TABLE OF CONTENTS 1
2
3
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
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
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 ().
Nile River Delta forms where Nile River empties into the Mediterranean Sea.
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: 2|Page
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. 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
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 3|Page
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. Key Characteristics:
High wave energy Open coasts Strong longshore currents Non-marine, swamp to Eolian dune Arcuate to strand-parallel sand dominated facies, barrier island sequences
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). Key Characteristics:
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High tidal range Extensive lower delta plain/tidal mudflats Shore perpendicular, elongate sand dominated facies, tidal channel deposits
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 siltclay. 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 6|Page
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 mouthbar and delta-front facies however if the delta prograde into deeper water then the mouthbar 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 8|Page
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 deltafront 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. Key Characteristics: 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. Key Characteristics: 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 marinederived sediment may prograde into the central lagoon. Key Characteristics: sand/gravel material built by wave action 10 | P a g e
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. Key Characteristics: 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. Key Characteristics: fine grained deposition similar to mud flats containing organic material typically vegetated salt marsh areas 11 | P a g e
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. Key Characteristics: sand/gravel deposition (bioclastic debris is common) Dune bedforms created and migrated with the tidal currents to generate crossbedded sandstone beds mud drapes Herringbone crossbedding (uncommon)
2.3 Estuary Characteristics
Lithologies Mineralogy
Texture
Bed Geometry
Wave Dominated Estuaries Beach/ Barrier Lagoons System sand and mud with some conglomerate sand mature quartz sands and shelly variable sands fine-grained, well sorted, well moderately to rounded clasts poorly sorted thinly bedded mud elongate lenses with thin sheets and lenses of sand
Sedimentary Structures
low-angle stratification and wave reworking
may be laminated and wave rippled
Paleocurrents
mainly waveformed structures
rare, not diagnostic
Fossils
robust shelly debris
Colors
not diagnostic
Facies associations
associated with coastal plain, lagoonal or shallow-marine facies
monospecific assemblages of hypersaline or brackish tolerant organisms dark due to anaerobic conditions associated with coastal plain or beach barrier deposits
Tide Dominated Estuaries Tide Channel Tidal Flat System mud, sand and mud and sand conglomerate variable
clay and shelly sand
well sorted in high energy settings
fine-grained, not diagnostic
lenses with erosional bases
tabular muds with thin sheets and lenses of sand
cross-bedding and crosslamination and inclined heterolithic stratification bimodal in tidal estuaries
bimodal in tidal estuaries
shallow marine
shallow marine fauna and salt marsh vegetation
not diagnostic
dark due to anaerobic conditions
overlain by fluvial, shallow marine, continental or delta facies
overlain by shallow marine or continental facies
Characteristics of Coastal and Estuarine Systems 12 | P a g e
ripple crosslamination and flaser/lenticular bedding
2.4 Other Classifications of Estuaries 2.4.1 Classification of the basis of Tidal Range Tidal Range (meters)
Name Microtidal
<2
Mesotidal
2–4
Macrotidal
4–6
Hypertidal
>6
Examples Limfjord, Isefjord (Denmark); Breydon Water, The Fleet and Portland Harbour (UK); Cancun Bay (Mexico); Tampa Bay, Mississippi, Laguna Madre, (US); Coastal lagoons Clyde, Dornoch, Cromarty, Ythan, Tay (Scotland); Orwell, Stour, Southampton, Lymington (England) Neath, Conwy (Wales); Mersey, Tyne, Thames, Dart (England); Lossie, Forth, (Scotland); Yellow Sea (China); Delaware (US) 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
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