Delta and Estuary

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TABLE OF CONTENTS

1 Deltaic Environment ...................................................................................................... 2

1.1 Introduction ............................................................................................................ 2

1.2 Controls on Delta .................................................................................................... 2

1.3 Identification of Delta Shape ................................................................................... 2

1.4 Classification of Delta .............................................................................................. 3

1.5 Deltaic Sub environments and the Mississippi Delta................................................ 5

1.6 Sedimentary Features Common to Deltaic Deposits ................................................ 6

1.7 Deltaic Facies .......................................................................................................... 7

1.8 Syndepositional Deformation in Deltas ................................................................... 8

1.9 Delta Characteristics ............................................................................................... 9

2 Estuary Environment ..................................................................................................... 9

2.1 Wave Dominated Estuaries ..................................................................................... 9

2.2 Tide Dominated Estuaries ..................................................................................... 11

2.3 Estuary Characteristics .......................................................................................... 12

2.4 Other Classifications of Estuaries .......................................................................... 13

3 References .................................................................................................................. 14

3.1 Book References ................................................................................................... 14

3.2 Web References .................................................................................................... 14

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Transitional Environment

1 Deltaic Environment

1.1 Introduction Deltas are accumulations of sediments that form where rivers empty into bodies of quieter

water. When flowing water enters a lake or ocean basin, its speed slows. The slower water

drops its sediments to the bottom of the basin and they accumulate to form a delta.

Deltas are important areas for agriculture, fishing, and vital petroleum reserves. Deltas are

also areas rich in wildlife, especially migratory birds. The term delta was first applied in 450

BC by the Greek historian Herodotus, who thought that sedimentary deposits at the mouth

of the Nile River resembled an inverted Greek letter—delta ().

1.2 Controls on Delta Main parameters that control delta shape are:

The size of the river

The type and amount of sediments it carries

The energy associated with waves and currents in the basin where sediments are

deposited

Other parameters that may influence are:

Climate

Relief

River mouth processes and time variations

1.3 Identification of Delta Shape In order to identify the delta, draw the sketch of tributaries and coastline on paper and

then:

Nile River Delta forms where Nile River empties

into the Mediterranean Sea.

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For river-dominated delta, look for thin fingers of the delta protruding into the basin.

It also helps to consider if the delta is located in a protected area, away from strong

currents or waves.

A smooth coastline is the main characteristic of wave-dominated deltas.

Tide-dominated deltas generally show wide lobes of land perpendicular to the coast.

Sometimes the delta shows all of these characteristic in which case we define it through its

most dominated character.

1.4 Classification of Delta On the basis of the shape, deltas are classified into three major types. These are:

1. River-domination delta. 2. Wave-dominated delta. 3. Tide-dominated delta.

1.4.1 River-dominated Delta

the type of delta system we have been describing above is dominated by a river that builds a

delta pretty much willy nilly into the sea. River dominated deltas are therefore those where

other effects, such as much reworking by waves or by tides is minor. These deltas tend to

build delta lobes out into the sea. These lobes might have little more than the distributary

channel and its levee exposed above sea level in which case the delta looks something like a

bird's foot. Other times more of the flood plain between individual distributary channels is

exposed above sea level, in which case the delta can have more of a lobed shape.

1.4.2 Wave-dominated delta

Where rivers dump into the sea in areas of significant wave build up. The action of waves is

to constantly rework the delta front. Sediment is carried off down the longshore drift

direction. This may cause the delta to have a more cuspate shape, where beveled by wave

Key Characteristics:

Upper Delta Plain (above high tide)

Meandering river systems

Fresh water lakes & swamps

Lower delta plain (b/w the tides)

Distributary channels

Inter-distributary bay fill

Levees

Subaqueous Delta (Delta Front) below low tide

Distributary mouth bar - bar finger

sands

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action. The shoreline down drift of the river mouth may have much better developed and

extensive beaches and even sandy spits can form in the down drift direction. Muds carried

down the river to the delta can get carried much farther down drift leaving large areas of

muddy shoreline in those areas away from the river mouths.

