Saline Quaternary Continental Lakes

Lake Eyre

Lake Eyre is an ephemeral playa-salt lake that lies asymmetrically in the southern portion of one of the largest internal drainage basins in the world (1.14 million km2) and covers one-sixth of the Australian continent (3.53). Drainage topography is flat, with extensive areas of sandy and stony deserts. There are regions with some relief near the boundaries of the drainage basin, but only 30% of the area is elevated more than 250 meters above sea level. All streams flowing into Lake Eyre are characterized by extreme variations in discharge and flow duration. The lowest part of the lake bed is some 15 meters below sea level.

 

Mean annual runoff across the basin is 4 km3, equivalent to a water layer 3.5 mm in depth, this is the lowest runoff level of any major drainage system in the world. The Lake Eyre Basin is large but dry, its drainage spans a number of climatic zones, including the souther edge of a tropical monsoon system to the north and the mid-latitude westerly circulation belt to the south. Some 5 x 105 km2 of the drainage basin receives less than 150 mm of rainfall per year (on average). The highest rainfall, with annual averages around 400 mm, occur in the northern and eastern margins, where rainfall comes from the edge of the monsoon belt.

 

That is,  northern end of the Lake Eyre catchment lies barely on the periphery of the planetary monsoon system. The Northern Australian monsoon is erratic both in space and time. Recent lake floods are related to La Nina phases of the El Nino-Southern Oscillation (ENSO) and are out of phase with the Pacific Dry Zone rainfall and El Nino events. Changes in the intensity of the monsoon have controlled the frequency and permanency of Lake Eyre waters throughout much of the Quaternary (see Warren, 2016; Chapter 3 for detailed literature compilation of the lake geology).

 

Lake Eyre, South Australia. Image on left taken on January 1, 2000 at a time prior to a major flooding event. Partly cloud-obscured image on right taken on April 6, 2000, after a major monsoon-fed flood influx into lake from northern part of lake. The elongate interdunal sabkhas and pans in the northern part of the image constitute the Kallakoopah Pans. Note the water-filled Warburton Channel transecting the northern lake floor (Landsat 7 satellite images acquired and processed by ACRES).

Salar di Uyuni, Central Altiplano, Bolivia, showing salt is dominated by white halite crust with Rio Grande fan delta in southwest (delta region indicated by green rectangle; Bing® image automounted and scaled in MapInfo®).

Salar di Uyuni, Bolivia

The Salar of Uyuni, 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.

 

A 121 m deep well, drilled in the central salar, intersected a complex evaporitic sequence made up 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 obvious lacustrine features and thicken upwards as salt beds thin. The well indicates a longterm hydrology where tectonics, as well as climate, controlled the level of the regional watertable (see Warren, 2016; Chapter 3 for detailed literature compilation of the salar geology and tectonics)

 

Sua (Sowa) Pan, Makgadikgadi Basin, Botswana

Deepest of a three pan complex separated by dune covered ridges. Pan surfaces are made up of brine-saturated sand and clay layers, with surface efflorescence of trona and halite. The main pan is seasonally flooded by Nata River and has a trona, brine processing plant that commenced operations in 1991. Salt-cake is a possible byproduct when prices make it economic. The pan produces ~ 300 kty of soda-ash and 600 kty of halite for South African market. (see Warren, 2016; Chapter 12 for a more detailed literature compilation)

 

Currently, Sua Pan is a seasonal salt lake or saline pan; it fills with water during the summer rainy season and retains water until April or May.

Salar de Atacama, Chile. A) Saline pan facies associated with the influx of the San Pedro River along the northeast side of the salar, the sulphate-rich marginal zone along the east and northeast of the salar and the salt-crusted halite facies dominating the southern portions of the salar. B) Laguna de la Piedra in the gypsum flat. Shows circular doline features indicating these zones of perennial brine ponding are karst associated. C) Laguna Tebiniquiche, cuspate outline of perennial water body indicates dissolution karst feature. D) some of the potash/lithium pans on the halite nucleus as are the brinefield wells and pipelines that feed the pans. Image scaled in MapInfo from a Bing® upload.

Salar di Atacama, Chile

Salar de Atacama is the lowest drainage sink in an endorheic basin in the pre-Andean depression of the Atacama Desert and contains more than a kilometre of interlayered halite and siliciclastics. At an elevation of 2,300 m above sea level with an area of 3,000 km2, it is the third largest saline pan in the world and an excellent barometer of Pleistocene climate change in the southern hemisphere.

 

Today the San Pedro River forms an ephemeral stream feed along the western margin of the northern sector of the salar and defines the edge of a saline mudflat dominated by sulphate, carbonate and siliciclastic sediments. The mudflat surface is dominated by brown halite-rich crusts and is underlain by silty clays with sand, displacive halite and locally organic-rich mud. A “sulphate marginal zone” dominated by ephemeral streams, eolian dune fields, sand sheets, dry mudflats and spring-fed ponds and marshes, forms the eastern edge of the salar and lies to the east of the saline mudflat . This northern region captures the majority of the siliciclastics entering Salar de Atacama.

Typically, the watertable is between tens of centimeters and several meters below the deflationary salar surface. Uppermost sulphate zone sediments are typically covered by, and cemented with, efflorescent sugary gypsum and minor halite. When trenched these gypsum crusts form discontinuous bowl-shaped concave-upward layers tens of centimeters in diameter.  A deep core (No. 2005) through the halite nucleus facies recovered some 100 meters of evaporitic sediment and sampled some 100,000 years of salar sedimentation. Deposition was dominated by arid conditions similar to today, with two significant wetter intervals. From 100 m to 62.8 m the core is composed of clear interlocking halite with patches of millimetre-scale sugary halite. These textures are near identical to those forming at the surface today and indicate dry mostly subaerial accumulations. From 62.8-47.0 m (75.7-60.7 ka) there is a vertical succession passing from mudflat gypsum to mudflat/shallow pond to subaerial halite and mudflat deposits that represent a progressive increase, then a decrease and finally an increase in water supply to Salar de Atacama. This occurred within what were moderately wetter conditions (saline lakes and saline mudflats) than the subaerial conditions of today. A similar somewhat wetter interval occurred from 53.4 to 15.3 ka, with the wettest perennial lake interval from 26.7 to 16.5 ka. Short relatively wet periods (chevron halite) also occurred in the Holocene from 11.4 to 10.2 ka and from 6.2 to 3.5 ka. The remaining sections are dominated by halite textures similar to the subaerial dominated capillary fringe conditions of today (see Warren, 2016; Chapter 3 for detailed literature compilation).

Chaganur Salt Lake, China

The Chaganur (Chagonnor) soda ash  lake deposit, discovered in 1966, is located in the central part of the Inner Mongolia Autonomous Region, some 80 km southeast of Erlianhot. At an elevation of 1000 meters, it is a lacustrine sump in the lowest part of an arid deflation basin, situated at an erg edge, in the cool arid steppes (BSk) of the Inner Mongolian plateau. It is made up of a smaller northeastern sub-basin (East Chaganur) and a larger southwestern sub-basin (West Chaganur) joined by a narrow channel. Two intermittent rivers discharge into the lake, one from the northeast and the other from the southeast. The lake is also fed by direct precipitation. Chagannur is the largest of a series of 16 alkaline salt lakes in the region (e.g. Hushunur, Wulannur, Huguonur, Halefushuyingnur, Muyingnur, Wulannur, Wenduobunur, South Chaganur) that are relicts of a formerly more extensive  palaeolake Chagannur.

