Deep-water sedimentary systems

Question 1

Define a deep-water sedimentary system

Answer 1

Sedimentary systems lying predominantly below the storm wave base (10s-100s metres)

Question 2

What is the main control on deep-water sedimentary systems? What does this result in?

Answer 2

Gravity causes low topography
Once sediment reaches the bottom, it is harder for them to leave it

Question 3

Give a reason for a grain not reaching a deep-water sedimentary system

Answer 3

Getting trapped in fluvial systems

Question 4

What was the Grand Banks earthquake (1929) first evidence of?

Answer 4

Turbidity currents

Question 5

Describe the Grand Banks earthquake (1929)

Answer 5

Triggered a submarine landslide associated with a 20km strike slip fault

Question 6

Why is it important to predict submarine landslides?

Answer 6

Global communication links (submarine cable systems)

Question 7

Give the six physical processes of sediment transport in deep water in order of increasing fluidity and mobility of material

Answer 7

Creep, slide, slump, laminar flow, turbulent flow, grain fall/flow

Question 8

What are sediment gravity flows important for?

Answer 8

Moving coarse sediment to deepwater environments

Question 9

What are the two types of sediment gravity flow?

Answer 9

Fluidal flows (turbidity currents)
Laminar flows (debris flows)

Question 10

Compare turbidites and debrites

Answer 10

Turbidites are better sorted because they decelerate rather than immediately stopping

Question 11

Describe turbidty currents

Answer 11

Newtonian rheology (viscosity is constant)
Particles settle in the absence of turbulence

Question 12

What are the two type of turbidity currents?

Answer 12

Hypopycnal (flows on water)
Hyperpycnal (sinks in water)

Question 13

What triggers submarine landslides?

Answer 13

Earthquakes and oversteepening

Question 14

Give four examples of gravity flow deposits, other than turbide and debrite

Answer 14

Hyperpycnite, pelagite, hemipelagite, channel fill

Question 15

What produces the Bouma sequence?

Answer 15

Idealised low-density turbidites

Question 16

How do turbidites vary from the ‘classical’ fan model?

Answer 16

They actually have complex vertical and lateral distribution of facies

Question 17

Give three characteristics of thick-bedded turbidites

Answer 17

Bed amalgamation, clasts, mix of facies

Question 18

Describe the clasts of thick-bedded turbidites

Answer 18

Intraformational mudflakes, extraformational grains

Question 19

Describe the mix of facies of thick-bedded turbidites

Answer 19

Thin- and thick-bedded turbidtes, debris flow deposits, coaser grains than outside channels

Question 20

Where are thin-bedded turbidites deposited

Answer 20

Often deposited on levees (internal/external to channel belt) or basin plain

Question 21

Describe thin-bedded turbidites deposited on the external levee

Answer 21

External levee sands typically thin upwards and are dominated by climbing ripples and lack erosional structures

Question 22

Describe thin-bedded turbidites deposited on the internal levee

Answer 22

Internal levee sands have multiple ripple sets, a complex palaeocurrent, and are poorly organised

Question 23

Describe thin-bedded turbidtes deposited on the basin floors

Answer 23

Dominated by single ripple form sets

Question 24

What is a hybrid flow?

Answer 24

A flow that has turbidity and debris flow elements or one that is transitional between the two

Question 25

What types of flow produce hybrid beds?

Answer 25

Composite flow and transitional flow

Question 26

Give an example of how a hybrid bed can form

Answer 26

Turbidity flow eroding into seabed, mud is incorporated, causing the flow to collapse, the flow is now transitional between turbid and laminar

Question 27

Define a debris flow

Answer 27

High conc (>40%) mixture of fluid and sediment that flows downslope due to gravity
Particles supported in flow by sediment and pore pressure

Question 28

Describe debris flow

Answer 28

Non-Newtonian rheology, has a buoyancy force, flow stops suddenly, the debrite reflects the flow content (poorly sorted)

Question 29

Where do debris flows frequently occur?

Answer 29

Within canyons or channels in deep-water settings

Question 30

Why can it be difficult to distinguish debrites in cores?

Answer 30

They can be difficult to discern from bioturbated textures and local remobilisation
Large clasts can be difficult to distinguish from laterally persistant mudstones

Question 31

Describe slides and slumps

Answer 31

Dominated by intact, but rotated, bedding (unlike debris flows)

Question 32

Where do slides and slumps occur?

Answer 32

Mainly on slopes but also sides of levees as a result of oversteepenig or flow undercutting

Question 33

Give two limitations of the interpretions of slides and slumps

Answer 33

Can be difficult to recognise in core and the distance travelled may be difficult to discern

Question 34

What can slides and slumps be examples of?

Answer 34

Mass transport deposits (MTD) when they occur in the subsurface or on the seafloor

Question 35

What are three main sections of slope failure?

Answer 35

Rear scarp (source/head) (extensional forces), debris (diplaced blocks), debris apron (accumualation) (compressional forces)

Question 36

Define thermohaline currents

Answer 36

Temperature and salinity driven flows
Powerful enough to move sediment on seafloor

Question 37

How do thermohaline currents rework a channel with a lobe?

Answer 37

Lobe can be broken up and carried in the direction of flow

Question 38

What deposits can thermohaline currents build?

Answer 38

Contourites
These act as topography and can deflect later turbidity currents

Question 39

How can thermohaline currents affect turbidity flows?

Answer 39

Can capture turbidity flow coming down a slope and carry it off in the direction of teh bottom current