Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks

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DELTAS AND ESTUARIES

195

offshore the distributary mouths. Mud diapirs lead to very

variable thicknesses in the mouth bar sands, which thicken into

withdrawal synclines between diapirs.

Horizontal fiowage of overpressured muds leads to extru-

sion of muds at the base of the prograding slope as imbricate

thrusts and folds. The same motion is partly responsible for

extensional stresses that drive sedimentary growth faults in

deltaic successions.

Owen, G., 1987. Deformation processes in unconsolidated sands. In

Jones,

M.E., and Preston, R.M.F. (eds.), DeformationofSediments

and Sedimentary Rocks. Geological Society of London, Special

Publication, 29, pp. 11-24.

Winker, CD., and Edwards, M.B., 1983. Unstable progradational

clastic shelf margins. In Stanley, D.J., and Moore, G.T. (eds.), The

Shelfhreak: Critical Interfaee on Continental Margins. Society of

Economic of Paleontologists and Mineralogists, Special Publica-

tion, 33, pp. 139-157.

Growth faults

Sedimentary growth faults occur in many deltaic successions.

They are large-scale features and commonly require excep-

tional exposures to be readily identified at outcrop. In small

exposures, they may be confused with later tectonic faults

though a lack of mineralization and sediment smearing on the

fault plane may suggest a syndepositional origin. Growth

faults are more commonly imaged in seismic data where

upward truncation by overlying undisturbed sediments can be

a criterion for their recognition. They are caused by shear

failure at discrete surfaces within the sedimentary pile, and

they commonly occur as listric surfaces that sole out on a basal

decollement. In smaller examples, confined within a single

progradational cycle, the basal surface may be a thin bed of

particularly fine-grained mudstone. In larger examples, which

penetrate several progradational units within a delta complex

at a continental margin, the decollement may be within a tJiick

interval of overpressured muds. Lateral flowage of those muds

toward the free surface slope and gravitational forces on the

hanging wall block both help to drive the movement of that

block down slope. Growth faults are characterized by slow and

continual movement and are aided by progressive sediment

loading. Their hanging wall areas act as local depocenters in

which thickened delta-front sediments are deposited and

preserved. With continued progradation of individual deltas

or of the continental slope the position of active faulting shifts

progressively basinwards.

Large-scale examples imaged on seismic at continental

margins show complex fault geometries with both synthetic

and antithetic fault elements. In plan, the major faults have

cuspate traces, concave toward the basin. Protracted fault

movement and the listric geometry of the faults leads to roll-

over anticlines in the hanging walls which may be important

hydrocarbon traps as they also coincide with enhanced

reservoir thickness. Clay smearing on the fault surfaces may

enhance the hydrocarbon seal.

John D. Collinson

Bibliography

Allen, J.R.L., and Banks, N.L., 1972. An interpretation and analysis of

recumbent-folded deformed cross-bedding. Sedimentology, 19:

257-283.

Collinson, J.D., 1994. Sedimentary deformational structures. In

Maltman, A. (ed.). The Geological Deformation of Sediments.

London; Chapman & Hall, pp. 95-125.

Edwards, M.B., 1976. Growth faults in Upper Triassic deltaic

sediments, Svalbard. Bulletin ofthe American Association of

Petro-

leum Geologists, 60: 341-355.

Jones,

M.E., and Preston, R.M.F. (eds.), 1987. Deformation of Sedi-

ments and Sedimentary Rocks. London: Geological Society of

London, Special Publication, 29.

Lowe, D.R., and Lopiccolo, R.D., 1974. The characteristics and

origins of disb and pillar structures. Journal of Sedimentary

Petrology, 44:

484-501.

Cross-references

Bedding and Internal Struetures

Convolute Lamination

Debris Flow

Deformation of Sediments

Dish Structure

Flame Structure

Fluid Escape Structures

Gases in Sediments

Liquefaction and Fluidization

Load Structures

Pillar Structure

Ball-and-Pillow (Pillow) Structure

Pseudonodules

Slide and Slump Struetures

Syneresis

DELTAS AND ESTUARIES

Definition of a delta

A delta is a discrete bulge of the shoreline formed at the point

where a river enters an ocean, sea, lake, lagoon or other

standing body of water (Figures D4 and D5). The bulge is

formed because sediment is supplied more rapidly than it can

be redistributed by basinai processes, such as waves and tides.

