SAN FRANCISCO VOLCANIC FIELD

NORTHERN ARIZONA

The obsidian sources within this field are, perhaps, the best studied in the Arizona, at least early on (Jack 1971; Peterson 2013; Robinson 1913; Sanders et al. 1982; Schreiber and Breed 1971; Shackley 1988, 2005). Although the chemical variability within sources in this field is generally less than in the mid-Tertiary sources to the south, the magmatic relationships between some of these sources as previously reported are probably in error (Jack 1971; Sanders et al. 1982).  Based on K-Ar dates, the obsidian and rhyolite from these sources varies between 2.70 Ma at Government Mountain to 1.10 from the obsidian on the San Francisco Peaks (McKee et al. 1998).

  Sources of archaeological obsidian in the San Francisco Volcanic Field

 

The coalesced rhyolite domes known as Government Mountain (view from the south, 1980s)

Government Mountain. Sections 24, 25 R4E, T22N and Sections 19, 20, 29, 30, R5E, T22N USGS Parks 7.5' Quad, Kaibab National Forest, south central Coconino County, Arizona (updated 6.25.98). This is, as noted earlier, perhaps the best known contemporary obsidian source in Arizona. Shackley (2005), however, showed that RS Hill glass may be equally common in Preclassic Hohokam contexts in central Arizona. Government Mountain is a single rhyolite dome that exhibits rhyolite/tuff/obsidian within the alluvium around the base of the dome structure. Even after years of modern and prehistoric collection, nodules up to 15 cm in diameter are common. Most nodules exhibit little cortex, per se, but some are covered with ash from the alluvium or exhibit a thin gray or brown weathered surface. The glass is aphyric, but the fabric is megascopically granular and distinctive, apparently from microphenocrysts of alkali feldspar and iron oxide (Burton 1986; Wolfe et al. 1987:13).  Some lighter gray and more vitreous material was recovered on the south slope. Thin flakes are cloudy gray when viewed with transmitted light. Secondary deposition occurs only within a short distance (2 km) around the base and is most noticeable south into Government Prairie.

A K-Ar at 2.700.08 Ma was derived from obsidian on the west side of the dome, where much of the obsidian for this analysis was collected (McKee et al. 1998).

Reduced cobbles and flakes are very abundant and densities of cultural obsidian reach 200 per 5m2 in places, especially on the west and south slopes; some portion of this, however, is modern reduction from local knappers. Bifacial core preforms also occur sporadically on the dome, and most are broken (usually lateral snap-fractures). No other artifactual material was observed. Published references are the same as for the previous San Francisco sources, although Robinson only mentions it in passing (1913:67).

Update June 2004: I sampled the dome complex toward the top of the domes and on the north side. Analysis of many archaeological data from this source suggested lower Rb values and higher Zr values than the original 10 source standards.  At the base of the north side of the complex a number of banded nodules were located.  Some of these exhibited the lower Rb values evident in the archaeological record.  The table below reflect the mean and central tendency for the original 10 source standards and 14 additional samples from near the top of the dome complex and the north side.

Update August 2014: With Jeff Ferguson (U Missouri) and Kathy Butler, re-investigated the dome complex around the circumference and up on the west slope.  Outcrops of ashy, perlitic lava did exhibit obsidian strata, and like many rhyolite domes that produced obsidian, much of the obsidian zone appears to be eroding downslope (see images below).  One additional sample (080514-2-1) was analyzed and included here.

Collection localities at Government Mountain (1985-2014).  Fs=Forest road designation.

 

   Ashy perlitic lava with obsidian strata above rock hammer at collection 080514-2.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                                             Close-up of obsidian strata from above image.

