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1B Earth A > Sedimentary Basins - L15-20 > Flashcards

Sedimentary Basins - L15-20 Flashcards

1
Q

Why study sedimentary basins

A

tells us how lithosphere can deform and also when

how lithosphere supports load

usually survive longer than features of compression - don’t erode away like mountains

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2
Q

How do we study basins

A
  1. acoustic imagery
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3
Q

How do we know that the basin grows over time rather than just create a hole and then fill in

A

bore holes and sediment analysis shows all of the layers are formed in shallow seas - therefore deposited in shallow so basin grows overtime

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4
Q

What is a sedimentary basin

A

a hole with sediments in

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5
Q

How deep can basins get

A

up to 20km of sediments

- Dnieper-Donets basin, Ukraine

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6
Q

the COST wells

A
  • drilled in sed of US mid atlantic margin in 70s
  • 4.9km deep
  • use fossils to determine age and water depth of deposition
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7
Q

How do we analyse basins

x4

A
  1. Surface sampling and mapping
  2. Boring holes
  3. Acoustic experiments
  4. Remote geophysical measurements
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8
Q

How do you surface map basins, is it good or bad

A

only possible if uplifted and deformed

not very often

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9
Q

Boring holes

A

drilling holes and taking cores
- expensive
needed for ages of sediments and water depths at time of deposition in undeformed

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10
Q

Acoustic experiments

A

expensive but good imagery of subsurface structure

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11
Q

Remote geophysical measurements

A

earthquake analysis and gravity anomalies

  • cheaper but give indirect information, need lots of inference
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12
Q

Basins that have been uplifted in UK

A
  1. Midland valley of Scotland - carboniferous basin
  2. Pennine basin - carboniferous
  3. Welsh basin - Lower Paleozoic
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13
Q

Subsidence

A

the ground sinking

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14
Q

Backstripping

A

the process by which we can standardise subsidence by correcting compaction and isostatic loading by later sediments

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15
Q

How dow e compare the subsidence of different basins

A

by standardising this subsidence by producing curve as if the basin contained air or water instead

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16
Q

COmpaction

A

reduction in porosity

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17
Q

When and why does amplification of subsidence occur

A

as basin fills with more - due to consequence of isostacy

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18
Q

5 types of basin

A
  1. extensional
  2. Flexural
  3. Subduction zone basin
  4. Impact basins
  5. Mystery Basins
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19
Q

Assumption when backstripping

A

the layers were homogenous sediments even if the sediments where different in each layer

  • this not always a thing
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20
Q

How much slip over what sized fault is mag 6

A

1m slip over 25km fault

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21
Q

Basin in Greece - what controls topography and why

A

12mm per year extension so Greece topography controlled by extension as faster than rate of erosion

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22
Q

Aegean sea crust

A

continental crust which is thinner at 20km than rest of crust - found from acoustic refraction

heat flow onshore roughly double than onshore

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23
Q

2 effects of stretching lithosphere

A
  1. thinning crust means subsidence - depends on the initial crustal thickness
  2. Thin mantle lithosphere = UPLIFT - the amount depends on the thinning of this
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24
Q

At what crustal thickness does the two effects of stretching balance

A

15km

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25
Q

What dominates when stretching oceanic crust

A

the thinning of the mantle lithosphere, so the uplift wins as less than 15km crustal thickness

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26
Q

What dominates when stretching continental lithosphere

A

the subsidence as more than 15km thickness of crust

27
Q

What happens after lithosphere stretching

A

more subsidence - from thermal equilibrium being reached

28
Q

Syn-rift vs post-rift phase

A

syn-rift is the subsidence while actively rifting

post-rift phase is the subsidence due to thermal balancing occurring but the plate isn’t thinning anymore via stretching

29
Q

Example of old and mature sedimentary basin

A

North Sea Basin

30
Q

How does continent stretch if it doesn’t plastically deform

A

by normal faulting - looks like tipping blocks

31
Q

What happens if stretching keeps going

A

thinned so much that decompression melting occurs

mid ocean ridge forms eventually

basin rifted and each side called passive

32
Q

Thickness of lithosphere

33
Q

Flexure

A

assume that over long timescales - lithosphere responds to long loading like a floating elastic plate

34
Q

How do gravity anomalies associate with flexure

A

positive anomaly on the local mass excess - like a mountain

negative anomaly where the local mass is uncompensated - the flexural basin

35
Q

Example of using end loading of a beam

A

subduction zones with the trench being the end with the load

- there is an outer rise where the bend is compensated

36
Q

Example of mid-plate loading

37
Q

Flexural/Foreland basins

A

15-20%

e.g the Ganges basin

38
Q

Exhumation of foreland basins

A

when the load is eroded, the basin is exhumed in flexural unloading

39
Q

Strike slip basins

A

occur when strike-slip faults jog - so theyre offset

40
Q

Issue with data collection of foreland basins

A

usually on land or half on land and this is harder to get data and it will be less accurate

41
Q

What are strike-slip basins a subset of

A

extensional basins

42
Q

Example of strike-slip basin

A

Dead Sea between Jordan and Israel

  • Sinai plate (Israel)
    moves southward
  • Arabia Plate (Jordan) moves northward
43
Q

Susidence of strike slip basins

A

usually rapid and localised

44
Q

Back-arc basins (those associated with subduction zones)

A

can be called an extensional basin subset

  • Western Pacific

caused by slab roll back

45
Q

What is slab rollback

A

the movement of the subducting part of the plate to move back on itself

  • when slab-perpendicular component of negative buoyancy overcomes pressure field in interior mantle wedge
46
Q

Where is slab rollback more likely

A

short subduction zones - as the mantle behind the rolling slab can move

47
Q

Example of back arc basin with rollback

A

Scotia arc

48
Q

Mystery basins

A

have record of crustal extension and long-duration susidence records
- but in very old regions of thick lithosphere

  • also bullseye shape so no obvious extensional direction
49
Q

Example of mystery basin

A

Michigan basin

50
Q

Impact basins

A

most common type of basin in solar system

- caused by asteroid collision with planet

51
Q

Size range of impact basins

A

millimeters to thousands of km

52
Q

what is the K-T boundary

A

The boundary between the Cretaceous and Tertiary (The now K-Pg so not tertiary but Paleogene boundary)

53
Q

What did Luis and Walter Alvarez find at the K-T boundary

A

Iridium anomaly

54
Q

Why is it rare to get iridium at Earths Surface

A

Strong siderophile - so accumulates in core during planetary accretion

55
Q

How did we know the location. of the impact crater?

A

Tektites and shocked quartz

gravity anomalies

radially distributed cenotes

56
Q

Cenotes

A

surface connections to subterranean water bodies

57
Q

features of impact basins

A

inner and outer ring

58
Q

Suevite

A

rock consisting of partially melted material, typically forming a breccia of glass and crystal or lithic fragments from impact event

59
Q

Coesite

A

polymorph of silicon dioxide formd at very high pressure and high temp

60
Q

How does compression of basins happen - eample of compressed basin

A

lithospheric shortening - orogeny

Like the Alps

61
Q

Basin inversion

A

When the normal faults formed during extension become reverse faults under compression

62
Q

Other form of regional uplift but not by lithospheric shortening

A

epeirogenic uplift

63
Q

epeirogenic uplift

A

regional uplift of hundreds of meters

  • caused by mantle convection
    so hot upwelling less dense and cause surface uplift
  • cold downwellings more dense than normal so cause depression

1B Earth A (5 decks)

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