files 5- handouts lec 15

7/28/2019 Files 5- Handouts Lec 15

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Geol 307: Sedimentation andStratigraphy

Semester 071

2007

Lecture 15Lecture 15

Deltas and estuariesDeltas and estuaries


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Deltaic environments Deltas form where a river enters a standing body of water (ocean, sea, lake) and

forms a thick deposit. The morphology of delta is divided into delta plain (delta top) and delta front. The delta plain is the subaerial part of a delta (gradational upstream to a

floodplain); the delta front (delta slope and prodelta) is the subaqueous component. Delta plains are commonly characterized by distributaries and flood basins (upper

delta plain) or interdistributary bays (lower delta plain), as well as numerouscrevasse splays.

Upper delta plains contain facies assemblages that are very similar to fluvialsettings.


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Delta front Mouth bars form at the upper edge of the delta front, at the mouth of distributaries;

they are mostly sandy and tend to coarsen upwards.

The delta slope is commonly 1-2 and consists of finer (usually silty) facies; themost distal prodelta is dominated by even finer sediment where deposition ismainly from suspension.

Progradation (basinward building) of deltas leads to coarsening-upwardsuccessions, and progradation rates depend on sediment supply and basin

bathymetry (water depth).


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The forms of modern deltas: a) the Nile delta, the original delta; b) the Mississippi delta, ariver dominated delta; c) the Rhone delta, a wave dominated delta; d) the Ganges, a tide

dominated delta.


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Rhone delta


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Controls on Delta

Galloways delta classification (1975): a triangular plot on which the relative importance ofwaves, tides and river processes factors is considered allows any modem delta to be classified

into one of :9 River-dominated delta (Mississippi delta)

9Wave dominated delta ( Rhone delta )

9Tide dominated delta. ( Ganges delta )

Controls-Tectonics

- Climate (vegetation)

-Eustatic sea levelchanges

Hinterland controls:

Slope, Relief & Discharge

Determines grain size

Basin controls :

Subsidence (depth control)

Waves & Tides(reworkingsediment)

Deltaregime

Delta

morpholophy

Fluvialdominated

Tidedominated

Wavedominated

Galloways delta classification

Deltafacies

The drawback of this classification is that some moderndeltas plot in the same part of the triangle but have verydifferent morphologies and characteristics.


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The 'Toblerone plot shown below is a modification of theGalloway triangle, adding another axis to show the grain size.

However, even this does not provide a comprehensiveclassification scheme as factors such as the slope and depthof the 'receiving basin' (the sea or lake into which the delta is

building) are not taken into account.


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If gradient of the river is low, most of its bedload is deposited before the water arrives

at the delta. Predominantly fine material (reworked and dispersed by waves, currentsand tides) is accumulated in delta.

With increasing river gradient, the flow velocity is greater and the proportion and sizeof the bedload increases. Delta at the mouths of braided rivers may be sandy or pebbly,

requiring more energetic basinal processes to remobilize them.

If the gradient of the hinterland adjacent to a lake or sea is very steep, an alluvial fanmay form and build out as a partially subaqueous feature.

In these cases the sediment body may be referred to as a fan delta; The term 'fan

delta' is often loosely applied to any coarse-grained delta, although the usual definitionis 'an alluvial fan which progrades into a standing body of water.

Grain size and hinterland slope controls


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Types of deltas

Delta morphology reflects the relative

importance of fluvial, tidal, and waveprocesses, as well as gradient andsediment supply

River-dominated deltas

Wave-dominated deltas

Tide-dominated deltas


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River-Dominated Delta


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River-dominated deltasOccur in microtidal settings with limited wave energy, wheredelta-lobe progradation is significant and redistribution ofmouth bars is limited


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Wave-Dominated Delta

Wave-driven longshore

drift redistributes sedimentalong the coast.


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Wave-dominated deltasAre characterized by mouth bars reworked into shore-parallel sand bodies and beaches


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Wave- & Tide-Influenced Delta


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Tide-dominated deltasExhibit tidal mudflats and mouth bars that are reworkedinto elongate sand bodies perpendicular to the shoreline


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Delta cycle & post and syndepositonal effects on deltasWhen the channel on the delta top changescourse, the former lobe is abandoned as a new

site of deposition is occupied.

River-dominated deltas tend to have the mostfrequent changes in position of the active lobe,butavulsion of channel course also occurs in

other delta types.The beds which mark the end of sedimentationon a delta lobe are known as the abandonmentfacies.

In the upper part of the delta plain these will bepeats or palaeosols which represent a low clasticsupply to this part of the plain now that activelobe progradation has moved elsewhere on thedelta.

Delta cycles: the facies succession preserveddepends on the location of the vertical profilerelative to the depositonal lobe of a delta


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Avulsion -- major shift in river course

during large flood


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Estuaries Estuaries are semi-enclosed coastal water bodies where fluvial and marine

processes interact. The mouths of many rivers are estuaries at present as a result of the

relative sea level rise which has occurred since the last ice age.

Under macrotidal regimes, the rise and fall of the tide and the ebb and flood

tidal currents establish two sub-environments within the estuarine setting:tidal channels and tidal mudflats


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Tidal channel The flow in the ebb and flood tides is funneled into single or multiple channels. Currents may

be powerful enough to cause scouring at he base, and gravelly debris including bioclasticdebris brought in by onshore currents may be left as a lag in the channel floor.

The flow within a tidal channel will move sand as a bedload and result in formation of bars. These bar deposits will show the characteristics of tidal sedimentation for example mud

drapes on the cross beds. Herringbone cross stratification only develops in regions where the ebb and flood flow

pathways overlap.

Alternation of sand and mud as bar deposits produce a pattern of bedding referred to asinclined heterolithic stratification is seen in tidal channel deposits as alternating layers of sandand mud which dip into the axis of the channel, perpendicular to flow.


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Tidal mudflats Away from the areas of strong tidal currents lie tidal mudflats. These regions are

flooded at high tide, and exposed when the tide falls. Fine sediment is carried in suspension across the mudflats on the rising tide,

depositing as the tide turns and falls.

Vegetation on the tidal flats traps sediment and mudflats are commonly sites of netaccumulation. The succession built up is predominantly mud with thin sand sheets

present if very high tides or storms wash coarser material across the flats.

Evidence of vegetation is normally abundant, as is bioturbation by the fauna whichlive on the nutrient-rich mudflats.


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Environments in tide dominated estuary

Tide-dominated estuaries have tidal

channels with bars and tidal mudflats that

contain tidal sedimentary structures (e.g.,

tidal bundles, heterolithic stratification)


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Wave-dominated estuaries are partly enclosed by a coastalbarrier and have well-developed bay-head deltas

Wave-dominated estuaries

Along coasts with smaller tidal ranges wave action is important in estuarine settings.

An inlet in the barrier allows water to pass through into the lagoon, a region ofshallow, low-energy sedimentation. At the mouth of the river a bay head delta forms.

This consists of delta top facies (channel and delta plain) which may be similar tothose found in other deltaic environments, but are confined by the incised valley of theestuary.


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Wave-dominated estuaries

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