Deltas are thus fundamentally regressive in nature, which

means that their deposits record a seaward migration or

progradation of the shoreline. The term delta has also been

used to describe any regressive deposit built by any terrestrial

feeder system into any standing body of water (Nemec, 1990 in

Colella and Prior, 1990). In this scheme, terrestrial feeder

systems can be alluvial (rivers, alluvial fans, braidplains, scree-

cones) or non-alluvial (volcanic lavas or pyroclastic flows). The

terms ebb- and flood-tidal delta have also been generally

applied to sediment accumulations that form around tidal inlet

channels in barrier-lagoon depositional systems (see Tidal

Inlets and Deltas), but these do not fit the above definition of

a delta because they are not linked to a river or terrestrial

feeder. This entry focuses on river deltas as a discrete

depositional system in which both environments and their

deposits will be described.

Definition of an estuary

Sedimentologists define an estuary (Figure D5) as the seaward

portion of a drowned river valley which receives sediment from

both fluvial and marine sources (modified after Dalrymple

etal., 1992). An estuary may be affected by tide, wave, and

river processes and is defined as extending from the landward

limit of tidal influence to the seaward limit of coastal infiuence.

The term estuary is also defined on an oceanographic basis

as a semi-enclosed body of marine water that is measurably

196

DELTAS AND ESTUARIES

terminal

dlstributapy

channels

Figure D4 Block didgram of a lobate, rivtT-dominated delta showing numerous channel bifurcations. Terminal distributary channels fed

triangular-shaped mouth bar deposits. Seaward progradation forms a series of otflapping inclined strata in dip-view, Tlie strike view shows a lens-

shaped sediment body with beds dipping away from the Hepositional axis of the delta lobe.

Increasing relative tidal

power-

Increasing relative wave

power-

Wave-dominated

Estuary

Lagoon

Embayed coasts with mixed sediment supply

Linear coasts

Lobate deltaic coasts supplied by rivers\\marine sediment supply

Tide-dominated

Sand

Mud Marsh

Figure D5 Comparison of regressive versus transgressive cuast.il depositiunal systems. Deltas form lobate sediment bodies. The character

and distribuiion of sediment in a delta is highly sensitive to the relative proportion of wave, tide, and fluvial processes. Differences in these

processes result in different shaped sand bodies. Also, note river-dominated delta at ihe head of the wave-influenced estuary. Deltas can thus form

smaller components of other types of coastal depusitional systems. Changes in sediment supply and sea level change can cause one type of

depositional system to evolve into another type (modified from Reading and Collinson. iy96 based on Heward,

1981,

and Boyd, Dalrymple and

Zaitlin,

1992),

DfLTAS AND ESTUARIES

197

diluted by land-derived Iresh water (Pritchard. 1967; Nichols

and Biggs. 1985). Sedimentologists tend lo reject this usage as

being too broad since it includes all brackish water environ-

ments including lagoons and many marine-in

flue

need delta

fronts whieh are not generally thought of as estuarine by

sedimentologists. The etymology of the word estuary also

means tides, so by definition all estuaries are formed adjaeent

to a marine body of water. There arc no entirely fresh-water

estuaries because measurable tides do not occur in lakes.

Consequently, drowned river valleys filled during rising lake

levels are not generally considered as estuarine.

In an estuary, the seaward portion of the valley is filled with

marine sediments and estuaries are fundamentally transgres-

sive in nature, unlike deltas, which are regressive. Deltas and

estuaries are nol mutually exclusive, however, because

regressive bayhead delta deposits ean readily form within the

up-valley reaches of an estuary (Figure D5). In this case, the

delta deposits would ibrtn a minor component of a larger.

overally transgressive estuarine valley-fill. If fluvial discharge

became high enough to "Hush out the valley" and begin to

form a broadly regressive deposit then the estuary would

evolve into a delta.

Estuaries form by the interaction of waves, tides and fluvial

processes. Two end members have been described (Figitre D5),

In lide-dominated estuaries (sueh as the Bay of Fundy in Nova

Scotia. Canada) the mouth of the estuary is kept open by

strong tidal currents. The center of the estuary tends to be

dominated by sandy bedforms and tidal bars whereas the

margins tend to be muddy tidal flats, ln wave-dominated

estuaries, the mouth of ihe esiuary is partly closed by a wave-

formed barrier island- A brackish lagoon or bay lies behind the

barrier and is commonly filled with flne-grained mud. At the

head of the bay. fluvial or tide-influenced bayhead delta

deposits form, which may be sandy or tnuddy. Mud

accumulates primarily in the so-called "central basin"

bounded by the barrier and bay head delta.

