River Deltas Flashcards
what are the 3 parameters that drive deltas?
Rivers, waves and tides.
When does a delta form?
Deltas form when prograding sediment bodies meet a standing body of water (sea or lake)
List the dominant external process controls on the plan-form appearance and sedimentary character or deltas formed where rivers meet the sea
Either waves tides or rivers.
Give two significant differences in sedimentary architecture (how it stacks in space) between river-dominated and wave-dominated deltas, noting why the difference occurs
More rapid transition from land to sea lithofacies and mouth bars will be absent from the record.
- Distributaries less common on wave dominated deltas
- Less channel avulsion, much simpler plan-form
- Mouth bars cannot build as large
what is the key thing about a mouth bar?
That it creates a sanding upwards sequence
Discuss how mouth bars form in a river
Mouth bars form where rivers meet the sea, they build up because as the two meet the velocity drops causing sediment to be deposited at the mouth in the form of a mouth bar.
Why do rivers split as they meet the sea?
Rivers split because the mouth bar is deposited in the middle, so the river splits and flows either side of it. The flow is meant to be even at both sides, however, most of the time the flow is stronger at one side than the other and so one part ends up getting blocked off.
Channel bifurcation occurs when?
During mouth bar formation it causes repeated channel bifurcation (splitting). This splitting is one of the factors which may cause the active lobe of a delta to switch position as it chokes itself up.
Discuss the difference between bifurcation and avulsion
bifurcation is the splitting of river channels where it becomes half its original size. Bifurcation channels are all active at the same time.
Avulsion is when the channel stays the same size but moves location. Although it may look like there is more than one channel, there is only one active at any time.
What causes a river to want to avulse?
When it is depositing sediment it becomes unstable so wants to avulse.
density of seawater varies with what?
Salinity and Temperature.
Density decreases with temperature rise.
Density increases with salinity
Why are density variations important when rivers meet the sea?
Because the less dense fluid will flow over the top of the more dense fluid, freshwater tends to be less dense with a density of 1 whilst seawater ranges from 1.025-1.30
what are the 3 conditions for density differences?
Hypopycnal flow; river less dense than sea, freshwater flows above seawater when they meet.
Homopycnal flow; river and sea are same density so thorough mixing occurs
Hyperpycnal flow; sediment makes flow denser than sea, as they meet the flow goes down and hugs the seabed as it is turbulent it won’t deposit sediment until velocity slows down.
Name what you would see as a result of hypopycnal flow
narrower mouth bars, bigger levees and distributaries cutting river channels
Name what you would see as a result of homopycnal flow
Classic full development of a delta
maximum no of distributary channels all uniform distribution
floodplain more marine influenced as you get closer to the sea.
Name what you see as a result of hyperpycnal flow
Lots of sediment dropped on the mouth bar causing steepening and gravitational collapse which triggers turbidity currents (mass flow deposits)
Bouma sequences
Discuss where and why delta lobes switch
Delta lobes switch near the mouth due to local rapid deposition as mouth bars may choke the channel
What is the backwater effect? what does it cause?
The backwater affects rivers 10-100s kms inland, when the river is in low flood stage the water is below sea level causing resistance of sea to start further inland decelerating the river much further inland than originally thought. This causes avulsion
Discuss wave dominated deltas
They are comparatively small with a large coastline, get a quicker transition from land to sea as the river sediment keeps getting moved further away creating strand planes.
Discuss tide dominated deltas
varies with tidal range, the bigger the tidal range the stronger the currents so the more likely it is to be tidal dominated. Ebb flow can be greater than the flood flow affecting end results. Mouth bars are elongated parallel to the river valley. Thick mud drapes on lateral accretion sets
Discuss deltas sub environments
Delta top - furthest away from sea, river dominated. waterlogged.
Lower delta plain - subaerial, waterlogged, distributary channels splitting.
Delta front - Subaqueous, mouth bars dominate, may be quite narrow as wave worked shoreface.
Pro-delta - Offshore and may be absent, muds present.
what features do you see in the lower parts of a river dominated delta?
Point bars, waterlogging, vegetation, peat mires.
As a delta builds out into the sea discuss what you may see.
You have muds further out then more sand coming in. Flat lamination of foreshore and then coal and river deposits on top.
Give two significant differences in sedimentary architecture between river dominated and wave dominated deltas, in each case noting why the difference occurs.
Answer would draw on the fundamental
effects of wave action reworking the delta
front
Because mouth bars are consistently reworked along
the shore by wave action, WDD have:
– more rapid transition from land to sea lithofacies, and
– mouth bars absent from record;
– distributaries much less common on WDD, much
simpler plan form, less channel avulsion, less complexity
all round;
– mouth bars cannot build as large, effectively flushing
out river mouth and preventing channel bifurcation
– apart from the high sediment availability, looks much
like coastline away from river
