What is a delta?

"discrete shoreline protuberances formed where rivers enter oceans, semi-enclosed seas, lakes or lagoons and supply sediment more rapidly than it can be redistributed by basinal processes" (Elliott, 1986)

Deltas have different influences - river vs waves vs tides.

What features do all these deltas have in common?

• they are connected to the land; built out into the water
  
• therefore partly subaerial; partly subaqueous
  

Sites of deposition on a delta can be divided into sub-environments that will have distinctive characteristics and facies associations.

• delta plain (where river processes dominate)
  
• delta front (where river and basinal processes are both important)
  
• prodelta (where basinal processes dominate)
  
  

Delta Plain

• distributary channels
• a wide variety of non-marine to brackish environments (swamps, marshes, tidal flats, interdistributary bays)
  
  

Delta Front

• distributary mouth bars
  
  

Prodelta

• fine sediment (suspension)
• merge with sediments of the basin floor
• may be graded beds

Delta forming processes

• where the river outflow is more dense (hyperpycnal)
  
• where the river outflow and basin water are equally dense (homopycnal)
  
• where the river outflow is less dense (hypopycnal)
  
  

hyperpycnal

• where sediment laden streams enter freshwater lakes
• delta tends to be small, steeply dipping and elongate
• fine-grained material may be carried far basinward by density currents
  
  

homopycnal

• common in freshwater deltas
• deltas with steeply dipping foreset beds and less steep topsets and bottomsets (Gilbert-type deltas)
  

hypopycnal

• common where freshwater enters seawater
• sediment carried further seaward as a bouyant plume
• less steep delta front deposits and more extensive deltaic deposits
  

Facies successions

Progradation of a delta lobe produces a single, relatively thick (1-100 m) coarsening upward facies succession

Abandonment of a delta lobe produces a thin transgressive succession overlying the coarsening upward profile

River-dominated deltas

Prodelta

• mud and silts tend to be massive to well stratified and may show graded bedding
  
• bioturbation
  
• soft sediment deformation structures
  
  

Delta front

• fluvial processes (unidirectional current ripples and cross-bedding)
  
• preserved organic matter
  
  

Delta Plain (channels and interdistributary areas)

• channel sequences
  
• interdistributary areas (floodplain mud, crevasse splays,sand bars, ridges or spits)
  

Wave dominated deltas

• prograding beach and beach ridge complexes
  
  
• wave produced structures (wave ripples and hummocky cross-stratification)
  
  
• prodelta tends to be more bioturbated, thinner and sandier
  
  
• delta plain channels
  
  
• interdistributary bays may be completely closed off by barrier/beach complexes, resulting in extensive back-barrier lagoons
  

Tide-influenced deltas

• delta front shows a coarsening upward succession but with a tidal influence (herringbone cross-bedding, reactivation surfaces)
  
  
• delta plain interdistributary bays - tidal processes (tidal flats, tidal channels)
  

Stratigraphic architecture

• prograding clinoform geometry
  
• clinoform surfaces dip seaward
  
  

Allocyclic controls in delta development

• may be hard to interpret in delta sequences unless they are very major
  
  
  
  

Anthroprogenic changes

• channel stabilisation (Mississippi and Po delta in Italy)
  
• leads to continued deposition in a lobe that may otherwise be abandoned.
  
• transgression due to loss of sediment after dam building (Nile delta and Aswan Dam)

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