 

At its maximum extent, which is thought to have been during the early to mid- Holocene, this palaeolake occupied an area of some 2,640 km2. The total area of the various lakes in the basin today is ~ 156 km2 and the area

Lake Chaganur a Quaternary trona lake, Inner Mongolia autonomous region, China.

of the current catchment is 2,800 km2. The basin, which is controlled by two faults running SW-NE, is a tectonic sump with a Cretaceous bedrock of sandstone and mudstone. Annual evaporation today is ten times that of precipitation. The natural soda beds in the Chaganur lake accumulated during climatically-induced shrinkage events, occupy an area of 21 km2 and are under-lain by some 20 m of sandy clay. The targeted deposit consists of nine soda beds with interstitial hypersaline brines that are interlayered with black muds (see Warren, 2016; Chapter 12 for detailed literature compilation).

Lake Dabuxum, China

The Qarhan Playa sump entrains nine perennial salt lakes: Seni, Dabiele, Xiaobiele, Daxi, Dabuxun (Dabsan Hu), Tuanjie, Xiezuo and Fubuxum north and south lakes. Dabuxum, which occupies the central part of the Qarhan sump, is the largest of the perennial lakes (~184 km2). Lake water depths vary seasonally from 20cm to 1m and are never deeper than a metre, even when flooded. Salt contents vary from 165 to 360 g/l, with pH ranging between 5.4 and 7.85.

 

Today the salt plain and pans of the Qahan playa are fed mostly from runoff from the Kunlun Mountains (Kunlun Shan), with a number of saline groundwater springs concentrated along the fault that defines an area of salt karst along the northern edge of the sump, especially north of Xiezuo Lake.

 

 

Bedded and displacive salts began to accumulate in the Qarhan depression some 50,000 years ago. Today, the surface salt crust consists of a chaotic mixture of fine-grained halite crystals and mud, with a rugged, pitted upper surface. Vadose diagenetic features, such as dissolution pits, cavities and pendant cements, form where the salt crust lies above the watertable. Interbedded salts and siliciclastic sediments underlie the crust to reach thicknesses of up to 75m .

 

Bedded potash, as carnallite, precipitates naturally in transient volumetrically-minor lake strandzone (stratoid) beds about the northeastern margin of Lake Dabuxum and as cements in Late Pleistocene bedded deposits in nearby Lake Tanje. Ongoing freshened sheetflow from the updip fans means the proportion of carnallite versus halite in the unit increases with distance from the Golmud Fan in both the layered (bedded) and stratoid (cement) modes of occurrence. At times in the past, when the watertable was lower, meteoric inflow was also the driver for the brine cycling that created the karst cavities hosting the halite and carnallite cements. Solid potash salts are not present in sufficient amounts to be quarried and most of the exploited potash resource resides in interstitial brines that are pumped and processed using solar ponds (see Warren, 2016; Chapter 12 for detailed literature compilation).

Lake Zabuye, China

Lake Zabuye, is located some 1000 km west of Lhasa, the Tibetan capital, and lies in the ET Köppen high-altitude tundra zone of the Tibetan Plateau. The lake is geologically interesting as the lithium content of the lake waters are so elevated that it is the only known lacustrine location in the world where lithium carbonate, zabuyelite, is a natural brine precipitate. When artificially concentrated, the crystallization sequence of primary salts from a Zabuye lake brine at 25°C is :

halite (NaCl) --> aphthitalite (3K2SO4·Na2SO4) --> zabuyelite (Li2CO3) --> sylvite (KCl) --> trona (Na2CO3·NaHCO3·2H2O) and thermonatrite (Na2CO3·H2O)

 

The lake’s brine is naturally supersaturated with NaCl and other salts, so millions of metric tons of halite, potash, trona, and other minerals have accumulated on the bottom of the lake in the past few thousand years .

 

Zabuye Lake is of significant economic value as it is a new type of exploited saline lacustrine deposit, which contains lithium and borate salts in addition to significant volumes of potash, halite, natron and Glauber’s salt. Lake waters also retain elevated levels of caesium, rubidium and bromine. Lake levels can vary by meters each year; in 2008 the water level was some 4422 m above sea level. At this level, the lake’s area is approximately 247 km2 and its salinity varies from 360 to 440 ‰, depending on seasonal differences in water input and evaporation rate. Lithium carbonate and sylvite precipitate in the lake, via a natural combination of brine concentration and cooling, but can also be induced in a processed-brine by an addition of soda ash (see Warren, 2016; Chapter 12 for detailed literature compilation).

Wadi el Natrun depression, Egypt

 

Natrun is found in large quantities in the beds of several Egyptian playa lakes in the Wadi el Natrun depression (e.g. Lake Natrun, and El Kab in the upper Nile, as well as Behiera in the nearby Libyan desert) and has been mined and traded from these localities for thousands of years. Writings as old as the reign of Rameses III (1198-1166 B.C.) refer to these deposits.

 

Lakes in Wadi el Natron depression have pH values of 8.5–9.5 and a salinities from 283 to 540 g/L. The main ionic components are sulphate, chloride, carbonate and sodium. Traces of magnesium were also present. The water of the lakes is of the Cl to SO4−Cl type. Increased Cl in Wadi El Natrun brines can increase metal solubility in the brine due to the formation of soluble chloro-complexes of trace elements. The metal concentrations in the brine decrease in the order: Pb>Cu>Cd>Ni>Zn >Fe>Mn.

 

The natron beds are also contain burkeite, gaylussite, trona, halite, northupite, pirsonite, thenardite in various lakes across the depression. In the  lowest lake in the Natrun chain there is a massive 1-m-thick thenardite bed located below the 0.5 m halite bed. All lakes are spring-fed depressions, with an ultimate sourced in Nile waters (see Warren, 2016; Chapter 12 for detailed literature compilation)

Gavkhoni Playa, Iran

The Gavkhoni playa, with an area around 550 km2, is located to the southeast of Esfahan in an intramontane basin in central Iran. Water enters this endoheic basin via the Zayandeh river, an ephemeral river outflow in the north, as well as via some small steams, ground water discharges and direct precipitation. The weather is cold in winter when temperatures decreases to around -7°C, but in mid summer the playa floor is hot and temperatures rise to about +42°C. Evaporation (about 3265 mm) exceeds precipitation (about 80 mm)

 

A few small sand dunes, covered by gypsiferous marl, occur in the surrounding saline sand flat. A soft, low-relief thin efflorescence of salt crust covers its surface. This salt crust includes a higher proportion of underlying sediment and adhesive aeolian sand and often exhibits an irregular network of puffy surface salts. The puffy ground is ephemeral and usually changes season to season. A few carbonate mounds (tufa) are found in the west of the sand dunes. Acicular gypsum is usually present as wavy layers, interbedded with aeolian sand layers along the eastern side of the sand dunes. Gypsum layers are soft and friable, not more than a few centimeters thick.

 

When the salt surface is exposed, the halite layers become buckled, break into a polygonal crust, and petee structures are formed. The polygonal pattern might be understood as a result of fracturing by volume reduction caused by either thermal contraction or desiccation. The buckling was caused by a net volume increase due to expansion during crystallization. The continued growth of the halite crystals just beneath the dry surface of the pan causes the lateral expansion of the surface crust, and leads to disruption of the crust into large polygons rimmed, by pressure ridges that over-ride each other. Pumping of subsurface brine, and subsequent evaporation along the cracks between the polygons, leads to precipitation of a spongy efflorescent halite  and rare ephemeral bitterns that can include tachyhydrite.

