Stop 22: Tocanao Quarry

There is a 30-40 m thick exposure of the Tocanao ignimbrite, 4.49 Ma the first phase of the Atana Formation at the Toconao quebrada. The quarry has become a national monument, and they now charge 1500/person and do not welcome geologists without permission. It is necessary to go to the Tocanao city official’s office with passports and verification of employment (business card) showing that you are a geologist in order to receive permission. All communication is done in Spanish.

The Tocanao ignimbrite within the quebrada contains a relatively crystal poor (< 10%) and lithic-poor matrix. Devitrified pumice clasts are randomly distributed throughout roughly 30% of the deposit and show distinctive elongated, tube-like vesicles and spherulites as evidence for devitrification. The pumice and the matrix contain copper-colored biotites and plagioclase as well as lithic enclaves from an older ignimbrite unit, possibly the Pujsa dated at 5.6 Ma.

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The top of the flow has a cemented sillar texture that is quarried as construction material known locally as “Piedra Blanca”. This vapor-phase altered texture was created by fusion processes that thermally indurated pieces of ash together and by the escape of hot gases through the conduit that form the elongated vesicles.

At the Toconao quebrada the ignimbrite is in contact with the underlying plinian fall deposit. The fall is mantling the surface of the underlying Pujsa ignimbrite, 5.6 Ma and has also incorporated some lithics from the Pujsa. The flow deposit as well as the fall layer are related to the La Pacana caldera that is roughly 20-30 km from this location. It is possible that both the Tocanao member and the subsequent Atana ignimbrite were the result of the eruption from the La Pacana caldera.

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You can drive down from the quarry itself and into the national monument, which is essentially a canyon comprised of ignimbrite. There is a nice trail that leads you along the edge the canyon along the deposit. In the distance you are able to see the Purico ignimbrite directly overlying the Toconao. The Atana/Talabre ignimbrites are not exposed here, which makes this the most distal part of the Purico deposit. Also, as you continue up the trail, you can see the Pujsa exposed beneath the Tocanao. If you take the trail down the canyon, it leads you to a river and beautifully lush vegetation.