TA2910 05 - Delta Coasts

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Challenge the future

DelftUniversity ofTechnology

M.E. Donselaar

Delta deposits

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Sedimentary environments

• Continental: fluvial (braided, meandering)

aeolian

lacustrine

• Coastal: deltaslinear (clastic, carbonate)

• Marine: shelf

deep marine sands

pelagic

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Subdivision coasts

• Dominant sediment type:

- Clastic coasts

- Carbonate coasts

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Clastic coasts: Morphology

• Linear coasts

Nile deltaBarrier island - Alabama

• Delta coasts

http://www.geosci.unc.edu/faculty/glazner/Images/Coastlines/BarrierIslands/BogueBanks.jpg

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Delta evolution - 2

• Deposition bed load

sediment at river mouth:

mouth bars

• Mouth bars form obstacle

• River is forced to take

another course: Change of

river bed

• Suspension load spreads farout into sea

Dr. John Anderson Georgia Perimeter College, Dunwoody Campus

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Delta evolution - 3

• Continued supply of sediment

to river mouth creates delta

platform

• Delta expands seaward over

its delta platformdelta slope

pro-delta

delta platform

coastal plain

The Open University (1989)

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Delta morphology

• River dominated deltas

• Wave dominated deltas

• Tide dominated deltas

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Relation delta morphology - processes


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River dominated deltas

• Delta protrudes far into the

marine basin

• Weak tidal and wave action,

hence:

• Delta sediment is not re-

transported and

• Delta shape not modified

Mississippi delta

http://cass.jsc.nasa.gov/images/soce/soce_S29.gif

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Wave dominated deltas - 1

• Along-shore transport by wave-

generated currents of sediment

that reaches the delta mouth:

• Formation of spits on both

sides

• Asymmetrical spits caused by

oblique approach of waves

(arrows)

• Delta protrudes into marinebasin

Ebro delta

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Wave dominated deltas - 2

• Waves transport delta

sediment laterally away from

distributary mouth

• Sediment deposited as beach

ridge complexes• Delta grows seaward by beach

ridge accretion

Wave-dominated Grijalva delta, Mexico

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Tide dominated deltas

• Strong tidal currents are

oriented perpendicular to the

coast and redistribute the

sediment

• Delta distributary channelsoriented parallel to each other

and perpendicular to the coast

• Delta does not protrude into

marine basin

http://www.escati.com/seasia_hi-def.htm

Irrawaddy river, Bay of Bengal

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Delta sub-environments - 1

distributarychannels

10 km

interdistributarybays

swamp

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Delta sub-environments - 2

levee

interdistributarybay

crevasse splay

2 km

distributary channel

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Water density & sedimentation

• Depends on densities of river and sea water:

- Hypopycnal flow: density of river water < density of seawater

- Hyperpycnal flow: density of river water > density of seawater

- Homopycnal flow: density of river water = density of seawater

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• Water with lower density forms wedge overlying water withhigher density

• Boundary: turbulence and mixing of water, hence:

• Energy loss and sedimentation

http://www.ozestuaries.org/indicators/In_turbidity_f.html



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Hypopycnal flow - 1

Density of river water < density of sea water


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Hypopycnal flow - 2


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Hypopycnal flow – 3: sedimentation

• In sub-aqueous levees formed at the side of the jet, wherespeeds are slowed by mixing

• in a distributary mouth bar, where a combination ofspreading and mixing reduce speed and competence

• Finer sediment is carried farther out and is deposited fromsuspension. It may be carried and distributed by offshorecurrents and is eventually deposited as prodelta muds. Thisresults in lower depositional slopes and more extensivedeposits

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Hyperpycnal flow - 1

Density of river water > density of sea waterForming of turbidity currents common. Mainly concerns small rivers

with a high sediment load during peak run-off

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Hyperpycnal flow - 2

• Where the river inflow is more dense than the basinal water(e.g., due to sediment concentration), sediment movesalong the base of the basin. It is usually deposited quiterapidly, but the finer portions may move farther as

sediment gravity (turbidity) flows. These are generallyassociated with coarser sediment loads, and resulting deltasare small, elongate and generally steeply dipping

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Homopycnal flow - 1

Density of river water = density of sea (or lake) water

Rapid mixture of two water types, and formation of Gilbert-type

deltas. Mainly concerns rivers that end in lakes with a low salinity

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Homopycnal flow - 2


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Delta shape in cross-section

Selley (1985)

normal to coast

parallel to coast

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Sedimentary sequences

• Seaward progradation of

delta:

• Delta platform deposits

gradually cover delta slope

and pro-delta deposits

• Coarsening upward successiondelta slope

pro-delta

delta platform

coastal plain


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Sequences - 1

• Vertical sequence varies with

location in the delta

environment

• Vertical sequence varies with

delta type (river-, wave- ortide-dominated)

Selley (1985)

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• Log response also varieswith location in the delta

Coleman & Prior (1982)

Sequences - 2

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Prograding delta succession

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Delta switching - 1

• Delta sediments build positive relief

• Sediment stack forms obstacle

• Entire delta switches to new position by upstream avulsion

• Accommodation increase in time: delta may return to(subsided) former position, hence:

• Autocyclic sequences

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Delta switching - 2

• Example: Mississippi delta

switching from positions 1-7

• Total time: 5000 y

http://www.utdallas.edu/research/fyi/060106/images/figure1.jpg

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Compulsory reading

• Nichols - Chapter 12:

• Sections 12.1 – 12.3

• Sections 12.4.5 – 12.4.7

•

Sections 12.5 – 12.7

TA2910 05 - Delta Coasts

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