Sierra La Breña

 This is the one source reported from Chihuahua that is in the Sierra Madre rather than the basin and range region. The probable genetic relationships between this source and Agua Fria over 40 km west in Sonora is discussed on the Agua Fria source page. These samples were collected by John Roney (BLM, Albuquerque). The following is from his field notes:

The purpose of these notes is to document an obsidian source located between Casas Grandes and La Cueva de la Olla in northwestern Chihuahua. On August 31, 1997 I drove the road between Ignacio Zaragosa (El Willy) and Colonia Juarez. Perlite occurred as bedrock outcrops intermittently between approximately 3339900N/763500E and 333680ON/768400E, a distance of about 6 km. In at least two places the outcrops were very extensive, consisting of ledges of bedrock several hundred yards or more in length. Samples of perlite were gathered at each of these locations (Numbers 1 and 2), but no knappable obsidian was found, and there were no flakes or other evidence of prehistoric activity [The source is located on the INEGI Ignacio Zamgoza Quad, H12B87, 1:50,000]. Inspection of several small drainages near the eastern edge of this area (Location 3) did yield several small Apache tears. Rudi and I spent about 45 minutes looking at fine gravel and coarse sands in the drainage bottoms just below the dam named Cuatro Caminos, where we found about a dozen small nodules of obsidian. Presumably these eroded out of the perlite deposits noted above. All were found in drainages on the northern edge of a 100 meter by 300 meter alluvial flat, which suggests that they may have originated near Cerro Lola. The largest was 3 x 2.5 x 2.0 cm, three others were about 2 cm in diameter, 4 were about 1 cm in diameter, and two were about 0.5 cm in diameter. (Two other small pieces are probably perlite.) There was no evidence that obsidian had been exploited prehistorically at this location (John Roney, personal communication).

The elemental concentrations indicate a calc-alkaline rhyolite unlike the peralkaline lavas Antelope Wells and Lago Barreal and similar to the other Sierra Madre source Agua Fria reported here from Sonora.

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Revised: 20 March 2015