Seds - Neritic and Turbidites

Question 1

Meaning of Neritic?

Answer 1

along the continental shelf

Question 2

Turbidity current

Answer 2

• Turbidity currents are submarine
• Primary way of moving mass off the continental shelf
• Fluid is a lubricant.
• Sand avalanches as a good analogy
• Smaller grains transported further.
• Smallest sediment suspended in water column as turbidity plume.
• Primary mechanism of transporting sediment from continental slope to the abyssal plain.
• Located at shelf edge due to gravitational instability.

Question 3

Boumba Sequence

Answer 3

• Turbidite is rock sequence left over after turbidity current
• 2 planar lamination phases
o B = Faster = too fast to shake up fine particles
o D = Slower phase – grains not going fast coming out of suspension form second layer of lamination
• Fines upwards
• Very rare that whole sequence is found in nature – more commonly layers are missing

Question 4

Shelf morphology:

Answer 4

• Shelf gradient approximately 1 degree
• Slope gradient approximate 4 degrees
• Neritic energy fence – small slope on shelf where it is difficult to move mass; once at edge it can move down gradient; therefore, an activation energy is needed
• To start turbidites you need rare high energy events too get over energy fence

Question 5

Barrier Reef Facies association:

Answer 5

Reef Slope
Reef Framework
Backreef/ tidal flat
• Need associated facies to distinguish reef type.
• Reef talus production enhanced by wave action.
• Back-reef sheltered from all but largest storm waves.
• Even incomplete waves act as breakwaters for waves

Question 6

Sources of shelf sediment:

Answer 6

• Shelves store sediment for long periods of time.
• Rivers are primary source of terrigenous sediment.
• Littoral ‘energy fence’ means most shelf sediment is mud.
• Tempestites are a source of sand for turbidites.
• Net Tidal currents (approx. 0.5 m/s)

Question 7

Spectrum of gravity flow systems:

Answer 7

• Turbidity currents are the most geologically important of the gravity mass transport systems.
• Olistholiths: A detached block that is a component of an olistotrome.
• Big coherent unit which isn’t in sequence – has fallen off but is mappable
• Block of reef in wrong place
• Olistotrome: A chaotic, mappable, melange of mud and blocks formed by gravity driven slip down-slope.

Question 8

Submarine fan morphology:

Answer 8

• Multiple lobes.
• Sediment supply determines lobe switching.
• Surface levees and channels
• Large scale morphology = alluvial fan like
• Smaller scale = meandering river