sign in sign up
<<
Home , Djibouti (city), Erythraean Sea, Salar de Uyuni, Salt lake, Salt pan (geology)

www.saltworkconsultants.com

Salty MattersJohn Warren - Tuesday January 31, 2017 and : Part 1 of 2 - Are modern the key?

(Figure 1b). But, in terms of distribution and the deposits and deserts economics of the associated , this climatic generalization Much of the geological literature presumes that thick sequences related to annual rainfall conceals three significant hydrological of bedded Phanerozoic evaporites accumulated in hot arid zones truisms. All three need to be met in order to accumulate thick tied to the distribution of the world’s deserts1 beneath of sequences of bedded salts (Warren, 2010): 1) For any substantial descending air within Hadley Cells in a latitudinal belt that is typ- volume of evaporite to precipitate and be preserved, there must ically located 15 to 45 degrees north or south of the equator (Fig- be a sufficient volume of cations and anions in the inflow ure 1a: Gordon, 1975). As this sinking cool air mass approaches to allow thick sequences of salts to form; 2) The depositional the landsurface beneath the descending arm of a Hadley Cell it setting and its climate must be located within a longer term basin warms, and so its moisture-carrying capacity increases. The next hydrology that favours preservation of the bedded salt, so the two articles will discuss the validity of this assumption of evap- orites tying to hot arid desert belts in the wind belts, first, by a consid- eration of actual Quaternary evaporite Stratosphere distributions as plotted in a GIS data- 15 km Tropopause base with modern climate overlays, Subtropical Jet then in the second article via a look at ancient salt/climate distributions. Polar Jet Significant volumes of Quaternary Hadley evaporite salts are normally interpret- Ferrel ed as being allied to the distribution Cell Polar Cell of the world’s hot arid deserts (Figure Cell 1b). In a general way this is true, but, as Warren (2010) shows, the correla- tion is an oversimplification. A hot Pole 60° 30° (Trade winds) Equator arid desert does not necessarily equate A. Desert belt (ITCZ) to occurrences of laterally extensive bedded evaporites; there must also be a significant long-term inflow Australia 83% to the evaporative sump, incoming 20 64% Asia 39% waters may be meteoric, marine, a 17% hybrid and perhaps the sump is fed North America 16% coming from dissolution of earlier formed salts in the drainage 10 basin, including diapiric salt (Table 1; Warren, 2016).

Actually, there are different ways of (%) covered Ground 0 defining a desert and by implication 90 60 30 0 30 60 90 N S its associated evaporites. One ac- Latitude cepted approach is to define desert as B. a terrestrial area receiving less that Figure 1. Hadley celles and deserts A) Longitudinal cross section through the ’s atmosphere 250 mm (10 inches) of annual pre- showing major circulation cells, including jet streams. ITCZ = intertropicl convergence zone. Belts cipitation. Using this definition some of cool dry descending air at 30° N and S of the equator create the main arid zones of the world. 26.2% of the world’s landsurface is B) Latitudinal distribution of the world’s modern deserts. Inset gives proportion of each conti- nent that is arid or semiarid. (after Warren, 2016).

Page 1 www.saltworkconsultants.com

Af Am As Aw BSh BSk BWh BWk Cfa Cfb Cfc Csa Csb Csc Cwa Cwb Cwc Dfa Dfb Dfc Dfd Dsa Dsb Dsc Dwa Dwb Dwc Dwd EF ET