Distinguishing deltas, barriers, estuaries, and

strandplains

Most of the sediment in a delta is derived directly from ihe

river that feeds it; in contrast with estuaries in which much of

the sediment is derived from the marine realm and in which

deposits are fundamentally transgressive. In barrier-island

systems (Figure D5). sediment is supplied by alongshore

transport. Alongshore transport occurs beeause waves ap-

proaching the shoreline are deflected and migrate parallel to

the shoreline eausing longshore drift. The longshore drift

system may carry sand and gravel, which can be deposited as

linear shoreface deposits, or they may carry large quatitities of

fluidt/ed mud lhat can be deposited to form muddy eoastlines.

In mud-dominated coastlines, silt, sand, or shelly material may

be winnowed out in the intertidal zone, forming thin and

narrow beaeh deposits termed cheniers. Barrier-islands may

also form components of wave-influeneed deltas. Rivers can

act as a baffle or groin thai causes sediment carried in the

longshore drift system to be deposited on the tipdrift side of

the distributary channel (see wave-dominated delta in

Figure D5),

Where basinai processes redistribute sediment to the point

that the lluvial source and delta morphology can no longer be

recognized, more general terms such as paralic. strandptain or

eoastal plain may be preferable.

Scale and importance of deltas

Deltas occur at a wide variety of scales ranging from basin-

scale depositional systems, such as ihe modern Mississippi

delta with an area of about 28..SOOkm". to sttialler components

of other depositional systems, such as bayhead deltas within

estuarine or lagoonal systems. Many large deltas, such as the

Danube in Romania and the Mississippi in thcGulf of Mexico,

contain several seales of delta lobes associated with the faet

that as a ehannel splits into several smaller channels, each new

channel can feed its own lobe.

A large number ofthe earth's peoples live on or near deltaic

coastlines and they are thus important froin an environmental

perspective. Ancient deltas are also eeonomically important

beeause they are eommonly associated with major coal, oil and

gas reserves. As a consequenee, much has been written about

deltas,

although there is still much interest in new delta

researeh. There has recently been an increase in documented

examples of delta types that have been historically lacking,

such as tide-iniluenced systems (e,g,. Maguregui and Tyler.

1991:

Reading and Collinson. 1996; Willis el al.. 1999).

Readers are referred to several summaries for general infor-

mation on deltas (Reading and Collinson. 1996; Bhattacharya

and Walker, 1992; Colefla and Prior.

199(1;

Whateley and

Pickering. 1989; Coleman and Prior. 1982: Broussard, 1975)

and estuaries (Nichols and Biggs. 1985; Dalrymple elal.. 1992;

Dalrymple etat.. 1994).

History of delta research

Historical overviews of delta research are provided by

Bhattacharya and Walker (1992) and Reading and Collinson

(1996) and summarized here. The concept of

delta dates back

to the time of Herodotus (r. 400 BC) who recognized that the

alluvial plain at the mouth of the Nile had the form of the

Greek letter A. The first study of ancient deltas was that of

Gilbert, 1885 {see Bhattaeharya and Walker. 1995), who

described Pleistocene fresh-water gravelly deltas in Lake

Bonneville. Utah, Gilbert reeognized a threefold cross-

sectional division of a delta into topset. loreset and bottotnset

deposits (note sigmoidal stratal geometry in Figures D4 and

D7).

Barrel! (1912 ihitl.) extended these subdivisions to the

tnuch larger scale of the Devonian Catskill wedge in the

Appalachians, and also provided the first explicit definition of

the essential features of an aneient delta deposit. Barrell

considered the recognition of overlying nonmarine facies

crucial in recognizing ancient deltas, although recent research

demonstrates that in a significant number of deltas, non-

marine topset facies may nol be preserved (e.g.. Bhattacharya

atid Willis. 2001). Barrell actually wrote his 1912 paper in

order to address uhat he perceived as an over-application of

an estuarine interpretation to many ancient sedimentary

successions that contained a marine to non-marine transition

and that he believed should be better interpreted as deltas.

The Mississippi river and its deltas have long been a focus

for understanding continental-seale delta systems (see sum-

mary of early papers by LeBlanc, 1975). Scruton (1960, see

LeBlanc. 1975) reeognized that deltas arc cyclic in nature and

eonsist of

progradational constructive phase usually followed

by a thinner retrogradational destructive phase coinciding with

delta abandonment. He also illustrated a vertical facies

succession of coarsening- and sandier-upward facies related

to the progradation of bottomset. foreset and topset strata

198 DELTAS AND ESTUARIES

(Figure D7). Kolb atid Van Lopik (1966. sec LeBlanc. 1975)

surnniarized much ofthe work that described the way in which

the Mississippi river constructed its delta plain over the past

9000 years, suggesting that the development was autocyclic.

This work was critical in establishing the idea lhat apparently

cyclic vertical successions of sedimentary strata can resull frotn

the intrinsic way in which river channels naturally avulse and

eause switching of delta lobes,

Colernan and Wright (in Broussard, 1975) cotnpiled data on

34 modern deltas and developed a sixfold classification based

on sand distribution patterns. One of the most widely used

classification schemes still used today is that of Galloway (in

Broussard. 1975), who subdivided deltas according to the

dominant processes controlling their morphology; rivers,

waves and tides (Figure D5). This scheme has since been

expanded to include grain calibre (Orton and Reading, 1993;

Reading and Collinson. 1996). This schetne has also been

extended to show how deltas, estuaries and barrier-lagoons

may evolve into one another in the context of changing sea-

level and sediment supply (Dalrymple etal... 1992).

Improvements in seadoor-imaging and seismic data acquisi-

tion led to the recognition ofthe abundance and importance of

synsedimentary deformation in the subaqueoits parts of

modern deltas (Coleman. Prior and Lindsay, 1983; Winker

atid Fdwards, 1983; Bhattaeharya and Walker. 1992), Similar

features have now been recognized in ancient deltas (e,g,.

Bhattacharya and Davies, 2001).

More reeently the evolution of modem deltas has been

interpreted in the context of sea-level changes and plate

tectonics, rather than just sediment supply (e.g., Domingucz

etal.. 1987; and Boyd. elal.. 1989; see Bhattacharya ^and

Walker. 1992; Hart atid Long, 1996). This resulted in

widespread abandonment of the idea that apparently cyclic

sedimeniary successions are aiitocyelie. but rather are con-

trolled by allocyclic processes such as subsidence, sea level

ehange, and climate change (see Cyelie Sedimentation).

Integration of basin-scale seismic stratigraphic concepts with

ideas about cyelie sea-level change were applied to many

ancient deltas in the development of sequence stratigraphy (e.g..

Galloway. 1989; Van Wagoner et al.. 1990; Bhatlaeharya.

1993).

More recent work is beginning to elucidate the nature

of tide-infiuenced deltas thai have heretofore been under-

recogni/ed in ancient sedimentary rocks because of lack of

well-studied modern or ancient examples (e.g., Maguregui and

Tyler. 1991; see Reading and Collinson. 1996; Jenette and

Jones.

1995; Willis t'/«/.. 1999).

River mouth processes

A delta fortns when a river of sediment-laden freshwater enters

a standing body of water, loses its competence to carry

sediment, and deposits it. Much of the active sand deposition

occurs in distinctive distributary mouth

bars,

formed directly at

the mouth ofthe channel (Figure D6), The mouth bars form

naturally as a consequence of the decrease in discharge and

bed shear stresses assoeiated with the loss of flow cotnpelence.

although very little work has been done that quantifies the

mechanics of how mouth bars scale to the flow behavior at the

ri\er tnouth. River mouths may be deflected downdrift by

waves and can be scoured and modilied by tidal currents

(Figure D5).

The general form of the deltaie deposit depends upon (1)

whether the river outflow is more dense (hvpcrpycnat flow).

