1. GE0-3112 Sedimentary processes and products
Lecture 9. Deltas
Geoff Corner
Department of Geology
University of Tromsø
2006
Literature:
- Leeder 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. 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.
Alluvial - deltaic system
Alluvial fans
River
Delta
Fan delta

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. Delta subenvironments
Subdivision according to:
Dominant process
Delta plain (river)
Delta front (river and basin)
Prodelta (basinal)
Morphology
Delta plain (plain)
Delta slope (slope)
Prodelta (base)
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 Mixed wave-tide 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 ( 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 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. Photo: Raymond Eilertsen 2000
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