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Regional Geology

The Andean Highlands are underlain principally by volcanic flows, tuffs and breccias from Pliocene to Holocene in age. The older volcanic rocks are chiefly of volcanic composition, predominantly and esitic composition with lesser basalt and rhyolite, and are thought to have erupted from fissures and volcanic centres. The younger volcanic rocks are chiefly and esitic to basaltic and are associated mainly with volcanic cones or strata-volcanoes. There are an estimated 800 volcanoes in northern Chile, more than 30 of which are in excess of 5800 metres in elevation. Locally ridges and hills of Jurassic to Cretaceous age, comprising sedimentary and volcanic rocks as well as Cretaceous plutonic rocks rise above the younger terrain. Deep valleys that drain westward across the front of the Andes also expose underlying Jurassic and Cretaceous age rock. As most of the Andes in northern Chile are within the region of interior drainages, most of the erosional products of the region have remained within the closed basins. Sedimentary basin fill includes alluvium, lake and playa sediments, salars, wind-blown soil and volcanic ash. Many of the basins in the Andean Highlands appear to be a result of faulting. The eastern boundaries of Salar de la Isla for example exhibit distinctly linear sides, possibly as a result of faulting. Many of the Andean Highlands salares are also asymmetrically situated within their basins as a result of faulting and tilting.

The geology of the northern Chilean salares is described by Stoertz and Ericksen (1974) who described the region as having an exceptional variety of types of hard saline crusts that are rare in other deserts.

Six general types of saline crusts are described:

1. Layered massive rock salt with a rugged surface
2. Slabby or nodular silty rock salt
3. Rugged gypsum or anhydrite
4. Massive coarsely crystalline rock salt
5. Smooth rock salt
6. Silty, nitrate-bearing saline crust

Most of the lagoons sampled during the recent property visit are shallow (less than 25 cm in depth) and are underlain by a smooth salt crust. Rugged gypsum beds were noted on the east margin of Salar de Agua Amarga but were not noted elsewhere. The salares are largely filled with detrital material derived from weathering of the surrounding highlands.