Evaporites

Question 1

What is an evaporite?

Answer 1

• Chemogenic
o Most abundant type of chemogenic rock
• Has a massive structure (can’t pick out particular grains)
• Rocks derived from evaporated water (mostly seawater)
o Seawater composition hasn’t changed much through time so expect similar evaporites through time

Question 2

Evaporite minerals

Answer 2

•	Halite most abundant mineral – seawater is mostly Na+ and Cl- in its salinity composition – cubic structure
•	Gypsum
o	CaSO4.2H2O 
o	Twinned Rhombus’s
•	Anhydrite 
o	CaSO4 
o	Chicken-wire/ mesh pattern 
	Due to shrinkage of water
	Causes topography and mud etc fills in gaps
o	Never comes straight out of water 
o	Alteration product of gypsum
	Further heating and loss of water
o	Nodules of anhydrite form at subsurface and host sediment is left as stringers in- between.

Question 3

Evaporite Precipitation?

Answer 3

• Would need 182km of seawater to dry up instantly to build up 2km of evaporites
• Not instant – constant depositional process – repeated sequences being laid down
• Need a balance of recharge and evaporation to form evaporites
• Hypersaline conditions, evaporation and difference in hydraulic head for recharge
• Evaporation causes decrease in water and therefore a pressure difference so water drawn in through barrier
• Perfect conditions in Messinian and Zechstein

Question 4

Controls on evaporite precipitation?

Answer 4

Climactic Controls:
• More evaporites in colder climate due to lower sea level and therefore evaporation and hypersaline conditions are met– Med
• Cyclicity of sea level points towards sea level being most likely

Tectonic Controls:
• Movement of Gibraltar creating barred basins
• Lack of explanation for cyclicity means it is less likely an explanation

Question 5

Evaporite precipitation sequence?

Answer 5

• Much more gypsum and anhydrites than halite’s
• Precipitated salt sequence – order of precipitation
• Reducing volume of brine (water with salt) and increase density, different minerals are precipitated
• Sequence:
 First to come out is calcite
• Not much to come out as not much calcium in seawater
 Next is gypsum
 Halite comes out at lower water concentrations
 Magnesium based minerals are last to come out and therefore are rarely precipitated from seawater
• Sequence of lowering water concentration and then recharging – leads to water being in zone of gypsum precipitation for longer
• Halite is actually more concentrated in seawater (ion activity product) but has a much higher solubility (100,000x more than gypsum) and therefore will stay in solution much longer
 Results in gypsum having a much higher saturation level which leads to its preferred precipitation
o Anhydrite is soluble also (2nd highest saturation state) but is derived from gypsum and therefore does not come out second

Question 6

Environments they are found in

Answer 6

Saline Pan
• Essentially a barred basin
• Thick layers of salt
• Salt flats – White sands – Gypsum – America
Sabkah
• Not bared
• Shallow seawater – salt laid down isn’t always covered by sea
• Tides causes recharge
• Dregs of water evaporating leading to small accumulations
• Not thick layers but mixed in with other minerals
Lagoon
• Isolated basins on coastline
• Some recharge – not continuous connection to sea

Question 7

Evaporites and sequence stratigraphy?

Answer 7

• Sequence stratigraphy effected by evaporitic drawdown as well as sea-level.
• Highstands have greater integration of clastic and carbonate lithologies within the evaporite sequence.
• Sequence stratigraphy demonstrated within the Permian Zechstein salts.
• Works based on mineral identification
• Sea level rise + fall
• Clay = high
• Sequence completely flooded – only clastic – no evaporates
• Salt = low
• Low sea level means evaporation and precipitation conditions can be met
• Intermediate between sea level change will be anhydrite
• Gypsum has stopped coming out and has started changing to anhydrite