Jawad Bashir(Thunder)

Bahria university

Presents

DELTA AND ESTUARIES

DELTA AND ESTUARY

DELTA

The term "delta," the Greek character ▲, was used to describe the mouth of the Nile by Herodotus nearly 2500 years ago. This term is still used by geographers and geologists alike. A modern definition cites a delta as "the subaerial and submerged contiguous sediment mass deposited in a body of water (ocean or lake) primarily by the action ofa river" OR

A delta can be defined as a ‘discrete shoreline protuberance formed at a point where a river enters the ocean or other body of water’ and as such it is formed wheresediment brought down by the river builds out as a body into the lake or sea.

HOW DELTAS FORMED

A delta forms where a jet of sediment-laden water intrudes a body of standing water. Current velocity diminishes radially from the jet mouth, depositing sediment whose settling velocities allow grain size to diminish radially from the jet mouth. Sedimentation around the jet mouth builds up to the air/water interface, but the force of the jet maintains a scoured channel out through the sediment. Ridges on either side of the distributary channel are termed as “Levees” termed

As sedimentation continues, the delta progrades out into the standing body of water. Three main morphological units appear. The delta platform is the subhorizontal surface nearest the jet mouth. It is basically composed of sand and is traversed by the distributary channel and its flanking leevs.

The delta platform grades away from the source into the delta slope on which finer sands and silts come to rest. This in turn passes down into the prodelta area on which clay settles out of suspension. A vertical section through the apex of a delta thus reveals a gradual vertical increase in grain size. At the base the prodelta clays grade up through delta slope silts into sands of the delta platform. Classically, these three elements have been termed the bottomset, foreset, and topset, respectively.

Eventually a distributary channel extends so far that its mouth becomes choked withsediment. At a point of weakness the lev6e bursts and a new distributary system is established. The abandoned distributary is choked by suspended sediment, and the wholeabandoned lobe sinks beneath the water as it compacts. This ideal delta model consists ofa series of interdigitating lobes, each one showing a gradual upward increase in grainsize, and a decrease in grain size from its point of origin.

Controls On Delta (Environments and Facies).

The supply of the sediment is determined by the nature of the hinterland, with the climate influencing the weathering and erosion processes and the discharge, the amount of water in the rivers, while there are tectonic controls on the topography, especially the gradient of the river and the effect this has on the grain size of the material carried. The relative importance of processes that rework the sediment

in the basin is controlled by climatic and geomorphologic factors: tidal range is determined by the local shape of the basin, while the wave activity is influenced by climate and the size of the water body. Thedepth of the water in the basin is also importantbecause it influences the effects of wave and tide processes and also controls the overall geometry of the delta body: if the delta is building out into shallow water it will spread further out into the basin than if the water is deeper

Classification Of Deltas

1. Grain Size and Sediment Supply2. On bases of Geomorphology(Shape and size)3. Modern Delta systems(Process controlled)

GRAIN SIZE AND SEDIMENT SUPPLY

Deltas are now commonly classified in terms of the dominant grain size of the deposits and the relative importance of fluvial, wave and tide processes This scheme can be applied to modern deltas and is useful because the characteristics of the deposits formed by different deltas within it can be used as a basis for classifying strata that are interpreted as delta facies.

MORPHOLOGY(SHAPE AND SIZE)

There are four main types of deltas classified on the bases of shape and size.

1. Bird’s Foot Delta2. Cuspate Delta3. Arcuate Deta4. Estuarine Delta

1. BIRD’S FOOT DELTA

A delta with long, projecting distributary channels that branch outward like the toes or claws of a bird. A bird's foot delta forms where sediment is deposited in relatively calm offshore waters. An example of a bird's foot delta is the Mississippi river delta.

2. Arcuate Delta

An arcuate delta forms when a river meets the sea in a place where the waves, currents, and tides are strong. It is often bow shaped and has a number of distributaries flowing across it. An example is the Nile delta of Egypt. Found in areas where longshore drift keeps the seaward edge of the delta trimmed and relatively smooth, is only one form of delta

3. Estaurine Delta

When the mouth of a river enters the sea and is inundated by the sea in a mix with freshwater and very little delta, it is called an estuary. An example of a estuarine delta is the Seine river delta in France or the Mackenzie river delta in Canada

4. Cuspate Delta

Tooth-shaped delta in which a single dominant river builds the delta forward into a lake or sea. A cuspate delta is formed when a river drops sediment onto a straight shoreline with strong waves. Waves force the sediment to spread outwards in both directions from the river's mouth making a pointed tooth shape with curved sides. An example is the Tiber delta in Italy.