1.4.3 Tide-dominated delta

Where river mouths hit the sea in areas affected by large tidal ranges, the delta shape can

be extensively reshaped by the twice a day flood and ebb tidal currents moving in and out of

the river mouth. This usually happens in bays and estuaries where the river mouth is

protected from much wave activity. The relentless in and out currents of tides can sculpt the

sediment into elongate tidal bars, such as seen in the lower left delta of the first figure

above. At the head of the bay there may be a classic looking delta, in this location referred

to as a bay-head delta, but farther seaward is a zone of lots of tidal bars, islands and inlets

caused by tide reworking. A vertical stratigraphic section through this type of deposit will be

dominated by lots of muds and sands that show bidirectional (ebb-directed and flood

directed) cross bedding and not much evidence of wave reworking (e.g. beaches) nor or

strongly prograding rivers (e.g. river dominated lobate deltas).


High wave energy

Open coasts

Strong longshore currents

Non-marine, swamp to Eolian

dune

Arcuate to strand-parallel sand

dominated facies, barrier island

sequences


High tidal range

Extensive lower delta plain/tidal

mudflats

Shore perpendicular, elongate

sand dominated facies, tidal

channel deposits

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1.5 Deltaic Sub environments and the Mississippi Delta

1.5.1 Deltaic Plain

The subaerial plan of the delta serves to distinguish it from the alluvial environment. The

alluvial-deltaic boundary occurs where the river begins to bifurcate and form distributaries.

This boundary coincides with the downstream change in river gradient and with the

prominent downstream decrease in grain-size ranges.

The subaerial part of the delta is referred to as the deltaic plain and is subdivided into an

upper plain containing fresh water deposits and a lower plain containing brackish water

deposits. The upper deltaic plain features are similar to those of the lower alluvial plain. The

principal morphological features of the deltaic plain are flood basins containing numerous

lakes and bays and distributary meander belts and/or straight channels flanked by natural

levees.

1.5.2 Pro Deltaic Plain

The offshore subaqueous part of the delta is referred to as the pro deltaic plain

environment in which the bulk of deltaic sediments are deposited. It is delineated in part by

its topographic form, the delta front bulge, and by nature of its sediments and microfauna.

The topographic form of the subaqueous parts of other deltas is not as apparent as that of

the Mississippi. The pro deltaic plain environment includes the deltaic plain fringe and

deltaic plain distal subenvironments.

1.5.2.1 Pro Deltaic Plain Fringe

The fringe includes the beaches, barrier islands, distributary mouth bars, interdistributary

bays, the nondescript delta-front platform, and the uppermost delta front slope. The

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Mississippi fringe area includes numerous beaches and barriers and extends from the shore

to approximately the 60-foot contour.

1.5.2.2 Pro Deltaic Plain Distal

The area of the distal subenvironment includes the middle and lower delta-front slope and

the delta-front toe.

1.6 Sedimentary Features Common to Deltaic Deposits Sedimentary features and their sequential relationships are very important criteria for the

recognition of lower deltaic plain and pro deltaic plain environments of deposition. The data

for sedimentary features east of the Mississippi Delta have been taken from Moore and

Scruton (1957).

1.6.1 Delta Plains

1.6.1.1 Distributary Channel Fill and Natural Levee

Complete information on the sedimentary features of channel fill deposits is not available.

Giant ripple bedded, small ripple bedded, and laminated and interbedded sand, silt, and clay

have been observed. Of these features, the laminated and interbedded sands, silts, and

clays are most common. The natural levee deposits which flank the channel consist of

interbedded, laminated, small ripple bedded, and contorted sand, silt, and clay. These may

be oxidized and disturbed since they are exposed to weathering during lower water stages.

1.6.1.2 Point Bar (Rio Grande Delta)

The finest grain sizes occur in the uppermost deposits. The middle and lower point bar

deposits of the Rio Grande distributaries are approximately equal in grain size, and the

sands are well to very well sorted. The bedding sequence downward grades from small

ripple bedded to horizontally bedded and giant ripple bedded sands. Poorly bedded gravel

deposits do not occur in over 200 borings in the point bar deposits of the Rio Grande Delta.

However, plentiful gravel is available to the alluvial sections of the stream less than 30 miles

distant. A small decrease in sand size with depth and the absence of a basal gravel zone in

point bar deposits appear to be indicative of a deltaic plain environment.