 

Elemental concentrations for Government Mountain source standards. All measurements in parts per million (ppm). GM prefix samples collected in the 1980s. 06 or 08 prefix are date and sample collection numbers.*

Sample Ti Mn Fe Zn Rb Sr Y Zr Nb Ba Pb Th
080514-2-1 792 473 9536 54 106 81 21 73 51 363 30 13
061504-1-1 609 464 7421 53 99 74 17 79 48 379 29 18
061504-1-2 592 452 7115 49 99 71 21 77 47 386 27 9
061504-1-3 616 529 8131 61 107 73 22 86 49 369 32 10
061509-1-4 600 514 8332 59 108 73 18 91 50 359 34 10
061509-1-5 613 531 8305 61 106 75 18 84 48 334 31 7
061509-1-6 721 532 8255 58 108 72 18 85 49 383 34 13
GM-11 628 533 8259 56 105 78 23 85 51 351 33 10
GM-12 630 496 7998 57 107 74 19 86 51 397 32 14
GM-13 627 546 8417 62 105 77 20 86 52 367 31 9
GM-14 652 544 8576 65 107 79 19 86 53 378 33 9
GM-15 641 507 8005 59 105 74 22 83 53 396 31 12
GM-16 646 487 8026 57 108 75 18 82 49 322 30 10
GM-17 607 475 7605 55 101 75 17 83 48 380 29 13
GM-18 614 504 8126 57 106 76 20 82 51 375 32 8
GMA    564 9037   111 78 19 84 54 348    
GMB    601 8946   111 79 21 79 52 351    
GMC    540 8484   109 77 18 82 55 350    
GMD    528 8410   107 79 21 82 53 362    
GME    603 9188   116 84 20 82 54 344    
GMF    554 8821   109 78 19 82 52 374    
GM1    537 8477   108 78 19 81 55 364    
GM2    492 8347   107 80 19 84 51 366    
GM3    550 8559   106 79 22 81 54 369    
GM4    468 7955   104 75 18 78 48 351    

 

Mean and central tendency for data in table above

 

 Oxide values for one Government Mountain sample

Sample

SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

MnO

Na2O

TiO2

Government Mtn

 

 

 

 

 

 

 

 

 

080514-2-1

75.87

13.431

0.7936

0.9335

4.317

<.001

0.0809

4.43

<.001

RGM1-S4

74.00

13.073

1.4097

2.163

4.885

<.001

0.0482

3.91

0.258

 * Cow Canyon and Government Mountain exhibit similar chemistry in a number of elements. They are megascopically distinctive and Ti, Zr, Nb, and Ba effectively separate these two sources.

Sitgreaves Mountain. Sections 7,13,14 R3E, T23N USGS Williams 15' Quad; Sections 16,17,19,20,24,25,31,32,36 R4E, T22N USGS Moritz Ridge and Parks 7.5' Quads. Kaibab National Forest, south central Coconino County, Arizona. Sitgreaves Mountain (or Peak) is the largest single mass of rhyolite in the San Francisco field (Robinson 1913). Abundant eroded cobbles of obsidian are present around the flanks of this mountain. Some of the cobbles are up to 20 cm in diameter. The nodules are part of a rhyolite/tuff/obsidian alluvium and densities up to 10 per m2 were recorded. No obsidian was recorded 'in situ' or in large blocks toward the peak. There appears to be no appreciable secondary deposition, although the eroded material in Spring Valley is chemically identical to both Sitgreaves and RS Hill. The rounded nodules exhibit a tan to brown cortex and a lustrous, but somewhat micro-granular interior. The glass is slightly vitrophyric with uncommon alkali-feldspar phenocrysts up to 3 mm in diameter. The glass is near opaque and gray with thin flakes exhibiting a cloudy gray pattern. This material, while similar to RS Hill and Government Mountain obsidian does not reduce as efficiently and the large sparse phenocrysts hamper control. Obsidian dated by K-Ar on the southeast side of Sitgreaves near RS Hill yielded a date of 2.84 0.02 Ma (McKee et al. 1998).

The nodules were frequently reduced at the source and flakes occur throughout the source area, but no intensive reduction areas were located. Published sources include Jack (1971); Moore et al. (1960); Robinson (1913); Sanders et al. (1982); Schreiber and Breed (1971).

RS Hill. Sections 15, 16, 21, 22, 28, 27 R4E, T22N USGS Parks 7.5' Quad, Kaibab National Forest, south central Coconino County, Arizona. This is a relatively small rhyolite dome that produced the largest nodules recovered in this study (35 cm in diameter). The nodules are similar to those from Sitgreaves, but not as eroded and therefore generally more angular. Gray cortex predominates. The fabric is also megascopically vitrophyric and micro-granular with sparse alkali-feldspar phenocrysts up to 3 mm in diameter. The glass however, is frequently blacker, more vitreous and a higher quality knapping obsidian. The fabric looks megascopically very similar to Government Mountain, but is not entirely aphyric. Secondary deposition occurs as slope wash into the Spring Valley Wash system, but the material is only transported a few kilometers south. Reduction of the nodules is very common and flake densities approach 100 per 5m2 in places. Published references are the same as for Sitgreaves.