Figure 2. Quaternary evaporites with areas >250 km2 and Köppen climate setting. Centroids are plotted on the earth surface where an evaporite depositional setting (sabkha, salina, playa, saline , saline soil) extends across an area of more than 250 km2 This plot set is overlain on a Köppen climate base (after Kottek et al., 2006). Main : A; tropical, B; arid, C; warm temperate, D; snow, E; polar. Precipitation: W; desert, S; steppe, f: fully humid, s: summer dry, w; winter dry, m; monsoonal. : h; hot arid, k; cold arid, a; hot summer, b; warm summer, c; cool summer, d; extremely continental, F: polar frost, T; polar tundra. accumulating salt mass can pass into the burial realm; 3) There the same hydrological consideration that is required for evap- must be a negative balance in the basin with the potential orite salts to form. That is, a desert is an area of land where for more water to leave the local hydrological sump than enter. annual precipitation (inflow) is less than potential evapotrans- When using a rainfall (precipitation) based definition of desert, piration (outflow). This definition of a desert is the one used the significance of these three simple hydrological axioms and by Köppen (1900). With slight modification, his climatological the consequences, as to where bedded evaporites accumulate, scheme is still in widespread use to breakout the various climatic is lost in the generalization that “evaporites form in the world’s zones across the world’s landsurface (Kottek et al., 2006). Using deserts.” a Köppen climate base, figure 2 plots worldwide occurrences of modern saline depositional systems with areas greater than 2 Continental-interior evaporites 250 km . Table 1 compares characteristics of some of the larger In an evaporite context it is better to define and plot saline hy- Quaternary bedded evaporite settings in marine edge and conti- drologies within a climatic framework where deserts are given nental interiors. These regions contain evaporite salts accumu- lating in saline soils, sabkhas, , saline , playas and salt flats, with more than 5000 metres 1% textural forms ranging from; isolated 4500-5000 6% B. crystals and nodules in a terrigenous 4000-4500 3% matrix, to salt crusts, to stacked beds 3500-4000 4% of salts that can be more than 10 me- 3000-3500 4% tres thick. The majority of the plotted Cfa 1% 2500-3000 3% saline areas are in arid regions, as de- Csa 1% 2000-2500 1% fined by Köppen (Zone B), but not all Dfb 1% 1500-2000 2% such areas of widespread salts are in Dsb 1% 1000-1500 9% deserts (as defined by Köppen) and Aw 1% 500-1000 13% Cwb 1% not all are in hot climates. 0-500 43% Dfa 1% The range of large (>250 km2) saline Dwc 2% below sealevel 12% systems in the world’s arid BSh 9% 0 10 20 30 40 50 60 70 80 ET 11% OCCURRENCES is more climatically diverse than just BSk 13% evaporite occurrences within a hot BWk 23% arid desert (BWh), although such as- BWh 38% sociations do constitute some 38% of saline occurrences (Figure 3a; War- A. 0 10 20 30 40 50 60 70 80 OCCURRENCES ren, 2010; 2016). Cold arid deserts Figure 3. Occurrences of saline areas (>250 km2) as bar charts showing percentage occurrences in A) climate zone (BWk) host 23% of large saline oc- and B) elevation ranges (metres level). currences, making a combined total Page 2 www.saltworkconsultants.com

