GE0-3112 Sedimentary processes and products

1. Lecture 4. Sedimentary structures II – sediment massive flows GE0-3112 Sedimentary processes and products Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 10, 11, 12. 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

30. \

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