3.2- Alluvial And Fluvial Sediments

Question 1

What is an alluvial fan?

Answer 1

It is a fan or cone shaped deposit of sediment built up a river as it decreases in velocity (often associated with change in slope)

Question 2

How can alluvial fans form?

Answer 2

• Can form from accumulations of sediment formed by deposition as trunk streams emerge from upland drainage basins into some sort of lowland basin.
• They have long profiles that concave upwards \__
• They have cross profiles that convex upwards (n)

Question 3

Where are alluvial fans common?

Answer 3

• High relief areas eg. Base of mountain ranges or below fault scarps
• Sparsely vegetated areas with infrequent but violent floods
• Humid areas with intense rainfall

Question 4

What happens when alluvial fans become inactive?

Answer 4

They become vegetated

Question 5

What can be used to date alluvial fans?

Answer 5

• Vegetation and sediment
• Can see the uplift of the mountain chain and weathering rate

Question 6

What are dry fans?

Answer 6

Those in semi-arid climates where flow over fan surface may be regarded as ephemeral

Question 7

What are wet fans?

Answer 7

Fans subject to perennial flow and where stream flow is an important mechanism of transport and deposition

Question 8

What is the most important environment of fan deposition?

Answer 8

In fault-bounded sedimentary basins where periodic fault movement enables subsidence and hence preservation of the fan sediments to occur.

Question 9

Where is an example of merging fan systems?

Answer 9

Death Valley

• There are multiple sediment influx centres

Question 10

What are characteristics of an alluvial fan?

Answer 10

• Poorly sorted and dominated by gravels
• Show down-fan decrease in grain size/bed thickness
• Show down fan increase in sediment sorting

= poorly sorted at the top and finely sorted at the bottom

Question 11

What are the 2 processes of deposition in alluvial fans?

Answer 11

1. Debris flow-dominated fans
2. Stream flow-dominated fans

Question 12

What are the characteristics of debris flow dominated fans?

Answer 12

• Characterised by lobes
• Poorly sorted, coarse sediments and muddy matrix

(A debris flow on an alluvial fan = the conglomerate is poorly sorted with larger clasts surrounded by a matrix of finer sediment)

Question 13

What are debris-flow dominated fans?

Answer 13

• When there is a dense mixture of water and sediment, transport and deposition are by debris flow, a viscous slurry of material that spreads out on the fan surface as a lobe.
• Debris flows do not travel far and a small relatively steep alluvial cone is built if this is the dominant process.

Question 14

What are stream flow dominated fans?

Answer 14

When more water is available, the mixture of sediment is more dilute, deposition will either be in confined sheet floods, or flow will be constrained to channels on the surface.

Dilute, water-lain fan deposits form fans with shallower slopes and greater radial extent (around 10km).

(Sheet floods on an alluvial fan can be seen in rock stratification)

Question 15

What are the characteristics of stream flow dominated fans?

Answer 15

• They are characterised by more sheet like deposits of gravel, sand and silt
• They are moderately well sorted, cross-bedded, laminated or structureless

Question 16

What are the processes of deposition determined by?

Answer 16

• Availability of water (climate)
• The amount and type of sediment being carried from the feeder canyon
• The gradient of the fan surface

Question 17

What are the importance of alluvial fans?

Answer 17

• They provide insight into source rocks from the eroded mountain chain
• Fans provide evidence of earths early history as they were more common before the rise of land plants in the Devonian

Question 18

How are alluvial fans important regarding water?

Answer 18

They are very porous and permeable fan deposits are the primary source of groundwater, which is used for irrigation and for water supply.

(Especially in arid or semi-arid climates)

Question 19

How are alluvial fans important in places like Iran and Iraq?

Answer 19

• Settlements are built on the edges of fans

They can exploit alluvial fans for their water supply. (Economies rely on fans)

• A Kariz is a system for transporting water from an aquifer or water well to the surface through an underground aqueduct. This prevents water being lost to evaporation on the surface.

Question 20

What is the Witswatersrand Basin in South Africa?

Answer 20

It is a placer deposit or placer which is an accumulation of valuable minerals (diamond and gold) formed by gravity separation from a specific source rock during the sedimentary processes.

