Salar de Uyuni, Bolivia

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Salar di Uyuni, showing extent of salt crust and a number of water-filled brine sinks (dolines) about the salar edge (ojo del diablo)

The Salar of Uyuni (or Salar de Tunapa), at an elevation of more than 3600 meters above sea level in the central Bolivian Altiplano, is an active ephemeral saline lake (salt-crusted saline pan) where substantial salt thicknesses have accumulated across an area of a little less than 10,000 km2, making it the world’s largest salt-filled saline pan. It contains significant volumes of lithium-enriched brine.

A 121 m deep well, drilled in the central salar, intersected a complex evaporitic sequence of 12 saline pan intervals separated by 11 mud layers deposited mostly as saline mudflats or sabkhas. The thick saline pan beds are composed of stacked salt crusts and alternate with thinner mud units in the lower half of the well. The mud beds show distinct lacustrine features and thicken upwards as salt beds thin. The well indicates a long-term hydrology where tectonics and climate controlled the level of the regional watertable (see Warren, 2016; Chapter 3 for detailed literature compilation of the salar geology and tectonics).

Many parts of the lake surface about the lake edge still show evidence of salt dissolution and crust recycling. Circular holes appear in the lake crust, especially about the salar edge; surface expression of the holes can be a few tens of centimetres wide, but they are typically underlain by brine-filled swallow holes opening out into salt cavities that are metres deep and wide. It makes driving on some parts of the salar edge quite hazardous. The local name for these small holes at the surface is “Eye of the Devil” (Ojo del Diablo). Local karst development about the edge of a bedded salt mass is an early diagenetic overprint common to many modern and ancient saltpans. It is driven by changes in the lake watertable and the encroachment of regions of less saturated water into the margins of the bedded salt.

The main fluvial feed to the salar is from the Rio Grande, which enters the salar depression in its southwestern part. There it has built up a 300-km2 delta, characterised by a lens of fresher water and sediment that intertongues with the halite crust unit (Figure). The delta itself is a complex of silt and clay with local sand lenses indicating fluvial channel fills. Clays are mostly smectite with some illite and minor kaolinite. Overall, the delta prism is characterised by low porosity and permeability. Paludal muddy lacustrine sediments underlie both the delta prism and the halite crust facies. The contact between the delta wedge and the halite unit is a brine interface and the watertable of the delta prism today lies close to the surface. 


Rio Grande Delta in southern Salar de Uyuni. A) Scaled satellite image downloaded as Bing® image and mounted in MapInfo®. B) Stratigraphic cross section and associated B-Li content of shallow brine in the southwestern portion of Salar di Uyuni in the Rio Grande delta. Arrows indicate brine flow, Brine chemistry shows a peak in both B and Li at the chemical interface between halite saturated lake brines and the delta prism, but the ulexite precipitates by capillary evaporation at the brine table interface beneath the delta prism (after Risacher and Fritz, 1991). See Figure 3.40 for location of delta.

The shallow water table converts much of the delta prism, in areas situated some distance from the fluvial channels, into an evaporitic mudflat, where capillary evaporation is driving fluid crossflow. This, in turn, stimulates a slow circulation in the underlying mixed groundwaters waters and so precipitates calcite, gypsum and ulexite (NaCaB5O9.8H2O) toward the edge of the delta prism (Figure). This has created one of the most significant borate accumulations in Bolivia, where the ulexite interval in the delta prism has an average thickness of 0.1 m over a 120 km2 area with a water content of 50% (Risacher and Fritz, 1991). Total boron reserves in this area are estimated to be around 1.6 x 106 tons.

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Flat nature of the salt surface (varies less than 1 m across the lake surface) and is underlain by stacked salt crusts. The monotony of the salt surface is indictive of the horizontal nature of the underlying water table and the growth of the crust via capillary evaporation

The thick saline pan beds making up the salar fill are composed of stacked salt crusts and alternate with thinner mud units in the lower half of the well. The mud beds show prominent lacustrine features and thicken upwards as salt beds thin. A 121 m long well drilled into the salar indicates a longterm hydrology where tectonics, as well as climate, controlled the level of the regional watertable.

Bromine contents of the recovered pan halites range from 1.3 to 10.4 mg/kg (Figure). Such low values mean the various halite beds were not precipitated by simple evaporation of meteoric and surface waters. This would be indicated by Br contents of tens of mg/kg in the pan halites. Rather the brines were created by the dissolution of older evaporites, which subcrop in the drainage basin.