1.6.1.3 Flood Basin

Black marsh and shallow small lake deposits consist of massive, disturbed, organic-rich silt-

clay.

1.6.2 Pro Deltaic Plain

1.6.2.1 Fringe

Most beach and spit, and shallow river mouth bar sands are massive or laminated. Cross

bedding, small and large, may be common in the bar sands. They are deposited where wave

action and currents are relatively strong and continuous. Laminated, small ripple bedded,

and interbedded sand, silt, and clay with very few shells occur in the deltaic fringe

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environment where currents and wave action are weaker. The bedding is frequently

repetitious and each unit represents either flood stages, storms, or tides. The occurrence of

the laminated and interbedded deposits was also indicated by the vertical stripes

In the deeper waters of the deltaic fringe environment, the silt and clay layers are thicker

than the sand layers.

Fecal pellets are frequently found in the fringe environment. The pellets are usually

lenticular to cylindrical in shape. They vary in size, but in most cases are under 5 mm in

length. They are generally darker than the containing matrix and are often olive-gray in

color.

1.6.2.2 Distal

Massive silt-clays were seen which are representative of the distal environment.

Sedimentary Features of Mississippi Delta

1.7 Deltaic Facies

1.7.1 Effects of Grain Size

If the river carries fine-grained particles, then the deposition at the mouth bar would be

relatively very small as most of the particles were carried away and deposit forming massive

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prodelta. On the contrary, if the river is carrying coarse-grain sediments then it form

extensive mouth bar deposition.

1.7.2 Effects of Water Depth

If the delta progrades into shallow water, then it will spread out and make extensive mouth-

bar and delta-front facies however if the delta prograde into deeper water then the mouth-

bar deposition would be restricted to limited shallow portion and most of the sediment

flows into the deeper portion.

1.8 Syndepositional Deformation in Deltas The delta front is a slope that can vary from about 1° in mud-rich settings to over 30° in

coarse-grained deltas. Even the very low angle slopes are potentially unstable and mass

movement of loose, soft sediment on the delta slope is common. Debris flows, slumps and

slides that consist of remobilised delta front deposits reworked and remobilised occur

andmay be seen as part of the succession in deltaic facies. The slumps and slides can be

large-scale, involving the movement of bodies of sediment tens of metres thick and

hundreds of metres across. The surfaces on which the slides move are like faults, and these

features are often regarded as growth faults, synsedimentary deformation structures.

Further instabilities also arise as a result of the relatively rapid accumulation of sediment on

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a delta: coarser, and relatively denser sediment of the delta top is built up on top of muddy,

wet and less dense delta-front facies and the result is the formation of mud diapirs.

1.9 Delta Characteristics Lithologies: conglomerate, sandstone and mudstone

Mineralogy: variable, delta-front facies may be compositionally mature

Texture: moderately mature in delta-top sands and gravels, mature in wave-reworked delta-

front deposits

Bed Geometry: lens-shaped delta channels, mouthbar lenses variably elongate, prodelta

deposits thin bedded

Sedimentary Structures: cross-bedding and lamination in delta-top and mouth-bar facies

Palaeocurrents: topset facies indicate direction of progradation, wave and tidal reworking

variable on delta front

Fossils: association of terrestrial plants and animals of the delta top with marine fauna of

the delta front

Colour: not diagnostic, delta-top deposits may be oxidized

Facies Associations: typically occur overlying shallow-marine facies and overlain by fluvial

facies in an overall progradational pattern.

2 Estuary Environment An estuary is the marine-influenced portion of a drowned valley (Dalrymple et al. 1992). A

drowned valley is the seaward portion of a river valley that becomes flooded with seawater

when there is a relative rise in sea level. Sediment supply to the estuary is from both river

and marine sources, and the processes that transport and deposit this sediment are a

combination of river and wave and/or tidal processes. They differ from delta as all the

sedimentation would happen in the drowned valleys.

The estuarine environment is characterized by having a constantly changing mixture of salt

and freshwater, and by being dominated by fine sedimentary material carried into the

estuary from the sea and from rivers, which accumulates in the estuary to form mudflats.

