Morphology of the ocean floor

• gradient of the shelf is usually less than 1:500 or 1-0.1^o
• gradient of the slope is 1-6^o
• gradient of the rise is about 4^o or between 1:90 and 1:700
• gradient of the abyssal plain is less than 1:1000

• submarine fans build up on the rise and abyssal plain at the foot of the slope and at the end of large submarine canyons

Morphology of modern fans

• some fans are very large eg the Indus fan with a volume of 1 x 10⁶ km³

• large submarine channel which feeds sediment onto the fan

• slides and slumps from the slope = mass gravity deposits

• early models suggested that fans comprised
  • the upper fan with a single channel with levees
  • the mid fan with suprafan depositional lobes at the end of channels
  • the flat lower fan without channels
• the upper fan is cut by a main channel with well developed levees

• slump and debris flow complexes from slope or channel levees

• channels are highly sinuous and move across the fan

• no depositional lobes at the end of channels

• well developed levee complexes - prone to slumping and collapse

• many channels are filled with mud

• channel complexes become smaller down fan

• the lower fan is characterised by sheet-like sand bodies with few or no channels

Processes operating on fans and sea floor

turbidity currents

• type of density current where the increased density is due to sediment load

• the current is turbulent and sediment is carried in suspension

• as the flow slows down, coarser grains settle towards the base and finally freeze in place

• form regular sequences of interbedded sandstone and mudstone

• sharp bases with erosional and deformational structures

• show graded bedding with upward fining

• Bouma sequence - divided into 5 zones

fluidised flow

• upward drag exerted by moving pore fluids exceeds the weigh of the grains

• sand bed expands, porosity increases and the sand becomes fluid supported

• form thick, non-graded, clean sands with abrupt bottoms and tops; dewatering structures (sand pipes and dish structures)

grain flow

• liquified, cohesionless particle flows in which the intergranular friction between sand grains is reduced by their continuous agitation (particles intermittently hit each)

• need a steep slope and a confined space to retain the high pore pressures (canyons)

• well sorted and clay free; massive; abrupt bases, loading structures; sharp tops

debris flow

contour currents

• contour currents rework ocean bottom sediments ---> contourites

• due to geostrophic flow - tend to parallel the continental rise

• rework fine sand and mud into small ripples
  

Facies found on deep sea fans

Classical turbidites

• thick bedded or proximal
• thin bedded or distal
• CCC turbidites- climbing ripples, convolute lamination, rip up clasts
• high rates of sedimentation; erosive currents
• deposited high on channel margins or levees

Massive sandstones

• little grading,up to many metres,erosive bases,water escape structures
• found within the channel
  

Pebbly sandstones

• well graded, stratified (plane or cross), imbricated pebbles
• channelised and laterally discontinuous
  

Conglomerates

• graded or stratified
• clasts may be imbricated
  

Pebbly mudstones

• clasts in a muddy matrix
• deposits of debris flows

Facies successions and models

classical fan models

A classical fan model and the range of facies is shown in the diagrams.

prograding lobes

two repeated sequences: thinning upwards and thickening upwards cycles

thickening upwards cycles = progradation of the suprafan lobes

thinning upward cycles = filling of abandoned submarine channels due to lobe abandonment

OR

gradual lobe shifting

OR

fining upwards on a channel levee

models do not match will the successions and morphology found on modern fans

importance of channel levee complexes

• seismic profiles show large channel complexes on both the upper and mid fans

• some channel fill sequences up to 580 m thick

• channel levee systems vary from 150 -500 m in height and in width from 30-250 km

• become smaller down fan

• slumps and slide failures - common on levees (some scars up to 250 m high and deposits can have volumes of 300-1500 km³ and have travelled for 200-300 km)

• mass transport complexes = products of large debris flows

Allostratigraphy

recent models relate fan cycles to eustatic sea level changes

1. lowering of relative sea level initiates a phase of fan growth

• deposition near the shelf edge may cause slope failure and canyon initiation
  
• deposition of mass transport complexes
  

2. fan builds up during lowstand - channel levee complexes develop

• sand deposition on lower fan
  
• bypass the channel systems, or
• sands move via less sinuous channels early in fan development
  

3. Mud deposition during transgression

• channels fill with mud
  
• slumps and debris flows from the slope and levees complexes
  

4. during highstand the fan surface is blanketed by mud

5. deposition of sediment via turbidity currents (one flow every two years)

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