Evaporite textures as hydrological indicators


Brine stratification, halocline setup, reflux hydrology and depositional texture responses

 Layered brine lakes and seas form wherever a somewhat less saline water mass (less dense) sits atop a more saline water body. Lower specific heat values inherent to more saline brines in such a stratified brine mass set up a thermal inversion in the water mass. It takes less solar energy to heat a more saline brine to the same temperature as a less saline brine. This means the lower more saline brine is hotter than the overlying brine layer in a system known as heliothermically stratified brine mass.

In a density and thermally stratified brine, the overlying water body must be holomictic for bottom-nucleated salts to accumulate across the subaqueous floor of a brine mass and for dense, saturated brine to sink into underlying sediments. Holomixis means a near homogenous distribution of brine density, temperature, and salinity throughout the brine mass, with internal mixing being ongoing and mostly maintained by wind movement. In contrast, a meromictic brine body is internally stratified, with a lower more-saline, denser, warmer water mass separated across a halocline from an upper, less saline, less-dense, cooler water mass. A longterm halocline hinders chemical or physical changes in the underlying denser waters, so shutting down bottom nucleation, as well as slowing and ultimately stopping brine reflux. A permanently stratified system is ectogenic, while a brine column that is temporarily stratified is endogenic. 


Importance of density stratification in controlling evaporite textures

Holomixis permits the deposition of a coherent salt layer across the whole basin floor, beneath both shallow and deep brine columns. Density stratification allows evaporitic salts to crystallise only in the upper water mass or at the brine-brine interface, so bottom nucleation tends to occur on the shallower lake floor, where it lies above the halocline. That is, long-term (ectogenic) column stratification means bottom nucleation of salts can only occur where the upper salt-saturated brine-mass intersects the sediment bottom, with a pelagic settling of salts occurring deeper out in the depositional basin, as in the Dead Sea before February 1979 (Warren, 2016). The bottom growth of crystals cannot happen on a deep bottom located beneath a density-stratified system as there is no mechanism to drive ongoing supersaturation in the lower water mass. For the same reason, constant brine reflux driving sinking of a dense brine into sediments beneath the floor of the evaporite basin can only proceed if significant regions of the overlying brine mass are holomictic.

When salts are accumulating beneath a holomictic brine mass, textures in bottom nucleates is controlled by the stability of the overlying brine column. When the overlying column is deep (>30-100m) then, other than areas on the deep bottom of local phreatic spring-fed outflows, there is no general hydrochemical mechanism to drive fluctuations in bottom-brine chemistry. The resulting deep bottom precipitates tend to be monomineralogic crystal clusters, possibly encased by re-transported material washed in from the shallower surrounds. In contrast, when the overlying brine column is shallow (<30m and typically <5-10m) then the chemistry and stability of the brine vary on a shorter-term (daily-weekly) basis, so more layered bi-mineralogic bottom-nucleates can accumulate as layered to laminated salt beds. In addition, all evaporite sediments can be reworked by bottom currents, with similar textures to those that characterise siliciclastic and mechanically-modified carbonate sediments. Deposition of capillary salts (sabkha deposits) occurs in subaerial settings, wherever the saline capillary zone intersects the land surface.


Significance of primary evaporite depositional textures as indicators of brine hydrology. 

Primary evaporites precipitate with distinct textures in several hydrologically-contrasted settings, contingent on brine stability and rates of temperature and salinity change in the mother liquor. Sometimes crystals remain where they precipitate, other times they are mechanically or geochemically reworked, or undergo partial degrees of dissolution and fractional recrystallisation.

Crystals may first precipitate at the air-brine interface in rafted crystal clusters that then sink to form cumulate beds, or can be blown to the strandzone (e.g., halite rafts crystallising at the air-brine interface in Lake Guilietti in Ethiopia). Then again, immediately after they crystallise, precipitates can sink from the uppermost water mass to ultimately collect as pelagic accumulations (cumulates) on the shallow or deep brine floor - “rain from heaven” deposits. Seasonal or longer-term changes in the chemistry and salinity in the upper water column means many such pelagic deposits are mm-scale laminates made up of mineral doublet or triplet layers. In a holomictic shallow water mass, coarse cm-dm scale crystals can form as bottom-nucleated inclusion-entraining precipitates, typically at the base of a water column that is tens of centimetres to meters deep. Such crystals, be they gypsum swallowtails or halite chevrons, tend to be composed of alternating inclusion-rich and inclusion-poor laminae and micro-laminae, reflecting rapid changes in chemistry or temperature of the overlying shallow holomictic water mass.

If a brine column remains both supersaturated and holomictic to greater depths, then evaporites can accumulate at the deepwater base of a brine column that is hundreds of meters deep. This is the case today in the North Basin of the Dead Sea in the Middle East  where a salt bed, made up of a meshwork of inclusion-free, randomly-aligned cm-scale halite crystals, is accumulating and has been doing so since the North Basin brine column became holomictic in February 1978. Before 1978 and for at least the preceding 400 years the brine column of North Basin (>370 m water depth) was a meromictic density-stratified hypersaline system, with pelagic mm-scale laminites accumulating on the same deep bottom, composed of alternating calcite and aragonite lamina, along with minor cm-scale gypsum crystal clusters or rosettes. The lack of inclusions in the halite mesh on the present-day deep bottom reflects the greater stability of chemical conditions on the bottom. The corollary is that growth-aligned evaporite crystals, rich in entrained inclusions of brine (e.g. chevron halite in Death Valley, California, or carbonate pellets encased in swallowtail gypsum in Marion Lake, Australia) indicate precipitation in much shallower water depths (decimetre to metre depths). That is, inclusion-rich aligned bottom-growth textures form in well-mixed brines that are shallow enough to experience short term changes in saturation and growth rate, coupled to short term changes in water temperature or salinity.

If currents and waves rework the bottom crystals, then ripple structures and dune forms can be the dominant texture in the accumulating evaporite bed. The presence of evaporite equivalents of carbonate ooids, namely gypsolites and halolites, indicate oscillating bottom currents at the time the crystals grew. Brine floor instability, related to seismic events, halokinesis or tectonism, can also lead to the formation of slumps, brine escape and debris flow textures (seismites) in a salt bed deposited at any water depth. The lowering of suprasalt brine column salinity or the lowering of the regional watertable can create karst cones and breccias in an evaporite bed.

Evaporite salts can crystallise as early diagenetic precipitates in the shallow subsurface some mm to cm below the sediment surface, as occurs today in the capillary zones of many marine coastal and continental mudflats and sabkhas (where displacive and poikilotopic crystals, nodules and efflorescent crusts are typical). Some initial crystals precipitates can be dissolved and recycled through the nearsurface sediment column, especially in arid and semi-arid settings, to create pedogenic profiles; as in the gypsite soils atop gypsum lunettes in southern Australia or the nitrate-rich salt solids of the hyperarid Atacama Desert. 

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