Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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. facies associations – typically occur overlying shal-

low-marine facies and overlain by fluvial facies in an

overall progradational pattern.

FURTHER READING

Bhattacharya, J.P. (2006) Deltas. In: Facies Models Revisited

(Eds Walker, R.G. & Posamentier, H.). Special Publication

84, Society of Economic Paleontologists and Mineralogists,

Tulsa, OK; 237–292.

Bhattacharya, J.P. & Giosan, L. (2003) Wave-influenced

deltas: geomorphological implications for facies recon-

struction. Sedimentology, 50, 187–210.

Boyd, R., Dalrymple, R.W. & Zaitlin, B.A. (1992) Classifica-

tion of clastic coastal depositional environments. Sedimen-

tary Geology, 80, 139–150.

Colella, A. & Prior, D.B. (Eds) (1990) Coarse-Grained Deltas.

Special Publication 10, International Association of Sedi-

mentologists. Blackwell Science, Oxford, 357 pp.

Reading, H.G. & Collinson, J.D. (1996) Clastic coasts. In: Sedi-

mentary Environments: Processes, Facies and Stratigraphy

(Ed. Reading, H.G.). Blackwell Science, Oxford; 154–231.

delta channel (dc)

delta plain (dp)

delta front (df)

prodelta (pd)

mouth bar (mb)

interdistributary bay (ib)

Fig. 12.23 Delta cycles: the facies succession preserved depends on the location of the vertical profile relative to the

depositional lobe of a delta.

198 Deltas

Clastic Coasts and Estuaries

The morphology of coastlines is very variable, ranging from cliffs of bedrock to gravelly or

sandy beaches to lower energy settings where there are lagoons or tidal mudflats. Wave

and tidal processes exert a strong control on the morphology of coastlines and the

distribution of different depositional facies. Wave-dominated coasts have well-

developed constructional beaches that may either fringe the coastal plain or form a

barrier behind which lies a protected lagoon. Barrier systems are less well developed

where there is a larger tidal range and the deposits of intertidal settings, such as tidal

mudflats, become important. A very wide range of sediment types can be deposited in

these coastal depositional systems and in this chapter only terrigenous clastic environ-

ments are considered: carbonate and evaporite coastal systems are covered in the

following chapter. Estuaries are coastal features where water and sediment are supplied

by a river, but, unlike deltas, the deposition is confined to a drowned river valley.

13.1 COASTS

Coasts are the areas of interface between the land and

the sea, and the coastal environment can comprise a

variety of zones, including coastal plains, beaches,

barriers and lagoons. The shoreline is the actual

margin between the land and the sea. Coastlines can

be divided into two general categories on the basis of

their morphology, wave energy and sediment budget.

Erosional coastlines typically have relatively steep

gradients where a lot of the wave energy is reflected

back into the sea from the shoreline (a reflective

coast, Fig. 13.1): both bedrock and loose material

may be removed from the coast and redistributed by

wave, tide and current processes. At depositional

coastlines the gradient is normally relatively gentle

and a lot of the wave energy is dissipated in shallow

water: provided that there is a supply of sediment,

these dissipative coasts can be sites of accumulation

of sediment (Woodroffe 2003).

13.1.1 Erosional coastlines

Exposure of bedrock in cliffs allows both physical and

chemical processes of weathering: oxidation and

hydration reactions are favoured in the wet environ-

ment, and the growth of salt crystals within cracks of

rocks sprayed with seawater can play an important

role in breaking up the material. Material accumulates

at the foot of cliffs as loose clastic detritus and occa-

sionally as large blocks when whole sections of the cliff

face are removed. Cliff erosion may result in wave cut

platforms (Fig. 13.2) of bedrock eroded subhorizon-

tally at beach level. Wave action, storms and tidal

currents will then remove the debris as bedload, as

suspended load or in solution. This contributes to the

supply of sediment to the marine environment, and

away from river mouths can be an important source

of clastic detritus to the shallow marine realm.

13.1.2 Depositional coastlines

A coastline that is a site of accumulation of sediment

must have an adequate supply of material to build

up a deposit. The sources of this sediment are from the

marine realm, either terrigenous clastic detritus

reworked from other sources or bioclastic debris.

The terrigenous material ultimately comes from riv-

ers, with a small proportion of wind-blown origin and

from direct erosion of coastlines. This sediment

is brought to a depositional coastline by tidal, wind-

driven and geostrophic currents (11.4) that trans-

port material parallel to shorelines or across shallow

seas. Wind-driven waves acting obliquely to the

shoreline are an important mechanism of trans-

port, creating a shore-parallel current known as

longshore drift. Shallow seas are generally rich

in fauna, and the remains of the hard parts of

these organisms provide an important source of bio-

clastic material to coastlines.

