LOS SITIOS DEL AGUA, NORTHERN SONORA   

                                                                                                                                       

   

 

 

 

 

 

 

 

The vent and dome at Los Sitios del Agua

 

 

 

 

 

 

 

 

 

   

   

Based on a 1992 analysis of 71 obsidian artifacts from various sites located in Organ Pipe Cactus National Monument, Steve Shackley noted 10 artifacts that exhibited an elemental chemistry different from any yet identified in the Southwest (Shackley 1992, 2005).  Since then, this unlocated source called AZ Unknown A, has been found in archaeological sites of various age throughout southern Arizona, as far north as the Phoenix Basin (Shackley 2005; Shackley and Tucker 2001).  While exploring Historic era trails in northwestern Sonora, Mexico, members of the Ajo Chapter of the Arizona Archaeological Society discovered a previously unknown source of obsidian and other archaeological sites adjacent to the Rio Sonoyta in northern Sonora (Figure 1). This newly discovered source, called Los Sitios del Agua, is AZ Unknown A and finally locates this most vexing unknown obsidian sources in the region and provides important source provenance data for the Archaic through Historic periods in the southern Southwest.

                                                                            

THE DISCOVERY

 During the spring of 2006, several members of the Ajo Chapter of the Arizona Archaeological Society were exploring back roads in northwestern Sonora, Mexico. After emerging onto a wide flat terrace immediately above the floodplain of the Rio Sonoyta, nodules of obsidian were observed among the gravels covering the terrace. During a return visit in the fall of 2006, an obsidian quarry was discovered in the hills at the northern edge of this same terrace. Based on the results of a trace element analysis of two samples conducted by the Northwest Research Obsidian Studies Laboratory in Corvallis, Oregon it is known that the characteristics of this obsidian (Table 1) are the same as those from Los Vidrios Obsidian Quarry (LV) (Skinner 2007a) as originally reported by Shackley (1988, 1995, 2005).

In the spring of 2007, exploration along the banks of the Rio Sonoyta approximately 8-9 km downstream from the LV quarry revealed obsidian marekanites littering the sides of a rhyolite dome complex. The trace element characteristics of four samples of this obsidian (Table 1) did not match those from LV, but instead were the same as those classified as “Unknown 1" by Skinner (2007a, 2007c) and as AZ Unknown A discovered by Shackley (1992, 1995, 2005). The obsidian source for these samples previously known as Unknown 1 and AZ Unknown A has now been located and will be named the Los Sitios del Agua Obsidian Quarry (LSA).  In October 2008 Rick and Sandy Martynec and Steve Shackley re-visited the rhyolite dome complex for two days, recorded and mapped the LSA source again, and collected over 300 marekanite samples for study at Berkeley.  While it is considered part of the same volcanic field as Los Vidrios, the composition, that of a mildly peralkaline rhyolite, is quite different (Vidal-Solano et al. 2008; see http://swxrflab.net/los_sitios_del_agua.htm).  The rhyolite dome is also called Vidrios Viejos and as mentioned above has an ⁴⁰Ar/³⁹Ar date of 14.27±0.15 Ma statistically similar to Los Vidrios (Vidal-Solano et al. 2008:697.

                                                                                              

THE OBSIDIAN AT LOS SITIOS DEL AGUA

 

Los Sitios del Agua is a probable Tertiary Period dome complex oriented in a northwest-southeast arc (see Fig. 2).  Typical of Tertiary rhyolite dome complexes that produce obsidian, Los Sitios del Agua exhibits perlitic lava bodies with abundant remnant marekanites, some as large as seven to eight centimeters, and many near five centimeters (Shackley 2005; Fig. 3 here). 

As with Los Vidrios, the Los Sitios del Agua domes appear to be part of a bimodal volcanic field known as the Sierra Pinacate Volcanic Field that consists of initial Tertiary events, mainly rhyolite in composition followed by mainly mafic eruptive events to the west of the earlier rhyolite during the Quaternary including 10 maar volcanic events (Donnelly 1974; Gutmann 2007; Gutmann et al. 2000).  Virtually no geological research has been directed toward the rhyolite on the eastern portion of the volcanic field other than the initial work by Shackley (1988, 1995, 2005).

 

 

 

 

 

 

 

 

The Los Vidrios dome complex west of Los Sitios del Agua

 

 

 

 

 

 

 

 

 

 

 

 

 

The majority of obsidian at the LSA quarry does not appear to have been formed in the same manner as that at Los Vidrios. Virtually all of the LSA marekanites are remnant marekanites in perlitic lava.  None of the marekanites were produced by ash flows as in the Los Vidrios case (Shackley 1988; Figure 3 here).  In addition to the angular quality of the obsidian from the LSA quarry, there are color variations within the site. Scattered about and localized in at least one area are light grey marekanites of obsidian.  Even more startling is a small quarry near the center of the LSA quarry where beautiful, jade green colored obsidian was apparently mined based on what appears to be a small prospect; this green obsidian has been termed Kate’s Green after its discoverer.  When tested by Skinner (2007c) this green obsidian was found to have nearly the same trace element composition as the surrounding jet black obsidian.  Analysis of source standards here in the Berkeley lab indicates substantive differences in barium in the green samples, however (Tables 2 and 3).  The other measured elements are similar (Table 3).  It is obvious that this unique colored obsidian held a special appeal. It still does today.  The other measured elements are similar.  It is obvious that this unique colored obsidian held a special appeal. It still does today.  The composition as reported by Vidal-Solano et al. using plasma-atomic emission spectroscopy matches well with the EDXRF data from this lab (2008:695).