Dalrymple et al. has divided the estuary environment into two broad categories which are:

2.1 Wave Dominated Estuaries An estuary developed in an area with a small tidal range and strong wave energy.

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Wave Dominated Estuary

They are further divided into:

2.1.1 Bay-head delta

The bay-head delta is the zone where fluvial processes are dominant. As the river flow

enters the central lagoon it decelerates and sediment is deposited. The form and processes

of a bay-head delta will be those of a river-dominated delta because the tidal effect is

minimal and the barrier protects the central lagoon from strong wave energy.


coarsening up progradational deposition

channel and overbank facies over sands deposited at the channel mouth

overlies fine-grained deposits of the central lagoon

2.1.2 Central lagoon

Here the river flow rapidly decreases and the wave energy is mainly concentrated at the

barrier bars. As a result similar conditions are formed at the central lagoon as were at the

normal lagoon.


fine-grained deposition, often rich in organic material

receive influxes of sand forming wave-ripples form

may also be draped with mud

2.1.3 Beach barrier

Formed at the outer edge of estuary environment where wave action reworks marine

sediment (mainly by long offshore drift). An inlet allows the exchange of water between the

sea and the central lagoon, and if there is any tidal current, a flood-tidal delta of marine-

derived sediment may prograde into the central lagoon.


sand/gravel material built by wave action

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100 m to few kilometers in width

few hundred meters to tens of kilometers in length

2.2 Tide Dominated Estuaries The estuary environment which develop in areas of high tidal range. Due to high tide range

the funnel shape of an estuary tends to increase the flood tidal current strength, but

decreases to zero at the tidal limit, the landward extent of tidal effects in an estuary. The

river flow strength decreases as it interacts with the tidal forces that are dominant.

Tide Dominated Estuary

They are further divided into:

2.2.1 Tidal channels

Point bars form on the inner banks of meander bends in the same way as purely fluvial

systems, but the tidal effects mean that there are considerable fluctuations in the strength

of the flow during different stages of the tidal cycle: when a strong ebb tide and the river act

together, the combined current may transport sand, but a strong flood tide may completely

counteract the river flow, resulting in standing water, which allows deposition from

suspension.


hetrolithic deposition at point bars

alternating layers of sand and mud dipping in to the axis of the channels

2.2.2 Tidal flats

Tidal flats are flat area lying adjacent to the tidal channels cover with sea waters during high

tides and subaerially exposed during low tides.


fine grained deposition similar to mud flats containing organic material

typically vegetated salt marsh areas

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cut by tidal creeks (sandy sediments) and act as the conduits for water flow during the tidal cycles

2.2.3 Tidal bars

Formed at the outer part of the estuary where tides have maximum effects. When the effect

of tide dies, the coarser sediments deposit.


sand/gravel deposition (bioclastic debris is common)

Dune bedforms created and migrated with the tidal currents to generate cross-bedded sandstone beds

mud drapes

Herringbone crossbedding (uncommon)

2.3 Estuary Characteristics

Wave Dominated Estuaries Tide Dominated Estuaries

Beach/ Barrier System

Lagoons Tide Channel

System Tidal Flat

Lithologies sand and

conglomerate mud with some

sand mud, sand and conglomerate

mud and sand

Mineralogy mature quartz

sands and shelly sands

variable variable clay and shelly sand

Texture well sorted, well rounded clasts

fine-grained, moderately to poorly sorted

well sorted in high energy settings

fine-grained, not diagnostic

Bed Geometry elongate lenses thinly bedded mud

with thin sheets and lenses of sand

lenses with erosional bases

tabular muds with thin sheets and lenses of sand

Sedimentary Structures

low-angle stratification

and wave reworking

may be laminated and

wave rippled

cross-bedding and crosslamination

and inclined heterolithic stratification

ripple cross-lamination

and flaser/lenticular bedding

Paleocurrents mainly wave-

formed structures rare, not diagnostic

bimodal in tidal estuaries

bimodal in tidal estuaries

Fossils robust shelly debris

monospecific assemblages of

hypersaline or brackish tolerant

organisms

shallow marine shallow marine fauna

and salt marsh vegetation

Colors not diagnostic dark due to anaerobic conditions

not diagnostic dark due to anaerobic

conditions

Facies associations

associated with coastal

plain, lagoonal or shallow-marine

facies

associated with coastal

plain or beach barrier deposits

overlain by fluvial, shallow marine, continental or

delta facies

overlain by shallow marine or continental

facies

Characteristics of Coastal and Estuarine Systems

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2.4 Other Classifications of Estuaries

