Deepwater sediments

Question 1

why does so much sediment accumulate on the continental shelf?

Answer 1

Because thats where the river meets the sea.

Question 2

Are canyons always connected to rivers? Discuss why or why not. Discuss the flow.

Answer 2

They are not always connected to rivers, as they can be fed directly or indirectly by rivers. They can be the place of sediment transport and accumulation.
Intermittent or hyperpycnal flow.

Question 3

How can material be moved down the continental slope? What causes this to occur?

Answer 3

Through submarine slumps and slides, move material from further up the slope down and can cause tsunamis. This is a gravitational collapse of the slope and is triggered by sediment overloading.

Question 4

slide and slump deposits create what?

Answer 4

Mass transport complexes

Question 5

discuss density in association to slides and discuss slides in general

Answer 5

The surrounding fluids are denser as we are at depth, get thrust faults at the base where the sediment starts to accumulate whilst rotational faulting causes the sliding to occur in the first place.

Question 6

detached blocks from sliding are called what?

Answer 6

Olistostromes

Question 7

How does sediment supply get into canyons?

Answer 7

Some from small rivers, indirect movement across the shelf driven by wind or storm events pushes sediment into the canyon.

Question 8

channels can be ….. or ……. on the slope of the basin floor

Answer 8

Channels can be incised or gradational on the slope of the basin floor.

Question 9

what features to channels and rivers share?

Answer 9

Meanders, levees, fine grained overbanks

Question 10

Briefly describe the 4 steps to canyon creation

Answer 10

1. early phase of canyon creation
2. Sediment backfills and is eroded
3. Early backfill
4. Canyon filled up

Question 11

Where are submarine fans created? discuss these features.

Answer 11

The base of the continental slope or on the slope outside canyons as you come off the continental shelf.
They are created by a release from confinement and can spread out over a small or large distance. Morphology depends on the dominant grain size.

Question 12

Give an example of a single and multi point source related to submarine fans

Answer 12

Single point: canyon or hyperpycnal feed

Multi point: slope collapse

Question 13

Discuss mud rich fans

Answer 13

Mud rich fans have a large lateral extent, they are dominated by grain size; fine grained muds and silts grow the largest.

Question 14

Discuss mixed grain size fans

Answer 14

Varied grain sizes, smaller sized. Canyon is incised

Question 15

Discuss Sand rich fans

Answer 15

Smallest fans with the coarsest grains. Can be sheet like further out and have more channel features if more proximal.

Question 16

State the 3 types of submarine fans in size order. Discuss what may change their morphologies and characters.

Answer 16

Mud rich fans are largest (1000km), mixed grain size fans (100km) and sand rich fans are the smallest (10km). Morphologies may be changed by RSL changes, whilst transgressions may change the character of the fans as it changes the sediment supply.

Question 17

Discuss CCD, what depth does it occur at.

Answer 17

Carbonate compensation depth; the depth below which all carbonate goes into solution. >4Km = CCD where all carbonate is in solution, carbonate begins to dissolve >3km.

Question 18

Discuss how turbidity currents move

Answer 18

They are less buoyant than the sea and more dense so they stay at the bottom, they flow in a turbulent motion which keeping sediment in suspension. It stops quickly and collapses.

Question 19

What triggers turbidity currents?

Answer 19

slope failure, subaerial events

Question 20

Discuss the difference between gravity driven and mass flow deposits

Answer 20

Gravity driven is when gravity is the force making the flow move. Mass flow deposits is when the sediment and fluid are moving together as one, the flow is usually fluid but can be subaqueous

Question 21

Behaviour of a flow is controlled by what?

Answer 21

The density of a flow and if it is cohesive or not

Question 22

discuss the difference between cohesive and non-cohesive flows.

Answer 22

cohesive flows have matrix strength binding the sediment.

Non-cohesive flows have water between the grains.

Question 23

What are the different types of density. Can these change?

Answer 23

hyper-concentrated (> 25% sed), concentrated (10-15% sed) or turbidity currents (<10% sed). The type can change over space and time due to erosion or deposition of sediment

Question 24

Discuss a debrite. What is a linked debrite?

Answer 24

Debrites are large debris deposits which conglomerates have left behind, they form from debris flows.

Linked debrites are conglomerates with structureless sand deposited on top of one another. The sand is on the bottom with the conglomerate on top.

Question 25

Discuss a contourite

Answer 25

Thermo-haline (response to temp and salinity) induced deep water bottom current that follows the continental slope.

Question 26

Discuss the complexity of these debrite flows.

Answer 26

At the base there is evidence for bedload transport, so there is much turbulence at the top of it keeping everything in suspension.

Question 27

The deposition of a coarse fraction (debrite freezing) does what?

Answer 27

Changes the flow density. If it turns to a low density flow it wont leave much behind.

Question 28

Discuss bouma sequences

Answer 28

They are a sequence that is always in the same order, you don’t always see every part of the sequence. The high density end of the specturm is from A-B which is has structureless sand or laminations representing high velocity on top, C has ripples due to rapid sediment deposition it is low density. D has settling from suspension, E is deposited between events, it is pelagic or hemipelagic. The mud is deposited later on. At the base of the sequence there may be flute casts or tool marks as evidence of erosion and rapid transport before rapid deposition.

Question 29

Are turbidity currents always in deep water?

Answer 29

No you need evidence such as trace fossils to say it is deep water

Question 30

Where can you get bouma sequences occurring?

Answer 30

Anywhere that is sheltered and below wave base, therefore these can occur in lakes or the sea if waves are not disturbing them.

Question 31

Can you get gravel turbidites?

Answer 31

Yes, if there is the right sediment available

Question 32

How are dish structures created

Answer 32

When sediment is deposited quickly water escapes dragging the sediment upwards creating a dish structure.

Question 33

Why are there no ripples or plane bed lamination in turbidites?

Answer 33

There is less bedload traction

Question 34

Name the mass transport deposit types, try to name them all but ideally remember 3

Answer 34

Escarpment, debris flows, thrust blocks, outrunner blocks, rotated blocks, glide blocks,

Question 35

Discuss the linkage between submarine fans and RSL.

Answer 35

When sea level is high, not alot of sediment goes out into the sea, as RSL drops you trigger deposition into the submarine fan.

Question 36

When in relation to RSL are submarine fans the most active?

Answer 36

They are the most active when RSL is dropped as you can feed sediment into the canyon system (fan) easier.