Lake Magadi

Lake Magadi lies within Kenya, slightly to the north of Lake Natron and  at the bottom of a steep-sided valley, which is the lowest point in the eastern or Gregory Rift Valley. The lake extends roughly 20 km N-S and is up to 6 km wide. Both lakes lie at an altitude of some 600 m above sea level and are surrounded by plateaus and active natrocarbonatite  volcanoes, reaching to more than 3,000 m asl. With an area of only 90 km2, Lake Magadi is one of the most saline, but also is one of the smallest, alkaline lake sumps in the Rift Valley.

 

There is  no perennial stream flow into Lake Magadi, but it has numerous warm surface springs feeding saline brines to the perennial lagoon portions of the Magadi depression. Trona accumulates in the central portions as flat pavements composed of stacked saline pan crusts. The Lake Magadi trona pan is some 74 km2 in area and is 7–50 m thick, known locally as the “Evaporite Series.” It is made up of cm-scale stacked trona-detrital couplets . Light-colored trona layers are composed of rosettes and splays of upward pointing, growth- aligned, trona crystals. Thin bands of finer trona, colored by dark, mainly windblown dust separate the crust layers from each other. Even as it stacks and dissolves, each layer retains some porosity between interlocking growth-aligned crystal splays.

 

Trona saturation is achieved when the shrinking brine sheets are 2–10 cm deep. At that stage the trona prisms grow upward from the pan surface as cm-scale aligned elongate blades and splays. Interlocking of the crystal splays give the accreting trona sheet an inherent strength that allows it to support expansion polygons or pressure ridges, with saucers up to several tens of meters wide and ridges up to a metre high. Ridging is the result of pavement overthrusting, driven by the sideways expansion of trona layers jostling for space in the various growing crystal pavements. The resultant cracking and thrusting of the layers cuts the trona pavement up into a series of overthrust-edged saucers, with small bright pink brine pools in the centre of each polygon; the color is the result of a haloarchaeal bloom (see Warren, 2016; Chapters 3 and 12 for detailed literature compilation)

 

Lake Magadi. A) Geological setting of Lake Magadi. The well position locates the core in the High Magadi beds.  B) Aerial view looks north and shows dark coloured moat facies of perennial lagoon defining lake edge.

Issyk Kul, Kyrgyzstan

Issyk-Kul (also Ysyk Köl, Issyk-Kol) is an endorheic lake in the northern Tian Shan mountains in eastern Kyrgyzstan. It is the tenth largest lake in the world by volume (though not in surface area), and the second largest saline lake after the Caspian Sea. Issyk-Kul means "warm lake" in the Kyrgyz language; although it is surrounded by snow-capped peaks, it never freezes.

 

Issyk-Kul Lake is also the second largest perennial mountain lake in the world, behind Lake Titicaca in South America. It lies at an altitude of 1,607 meters with waters reaching 668 meters in depth. Some 118 rivers and streams flow into the lake; the largest are the Djyrgalan and Tyup rivers. It is also fed by springs, including many hot springs, and snow melt.  The lake's southern shore is dominated by the Teskey Ala-Too Range of the Tian Shan mountains. The Kungey Alatau of the Tian Shan runs parallel to the north shore.

 

Like Lake  Van in Turkey,  it is located in a foreland depression created by the Neogene collision of India with Eurasia. Its intermontane situation in an adiabatic shadow explains its arid cool steppe Koepen climatic setting (Bsk). The lake has no current outlet, but some hydrologists hypothesize that lake-fed groundwaters water seep into the Chu River. The bottom of the lake contains the mineral monohydrocalcite. Currently,  lake  salinity is approx. 6,000 ppm— compared to 35,000 ppm salinity of typical seawater— and, although the lake level is still currently some 8 meters (26 ft) higher than in medieval times, its level now drops around 5 cm per year due to water diversion for agricultural use.

 

 

 

Dead Sea, Middle East

The Dead Sea water surface defines what is the deepest continental position (−415 m asl) on the earth’s current terrestrial surface. It is our only modern example where bedded evaporitic sediments accumulating on the floor of a brine body, where water depths are measured in hundreds of meters. This salt-encrusted depression is 80 km long and 20 km wide, has an area of 810 km2, is covered by a brine volume of 147 km3 and occupies the lowest part of a drain-age basin with a catchment area of 40,650 km3 .

 

However, falling water levels in the past few decades mean the permanent water mass now only occupies the northern part of the lake, while saline anthropogenic pans occupy the southern basin so that the current perennial “Sea” is now only some 50 km long. Rainfall in the region is 45–90 mm, evaporation around 1,500 mm, and air temperatures between 11 and 21°C in winter and 18–40°C in summer, with a recorded maximum of 51°C. The subsiding basin is surrounded by mountain ranges to the east and west, producing an orographic rain shadow that further emphasizes the aridity of the adjacent desert.

 

So, until recently, the basin floor was physiographically divisible into two contiguous and permanent brine-covered water bodies, the Northern and Southern basins, largely separated by a diapir-cored shallow sill, the Lisan Ridge and joined at the Lisan Straits. Top of Miocene salt is 100–120 m below the surface in the Lisan Peninsula and a few meters below the surface at Mt. Sedom. The land surface of the Lisan Peninsula is slowly rising and being karstified, at a rise rate of a few mm per year, driven by salt flow. Widespread at-surface evidence of this rise is swamped by the changing of the depositional and hydrological system, driven by fluctuating lake water levels. Continuing falling water levels since the 1930s means that today the North Basin is the only permanent natural water mass, with waters some 320 m deep. The Southern Basin is today a saline pan/mudflat, which would be a subaerially exposed plain, except that brine-filled saltworks now cover much of the former Southern Basin lake. Perennial brine sheets are artificially maintained in these pans by continually pumping of brines from the Northern Basin on both the Israeli and Jordanian sides of the Dead Sea (for more detail refer to Warren, 2016, Chapter 4)

Sabkha Matti, Middle East

Sabkha Matti is one of the largest tectonically-induced sabkha depressions in the region. There, capillary salts (gypsum and halite) are growing in inland sabkha zones, where shallow water-tables occur across large expanses hosted within deflationary eolian sand sheets. Coast-parallel supratidal salt flats of the marine margin of the sabkha grade southwards into inland flats of the continental eolian sabkha with no distinct break in surface topography. The inland sabkha (BWh), with an area of 2,950 km2 , extends southwards for about 150 km and, in this distance, the land surface rarely rises more than 40–60 m above sea level. There is little information available on rainfall for the inland areas of Sabkha Matti, but measurements taken on the coast show that average annual rainfall is less than 40 mm and further inland it is less.

 

Like the Rann of Kutch, Sabkha Matti occupies a former fluvial valley, which in the Miocene was a fresh water river system. Today this region of saline mudflats is a groundwater sump to the Qatar Arch. It also defines the margin to the modern accretionary eolian sabkha sheet and so is adjacent to a set of dry mudflats and sand sheets. This in turn passes downwind into an active sand sea (Rub Al Khali portion of the Arabian Erg) that is ultimately piling up sand at the foot of the Oman Mountains

 

Evaporation of occasional ephemeral brine sheets and reform the salt crust, which for most of the year covers large areas of Sabkha Matti. Somewhat thicker salt crusts (1 ± 5 cm), with large megapolygonal pressure ridges characterize the Sabkha Matti surface situated within 5–10 km of the modern coast. Further inland the Sabkha Matti surface is characterized by thinner salt crusts (≈1 cm) and smaller, lower-relief, pressure ridges, along with more numerous blisters and petees interacting with windblown sediments to form characteristic haloturbated "tiger-skin" structures.

