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Deposit Type
Very particular conditions allowed the formation of the caliche deposits in the Atacama Desert. Such deposits have been poorly studied, as compared with more ubiquitous deposit types. Soil development in the Atacama is almost unique on Earth, because it is one of the few places where soils primarily contain large concentrations of nitrates, as well as significant concentrations of perchlorate, iodate, and phosphate salts. Only in the Antarctic have relatively similar soils been identified. The presence of the corresponding anions is mainly derived from photochemical reactions in the atmosphere, and their accumulation in the Atacama is the result of the nearby presence of a major volcanic chain, the extreme aridity and the stability of landscape surfaces.
Currently there is little agreement regarding the timing for the initiation of hyperarid climatic conditions in the Atacama Desert of northern Chile. Some argue that the Atacama became the current hyperarid desert around 15 million years ago, and that this desiccation is the result of Andean uplift to high elevations. However, others argue that the onset of hyperaridity in the Atacama commenced around 4 million years ago and is related to global climate cooling and not to Andean uplift.
The Chilean nitrate beds were formed due to the deposition of nitrates and other saline constituents in salars and ephemeral lakes in the present Atacama Desert. Windblown salts from these playa-type deposits were deposited in a belt along the western side of the Atacama Desert depression over the past 10 to 15 million years. Some iodine and saline constituents were also deposited from ocean fogs and rainfall from the west. In spite of the fact that the average annual rainfall is nominally less than 1 mm, with as much as 20 years between rain showers, the dominant land-forms of the Atacama Desert resulted from water erosion. Infrequent rainwater leaching and shallow deposition in the alluvial fan sediments built up shallow concentrations of saline minerals. Recurrent deliquescence and leaching has continued to redistribute the iodine and saline minerals deeper and/or downslope with time. However, economic concentrations of iodine and nitrate minerals are probably restricted to areas that are either (1) capped with an impermeable layer or (2) raised and well-drained, so that there are little or no descending fluids to dissolve and disperse them. This could explain the lateral limitation of the commercial-grade iodine and saline mineralization.
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