SOUTH SAUCEDA MOUNTAINS
An artist's rendition of Casa Grande (Compound A) during the Classic Period. Sauceda Mountains obsidian was a dominant raw material choice during this period.
(Rebecca Leer, National Park Service drawing)
Coalesced rhyolite domes in the South Sauceda Mountains, Arizona
Sections 20,21,22,27 through 35 R3W, T9S; Section 36 R4W, T9S; and adjacent area (no sections delimited) in R3W, T10S USGS Tom Thumb 7.5' Quad, Barry M. Goldwater Range, southwest Maricopa County, Arizona.
This is a previously unknown marekenite source first surveyed in September 1986. At the south end of the Sauceda Mountains, as part of the Tertiary Sauceda Volcanic Field, are a series of north to south arcing rhyolite domes (Wilson et al. 1957). Similar to Vulture, many of the rhyolite units exhibit devitrified and somewhat vitrophyric glass (vitrophyre) and perlite toward the outer margins due to more rapid cooling. No nodules were found in situ within the perlite or vitrophyre, again like Vulture. This suggests that many of these Tertiary marekenites were pyroclasts in the tuff eruptions rather than remnants in the perlite as at Superior and Los Vidrios.
The source is chemically bimodal, particularly in strontium and to a lesser extent zirconium (see figure below). The bimodality may be due to magmatic differentiation and/or significant time depth between eruption. The bimodality is nearly 50/50 and nodules of both modes are intermixed within a conglomerate and developing pavement consisting of rhyolite, perlite, vitrophyre, basalt, and obsidian very similar to Vulture. It is impossible, at this point, to determine which rhyolite structure(s) contribute to Sauceda modes A or B. Sauceda is more precisely termed a chemical group, since there is an unusual level of variability among some elements. This is not a significant problem archaeologically since nodules of both modes are mixed within the conglomerate of the source area, and selection by prehistoric knappers corresponded to the quality of the material which has no relation to the source chemistry in this case. Secondary deposition follows the course of Sauceda Wash for at least 20 km north and perhaps as far as Gila Bend 35 km north. Nodule density ranges up to 30 per 5m2 probably the prehistoric density at Vulture and other marekenite localities before they became popular rockhound collection points---not possible until recently on Luke Air Force Range. The largest nodule recovered measures 11 cm in diameter. Nodules 5 cm are quite common and a few 8 to 9 cm were recovered. The range of nodule sizes at Sauceda is probably similar to the pre-Anglo Southwestern marekanite sources before collection of "Apache Tears" became popular, although the bipolar reduced nodules at Vulture and Tank Mountains suggest that large nodules were relatively rare at these sources. Nodule size, as discussed later, may have been one of the factors influencing selection by prehistoric knappers.
The color and opacity is as variable as Vulture if not more so. All the glass is aphyric and the most common color is a translucent green brown, but black opaque, green and black banded, and even some light gray/black banded nodules are represented. Color or opacity does not correlate with the bimodal chemistry. Cortex varies from a waterworn gray in the washes to a velvet-like surface on the conglomerate. This obsidian makes excellent knapping raw material, equal to any in the Southwest.
The prehistoric procurement pattern is somewhat unusual here. Rejected flakes and bipolar cores occur throughout the conglomerate and in the braided Sauceda Wash system, but at very low densities. However, near the rhyolite domes, flake and core fragment densities approach 50 per m2 in some areas. Perhaps more interesting, virtually no unreduced nodules remain near the rhyolite/perlite domes. It appears that there was some preference for nodules close to the perlite rather than in the alluvium, possibly due to higher density and size. This pattern was not present at any other source in the Southwest, and was particularly noticeable in Section 36 R3W, T9S. There are no published sources on this locality other than the county geological map (Wilson et al. 1957).
In May 1996, samples submitted by Bruce Masse and Adrienne Rankin of the Department of the Air Force, Luke Air Force Range from west of the Sauceda Mountains further defined the chemical differentiation within the Sauceda magma chamber. This is evident in the matrix plot below and suggests the possibility of separate temporal events and/or vents located west and south of the Sauceda Mountains.
Matrix plot of samples from west and south sections of the Sauceda Mountains indicating the chemical differentiation.
Elemental concentrations for Sauceda Mountains source standards. Samples with a "W" prefix from west of the Sauceda Mountains, all others from the southern end of the range. All measurements in parts per million (ppm).
Mean and central tendency for the data in above table in ppm.
This page maintained by Steve Shackley (firstname.lastname@example.org).
Copyright © 2001 M. Steven Shackley. All rights reserved.
Revised: Wednesday, 17 October 2018
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