MULE CREEK REGIONAL SOURCE

ARIZONA AND NEW MEXICO

Perlitic lava dated to 17.5 mya at the Antelope Creek locality of the Mule Creek Regional Source, western New Mexico

Mule Creek Caldera and Ash-flow Sheet

Collections from: Sections 1,2,3,10,11,12 R21W, T14S; Sections 5,6 R20W, T14S USGS Mule Creek 7.5' Quad, Gila National Forest, Grant County, New Mexico; and a road cut at the junction of AZ Hwy 78 and Coal Canyon R32E, T14S (no sections delimited), Greenlee County, Arizona.

Mule Creek is probably the geographically largest obsidian source in the Southwest. The obsidian is found in a very extensive late Tertiary ash-flow sheet that covers portions of Greenlee County, Arizona, Catron and Grants Counties, New Mexico and may extend over to the Mogollon Mountains to the east (see Gwynn Canyon source below) (Rhodes and Smith 1972; Weber and Willard 1959). Samples collected from the Mule Creek area in New Mexico and near Ash Peak, Arizona are chemically identical, while some samples from Mule Creek and Gwynn Canyon are similar. The nodules, up to 10 cm in diameter, may have been pyroclasts within the large ash flow. The nodule density in some areas reaches 100 per 5m², especially on the top of the ash hills. Erosion into the San Francisco and Gila River systems is very possible.

The aphyric glass ranges from opaque black to translucent smoky gray with some gray banding. In 200 specimens collected, three are mahogany-brown and black banded similar to Slate Mountain (Wallace Tank) material. Some of the cortex exhibits a silver sheen, but most is a thin black-brown. The material is a fair medium for tool production, but is very brittle much like Los Vidrios. The pressure reduction potential is, however, very good. Interestingly, Mule Creek obsidian was extremely rare in Arizona Archaic sites, while Cow Canyon and Gwynn Canyon was very common (see Shackley 1990:Chapter 8). This is probably a function of the lower quality of Mule Canyon material, particularly since Gwynn Canyon is further from the Arizona sites.

Marekenites in perlitic lava matrix at the Antelope Creek locality (shown above) New Mexico

The overall density of obsidian flakes and cores was relatively low (<1 per 20 m²), probably due to the sheer extent of the deposit. A few rejected biface fragments (laterally fractured) and utilized flakes (<10) were recorded. Published references include Findlow and Bolognese (1982a and 1982b), Rhodes and Smith (1972), and Weber and Willard (1959).

[updated 1995] One of the most startling recent discoveries is the chemical variability in the Mule Creek obsidian. In the earlier study, I noted two "outliers" collected at Mule Creek with significantly higher rubidium concentration values (Shackley 1988a:767). These outliers have now been identified as a distinct chemical group, often mixed in the regional Gila Conglomerate with three other chemical groups. The geology in the area is complex and has been studied by Ratté, and others for some time (Brooks and Ratté 1985; Ratté 1982; Ratté and Brooks 1983, 1989; Ratté and Hedlund 1981; Rhodes and Smith 1972). Primary in situ perlite localities for three of the chemical groups have been located.

Ashy, perlitic lava at the Mule Mountain Locality

At least four distinct chemical groups are evident, distinguished by Rb, Y, Nb, and Ba, and a lesser extent Sr, and Zr concentration values, and are named after the localities where marekanites have been found in perlitic lava: Antelope Creek; Mule Mountains; and Mule Creek/North Sawmill Creek all in New Mexico. Additionally, during the 1994 field season, a fourth sub-group was discovered in the San Francisco River alluvium near Clifton, Arizona and in older alluvium between Highway 191 and Eagle Creek in western Arizona north of Clifton. While in situ nodules have not yet been found they are certainly located somewhere west of Blue River and north and west of the San Francisco River since none of this `low zirconium' sub-group was discovered in alluvium upstream from the juncture of the Blue and San Francisco Rivers. The genetic relationship is apparent in the trace element matrix plot (Shackley 1995:Figure 2), and signifies the very complex nature of the Mule Creek silicic geology, with subsequent depositional mixing in the Gila Conglomerate. Glass at other Tertiary sources in the Southwest, such as Sauceda Mountains and Antelope Wells, also appear to exhibit more than one chemical mode, although not as distinct as Mule Creek (LeTourneau 1994; Shackley 1988a, 1990). The Mule Creek case is unusual because the chemical groups are not always spatially discrete and occur together in the extensive Gila Conglomerate which is mainly composed of Mule Creek rhyolite and tuffs in the area where the marekanites do occur (see Ratté and Brooks 1989).

Table 1. Elemental composition of Mule Creek Regional Source Groups. All measurements in parts per million (ppm).

Three-dimensional plot of Rb, Zr, and Ba indicating the compositional differentiation of the sub-sources in the Mule Creek Source Region.

Sr, Rb, and Zr concentration plot of Mule Creek and Gwynn Canyon source data. Note the possible genetic similarity between the Mule Creek and Gwynn Canyon sources, possibly due to the similar (Mogollon-Datil Province) crustal origin. 

Study area with selected archaeological sites (filled circles), obsidian sources (filled triangles), modern communities (filled squares), and modern drainage system. Mule Creek Chemical Group "in-situ" localities = 1 Antelope Creek; 2 Mule Mountains; 3 Mule Creek/N. Sawmill Creek. Source localities linked to appropriate pages or data.

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright © 2002 M. Steven Shackley. All rights reserved.
Revised: 27 May 2004

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