GE0-3112 Sedimentary processes and products

1. Lecture 9. Deltas GE0-3112 Sedimentary processes and products Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 22. River deltas.

2. Contents • 3.1 Introduction - Why study fluid dynamics • 2.2 Material properties • 2.3 Fluid flow • 2.4 Turbulent flow • Further reading

3. Importance of deltas • Depocentres at the land-sea junction. • Source and reservoir for hydrocarbons. • Sites of human habitation and resource utilisation.

4. Alluvial fans Fandelta Delta River Alluvial - deltaic system What is a delta? • A river delta is the sediment accumulation at the mouth of a river. • A fan delta is the delta of an alluvial fan. NB. There is a gradation between river deltas and fan deltas in the sense that alluvial plains and alluvial only differ depositionally with respect to their degree of confinement.

5. Eg of river and fan delta

6. Prerequisites for delta formation • Delta formation depends upon the balance between sediment supply by the river and removal by basinal processes. • High constructive and destructive deltas.

7. High constructive and destructive deltas • High constructive (e.g. Mississippi) • Low constructive (e.g. Amazon)

8. Factors influencing delta morphology • Supplying basin • Discharge regime • Sediment caliber • Sediment volume • Receiving basin • Bathymetry • Waves and tides • Relative sea level • Tectonics, isostasy • Eustasy • Climate

9. Dominant process Delta plain (river) Delta front (river and basin) Prodelta (basinal) Morphology Delta plain (plain) Delta slope (slope) Prodelta (base) Delta subenvironments Subdivision according to: Delta slope or delta front or 'prodelta' Delta plain Delta front Prodelta Delta lip Delta toe

10. Prosesses at the river mouth • Outflow type • Hypopycnal (less dense; buoyancy dominated) • most marine deltas (coarse to fine) • Homopycnal (equal density; friction dominated) • suspension-rich flow into lakes and the sea • Hyperpycnal (more dense; inertia dominated) • 1) underflows in lakes • 2) hyperconcentrated flows (flood discharge) in sea

11. Outflow jets and mouth bars • Inertia-dominated. • Friction dominated. • Buoyancy dominated.

12. Inertia-dominated jets • Homopycnal flow. • Turbulent diffusion. • Deep water • Lunate mouth bar.

13. Friction-dominated jets • Shallow water. • Frictional drag with bottom. • Homo-/hypo-/hyperpycnal flows? • Mid-ground distributary bars.

14. Buoyancy-dominated jets • Hypopycnal flows. • Salt-wedge development. • Crescentic mouth bar?

15. Wave and tide effects

16. Subaqueous processes • Suspension settling from overflow/interflow plume. • Underflows and turbidity currents • Mass movement and slope failure • Grain flows • Debris flows • Slumps • Creep

17. Clastic and organic deposition

18. Delta types and classification Classification criteria: • Dominant process (river-wave-tide) • Shape (lobate, cuspate, birdfoot) • Grain-size (coarse, fine)

19. Delta case histories • River-dominated • Mississippi • Mixed wave-tide dominated • Niger • Wave-dominated • Nile • Tiber • Tide-dominated • Ganges-Brahmaputra • Mahakam • Fly • Fjord deltas

20. Mississippi delta • River-dominated regime. • Birdfoot morphology (modern delta). • Well-developed buoyancy forces. • Low tidal range (c. 0.3 m); moderate wave energy. • Fine-grained sediment load. • Gentle near-shore gradient. • Several Holocene progradational lobes.

21. River-dominated deltas • Low-moderate wave energy, micro- to mesotidal • Lobate to birdsfoot • Examples: • Mississippi • Braidplain deltas • Some fjord deltas

22. Mississippi – progradation history • Seven Holocene delta lobes: 1 Maringouin/Sale Cypremont (7,5-5 ka) 2 Cocordie 3 Teche (5,5-3.8 ka) 4 St. Bernard (4-2 ka) 5 Lafourche (2.5-0.8 ka) 6 Plaquemine 7 Balize (1-0 ka) Achafalaya (50 – 0 yrs)

23. Modern birdsfoot delta

24. Delta lobe development

25. Mixed wave-tide dominated deltas • High wave energy, mesotidal • Lobate shape • Barrier beach and tidal channels • Examples: • Niger

26. Niger delta • Mixed tide/wave-dominated regime. • Braided R.Niger divides into tide-dominated distributary channels. • Deep offshore. • Cenozoic history (9-12 km thick).

27. Niger delta subenvironments • Upper deltaic floodplain. • Lower deltaic mangrove swamps. • Tidal channels/ mouth bar sands. • Coastal barrier sands. • Offshore/prodelta mud.

29. Niger delta facies • NB. Numerous growth faults at depth

30. Niger delta structure & development • Highstand • Transgression • Lowstand

31. Wave-dominated deltas • High-wave energy, microtidal. • Lobate to cuspate shape. • Fringing barrier-beach system. • Examples: • Nile, Egypt • Tiber, Italy • Rhône, France Rhone delta, France

32. Nile delta • Wave-dominated (eastflowing currents and longshore drift). • Microtidal. • Lobate-shaped with cuspate outlet cones. • Aswan dam (1964) halted sediment supply – coastal erosion and land reclamation. • Complex L. Pleistocene - Holocene history.

33. Nile delta subenvironments and facies • 500 km long barrier-beach complex. • Cuspate oulet lobes (Rosetta and Damietta). • Back-barrier lakes and lagoons.

36. Nile delta progradation history • LGM (20 ka) – sandy incised braidplain on shelf. • after 8 ka – postglacial transgression and floodplain deposition. • several prehistoric abandoned distributary courses. 1 Early Holocene (c. 10 ka) 3 Historical 2 Modern

37. Tiber delta • Wave-dominated regime. • Cuspate shape. • Bay-head delta before postglacial s.l. rise. • Rapid progradation last 500 yrs (5 km).

38. Tide-dominated deltas • Macrotidal coasts (range >4 m). • Dense network of tidal channels. • Coast-normal, linear, tidal current ridges offshore. • Examples: • Ganges-Brahmaputra, Bangladesh • Mahakam, Indonesia • Fly, Gulf of Papua Irrawaddy delta, Myanmar

39. Ganges- Brahmaputra, Bangladesh

40. Mahakam, Indonesia

41. Fjord deltas • Coarse-grained, steep faced (Gilbert deltas) • Confined to unconfined (straight fronted to lobate). • Mixed fluvial-wave-tide influenced. • Examples: • Alta delta (unconfined, moderate wave energy) • Tana (semi-confined, moderate wave energy) • Målelv (confined, low wave-energy).

42. Alta delta

43. Tana delta

44. Målselv delta Photo: Raymond Eilertsen 2000

45. Målselv delta

46. Fjord-delta structure and facies Topset-foreset units in a gravelly fjord-head delta

47. Further reading