Virtual Altiplano

As you continue to drive east on the international road toward Argentina, the Atana ignimbrite changes its dip direction. Its dominant dip direction was west and it suddenly dips to the east, suggesting that the road crosses over the La Pacana caldera.

There is an obvious exposure along the international road that is a great indroduction to the deposits that dominate the northeast region of Chile. When looking to the south, the lowermost unit is the Atana ignimbrite, 4.09 Ma. Above the Atana is a fall deposit that is overlain by the Tara ignimbrite, 3.68 Ma. The Filo Delgado ignimbrite is the uppermost unit exposed.

The Filo Delgado ignimbrite is younger than Purico (1.35 Ma) and is the youngest unit of the La Pacana caldera. It is exposed on the east side of the La Pacana caldera and is stratigraphically related to the Upper Purico that is exposed on the west side of the caldera.

The Filo Delgado is a thin crystal-poor unit that contains welded pumices and distinctive crystal-poor, obsidian fiamme. The ignimbrite is characterized as lenticulite, because the matrix is crystal rich but the obsidian-like lenses have minimal crystal content. Also the entire ignimbrite is welded and has a blocky texture.