Week 4

Question 1

Evaporites =

Answer 1

Chemogenic rock

Question 2

Evaporite minerals

Answer 2

HALITE
NaCl

GYPSUM
CaSO4.2H2O

ANHYDRITE
CaSO4

(Sylvite; KCl)
(Epsomite; MgSO4.7H2O)

Question 3

How does evaporite composition occur?

Answer 3

From seawater

• mixes geologically fast (~1000years) compared to terrestrial
• composition therefore more constant than terrestrial water
• lots of NaCl (although N.B. this doesn’t dominate evaporite composition???)

Question 4

Marine brine evolution over time - observations

Answer 4

1. Only top 3 in record
   = most evaporation not complete due to recharge
   i.e. not much time spent in ‘halite zone’
2. Gypsum > halite
   = despite high conc, halite’s high solubility product = less saturated
3. Anhydrite < gypsum
   = low solubility product BUT doesn’t precipitate directly

Question 5

Chicken wire/mesh texture

Answer 5

Classic texture of anhydrite
Due to lack of water

• nodules form at subsurface
• host sediment = stringers in between

Question 6

Anhydrite/gypsum relationship

Answer 6

Anhydrite – expands –> gypsum

Gypsum –> shrinks –> anhydrite

Question 7

Ancient evaporite deposits; examples

Answer 7

MIOCENE MESSINIAN SALINITY CRISIS

• 1 million km3
• 2km thick
• 5% total ocean salt budget
• 1 million years

PERMAIN ZECHSTEIN SALT

• 2.2km
• diapirs 4km thick
• 5% total ocean salt budget
• 7 million years

Question 8

Salt diapir =

Answer 8

reworked evaporite

Salt is mobile, so if squashed by overburden = diaper somewhere else

Question 9

Ancient evaporite deposit paradox

Answer 9

1km of seawater should make 11m of evaporite but we are getting km thick deposits - doesn’t make sense?

Question 10

How do thick evaporite deposits form?

Answer 10

CONTROLS = tectonics + climate (think about their balance = important)

• isolated brine water body = progressive concentration in e.g. barred basin w/ porous/clean barrier like sandstone
• climate e.g. ice age = reduces SL = ISOLATED
• water level drawdown due to initial evaporation
• differential head = further water supply

Question 11

Evaporite depositional settings

Answer 11

Coastal/continental saline pan

Coastal/continental sabkha

Marine marginal shelf/lagoon

Barred-basin: deep to shallow

Barred basin: dessicated

Question 12

Order of precipitation

Answer 12

Calcite
Gypsum
Halite
Halite/chlorite solid solution
Epsomite
Sylvite
Carnalite

Question 13

Sabkha =

Answer 13

Area of coastal flats subject to periodic flooding and evaporation = accumulation of aeolian clays, evaporites, and salts

Question 14

What is evaporite sequence stratigraphy affected by

Answer 14

1) evaporitic drawdown
2) SL

As SL moves from low –> high

Salt – anhydrite – clay (more integration of clastic/carbonate material

Question 15

Continental brine composition; controlling factors

Answer 15

RUNOFF

• climate
• catchment size

CATCHMENT BEDROCK
- supply of CO32-

CHEMICAL WEATHERING

• T
• precipitation pH

Question 16

Other chemogenic sediment examples

Answer 16

BIFs

Bauxites

Phosphorites

Manganese ores

Sinter deposits = hot springs/water rich in minerals

~Limestones

Question 17

When does uniformitarianism not work?

Answer 17

AZOLLA = freshwater fern grew rapidly in Arctic waters then sank to bottom = -CO2

BIFS - O2 now stable

Question 18

Varve =

Answer 18

Thin layer of clay/silt, represents deposition from a single year (summer/winter) in a lake

• can be measured to determine chronology of glacial sediments