Evaporites

If you evaporate a 1000m column of sea water only 1.5m of evaporites are produced. Given this, how would you account for the 2km thickness of Permian Zechstein deposits? In your answer make reference to the sequence of minerals you would expect to be deposited while evaporating sea water.

Evaporites are bedded deposits crystallized from hyper-saline brine. Brine is formed when the evaporation rate is faster than the influx or refresh rate. Brines contain highly soluble ions, like sulfates (gypsum and anhydrite) and halites (halite, sylvite, and carnallite), with traces of borates, silicates, nitrates, and sulfo-carbonates. In laboratory settings, the least soluble constituents crystallize first: gypsum and anhydrite precipitates when the brine is reduced to 20% of the original volume, halite and sylvite precipitate when the brine is reduced to 10% of the original volume, and bitter borates and nitrates crystallize last. Real life is more complicated, particularly with alternating evaporating and replenishing sequences changing the brine concentrations, but bulls-eye patterns with the least soluble minerals along the rims and bottom of the deposit grading in to the most soluble minerals at the center and top are still sometimes observed.

Environments
Arid environments — hot with limited precipitation — are ideal for developing brines. Semiarid playa lakes, sabkhas (supratidal flats), salt pans, estuaries, and lagoons are all environments where brines forms. Evaporites have uneven spacial and temporal distribution, with widespread appearance in broad, shallow basins with minimal competing sediment flux in arid climates. Although we have no modern analogues, huge evaporate deposits suggest that ancient environments contained large evaporating basins. Because suitable climate conditions are usually between 10° and 30° latitude (a global band of desert), evaporate deposits can assist in reconstructing prehistoric continent positions.

Evaporites in non-marine settings — closed lakes, ephemeral lakes, or playas in arid or semi-arid climates — are most likely to demonstrate a bulls-eye pattern of minerals, with gypsum rims and bitter borate centers. The most common trace minerals are borax, trona, epsomite, gaylussite, and glauberite.

Evaporites in shallow marine settings can be divided into supra- and intratidal deposits, and marine shelves. In both instances, the brine is refreshed by seawater and groundwater, and deposits are layered by adjacent facies during as sea level changes adjust shoreline position.

Sabkha are common in modern supra- and intratidal settings, with evaporites intermixed with siliclastic debris washing down from inshore erosion, and offshore sand tossed up by storms.  Sabkha commonly demonstrate chicken wire structure, elongate anhydrite clumps separated by strings of carbonate or siliclastic mud.

Shallow marine shelves with water less than 5 metres depth produce evaporate deposits hundreds of metres thick extending thousands of kilometres along the shoreline. The deposits consist of mostly gypsum and not the more soluble minerals like halite, which suggests the brine is continually evaporated to between 70% and 90% of the original volume, then replenished with an influx of water. Fine internal lamination indicates the brine must be very calm and undisturbed during evaporation. To produce large, laminated gypsum deposits, the formation environment must have a physical barrier to isolate the brine, with replenishment during high tides or storm surges. Evaporation and concentration of the brine would take place at low tide and during daily temperature peaks, with producing crystallization recording partial or incomplete evaporation sequences. The water must have been extremely shallow, centimetres to metres deep, so that evaporation would produce high salinity within hours. These deposits are produced in shallow brine pools, estuaries, lagoons, and salt flats.

Similar deposits, but without lamination and with graded clasts and slumps, are deep marine deposits formed in deep basins. The basins could be filled with water 40 to 600 metres deep. If the basin were full, evaporating at the edges could form evaporites that are transported to the centre of the basin by slumps. Alternately, the basin could be only partly filled with shallow water, a seepage sill isolating the basin. An example of this form is a Miocene deposit located in the Mediterranean, a bulls-eye pattern 2,000 metres thick, reflecting at least 40 full evaporation cycles.

Permian Zechstein deposits
The Permian Zechstein deposits are 2,000 meters thick of evaporate deposits under the modern Mediterranean sea. Until the Zanclean flood approximately 5.33 million years ago, the Gibraltar Strait was blocked, isolating the Mediterranean basin from the oceans. The period of isolation formed a deep marine evaporite deposit, where isolated shallow basins with rare influxes of water from seepage, extreme high tides, storm surges, and rain provided shallow brine lagoons and mudflats that could rapidly evaporate. Over many, many, many years (300,000 years) of influx-evaporation cycles paired with basin subsidence, a thick deposit could form. The bulls-eye mineralogy indicates at least 40 evaporate cycles, while the lithology of evaporites interbedded with carbonates and siliclastics bound at least five major depositional sequences. The blocking sill finally broke, leading to a rapid refill of the basin in as little as a decade.

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