MODERN DELTA TYPES (PROCESS CONTROLED)

1. River Dominated Deltas2. Nile (Original Type) Deltas3. Wave Dominated Deltas4. Tide Dominated Deltas

1. NILE DELTA

2. RIVER DOMINATED DELTA

River-dominant deltas are found where rivers carry so much sediment to the coast that the deposition rate overwhelms the rate of reworking and removal due to local marine forces. In regions where wave energy is very low, even low-sediment-load rivers can form substantial deltas

3. WAVE DOMINATED

At wave-dominant deltas, waves sort and redistribute sediments delivered to the coast by rivers and remold them into shoreline features such as beaches, barriers, and spits. The morphology of the resulting delta reflects the balance between sediment supply and the rate of wave reworking and redistribution.

4. TIDE DOMINATED

At the river mouths, mixing obliterates vertical density stratification, eliminating the effects of buoyancy. For part of the year, tidal currents may be responsible for a greater fraction of the sediment transporting energy than the river. As a result, sediment transport in and near the river mouth is bidirectional over a tidal cycle

DELTA ENVIRONMENTS AND SUCCESSIONS

Marine deltas form at the interface of continental and marine environments. The processes associated with river channel and over bank settings occur alongside wave and tidal action of the shallow marine realm. Flora and fauna characteristic of land environments, such as the growth of plants and the development of soils, are found within a short distance of animals that are found exclusively in marine conditions. These spatial associations of characteristics seen in modern deltas occur as associations of facies in the stratigraphic record. Deltas can therefore be considered in terms of sub environments, divisions of the overall delta environment in which these combinations of processes occur.

Delta-top subenvironments

Deta Top/Plain

Deltas are fed by a river or an alluvial fan and there is a transition between the area that is considered part of the fluvial/alluvial environment and the region that is considered to be the delta top or delta plain. Delta channels can be as variable in form as a river and may be meandering or braided, single or divided channels. Branching of the river channel into multiple courses is common, to create a distributary pattern of channels across the delta top. The coarsest delta-top facies are found in the channels, where the flow is strong enough to transport and deposit bedload material. These may be vegetated under appropriate climatic conditions and in wet tropical regions large, vegetated swamps may form on the delta top. These may be sites for the accumulation of peat, although if there is frequent overbank flow from the channel the deposit will be a mixture of organic and clastic material to form a carbonaceous mud.

Interdistributary Bays

On deltas where the channels build out elongate lobes of sediment, sheltered areas of shallow water may be protected from strong waves and currents. These sheltered areas along the edge of the delta top are called interdistributary bays

Delta-front sub environments

Sub Aqueous Mouth

At the mouth of the channels the flow velocity is abruptly reduced as the water enters the standing water of the lake or sea. The delta front immediately forward of the channel mouth is the site of deposition of bed load material as a sub aqueous mouth bar.

Delta slope

The current from the river is dissipated away from the channel mouth and wave energy decreases with depth, leading to a pattern of progressively finer material being deposited further away from the river mouth. the delta slope, is often shown as a steep incline away from the delta top, but the slope varies from only 1 or 2 degree in many fine-grained deltas to as much as 30 degree in some coarse-grained deltas.

