PYRAMID MOUNTAINS, SOUTHWESTERN NEW MEXICO

In 2023 an analysis of obsidian artifacts from the Classic Mimbres West Baker Site in Hidalgo County at the northern edge of the bootheel, southwestern New Mexico just south of the Pyramid Mountains, indicated that over 71% of the artifacts were produced from what appeared to be Chihuahua Unknown A, or very compositionally close to the elemental concentrations of that “unknown” source.

            The first approximation for the source of this material was that it was a relatively local source, possibly quite near the site given that: 1) it has never been seen in regional archaeological contexts beyond the two called Chihuahua Unknown A from LA 132649 in southern New Mexico (Shackley 2002) ; and 2) is a very dominant raw material in the site with over 70% of the artifacts produced from this source.  The logical suspect would be one of the Oligocene (24–30 Ma) or Eocene (31–36 Ma) rhyolite formations in the Pyramids (Deal et al. 1978; Flege 1959).  Given the rather great age of these rhyolites, it is expected that the nodule size will be small as seen in Sample 16153-4005 a split bipolar core that is only 22.2 mm in diameter recovered from the site.  This is within the range of marekanite size at the Nutt Mountain source in Sierra County, New Mexico just to the north that is 31.74±0.13 Ma, and a member of the Mogollon-Datil Volcanic Province, as are the rhyolites in the Pyramid Mountains given the age of the formations (Shackley et al. 2018; see Nutt Mountain description).  The obsidian assigned to this unknown source is a high silica rhyolite typical of these obsidians in the Mogollon-Datil volcanic Province (Deal et al. 1978; Shackley et al. 2018).  While most elements are similar between the Chihuahua Unknown A and Pyramid Mountains, they can be discriminated with Zr and Ba, and to a certain extent Fe (see Table and Figure below).  It seemed reasonable to assume that Chihuahua Unknown A was somewhere in the border area, and as noted above it was not yet clear whether the other group is from the Pyramid Mountains in the bootheel of southwestern New Mexico.  The composition of these two unknowns is similar enough that they could be from the same magma source, but as of that time they had not been recovered in the same archaeological context, and Zr and Ba are very different (Table below).

Mean values for selected elements of Chihuahua Unknown A (n=17) and Pyramid Mountains mean values (n=28).  All measurement in parts per million (ppm).

Ba/Zr bivariate plot of Chihuahua Unknown A and Pyramid Mountains assigned artifacts.  Confidence ellipses at 95%.

            In March of 2026 an attempt to locate the possible source of the West Baker obsidian was, in part, successful.  The sediments to the southeast and south of Pyramid Mountains were examined over a day, and at the southern area one small marekanite was recovered measuring only 9.4 X 7.2 mm in diameter (Sample 031226-2-1).  XRF analysis indicated that that the composition on six of the elements including Ti, Rb, Sr, Y, Nb and Ba were well within the composition of the archaeological artifacts from West Baker.  Zirconium was slightly lower, but the composition of this marekanite was likely not Chihuahua Unknown A, as discussed above.  Much of the property surrounding the Pyramid Mountains is privately held, and access is not yet acquired.  However, as with the Animas Mountains case, it is quite likely that the southern Pyramid Mountain rhyolite (obsidian) is the source of the West Baker artifacts.  Further research and access to the land around the south Pyramid Mountains will hopefully locate the primary source of this rare obsidian source.  Again, it is possible that the Pyramid Mountain obsidian and Chihuahua Unknown A are from the same magma source, even though Zr and Ba are quite variant - only time will tell.

Pyramid Mountain source marekanite (031226-2-1) and the West Baker Pyramid Mountain “unknown” assigned artifacts.

     

Ba/Ti. Nb/Y, and Sr/Rb bivariate plots of the West Baker obsidian artifacts and the 031226-2-1 marekanite.  Confidence ellipses at 95%.

This page maintained by Steve Shackley (shackley@berkeley.edu).
Revised: 22 June 2026

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