GE0-3112GE0-3112 Sedimentary processes and products Sedimentary processes and products

Lecture 4. Sedimentary structures II – sediment Lecture 4. Sedimentary structures II – sediment massive flowsmassive flows

Geoff CornerGeoff CornerDepartment of GeologyDepartment of GeologyUniversity of TromsøUniversity of Tromsø20062006

Literature:Literature:- Leeder 1999. Ch. 10, 11, 12. - Leeder 1999. Ch. 10, 11, 12. Cohesive sediments, sediment gravity flows and Cohesive sediments, sediment gravity flows and

deformation structures deformation structures

ContentsContents

►4.1 Cohesive sediment transport and 4.1 Cohesive sediment transport and erosion.erosion.

►4.2 Sediment gravity flows4.2 Sediment gravity flows►4.3 Soft-sediment deformation4.3 Soft-sediment deformation

4.1 Cohesive sediments4.1 Cohesive sediments

Clay and cohesionClay and cohesion

►Atomic attractions between small clay Atomic attractions between small clay particles impart cohesive strength.particles impart cohesive strength.

►Positive-negative electrical charges Positive-negative electrical charges are ’amplified’ in suspensions in are ’amplified’ in suspensions in seawater a strong electrolyte) causing seawater a strong electrolyte) causing particle attraction (flocculation).particle attraction (flocculation).

►Flocculation is enhanced by organic Flocculation is enhanced by organic matter. matter.

Cohesive bed processesCohesive bed processes

Settling velocity vs. floc size and Settling velocity vs. floc size and sediment concentrationsediment concentration

Cohesive bed erosionCohesive bed erosion

►Erosion may take the form Erosion may take the form of:of: direct particle-by-particle direct particle-by-particle

erosion where floc-floc bonds erosion where floc-floc bonds must be broken.must be broken.

erosion (re-entrainment) of erosion (re-entrainment) of freshly depsited water-rich freshly depsited water-rich suspension from the bed.suspension from the bed.

mass erosion due to failure mass erosion due to failure above a slide plane.above a slide plane.

Erosional bedforms – flute Erosional bedforms – flute marksmarks

4.2 Sediment gravity flows4.2 Sediment gravity flows

Grain-flow avalanchesGrain-flow avalanches

►Movement by shear between grains Movement by shear between grains and against underlying surface.and against underlying surface.

► Interstitial fluid plays little part in the Interstitial fluid plays little part in the motion.motion.

►May involve boulder to sand size; also May involve boulder to sand size; also snow.snow.

►Low friction/high energyLow friction/high energy

Avalanche depositsAvalanche deposits

Fluidization and kinetic Fluidization and kinetic filteringfiltering

►Acoustic Acoustic fluidizationfluidization of high energy of high energy avalanches?avalanches?

►Kinetic filteringKinetic filtering (sorting) causes (sorting) causes inverse grading.inverse grading.

Debris flowsDebris flows

►Extreme form of hyperconcentrated flow.Extreme form of hyperconcentrated flow.►Typically silt- to boulder-size particles Typically silt- to boulder-size particles

set in a matrix of clay-grade fines and set in a matrix of clay-grade fines and water.water.

► Involve transmission of both solid and Involve transmission of both solid and fluid stresses (excess pore pressure).fluid stresses (excess pore pressure).

►Excess pore pressure gives low shear Excess pore pressure gives low shear strength (low friction).strength (low friction).

►Density 1.8-2.3; velocity >10 m/s not Density 1.8-2.3; velocity >10 m/s not uncommon. uncommon.

Debris-flow depositsDebris-flow deposits

►Plug flow gives no/little shear fabric or Plug flow gives no/little shear fabric or sorting.sorting.

►Traces of shear fabric may develop at Traces of shear fabric may develop at base and margins.base and margins.

►Plug flow leaves central channel bordered Plug flow leaves central channel bordered by high levees.by high levees.

Debris-flow depositsDebris-flow deposits

►Surging may give weak stratification.Surging may give weak stratification.►Waning flow may give normal grading.Waning flow may give normal grading.►Kinetic filtering may give inverse Kinetic filtering may give inverse

grading.grading.

Levees and lobesLevees and lobes

Turbidity flowsTurbidity flows

►Cause by movement of density current Cause by movement of density current in water or air downslope.in water or air downslope.

►Triggered by:Triggered by: Evolve from slides/slumps.Evolve from slides/slumps. Hyperpycnal flows from rivers mouths.Hyperpycnal flows from rivers mouths. Longshore drift to submarine canyonsLongshore drift to submarine canyons (Pyroclastic flows; snow avalanches)(Pyroclastic flows; snow avalanches)

TurbiditesTurbidites

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4.3 Soft-sediment 4.3 Soft-sediment deformationdeformation

►Liquefaction: change of state from Liquefaction: change of state from solid to liquid.solid to liquid.

►Pore pressure rather than grain Pore pressure rather than grain contacts supports the material.contacts supports the material.

►Caused by:Caused by: Shock (earthquake, tides, etc.)Shock (earthquake, tides, etc.) Fluid addition (fluidization).Fluid addition (fluidization).

Deformation structuresDeformation structures

►Liquefaction structures:Liquefaction structures: Sand volcanoesSand volcanoes Convolute laminationConvolute lamination Pillar and dish structuresPillar and dish structures

►Slides and growth faultsSlides and growth faults►SlumpsSlumps►Skrinkage cracksSkrinkage cracks

Sand volcanoesSand volcanoes

Convolute laminationConvolute lamination

Dish structuresDish structures

Growth (listric) faultsGrowth (listric) faults

SlumpsSlumps

Further readingFurther reading