Turbidite =
Sequence left behind by a turbidity current
Turbidity current =
- submarine mass transport
(shelf edge provides gravitational instability) - fluid = lubricant
Primary mechanism of transporting sediment from continental slope onto abyssal plain
Form of a turbidity current
Smallest sediment suspended in water column = turbidity plume
Head = before main pulse?
Bouma sequence explained
Initially so fast = laying down LAMINATIONS
Slowed = affected by different currents = RIPPLES
Just settling taking place = PLANAR LAYERS
Bouma sequestion
Massive/graded sand
Plane // laminae sand
Ripples/waves/lamination/convoluted laminae of sand/silt
Laminated silt/mud
Laminated mud
Shelf vs slope angles and their impact
Shelf ~1’
Slope ~4’
Neritic energy effect to overcome
Shelf =
intermediate storage (for thousands/millions of years)
What is the shelf itself made up of
Carbonates
N.B. Therefore likely to find carbonates in turbidites (rather than clastics) due to low transporting distance!
What affects carbonate deposition
Light (=depth)
T
Waves
Tides
Types of carbonate shelf
- Reef
- Unrimmed (open shelf)
- Mounds
diagrams
Reef types
Fringing
Barrier
Atoll
- lithified at shallow depths = preserved
Components of a reef
“Barrier Reef Facies Association”
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
Reef composition through time
Has changed
Initially = amalgamations of algal material
Skeletal reefs have come and gone with aragonite (:() and carbonate (:)) seas
Why do narrow shelves occur?
Due to sediment damming
Common at collision margins
More found in Atlantic than Pacific
Basement ridge =
Shelf itself just the sediment deposited on top
Shelf sediment sources
RIVERS
- primary terrigenous sediment source
- littoral energy fence = mostly mud
TEMPESTITES
- turbidite sand source
TIDES
- tidal currents ~0.5 m/s
STORMS AND WAVES
How do storms provide a source of shelf sediment?
Bring sediment further up onto continental crust
Then eroded and transported into turbidites
How do waves provide a source of shelf sediment?
“Sand waves”
Long form ripples
- amplitude <=15m
- wavelength <=600m
Low wave action
Diagnositic tidal asymmetry
- slow creep of material onto continental crust
Issues with the Bouma sequence
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)
Traction mat =
very fine layer of mud that massive blocks can slide on
- analogous to water for fine grains
“A spectrum of gravity flow deposits”
Rockfall
Sliding
Slumping
Mass flow
Turbidity current
Olistolith =
Big, coherent unit moved by olistrome but big enough to map (100s m)
MISLEADING
Olistrome =
Chaotic, mappable melange of mud/blocks due to gravity = downslope
Sedimentary logs of turbidites
Coarse –> medium –> fine
- depends on sediment availability/distance from source
- can be coarse/fine-grain dominated