River-borne suspended load enters the relatively still water of the lake or sea to form a sediment plume in front of the delta. Fresh river water with a suspended load may have a lower density than saline seawater and the plume of suspended fine particles will be buoyant, spreading out away from the river mouth. As mixing occurs deposition out of suspension occurs, with the finest, more buoyant particles traveling furthest away from the delta front before being deposited in the prodelta region. Gravity currents may also bring coarser sediment down the delta front and deposit material as turbidites

Deltaic successions

The definition of a delta includes the concept of progradation, that is, deposition results in the sediment body building out into the lake or sea. The sedimentary succession formed will therefore consist of progressively shallower facies as the prodelta is overlain by the delta front, which is in turn superposed by mouth-bar and delta-top sediments. The succession formed by the progradation of a delta therefore has a shallowing-up pattern, a series of strata that

consistently shows evidence of the younger beds being deposited in shallower water than the older beds they overly . In the delta-front subenvironment the deepest water facies, the prodelta deposits, are the finest grained as they are deposited in the lowest energy setting. In a shallowing-up succession they will be overlain by sediments of the delta slope, which will tend to be a little coarser, and the shallowest facies will be those of the mouth bars, which are typically sandy or even gravelly sediment. The beds formed by delta progradation will therefore show a coarsening-up pattern

VARIATIONS IN DELTA MORPHOLOGY AND FACIES

The combinations of factors that control delta morphologies give rise to a wide spectrum of possible delta characteristics. the Mississippi Delta is fine-grained and river dominated, the Rhone Delta is mixed sand and mud and is wave-dominated, the Skeidarasandur is mainly gravelly with river and wave influence, and so on. Even with all the possible positions within that plot, there is also the additional variable of water depth to be added. Every modern delta will have individual characteristics due to the different factors controlling its form, and it may be expected that the deposits of ancient deltas will be similarly variable.

1. Effects of grain size: fine-grained deltas

The deposits on a delta will include a high proportion of fine-grained material if the fluvial system supplying it is a mixed-load river. Low gradient, mixedload river channels characterise the lower tracts of large river systems. Large rivers like these carry sediment that is delivered to the delta as sandy bedload and a large suspended load of silt and clay. Sand

deposition on the delta top is concentrated in the delta channels and on adjacent levees, while the bulk of the delta plain and any interdistributary bay areas are regions of mud accumulation . The proximal mouth bars may also be sandy, but the rest of the delta slope and prodelta receive sediment fall-out from the plume of suspended sediment that issues from the river mouth . The delta front may also be the site of mass flows: the wet, muddy sediment brought down by the river may be transported by turbidity currents to deposit as turbidites on the lower part of the delta front, in the prodelta area and beyond. The proportion of sand in the delta deposits increases if the feeder river provides more bed load sediment. Sandy bed load rivers also transport material in suspension, but the delta environment

becomes a setting for deposition of sand in channels, as over bank splays on the delta top and as shallow marine deposits in the upper part of the delta front. Extensive sand bodies form as mouth bars, perhaps reworked by wave and tide action.

1. Effects of grain size: coarse-grained deltas

Coarse-grained deltas, also referred to as fan deltas, are fed by pebbly braided rivers or alluvial fans. They form adjacent to areas of steep relief, where streams in the catchment areas of the rivers flow down steep slopes carrying coarse material into rivers or on to alluvial fans that prograde into a lake or the sea. Settings such as the faulted margins of rift basins are typical sites for coarse-grained deltas to form. Progradation of a coarse-grained delta across a shallow lake or sea floor results in a coarsening-up succession from finer sands deposited furthest offshore through coarser sands, granules, pebbles and even cobbles or boulders at the top of the delta-front succession, which is then overlain by coarse fluvial or alluvial fan facies of the delta top. Coarse-grained deltas that display these characteristics have been classified as ‘shelf-type fan deltas’ by Wescott & Ethridge (1990).

2. Water depth: shallow- and deep-water deltas

A delta progrades by sediment accumulating on the sea floor at the delta front where it builds up to sea level to increase the area of the delta top. For a given supply of sediment, the rate at which the delta progrades will depend on the thickness of the sediment pile that must be created to reach sea level. Delta progradation will hence occur at a greater rate if it is building into a shallow sea or lake, and the area covered by a delta lobe will be greater because it forms a thin, widespread body of sediment. In contrast a delta building into deeper water will form a thicker deposit that progrades at a slower rate

A delta building into shallow water will tend to have a large delta-plain area. If the climate is suitable for abundant plant growth, peat mires may develop on parts of the plain away from the delta channels and delta successions that have developed in a shallow-water setting may therefore include coal beds. The delta-front facies will all be deposited in shallow water, and hence will be strongly influenced by processes such as wave action . Sandy and gravelly deposits are therefore likely to be relatively well sorted.