Name Köppen Area (km2) Elev. (m) Diapirs and not located in polar or near-polar high- er latitudes. The lakes and saline pans of the Continental interior high plateaus of the () and the Australia BWh 8528 -15 No Himalayas (Tibetan Plateau) typify this style Salar de BWk 9,654 3650 Yes of tundra (ET) evaporite. Water may be com- monplace in the ET zone, but is there mostly Sua Pan (Makgadikgadi Pans) BSh 3,307 888 No as ice, and cryogenic salts are commonplace Chile BWk 3,065 2250 Yes (see Salty Matters, Feb 24, 2015). The re- Chagannur BSk 15 968 No maining where significant evaporite volumes are found, some 6% of the total of Dabuxum Lake China BWk 9 2681 Yes large saline occurrences is a group of deposits Zabuye Salt Lake China ET 251 4426 No defined by continental interior snow climates Wadi Natrun BWh 20 -23 No (group D), some with hot dry summers with solar evaporites alternating with dry winters Gavkhoni Playa Iran BWk 477 1450 Yes favouring the possible accumulation of cryo- Lake Magadi Aw 111 643 No genic salts (e.g. , USA). Issyk Kul Kyrgystan BSk 6,235 1619 No In the Northern Hemisphere the occurrence Middle East BSh 1,023 -415 Yes of large evaporite systems within arid deserts Sabkha Matti Middle East BWh 2,954 30 No and steppe climates (BW and BS settings) ex- tends much further south toward the equator Umm Al Sammim BWh 2,969 81 Yes and much further poleward (from 5-55°N) Chott el Djerid (Jerid) BWh 5,728 14 Yes than the narrower range of large evaporite Badwater Pan USA BWh 54 -75 No occurrences and associated climates in the southern hemisphere (Figure 4). This hemi- USA BWh 929 -74 No spheric asymmetry in evaporite occurrence Great Salt Lake USA Cfa 4,998 1280 No is mostly a response to world-scale adiabatic Marine margin effects associated with the collision of India with Eurasia and growth of the Himalayas. Lake Macleod Australia BWh 2,067 -0.6 No Today, a Cainozoic mountain range, centred Lake Macdonnell Australia BSk 108 -0.5 No on the Himalyas, diverts world-scale atmo- Lake Asal BWh 54 -155 No spheric air flows from the more north-south trajectory, usually associated with Hadley Dallol saline pan BWh 344 -115 Yes Cell circulation. For example, the Kunlun Sabkhat Maradah BWh 473 -0.5 No Mountains, first formed some 5.3 Ma,- pre Oje de Liebr Mexico BWh 373 -0.1 No vents moisture from the Indian Monsoon reaching much of the adjacent Tibet Plateau. Sabkha UAE BWh 1,655 0.1 No Its adiabatic rain shadow creates the Takla- Large bodies of makan desert, the second largest active sand (Sea of Islands) Kazakhstan BWk 58,178 40 No desert in the world (BWk). Kara-Bogaz-Gol (water filled) Turkmenistan BSk 17,528 -30.5 No The uplift of the Himalayas also creates a Kara-Bogaz-Gol (remnant) Turkmenistan BSk 919 -29 No dry easterly jet stream, moving arid cool air across the Tibet Plateau, around the northern Table 1: Climatic setting, areas and elevations (msl) of various regions with significant side of the Himalayas, and then equatorward volumes of Quaternary-age evaporites across the Arabian Peninsula toward of 61% for large evaporite accumulation (area >250 km2) occur- where it descends and gains heat. That is, this rences in modern arid deserts (BW group), while the arid steppes stream of cool southwesterly-flowing dry air warms as it moves (BSh and BSk) host another 22%. In total the world’s arid cli- across the Eastern Mediterranean land areas and so heightens matic zones host 83% of today’s larger evaporite occurrences existing aridity. This helps create an adiabatic desert zone that (Figure 3b). today ranges across Arabia and northern Africa almost to the Equator (Figures 5). This leaves another substantial, but not widely recognized, cli- mate zone where significant volumes of Quaternary evaporites In the southern hemisphere, the uplift of the Andes has formed can accumulate, this is the polar tundra (ET); an environment high intermontane depressions and the allied adiabatic aridity where some 11% of large evaporite areas occur. In terms of that are cooler with lower evaporation rates and higher relied in evaporite volumes, the polar tundra (ET) is typically an arid high the immediate basin compared to groundwater depressions in altitude belt, mostly in the Horse Latitude (Trade Wind) belts, flatter lower-elevation continental interior deserts like the Saha- ra. This higher stability hydrology favours salars over dry - Page 3 www.saltworkconsultants.com

Aw 5000 BSh BSk 4000 BWh BWk 3000 Cfa Csa 2000 Csb Cwb Eleevtion (m) Dfa 1000 Dfb Dsb 0 Dwc ET 1000 40 30 20 10 0 10 20 30 40 50 60 Latitude Figure 4. Large saline areas (>250 km2) on the earth’s landsurface in terms of Köppen climate zone versus latitude and eleva- tion.

Turkestan Tien Shan desert Taklamakan Kulun Shan

Saharandesert

Iranian Himalaya desert

Thar desert

Arabian desert

Somali desert

Figure 5. Adiabatic funnelling in uenced large scale atmospheric circulation so that the northern hemisphere desert belt extends to within 5° of the equator.

