Alluvial Fans

Question 1

Happens because

Answer 1

of an abrupt change in gradient, resulting in instant loss in velocity and straight deposition.

Question 2

Grading:

Answer 2

Coarser at the top and finer material deposited last

Question 3

Why fan shaped?

Answer 3

• There are multiple channels, but not all active at the same time due to avulsion
• Continuous deposition from active channels, resulting in shallowing gradients and thus leading to avulsion.
• will go into whatever shape is available

Question 4

Sediment transport processes:

Answer 4

Predict how the deposits will occur:

• Current
• Debris
• Grain flow
• Hyper concentrated flood flow

Question 5

Debris Flow dominated fans:

Answer 5

• Distinct channels
• Hummocky surface (freezes from bottom up)
  Steep slopes

Question 6

Debris flow deposits:

Answer 6

• Very poorly sorted
• Matrix supported
• angular to subrounded
• Massive to indistinct bedding - bedding hard to find

Question 7

Stream flow dominated fans:

Answer 7

• shallow slopes
• Distinct channels
• Vegetation common
• Smoother surfaces
• Water running pretty constantly or a pretty dilute flow
• River is flowing down the district channels (rivers don’t move as much as we maintain the stop banks)

Question 8

Stream flow deposits:

Answer 8

• clast supported
• well rounded to subangular
• Moderately sorted
• Indistinct to well bedded
• Expect to see vegetation and soil forming on the slope because of gentleness

Question 9

Facies Model

Answer 9

• migrating channels in fan shape
  Coarse at proximal to fine at distal
• Channel numbers increase from proximal to distal
• Channels filled with different deposits depending on process (debris flow or stream flow

Question 10

Debris/mud flow fans facies model

Answer 10

• mix of mud and coarser sediment

- lower water/sediment

Question 11

Fluvial/sheet flow fans:

Answer 11

Deposits of coarser sediments; favoured by higher water/sediment ratio

Question 12

Fan size and morphology:

Answer 12

• Depend on catchment area, rock types, rainfall, gradient and sedimentary processes.
• if the drainage basin is really big you might have time to collect enough water to have a constant flow of water. Small catchment often means that you have more sediment than water

Question 13

Interacting fans

Answer 13

• seldom in isolation
• Adjacent fans interact
  Swamps between fans
• Lakes or rivers on valley floors

Question 14

Holden Fan on Mars:

Answer 14

• Crater can give us relative age but not much in relation to the facies model
• Lumpier surface suggests Debris Flow
• Channels more distinct near apex (narrow part of alluvial fan)

Question 15

Screeslope/Talus Cone - grain flow sediment transport:

Answer 15

• Very steep slope
• Few to no distinct channels
• Smooth surface
• Might get mistaken for alluvial flow in records

Question 16

Talus Cone/Screeslope - grain flow deposits

Answer 16

• clast supported -little matrix
• Angular Breccia
• Moderately Sorted
• No bedding
• Rarely Preserved

Question 17

Landslide formed fans

Answer 17

• Smooth face at head scarp
• hummocky surface of fan
  few or no channels
• More intact blocks uphill
• Can create fans from landslides
• Smooth surface at top but not a gully feeding it

Question 18

Channel Avulsion

Answer 18

Channel changes from to another.

• Usually happens in a flood .
• Bed is getting shallower and shallower until the channels move to a steeper slope

Question 19

Avulsion:

Answer 19

• catastrophic change in channel location. Usually avulses to a steeper gradient slope

Question 20

Current transport process:

Answer 20

Constant flow through the channel

Question 21

Debris transport process

Answer 21

Intermittent flow through channel. Viscous (high sediment to water ratio)

Question 22

Hyper concentrated flood flow (HFF):

Answer 22

Like a turbidity current in air