512 14671 128 131 23 99 26 860
BT2  1230 546 14437 137 129 25 100 30 924
BT3  1064 499 13472 133 118 21 94 29 984

 

References Cited

     Bush, Mark M., 1986,  The Geology of Round Mountain, A Bimodal Volcanic Field in Northwest Arizona.  Unpublished Master's Thesis, Department of Geology, State University of New York, Buffalo.

            Cameron, Catherine M., and Robert L. Sappington, 1984,  Obsidian Procurement at Chaco Canyon, A.D. 500-1200.  In Recent Research on Chaco Prehistory, edited by W.J. Judge and J.D. Shelberg, pp. 153-160.  Reports of the Chaco Center 8, Division of Cultural Research, National Park Service, Albuquerque.

            Lesko, L., 1989, A Reexamination of Northern Arizona Obsidians.  Kiva 54(4):385-399.        

     Moore, Richard T., Eldred D. Wilson, and Robert T. O'Haire, 1960,  Geologic Map of Coconino County, Arizona.  Bureau of Mines, University of Arizona, Tucson.

            Shackley, M.S., 2005, Obsidian: Geology and Archaeology in the North American Southwest.  University of Arizona Press, Tucson.

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright 2014 M. Steven Shackley. All rights reserved.
Revised: 19 August 2015

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