Painted Rock Mountains (dacite/trachydacite obsidian)

            This source in southwest Maricopa County, Arizona was not included in the discussion in 2005 since it had not yet been recovered in archaeological context (Shackley 2005).  A long-term project in western Arizona focused on the Patayan and Hohokam of western Arizona by Aaron Wright of Archaeology Southwest indeed recovered Painted Rock Mountains obsidian (Shackley 2021; Wright and Hopkins 2016).  In the 1990s four samples of very small marekanites were submitted by a CRM company from near the spillway of the dam, and in 2023 Archaeology Southwest submitted another sample from the same area.

            In December 2024, Aaron Wright, Chris Loendorf and I examined an area to the northwest of Painted Rock Reservoir dam where Wright recovered samples a few years earlier.  Very small marekanites (all ≤ 21 mm in diameter) were recovered within and eroding from a platy volcanic flow mapped by Skotnicki (1993) as a dacite: “Hard, dense, platy dacite flows.  Typically maroon or gray…Forms the south side of the spillway…and forms laminated outcrops. Locally vitrophyric, such as the hill downstream from the dam.” (Skotnicki 1993).

            This accurately describes the environment from which marekanites were recovered both in-situ and eroding downhill from the dacite in 2024.  An analysis of a vitrophyric marekanite embedded in the dacite indicates that the matrix is a trachydacite to dacite as described by Skotnicki (1993; see figure herein below).  This is a rare obsidian produced not from rhyolite but by a somewhat high silica dacite/trachydacite (see Figure below).  Trachyte obsidian does occur rarely and his been recorded in Sonora, as well as Hawai’i on the Big Island (McCoy 2011; Shackley 2024).  Since the proportion of silica (SiO₂) is relatively low (< 70 weight percent) although aluminum (Al₂O₃) is relatively high here (17-20 wt. percent), the marekanites are quite small (see Shackley 2005).  As discussed in Shackley (2005:10-15), silica and aluminum function as the glass formers and if H₂O is relatively low (< 1 wt. percent), glass can form, however even if silica is too low, but in this case aluminum high, small marekanites can form.  While this is not a rhyolitic obsidian the Painted Rock Mountain obsidian is knappable as seen in the debitage in the Pioneer/Colonial Hohokam contexts nearby. Given the small marekanite size, it is not surprising that this source has not been detected any distance from the source.

Oxide values for the two trachydacite/dacite (samples 6 and 7), the basalt overlying the trachydacite (samples 8 and 9), two rhyolite (obsidian) standards NBS-278 and RGM-1 and recommended values.  See TAS plot below.

nr = not reported

TAS plot of the Painted Rock Mountain rocks recovered at the collection locality, and the two rhyolite (obsidian) standards NBS-278 and RGM-1.  Sample 120824-6 is a trachydacite matrix with an embedded vitrophyric obsidian marekanite, possibly yielding the slightly higher silica than another rock sample in the same area sample 120824-7.  The basalt and basaltic andesite is from the overlying later rock as mapped by Skotnicki (1993), his Tb unit as a “mesa capping flow”.

Elemental concentrations for the Painted Rock Mountains trachydacite obsidian, analysis of rhyolite (obsidian) standards NBS-278 and RGM-1 and recommended values.  "PR" samples are from earlier collections, and "120824" samples from the 2024 collection.

Painted Rocks Mountains obsidian mean and central tendency from the data in table above (121224 analysis)

REFERENCES

McCoy, M.D.

2011 Geochemical Characterization of Volcanic Glass from Pu‘u Wa‘awa‘a, Hawaii Island. Rapa Nui Journal 25:41–49.

Shackley, M.S.

2021 Source Provenance of Obsidian Artifacts from a number of Hohokam and Patayan Contexts in Western Arizona.  Report prepared for Aaron Wright, Archaeology Southwest, Tucson, Arizona.

            2005  Obsidian: Geology and Archaeology in the North American Southwest.  University of Arizona.20

2024 Obsidian Sources and Source Provenance of Archaeological Obsidian in northern Sonora: the Known and Unknown.  In Reframing and Expanding Sonoran Archaeology, edited by H. M. Claypatch and E. Villalpando Canchola, Kiva 90:293-305.

Skotnicki, S.

1993 Geologic Map of the Painted Rock Mountains, Maricopa County, Arizona.  Open-File Report 93-7, Arizona Geological Survey.

Wright, A.M. and M.P. Hopkins

2016  The Great Bend of the Gila: Contemporary Native American Connections to an Ancestral Landscape.  Archaeology Southwest Technical Report 2016-101.  Tucson, Arizona.

Back to the SW obsidian source page

To the EDXRF Lab home page