Page 4 www.saltworkconsultants.com

AW BSh BSk BWh BWk Csa *Dfb Dwc *EF *ET 80 North 2 2 2 2 5 2 2 1 1 1 8 2 8 5 1 1 2 8 6 6 4 4 2 8 2 5 5 6 1 1 7 1 1 1 1 1 1 6 5 1 8 3 1 1 2 2 1 1 1 1 6 1 3 1 1 7 3 1 1 60 7 3 3 8 6 40

20

0

-20 Latitude (degrees) Latitude

-40

-60

South 535m 696m 892m 900m -415m 122m 1081m -29m 113m 700m -155m 279m -70m -118m 103m 5m -23m -70m 600m 2250m 900m 1400m 3650m 180m 493m 2675m 2645m 1300m 40m 836m 1270m 700m 712m 650m 500m 650m 292m 4400m 3200m 174m 90m 4300m 1800m 4300m 4636m 4572 3650m 4200m -80 on ea r Acigol e Lake anda Aral Sea V Lake Asal e Muerto Seni Lake adi Natrun Lake Salton Sea r alley Playa Lake Qarun Lake Nat r W Owens Lake Cachilaguna Searles Lake Lake Magadi Lake Giulietti Kuchuk Lake Chaplin Lake eat Salt Lake Sybouts Lake Big Quill Lake Dallol Dabuxun Lake Laguna Salinas G r Laguna del Rey Ingebright Lake Dead Sea Brine Kara Bogaz Gol Sua (Sowa) Pan Bristol Horseshoe Lake Whitesho r Salar de Carmen Rio Grande Salar entino Ameghino Zabuye Salt Lake Salar de Atacama Otjiwalunda playa Laguna Honda Sur Laguna Santa Maria Clayton V Flo r Hobbs mirabilite a Atacama nitrate fields Laguna Hedionda Sur Changannur Salt Lake

Koeppen Climate Zone Salar de Homb

AW= Equatorial desert Don Juan , Lake Salars inland of Antofagasta BSh = Arid, steppe, hot arid BSk = Arid, steppe, cold arid BWh = Arid, desert, hot arid BWk = Arid, desert, cold arid Dominant exploitable salt Csa = Warm temperate, steppe, hot summer 1 = Salt cake (Na-sulphates) assemblage Dfb = Snow, fully humid, warm summer 2 = Soda ash (Na carbonates) assemblage Dominant Exploitable Brine Dwc = Snow, desert, cool summer 3 = Borate Assemblage 6 = brine EF = Polar frost 4 = Nitrate Asemblage 7 = brine ET = Polar tundra 5 = Potash Assemblage 8 = CaCl brine Figure 6. Quaternary continental lacustrine saline deposits plotted against their latitude and Köppen climate zone (see Figure 2). The brown shade indicates “horse latitude” belts situated some 25-40° north and south of the equator. Exploited or exploitable commodi- ties are listed as a set of numeric keys for each deposit, in some lakes the exploited deposits underlie or form about the margins of the current lake/playa and indicate earlier (Pleistocene) or marginward increases in (after Warren, 2016; see Chapters 11 and 12 for details). Asterisk indicates settings where cryogenic salts dominate the natural precipitates, i.e. saline lakes within the D and E climate settings (after Warren, 2010). flats, as typified by Salar de Atacama and Salar de Uyuni. Ata- margins, especially if adjacent to mountain belts (e.g. the vari- cama has a Quaternary saline sediment fill made up of a more ous circum Saharan chotts, playas and sabkhas adjacent to Atlas than 900 m thickness of interlayered salt and clay, while Uyuni Mountains), or 2) to ancient inherited paleodrainage depressions holds a more than 120 m thick interval of interbedded salt and (as in the majority of the interior salt lakes of Australia or the clay infill, with areas of 3,064 km2 and 9,654 km2 and elevations southern Africa pans). Another hot arid desert (BWh) evaporite of 2250 m and 3650 m, respectively (Figure 7a). These salars are association is defined by termination outflow rims of deep arte- the two largest known examples of Quaternary bedded ac- sian systems, as in Lake Eyre North, Australia (8,528 km2, with cumulation, worldwide. Yet neither resides in hot arid desert set- an ephemeral halite crust up to 2 m thick in its southern portion). tings (BWh); both are located in cold arid deserts (BWk) and in Similar, deeply-circulating, meteoric artesian hydrologies help actively subsiding, high altitude (>2500m) intermontane (high explain the distribution of chotts in the BWh zone of NE Africa. relief) endorheic depressions. Unlike Atacama and Uyuni in the Andes, none of these mod-

