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. This is the only, as yet, known source in the Mount Floyd field. 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. 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, 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, but the material was probably 'test knapped' everywhere. A sandstone two-handed mano was the only other artifact type recorded.

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

Raw elemental concentrations for Partridge Creek (Round Mountain) source standards. All measurements in parts per million (ppm).

[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). The Partridge Creek (Round Mountain) source still remains as originally described (Shackley 1988a:754-755). 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 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.

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

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).

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

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright © 2001 M. Steven Shackley. All rights reserved.
Revised: 16 December 2009

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