GE0-3112 Sedimentary processes and products Lecture 9. Deltas 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.

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

Eg of river and fan delta

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

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

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?

Wave and tide effects

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

Clastic and organic deposition

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

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

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)

Modern birdsfoot delta

Delta lobe development

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

Ganges- Brahmaputra, Bangladesh

Mahakam, Indonesia

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).

Alta delta

Tana delta

Photo: Raymond Eilertsen 2000 Målselv delta Photo: Raymond Eilertsen 2000

Målselv delta

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

Further reading