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Sedimentology > Week 8 > Flashcards

Week 8 Flashcards

1
Q

Turbidite =

A

Sequence left behind by a turbidity current

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2
Q

Turbidity current =

A
  • submarine mass transport
    (shelf edge provides gravitational instability)
  • fluid = lubricant

Primary mechanism of transporting sediment from continental slope onto abyssal plain

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3
Q

Form of a turbidity current

A

Smallest sediment suspended in water column = turbidity plume
Head = before main pulse?

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4
Q

Bouma sequence explained

A

Initially so fast = laying down LAMINATIONS

Slowed = affected by different currents = RIPPLES

Just settling taking place = PLANAR LAYERS

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5
Q

Bouma sequestion

A

Massive/graded sand

Plane // laminae sand

Ripples/waves/lamination/convoluted laminae of sand/silt

Laminated silt/mud

Laminated mud

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6
Q

Shelf vs slope angles and their impact

A

Shelf ~1’

Slope ~4’

Neritic energy effect to overcome

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7
Q

Shelf =

A

intermediate storage (for thousands/millions of years)

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8
Q

What is the shelf itself made up of

A

Carbonates

N.B. Therefore likely to find carbonates in turbidites (rather than clastics) due to low transporting distance!

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9
Q

What affects carbonate deposition

A

Light (=depth)

T

Waves

Tides

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10
Q

Types of carbonate shelf

A
  1. Reef
  2. Unrimmed (open shelf)
  3. Mounds

diagrams

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11
Q

Reef types

A

Fringing

Barrier

Atoll

  • lithified at shallow depths = preserved
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12
Q

Components of a reef

“Barrier Reef Facies Association”

A

Reef slope

  • talus production
  • agitated waters
  • waves breaking

Framework

  • highest wave energy
  • highest range of organisms

Backreef

  • sheltered from all but largest storm waves
  • low diversity
  • ?hypersaline
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13
Q

Reef composition through time

A

Has changed

Initially = amalgamations of algal material

Skeletal reefs have come and gone with aragonite (:() and carbonate (:)) seas

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14
Q

Why do narrow shelves occur?

A

Due to sediment damming
Common at collision margins

More found in Atlantic than Pacific

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15
Q

Basement ridge =

A

Shelf itself just the sediment deposited on top

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16
Q

Shelf sediment sources

A

RIVERS

  • primary terrigenous sediment source
  • littoral energy fence = mostly mud

TEMPESTITES
- turbidite sand source

TIDES
- tidal currents ~0.5 m/s

STORMS AND WAVES

17
Q

How do storms provide a source of shelf sediment?

A

Bring sediment further up onto continental crust

Then eroded and transported into turbidites

18
Q

How do waves provide a source of shelf sediment?

A

“Sand waves”

Long form ripples

  • amplitude <=15m
  • wavelength <=600m

Low wave action

Diagnositic tidal asymmetry
- slow creep of material onto continental crust

19
Q

Issues with the Bouma sequence

A

Not always a complete sequence due to:

  • material availability
  • distance from source (far = large already deposited)
  • top band often eroded (e.g. flute casts indicate)
20
Q

Traction mat =

A

very fine layer of mud that massive blocks can slide on

  • analogous to water for fine grains
21
Q

“A spectrum of gravity flow deposits”

A

Rockfall

Sliding

Slumping

Mass flow

Turbidity current

22
Q

Olistolith =

A

Big, coherent unit moved by olistrome but big enough to map (100s m)

MISLEADING

23
Q

Olistrome =

A

Chaotic, mappable melange of mud/blocks due to gravity = downslope

24
Q

Sedimentary logs of turbidites

A

Coarse –> medium –> fine

  • depends on sediment availability/distance from source
  • can be coarse/fine-grain dominated
25
Q

Debris =

A

Rounded at bottom of sedimentary log
Represents traction mat (quick change)

  • initial period of coarsening up then well sorted and fining
26
Q

Submarine fan morphology

A

Multiple lobes

Surface levees/channels

Wedge
- more pronounced due to high energy

Large scale = look like alluvial fan

Smaller scale = rivers with levees/floodplain

27
Q

Submarine fans; varying scales

A

Mud rich = 1000km

Mixed = 100km

Sand rich = 10km

28
Q

Repeated pulses in submarine fans

A

Feeds system

So difference in climate = different sedimentary flux response

29
Q

Controls on submarine fan cycles

A

Low SL = river carries material further
= low energy required for “littoral energy fence”
= more turbidites

LOW SYSTEMS TRACT

30
Q

What does the disappearance of turbidites indicate?

A

SL rise

Sedimentology (9 decks)

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