Revisit and resampling 4 August 2014: Surveyed and sampled up to near the top of the RS Hill dome.  Collected samples on the east and west slope, as well as the top of the ridge (the 080414-4 samples below).  Nodule size on top of dome was up to over 10 cm, with some near 15 cm in very high density.  Due to managed fires on Sitgreaves Mountain, we were denied access, but the samples from the west side of RS Hill are up against Sitgreaves (see map here).  Given the complete overlap of elemental concentrations between Sitgreaves and RS Hill this isn't an issue for source assignment.  Certainly RS Hill is a vent dome from the larger Sitgreaves Mountain dome complex.  The original (1980s) RS Hill samples (RSW for west slope, and RSS for southslope) were collected along the forest roads (see image below).

Mean and central tendency for RS Hill/Sitgreaves samples

Raw elemental concentrations for Sitgreaves Mountain (SM) and RS Hill (RS) source standards. All measurements in parts per million (ppm).

Sample Ti Mn Fe Zn Rb Sr Y Zr Nb Ba Pb Th
SM1 415 374 9906   410 7 94 174 269 42    
SM2 529 417 10335   402 8 95 173 259 43    
SM3  316 381 9091   385 9 87 176 254 45    
SM4  300 424 9255   391 8 86 174 257 44    
SM5  371 397 8827   376 8 83 165 246 45    
SM6  317 462 9609   408 4 89 180 262 46    
SM7  282 462 10012   406 6 88 174 263 43    
SM8  290 444 9621   387 5 84 168 251 48    
SM9  277 413 9346   387 6 86 177 252 41    
SM10  276 456 9718   404 5 89 183 262 41    
RSW1 450 427 10543   424 0 100 180 265 45    
RSW2 358 430 10723   423 8 93 176 271 45    
RSW3 507 354 9822   396 6 92 168 260 47    
RSW4 491 421 10610   415 7 94 178 264 43    
RSW5 482 404 10332   411 0 94 175 263 44    
RSS1  285 427 9907   410 7 88 172 264 45    
RSS2  210 420 9890   409 4 90 179 268 46    
RSS3  291 476 9766   405 8 84 176 260 43    
RSS4  231 427 9782   409 6 88 181 260 43    
RSS5  232 424 9335   388 9 82 174 256 42    
RSS6  295 442 9554   407 4 86 176 272 41    
080414-4-10 728 424 10255 153 386 10 89 164 259 8 80 57
080414-4-9 761 401 10239 155 386 10 87 162 260 0 81 46
080414-4-8 736 416 10349 158 398 9 89 160 258 0 84 43
080414-4-7 756 412 10311 157 387 10 91 166 258 0 79 50
080414-4-6 916 418 10755 153 384 10 89 163 268 0 81 46
080414-4-5 777 435 10995 171 415 8 87 171 265 20 87 50
080414-4-4 765 366 9636 139 367 13 84 155 253 0 70 41
080414-4-3 751 403 10191 151 384 11 86 164 252 0 78 50
080414-4-2 729 402 10388 156 394 8 86 157 261 1 86 54

 Oxide values for one RS Hill sample

Sample

SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

MnO

Na2O

TiO2

RS Hill

 

 

 

 

 

 

 

 

 

080514-4-2

76.21

13.017

0.3688

1.0632

4.171

<.001

0.0569

4.95

<.001

RGM1-S4

73.96

13.103

1.3888

2.181

4.823

<.001

0.0437

3.94

0.319

 

RS Hill and Sitgreaves Mountain with relevant features and collection localities in August 2014

Kendrick Peak. Sections 1,2,3 R5E, T23N; Sections 35,36 R5E, T24N USGS Kendrick Peak 7.5' Quad, Kaibab and Coconino National Forests, south central Coconino County, Arizona. Kendrick Peak is a large composite cone consisting of five lavas (Robinson 1913). The rhyolite/obsidian is located on the northeast and east slopes. Robinson describes a "thick flow of black, lustrous obsidian" on the northeast divide (1913:55). This was not relocated on the survey, but abundant nodules up to 10 cm in diameter were collected from the east and northeast slopes near Crowley Park. Nodule densities up to 50 per 5 m2 were recorded especially toward the northeast divide. Secondary deposition occurs down into the Crowley Park meadow 5 km east. Obsidian from the Kendrick Peak dome yielded a K-Ar date of 1.90 0.25 Ma (McKee et al. 1998).

