Presentation on theme: "GE0-3112 Sedimentary processes and products Lecture 4. Sedimentary structures II – sediment massive flows Geoff Corner Department of Geology University."— Presentation transcript:
GE0-3112 Sedimentary processes and products Lecture 4. Sedimentary structures II – sediment massive flows Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 10, 11, 12. Cohesive sediments, sediment gravity flows and deformation structures Cohesive sediments, sediment gravity flows and deformation structures
Contents ► 4.1 Cohesive sediment transport and erosion. ► 4.2 Sediment gravity flows ► 4.3 Soft-sediment deformation
4.1 Cohesive sediments
Clay and cohesion ► Atomic attractions between small clay particles impart cohesive strength. ► Positive-negative electrical charges are ’amplified’ in suspensions in seawater a strong electrolyte) causing particle attraction (flocculation). ► Flocculation is enhanced by organic matter.
Cohesive bed processes
Settling velocity vs. floc size and sediment concentration
Cohesive bed erosion ► Erosion may take the form of: direct particle-by-particle erosion where floc-floc bonds must be broken. erosion (re-entrainment) of freshly depsited water-rich suspension from the bed. mass erosion due to failure above a slide plane.
Erosional bedforms – flute marks
4.2 Sediment gravity flows
Grain-flow avalanches ► Movement by shear between grains and against underlying surface. ► Interstitial fluid plays little part in the motion. ► May involve boulder to sand size; also snow. ► Low friction/high energy
Fluidization and kinetic filtering ► Acoustic fluidization of high energy avalanches? ► Kinetic filtering (sorting) causes inverse grading.
Debris flows ► Extreme form of hyperconcentrated flow. ► Typically silt- to boulder-size particles set in a matrix of clay-grade fines and water. ► Involve transmission of both solid and fluid stresses (excess pore pressure). ► Excess pore pressure gives low shear strength (low friction). ► Density 1.8-2.3; velocity >10 m/s not uncommon.
Debris-flow deposits ► Plug flow gives no/little shear fabric or sorting. ► Traces of shear fabric may develop at base and margins. ► Plug flow leaves central channel bordered by high levees.
Debris-flow deposits ► Surging may give weak stratification. ► Waning flow may give normal grading. ► Kinetic filtering may give inverse grading.
Levees and lobes
Turbidity flows ► Cause by movement of density current in water or air downslope. ► Triggered by: Evolve from slides/slumps. Hyperpycnal flows from rivers mouths. Longshore drift to submarine canyons (Pyroclastic flows; snow avalanches)
4.3 Soft-sediment deformation ► Liquefaction: change of state from solid to liquid. ► Pore pressure rather than grain contacts supports the material. ► Caused by: Shock (earthquake, tides, etc.) Fluid addition (fluidization).