Figure D6 Se<ivvard mij^riifion Itermed progradation or regression) of a

delta builds an upward coarsening facies succession, thai shows a

transition from marine inlo nonmarine lopset deposits (modified after

Scrulon,

19()0). Topset facies may be eroded by waves

ihe sea floods

back over ihe land during transgression.

equally dense {homopvcnal) or less dense {hypopycnal) than the

standing body of water (Figure D6). and (2) the extent to

which the deposits are reworked by wave and tidal processes,

although tidal processes are insignificant in lakes. At any given

river mouth, inertial. frictional and buoyant forces may be

operative in varying proportions (Figure D6), In hyperpycnal

deltas that have exceedingly high sediment concentrations at

the river rnouth. the slurry-like mixture of sediment and water

just keeps on moving downslope as a seditnent gravity fiow

(see sediment gravity flow). These sediments can end up being

deposited in deepwater systems. Hyperpycnal rivers tend to be

small and/or "dirty" such as the modern Sepik River, in Papita

New Guinea, the Eel River in Northern California, and Ihe

rather larger Yellow River (Yangste) in China (Mulder and

Syvitsky. 1995). In hypopyenal settings buoyant, less dense

fresh water rides over a marine salt-wedge and carries

suspended mud far offshore. Sand tends to be segregated

and deposited at the mouth of the channel as a distinct sandy

mouth bar with muddier sediments deposited farther offshore

in the prodelta area (Figure D6). In homopycnal deltas a great

degree of mixing occurs and frietional forces cause rapid

deposition of all ofthe tnud. sand, and/or gravel carried by the

river (Figure D6). This results in deposition of a large poorly

sorted '•Gilbcrt-delta'"-type mouth bar commonly with angle-

of-repose foresets (Figure D6).

Rapid sedimentation in all delta types can cause sediment

instability at the river mouth (see Synsedimentary Structures

atid Growth Faults). Slumping commonly occurs after the

mouth bar builds up to a slope that exceeds the failure

threshold. Mouth bars are especially susceptible to failure

because of the high amount of water trapped within the

sediment thai causes high pore pressure. Slumped seditnent at

the river mouth can evolve into various types of sediment

gravity flows and are an important way of providing sediment

to the deep oceans.

Many rivers will experience dramatie changes in discharge

as a function of seasonal climate change or as a result of major

stortns. Sediment discharge and sediment caliber of the river

plays an itnportant role in determining the sediment eoneen-

tration discharged at the river mouth (Figure D6). These

changes in sediment concentration ean cause a river to ehange

from hypopycnal to hyperpyenal. even in fully marine settings

(Mulder and Syvitsky. 1995), Recent work has attempted to

distinguish the relative importance of these processes in

Subaqueous

levee

HYPOPYCNAL FLOW

BUOYANCY

SALT WATER

ENTRAINED

UPWARDS

.Bar -. Bar ';V;-: Distal i-I,

back i; crest i

^ bar

Prodelta

TRANSVERSE

FLOW

CONVERGENCE

1000 m

Levee

Bar finger sands

FLOW CONVERGENCE

Low tide

FRICTION \ BUOYANCY

D \ O

100 m

Mouth —

- . C

^ rippled sand

rSn Pfodetta

100 nr\

CHANNEL

BIFURCATION -V

;,;.?"

FRICTION

\ TURBULENT

\ FRICTION \ DIFFUSION

\ WAVES \ AND

\'-:^--i:

BUOYANCY

• \

Channel

bar

tide

'' lOOm =—^ — _- _•

INERTIA-DOMIfJATEO

HOMOPYCNAL FLOW

FORMATION OF

GILBERT-TYPE FORESETS

HYPERCVNAL FLOW

SEDIMENT GRAVITY FLOWS

porta

BSSSS

4 I 1 • '

••'•

•

> 4 > < <

, > I I I 4

ensity

Figure D7 River Mouth processes and their deposits. Hypopycnal flow (lop piineh is defined where int'lowing w.3ter (typitolly fre^h) is less ctense thdn

waler in the receiving Ijasin (typitally salinel. this favors the formation of a salt wedge and elongate bar-finger type mouth bars. The less dense fresh

waler is buoyed-up by the denser seawater and mud is carried far offshore. These delt.i fronts are typically referred to as buoyancy dominaled. As the

sediment becomes coarser-grained (mickile p.]nelip more friction is generated. This is common where the density of inflowing and receiving waters are the

same (homopycnall such as in many lacustrine deltas and in rivers wilh high sediment loads. These delta fronts are sometimes referred to as friclion-

dominated and favor developmeni of triangular (radiall mouth bars. In rivers with exceptionally high sediment concentrations (lower panel), especially

small muddy rivers, hyperpycnal conditions may result. In these cases, sediment-laden water is more dense than water in the receiving t>asin. The

inflowing mixture of water and sediment may have enough inertia to continue flowing down the sea bed as a sediment gravity flow. These types of delta

tronls are said to be inertia-dominaled. These types of deltas may be linked to deep water depositional systems and provide

means of supplying sediment

to submarine fan de|)osits. These mouth bar types are not mutually exclusive. Rivers can change from hypopycnal to hyperpytnal as discharge increases

during exceptional flooding events and the degree of buoyant, frictional and inertial forces can vary across a bar (middle and lower panel). Modified

atter Reading and Coliinson, 1996, based on Orton and Reading, 1993.