In deeper water, a greater proportion of the sediment will be deposited in the lower part of the delta slope as a thicker coarsening-up succession is generated during delta progradation . The area of the delta top will be relatively small, with less potential for the development of widespread finegrained delta-plain facies and mires. Wave-reworked mouth-bar facies will be limited in extent because of the small area of shallow water where wave action is effective.

3. Coarse-grained deep-water deltas

The combination of a supply of coarse sediment and a steep basin margin results in a particular delta form that is unlike all other deltas and therefore merits a special mention . They even have a special name ‘Gilbert-type deltas’, named after the American geologist G.K. Gilbert who first described deposits of this type in 1895. Gilbert-type deltas have a characteristic three-part structure.

The topset (the delta top) is a subaerial to shallowmarine environment, with gravels deposited by braided rivers and, in some cases, reworked by wave processes at the shoreline. In front of the topset lies the foreset (the delta front), which is very distinctive because the beds are at a steep angle, typically up to around 30 degrees and close to the angle of rest of material. Deposition on a foreset occurs by two mechanisms (Nemec 1990b): debris flows of poorly sorted gravel mixed with sand and mud, and well-sorted gravels deposited by a grainflow (avalanche) process. Slumping is often seen on the delta because the steep slopes of the foresets can become unstable. At the base of the foreset slope sediments are finer, comprising mud, sand and some gravel, which lie approximately horizontally and are the products of turbidites and suspension deposition in a prodelta setting, known in this context as the bottomset. As a Gilbert-type delta progrades, the foreset builds out over the bottomset and in turn the foreset is overlain by topset facies: the resulting deposit is in the form of a sandwich of steeply-dipping conglomeratic strata between layers of horizontal beds of conglomerate and sandstone.

ECONOMIC ASPECTS OF DELTAIC DEPOSITS

Ancient deltaic deposits are extremely important economically. They host most of theworld's coal, and many major petroleum provinces. Deltas make excellent petroleumprovinces because they fulfil all the conditions necessary for petroleum source bed formation, petroleum generation, and entrapment.

The deltaic process is a way of depositing lobes of sand (potential reservoirs) into envelopes of organic-rich marine muds (potential source beds). Deltaic environments deposit many potential stratigraphic traps, including mouth bars, barrier bars, and channels. Rapid deposition often leads to overpressuring. This may generate diapiric traps and roll-over anticlines. Deltas need a basin, or at least some subsidence, before they may form. Subsidence implies crustal stretching and thus increased heat flow. This expedites the maturation of source beds. No wonder then that ancient deltas are major.petroleum provinces. The Tertiary Niger Delta and the Tertiary Gulf Coast province of the USA are two classic examples

Once a deltaic petrolum accumulation has been found, however, sedimentology mustbe applied to develop it efficiently. The earlier discussion of fluvial reservoirs introducedthe problems of mapping channels, first trying to establish their continuity deterministically, but then often having to resort to modelling the reservoir statistically. Similar situations are encountered in deltaic reservoirs. Here the problems are complicated by the fact that, not only may there be downslope trending channels, but there may also be shallow marine sands elongated perpendicular to the distributaries.

RECOGNITION OF DELTAIC DEPOSITS

A key feature of many deltas is the close association of marine and continental depositional environments. In delta deposits this association is seen in the vertical arrangement of facies. A single delta cycle may show a continuous vertical transition from fully marine conditions at the base to a subaerial setting at the top. This transition is typically within a coarseningupwards succession from lower energy, finer grained deposits of the prodelta to the higher energy conditions of the delta mouth bar where coarser sediment accumulates.

DELTA TOP DEPOSITS

The delta top contains both relatively coarse sediment of the distributary channel as well as finer grained material in overbank areas and interdistributary bays. The channel may be recognised by its scoured base, a fining-up pattern and evidence of flow, which will be unidirectional unless there is a strong tidal influence resulting in bidirectional currents.The delta top will show signs of subaerial conditions, including the development of a soil. Deposits in the sheltered interdistributary bays may show thin bedding resulting from influxes of sediment from the delta top and symmetrical ripples due to wave action.

