GRANTS RIDGE

AND

HORACE MESA

MOUNT TAYLOR VOLCANIC FIELD

NORTHWESTERN NEW MEXICO

Landsat photo of the Mount Taylor Volcanic Field.  East Grants Ridge is in the lower left with the late mafic table land known as Horace Mesa to the right of East Grants Ridge.

East Grants Ridge coalesced domes with Horace Mesa in left rear background from La Jara Mesa.

The "Grants Ridge" source of archaeological obsidian in the Mount Taylor Volcanic Field in northwestern New Mexico has just been systematically sampled and analyzed this year (1997). Previous chemical analyses by Baugh and Nelson (1987) and others have generally been based on grab samples from the East Grants Ridge area. Our lab's recent analysis of archaeological obsidian from the Zuni and Hopi areas suggested that, unlike the somewhat vitrophyric glass from Grants Ridge, prehistoric knappers preferred an aphyric glass that while chemically similar, does not elementally covary with samples from Grants Ridge. Systematic survey and sampling this summer resulted in the discovery of another source on Horace Mesa to the east of East Grants Ridge. These nodules up to 8 cm in diameter are aphyric and are a better medium for tool production. The chemistry differs in a number of incompatibles, but appears to be derived from the same magma source of high silica rhyolite, a late Tertiary and early eruptive phase in the Mount Floyd field. A complete major, minor, and trace analysis has been completed using the new Philips PW2400 WXRF in the lab and will be published in Shackley (1998).  The Mount Taylor obsidian appears to be yet another example of chemical gradients in silicic melts that have archaeological relevance (see Shackley, 1995).

Figure 1. Selected topographical and geological map of study area. Contours are approximate, but accurate at points noted. All measurements in meters. Tr = Plio-Pleistocene rhyolite; Qb=Quaternary basalt; Qta=Quaternary andesite. Collection localities noted by date/sample number (i.e. 081897-1). Fr=Forest Road. Map based on Dillinger (1990), Hunt (1938), and Lipman and Moench (1972).

Raw elemental concentrations for Grants Ridge/Horace Mesa source standards. All measurements in parts per million (ppm).

Selected WXRF oxide values (wt.%) for two Mount Taylor field source standards^a, the Baker & Ridley (1970) "average" analysis of rhyolite pumice and glassy flow from Mount Taylor^b, followed by analysis of RGM-1, USGS rhyolite rock^c, and recommended values in Govindaraju (1994).

^a 081897-2-11 is from the Grants Ridge locality, and 081997-1-7 is from the Horace Mesa locality noted on Figure 1. Both samples analyzed by WXRF. Analytical methods and instrument settings in Shackley 1998.

^b This is an "average" analysis of an unknown number of samples from rhyolite pumice and "glassy flow", presumably from the Mount Taylor stratovolcano (Baker & Ridley 1970:109).

^c This is a single analysis by WXRF of a fused disk standard. Analytical methods and instrument settings in future publication.

Zr, Y, Nb three dimensional plot of Horace Mesa and Grants Ridge source standards.

.

Y versus Nb plot of Horace Mesa (upper group) and Grants Ridge (lower group) source standards. Error bars = 1st standard error of measurments within population.

Discriminant classification results for original and cross-validated source standard groups using Mn, Y, Zr, and Nb variables.

REFERENCES

Baker, I., and Ridley, W.I. (1970). Field evidence and K, Rb, Sr data bearing on the origin of the Mt. Taylor volcanic field, New Mexico, U.S.A. Earth and Planetary Science Letters 10, 106-114.

Baugh, T.G., and Nelson, F.W., Jr. (1987). New Mexico obsidian sources and exchange on the southern Plains. Journal of Field Archaeology 14, 313-329.

Dillinger, J.K. (1990). Geological map of the Grants 30' X 60' quadrangle, west-central New Mexico. Map C-118-A, Denver, U.S. Geological Survey.

Govindaraju, K. (1994). 1994 compilation of working values and sample descriptions for 383 geostandards. Geostandards Newsletter 18.

Hunt, C.B. (1938). Igneous geology and structure of the Mount Taylor Volcanic Field, New Mexico. Washington, DC, U.S. Geological Survey Professional Paper 189B.

Lipman, P.W., and Moench, R.H. (1972). Basalts of the Mount Taylor Volcanic Field, New Mexico. Geological Society of American Bulletin 83, 1335-1344.

Shackley, M. S. (1995). Sources of archaeological obsidian in the greater American Southwest: An update and quantitative analysis. American Antiquity 60, 531-551.

Shackley, M.S. (1998).   Geochemical differentiation and prehistoric procurement of obsidian in the Mount Taylor Volcanic Field, northwest New Mexico.  Journal of Archaeological Science 25:1073-1082. 

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright © 2001 M. Steven Shackley. All rights reserved.
Revised: Friday, 12 March 2004 03:03:24 PM -0800

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