Alluvial Fans
What is the facies model like for the overbank deposits?
Finer-grained sediments that could contain rootlets. Evidence of flooded can sometimes be seen between these depositions, characterised by planar bedding or ripples.
What is the stratigraphy like in the facies model?
There will be a lag of pebbles at the bottom of the stratigraphy, followed by trough bedding which formed in flood periods. As flow decreased, planar bedding formed. Ripples can also be seen on bars.
How are intramontane basins formed?
They are formed where neighbouring alluvial fans feed into a closed-system valley.
What are the sediment bars like at the distal end of a braided river?
They are generally sand, but the rivers still carry very fine-grained sediments.
At the proximal end of a braided river, what are the sediment bars like that divided the river flow?
They are gravel dominated, but are not long enough to establish plants.
Why are alluvial fans significant both palaeoclimatically and palaeotectonically?
They can describe depositional and weathering conditions, as well as local topography and sediment constituents.
What is an alluvial fan?
A semi-circular to conical accumulation of coarse-grained sediments.
What is a crevasse splay?
An area of coarse grained sediments brought onto the inter-bank area when a levee breaks.
What are stream channel deposits?
Areas of sand between major flood fan deposits.
Where are braided rivers generally found?
At the toe of alluvial fans.
What is avulsion?
Avulsion is where a channel is abandoned, and a new, straighter channel is created.
In the facies model, what bedforms can be seen?
Bar surfaces can be seen. They can have straight crests, which form in shallow water, or sinuous crests, which form in deeper water. They form planar and trough cross bedding.
In sinuous rivers, where does deposition and erosion occur?
Deposition occurs on the inside of a meander, and erosion occurs on the outside.
How do oxbow lakes develop?
Erosion of the river causes a loop to cut across, taking the shorter route. The old loop becomes abandoned by the river flow.
How does lateral migration occur?
Erosion on the outside of the bend, and deposition on the inside cause the meander to migrate in that direction.
Where do debris flow fans forms?
In dry, harsh environments.
Where is grain size the largest in a sinuous river?
In the thalweg - the line of fastest flow.
What kind of alluvial fans do sheetflood flows produce?
Lower relief, wet fans.
What are the deposits of sheetflood fans like?
Moderately sorted, clast supported sediments that are normally graded. Contain imbricated clasts.
What kind of deposits do debris flows form?
Poorly sorted, matrix supported sediment with random clast orientation and no sedimentary structures.
How do alluvial fans form?
Steep channels and other sediment sources feed out onto flat planes. The energy of the system drop dramatically, leading to the deposition of more coarse-grained sediments.
What are the 4 river forms?
Straight, sinuous, anastomosing and braided.