Halite from one such nearby gypsum-capped diapir has a very low Br content of 2 mg/kg, strongly suggesting that most of the halite deposited at Salar di Uyuni originated from the leaching of similar diapiric salt. The ubiquity of this salt source is indicated by the numerous gypsum-crested salt diapirs, which still outcrop in the Altiplano (Figure). 


Salar de Uyuni, Andean Altiplano. A) Locality map showing hydrological relations with drainages of other salars and lakes of the Altiplano as well as its proximity to diapirs. The dissolution of diapir salts has sourced much of the salt accumulating in the Salar de Uyuni. B) Lithology and Br profiles from a 121 metre well in Salar de Uyuni (after Risacher and Fritz, 2000).

The very low and reasonably constant Br content (1.6-2.3 mg/kg) of the lower salt beds, along with an abundance of detrital minerals, suggest the deeper and thicker halite beds were deposited as saline pan intervals in ephemeral lakes (Figure). Thereafter, perennial water episodes of increasing duration flooded the central Altiplano. This change is tied to an increase in the volume of dilute meteoric waters entering the Uyuni drainage basin and in the proportion of mud beds in the salar core. These waters had lower Cl/Br- ratios, indicated by an increase in Br to 6-10 mg/l in the pan halite beds between 10 and 40 m depth (Figure). The change may have also coincided with the exhaustion of diapiric halite as a source of ions, perhaps related to a decrease in the intensity of tectonism that was driving diapiric salt to the surface. Paleolake levels in the central Altiplano rose at this time, while those in the northern Altiplano (the Titicaca basin) were simultaneously falling. This reflects progressive fluvial erosion of the threshold between the northern and the central Altiplano, which lowered the levels of the northern lakes and allowed more dilute waters with lower Cl/Br- ratio to flood the central Altiplano and so move into Salar de Uyuni. The three uppermost salt crusts show a decrease in bromine content, which may reflect a recent modification to the inflow regime. The salt of the uppermost saltpan layer began to accumulate around 10,000 years ago and continues today. The halite shows a bromine distribution that is low in the shallow parts of the bed (≈ 2 mg/l) but unexpectedly increases with depth to values over 8-10 mg/l. This increase is thought to indicate an episode of exposure and recycling of earlier formed marginward salt crust into the current brines (Figure). 

Regionally, the history of the Salar is associated with a sequential transformation between several vast lakes (Figure A). Some 30,000 to 42,000 years ago, the area was part of a Pleistocene mega-lake, Lake Minchin. With the climatic changes tied to the end of the Pleistocene, Lake Minchin transformed into paleo-lake Tauca, with a maximum water depth of 140 meters, and an estimated age of 13,000 to 18,000 or 14,900 to 26,100 years. The youngest paleolake was Coipasa, which is radiocarbon dated at 11,500 to 13,400 years ago. as paleolake Coipasa shrank, it left behind two modern perennial lakes, Poopó and Uru Uru, and two major salt flats, Salar de Coipasa and the larger Salar de Uyuni, which is roughly 100 times the size of the Bonneville Salt Flats in the USA. Today, Lake Poopó is a neighbor of the much larger Lake Titicaca and during the wet season, Titicaca overflows and discharges into Poopó, which in turn, can flood Salar De Coipasa and Salar de Uyuni

As paleo-lake Uyuni dried some 10,000 years ago, the resulting salt crust was initially distributed over a wider area than the crust of the present Uyuni pan and it covered the topographic surface of the salar depression up to the level of the lake where halite brines first reached saturation. Ongoing lowering of the Holocene watertable and the associated dissolution of the higher parts of the salt crust about the more marginward parts of the pan depression, recycled ions (including Br) from crusts about the margins of the lake into the brines feeding the lowered brine levels located further toward the salar centre. Ongoing recycling explains the progressively reduced Br contents in successive pan crusts in the more central parts of the salar. Clearly, the increasing aridity and lowered watertable that typifies the current episode of saline pan aggradation has cannibalised earlier versions of itself as ongoing longterm drying of the lake raised marginward crusts above the salar watertable. Many parts of the lake surface about the lake edge still show evidence of ongoing salt dissolution and crust recycling
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Merged dissolution dolines (ojo del diablo) indicate an inflow of fresher water about the edges of the salar
Pit near edge of salar, dug in the Uyuni salt crust, showing a widespread underlying brine-filled karst layer (ojo el diablo). 

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