9 - Deep-water Sedimentary Systems: Controls and environments

What are the physical processes of sediment transport in deep water?

• Six classes ranging from creep of consolidated material to fall of completely unconsolidated sediment

• Increasing mobility of material and grain freedom from creep to fall

• Sediment gravity flows are most important for moving coarse sediment to deep water environments

  • Fluidal flows (turbidity currents)

  • Laminar flows (debris flows)

    

What are turbidity currents?

Definition: A turbidity current is a flow driven by the action of gravity upon the density excess of a sediment laden fluid within an ambient fluid. Sediment is maintained in suspension by the upwards (and chaotically) directed turbulent flow cells.

• Flows have a dominantly Newtonian rheology

• In the absence of turbulence, particles will settle from suspension

• In the absence of sediment, the flow will stop (unlike a river)

• Deposit character is directly linked to the rate of deposition, which, in turn is linked to the steadiness of the flow.

• The deposits of turbidity currents are termed Turbidites

  

How are turbidity currents initiated?

• Submarine landslides (delta collapse, canyon collapse) triggered by earthquakes or over-steepening

• Homopycnal and hyperpycnal flow

What are the types of gravity flow deposits?

• Turbidity current: turbidite. Turbidites don’t flow!

• Debris flow: debrite

• Hyperpycnal flow: hyperpycnite

• Pelagic sediment: pelagite

• Hemipelagic sediment: hemipelagite

• Channel: channel fill etc.

Describe Low Density Turbidites

Pelagic interval:

• Clayey marl (mixture of clay and calcium carbonate) Results from accumulation of floating organisms such as foraminifers.

Te, Pelitic interval:

• Clayey sediments with no distinct sedimentary structures. Sand content decreases upward.

Td, Upper interval of parallel lamination:

• Parallel lamination in very fine sand and silty clay generated during lower stage flow regime current conditions.

Tc, Interval of current ripple- and deformed convolute-lamination:

• Small-scale current ripples (<5cm high) in fine grained sand & silt. Deformation caused by escaping water and sediment.

Tb, Lower interval of parallel lamination:

• Thick parallel lamination in (occasionally graded) sand generated during upper stage flow regime current conditions.

  

Ta, Graded/massive interval:

• Upward fining in grain-size caused by gradual decline in current velocity. No apparent sedimentary structures.

  

Describe High Density Turbidites

Bouma Ta, Graded/massive interval: Upward fining in grain-size caused by gradual decline in current velocity. No apparent sedimentary structures.

• S1, Traction sedimentation stage: “Proximal”, pebbly sandstones. Plane beds, flat and oblique lamination and cross stratification. Locally erosive.

  

• S2, Traction-carpet stage: Inversely graded  coarse-grained sand and granule layers. Individual layers are 5-15 cm thick.

  

• S3, Suspension-sedimentation stage: Massive with or without water escape structures such as dish and pillars

  

• Tt, Traction structures: Medium- and fine-grained sandstones with deformed flat lamination and large-scale cross-stratification.

  

What is the distribution of high- & low-density turbidites?

Describe thick-bedded tubidites

Within or outside channel deposition?

Evidence of channel deposition?

• Bed amalgamation

• Clasts

  • Intraformational mudflakes

  • Extraformational grains

• Mix of facies

  • Thin- and thick-bedded turbidites

  • Bypass-parts of deposits

  • Debris flow deposits

  • Coarser grains than outside channels

Describe thin-bedded tubidites

• Levee (internal or external) or basin plain turbidites - or no genetic context at all??

• External levee sands generally classical turbidites which thin upwards

  • Dominance of climbing ripples

  • Lack of erosional structures

  • Laterally continuous from channel to pinch out

• Internal levees = multiple ripple sets, palaeocurrent complexity, poorly organized

• Basin floor turbidites will be dominated single ripple form sets

  

How do hybrid beds form?

What are debris flows?

Definition: A debris flow is a high concentration (typically >40%) mixture of fluid and sediment which flows downslope due to gravity. Particles are supported in the flow by the sediment and pore pressure in the flow.

• Flows have a dominantly non-Newtonian rheology

• Particles are supported by the flow matrix strength and pore fluid pressure which creates a buoyancy force

• Deposit character is directly linked to the flow content. Flows freeze ‘en-masse’ and the deposit represents what the flow was at that time.

• The deposits of debris flows are termed Debrites

  

What do debris flow deposits look like?

• Chaotic mixtures of different grain types and sizes

• Poorly sorted or…?

• Inverse grading may occur

• Clasts are disorganized

• Occur frequently within canyons or channels in deep-water settings

  

What are slides and slumps?

• Dominated by intact, but rotated bedding (contrasts debris flows)

• Occur mainly on slopes, but also front- and backsides of levees as a result of oversteepening or flow undercutting

• May be difficult to recognize in core

• Distance travelled may be difficult to ascertain

• In subsurface and seafloor often termed ‘mass transport deposits’ or MTDs. Infers a range of processes.

  

What do turbidites, hybrid event beds and debrites look like in core?

Turbidites:

Hybrid Event Beds:

Debrites: