The Atacama Desert forms one of the major hyper-arid deserts of the world and it is known to be the driest place on Earth. The extreme aridity of the climate and unusual mineralogy of the regolith in the Atacama region is of considerable interest because of its value as an analogue to the Martian surface. The sedimentary succession in the Atacama Desert records deposition under an arid to semiarid climate from the late Jurassic (150Ma) to the present day (Hartley, 2005). Comparison with other long-lived deserts such as the Namib, Sahara and Australia deserts suggests that the Atacama is also the oldest extant desert. Arid environments account for 30% of the global land surface, a figure that is increasing annually owing to desertification (Hartley, 2002). The present day location of the Atacama Desert within the dry subtropical climate belt is the principal cause of aridity (Fig. 1). This extreme aridity has resulted in a number of usual and unique features. These include the very low rates of erosion and accumulation of a range of unusual salts, including perchlorates, iodates, and nitrates in the soils as well as the more common halite, gypsum and anhydrite (Clarke, 2005). To understand the history of aridity in the Atacama Desert and its relevance to arid zone morphogenesis, regolith formation, and supergene mineralization, it is vital to approach this subject from a whole regolith perspective. Knowing about these approaches is critical for effective mineral exploration in the area because it hosts many of the world’s most significant porphyry copper-gold ore deposits (e.g. Chichicamata, the largest open pit copper mine in the world).
Sedimentology of the Atacama Desert
Miocene to Pliocene sedimentary rocks in northern Chile occur within two fault-bounded basins, the Central Depression (including an eastward extension, the Calama basin) and the Preandean Depression (Fig. 2) (Hartley and Chong, 2002).
The north-trending Central Depression extends throughout northern Chile into Southern Peru and it is separated from the Coastal Cordillera to the west by the Atacama fault zone. Coastal Cordillera is characterized by high relief on all scales and active faulting along the Atacama Fault. This high relief contrasts with the smooth, duricrusted surfaces further inland.
There are a series of transpressional faults within the eastern margin that separate the depression from the Precordillera. The Preandean Depression consists of a series of endorheic basins developed between the eastern, thrust-bounded margin of the Precordillera and the Miocene to Holocene volcanic arc of the High Andes (Hartley and Chong, 2002). Sedimentation was continuous throughout the Miocene to the present in the Preandean Depression. From the Oligocene to early Miocene, fluvial, alluvial-fan and playa-lake sediments were deposited under a semiarid climate.
History of Aridity
The primary source of data on the history of aridity in the Atacama Desert is in the mostly terrestrial sediments of the region (Clarke, 2005). Table 1 shows the most relevant sedimentological and stratigraphical data. Stratigraphic Units
Unnamed units in Salar
Evaporites and clastics
Evaporitic lakes (such as
Salar de Atacama)
Hilaricos and Soledad
Anhydrite, halite, gypsum
Continental playa lake San Pedro Group
Mudstone, gypsum, anhydrite,
Continental evaporitic lake
andesitic volcanics, gypsum,
Westerly derived alluvial
fans feeding into playa lakes
limestones, gypsum, halite casts.
Coastal saline lagoons
Limestone, sandstone, calcareous and bituminous shale, anhydrite,
Marine to evaporitic
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