Depositional Environments

Question 1

What is ‘Walther’s Law’? [1894]

Answer 1

Particular facies which are side by side at any one time will come to overlie one another through time in the form of a sedimentary succession. In other words, depositional environments migrate laterally so sediments as part of one environment will come to lie on top of another as part of a vertical system.

Question 2

What does the term ‘prograding’ mean and to what setting does it apply?

Answer 2

Prograding is the progression of sediment into the sea from the continuous build up of sediment. It applies to shallow marine settings.

Question 3

Which factor is the best to be used to define a desert? And why should temperature not be used?

Answer 3

Temperature is not a good factor to base the classification of desserts on, as most deserts are actually cold. Instead, we should use the aridity, otherwise known as the lack of rainfall. Aridity causes a lack of vegetation which is known to be common for deserts.

Question 4

What are the levels of rainfall for semi-arid and arid environments?

Answer 4

Semi-arid = 500-250mm/yr

Arid =

Question 5

What are hot, arid environments characterised by?

Answer 5

They are characterised by a moisture deficits, where the evaporation level exceeds the precipitation level.

Question 6

What are the four controls on the location of a desert?

Answer 6

Location within a continent, the distance from oceans, the location of mountain ranges and atmospheric circulation.

Question 7

What are the typical sediments transported through aeolian processes?

Answer 7

They will have a typical grain size of 0.1-1mm, as air has a low density and viscosity so cannot carry large sediments. They will also be well rounded as wind currents are effective sculpting agents through mechanical abrasion. Grains will also appear to be frosted [covered in a polish] due to small scale solution.

Question 8

What are the main types of aeolian lamination? [there are three]

Answer 8

Grainfall - grains from temporary suspension fall on the lee side of a slope
Grainflow - sand that has accumulated at the top of the lee slope is unstable, so avalanches down the lee side
Wind ripples - grains move by saltation and creep and patches of sand build up, lining up perpendicular with the wind direction

Question 9

What is the difference in profiles of wind and water ripples?

Answer 9

Water ripples have steeper sides and a larger height, whereas wind ripples are much shallower and have asymmetric profiles

Question 10

What are barchan dunes?

Answer 10

These are crescent shaped dunes where the crests are orientated at 90-15 degrees to the wind direction. They are sand moving dunes and have a low preservation potential.

Question 11

What are transverse dunes?

Answer 11

They have straight crests that are orientated at 90-75 degrees to the wind direction. They are sand trapping dunes and have a high preservation potential.

Question 12

What are the interdune sedimentary structures for wet, damp and dry conditions?

Answer 12

Wet - channels, small deltas, wavy laminae, contorted structures
Damp - algal structures, evaporite structures, rain impact ripples, adhesion laminae
Dry - wind ripples, dune cross beddings, lags, deflation scours

Question 13

What is the difference between fluvial and alluvial?

Answer 13

Fluvial - just the river channel

Alluvial - the whole system including the channels and the floodplains

Question 14

What are alluvial fans?

Answer 14

Cones of detritus that form at a distinct break in slope at the edge of an alluvial plane. They can also form in environments such as deltas. They have a fan apex, fan head trench and a fan toe.

Question 15

What is a debris flow dominated alluvial fan? Where do they occur?

Answer 15

A dense mix of water and sediment, so a viscous slurry will spread onto the fan as a lobe. They occur in humid/semi-arid climates and have smaller steeper catchment areas that encourage a dominance of debris flows.

Question 16

What is an important control on the form of alluvial fans?

Answer 16

Source rock characteristics - if the source rocks [within the catchment area] are of a muddy dominance, then there will be a muddy debris flow.

Question 17

What sort of deposits are found from debris dominated alluvial fans?

Answer 17

Poorly sorted, matrix supported, randomly orientated coasts, no sedimentary structures

Question 18

What is a sheetflood deposit? When do they occur?

Answer 18

Poorly sorted, showing normal grading. They have an erosive base with imbricate structures from left to right.
They occur when there has been an increase in water within the flow.

Question 19

What is a facies?

Answer 19

A body of rock that has specific characteristics and is defined by lithology, colour, bedding, grain size, fossils and sedimentary structures.

Question 20

What is the difference between proximal and distal sediment?

Answer 20

Proximal - coarse alluvium in braided channels

Distal - fine sediment in meandering channels and over bankjflood deposits

Question 21

What are anastomising rivers?

Answer 21

Rivers that flow as multiple interconnected channels which are separated by areas of the floodplain. They form in areas where the banks are stabilised by vegetation and do not show migration of channels; the vegetation roots cause the channels to remain fixed.

Question 22

Why are anastomising rivers difficult to find in the geological record? What is an example of an anastomising river?

Answer 22

It is difficult to prove that more than one channel were active at the same time. An example is the Saskatchewan river in Canada.

Question 23

What is a meandering river?

Answer 23

These are rivers that have a continuously curved path due to erosion and deposition with different current speeds along its course. Sediment is deposited along the inner bank and the outer bank is scoured.

Question 24

What are point bars and the thalweg?

Answer 24

Within a meandering river, the current on the inner bank is the slowest so will deposit sediment in the form of a point bar, building up a lateral accretion surface. The thalweg is the fastest current, which scours out the outer bank as it flows along, creating steep outer banks for the river.

Question 25

What are braided rivers? Give an example.

Answer 25

The sediment within these rivers is carried by saltation and traction, known as the bed load of the river. The bed load is deposited as bars of sand or gravel in the channel which divide up the water. An example is the Brahmaputra river in India.

Question 26

What are the four types of bars found in braided river systems?

Answer 26

Longitudinal bars - elongate, parallel to the channel
Transverse bars - spread across the channel
Compound bars - mix of sand and gravel
Linguoid bars - crescent shaped bars that point downstream

Question 27

What is the difference in flow patters across linguoid bars when there is high or low discharge?

Answer 27

High discharge - flow is directed over the bars

Low discharge - flow is concentrated between the bars

Question 28

What is a key feature that can be formed during flooding?

Answer 28

Natural levees can build up during the rise and fall of sea level. The coarsest sediments deposit at the channel edges and the finer sediments are deposited over the outer part of the floodplain.

Question 29

What is a crevasse splay? What is a typical structure that crevasse splays form?

Answer 29

If natural levees break, water laden with sediment is carried onto the floodplain, creating a crevasse splay. The typical structure formed is climbing cross ripple laminae.