Flows belonging to the Tacaza group are found throughout the south-central Andes, however the only portion that appears in the Colca region belong to the older Tacaza with dates in the range of 30.21±0.73 Ma and 26.51±0.6 Ma. At 1900m thickness these Colca lava flows are the thickest Tacaza layers in the larger region (Palacios, et al. 1993: 86).

Figure 4-17. Andesitic Tacaza deposits with breccias and tuff outcrops in the Quebrada de los Molinos drainage. The Chivay obsidian source in later Barroso deposits is found high above, on the right side of the photo.

In the Colca area, these deep deposits of lavas and breccias consist of andesites and trachybasalts (containing higher feldspar content) intercalated with tuff bands. The Tacaza layer appears predominantly on the western half of our survey zone.