 

Umm as Samim, Oman

Umm as Samim (“Mother of Worries”) sabkha is fed sediment from dunes to the west, alluvium from the Oman Mountains to the east, and groundwater from the limestone aquifers of the Tertiary Umm Er Radhuma Formation to the east and south. At around 3,000 km2 in area (55 m elevation), Umm as Samim is one of the largest sabkhas on the Arabian Peninsula. It forms a groundwater sump along the boundary of the eastern edge of the Arabian erg and the alluvial fans to the east, with many widyan draining from Al Hajar (Oman Mountains). Wadi Aswad, Wadi Musallim, Wadi Majhul, Wadi Umayri and Wadi Haliban all periodically feed the Umm as Samim sump.

 

Units hosting Holocene capillary salts at Umm as Samim are typically less than a few meters thick. Today, the water source is dominated by alluvial seeps, but prior to 6,000 years ago it was a much less saline hydrology. Then the sump was a perennial lake. The initial closed sabkha depression or playa formed via deflation in the gypsiferous ‘Fars Group’ sometime prior to 30 ka. During the Late Pleistocene, from 30 to 20 ka, this closed depression became a saline lake, and fluvial flow brought in detrital silt and clay. A drier climate prevailed from 20 to 15 ka and the lake changed to a sabkha. Wetter conditions returned towards the end of the Pleistocene, from 15 to 12 ka, and a perennial saline lake was re-established, whereby halite was dissolved but gypsum still remained. In the interval from 12 to 9 ka, sabkha conditions were again established with deposition of displacive gypsum and minor halite in a sandy matrix. During the Early Holocene wet phase, from 10 to 5.5 ka, a saline lake formed again, only to terminate in the arid capillary gypsum/halite conditions from 5.5 ka to the present-day.

 

Chott el Djerid, Tunisia

Chott el Djerid in southern Tunisia is an endorheic playa where ephemeral carnallite accumulates as efflorescences and rare intercrystalline cements in the uppermost parts of ephemeral halite crusts in lowermost, more central portions of the saline pan facies.  The main depression has an elongated arm (Chott Fejej) that stretches eastward toward f Gabes. To the north, beyond the sands flats are the low-relief alluvial fans of the Atlas foothills. To the south of Chott el Djerid is the transition into the Great Eastern Erg of the Saharan Desert, the outer edges of which are characterized by low-relief, isolated sand mounds that mostly lack well-defined slip faces and are separated by interdunal sabkha.

 

The chott, with an elevation typically more than 10 m below sea level on the salt-flats, has an area of approximately 5,360 km2 in a drainage basin with an area of 10,500 km2. Mean annual rainfall for the area is 80–140 mm, the mean annual temperature is 21 °C and evaporation, which is highest between May and September, has a mean annual value of 1,500 mm.

 

 

Chott el Djerid is the largest in a zone of chotts formed where a series of regional aquifers emerge to create a region of discharge playas along the northeast extremity of the Bas Saharan Artesian Basin. It is a groundwater sump region in a tectonically quiescent area of the North African craton. The encompassing chott-hosting depression extends over much of the Saharan desert into Morocco and Libya and encloses the northern end of the Grand Erg Oriental.

 

The lowest and more central part of the deflationary depression that is Chott el Djerid is now a halite pan outlined by a thin ephemeral halite crust atop Quaternary clays and sand aquifers that host saline pore waters. It is surrounded by a gypsum mudflat, which in turn pass marginward into a broad evaporitic mudflat and dry sandflat. Much of the mudflat surface surrounding the halite-encrusted portion is covered by gypsum petees and adhesion ripples. Parts of the sandflat are transitional into aeolian-overprinted sediments with characteristic wind-blown sand streaks, wind-rippled sand sheets mounds and shadow dunes, all of which are cross cut by wadi channels that feed surface water into the central depression during times of flood. Like the interior salt lake basins along the southern edge of the Great Artesian Basin of Australia, the chotts are deflationary rather than accretionary depressions. They have lost something like 15–20 m of late Quaternary fill via eolian deflation to the adjacent Sahara desert erg.

Badwater Pan, Death Valley, USA

Death Valley, California, is probably the best known playa of the Basin and Range setting in the USA, yet Death Valley is a very steep-sided and unusual lacustrine depression. Its floor is some 86 m below sea level, making it the lowest  continental locality in the Western Hemisphere.  In its current form it is a saline pan rather than a perennial halite accumulating lake, but in the Late Pleistocene it was a deep perennial system. Although well studied, the volume of salts accumulating in Death Valley is relatively minor in terms of the total volume of salt in saline lacustrine settings of the world.

 

Its steep sides mean it can become a perennial saline lake with relatively minor changes in climate. In many areas of Death Valley the coarse-grained debris flows of the alluvial fan directly abut the salt crusts of the sabkha and the saline pan. Sandflats and dry mudflats, so obvious in most Basin and Range playas, are poorly developed or absent in Death Valley. This too reflects the steep valley walls and its relatively high watertable in relation to the lake surface. It is a rapidly subsiding saline lacustrine system in which more than 50% of the Quaternary sediment fill beneath the lowest parts of the lake floor, as at Badwater Pan, are dominated by saline lake and pan halite rather than matrix-rich evaporitic mudflat sediment  (sabkha).

 

Core intervals through saline pan sediments at Badwater are composed of inter-bedded halite, chaotic muddy halite, and mud. The halite contains abundant vertical dissolution pipes, cemented with clear halite, indicating the watertable periodically dropped

below the surface of the stacking halite crusts. These sediments record repeated flooding by dilute waters, dissolution of subaerially exposed surface salt crusts, deposition of mud from suspension, precipitation of halite during the saline lake phase, and cementation by diagenetic halite. Such deposits document a relatively wet climate with a high ratio of water inflow to evaporation compared to today.

 

Ostracods in calcitc mud layers represent the least saline, deepest lake phases when, based on the position of beach deposits in the valley walls, water depths in the deepest part of the lake were as much as 90 m above the current water table. Halite layers are made of fine-grained cumulates and clear, vertically oriented crystals precipitated during shallower, perennial lake stages. The near complete absence of syndepositional dissolution textures in saline minerals precipitated in the perennial saline lake interval, imply that the accumulating salts were permanently protected from dissolution by an overlying perennially-saline at times holomictic water column. (see Warren, 2016, Chapter 3 for detailed literature synthesis)

 

Salton Sea, USA

The Salton Trough is a continental rift zone at the head of the Gulf of California, It defines the transition from the offshore divergent tectonics of the East Pacific Rise to the onshore strike-slip dextral tectonics of the San Andreas Fault System. Infilling of the rift by the bi-polar Colorado River delta for the past 4 Ma has isolated the northern part of the trough, forming the closed Salton Basin in an orographic desert.

 

The current Salton Sea was created anthropogenically in 1905 when a canal, designed to bring irrigation water from the Colorado River into the Imperial Valley, was breached during spring floods. The entire volume of this large river was then channeled into the Salton Sink, which at the time was an evaporitic mudflat/pan with a surface more than 8 0 m below sea level. It was 2 years before the breach was plugged and the river diverted back to its natural course, but, by that time the Salton Sink had become the Salton Sea, a huge saline inland lake extending over several hundred square kilometers. Much of the salinity was the result of dissolution of the salt crust that previously occupied the lower portions of the sink.

