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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. Contents. 3.1 Introduction - Why study fluid dynamics 2.2 Material properties 2.3 Fluid flow
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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.
Contents • 3.1 Introduction - Why study fluid dynamics • 2.2 Material properties • 2.3 Fluid flow • 2.4 Turbulent flow • Further reading
Importance of deltas • Depocentres at the land-sea junction. • Source and reservoir for hydrocarbons. • Sites of human habitation and resource utilisation.
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.
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.
High constructive and destructive deltas • High constructive (e.g. Mississippi) • Low constructive (e.g. Amazon)
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
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
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
Outflow jets and mouth bars • Inertia-dominated. • Friction dominated. • Buoyancy dominated.
Inertia-dominated jets • Homopycnal flow. • Turbulent diffusion. • Deep water • Lunate mouth bar.
Friction-dominated jets • Shallow water. • Frictional drag with bottom. • Homo-/hypo-/hyperpycnal flows? • Mid-ground distributary bars.
Buoyancy-dominated jets • Hypopycnal flows. • Salt-wedge development. • Crescentic mouth bar?
Subaqueous processes • Suspension settling from overflow/interflow plume. • Underflows and turbidity currents • Mass movement and slope failure • Grain flows • Debris flows • Slumps • Creep
Delta types and classification Classification criteria: • Dominant process (river-wave-tide) • Shape (lobate, cuspate, birdfoot) • Grain-size (coarse, fine)
Delta case histories • River-dominated • Mississippi • Mixed wave-tide dominated • Niger • Wave-dominated • Nile • Tiber • Tide-dominated • Ganges-Brahmaputra • Mahakam • Fly • Fjord deltas
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.
River-dominated deltas • Low-moderate wave energy, micro- to mesotidal • Lobate to birdsfoot • Examples: • Mississippi • Braidplain deltas • Some fjord deltas
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)
Mixed wave-tide dominated deltas • High wave energy, mesotidal • Lobate shape • Barrier beach and tidal channels • Examples: • Niger
Niger delta • Mixed tide/wave-dominated regime. • Braided R.Niger divides into tide-dominated distributary channels. • Deep offshore. • Cenozoic history (9-12 km thick).
Niger delta subenvironments • Upper deltaic floodplain. • Lower deltaic mangrove swamps. • Tidal channels/ mouth bar sands. • Coastal barrier sands. • Offshore/prodelta mud.
Niger delta facies • NB. Numerous growth faults at depth
Niger delta structure & development • Highstand • Transgression • Lowstand
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
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.
Nile delta subenvironments and facies • 500 km long barrier-beach complex. • Cuspate oulet lobes (Rosetta and Damietta). • Back-barrier lakes and lagoons.
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
Tiber delta • Wave-dominated regime. • Cuspate shape. • Bay-head delta before postglacial s.l. rise. • Rapid progradation last 500 yrs (5 km).
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
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).
Målselv delta Photo: Raymond Eilertsen 2000
Fjord-delta structure and facies Topset-foreset units in a gravelly fjord-head delta