The Pastos Grandes Caldera, measuring roughly 60km N-S and 30km E-W, is the source of 2 known ignimbrite-forming eruptions. The first eruption occured approximatley 5.45 million years ago, erupting the Chuhuilla Ignimbrite. The second eruption occured approximately 2.89 million years ago, erupting the Pastos Grandes Ignimbrite. The ignimbrites account for 1200 km3 and 1500 km3 total volumes respectively. A third ignimbrite, the Alota ignimbite, was erupted from the Cerro Juvina ignimbrite shield to the northeast of the caldera. The goals for this field season were to map the relationship between the ignimbrite and the eastern scarp (outlined in orange) of the caldera (1), determine the relationship between the ignimbrites and Cerro Chuhuilla, a large dome on the northeastern scarp (2), collect samples from the post resurgent domes inside the caldera (3), map the resurgent dome (4), and become familiar with the intracaldera vs outflow facies of the 2 ignimbrites (5). The overall goal of the field work is to map the caldera in detail to identify eruptions of all scales sourced from the complex.
The major features of the Pastos Grandes caldera system are highlighted above. The eastern extent of the caldera is traced in orange and the routes through the area are traced in blue (Pastos Grandes Caldera Loop), yellow (Alota/Villa Mar Road), and red (Cerro Chuhuilla Road). Note that in many cases the roads had to be left behind to forge new paths. for a more Numbers denote the location of the each of the stops.
A view of the outflow facies of the Pastos Grandes Ignimbrite (top) and the Chuhuilla Ignimbrite (bottom) with the Alota Ignimbrite, erupted from nearby Cerro Juvina at 5.23 Ma, in between. This stratigraphic sequence is common on the northeastern flanks of the caldera.
A view northeast from the resurgent dome to the eastern scarp of the caldera. The distance is nearly 17.5km covering the entire width of the eastern moat. Intracaldera Pastos Grandes Ignimbrite covers the base of the scarp in the distance.
The towns of Alota and Villa Mar served as our base camps when looking at outcrops along the collapse scarp. Both towns are quite small, but had more than enough aminities to keep us comfortable. Although, at that point the only thing we needed was a bed!
The streets of Alota had the feel of the set of a western movie.....
and our hostal was a little crowded from time to time.
The first goal of the field season was to determine the relationship between the scarp itself and the Pastos Grandes Ignimbrite. Questions regarding the area revolve around the age of the scarp. Is the scarp a remnant of the Chuhuilla Ignimbrite eruption, or is it the age of Pastos Grandes? In the field, the goal was simply to determine the contact relationship; the exact age of the scarp will be determined by age dating techniques in the lab. An excellent contact relationship, shown below. can be found just west of the town of Villa Mar.
In the Pastos Grandes Ignimbrite, pumice is rare but found to have phenocrysts quartz, plagioclase, biotite, and sanidine (Salisbury et al., 2010). The intracaldera facies commonly contains dense juveniles, and fiamme.
A view West towards the collapse scarp. From this vantage point the scarp lavas can be seen through a small canyon cut in the Pastos Grandes Ignimbrite.
Looking North at the scarp lavas which are covered by the outflow of the Pastos Grandes Ignimbrite. A yellow line highlights the contact.
Looking East from the collapse scarp through a valley cut in the Pastos Grandes Ignimbrite. From here the entire outflow facies is visible (all 100m in this location).
Back at the hostal in Villa Mar with the Pastos Grandes Ignimbrite looming overhead. (Yes that is a plane on top of the ignimbrite)
The second goal was very similar to the first. It is known that Cerro Chuhuilla is of roughly the same age as the Chuhuilla Ignimbrite. However, was all of Cerro Chuhuilla erupted at the same time? Was activity here continuous from the time of the Chuhuilla Ignimbrite to the eruption of the Pastos Grandes Ignimbrite? Again, contact relationships were determined in the field, with precise dating of the lavas to be done back in the lab. Multiple lavas were discovered on Cerro Chuhuilla; it was decided that much more detailed mapping will be needed in future field seasons.
In the Chuhuilla Ignimbrite, the pumice is crystal rich with large purple quartz (>5mm) and smaller (≤5mm) biotite. The pumice also contains dense crystal rich enclaves of euhedral plagioclase, quartz, biotite, and amphibole (Salisbury et al., 2010.)
From the eastern moat of the caldera, Cerro Chuhuilla (right) and Cerro Chulucani (left) can be seen to the North.
From the eastern flanks of Cerro Chuhuilla, the Chuhuilla ignimbrite can be seen sprawling out towards the town of Alota, nearly due East of the dome. Cerro Chuhuilla is not the source of the Chuhuilla Ignimbrite, rather it is part of the collapse scarp of the caldera that may date back to the eruption of the ignimbrite at 5.45Ma.
The third goal of the season was to collect samples of all of the Post-Resurgent Domes for age dating and chemical analysis. Very general mapping was done in the area; however, as with Cerro Chuhuilla, it was discovered that much more detailed mapping will be needed in future field seasons.
A map of the post resurgent lava domes and the resurgent dome structure. Smaller post resurgent domes (PRD) are on the flanks of the resurgent dome structure. The collapse scarp (orange line) can be seen North and East of Laguna Pastos Grandes.
Above and below are views of the lava domes in the middle of the Pastos Grandes caldera, as viewed from the West and northwest respectively. Laguna Pastos Grandes, in the forground of each photo, is a popular tourist attraction in the area.
Cerro Chascon is a part of the Chascon-Runtu Jarita Complex, which is a group of 10 lava domes dated at ~85ka. The northern domes are rhyodacites while to the south domes become more rhyolitic. Plagioclase and hornblende are common to all of the domes, yet sanidine, quartz, and biotite are also seen in the northern domes, while clinopyroxene is only present in the southern domes.
The fourth goal involved one very long day, and a very long hike, up and around the resurgent dome of the caldera. The idea was to get as familiar as possible with the facies associated with the dome and the structures associated with the collapse.
Above is a map of the resurgent dome, some of the lava domes are visible to the northwest. The red line indicates the placement of the cross sectional view below.
An East-West profile of the resurgent dome. The small flat block in the middle is likely a remnant feature from prior to the collapse of the dome. The small hump the the East is a post-resurgent dome named Sombrieta.
Views of the resurgent dome collapse from the East. On the left, the small flat block (sombriacita) is visible. On the right is a look at the internal structure of the dome. Lava flows that likely post-date collapse can be seen draping the collapse features.
Looking southeast from the resurgent dome to Volcan Uturuncu over 80km away. Uturuncu is an active strato volcano in the back-arc of this region. We met a lot of geologists and geophysicists working on and around Volcan Uturuncu.
While working on the resurgent dome we made it over the 5000m elevation mark! The peak of the resurgent dome is over 5700m (18,700ft) elevation.
The fifth goal of the season was quite simple in comparison to the previous days. The idea was to become more familiar with the intra caldera facies of the Pastos Grandes Ignimbrite, looking for any variation in the unit with proximity to the scarp. The results were quite boring, as the facies seems to be pretty unifom along the inner flanks of the caldera scarp.
The well-indurated intracaldera Pastos Grandes Ignimbrite contains dense juveniles and fiamme (flattened glassy pumice).
Looking West into Chile, across the western moat of the caldera which is completely filled with intracaldera Pastos Grandes ignimbrite.