 

Although this transformation was anthropogenic, the underlying Quaternary fill preserves a similar set of pan to saline lake alternations. High evaporation rates combined with an extremely variable hydrologic budget resulted in episodic infill by saline lake and lacustrine evaporite sediments.

In the modern Salton Sea there is a strong correlation between winds that overturn the stratified lake waters and significant fish and barnacle kills due to inimical waters (low oxygen, high H2S and ammonia) rising into the upper water column. Following death, fish carcasses float to the lake surface and deposit and accumulate along the Salton shore, which retains wave washed sediments that are rich in disarticulated fish and single bone and scale elements, along with a significant component of barnacle shells.

 

Within the Salton Sea geothermal system, rapid subsidence and rift-related magmatic intrusions at relatively shallow depth make up an arc of five Quaternary rhyolite domes. Related convective heating has flushed Plio-Pleistocene and younger sediments (fluvial and lacustrine deltaic matrices in a rift fill up to 1–4.5 km thick) with hypersaline chloride- and metal-rich brines, containing up to 26 wt% of dissolved solids. Seeps of mud, gas (CO2) and dominantly polyaromatic hydrocarbons are common in this geothermal field, where wells drilled for geothermal power record temperatures of up to 350°C at depths ranging from 1,500 to 2,500 m. Hydrocarbons appear to originate from hydrothermal heating rather than thermal maturation of organic compounds via burial. (See Warren 2016, Chapter 14 for further discussion of the Salton Sea geothermal system and its importance in improved understanding of base metal fluids)

Great Salt Lake, USA

Great Salt Lake, Utah, is a perennial water body and has been so for the last 30,000 years. It is the largest saline lake in the United States and one of the largest perennial saline lakes in the world, measuring some 120 km long and 55 km wide, with a water surface elevation that oscillates around 1,280 m above sea level. At this elevation the lake brines cover an area of 4,180 km2 and have a maximum depth of 10 m. In a typical year the water surface fluctuates around 30–60 cm, reaching its highest level in May-July (after snow melt) and dropping to its lowest level in October-November. Since 1847 the water level of Great Salt Lake has varied from a low of 1,277.5 m in 1963 to a high of 1,283.75 m in 1986–87. The 3.7-m rise in the level of the Great Salt Lake between 1982 and 1986, leading to the record historical high in lake water level, was at least partly related to the record rainfall and snowfall in its catchment during the 1982/3 El Niño event. Because the lake is so shallow and its floor so flat, small changes in water level correspond to large migrations of the strandline.

 

Great Salt Lake lies in an endorheic basin with no surface outlet. The main rivers entering the lake are the Bear River from the north, the Weber/Ogden River from the east, and the Jordan River from the south (Jones et al. 2009). Together with small volumes of bajada runoff, the rivers supply around two thirds of the water currently entering the lake. Direct precipitation into the lake supplies some 31 %,

while ground water supplies the remaining 3 %. The drainage basin of the lake today covers an area of about 57,000 km2. Water is lost from the lake mostly through evaporation, with something like 3,500 km3 of water evaporating annually. When inflow equals evaporation, the level of the lake remains constant. If inflow is greater or less than evaporation, the level of the lake will rise or fall, respectively.

Being situated along the Wasatch Front at the eastern edge of the tectonically active Basin and Range Province, means the size, shape, and location of the Great Salt Lake depression has been a response to variable intensities of subsidence and faulting, overprinted by the intense climatic vagaries of the Late Pleistocene. It currently straddles BSk and Cfa Köppen climate zones. Present-day Great Salt Lake is a remnant of a much larger and fresher water body that was once linked into the Pleistocene Lake Bonneville system, as were Utah Lake, Sevier Lake, and the Bonneville Salt Flats. Areas of perennial fluvial inflow into Pleistocene Great Salt Lake are defined by classic “bird’s-foot” or “Gilbert-style” hanging deltas. The downcutting of the current hydrology and the presence of Pleistocene shoreline terraces on higher levels of the delta prism suggest deltas were most active during Pleistocene lake full stages (Lake Bonneville time) when the lake was not accumulating evaporite salts.

 

Modern strandzone deposits are mostly shorezone spits and bars, which are dominated by aragonitic ooid and peloidal sands (mostly brine shrimp pellets). Carbonates also precipitate about the lake strandline in spring mounds, algal mounds, and in stromatolites. Older algal reef mounds are located further out in the lake and are now buried by lake muds; they grew earlier in the Holocene when lake water levels were lower than today. Oolite shoals today form sand waves that define the lake strandzone. Individual ooids are aragonite with a radial crystal structure. When and how the individual ooids grew is not well understood, but halobacteria and organic matrices are thought to play a role. Laminated mudstones, in large part composed of the faecal pellets of brine shrimp, are currently accumulating further out in the lake. The pellets are a mixture of aragonite and detrital matter, mostly quartz, clays and organics. The brine shrimp Artemia gracilis thrives in the lake, it is a filter feeder that flourishes in spring and summer. It supplies a seasonal rain of faecal pellets to the lake bottom and its droppings represent a sampling of the mineral matter suspended in the water column. This implies aragonite is a seasonal precipitate within the upper part of the meromictic water column. Rippled sand lamina alternate with laminar mud in the deeper water deposits of Great Salt Lake, with many deeper beds disrupted by the growth of now-dissolved evaporites

Marine Margin Salinas and Sabkhas

Lake Macleod, Australia

Lake Macleod, West Australia, is a halite-filled salina some 120 km long and 40 km wide, lying just to the north of Shark Bay. Climate is arid, with average rainfall around 230-260 mm and annual potential pan evaporation in excess of 2,600 mm (BWh Köppen Climate zone). The current saltflat surface of much of Lake Macleod still lies about a metre below sea level indicating evaporative drawdown and a groundwater feed from the nearby Indian Ocean.

 

Some parts of the lake depression, especially near the marine seeps, are still covered with semipermanent water sheets. In its northern part, and across the current lake surface, the lake fill is a Holocene gypsarenite unit; it is as much as 5 meters thick along the northern and western (seaward) sides of the lake. To the south and beneath the laminated gypsum unit is a widespread halite bed up to 7 m thick. It is made up of stacked halite crusts, dominated by growth-aligned chevrons and cornets. Brines from this halite unit in the south are pumped into saltwork ponds to produce halite for the chemical industries of southeast Asia. Gypsum was dredged in the central part of the lake and mostly used for wallboard manufacture. Both the gypsum and the halite units overlie a thin unit of Holocene lagoonal carbonate deposited prior to the depression’s transition from marine embayment to coastal seepage lake.

 

Lake Macleod never possessed a surface (hydrographic) connection to the ocean at any time in its salt-accumulating Holocene history. Seawater seeps onto the subsealevel lake floor via a series of marginward carbonate-precipitating spring pavements and beds, fed from fractures and caves in the underlying and surrounding calcareous coastal dune aquifer. As the escaping water exits the carbonate-dune aquifer it degasses and concentrates to salinities where it precipitates seepage or strandzone carbonates, including aragonite pisolites and tepees, especially about seeps along the eastern and northern edges of the Macleod salina.