The form of a depositional c oastline is deter-

mined by the supply o f sediment, the wave energy,

thetidalrangeandtheclimate.Climateexertsa

strong control on coasts that are primarily sites of

carbonate and evap orite deposition, and these

environments are considered in Chapter 15.

Along clastic coastlines a beach of sandy or grav-

elly material forms where there is a sufficient sup-

ply of clastic se diment and enough wave energy to

transport the material on th e foreshore. The form

of the beach, and the develo pment of bar rier sys-

tems and lagoo ns, is dependent on whether the

coastline is in a micro-, meso- or macrotidal

regime. Sea-level changes also s trongly influence

coastal morphology. In the following sections the

processes related to beach formation are first consid-

ered, followed by a description of the morphologies

that can exist in wave-dominated and tidally influ-

enced coasts.

Dissipative coast

Reflective coast

Gently sloping flat beach, broad surf zone

Steep beach with berm, narrow surf zone

Typically sand

Typically coarse sand and gravel

Fig. 13.1 Reflective coasts are usually

erosional with steep beaches and a

narrow surf zone. Dissipative coasts may

be depositional, with sand deposited

on a gently sloping foreshore.

Fig. 13.2 An erosional coastline: wave action has eroded

the cliff and left a wave-cut platform of eroded rock on

the beach.

200 Clastic Coasts and Estuaries

13.2 BEACHES

The be ach is the area washed by waves breaking o n

the coast. The se award p art of the beach is t he

foreshore ( 11.1), which is a flat surface where

waves go back and forth and which is gently dipping

towards the sea (Fig. 13.3). Where wave energy is

sufficiently strong, sandy and gravelly material may

be continuously reworked on the foreshore, abrading

clasts of all sizes to a high degree of roundness, and

effectively sorting sediment into different sizes (Hart &

Plint 1995). Sandy sediment is deposited in layers

parallel to the slope of the foreshore, dipping offshore

at only a few degrees to the horizontal (much less

than the angle of repose). This low-angle stratification

of well-sorted, well-rounded sediment is particularly

characteristic of wave-dominated sandy beach

environments (Clifton 2003, 2006). Grains are typi-

cally compositionally mature as well as texturally

mature (2.5.3) because the continued abrasion in

the beach swash zone tends to break down the

weaker clasts.

On gravel beaches the water washed up the beach

by each wave tends to percolate down into the porous

gravel, and the backwash of each wave is therefore

weak. Clasts that are washed up the beach will there-

fore tend to build up to form a storm ridge at the top

of the foreshore, a back-beach gravel ridge that is a

distinctive feature of gravelly beaches. The clast com-

position will vary according to local sediment supply,

and may contain terrigenous clastic, volcaniclastic or

bioclastic debris.

At the top of the beach, a ridge, known as a berm,

marks the division between the foreshore and back-

shore area (Fig. 13.3). Water only washes over the

top of the berm under storm-surge conditions. Sedi-

ment carried by the waves over the berm crest is

deposited on the landward side forming layers in the

backshore that dip gently landward. These low-angle

strata are typically truncated by the foreshore strati-

fication, to form a pattern of sedimentary structures

that may be considered to be typical of the beach

environment (Figs 13.3 & 13.4). The backshore area

may become colonised by plants and loose sand can

be reworked by aeolian processes.

Wave action in the lower part of the foreshore can

rework sand and fine gravel into wave ripples that

can be seen on the sediment surface at low tide and

can be preserved as wave-ripple cross-lamination.

However, wave-formed sedimentary structures on

the beach may be obliterated by organisms living in

the intertidal environment and burrowing into the

sediment. This bioturbation may obscure any other

sedimentary structures.

Coastal plain

Foreshore

Beach dune ridge

Beach

Foreshore

Backshore

Berm

Fig. 13.3 Morphological features of a beach comprising a beach foreshore and backshore separated by a berm; beach dune

ridges are aeolian deposits formed of sand reworked from the beach.

Beaches 201

13.2.1 Beach dune ridges

Aeolian processes can act on any loose sediment

exposed to the air. Along coasts any sand that dries

out on the upper part of the beach is subject to rework-

ing by onshore winds that may redeposit it as aeolian

dunes (8.6.1). Coastal dunes form as ridges that lie

parallel to the shoreline and they may build up to

form dune complexes over 10 m high and may stretch

hundreds of metres inland (Wal & McManus 1993).