 

SOURCE DESCRIPTION 

            Although most of the surface obsidian at the LSA quarry exhibits a shiny, almost freshly broken appearance, only a few lithic reduction stations were noted. While examination of the site revealed scattered reduction flakes and cores, there are a few locations where numerous nodules have been tested by the bipolar reduction method.  Here amongst the layers of debitage can be found  large, thin primary flakes, some are quite long exceeding 5 cm in length. This undoubtedly was a characteristic desired for the production of the small, delicately made projectile points and cutting and scraping tools found on nearby sites. One is led to conclude from the lack of secondary and final pressure flaking debitage at the LSA quarry, that the larger and more desirable flakes were transported to other locations for further and final refinement.   Knapping experiments by Shackley noted that LSA obsidian is considerably less brittle than the nearby Los Vidrios obsidian that occurs in much larger quantities.  Production of a Desert Side-notched projectile point from the production of a bipolar flake took less than 20 minutes with excellent predictability.  As discussed below, this may be why Los Sitios del Agua is common in archaeological contexts to the north even though the source material is numerically inferior to Los Vidrios.

            Physically, the obsidian at the LSA quarry encompasses an area that measures approximately 800 meters north-south by 850 meters east-west; the base of the small hills are at 207 meters above sea level and rise to an elevation of 224 meters above sea level. The density of obsidian on the ground surface varies widely within the quarry from areas where there are no nodules to others where there are more than 500 in one square meter.  There appears to be virtually no secondary deposition into the Rio Sonoyta, at least as evident today.  This may be why it was not discovered by the earlier study in the 1980s (Shackley 1988).

 

 

The high surface density of marekanites at Los Sitios del Agua

 

XRF ANALYSIS

 

            Of over 300 marekanite samples collected from Los Sitios del Agua, 40 samples (» 13%) were analyzed by energy-dispersive x-ray fluorescence spectrometry for 16 elements, ten of which are reported here (Tables 2 and 3).  The laboratory and instrument protocol is here (see also Shackley et al. 2016). 

            Immediately apparent is that this rhyolite lava is a mildly peralkaline rock.  These volcanic rocks are generally seen as a division of volcanics where the proportion of alumina (AlO₂) is less than that of sodium and potassium oxides combined, but more importantly generally exhibits relatively high iron and zirconium values (Best 1982; Cann 1983; Hildreth 1981; Shackley 2005; see Figure 4 here).  These peralkaline volcanic rocks, including rhyolitic obsidian is common in rift regions were more mafic lava associated with the mantle is sampled and crustal volcanism occurs, such as the Great Rift Valley in eastern Africa.  In the Rio Sonoyta case, the elemental composition of Los Vidrios and Los Sitios del Agua couldn’t be more different (Figure 4).  There are no peralkaline obsidians north of the International border, but are quite common associated with the Sierra Madre Occidental, probably the largest rhyolite field in the world,  and the Chihuahuan Basin and Range just to the east of the Sierra of eastern Sonora and Chihuahua. (Gunderson et. al. 1986; Fralick et al. 1998; Shackley 2005).  In obsidian, peralkaline chemistry is objectified by very dark often opaque glass, and glass that is often green when viewed with transmitted light both caused by a high iron content.  The most archaeologically famous peralkaline glass in the New World is that from Sierra de Pachuca, in the state of Hidalgo, Mexico (Barker et al. 2002; Tenoria 1998).  The obsidian from Los Sitios del Agua Areas A, B, C, and E, are all translucent dark green similar to Pachuca, while the marekanites from Area D is an opaque light green -  all due to the relatively high iron.  This makes Los Sitios del Agua obsidian the most megascopically distinct of the Sonoran Desert obsidian sources.

 

LOS SITIOS DEL AGUA IN SITES NORTH OF THE BORDER

 

Besides the ten artifacts analyzed by Shackley from Organ Pipe Cactus National Monument just north of the border, obsidian from Los Sitios del Agua has also been recorded (as AZ Unknown A) in other sites as far north as the Phoenix Basin.  Table 4 exhibits the number of artifacts produced from Los Sitios del Agua in Arizona sites.  The vast majority of Los Sitios del Agua obsidian is from sites in southern Arizona, including sites in the Phoenix Basin.  It is almost always in assemblages where Los Vidrios obsidian also occurs, suggesting that marekanites from both sources were collected prehistorically together while traveling along the Rio Sonoyta as discussed above.  It was not found at the Preclassic site of Snaketown on the Gila River Indian Nation, but a few were recovered from surface sites on the Gila River Indian Nation (Shackley 2005; Table 4 here).  It also was not recovered from the Classic Period sites of Pueblo Grande or Casa Grande (Bayman and Shackley 1999; Peterson et al. 1997).

 

CONCLUSION

 

            The discovery of the Los Sitios del Agua obsidian source in northern Sonora solves one of the unlocated sources in the Southwest.  The glass proper, a green peralkaline obsidian, is unique in the Sonoran Desert for its color and just as unique chemically, and probably held value in prehistory.  Los Sitios del Agua is a small dome complex covering a much smaller area than the extensive dome complex of Los Vidrios, mostly across the Rio Sonoyta.  It does, however, occur in sites at least as far north as the Phoenix Basin and often in association with artifacts produced from Los Vidrios obsidian.  While these two Sonoyta River Valley obsidian sources were likely sampled by groups moving north or south through the area, it is not necessarily clear whether they can be used as a signature of shell exchange during any particular time period.  Reconnaissance of the valley by members of the Arizona Archaeological Society suggests that all time periods, particularly Preclassic through Historic periods are represented at sites in the region.