Today, the northern part of the lake is a perennial seepage pond, called Ibis Pond, where the perennial inflow water gathers in ponds that are over a metre deep. When the water first escapes into the pond it has normal to near normal marine salinities and supports a restricted population of halotolerant marine gastropods and small fish. A similar marine biota can be found in marine seepage ponds on the seaward side of gypsum-filled Lake Macdonnell of coastal South Australia. Spring-fed normal marine waters are thought to have supplied the Lake Macleod brine lake for the last 5,000 years, ever since the Australasian Sea rose to a level that was within a metre or two of its present level. At that time a series of laterally accreting beach ridges cut off the lake’s southern surface connection to the Indian Ocean (5,100 years ago). Since then, evaporative drawdown has pulled seawater into the lake depression and bedded salts have accumulated (see Warren 2016, Chapter 4 for more detail).

Lake Macdonnell, Australia

Lake Macdonnell, near the head of the Great Australia Bight in South Australia, is a smaller marine seepage salina compared to Lake Macleod, but with a similar Quaternary coastal carbonate dune-hosted marine seepage hydrology. It has an area of 451 km2, a 10 m-thick fill of Holocene bedded gypsum and is located in a milder BSk setting, compared to Lake Macleod.

 

Even so, annually, the Lake Macdonnell operation is quarrying more than 1.4 Mt of Holocene coarsely-crystalline near-pure gypsum and  producing more that 35,000 t of salt via by pan evaporation of lake brines. It is the largest gypsum mine in Australia. Gypsum is quarried by first pumping down the water level then using bulldozers and excavation equipment. The gypsum product is stockpiled for several years to allow halite to leach driven by low levels of natural rainfall. It is then loaded onto trains using front-end loaders and taken to port

 

Like Lake Macleod, it is currently evaporating seawater via a seepage-fed  drawdown hydrology. Yet, it has not had a hydrographic connection to the ocean for more than 4,000 years (see Warren 2016, Chapter 4 for more detail).

Lake Asal, Djbouti

Lake Asal, with a water surface some 155 m below sea level, is the lowest place in Africa and defines the lowest part of the Afar rift. The lake is small (area 52 km2), endoheic and shallow, with a maximum water depth of 30 m and an average depth of 7.4 m. The Afar depression contains a series of lakes and salt pans at various depths below sea level: Lake Badda (−50 m), Lake Assale (−118 m), Lake Bakili (−120 m), Lake Afrera (−111 m), Lake Acori (−94 m) and the lowest Lake Asal (−155 m).

 

Regionally, the area is extremely arid with average rainfall below 100 mm/year and evaporation of about 5 m/year (Köppen climate – BWh). Locally, in the Asal area, mean annual rainfall in the area is 175–200 mm/year and evaporation exceeds 3.5 m/year. The lake depression lies in the NW-SE oriented Asal – Ghoubbat al Kharab (Ardoukôba) rift extending northeast into the Gulf of Aden.

 

In the vicinity of Asal, the rift is subaerially exposed over a 12 km distance between Lake Asal and the Red Sea and is part of a tectonic system considered to be one of two examples where an oceanic ridge outcrops; the other being on the island of Iceland. This part of the rift extends southwest into the East African rift and is at the marine-fed end of what is our type example (Afar triangle) of a spreading centre (triple junction) in an arid part of world’s rift systems.

 

Although small, Lake Asal is one of the few modern examples of a marine-fed rift basin that is actively accumulating bedded evaporites. As such, it is the only Quaternary analogue for ancient marine-fed rifts including the numerous Mesozoic evaporitic lacustrine basins that underlie passive margins on either side of the Atlantic.

The lake surround today is completely continental with no surface connection to the ocean. Yet the fact its −155 m asl water level has changed little in the last few centuries argues it is fed a steady flow of seawater through the highly fractured 10 km-wide volcanic ridge that separates oceanic water at Ghoubbat al Kharab from the hypersaline waters of Lake Asal. It constitutes an excellent example of a drawndown marine seepage basin fed through a volcanic ridge).

 

Over the longer time frame of the past 10,000 years, lake levels have changed drastically). Some 10,000 year ago Lake Asal was almost dry in the late Pleistocene. A highstand lake level at 160 m asl existed in the early Holocene, between 8.6 and 6 ka, and the lake at that time drained to the Ghoubbat el Kharab, across a shallow sill at an elevation of some 4 m bsl, to the Gulf of Aden, both as surface and subsurface flows. Subsequent to this time span, the lake level dropped considerably to 150 m below sealevel enabling at the first time subsurface seawater infiltration from the sea to the lake across the fractured divide. Since then, the lake water salinity is controlled, mainly, by seawater input and high evaporation, responsible also for Ca-sulfate and halite deposition.

Dallol Saline Pan, Danakhil, Ethiopia

The Danakil Depression (aka Dallol Depression) of Ethiopia and Eritrea is an area of intense hydrothermal activity with potash occurrences related to rift magmatism, marine flooding, and deep brine cycling. It is part of the Afar Triple Junction and is located in the axial zone of the Afar rift, near the confluence of the East African, Red Sea and Carlsberg rifts. The depression is the northern part of the Afar depression and runs SSE parallel to the Red Sea coast but is some 50–80 km inland and separated from the sea by the Danakil Mountains. It is 185 km long, up to 70 km wide, with a floor more 116 m below sea level in the deeper part of the depression.

 

 

The Danakil Depression widens to the south, beginning with a 10 km width in the north and widening up to 70 km in the south . The basin floor in the northern part is the deepest and has elevations as low as 50–120 m below sealevel (Fig. 11.14c). A shallow volcanotectonic barrier behind Mersa Fatma on the coast prevents hydro-graphic recharge, or even substantial seawater seepage, into the current depression from waters of Howakil Bay, which is some 50–60 km WNW of Ito Aichil on the Red Sea coast.

 

In terms of daily temperature the floor of the Danakil is one of the hottest places on earth; year round the daily te-perature is above 34 °C and in summer every day tops 40 °C, with some days topping 50 °C. These high temperatures and a lack of rainfall place it at the hyper-arid end of the world desert spectrum and make it a well known example of a BWh Köppen climate zone.

A halite-floored elongate saltpan, known as the Dallol Pan, covers the deepest part of the Danakil Depression over an area some 40 km long and 10 km wide). The pan’s position is asymmetric; it lies near the depression’s western edge, some 5 km from the foot of the escarpment to the Balakia Mountains and the Ethiopian Highlands, but some 50 km from the eastern margin of the depression and today constitutes the deepest continental drainage sump of the Afar depression. The area located northeast of the main modern salt pan, is mostly an extensive gypsum plain. It defines a higher less saline lacustrine episode in the basin fill.

 

Bedded Pleistocene evaporites may underlie the entire Danakil depression, but younger lava flows of the Aden Volcanic Series and alluvium washed in from the surrounding bajada obscure much of the older Pleistocene sedimentary series in southern part of the depression beyond Lake Assale. A well-defined set of reef outcrops atop the eastern bajada defines a Late Pleistocene marine episode. The region beneath the Dallol pan sediments is considered highly prospective for a variety of potash salts (see Salty Matters, articles 1, 2, 3, 4).

Sabkha Maradah, Libya

The sabkha (chott) occurs in a zone of shallow groundwater related to the underlying regional anticlinal structure. It is located at the downslope end of a number of oasis springs.

 

The saltflat did produce small volumes (~21,000 tonnes) of KCl from brine pits dug in 1939 and there is a published reserve for a 15-square km area within the sabkha (chott) of 1.6 million tonnes of KCl.

