Deep-water sedimentary systems Flashcards
Define a deep-water sedimentary system
Sedimentary systems lying predominantly below the storm wave base (10s-100s metres)
What is the main control on deep-water sedimentary systems? What does this result in?
Gravity causes low topography
Once sediment reaches the bottom, it is harder for them to leave it
Give a reason for a grain not reaching a deep-water sedimentary system
Getting trapped in fluvial systems
What was the Grand Banks earthquake (1929) first evidence of?
Turbidity currents
Describe the Grand Banks earthquake (1929)
Triggered a submarine landslide associated with a 20km strike slip fault
Why is it important to predict submarine landslides?
Global communication links (submarine cable systems)
Give the six physical processes of sediment transport in deep water in order of increasing fluidity and mobility of material
Creep, slide, slump, laminar flow, turbulent flow, grain fall/flow
What are sediment gravity flows important for?
Moving coarse sediment to deepwater environments
What are the two types of sediment gravity flow?
Fluidal flows (turbidity currents)
Laminar flows (debris flows)
Compare turbidites and debrites
Turbidites are better sorted because they decelerate rather than immediately stopping
Describe turbidty currents
Newtonian rheology (viscosity is constant)
Particles settle in the absence of turbulence
What are the two type of turbidity currents?
Hypopycnal (flows on water)
Hyperpycnal (sinks in water)
What triggers submarine landslides?
Earthquakes and oversteepening
Give four examples of gravity flow deposits, other than turbide and debrite
Hyperpycnite, pelagite, hemipelagite, channel fill
What produces the Bouma sequence?
Idealised low-density turbidites
How do turbidites vary from the ‘classical’ fan model?
They actually have complex vertical and lateral distribution of facies
Give three characteristics of thick-bedded turbidites
Bed amalgamation, clasts, mix of facies
Describe the clasts of thick-bedded turbidites
Intraformational mudflakes, extraformational grains
Describe the mix of facies of thick-bedded turbidites
Thin- and thick-bedded turbidtes, debris flow deposits, coaser grains than outside channels
Where are thin-bedded turbidites deposited
Often deposited on levees (internal/external to channel belt) or basin plain
Describe thin-bedded turbidites deposited on the external levee
External levee sands typically thin upwards and are dominated by climbing ripples and lack erosional structures
Describe thin-bedded turbidites deposited on the internal levee
Internal levee sands have multiple ripple sets, a complex palaeocurrent, and are poorly organised
Describe thin-bedded turbidtes deposited on the basin floors
Dominated by single ripple form sets
What is a hybrid flow?
A flow that has turbidity and debris flow elements or one that is transitional between the two
What types of flow produce hybrid beds?
Composite flow and transitional flow
Give an example of how a hybrid bed can form
Turbidity flow eroding into seabed, mud is incorporated, causing the flow to collapse, the flow is now transitional between turbid and laminar
Define a debris flow
High conc (>40%) mixture of fluid and sediment that flows downslope due to gravity
Particles supported in flow by sediment and pore pressure
Describe debris flow
Non-Newtonian rheology, has a buoyancy force, flow stops suddenly, the debrite reflects the flow content (poorly sorted)
Where do debris flows frequently occur?
Within canyons or channels in deep-water settings
Why can it be difficult to distinguish debrites in cores?
They can be difficult to discern from bioturbated textures and local remobilisation
Large clasts can be difficult to distinguish from laterally persistant mudstones
Describe slides and slumps
Dominated by intact, but rotated, bedding (unlike debris flows)
Where do slides and slumps occur?
Mainly on slopes but also sides of levees as a result of oversteepenig or flow undercutting
Give two limitations of the interpretions of slides and slumps
Can be difficult to recognise in core and the distance travelled may be difficult to discern
What can slides and slumps be examples of?
Mass transport deposits (MTD) when they occur in the subsurface or on the seafloor
What are three main sections of slope failure?
Rear scarp (source/head) (extensional forces), debris (diplaced blocks), debris apron (accumualation) (compressional forces)
Define thermohaline currents
Temperature and salinity driven flows
Powerful enough to move sediment on seafloor
How do thermohaline currents rework a channel with a lobe?
Lobe can be broken up and carried in the direction of flow
What deposits can thermohaline currents build?
Contourites
These act as topography and can deflect later turbidity currents
How can thermohaline currents affect turbidity flows?
Can capture turbidity flow coming down a slope and carry it off in the direction of teh bottom current