2 ern BWh artesian systems preserve stacked decametre-thick salt Worldwide, distribution of most of the larger (>250 km ) Qua- beds, nor did they do so at any time in the Quaternary. Rather, ternary evaporite settings located in hot arid (BWh) desert set- the most extensive style of BWh salt in meteoric-fed artesian tings, tie either to; 1) endorheic river terminations along desert outflow zones is as dispersed crystals of gypsum and halite in Page 5 www.saltworkconsultants.com

a terrigenous redbed matrix (sabkha) or as visually impressive Relatively few marine-fed evaporite regions exist today with ar- large ephemeral saline flats and pans covered by metre-scale eas in excess of 250 km2 (Table 1; Figures 6, 7b). By definition, salt crusts that dissolve and reform with the occasional decadal in order to be able to draw on significant volumes of , freshwater (Warren 2016; Chapter 3). Sediments of such these basins must operate with a subsealevel hydrology. This al- continental groundwater outflow zones are typically reworked lows large volumes of seawater to seep into the depression and by eolian processes and, due to a lack of long term watertable evaporate. It also means most of these deposits are located near stability, the longterm sediment fill is matrix-rich and - evap the continental edge where a freestanding mass of seawater is orite-poor, with the Quaternary sediment column typified by not too distant episodes of deflation, driven by 10,000-100,000 year cycles of glacial-interglacial climate changes. The largest known deposit of this group is Lake Macleod on the west coast of Australia, with an area of 2,067 km2 and containing It seems that to form and preserve laterally-extensive decame- a 10m-thick Holocene gypsum/halite bed (Figure 7b). It hosts a tre-thick stacked beds of halite in a Quaternary time-framework saltworks producing some 1,500,000 tonnes/year of halite from requires an actively subsiding tectonic depression in a cooler lake brines in a BWh setting (Warren, 2016). A smaller marine high-altitude continental desert, where and evapo- seepage example, with a similar Quaternary coastal carbonate ration rates are somewhat lower than in BWh settings, allowing dune-hosted seepage hydrology, is Lake MacDonnell near the brine to pond and remain at or near the surface for longer periods head of the Great Australia Bight. It has an area of 451 km2, a (Figures 6, 7a). But perhaps more importantly, all of the larger 10m-thick fill of Holocene bedded gypsum and is located in a regions of the Quaternary world, where thick stacked bedded milder BSk setting, compared to Lake Macleod. Even so, annu- (not dispersed) evaporites are accumulating, are located in con- ally, the Lake MacDonnell operation is quarrying more than 1.4 tinental regions with drainage hinterlands where dissolution of million tonnes of Holocene coarsely-crystalline near-pure gyp- older halokinetic marine-fed salt masses are actively supplying sum and producing more that 35,000 tonnes of salt via by pan substantial volumes of brine to the near surface hydrology. This evaporation of lake brines. halokinetic-supplied set of deposits includes, Salar de Uyuni and Salar de Atacama in the Andean Altiplano, the Kavir salt lakes of Interestingly, when the climatic settings of Holocene coast- Iran, the Qaidam depression of China and the Dead Sea (Table al salinas of southern and western Australia are compared, all 1). show similar interdunal sump seepage hydrologies with uncon- fined calcarenite aquifers, yet it is clear that gypsum evaporite beds dominate in BSk and lower precipitation levels, with more Marine-edge evaporites carbonate, in Csb coastal settings typified by hot dry summers. Thick sequences of stacked Quaternary evaporite beds with a Halite dominates the marine-fed bedded fills in BWh coastal set- marine-brine feed are far less common and far smaller than me- tings, while Coorong-style meteoric-fed carbonates dominate in teoric/halokinetic Quaternary continental evaporite occurrences.

