1. 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

2. Contents ► 4.1 Cohesive sediment transport and erosion. ► 4.2 Sediment gravity flows ► 4.3 Soft-sediment deformation

3. 4.1 Cohesive sediments

4. 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.

5. Cohesive bed processes

6. Settling velocity vs. floc size and sediment concentration

7. 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.

8. Erosional bedforms – flute marks

12. 4.2 Sediment gravity flows

13. 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

14. Avalanche deposits

15. Fluidization and kinetic filtering ► Acoustic fluidization of high energy avalanches? ► Kinetic filtering (sorting) causes inverse grading.

16. 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.

21. 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.

22. Debris-flow deposits ► Surging may give weak stratification. ► Waning flow may give normal grading. ► Kinetic filtering may give inverse grading.

23. Levees and lobes

25. 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)

28. Turbidites

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31. 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).

32. Deformation structures ► Liquefaction structures:  Sand volcanoes  Convolute lamination  Pillar and dish structures ► Slides and growth faults ► Slumps ► Skrinkage cracks

33. Sand volcanoes

34. Convolute lamination

35. Dish structures

36. Growth (listric) faults

37. Slumps

38. Further reading