Week 4 Flashcards

1
Q

Evaporites =

A

Chemogenic rock

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2
Q

Evaporite minerals

A

HALITE
NaCl

GYPSUM
CaSO4.2H2O

ANHYDRITE
CaSO4

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

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3
Q

How does evaporite composition occur?

A

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???)
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4
Q

Marine brine evolution over time - observations

A
  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
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5
Q

Chicken wire/mesh texture

A

Classic texture of anhydrite
Due to lack of water

  • nodules form at subsurface
  • host sediment = stringers in between
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6
Q

Anhydrite/gypsum relationship

A

Anhydrite – expands –> gypsum

Gypsum –> shrinks –> anhydrite

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7
Q

Ancient evaporite deposits; examples

A

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
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8
Q

Salt diapir =

A

reworked evaporite

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

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9
Q

Ancient evaporite deposit paradox

A

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

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10
Q

How do thick evaporite deposits form?

A

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
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11
Q

Evaporite depositional settings

A

Coastal/continental saline pan

Coastal/continental sabkha

Marine marginal shelf/lagoon

Barred-basin: deep to shallow

Barred basin: dessicated

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12
Q

Order of precipitation

A
Calcite
Gypsum
Halite
Halite/chlorite solid solution
Epsomite
Sylvite
Carnalite
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13
Q

Sabkha =

A

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

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14
Q

What is evaporite sequence stratigraphy affected by

A

1) evaporitic drawdown
2) SL

As SL moves from low –> high

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

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15
Q

Continental brine composition; controlling factors

A

RUNOFF

  • climate
  • catchment size

CATCHMENT BEDROCK
- supply of CO32-

CHEMICAL WEATHERING

  • T
  • precipitation pH
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16
Q

Other chemogenic sediment examples

A

BIFs

Bauxites

Phosphorites

Manganese ores

Sinter deposits = hot springs/water rich in minerals

~Limestones

17
Q

When does uniformitarianism not work?

A

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

BIFS - O2 now stable

18
Q

Varve =

A

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