Cortex is generally absent with just gray-black weathered glass. The glass is vitrophyric with abundant feldspar phenocrysts up to 2 mm in diameter. The color is gray-black and nearly opaque. Thin flakes show the distinctive cloudy gray translucent character common in San Francisco obsidians.

The vitrophyric and nearly devitrified fabric in most specimens hampers control during reduction. Kendrick is not a good raw material for tool production, and was not detected in any sites in this study. Interestingly, prehistoric reduction of nodules was common, occurring everywhere on the slopes. Artifactual density was difficult to estimate in the heavy pine duff. Published references are the same as previous San Francisco obsidians.

Note: Elemental concentrations for Kendrick Peak, San Francisco Peak, and O'Leary Peak are not presented here due to their rarity in archaeological context. See Shackley (1995) for these data.

Slate Mountain (Wallace Tank). Southeast corner of Section 2 R5E, T24N USGS Kendrick Peak 7.5' Quad, Coconino National Forest, south central Coconino County, Arizona. Jack (1971) and Schreiber and Breed (1971) both identify Slate Mountain as the source of obsidian here, but it is actually derived from a small rhyolite/obsidian dome at the base of the south slope at Wallace Tank. The dome measures a mere 100 meters in diameter by 10 meters high; the structure, however, is practically entirely composed of equal proportions of black and mahogany-red and black nodules. A vertical adit dug into the top of the dome to a depth of 2 meters exhibits a series of alternating layers of rhyolite and reddish perlite that contains mahogany-red and black nodules up to 10 cm in diameter. The nodules are as dense as 10 per m2 on the surface of the dome. The cortex and fabric are similar to Kendrick, but the glass is more vitreous, the phenocrysts are smaller (<2mm) and less dense, and half the nodules are a mixed mahogany-red and black. Secondary deposition does not occur more than a kilometer away from the dome structure. Rhyolite from the Slate Mountain dome yielded a K-Ar date of 1.57 0.04 Ma (McKee et al. 1998).

Prehistoric reduction of the nodules is very common (up to 20-50 flakes and cores per m2) and there appears to be no preference for the reddish or black nodules, although subsurface deposits suggest that the red material may have been more common at one time. One small plainware sherd was recovered as well as two non-Slate Mountain obsidian artifacts: A unifacially retouched flake made from Government Mountain material and a biface distal end fragment made from RS Hill material. Published references include all the above for San Francisco sources in addition to Mintz (1942).

SLATE MOUNTAIN (WALLACE TANK) 

Element      Mean    Std Dev   Minimum   Maximum       N   

Ti          649.86      72.71     553.2     773.9     10

Mn          446.30      24.69     424.8     501.0     10

Fe        11617.40     450.53   11178.6   12643.1     10

Rb          115.26       3.91     109.9     122.9     10

Sr           67.38      10.07      61.1      94.8     10

Y            21.90       1.89      18.2      24.7     10

Zr          143.16       4.49     134.5     149.4     10

Nb           45.84       2.51      40.6      48.9     10

Ba          753.62      24.94     716.9     792.4      9

 

San Francisco Peaks (Fremont-Agassiz Saddle). Northeast corner of Section 5 R6E, T22N USGS Humphrey's Peak 7.5' Quad, Coconino National Forest, south central Coconino County, Arizona. This source is located above 3300 m in elevation on the northwest slope of Fremont Peak near the saddle. The obsidian, located within a rhyolite lava that measures less than 20 m across, occurs as a large vitrophyric block about 5 meters in diameter with abundant eroding nodules up to 15 cm in diameter. The cortex is variable from ashy-rhyolite to weathered glass, and the glass itself is extremely vitrophyric with abundant alkali-feldspar phenocrysts up to 4 mm in diameter with accessory zircon and rare earth titanosilicates (see Burton 1986). Many nodules are interbanded with ash. The color is gray-black and translucent around the margins. The material will not allow controlled fractures and is generally useless for biface manufacture, although flakes can be removed on the more vitreous materials. This source has not appeared in archaeological contexts known to me. There appears to be some prehistoric reduction here, but the density is light and many 'flakes' could have been produced by freeze-thaw processes. Published references include Jack (1971), Moore et al. 1960, Robinson (1913), and Schreiber and Breed (1971).  The rhyolite flow of Core Ridge on San Francisco Mountain yielded a K-Ar date of 1.10 0.2 Ma (McKee et al. 1998).