DELTA FRONT DEPOSITS

The shallower water deposits of the delta front may be extensively reworked by wave and/or tidal action resulting in cross-stratified mouth-bar facies. The geometry and extent of the mouth-bar sand bodies will be determined by the relative importance of river, tidal and wave processes.

PRO DELTA DEPOSITS

Deeper, lower delta slope deposits and prodelta facies are finer grained, deposited from plumes of suspended material disgorged by the river, or as turbidites that flowed down the delta front.

Deltaic deposits are almost exclusively composed of terrigenous clastic material supplied by rivers. However, there are examples of deltas formed by lavas and volcaniclastic material building out into the sea, and these are not fed by water, but by the volcanic processes: the term ‘non-alluvial delta’ may be applied to these deposits.

Palaeontological evidence from fauna and flora can be important in the recognition of the marine and continental sub environments of a delta. A distinct fauna tolerant of brackish water may be found near the mouths of channels and in the interdistributary bays where fresh and marine water mix. The mixture of shallow-marine, brackish and freshwater fauna plus coastal vegetation is also characteristic of deltaic environments. The contrast between fresh and saline water is not present in deltas formed at the margins of freshwater lakes and in these settings the recognition of the delta must be based on the facies patterns.

Characteristics of deltaic deposits

1. Lithologies – conglomerate, sandstone and mudstone2. Mineralogy – variable, delta-front facies may be compositionally

mature3. Texture – moderately mature in delta-top sands and gravels,

mature in wave-reworked delta-front deposits4. Bed geometry – lens-shaped delta channels, mouthbar lenses

variably elongate, prodelta deposits thin bedded5. Sedimentary structures – cross-bedding and lamination in

delta-top and mouth-bar facies6. Palaeocurrents – topset facies indicate direction of progradation,

wave and tidal reworking variable on delta front7. Fossils – association of terrestrial plants and animals of the delta

top with marine fauna of the delta front

8. Colour – not diagnostic, delta-top deposits may be oxidized

9. Facies associations– typically occur overlying shallow- marine facies and overlain by fluvial facies in an overall progradational pattern.

ESTUARIES

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. They are regions of mixing of fresh and seawater. 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. An estuary is different from a delta because in an estuary all the sedimentation occurs within the drowned valley, whereas deltas are progradational bodies of sediment that build out into the marine environment. A stretch of river near the mouth that does not have a marine influence would not be considered to be an estuary. Estuaries are common features at the mouths of rivers in the present day because since the last glacial period there has been a relative rise in sea level.

Two end members are recognised (Dalrymple et al. 1992): wave-dominated estuaries and tide-dominated estuaries, with a range of intermediate forms in between. In addition to these two basic process controls, the volume of the sediment supply and the relative importance of supply from marine and fluvial sources also play an important role in determining the facies distributions in an estuarine succession. The extent of estuarine deposits will depend upon the size of the valley and the depth to which it has been flooded. Modern estuaries range from a few kilometres to over 100km long and from a few hundred metres to over 10 km wide. The thickness of the succession formed by filling an estuary is typically tens of metres. Sedimentation in an estuary will eventually result in the drowned valley filling to sea level and, unless there is further sea-level rise, the area will cease to have an estuarine character. If there is a high rate of fluvial sediment supply, deposition will start to occur at the mouth of the river and a delta will start to form. Under conditions where the marine processes are dominant, the river mouth will become an area

of tidal flats if tidal currents are strong, or the sediment will be reworked and redistributed by wave processes to form a strand plain. An estuary is therefore a temporary morphological feature, existing only during and immediately after transgression while sediment fills up the space created by the sea-level rise.

Wave-dominated estuaries

An estuary developed in an area with a small tidal range and strong wave energy will typically have three divisions: the bay-head delta, the central lagoon and the beach barrier.

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. A coarsening-up, progradational succession will be formed, with channel and overbank facies building out over sands deposited at the channel mouth, which in turn overlies fine-grained deposits of the central lagoon.

