Fluvial Systems Flashcards
(17 cards)
Fluvial Subenvironments
– Point bars and lateral accretion complexes
– Chute bars
– Channels and their fills
– Braid bars of braided rivers
– Natural levees and crevasse splay deposits
– Floodplain deposits
– Oxbow lakes
Sediment characteristics of fluvial systems
- Sand, gravel and mud (floodplains)
- Moderately- to poorly-sorted
- Point bars and braid bars
– Fining-upward - Migration of channels
– Fining-upward succession - Channel lags are overlain by
– Point bar deposits
– Floodplain deposits
– Multiple episodes of migration = stacking of such successions - Multiple episodes of channel shifting and bar migration (braided rivers)
– Vertical stacking of bar deposits (cyclic sequences)
Alluvial fans
(shape, occurence, sediments)
- Network of branching distributary channels
- Coned-shape to arcuate in plan view
- Convex-up xs-sectional profile; Concave-up long profile
- Fairly steep depositional slopes
– Greatest at the fan apex - common in areas of…
– high relief; base of a mountain range - Abundant supply of sediment is available
- Sediments are usually …
– Poorly-sorted and abundant gravel-size (w muddy matrix)
– downfan ↓ in grain size & bed thickness
– downfan ↑ in sediment sorting
Alluvial fans and outwash plains
Sparse vegetated arid, semiarid regions
– Sediment transport:
* Rare, irregular (catastrophic)
–>Sudden cloudburst
* merge downslope into desert-floor env. with internal drainage (e.g. Playa lakes)
Humid and glaciated areas
– Where rainfall or meltwater is intense
– May merge downslope with
* alluvial or deltaic plains
* Beaches or tidal flats
* May build into lakes/ocean (fan delta)
Debris-flow-dominated fans
source area of high relief, flows down narrow channel, spreads out at bottom of channel
Flow expansion
Stream power ↓ = loss of competence
Hence, leads to deposition
changes along fan:
longitudinal bars - fine gravel mid fan
transition - medium gravel mid fan
sheet bars - upper fan
Stream-flow dominated fans
Generally larger fans
Sheet-like deposits
Gravel, sand, and silt
Alluvial fan successions
Many beds may display no obvious vertical grain-size trends
* Others may become finer or coarser upward
* Thickening- and coarsening-upward
successions
– Active fan progradation
* Thinning- and fining-upward successions
– Inactivity or fan retreat
* Successions may be»_space; 100 m in thickness
* Grade laterally into other environments
Braided – bedload channels
Multiple channels separated by bars and islands
* Steep gradients
* High width/depth ratios (> 40)
– Wide, shallow stream bed (e.g. Brahmaputra = 50:1 – 500:1)
* Low sinuosity (1.2 – 1.2)
* Tend to migrate laterally
* Channel fills are coarse-grained
– Little suspended load material
proximal braided river (lower alluvial fan)
- Longitudinal gravel bars
- Lateral migration of bars leads to tabular sheets of gravel
braided river & distal braid plane
- Dunes
- Ripples
- Wide channels
- Flat linguoid sand bars
- Wide floodplain rarely inundated by flash floods
e.g. Distal outwash plains of glaciated areas
When glaciers and volcanoes meet…
Outburst floods - geohazard
Large outwash (braided) plains - meltwater
Conglomeratic alluvial facies
Meander –mixed-load systems
- Sinuosity (> 1.5)
- More floodplain deposits than braided rivers
- Channels are more stable (but they do
migrate!)
– Erosional bank
– Point bar deposit
– Natural levees
Channel floor
* Sediment lags
– Coarsest material transported by the river
* Peak flow
– May contain mud clasts or blocks eroded from the banks
– Flat, elongate bars
* Imbricated gravel
* Crudely laminated and planar xs-bedded gravelly sand
Architectural elements
LA = Lateral accretion unit
Main architectural element of point bars
The base is related to the channel, which is overlain by the LA due to channel migration
Point bars
- Deposition in areas of lower velocity turbulence
- Sediment moves up and out of the channel onto the bar
– Fining-upward sequences
* Sand on top of channel lags
– Internal structures
* From horizontal bedding (upper flow regime)
* To trough xs-bedding (lower flow regime)
– Lateral accretion complexes
* “epsilon” xs-bedding
* Ridge-and-swale topography
Chute bars
Generated during flood stage
Coarse-grained bedload material
Floodplain deposition
- Natural levee deposits
– Build up during moderate floods
– Horizontally stratified fine sands overlain by laminated silt/mud - Floodplain deposits
– Fine-grained sediments (through settling)
– Laminated beds (few millimeters)
– Plant debris
– Bioturbation (land-dwelling organisms or plant roots)
– Soil forming processes
– Backswamps and lakes when the wt is at the surface (humid
climates) - Crevasse-splay deposits
– Rapid sedimentation from traction/suspension
-> May resemble a Bouma sequence
– Grade into the fined-grained floodplain deposits
Facies associations
meandering river deposit:
- lateral accretion (point bar) facies
- floodplain (overbank) facies
Suspended-load systems
- High sinuosity single channels
- Great stability
- Low width/depth ratio (< 10)
- Gradient and stream power are usually low
- Channel fills
– High proportion of silts/muds - Silty levee deposits are well developed
Anastomosed rivers
Mud-rich sed. source
V. low river gradient
Seasonal water budget
V. low gradient in a
subsiding basin maintained for a long time
e.g. Foreland basin settings