Oje di Liebr, Mexico

The region located south of Guerrero Negro, around the head of the Laguna Ojo de Liebre, was once the largest sabkha complex in North America. Several smaller sabkhas occur elsewhere on both coasts of Baja California. Beginning in 1957, a significant part of the natural sabkha at Laguna Ojo de Liebre has been covered with ponds for solar concentration and crystallization, in turn making this operation the world's larger producer of industrial halite.

 

The border between the upper lagoon and the sabkha is marked by a low beach ridge of sand, composed mainly of quartz, plagioclase, and hornblende, with varying amounts of carbonate shell fragments and oolites. Sands elsewhere in the lagoon contain pelletal apatite, but none are found at this beach. The beach ridge is breached by a number of tidal creeks, which are bordered by a carpet of Salicornia and Spartina that makes them show as black patches in satellite imagery.

 

Many daily tides and especially semi-monthly spring tides flood an area behind the beach ridge, dominated by black, rubbery mats of cyanobacteria. The cyanobacterial mats originally covered an area of about 100 km2. The mats entrain interstitial aragonite, high-magnesian calcite, dolomite, and gypsum. Beyond the algal mat area, the salt flats were covered with a hard crust of gypsum and halite. These salt  flats originally had an area of some 500 km2 The lagoon side of this area was flooded by most tides, with evaporation sufficient to produce a visible floating film of gypsum and halite crystals. The shoreward parts of the salt flats were beyond the reach of normal tides, but apparently storm surges and persistent onshore winds were able to drive a layer of lagoonal water to the farthest reaches of the flats. This intermittent flood, 'probably one or more times a year, was documented by swash marks composed of shells, seaweed and other flotsam that wound across the flats. The resulting deposit of salt and gypsum varies from a few millimeters thick at its lagoonward edge, to a maximum of 2.5 m. The wide-ranging survey in 1916 showed salt/

gypsum thicknesses generally in the range of 10 to 20 cm. The mineralogy of the evaporite deposit in the former sabkha included gypsum of various morphologies; halite, polyhalite, celestite, magnesite and basanite. The salts deposited as a succession of irregular layers 0.2-2 cm thick, made up of light-colored evaporite minerals alternating with darker layers of organic debris and windblown sand.

 

Abu Dhabi Sabkha, UAE

The sabkha-dominated At Taf coastal strip is approximately 320 km (200 miles) long and the coastal sabkha belt is up to 24 km (15 miles) wide; on average it has prograded 1.5 km (1 mile) every 1000 years. Seaward slopes on the sabkha are low and range from 1:3,000 to 1:4,000, this means whenever water a few centimeters deep arrives on the sabkha, the strandline on the brine sheet will have migrated kilometers.

 

Water temperatures on the open platform in the vicinity of Abu Dhabi Island range from 23° to 34°C and from 15° to 40°C in the inner lagoon. Shallow waters in the lagoon can experience daily changes of 10°C. Air temperatures on the adjacent mudflat can be as low as 5°C in the winter and as high as 50°C in summer, with average temperatures

ranging from 23° to 33°C. Sediment surface temperatures on the sabkha are even more variable, with reported values up to 60° to 80°C atop dark-coloured microbial mats. Temperatures a few centimeters below the sabkha surface range from 22° to 40°C.

 

In summer, the sabkha is  extremely humid, especially at night, when the humidity reaches 100%. Salinities in the open Gulf are 3 to 4‰ above normal open marine salinities. Where coastal islands restrict exchange with the open Gulf, as in the vicinity of Abu Dhabi Island, lagoon salinities range between 54 and 67‰, near double that of the open ocean.

 

Prevailing onshore winds influence evaporation levels on mudflats along the southern coast of the Arabian Gulf. In winter the intensity of the onshore winds increases as the Shamal blows seawater and sediment from the lagoon onto the mudflat. At other times offshore winds blow continental dune sand into the back-side of the sabkha. Winter Shamals occur from November through March and tend to follow cold fronts. Shamals tend to set in with great abruptness and force, often with wind speeds of 40–50 km/h and gusts up to 100 km/hr. Both summer and winter Shamals are known, those in summer being less powerful. This dominant northwest to southeast airflow is a result of orography. Sharply rising mountains lie to the east and north of the Gulf, while gently rising mountains lie to the west and southwest effectively funnelling the low-level airflow. The Shamal is an equally important sedimentological driver in southern Gulf sedimentation as  the Caribbean hurricanes to the Bahamian platform.

 

The sabkha is furnished with matrix sediment from two sources, carbonates from the lagoon and siliciclastics from the hinterland. Many earlier sabkha models tended to ignore the eolian input, yet some areas of the mainland side of the Abu Dhabi sabkha (e.g. Al Du’yybaya sabkha) have quartz silt and sand as the major matrix material and it is the dominant unconfined aquifer supplying meteoric water to the coastal plain. Rainfall along the sabkha coast averages 70 mm/year and is very irregular, in some years no rainfall is recorded. In the 6-year period from 1958 to 1964 the highest annual reading on the sabkha was 6.73 cm, the lowest 0.33 cm. Although infrequent, desert rains can be torrential, especially in coastal sabkhas near the foot of the Oman Mountains, such as at Rams, where flooded wadis typically carry continental sediments as sheet floods many kilometers into the sabkha. Rain floodwaters can sometimes cover portions of the sabkha for one to three months and so dissolve and recycle surface and nearsurface halite/bittern crusts, as occurred in late 2003 and 2009.

 

Physiographically, the Abu Dhabi and other sabkhas in the Gulf are characterized by billiard table flatness, a direct reflection of control by capillary wicking, fed from the underlying watertable. The sabkha surface is a Stokes surface. In general, the watertable control to any sabkha gives them one of the most leveled forms of any arid landscape feature. Below this flat surface the Abu Dhabi sabkha preserves an evaporitic mixed-carbonate-siliciclastic stratigraphy. Deposition began some 7,000 years ago when a rapid transgression flooded interdunal (eolianite) depressions with a veneer of marine-reworked, predominantly quartzose sediments. About 4,000 years ago the Arabian Gulf shoreline reached its maximum Holocene strandline and the edge of this flooding event is defined by a line of well preserved beach-ridge sediment sometimes called the “Evans line.” This was followed by pulsed progradation of the sabkha sequence to its present configuration. The Abu Dhabi sabkha and other sabkhas along the Trucial coast  lie in a a very well documented depositional setting.  Further information and a comprehensive literature summary can be found in Warren, 2016; Chapter 3.

Large saline interior isolated lakes (seaways)

Aral Sea, Kazakhstan

The Aral Sea is a major saline lake in Central Asia, located at the boundary between Uzbekistan and Kazakhstan, now independent states of the former USSR. Until the early 1960s, the lake surface level was stable at about 53.5 m above sea level. At the time, the lake was the fourth largest on Earth, with the volume about 1070 km3 and the area over 65000 km2.

 

Since 1961, the Aral Sea has been continuously shrinking due to a combination of natural and anthropogenic factors. The latter is associated with unsustainable irrigation and diversions of water from the lake’s tributary rivers, Amudarya and Syrdarya. Today, the total level drop is about 24 m, and the remaining water body of the Aral Sea amounts to only about 10% of its original volume.

 

Accordingly, the salinity of the once brackish lake increased by a factor of magnitude, and now exceeds 100 g/kg. Today, the residual lake consists of two separate parts, referred to as the Small Aral Sea and the Large Aral Sea (Fig. 1). The former makes up the northernmost part of the original  sea and separated from the main body of the lake in 1989. The Large Aral Sea in itself has almost split into two parts, namely, the shallow to ephemeral (>3 m deep) but broad eastern basin and the smaller but relatively deep western basin (nowadays ~39 m) . In summer, the eastern basin acts as a huge evaporation pond; therefore, the salinity of the eastern basin is, generally, much higher than that of the western lobe. The two basins exchange water and properties through a narrow(*1 km) but rather deep (*5 m) connecting channel.

