Sediment Gravity Flows and their Deposits

Question 1

Sediment gravity flows

Answer 1

Fluid assisted movement of sediments that is gravity driven.

 Caused by a breakdown in grain packing.

 Occur when internal friction and cohesiveness is low

 Can occur in both subaerial and subaqueous environments

Question 2

turbidity currents - uniform type

Answer 2

uniform flows lack a turbulent head
-similar velocities as surge-type flows
-flow can occur on slope <1 degree
->sediment-laden rivers run into lakes (muddy rivers discharge)
-»Less likely in marine settings owing to lower density contrasts

occurs more in fresh water as it is less dense than salt water ->more easily reaches density contrast

Question 3

Structure of surge-type turbidity currents

Answer 3

The head
 Twice as thick as the rest
 Intense turbulence
 Divided transversely into lobes and clefts
 Coarser material, may be a region of erosion

 The body
 Flow is nearly steady and uniform in thickness
 The body flows at a faster velocity than the head
 Process of mixing
 Region of deposition

 The tail
 Thins abruptly away from the body and becomes more dilute

Question 4

Discovery of Turbidity currents and Turbidites

Answer 4

A turbidity current triggered by the 1929 grand banks earthquake off Nova Scotia appears to have traveled south across the floor of the Atlantic
 over a distance of >300 kilometers
 at a velocity up to 19msec-1, based on timed breaks in submarine telegraph cables

the combination of the Newfoundland Tsunami (ca. 1952), and early mapping of the ocean floor led to improved understanding of turbidity currents, in the 1950s

Question 5

Turbidity currents (steady, uniform type)

Answer 5

Uniform flows lack a turbulent head
 Similar velocities as surge-type flows
 Flow can occur on slope < 1 degree
 Sediment-laden rivers run into lakes (muddy rivers discharge)dsxzAQ
 Less likely in marine settings owing to lower density contrasts

Question 6

low and high density flows

Answer 6

low density flows
- 20-30% grains
-clay, silt, fine-medium sand
-suspended entirely by turbulence

high density flows
->30% grains
-may include coarse grained sands and pebble to cobble size clasts
-turbulence and hindered settling from high sediment concentrations

-the two types may occur along a single current