The basement beneath the chain is composed from Ordovician marine sediments that were later deformed by the Acadian orogeny. The La Pacana caldera is buried beneath Puntas Negras. Both the 3.2 ± 0.3 Patao ignimbrite and the 2.4 ± 0.4 mya Pampa Chamaca or Tuyajto ignimbrite from La Pacana may have been erupted from beneath Puntas Negras. the Tuyajto ignimbrite is 530,000 ± 170,000 years old.