A. B. Llica

Tahua Indian

Cygnet Pond Salar de Uyuni (3650 m elevation) Ibis Pond (marine seep) Dune eld Lake (Pleistocene) Macleod (-0.5 m) Uyuni

Volcanic terrain

Rio Grande 20 km delta 20 km

Figure 7. Largest modern bedded salt deposits. A) Continental: Salar di Uyuni, Central Altiplano, Bolivia, the high-altitude endoheic salar is dominated by white halite crust with the Rio Grande fan delta in the southwest of the salar. Numerous salt diapirs subcrop in the (Bing® image mounted and scaled in MapInfo®). B) Marine-fed: Lake Macleod, coastal West Australia. It is the largest evaporite- lled coastal salina in the world. Marine-fed brine springs and sheets are visible in the central western and northwestern strandzone, while anthropogenic solar are visible in the south. The interdunal depression only started to accumulate Holo- cene salts after its surface connection to the ocean at its southern end was cut o by longshore sand drift (Landsat image courtesy of NASA, mounted in MapInfo®).

Page 6 www.saltworkconsultants.com

5 km

Halite Pan

Gypsum Lake

Lake Asal (Lac ‘Assal) (-115 m elevation) 150 Ghoubbet al ? 100 ? Kharab 50 ()

0 sealevel

-50 Elevation (msl) Elevation -100 Fiale (A5) Ardoukôba (Asal) Rift -150 Gale leGoma 10 8 6 4 2 0 Lava Ridge 103 yr BP (A1-A4, A6)

Ghoubbet al SE NW Lake Asal Kadda Kharab depression Soma Ocean 0m halite gypsum marine Oued Doubié seepage 200m 0 10km hydrothermal influx

Older gypsum beds Oued Aroira Halite crust pavement Alluvial inflow (wadi)

Perennial Late Pleistocene lacustrine sediment brine lake ≈ 17,000 years b.p. (silts, clays, diatomites and travertines) Gypsum lake Oued Dafarré (subaqeous) Maximum extent of Lake Asal in the early Holocene

Spring inflow of seawater from Sea of Ghoubbet North A5 Oued Kalou Current flow in gypsum precipitating Kadda perennial portion Lake Asal Soma A4 A3 Dominant wind direction A6

0 5 km Figure 8. Holocene sediment distribution in Lake Asal, Republic of Djibouti (surface geology after Stieljes 1973). Marine in ow is via fractured basalts of the Kodda Soma. A3–6 indicate positions of geothermal test wells – A1 and A2 lie further to the south- west. Insets show water levels in past 10,000 years (After Gasse and Fontes 1989) and an idealised cross section from the ocean to the lake depression (Landsat image courtesy of NASA).

