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

Sedimentary Basins - L15-20 Flashcards

(63 cards)

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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
What dominates when stretching oceanic crust
the thinning of the mantle lithosphere, so the uplift wins as less than 15km crustal thickness
26
What dominates when stretching continental lithosphere
the subsidence as more than 15km thickness of crust
27
What happens after lithosphere stretching
more subsidence - from thermal equilibrium being reached
28
Syn-rift vs post-rift phase
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
Example of old and mature sedimentary basin
North Sea Basin
30
How does continent stretch if it doesn’t plastically deform
by normal faulting - looks like tipping blocks
31
What happens if stretching keeps going
thinned so much that decompression melting occurs mid ocean ridge forms eventually basin rifted and each side called passive
32
Thickness of lithosphere
120km
33
Flexure
assume that over long timescales - lithosphere responds to long loading like a floating elastic plate
34
How do gravity anomalies associate with flexure
positive anomaly on the local mass excess - like a mountain negative anomaly where the local mass is uncompensated - the flexural basin
35
Example of using end loading of a beam
subduction zones with the trench being the end with the load | - there is an outer rise where the bend is compensated
36
Example of mid-plate loading
hawaii
37
Flexural/Foreland basins
15-20% e.g the Ganges basin
38
Exhumation of foreland basins
when the load is eroded, the basin is exhumed in flexural unloading
39
Strike slip basins
occur when strike-slip faults jog - so theyre offset
40
Issue with data collection of foreland basins
usually on land or half on land and this is harder to get data and it will be less accurate
41
What are strike-slip basins a subset of
extensional basins
42
Example of strike-slip basin
Dead Sea between Jordan and Israel - Sinai plate (Israel) moves southward - Arabia Plate (Jordan) moves northward
43
Susidence of strike slip basins
usually rapid and localised
44
Back-arc basins (those associated with subduction zones)
can be called an extensional basin subset - Western Pacific caused by slab roll back
45
What is slab rollback
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
Where is slab rollback more likely
short subduction zones - as the mantle behind the rolling slab can move
47
Example of back arc basin with rollback
Scotia arc
48
Mystery basins
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
Example of mystery basin
Michigan basin
50
Impact basins
most common type of basin in solar system | - caused by asteroid collision with planet
51
Size range of impact basins
millimeters to thousands of km
52
what is the K-T boundary
The boundary between the Cretaceous and Tertiary (The now K-Pg so not tertiary but Paleogene boundary)
53
What did Luis and Walter Alvarez find at the K-T boundary
Iridium anomaly
54
Why is it rare to get iridium at Earths Surface
Strong siderophile - so accumulates in core during planetary accretion
55
How did we know the location. of the impact crater?
Tektites and shocked quartz gravity anomalies radially distributed cenotes
56
Cenotes
surface connections to subterranean water bodies
57
features of impact basins
inner and outer ring
58
Suevite
rock consisting of partially melted material, typically forming a breccia of glass and crystal or lithic fragments from impact event
59
Coesite
polymorph of silicon dioxide formd at very high pressure and high temp
60
How does compression of basins happen - eample of compressed basin
lithospheric shortening - orogeny Like the Alps
61
Basin inversion
When the normal faults formed during extension become reverse faults under compression
62
Other form of regional uplift but not by lithospheric shortening
epeirogenic uplift
63
epeirogenic uplift
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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