2.4.1 Classification of the basis of Tidal Range

Name Tidal Range

(meters) Examples

Microtidal < 2 Limfjord, Isefjord (Denmark); Breydon Water, The Fleet and Portland Harbour (UK); Cancun Bay (Mexico); Tampa Bay, Mississippi, Laguna Madre, (US); Coastal lagoons

Mesotidal 2 – 4 Clyde, Dornoch, Cromarty, Ythan, Tay (Scotland); Orwell, Stour, Southampton, Lymington (England)

Macrotidal 4 – 6 Neath, Conwy (Wales); Mersey, Tyne, Thames, Dart (England); Lossie, Forth, (Scotland); Yellow Sea (China); Delaware (US)

Hypertidal > 6 Bay of Fundy (Canada); Severn (UK); Seine, Somme (France)

2.4.2 Types of Estuaries (after Fairbridge and Davidson et al. 1991)

Fjords (Drowned glacial troughs): Fjord-type estuaries occur where valleys have

been deeply eroded by glaciation. Characterized by deep inner basins linked to the

sea by shallow entrance sills, for example, Sea lochs in West of Scotland, Fjords in

Norway, Sweden, Alaska, British Columbia, New Zealand, such as the coasts of

Norway, Western Scotland, Alaska, and New Zealand.

Fjards (Typical of glaciated lowland coasts): More complex than fjords, with a more

open and irregular coastline, for example, Solway Firth England/Scotland, eastern

Canada, and New England.

Rias (Drowned river valleys): Formed by subsidence of land and/or a rise in sea

level. Deep, narrow channels with a strong marine influence, for example, Estuaries

of Cornwall, England and Brittany, France.

Coastal plain estuaries: Formed by the flooding of pre-existing valleys. Unlike Rias,

these estuaries are often very shallow and filled with sediment so that extensive

mudflats and salt marshes occur. Commonest type of estuary in United Kingdom, for

example, Severn, Dee, Humber, Thames, England. Chesapeake Bay, Charleston

Harbor, Delaware Bay, USA.

Bar-built estuaries: Also drowned river valleys, but recent sedimentation has kept

pace with their drowning so that they have a characteristic bar across their mouths,

for example, Alde, England; Ythan, Scotland. Barnegat Bay, New Jersey, Laguna

Madre, Texas, Albufeira, Portugal, and most estuaries of North Carolina—Florida

coast. In many estuaries in South Africa and Australia the bar may seasonally close

the estuary, creating closed or blind estuaries.

Complex estuaries: Drowned river valleys of complex origin, typically a mixture of

glaciation, river erosion, and sea level rise, for example, Scottish Firths: Solway,

Moray, Dornoch, Tay, and Forth. San Francisco Bay is a complex estuary created by

tectonic activity (land movement due to faulting).

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Barrier beaches: Open coast system where a bar or barrier develops offshore, and

an estuary is thereby created behind the barrier. For example, North Norfolk Coast,

Lindisfarne, England.

Linear shore sites: Formed where the shore is sheltered, for example, by barrier

islands. Usually considered as a subdivision of a complex estuary. For example Essex

and North Kent coast, England.

Embayments: Large natural areas formed between rocky headlands that naturally fill

with soft sediments. For example, Carmathen bay, Wales, Morecambe bay, The

wash, England.

3 References

3.1 Book References

Gary Nichols – Sedimentology and Stratigraphy – 2nd Edition - 2009

3.2 Web References

http://www.classzone.com/books/earth_science/terc/content/investigations/es130

1/es1304page01.cfm

http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/Sedstrat6/sedlect_6.htm

www.geology.wmich.edu/barnes/geos435/12_G435.pps

www.logobook.ru/af/11051214/2365/0198525087_sample.pdf

http://www.searchanddiscovery.net/documents/Shell2/images/chptr4.htm

Delta and Estuary

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