Salar de Punta Negra

The Salar de Punta Negra lies in the eastern Antofagasta Region of Chile, which is an important source of copper and nitrate for the country.^[1] Before 1981, when a large mine opened in the area, access to the region was difficult.^[2] The name refers to a black lava flow on the eastern side of Salar de Punta Negra that was erupted during the Late Pleistocene.^[3]

The Salar has a surface of 230 square kilometres (89 sq mi),^[1] and is a playa with a polygonal clayey-salty surface that is in some places uplifted and occasional canals and ponds filled with saltwater.^[4] The salt deposits consist mainly of gypsum, halite and ulexite,^[5] they form crenulated rims^[6] and pressure ridges in some points.^[7] The playa appears to be somewhat asymmetric, lower in the northwest, probably a consequence of faulting.^[8]

Salar de Punta Negra lies at the centre of a converging drainage network^[9] and is surrounded by a bajada which often becomes steep where it meets the playa and is itself crisscrossed by channels that originate in dry valleys at the top of the bayada. Of these dry valleys, four of these in the Western Cordillera carry water seasonally^[10] and are known as Quebrada Zorros, Quebrada Zorritos, Quebrada Tocomar (or Toconar^[11]) and Quebrada Llullaillaco. These are often subject to flash floods which occasionally reach the playa surface in the form of mudflows.^[12] Additional valleys are the Quebrada El Salto and the Quebrada El Salado.^[13] The Río Frio enters the Salar from the south and gets its water from the Cordillera Domeyko.^[14] Most of the drainages do not carry water all the way to Salar de Punta Negra; the water seeps underground before reaching the Salar.^[15] Finally, fault-controlled springs such as Barrancas Blancas and water seeps at the eastern margin of Salar de Punta Negra discharge water.^[11]

It is part of a 77 kilometres (48 mi) long and 30 kilometres (19 mi) wide basin between the Western Cordillera with its volcanoes and the Cordillera Domeyko^[16] which has formed through tectonic processes in the Mesozoic and Cenozoic^[17] including block faulting and was then filled by Cenozoic deposits.^[10] In the Western Cordillera, the volcano Llullaillaco reaches a height of 6,725 metres (22,064 ft) while the average elevation of the depression amounts to 3,000 metres (9,800 ft);^[1] Llullaillaco is one of the highest volcanoes in the world.^[11] The basin has a surface area of about 4,263 square kilometres (1,646 sq mi);^[18] to the east of Salar de Punta Negra lie the Pampa El Salado, the Pampa San Eulogio and the Pampa del Chino.^[19] North of Punta Negra lies the smaller Salar de Imilac.^[15]

It was once believed that Salar de Punta Negra at one time contained a large lake, 55 kilometres (34 mi) long, 20 kilometres (12 mi) wide^[2] and 125 metres (410 ft) deep at least.^[20] Traces of the former lake such as lacustrine sediments, river deltas, shorelines and lacustrine terraces have been found.^[21] During the latest Pleistocene-Holocene, the Quebrada de las Zorras conveyed water from the mountains around Llullaillaco to Salar de Punta Negra.^[22] The lake eventually overflowed first into the Salar de Imilac farther north and then through the Quebrada de Agua Colorada into the Salar de Atacama;^[23] archeological findings have been made in proximity to the overflow pathways.^[20]

Later research indicated that there is no firm evidence of such a lake such as either lacustrine sediments^[24] or former shorelines, with supposed shorelines being instead berms formed by subsidence and faults. Likewise, typical fine sediments associated with water are only found on the eastern side of Salar de Punta Negra, where springs are active. The absence of a lake in Salar de Punta Negra is consistent with the fact that paleolakes with clear shorelines in the region only occur at elevations of over 3,500 metres (11,500 ft).^[25] Farther south, the Salar de Aguas Calientes and the Salar de Pajonales feature clear evidence of former lakes.^[26]

The Central Andes in Chile consist of five separate geological domains. From east to west these are the Western Cordillera with active volcanoes, the Pre-Andean Depression which contains a number of Tertiary basins such as the Salar de Atacama and the Salar de Punta Negra, the Cordillera de Domeyko, the Central Depression and finally the Cordillera de la Costa. With the exception of the Salar de Atacama, the geology of the Pre-Andean Depression is usually poorly known, as most geological research is focused on the eastern side of the Andes where oil reserves are suspected and on the copper-bearing domains of the western Andes.^[27]

The area is largely covered by Cenozoic sediments and volcanic rocks, but in outcrops a number of older formations can be observed: Devonian-Carboniferous marine deposits (Zorritas Formation), pyroclastic rocks (La Tabla Formation),^[16] both of which reach thicknesses of over 2 kilometres (1.2 mi),^[19] 300-280 million years old plutons, the Triassic sediment-volcanic Sierra Guanaco and the fluvial-lacustrine Sierra de Varas and the marine Rhaetian-Jurassic Profeta Formation.^[28] Finally, there are Paleogene deposits of mostly sedimentary or volcanic origin such as the Naranja Formation and the Pampa de Mulas Formation.^[29] Large fault systems such as the Escondida-Punta Negra fault delimit the Salar de Punta Negra basin to the west and the east; some eastern faults offset recent deposits.^[30]