Page 7 www.saltworkconsultants.com

similar interdunal coastal seepage depressions in the more the zone. supply much of the salt accumulating in the suprasealevel humid and somewhat cooler Csb settings of the Coorong region. sabkhas near Khobar in (BWh), Rann of Kutch in India (28,000 km2, BWh), Sabkha Matti (2,955km2, BWh) One of the most visually impressive marine seep systems in the in the Emirates, and Sabkha de Ndrhamcha (634 km2, BWh) in world is Lake Asal, immediately inland of the coast of Djibou- . And yet today all of these large sabkha occurrences ti, with an area of only 54 km2 it is much smaller than the 250 2 are located just inland of modern coastal zones and so, without km cutoff used for this discussion. It is located in a BWh cli- hydrological knowledge, are easily interpreted as marine. The mate, similar to that of the Danakil de- pression, contains subaqueous textured Ground surface gypsum and pan , and lies at the Evapotranspiration bottom of a hydrographically-isolated Soil moisture drives moisture changes basalt-floored depression with a brine zone lake surface some 115m below sealev- el (Figure 8; see Warren, 2016 Chapter Potential 4 for geological detail). This difference for Vadose Unsaturated deation zone Moisture in elevation between the nearby Red Sea zone distribution Matrix with air and and the lake floor drives a marine seep varying hydrology, so that seawater-derived water/brine Anhydrite in pores groundwater escapes as springs along the Stokes basaltic lake margin. Most of the cations surface Capillary < 1 m and anions in the seawater feed, that ul- (sabkha fringe surface) Water table timately accumulate as salts in the sub- Saturated 0 Saturation 100% (Phreatic surface) sealevel Asal sump, move lakeward via Phreatic zone zone Gypsum gravitational seepage along fractures in a basaltic ridge aquifer separating the Red Sea from the brine lake, in what is an ac- Hydrologic Zones tively extending continental rift that will shortly become and arm of the Red Sea. Matrix with water/brine lled pores That there are very few large marine-fed A. SABHKA HYDROLOGY bedded Quaternary evaporite systems is illustrated by summing the total area NONSTRATIFIED Salinity Temperature of large active continental-fed evapo- rite areas (>250km2) listed in Figure 2, tens of hypersaline which gives a total surface area world- metres wide in excess of 360,000 km2. In con- holomictic Dept h trast, the total area of large modern ma- bottom nucleates rine-fed bedded salt systems, at slightly Brine re ux less 10,000 km2, is more than an order of magnitude smaller. This low value for Salinity Temperature large coastal marine-fed evaporite occur- STRATIFIED rences is in part because many classic Fresher halocline thermocline coastal sabkhas, with characteristic dis- tens of persed evaporites in their supratidal sed- metres hypersaline Dept h iments and long considered to be arche- pelagic cumulates Meromictic or Oligomictic typal marine-fed groundwater evaporite No brine re ux heliothermal system, are now seen as mostly continen- tal brine-fed hydrologies. B. SALINA HYDROLOGY For example, the modern Abu Dhabi Figure 9. Hydrological classi cation in saline settings. A) Hydrological zonation in the sabkha system (1,658 km2; BWh) has saline mudat and its landward surrounds show sabkha evaporites tend to precipitate been shown by Wood (2010) to be a and be preserved in the hydrologic fringe known as the capillary zone. B) Water mass continental groundwater outflow area, zonation in a perennial brine lake or seaway. When the water mass is non-strati ed where most of ions precipitating as salts (holomictic) then a combination of bottom nucleation and brine reux can occur. in the supratidal zone are supplied by When the water mass is strati ed then on a less saline set of salt precipitate via evapo- ration in the upper water mass and settle to the botton or a another set of salts precip- upwelling of deeply-circulated meteoric itate via brine mixing at the halocline or brine interface. In this strati ed situation there waters, not seawater flooding (Warren, is no way a gravitational drive can be set up that will force denses bottom brines to 2016; Chapter 3). Similar continental reux into the sediments below lower denser water mass. hydrologies, resurging into the coastal Page 8 www.saltworkconsultants.com

