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Geology AS Level > SEDIMENTARY ROCKS > Flashcards

SEDIMENTARY ROCKS Flashcards

1
Q

Minerals

A

quartz - psychically tough so often forms sediments
feldspars and micas - alter due to chemical weathering and become clay minerals
calcite, gypsum and halite - evaporite minerals. Calcite is main mineral of limestone
haematite - sometimes found as cement (mineral that sticks clasts/grains together)

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2
Q

weathering and erosion

A

weathering - the break up of rock in situ, involving the gradual decay of the materials of the rock structure - leads to the whole rock being weakened

erosion - the break up of rock by various agents of erosion - particles then are transported and deposited by that agent of erosion

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3
Q

Chemical weathering

A

carbonation

  • rain + CO² = carbonic acid
  • CO² can be from air or soil
    carbonic acid dissolves limestone
  • calcite + carbonic acid = calcium + hydrogen carbonate ions
  • produces “KARST” landforms - e.g caves, potholes

hydrolysis

  • water enters the atomic lattice of feldspars and micas
  • mobilises the K, Ca, Na, etc and leaves clay behind
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4
Q

Mechanical weathering

A

exfoliation

  • “onion skin weathering”
  • due to expansion and contraction with big temperature changes (e.g deserts)
  • cracks form parallel to surface

freeze-thaw

  • water enters crack in rock
  • Water freezes, ice expands causing cracks to grow
  • repeats until rock breaks off

pressure release

  • erosion exposes underlying rocks which were compressed
  • underlying rocks expand, creating joints parallel to surface
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5
Q

Biological weathering

A
  • when animals/plants break rocks apart
  • e.g roots growing through rocks
  • e.g animals burrowing into the ground
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6
Q

Effects of climate on weathering

A
  • cold - mechanical (frost shattering)
  • temperate - mix
  • hot/dry - mechanical (exfoliation)
  • hot/wet - chemical
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7
Q

Clastic sedimentary rocks

A
  • conglomerates - rounded clasts
  • breccia - angular clasts
  • orthoquartzite - quartz grains and cement
  • desert sandstone - millet seed with iron oxide cement
  • arkose - >25% K-feldspar
  • greywacke - immature
  • clay - plastic
  • shale - layered
  • mudstone - not layered
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8
Q

Transportation of sediments

A
  • traction - pulled/rolls along bed
  • saltation - bouncing along bed
  • solution - dissolved into solution
  • suspension - floats in current

Transportation can be due to:

  • gravity - e.g rockfalls
  • wind - e.g saltation
  • rivers/sea - e.g currents, waves
  • glaciers
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9
Q

Hjulstrom curve

A

diagram in notes 6

Link between velocity of current and size of sediment transported

Particles need faster current to start moving than to keep moving

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10
Q

mature and immature

A

mature

  • well sorted
  • rounded clasts
  • one mineral
  • e.g sand dune

immature

  • poorly sorted
  • angular clasts
  • range of minerals
  • e.g glacial till
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11
Q

Shape of clasts

A
  • sphere - 3 axis all equal
  • rod - 2 axis equal, 1 longer
  • disc - 2 axis equal, 1 shorter
  • blade - 3 axis all unequal
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12
Q

Particle size

A

Wentworth-udden scale

  • diameter | phi | sediment
  • > 2mm | -2 to -8 | gravel, pebbles
  • 2 | -1 | sand (very coarse)
  • 1 | 0 | sand (coarse)
  • 0.5 | 1 | sand (medium)
  • 0.25 | 2 | sand (fine)
  • 0.125 | 3 | sand (very fine)
  • 0.0625 | 4 | silt
  • 0.0039 | 8 | clay
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13
Q

What controls sediment sorting

A

Sorting - the degree to which all particles are the same size

  • transport agent affects sediment size, sorting and roundness
  • energy of environment (higher = better sorted)
  • climate affects transport agent and weathering type
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14
Q

Sieving

A
  • weigh sediment
  • put in sieve stack (coarsest at top) and shake
  • weigh the contents of each sieve
  • display data as a table and histogram
  • calculate the cumulative mass
  • plot a cumulative frequency curve (cumulative mass % against phi)
  • find phi values for 84% and 16%
  • put data into coefficient of sorting formula
  • P = (phi84 - phi16) / 2
  • refer to table to see level of sorting
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15
Q

catastrophism

A
  • the theory that changes in the Earth’s crust have resulted mainly due to sudden violent and unusual short lived event
  • e.g floods, volcanism, earthquakes
  • accounts for extinction of species
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16
Q

gradualism

A
  • theory that changes are gradual and slow over time
  • model is applied in evolution where one species slowly transforms into another
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17
Q

uniformitarianism

A
  • theory that slow, incremental changes created Earth’s geological features
  • e.g erosion
  • geological processes observable now were acting in the same way in the past
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18
Q

facies

A
  • all the characteristics of a sedimentary rock produced by its environment of deposition
  • allows it to be distinguished from a rock deposited in an adjacent environment
19
Q

diagenisis and lithification

A

diagenisis - all processes acting on sediments at low temperature and pressure near the Earth’s surface

lithification - process of turning loose sediment into hard rock via compaction and cementation

