Alluvial Fans Flashcards
Happens because
of an abrupt change in gradient, resulting in instant loss in velocity and straight deposition.
Grading:
Coarser at the top and finer material deposited last
Why fan shaped?
- 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
Sediment transport processes:
Predict how the deposits will occur:
- Current
- Debris
- Grain flow
- Hyper concentrated flood flow
Debris Flow dominated fans:
- Distinct channels
- Hummocky surface (freezes from bottom up)
Steep slopes
Debris flow deposits:
- Very poorly sorted
- Matrix supported
- angular to subrounded
- Massive to indistinct bedding - bedding hard to find
Stream flow dominated fans:
- 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)
Stream flow deposits:
- 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
Facies Model
- 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
Debris/mud flow fans facies model
- mix of mud and coarser sediment
- lower water/sediment
Fluvial/sheet flow fans:
Deposits of coarser sediments; favoured by higher water/sediment ratio
Fan size and morphology:
- 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
Interacting fans
- seldom in isolation
- Adjacent fans interact
Swamps between fans - Lakes or rivers on valley floors
Holden Fan on Mars:
- 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)
Screeslope/Talus Cone - grain flow sediment transport:
- Very steep slope
- Few to no distinct channels
- Smooth surface
- Might get mistaken for alluvial flow in records
Talus Cone/Screeslope - grain flow deposits
- clast supported -little matrix
- Angular Breccia
- Moderately Sorted
- No bedding
- Rarely Preserved
Landslide formed fans
- 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
Channel Avulsion
Channel changes from to another.
- Usually happens in a flood .
- Bed is getting shallower and shallower until the channels move to a steeper slope
Avulsion:
- catastrophic change in channel location. Usually avulses to a steeper gradient slope
Current transport process:
Constant flow through the channel
Debris transport process
Intermittent flow through channel. Viscous (high sediment to water ratio)
Hyper concentrated flood flow (HFF):
Like a turbidity current in air
