4.5. Pliocene (Barroso group) obsidian in the Colca valley

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The obsidian that was widely used in the prehispanic central Andes occurs in Tertiary flows along the western cordillera. In the main part of the Colca valley, obsidian is found in the lower levels of Late Tertiary lava flows on both the north and south sides of the Colca river. East of the town of Chivay, the Chivay obsidian type has been observed to occur where Barroso flows contact the older Tacaza deposits (Burger, et al. 1998: 205). Barroso flows (TBa), clearly evident in photos shown in Figure 4-11 and Figure 4-15, extend atop Tacaza flows to the east of Chivay, and obsidian has been observed where layers described as "Tertiary intrusive, porphorytic" (T-po) extend atop Tacaza flows to the northwest of Coporaque (Figure 4-23). A fission track date on an obsidian sample collected east of Chivay confirms the Pliocene origins of these flows with a result of 3.52 ± 0.15 Ma (Poupeau and Labrin, 10 Oct 2006, pers. comm.). To date, research has shown that high quality obsidian flows in the Colca valley that occurred during the late Tertiary fall into two chemical groups: (1) the Chivay type found to the east of the town of the Chivay, and (2) the "Uyo Uyo" type that lies across the Colca River, west of the town of Coporaque. In the course of fieldwork in 2003 the Upper Colca project sampled obsidian from both locations and had results analyzed by the Missouri University Research Reactor, as will be described below.