20
Q

Diagenesis - pressure dissolution

A
  • load pressure is concentrated at the contact point between grains
  • minerals dissolve easily under stress
  • minerals enter solution
  • clay us insoluble and is concentrated in layers
  • dissolved minerals may recrystalise and form styolites

diagram in notes 7

21
Q

Lithification - compaction of coal

A

increasing in depth of burial

  • peat
  • lignite
  • bituminous coal
  • anthracite

Carbon levels increase, water/gas levels decrease

22
Q

Lithification - cementation of sandstones

A
  • biologically formed sediments are permeable
  • groundwater containing minerals in solution enter pore spaces and may precipitate the minerals, forming a cement which binds grains together
  • this reduces porosity + permeability and limits space for oil + gas storage
23
Q

Lithification - cementation of limestones

A
  • modern limestone are composed of unstable aragonite
  • over time becomes stable calcite
  • some ancient corals had hard parts composed of calcite
  • details in these fossils are better preserved than those composed of argonite which converted to calcite
24
Q

Sedimentary structures - cross bedding

A
  • way-up indicator
  • palaeo-current indicator
  • palaeo-environment indicator (desert dune)
25
Q

Sedimentary structures - graded bedding

A
  • ‘fining upward sequence’
  • occurs when mixed sediment settles in water column
  • way-up indicator
  • palaeo-environment indicator (bouma sequence in turbidite)
26
Q

Sedimentary structures - imbricate structure

A
  • imbricated pebbles lean in direction of current (palaeo-current indicator)
  • palaeo-environment indicator (fluvial)
27
Q

Sedimentary structures - salt pseudomorphs

A
  • evaporation of salt water creates cubes of halite
  • sediment drys out
  • when lake is filled in, crystals dissolve leaving a cast
  • palaeo-environment indicator (arid evaporation)
28
Q

Sedimentary structures - ripple marks

A
  • way-up indicator
  • palaeo-current indicator (asymmetrical)
  • symmetrical = oscillating current
  • palaeo-environment indicator (shallow sea, dunes)
29
Q

Sedimentary structures - desiccation cracks

A
  • sediments like clay shrinks when it dries, causing cracks
  • way-up indicator
  • palaeo-environment indicator (arid evaporation)
30
Q

Sedimentary structures - flute cast

A
  • caused by turbulent flow on the base of a bed
  • way-up indicator
  • palaeo-current indicator
  • palaeo-environment indicator (powerful turbidity current)
31
Q

Hot desert environment

A
32
Q

Desert facies - breccias and conglomerates

A
  • waddi conglomerates
  • arkose alluvial fans
  • poorly sorted
  • very immature (angular, range of rock types)
  • Red (iron oxide cement)
33
Q

Desert facies - aeolian sandstones

A

sand grains transported by wind and affected by attrition

  • texturally mature (well sorted, well rounded)
  • mineralogically mature (quartz)
  • frosted
  • cross bedding + desiccation cracks
34
Q

Desert facies - playa lake evaporites

A
  • temporary ‘playa lakes’
  • Water evaporates and dissolved ions become concentrated
  • least soluble precipitate first forming layers of evaporite minerals

CaGyHalK
- calcite, gypsum, halite, k salts

35
Q

Shallow siliciclastic environment

A
  • very mature (well rounded, well sorted sandstone/orthoquartzite)
  • symmetrical ripple marks
  • lateral variation
36
Q

Transgression and regression

A

transgression

  • sea floods onto land
  • fining upward sequence

regression

  • sea drains from land
  • coarsening upwards sequence
37
Q

Fluvial environments

A
  • lack of fossils
  • mix of conglomerate, sandstone and shales
38
Q

Fluvial facies - meanders

A
  • erosion on outside bend, deposition on inside bend (point bar)
  • channel will migrate laterally
  • shows cross bedding
39
Q

Fluvial facies - alluvial fans

A
  • river flows onto flat plain
  • velocity decreases so coarse sediments are deposited as alluvial fans
40
Q

Non-clastic sedimentary rock - biological origin

A

Limestones

  • fossiliferous limestone (bioclastics)
  • crinoidal limestone
  • chalk - composed of single cell algae

coal

  • result of coalification
  • peat, lignite, bitumous coal, anthracite
41
Q

Non-clastic sedimentary rock - chemical origin

A

limestone

  • micritic limestone - particles of calcium carbonate precipitate and settle in sea water
  • oolitic limestone - oolitith precipitated into shallow warm sea with constant waves. Sink at 1mm. Shows concentric layering

evaporites

  • minerals precipitated out of solution when salt water evaporates
  • calcite, gypsum, halite, k salts
42
Q

Carbonate rocks

A
  • > 90% calcite ( CaCO³) or dolomite
  • limestone or dolomite limestone
43
Q

Folk classification

A
  • based on type of grain and amount of sparite/micrite it contains
  • needs a microscope

Sparite - crystalline calcite cement
Micrite - microscopic crystal cement

  • intraclasts - intrasparite/intramicrite
  • ooliths - oosparite/oomicrite
  • fossils - biosparite/biomicrite
  • pellets - pelsparite/pelmicrite
44
Q

Dunham classification

A

-based on texture and mud content
-no grain identification needed (so easy to use in field)

  • mudstone - <10% grain
  • wackestone - >10% grain
  • packstone - grains are touching
  • grainstone - lack of mud

Geology AS Level (10 decks)

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