distinction between a water budget and a salt supply budget em- graphically closed, with the purer, most voluminous, examples phasizes a general observation in Holocene evaporite systems of 100m-thick evaporite successions located in tectonically ac- that, in order to define the main fluid carrier for the salt volume tive piggy-back depressions, with lake floors and hydrologies found in any supratidal part of an evaporitic depression, it is im- that are more than 2000 m above sea level. Their hydrologies portant to understand and quantify the of the groundwater are endorheic and strongly influenced by deeply circulating me- feed, be it by marine or nonmarine, in both continental-interior teoric waters. In addition, the better examples of thick stacked and marine-margin settings. It also means that surface runoff and bedded salts are supplied ions via the dissolution of older marine seawater washovers, while at times visually impressive, typical- halokinetic salts in the surrounding drainage basin (Table 1). ly do not supply the greater volume of preserved salts in most modern coastal sabkha systems. Yet, anyone working in ancient (pre-Quaternary) evaporites knows from simple arguments of salt volumetrics versus brine Due to the highly impervious nature of muddy sabkha sediment, sources, and the nature of the enclosing sediments, that they are an occasional seawater storm flood into a coastal margin mudflat part of a dominantly marine basin fill. This will be the focus does not mean the pooled seawater ever penetrates the underly- ing sabkha. Most of its solute load is deposited as an ephemeral References surface crust of halite that is a few centimetres thick atop the Gordon, W. A., 1975, Distribution by latitude of Phanerozoic sabkha (Wood et al., 2005). Based on Wood’s and other evaporite deposits: Journal of Geology, v. 83, p. 671-684. hydrological studies of modern coastal sabkhas in BWh settings, centred on the Middle East, it seems that the salt supply to most Köppen, W., 1900, Versuch einer Klassification der Klimate, modern coastal sabkha depressions is still not at hydrological vorzsugsweise nach ihren Beziehungen zur Pflanzenwelt: equilibrium with the present sealevel, which was reached at the Geographraphische Zeitschrift, v. 6, p. 593-611. beginning of the Holocene some 6,000-8,000 years ago. That is, modern zones of continental groundwater outflow along an arid Kottek, M., J. Grieser, C. Beck, B. Rudolf, and F. Rubel, 2006, coast can have watertables that are up to a metre of two above World Map of the Köppen-Geiger climate classification updat- sealevel and are strongly influenced by the resurgence of sea- ed: Meteorologische Zeitschrift, v. 15, p. 259-263. ward-flowing deeply circulating, continental groundwaters (it is Stieljes, L., 1973, Evolution tectonique récente du rift d'Asal: axiomatic that surface runoff and local meteoric recharge tend Review Geographie Physical Geologie Dynamique, v. 15, p. to be limited in BWh settings). In this situation resurging waters 425-436. in the sabkha tend to have a dominantly continental ionic supply for their preserved salts. Warren, J. K., 2010, Evaporitic source rocks: mesohaline re- sponses to cycles of “ or feast” in layered brines, Qua- Lastly, in terms of the climatic and hydrological pre-requisites ternary carbonate and evaporite sedimentary facies and their for the accumulations of thick bedded Holocene salt deposits, ancient analogues, John Wiley & Sons Ltd., p. 315-392. one must recognize that marine-margin sabkha do not have the same geohydrology as a near-coastal, hydrographical- Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3- ly-isolated seepage-fed sub-sealevel salina depression. A BWh 319-13511-3): Berlin, Springer, 1854 p. or BSk near-coastal salina is slowly but continually resupplied ions via springs fed by the effusion of seawater, moving under Warren, J. K., and C. G. S. C. Kendall, 1985, Comparison of a gravity drive (evaporative drawdown), through an aquifer that sequences formed in marine sabkha (subaerial) and salina (sub- is the barrier separating the salina depression from the adjacent aqueous) settings; modern and ancient: Bulletin American Asso- ocean. Until the depositional surface reaches hydrological equi- ciation of Petroleum Geologists, v. 69, p. 1013-1023. librium, it draws continually on the near limitless supply of ions Wood, W. W., 2010, An historical odyssey: the origin of solutes in the adjacent salty reservoir that is the world’s ocean. Salts in the coastal sabkha of Abu Dhabi, , Qua- growing displacively in the supratidal parts of a sabkha cannot ternary carbonate and evaporite sedimentary facies and their an- call on a drawdown hydrology and can only be supplied marine cient analogues, John Wiley & Sons Ltd., p. 243-254. salts from salt spray as it is blown inland or from waters washed over the sabkha by the occasional storm driven wash-over and Wood, W. W., W. E. Sanford, and S. K. Frape, 2005, Chemical breakout (Figure 9; Warren and Kendall, 1985). openness and potential for misinterpretation of the solute envi- ronment of coastal sabkhat: Chemical Geology, v. 215, p. 361- So what? 372. So, if we based our ideas of where ancient bedded evaporites formed on a strictly uniformitarian approach using Quaternary analogues, then the study of where bedded salts have formed best in the world over the last two million years leads to a conclusion that large bedded salt deposits are not marine fed. Rather, in the [1] The word desert comes from the Latin dēsertum (originally Quaternary, thick bedded salts form best in tectonically active, "an abandoned place"), a participle of dēserere, "to abandon." subsiding hypersaline groundwater sumps located in high-alti- tude high-relief cold-arid deserts. These depressions are hydro-

Page 9 www.saltworkconsultants.com

John Warren, Chief Technical Director SaltWork Consultants Pte Ltd (ACN 068 889 127) Kingston Park, Adelaide, South Australia 5049 www.saltworkconsultants.com

Page 10