• It is one of the greatest gold-producing areas of the world
• There are 6 major goldfields in the basin, gold is concentrated in ancient fan deposits from the source areas of granite that originally contained gold.
• There is evidence that each of these fields is associated with a wet fan that developed where a large river discharged from the source rocks.

Question 21

Where is gold and diamond found in an alluvial fan?

Answer 21

Can use fans to predict where gold and diamond deposits are as they are heavier and sink before other minerals like quartz which are transported further along the fan.

Question 22

What are the 2 main categories of fluvial systems?

Answer 22

1. Meandering (sinuous, single-channel)
2. Braided (Multiple channels)

Question 23

Where are braided channels common?

Answer 23

• In glaciated areas
• In tectonically active areas

Question 24

What is sinuosity?

Answer 24

The length of the stream channel to the length of the valley

Question 25

How is sediment transported round a bend in a river

Answer 25

The flow around bends is a helical flow.

• There is erosion on the outside of the bend and deposition on the inside of the bend
• Material eroded from the channel causes lateral accretion and is characterised by cross-bedding and general fining upward

Question 26

What is lateral accretion?

Answer 26

It is when sediment accumulates at the side of a channel or bank.

Question 27

How is an oxbow lake formed?

Answer 27

Continued meander migration leads to channel cut-off or river avulsion.

The lake is then isolated away from the river as sediment and clay deposits block it from the newer straight river channel.

Question 28

How are natural levees formed?

Answer 28

It is a form of vertical sedimentation.

• The river floods and sediment is deposited on the banks of the river. Coarsest at channel edge and finer sediments are travelled further onto the floodplain.
• Repeated flooding creates natural levees as more sediment is deposited each time.

Question 29

How are crevasse splay deposits made?

Answer 29

They are fan shaped sediment accumulations that are deposited as the result of a breaching of the natural levee.

When this happens, water carries sediment and it accumulates in a thin, fining upward and outward layer of fine sand and mud.

Question 30

What is a point bar?

Answer 30

Where sediment is accumulated at the inside bank of a river as velocity drops.

Question 31

What is avulsion?

Answer 31

It is rapid abandonment of a river channel and the formation of a new channel.

• Avulsions occur as a result of channel slopes that are much less steep than the slope of the river could travel if it took a new course (wants to take fastest route)
• They typically occur during large floods which carry the power necessary to rapidly change the landscape. Also dam removal.

Question 32

What are the controlling factors of fluvial channel morphologies?

Answer 32

• Grain size
• Type of sediment load
• Fluid discharge
• Discharge regularity
• Size of drainage area
• Bank stability (lithology, vegetation)

As well as regional slope and tectonic events

Question 33

What is the architecture of fluvial deposits determined by?

Answer 33

The rates of subsidence and frequency of avulsion

Question 34

What does more frequent avulsion lead to?

Answer 34

More lateral migration (deposits are spaced)

Question 35

What does less frequent avulsion lead to?

Answer 35

More lateral migration (Deposits are more closely stacked)

because the sediment that is transported by the river or stream is not spread out over a large area. When avulsions are infrequent, sediment tends to accumulate in a more limited area, creating thicker and more tightly packed deposits.

Question 36

What is the significance of fluvial sediments?

Answer 36

Can tell how oil and gas migrate and are going to be stored.

Can identify where aquifers will be

Sediments can be used for c02 sequestration

Question 37

What happens when there is a slow subsidence rate?

Answer 37

This means there is a gradual sinking of the land.

In areas with slow subsidence rates, the river channel may have enough time to adjust to the sinking land, by deepening its bed and widening its floodplain. As a result, the river may not overflow its banks as frequently or as severely as it would in areas with rapid subsidence.

Question 38

What happens when there is a fast subsidence rate?

Answer 38

This can cause the river to overflow its banks and flood the surrounding areas more frequently and more severely.

• Rapid subsidence can cause rivers to flood by lowering the elevation of the land relative to the river channel. When the land subsides rapidly, the river channel may not have enough time to adjust its course or deepen its bed, which can lead to the river overflowing its banks and flooding the surrounding area.

This can lead to changes in the rivers channel shape and flow patterns, as well as increased sediment accumulation.