 

As the salinity of the sea increased, the first precipitated compound was the calcium carbonate. For somewhat higher salinities, magnesium carbonate MgCO3 was also precipitated. The subsequent salinization led to precipitation of gypsum. which

started in the late 1980s when salinities exceeded 26–28 g/kg. Massive deposits of gypsum precipitated during the recent and ancient regressions of the Sea can be seen on and beneath the former lake bottom. Processes expected at even higher salinities than today include the precipitation of mirabilite Na2SO4 􀀁.10H2O, halite NaCl, glauberite CaSO4 􀀁.Na2SO4, and epsomite MgSO4.7H2O.

 

Prior to the desiccation onset in 1960, the autumn/winter deep convection was typically responsible for complete mixing and ventilation of the water column. Therefore, the entire column contained oxygen at high concentrations. No hydrogen sulphide content was ever reported for the pre-desiccation period, except, maybe, extremely rare and unconfirmed occurrences in limited deltaic areas with enhanced buoyancy-controlled stratification, although traces of H2S in Aral bottom sediments have been long known. This situation persisted through the initial stage of the desiccation in 1961–1991, but significantly changed between 1991 and 2002. By the early 2000s, enhanced density stratification had arisen, largely impeding vertical mixing and ventilation in the water bodies. As a consequence, the bottom portion of the water column turned anoxic and contained hydrogen sulphide.

Kora-Bogaz-Gol,Turkmenistan

 Kara Bogaz Gol lies in the cool arid steppes of Turkmenistan at the edge of the Caspian Sea with a brine surface some 29 meters below sea level. The waters of Karabogazgol are derived from the Caspian Sea via a narrow inlet that gives the Gulf its name. In the past 100 years water levels in Karabogazgol have been a metre or so less than the adjacent Caspian water surface, which has fluctuated between 25 and 29 meters below sea level.

 

For much of the Holocene, Karabogazgol (Bay) has been connected via a narrow 500m-wide inlet to the Caspian Sea, which flows water from the Caspian to Karabogazgol at a rate of 25-44 meters/minute. Natural inflow from the Caspian replaces water lost by evaporation from Karabogazgol (Bay) and when this happens the drawndown water level in the gulf is consistently 0.5-3 meters below that of the Caspian Sea. This makes it one of the few modern examples of a drawdown basin with a highly restricted hydrographic connection between one isolated seaway and another. It is sufficiently restricted to allow gypsarenite to accumulate today on the brine-covered lake bottom. Water level in the Caspian Sea levels have varied considerably over the 10,000 years and so Karabogazgol repeatedly lost its hydrographic connection with the Caspian whenever water levels fell below the entrance sill. In earlier times of cooler temperatures that allowed winter freezing of a perennial water body, beds some 3 to 8 metre-thick containing cryogenic sodium sulphate formed and are today composed of varying amounts of calcite-hydromagnesite-gypsum-halite-epsomite-mirabilite-glauberite.

In Turkmen, Karabogazgol means “lake of the black throat,” so named because the gulf is continually gulping down the waters of the Caspian. There is an interesting anthropogenic aspect to the natural loss of water from the Caspian to the Gulf and the periodic salting of Karabogazgol. Early last century (prior to 1929) the Caspian sealevel was around -26 masl and Karabogazgol was almost entirely water-filled (as it is today). The water-covered surface area was around 18,000 km2 and its maximum water depth was around 9 meters. The annual volume of water flowing through the inlet was high, somewhere between 18 to 26 km3 and the difference in level between waters of the Caspian and Karabogazgol was around 0.45 meters.

 

From 1929 until 1940 there was a 1.8 fall in the level of the Caspian Sea and by 1948 the Caspian level was -27.87 m asl. The volume of inflow to the Gulf fell to 12-14 km3/yr by the late 1940s and to 8 km3/yr in 1956. Differences in water level between Caspian and the Gulf increased to 3.17 m in 1947 and 3.80 m in 1955. By 1957-1959 the water-covered area in Karabogazgol had fallen to around 13,000 km2. By 1977 water level in the Caspian was -29.02 m asl, it had fallen fell by more than 3 meters since 1929 . The 1977 water surface in Karabogazgol was 4.5 meters lower than the Caspian and the strandline of southern Karabogazgol had retreated by 50-60 km in 50 years. This ongoing drop in water level in Karabogazgol since 1929 meant fisheries were threatened on the Caspian side and numerous salt works had developed in the increasingly exposed and accessible brine flats in the Karabogazgol depression.

 

The ongoing fall of the Caspian Sea level from 1929 and well into the 1970s, was thought by the Russian Government to be related to the continuous loss of water from the Caspian to Karabogazgol. After all, the climate in the Karabogazgol was arid, the annual evaporation rate was 1000-1500 mm and annual rainfall around 70 mm. Russian government scientists argued that damming the entryway would stop the Caspian spilling into the Gulf and so arrest the fall in the Caspian water level and the threat to fisheries. This notion of interfering in a natural saline system was opposed by a number of Turkmeni scientists who argued that the rise and fall of the Caspian and the variable filling in Karabogazgol were natural processes and had been noted in the texts of ancient Islamic scholars living in the region.

 

Ignoring the argument that rise and fall of brine level in isolated saline waters bodies, such as the Caspian, was the normal situation, a decision was made in 1977 by the central administration in Moscow that the entryway would be damned. Central administration scientists had concluded that sufficient water would remain in Karabogazgol after damming to allow the salt industry to continue its operations. The dam was not actually constructed for another 3 years. Ironically, the natural fall in the level of the Caspian had reversed in 1977 and had began to rise once more, with a corresponding rise in waters levels in the Gulf. From 1977 until 1995 the Caspian level rose by the same 3 meters it had dropped since the 30s. However a decision had been made back in 1977 to dam the entryway, so in 1980, even though the Caspian water level was rising once more, Russian engineers dammed the entry strait between the Caspian and Karabogazgol.

 

This complete loss of hydrographic connection to the Caspian did not have the desired effect, which was by then an irrelevancy, but egos and the authority of the Russian Empire was involved. Rather, the damming of the inflow resulted in rapid desiccation of Karabogazgol, so that by 1983 only a small brine-covered area remained in the lowest part of the gulf depression. Much of the former brine lake floor had turned into a saline dust bowl, blowing high levels of sodium salts onto downwind irrigated pasture lands.

 

By 1984 the villages surrounding Karabogazgol had turned into ghost towns. In 1984, in an attempt to restore the ecosystem, eleven large diameter pipes were laid across the dam so that Caspian water could be pumped back into Karabogaz at a controlled rate. In 1992, after the fall of the Soviet Empire, this approach was abandoned and the dam was completely demolished by order of the president of the newly independent Turkmenistan. As it refilled from 1992 to 1995, the gulf water level rose by some 6 meters and since then its water level has once again matched that of the Caspian Sea and it is once again fulfilling its role as a natural desalinator of the waters of the Caspian. Today, with the entryway connection to the Caspian restored, the water chemistry of the main open water body in the centre of the gulf is an Na-Mg-Cl brine, with a density of 1.2 g/cm3, and pH values that range between 7.2 and 9.

 

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