4.3.3. Oligocene and Miocene lavas
Oligocene and Miocene lavasTacaza Group
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.
Pichu Formation ash flows and the Castillo de Callalli
On the eastern side of the study area, the Tacaza formation is overlain with the Pichu formation consisting of sandy tuffs and white ignimbrites. Among these Miocene ignimbrites layers is the Castillo de Callalli formation, an ash-flow tuff that rises dramatically from Llapa river just upstream of Callalli and is a principal landmark in the Upper Colca region. In the INGEMMET study (Ellison and Cruz 1985) the Castillo de Callalli was assigned to the Pichu formation. This landmark is an approximately 400m hill of silicic ash-flow tuff ranging from densely welded to non-welded tuff. The layering in this formation has recently been subject to a more detailed study involving isotopic dating and phenocryst mineralogy (Noble, et al. 2003). The recent work shows that this formation is not a single stratigraphic unit, as presented in the INGEMMET study, but rather it consists of two layers separated by 16 Ma.
Figure 4-18. The lower section of the Castillo de Callalli is known as "Cabeza de León". Evidence of an LIP pukara was encountered on the summit [A03-935].
The lower part of Castillo de Callalli is adjacent to the main road to Callalli and follows the Llapa River. The Cueva de Quelkata, a rockshelter with a predominantly Terminal Archaic component (Chávez 1978) that was dynamited by the Majes Project road construction, is at the base of this formation. This lower section has well developed columnar jointing and is a densely welded, devitrified ash-flow tuff, and K-Ar dating on phenocrystic hornblende indicates that this lower flow is 20.7±0.6 Ma (Noble, et al. 2003: 33). The upper section is described as "partly welded vapor phase crystallized tuff with the physical characteristics of the distal part of an outflow sheet". Phenocrystic sanidine from this section yielded an40Ar-39Ar age determination of 4.72±0.02 Ma (Noble, et al. 2003: 35). The study suggests that the upper part of the Castillo de Callalli formation is associated with the Cailloma caldera to the north which erupted three times during the Pliocene. Further discussion of Miocene volcanism in the Orcopampa area of the Chila cordillera, between the Chivay and Alca obsidian sources, can be found in Swanson's (1998) geology dissertation.