Obsidian Sources

As part of the Quijos Valley Project lithic analysis, a brief survey of the area for possible obsidian sources was carried out in 2007. High concentrations of obsidian in the southern limits of the survey zone, as well as identification of obsidian cobbles in the rivers just beyond the southern limits by Cuéllar in 2002 suggested a nearby source. All major, and some minor, tributaries of the Río Cosanga north of, and including, the Río Aliso, and the east trending section of the Río Quijos were inspected, as were the lower reaches of the Ríos Borja and Oyacachi (Map). A secondary obsidian source was identified centered on the Río Aliso and the Quebrada Pumayacu Grande, both tributaries of the Río Cosanga, draining the eastern foothills of the Antisana volcano. Small cobbles of visually distinct obsidian also were recovered from gravels in the Río Bermejo, north of the Aliso-Pumayacu Grande area. Recently, Bellot-Gurlet and colleagues (2008) analyzed four obsidian samples recovered from the Ríos Aliso, Bermejo, and Cosanga using the particle induced X-ray emission (PIXE) and inductively coupled plasma (ICP) methods. Their results indicate that that these four samples represent two distinctive, and heretofore unknown, obsidian sources.

The majority of the obsidian cobbles identified at this secondary obsidian source were black or black and grey banded in color, often containing high quantities of phynocrystic inclusions and internal fracture planes. These black and banded cobbles ranged from very small gravels (< 5 cm in maximum size) to larger cobbles (10-30 cm in size). During replication experiments on cobbles of black and banded obsidian recovered during the source survey, bifaces were produced with some difficulty due to the overall brittleness of the obsidian. In an unnamed tributary of the Río Aliso numerous smaller cobbles (~10 cm) of high quality clear obsidian, free of phynocrysts, also were recovered. Replication experiments on cobbles of clear cobbles produced much better results in bifacial reduction, however smaller cobble size limited what could be produced. Cobbles of the clear obsidian were relatively scarce in comparison and no clear cobbles as large as the largest black cobbles were encountered. It is not currently known whether these two color types are chemically distinct.

A high quality variety of clear obsidian also is known from the Callejones obsidian flow located 3 km northeast of the Mullumica flow, along the eastern flanks of the Sierra de Guamani in the northern Cordillera Real (Bigazzi et al. 1992; Salazar 1992). This source may account for some of the clear variety encountered in the survey. Although no obsidian cobbles were identified in the gravels of the Río Oyacachi during the summer of 2007, if clear obsidian was entering the north trending section of the Quijos River Valley from the Cordillera Real, this main tributary would be a likely candidate for its distribution.

The visual characteristics of color, texture and type and size of inclusions suggest that the black, and possibly clear, obsidian artifacts recovered during the Quijos Valley survey came from this Aliso-Pumayacu secondary source area. As a result, the Quijos Valley data set has been analyzed on that assumption. Whether or not these cobbles were simply extracted from the Río Cosanga, and the Río Quijos north of its confluence with the Río Cosanga, or were transported through some form of exchange, direct procurement, or combination of all the above is not known. Nonetheless it is recognized that some proportion of the obsidian represented by the Quijos Valley survey assemblage may come from outside of the region, such as those known source in the Cordillera Real (Salazar 1992), whose visual description match some of the obsidian recovered in the Quijos region. Obsidian source samples and artifacts collected during 2002 and 2007 field seasons will be submitted for trace element analysis in the near future to address this issue.

Finally, cobbles of rhyolite and fine basalt were found throughout the gravel bars of the Río Quijos, while no cobbles of these materials were identified in the Río Papallacta, just upriver from its confluence with the Río Quijos. It is likely that these river cobbles are the source of the rhyolite and basalt artifacts found throughout the survey zone.

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