Vegetation (grasses, shrubs and trees) plays an

important role in stabilising and trapping sediment

(Fig. 13.5). The limiting factor in beach dune ridge

growth is the supply of sand from the beach. They

commonly form along coasts with a barrier system,

but can also be found along strand-plain coasts.

In a sedimentary succession these beach dune ridge

deposits may be seen as well-sorted sand at the top of

the beach succession (Fig. 13.6). Some preservation of

the roots of shrubs and trees that colonised the dune

field is possible, but the effect of the vegetation is often

to disrupt the preservation of well-developed dune

cross-bedding.

13.2.2 Coastal plains and strand plains

Coastal plains are low-lying areas adjacent to seas

(Fig. 13.7). They are part of the continental environ-

ment where there are fluvial, alluvial or aeolian pro-

cesses of sedimentation and pedogenic modification.

Coastal plains are influenced by the adjacent marine

environment when storm surges result in extensive

flooding by seawater. A deposit related to storm flood-

ing can be recognised by features such as the presence

of bioclastic debris of a marine fauna amongst depos-

its that are otherwise wholly continental in character.

Sandy coastlines where an extensive area of beach

deposits lies directly adjacent to the coastal plain are

known as strand plains (Fig. 13.7). Along coasts

supplied with sediment, beach ridges create strand

plains that form sediment bodies tens to hundreds of

metres across and tens to hundreds of kilometres long

and progradation of strand plains can produce exten-

sive sandstone bodies. The strand plain is composed of

Fig. 13.4 Foreshore-dipping and backshore-dipping strati-

fication in sands on a beach barrier bar.

Fig. 13.5 A beach dune ridge

formed by sand blown by the wind

from the shoreline onto the coast

to form aeolian dunes, here

stabilised by grass.

202 Clastic Coasts and Estuaries

the sediment deposited on the foreshore and back-

shore region. The backshore area merges into the

coastal plain and may show evidence of subaerial

conditions such as the formation of aeolian dunes

and plant colonisation.

13.3 BARRIER AND LAGOON

SYSTEMS

13.3.1 Barriers

Along some coastlines a barrier of sediment separates

the open sea from a lagoon that lies between the

barrier and the coastal plain (Fig. 13.8). Beach bar-

riers are composed of sand and/or gravel material and

are largely built up by wave action. They may be

partially attached to the land, forming a beach spit,

or wholly attached as a welded barrier that comple-

tely encloses a lagoon, or can be isolated as a barrier

island in front of a lagoon. In practice, the distinction

between these three forms can be difficult to identify

in ancient successions and their sedimentological

characteristics are very similar. Barriers (Fig. 13.9)

range in size from less than 100 m wide to several

kilometres and their length ranges from a few

hundred metres to many tens of kilometres (Davis &

Fitzgerald 2004). The largest tend to form along the

open coasts of large oceans where the wave energy is

high and the tidal range is small.

Fig. 13.6 A schematic graphic sedimentary log of

sandy beach deposits.

Barrier and Lagoon Systems 203

The seaward margin of a barrier island has a beach

and commonly a beach dune ridge where aeolian

processes rework the sand. Vegetation helps to stabi-

lise the dunes. On the landward side of the island the

layers of sand deposited during storms pinch out into

the muddy marshes of the edge of the lagoon. During

storm surges seawater may locally overtop the beach

ridge and deposit washovers of sediment that has

been reworked from the barrier and deposited in the

lagoon (Fig. 13.8). Washover deposits are low-angle

cones of stratified sands dipping landwards from the

barrier into the lagoon.

The conditions required for a barrier to form are as

follows. First, an abundant supply of sand or gravel-

sized sediment is required and this must be sufficient

to match or exceed any losses of sediment by erosion.

The supply of the sediment is commonly by wave-

driven longshore drift from the mouth of a river at

some other point along the coast and there may also

be some reworking of material from the sea bed

further offshore. Second, the tidal range must be

small. In macrotidal settings the exchange of water

between a lagoon and the sea during each tidal cycle

would prevent the formation of a barrier because a

chenier ridges

(former beach ridges)

coastal plain

strand plain

(beach attached

to coastal plain)

sea

Fig. 13.7 A wave-dominated coastline with a coastal plain bordered by a sandy beach: chenier ridges are relics of former

beach strand plains.

beach barrier

washover deposit

lagoon

coastal plain

sea

wave fronts

longshore drift

inlet

Fig. 13.8 A wave-dominated coastline with a beach-barrier bar protecting a lagoon.

