Seds - 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
• Gypsum
• Anhydrite

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
Sabkah
Lagoon
Barred Basin

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 + carbonates = 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

Question 8

Sedimentary Rock Types:

Answer 8

Clastic rocks 
• Sandstones, Mudstones etc. 
• Biogenic rocks 
• Limestones, Chert (from diatomite) etc. 
• Organic rocks
• Coals, Oil shales, skeletal limestone etc. 
• Volcaniclastic rocks
 • Ashes, ignimbrites etc. 
• Chemogenic rocks 
• Evaporites!

Question 9

Permian Zechstein salt

Answer 9

• Thickness: up to 2.2 km with diapirs of 4 km thick
• North Sea about 500m so how did 2.2km of salt form?
• Diapirs formed as they are forced out from overburden elsewhere ‘waterbed and weight analogy’
• 5% of the total ocean budget
• Laid down in approximately 7 million years

Question 10

Castile Evaporitic Varves

Answer 10

• Large concentration in Texas – same rock for hundreds of miles
• Yearly succession shown in rock – white bands show summer and dark bands show winter

Question 11

Saline Pan

Answer 11

 Lake near coast with occasional sea water flooding
• Essentially a barred basin
• Thick layers of salt
• Salt flats – White sands – Gypsum – America

Question 12

Sabkha

Answer 12

 Sea water floods into a sediment and evaporite precipitates as a sort of cement
• Leads to not clear beds but large crystals
• 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

Question 13

Lagoon

Answer 13

 Can partially dry out, don’t get large layers but growth from the bottom
• Isolated basins on coastline
• Some recharge – not continuous connection to sea

Question 14

Barred-basin

Answer 14

 Don’t get them often
 Lage scale salt pan
 Need often recharge

Question 15

Order of precipitation

Answer 15

• Calcite first – thin rim
• Gypusm – thicker inner rim
• Halite – centre thick circle

Question 16

Halite

Answer 16

most abundant mineral – seawater is mostly Na+ and Cl- in its salinity composition – cubic structure

Question 17

Gypsum

Answer 17

CaSO4.2H2O

o Twinned Rhombus’s

Question 18

Anhydrite

Answer 18

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 19

Evaporite precipitation history ref

Answer 19

Leader, 2011
Although several periods during the Phanerozoic have been marked by impressive occurences, e.g. opening of the Atlantic and the Red Sea ocean basins, and desiccation of the Miocene Mediterranean