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

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Presentation on theme: "GE0-3112 Sedimentary processes and products Lecture 9. Deltas Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch."— Presentation transcript:

1 GE Sedimentary processes and products Lecture 9. Deltas Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder Ch. 22. River deltas. 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 Dominant process ► Delta plain (river) ► Delta front (river and basin) ► Prodelta (basinal) Morphology ► Delta plain (plain) ► Delta slope (slope) ► Prodelta (base) Subdivision according to: Delta plain Delta front Delta slope or delta front or 'prodelta' 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 ( 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.

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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.

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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


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