204 Clastic Coasts and Estuaries

restricted inlet would not be able to let the water pass

through at a high enough rate. Barrier systems are

therefore best developed in microtidal (Fig. 13.8) and,

to some extent, mesotidal settings (Fig. 13.10). Third,

barrier islands generally form under conditions of

slow relative sea-level rise (Hoyt 1967; FitzGerald &

Buynevich 2003). If there is a well-developed beach

ridge, the coastal plain behind it may be lower than

the top of the ridge. With a small sea-level rise, the

coastal plain can become partially flooded to form a

lagoon, and the beach ridge will remain subaerial,

forming a barrier. For the barrier to remain subaerial

as sea level rises further, sediment must be added to

the beach to build it up, that is, the first condition of

high sediment supply must be satisfied.

13.3.2 Lagoons

Lagoons are coastal bodies of water that have very

limited connection to the open ocean. Seawater

reaches a lagoon directly through a channel to the

sea or via seepage through a barrier; fresh water is

supplied by rainfall or by surface run-off from the

adjacent coastal plain. If a lagoon is fed by a river it

would be considered to be part of an estuary system

(13.6). They are typically very shallow, reaching only

a few metres in depth.

Lagoons generally develop along coasts where

there is a wave-formed barrier and are largely pro-

tected from the power of open ocean waves (Reading

& Collinson 1996). Waves are generated by wind

blowing across the surface of the water, but the

fetch of the waves (4.4) will be limited by the dimen-

sions of the lagoon. Ripples formed by waves therefore

affect the sediments only in very shallow water. The

wind may also drive weak currents across the lagoon.

Tidal effects are generally small because the barrier–

lagoon morphology is only well developed along

coasts with a small tidal range.

Fine-grained clastic sediment is supplied to lagoons

as suspended material in seawater entering past the

barrier and in overland flow from the adjacent coastal

plain. Organic material may be abundant from vege-

tation which grows on the shores of the lagoon.

In tropical climates, trees with aerial root systems

(mangroves) colonise the shallow fringes of the

lagoon. Mangroves cause the shoreline to prograde

into the lagoon as they act as sites for accumulation of

Fig. 13.9 Beach-barrier bars along a wave-dominated

coastline.

lagoon

flood-tidal

deposit

ebb-tidal

deposits

sea

beach barrier

tidal flow

mud flats

coastal plain

Fig. 13.10 Morphological features of a coastline influenced by wave processes and tidal currents.

Barrier and Lagoon Systems 205

sediment and organic matter along the water’s edge.

In more temperate climates, saline-tolerant grasses,

shrubs and trees may play a similar role in trapping

sediment. Coarser sediment may enter the lagoon

when storms wash sediment over the barrier as

washover deposits, which are thin layers of sand

reworked by waves. Sand is also blown into the

water by onshore winds picking up material from

the dunes along the barrier.

An important characteristic of lagoons is their

water chemistry. Due to the limited connection to

open ocean, it is common for lagoon water to have

either higher or lower salinity than seawater. Low

salinity, brackish water (10.3) will be a feature of

lagoons in areas of high rainfall, local run-off of

fresh water from the coastal plain or small streams.

Mixing of the lagoon water with the seawater is

insufficient to maintain full salinity in these brackish

lagoons. In more arid settings the evaporation from

the surface of the lagoon may exceed the rate at

which seawater exchanges with the lagoon water

and the conditions become hypersaline (10.3), that

is, with salinities higher than that of seawater. If

salinities become very elevated, precipitation of

evaporite minerals will occur (3.2).

A lagoonal succession is typically mudstone, often

organic-rich, with thin, wave-rippled sand beds

(Boggs 2006) (Fig. 13.11). The deposits of lagoons

can be difficult to distinguish from those of lakes

with similar dimensions and in similar climatic set-

tings. The processes are almost identical in the two

settings because they are both standing bodies of

water. Two lines of evidence can be used to identify

lagoonal facies. First, the fossil assemblage may indi-

cate a marine influence, and specifically a restricted

fauna may provide evidence of brackish or hypersa-

line water. Second, the association with other facies

is also important: lagoonal deposits occur above or

below beach/barrier island sediments and fully marine

shoreface deposits.

13.4 TIDES AND COASTAL SYSTEMS

13.4.1 Microtidal coasts

Under microtidal conditions wave action can main-

tain a barrier system (Fig. 13.8) that can be more or

less continuous for tens of kilometres. Exchange of

water between the lagoon and the sea may be very

limited, occurring through widely spaced inlets and as

seepage through the barrier. Coarse sedimentation in

the lagoon will be largely restricted to washovers that

occur during storms. There is a strong likelihood of

the lagoon waters becoming either brackish or hyper-

saline, depending upon the prevailing climate.