Central Lagoon

The lowest energy part of the estuarine system is the central lagoon, where the river flow rapidly decreases and the wave energy is mainly concentrated at the barrier bar. The central lagoon is therefore a region of fine-grained deposition, often rich in organic material, similar to normal lagoonal conditions .When the central lagoon becomes filled with sediment it becomes a region of salt-water marshes crossed by channels. In wave-dominated estuaries, parts of the lagoon that receive influxes of sand may be areas where wave-ripples form and these may also be draped with mud.

Beach Barrier

The outer part of a wave-dominated estuary is a zone where wave action reworks marine sediment (bioclastic material and other sediment reworked by longshore drift) to form a barrier. The characteristics of the barrier will be the same as those found along

clastic coasts . 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.

Tide-dominated estuaries

Tidal processes may dominate in mesotidal and macrotidal coastal regimes where tidal current energy exceeds wave energy at the estuary mouth. The funnel shape of an estuary tends to increase the floodtidal 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. Three areas of deposition can be identified : tidal channel deposits, tidal flats and tidal sand bars.

Tidal Channels

In the inner part of the estuary where the river channel is influenced by tidal processes, the low-gradient channel commonly adopts a meandering form (Dalrymple et al. 1992). 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. The deposits in the point bar are therefore heterolithic, that is, they consist of more than one grain size, in this case alternating layers of sand and mud (Reineck & Singh 1972). This style of point-bar stratification has been called ‘inclined heterolithic stratification’, sometimes abbreviated to ‘IHS’ (Thomas et al. 1987). These alternating layers of sand and mud dipping in to the axis of the channel (perpendicular to flow) are a distinctive feature of tidally influenced meandering channels.

TIDAL FLATS

Adjacent to the channels and all along the sides of the estuary there are tidal flat areas that are variably covered with seawater at high tide and subaerially exposed at low tide. These are typically vegetated salt

marsh areas cut by tidal creeks that act as the conduits for water flow during the tidal cycles. The processes and products of deposition in these settings are the same as found in macrotidal settings

TIDAL SAND BARS

The outer part of a tide-dominated estuary is the zone of strongest tidal currents, which transport and deposit both fluvially derived sediment and material brought in from the sea. In macrotidal regions the currents will be strong enough to cause local scouring and to move both sand and gravel: bioclastic debris is common amongst the gravelly detritus deposited as a lag on the channel floor (Reinson 1992). Dune bedforms are created and migrate with the tidal currents to generate cross-bedded sandstone beds. Evidence for tidal conditions in these beds may include mud drapes, reactivation surfaces and herringbone cross-stratification. The mud drapes form as the current slows down when the tide turns, and the reactivation surfaces occur as opposing currents erode the tops of dune bedforms.

Recognition Of Estuarine Deposits

There are many features in common between the deposits of deltas and estuaries in the stratigraphic record. Both are sedimentary bodies formed at the interface between marine and continental environments and consequently display evidence of physical, chemical and biological processes that are active in both settings (e.g. an association of beds containing a marine shelly fauna with other units containing rootlets). The key difference is that a delta is a progradational sediment body, that is, it builds out into the sea and will show a coarsening-up succession produced by this progradation. In contrast, estuaries are mainly aggradational, building up within a drowned river channel. The base of an estuarine succession is therefore commonly an erosion surface scoured at the mouth of the river, for example, in response to sea level fall.

Characteristics of estuarine systems

Tidal channel systems

1. lithology – mud, sand and less commonly conglomerate2. mineralogy – variable3. texture – may be well sorted in high energy settings4. bed geometry – lenses with erosional bases5. sedimentary structures – cross-bedding and crosslamination

and inclined heterolithic stratification6. palaeocurrents – bimodal in tidal estuaries7. fossils – shallow marine8. colour – not diagnostic9. facies associations – may be overlain by fluvial, shallow

marine, continental or delta facies

Tidal mudflats

1. lithology – mud and sand2. mineralogy – clay and shelly sand3. texture – fine-grained, not diagnostic4. bed geometry – tabular muds with thin sheets and lenses of

sand5. sedimentary structures – ripple cross-lamination and

flaser/lenticular bedding6. palaeocurrents – bimodal in tidal estuaries7. fossils – shallow marine fauna and salt marsh vegetation8. colour – often dark due to anaerobic conditions9. facies associations – may be overlain by shallow marine or

continental facies

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