LOS VIDRIOS

NORTHERN SONORA

The mafic Sierra Pinacate Volcanic Field with the rhyolite structures of Los Vidrios to the east with the Rio Sonoita flowing to the south. The Tertiary rhyolite is substantially older than the Quaternary mafic eruptive events in the Sierra Pinacate. Landsat photo.

Coalesced rhyolite domes at Los Vidrios, east of the Sierra Pinacate Volcanic Field, northern Sonora. Rio Sonoita wash in foreground

Located between UTM 352 and 354 East and 29 and 30 North of the Puerto Peñasco H12-1 Mexican Topographic Quad 1:250,000 northwest Sonora, Mexico. This is another probable middle Tertiary glass source located about 8 to 12 km south of the US/Mexican border along the Rio Sonoita and on the eastern border of the predominately mafic Quaternary Sierra Pinacate Volcanic Field. The source is comprised of a northwest-southeast arcing series of rhyolite and rhyolite/obsidian domes. Nodules up to 10 cm in diameter are found in a rhyolite/tuff/obsidian conglomerate eroding into Rio Sonoita and embedded in vitrophyre within the rhyolite domes. Rhyolite/obsidian units, some up to 30 m by 40 m, are common on many domes. One small dome between UTM 29-30 North and 352-353 East (on the 353 line), consists of an alternating series of rhyolite and obsidian strata with nodules embedded within the glass, similar to Slate Mountain. Densities within the devitrified glass matrix are up to 50 per m², perhaps five times that if pea size nodules are counted. Within the conglomerate densities reach 10 to 20 per 5 m².

Secondary deposition consists of erosion through the Rio Sonoita river system south at least as far as the Sierra Blanca and perhaps 65 km to the Gulf of California. The thin cortex on the nodules varies from brown to gray and velvet-like in the conglomerate above the Rio Sonoita. The aphyric glass is very consistently black and very opaque even in the thinnest flakes. No banding or lighter shades were noticed in hundreds of nodules collected.

The knapping quality of this obsidian is good, but the glass is quite brittle and bipolar reduction sometimes resulted in 'exploding' nodules. Pressure flaking, however was quite successful when a flake was successfully removed and flakes were easily removed with minimum platform collapse. There are no known geological or archaeological references to this source other than a short mention in Lumholtz (1912:286).

Raw elemental concentrations for Los Vidrios source standards. All measurments in parts per million (ppm).

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright © 2001 M. Steven Shackley. All rights reserved.
Revised: 28 September 2008

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