sfvolfld

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 2.70±0.08 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.

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

* 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).

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

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.

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).

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 quite easily at this elevation. 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).

Major, minor, and trace elements for one sample of San Francisco Peak obsidian. Measurements in wt. % or parts per million as noted.

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).

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.

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.

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

Sample	Na2O	MgO	Al2O3	SiO2	P2O5	K2O	CaO	TiO2	V2O5	MnO	Fe2O3
	%	%	%	%	%	%	%	%	%	%	%
SFPK-1	5.343	0	12.703	74.104	0	4.3	0.232	0.063	0	0.069	2.828

	Cl	Zn	Rb	Sr	Y	Zr	Nb	Ba	Pb	Th
	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm
SFPK-1	1573	194	142	11	75	717	141	<1	67	28