13.4.2 Mesotidal coasts

With the increased tidal range of mesotidal condi-

tions, more exchange of water between the lagoon

and the sea is required, resulting in more inlets form-

ing, breaking up the barrier into a series of islands

(Fig. 13.10). These inlets are the pathways for the

tidal flows and the currents within them can be

strong enough to redistribute sediment. On the lagoon

side of the barrier sediment washed through the

channel is deposited in a flood-tidal delta

(Fig. 13.10). The water in the lagoon is shallow, so

Fig. 13.11 A schematic graphic sedimentary log of clastic

lagoon deposits.

206 Clastic Coasts and Estuaries

the sediment spreads out into a thin, low-angle cone

of detritus dipping very gently landwards. Bedforms

on the flood-tidal delta are typically subaqueous

dunes migrating landwards, which result in cross-

bedding with onshore palaeocurrent directions

(Boothroyd 1985). Ebb-tidal deltas form on the sea-

ward margin of the tidal channel as water flows out of

the lagoon when the tide recedes. Building out into

deeper water they are thicker bodies of sediment than

flood-tidal deltas and the direction of bedform migra-

tion is seawards. The size and extent of an ebb-tidal

delta is limited by the effects of reworking of the

sediment by wave, storm and tidal current processes

in the sea.

13.4.3 Macrotidal coasts

Coasts that have high tidal ranges do not develop

barrier systems because the ebb and flood tidal

currents are a stronger control on the distribution of

sediment than wave action. A depositional coast in a

macrotidal setting will be characterised by areas

of intertidal mudflats that are covered at high tide

and exposed at low tide. Water flooding over these

areas with the rising tide spreads out and loses

energy quickly: only suspended load is carried across

the tidal flats, and this is deposited when the water

becomes still at high tide. The upper parts of the tidal

flats are only inundated at the highest tides. The

incoming tide brings in nutrients and tidal flats are

commonly areas of growth of salt-tolerant vegetation

(xerophytes) and animal life is often abundant

(worms, molluscs and crustaceans in particular).

The deposits of this salt marsh environment

(Belknap 2003) are therefore predominantly fine-

grained clay and silt, highly carbonaceous because

of all the organic material and the animal life results

in extensive bioturbation. The vegetation on the tidal

flats tends to trap sediment, and mud flats are com-

monly sites of net accumulation. Tidal flats are often

cut by tidal creeks, small channels that act as

conduits for flow during rising and falling tides: the

stronger flow in these creeks allows them to trans-

port and deposit sand, resulting in small channel

sand bodies within the tidal-flat muds. Coarser sedi-

ment is also introduced onto the tidal flats during

storms, forming thin layers of sand and bioclastic

debris. Flaser and lenticular bedding (4.8) may occur

on the lower parts of the tidal mudflats, where

currents may be periodically strong enough to trans-

port and form ripples. These ripple-laminated sands

will occur interbedded with mud that drapes the rip-

ple forms.

13.5 COASTAL SUCCESSIONS

The patterns of sedimentary successions built up at a

coast are determined by a combination of sediment

supply and relative sea-level change. (As will be seen

in Chapter 23, these two factors are in fact dominant in

controlling the large- and medium-scale stratigraphy

in all shallow marine depositional environments.)

Prograding barriers and strand plains are those

that build out to sea through time as sediment is

added to the beach from the sea. A barrier will become

wider, and the inner margins may become more sta-

bilised by vegetation growth. A prograding strand

plain will result in a series of ridges parallel to the

coastline, chenier ridges (Fig. 13.7), which are the

relicts of former beaches that have been left inland

as the shoreline prograded (Augustinus 1989).

Retrograding barriers form where the supply of

sediment is too low to counteract losses from the

beach by erosion. Removal of sediment from the

front of the barrier reduces its width and, in turn, its

height. This makes the coast more susceptible to

washovers of sand occurring and the lagoon (or a

marsh behind a strand plain) will become partly filled

in. By this process the beach system will gradually

move landward.

Under conditions of slow relative sea-level rise the

beach may also move landward, but a lagoon will

also expand, flooding the adjacent coastal plain in

response to the sea-level rise. Through time these

transgressive barrier systems will build up a succes-

sion from coastal plain deposits at the base, overlain

by lagoon facies and capped by beach deposits of the

barrier system (Fig. 13.12). A similar transgressive

situation at a strand plain will result in coastal plain

deposits overlain by beach deposits.

13.6 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

Estuaries 207