O'Leary Peak/Robinson Crater. Section 10 R8E, T23N USGS O'Leary Peak 7.5' Quad, Coconino National Forest, south central Coconino County, Arizona (updated 6.25.98). This is a low density, low quality rhyodacite glass source on the eastern edge of the field. The nodules, up to 12 cm in diameter, were collected between O'Leary Peak and Robinson Crater. Rhyodacite units (Moore and Wolfe's Qoo unit) occur on the slopes of O'Leary and are surrounded by Sunset basalt cinders (see Moore and Wolfe 1976). The obsidian occurs sporadically in the cinders and rhyodacite, and the density is generally less than 1 per 5m2. The obsidian is highly vitrophyric (pitchstone). Most looks like a grey-black glassy rhyodacite. Preferred planes of cleavage are caused by bands of ash in many nodules, and the cleavage planes and phenocrysts do not allow successful knapping other than sporadic flake removals. No O'Leary Peak material has been located below the Mogollon Rim, but artifacts have been reported in the Sunset Crater area.

Interestingly, reduced cores and flakes do occur (less than 1 per 5m2) and two plainware and one black-on-white sherd were recorded. Additional published sources include Jack (1971), Moore et al. (1960), Schreiber and Breed (1971).

References

Burton, J.H. (1986). Selected Petrologic Applications of Back-Scattered Electron Imaging.  Unpublished Ph.D. dissertation, Department of Geology, Arizona State University, Tempe.

Jack, R.N. (1971) The source of obsidian artifacts in Northern Arizona. Plateau 43:103-114.

McKee, E.H., P.E. Damon, M. Shafiqullah, R.C. Harris, and J.E. Spencer (1998) Compilation of unpublished USGS and University of Arizona K-Ar dates of volcanic rocks of the San Francisco Volcanic Field, northern Arizona.  Arizona Geological Survey Open File Report 98-2, Tucson.

Mintz, Y. (1942) Slate Mountain Volcano-Lacolith.  Plateau 14:42-47.

Moore, R.B., and Wolfe, E.W. (1976).   Geological map of the eastern San Francisco volcanic field, Arizona.   Miscellaneous Investigations Series, Map I-953.  USGS.

Peterson, B.T. (2013) The geochemistry of the San Francisco Volcanic Field: anatomy of a mid-sized intraplate volcanic system from source to surface. In Rates and Mechanisms of Magmatic Processes: Isotopic and Geochronological Evidence. Ph.D. dissertation, Department of Earth and Planetary Science, University of California, Berkeley.

Roberts, T.M. (2008) Footprints and "fingerprints": a northern Arizona geochemical study of Archaic Period lithic procurement and mobility.  Masters Thesis, Department of Anthropology, Northern Arizona University, Flagstaff.

Robinson, H.H. (1913). The San Francisco Volcanic Field, Arizona.  U.S. Geological Society Professional Paper 76.  Washington, DC: U.S. Government Printing Office.

Sanders, S.C., Zahrt, J.D., and Bell, G. (1982). Trace and minor element analysis of obsidian from the San Francisco Volcanic Field using x-ray fluorescence.  Advances in X-Ray Analysis 25:121-125.

Schreiber, J.P. and Breed, W.J. (1971). Obsidian localities in the San Francisco Volcanic Field, Arizona. Plateau 43:115-119.

Shackley, M.S. (1988). Sources of archaeological obsidian in the Southwest: an archaeological, petrological, and geological study.  American Antiquity 53:752-772.

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

Wolfe, E.W., Ulrich G.E., Holm, R.F., Moore, R.B., and Newhall, C.G. (1987).  Geologic map of the central part of the San Francisco Volcanic Field, north-central Arizona. USGS Miscellaneous Field Studies Map MF-1959.

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

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