200 DFITAS AND ESTUARIES

cliissifying ancient river mouth sediments (e,g,. Martinscn.

1990;

see Reading and Collinson, 1996).

River mouth processes ean also be important in the

autoeyclic process ol" river avulsion that results in delta lobe

switching. In this process, the river builds a large deltaic edifice

that creates enormous form friction at the mouth of the river.

This friction causes a loss of discharge at the river mouth. In

addition, the river extends seaward, which decreases the

effective slope. The loss of discharge and decreasing slope

propagates up.stream and eventually causes the river to avulse.

At a more local scale, friction at the river mouth may cause

upstream deposition of sand. In a study of Cretaeeous-age

growth faulted deltaic strata in Utah, growth faults initiate

both upstream and downstream as the loci of rapidly deposited

sands shift (Bhattacharya and Davies. 2001).

Delta environments

No one environment is characteristic of a delta. In plan

view, deltas comprise three main environments: the delta plain

(where river processes dominate), the delta front (where river

and basinai processes are both important) and the prodelta

(where basinai proeesses dominate). In cross-scctionai views,

these three environments roughly coincide with the topset.

foreset and bottomset strata of early workers (Figure 1)7),

Delta plains usually contain distributary channels and a

wide variety of nonmarine to brackish, paralic environments

including swamps, marshes, tidal flats, and interdistrihutary

bays (Figure D4). The line (hat demarcates the landward limit

of tidal incursion of seawater is referred to as the bay line and

marks the boundary between the upper and lower delta plain.

In ancient settings, the bay line may be indicated by the

landward limit of marine or brackish-tolerant fossils or trace

fossils. Lacustrine deltas have no bayline as such, since there

are no significant tides in lakes.

Many deltas contain distributary channels that show

different sizes and shapes in dilTerent positions on the delta.

There is therefore no such thing as "a distributary channel"" in

many deltas. Typically, a trunk fluvial system will first avulse

at the point where the river becomes unconlined forming a

nodal avulsion point (Mackey and Bridge, 1995). Delta plain

channels tend to be few in number and are separated by wide

areas of interdistributary bays, swamps, marshes, or lakes in

the upper delta plain, although these interdistributary areas

can be replaced with channel deposits depending on the

avulsion frequency (Mackey and Bridge. 1995). Upper delta

plain channels may be exceedingly difficult to distinguish from

fluvial channels, especially in lacustrine deltas. In fact, these

types of distributaries are, strictly speaking, non-marine in

nature and have been described in ihe context oi fluvial

depositional systems (Mackey and Bridge. 1995). Distribu-

taries can show several orders

ot"

branching. The smallest scale

channels are referred to as terminal distributary channels and

are associated with the proximal delta front. Commonly these

are only a few meters deep and will tend to lie on top of

associated distal delta front deposits.

In many ancient examples, thick channelized deposits

overlying marine prodelta shales have been interpreted as

distributary channels (e.g., Rasmussen et a!., 1985) eroding

into their associated delta fronts. Some of these sandstones are

over 30

thick and cut into delta front sandstones that are

only

m or so thiek. Terminal distributaries should actually

be difficult to recognize in subsurface beeause they tend to be

shallow and completely contained within the sandy delta front

facies.

Some of these deeply incised channels are now

interpreted to be fluvially-incised valleys (e.g.. Dalrymple

etal.. 1994: Willis. 1997).

The delta ("ront is the site of much of the active deposition,

particularly at the mouths of distributary channels, where the

eoarsest sediment is deposited in distinct bars. Much of the

deposition occurs in distributary mouth bars (also referred to

as stream mouth or middle-ground bars) separated by shallow

"terminal"" distributary channels (Figures D4 and D6), Mouth

bars can be on the order of several hundred meters wide and

several kilometers long, such as in the Atchafalaya delta in the

Gulf of Mexico (e.g.. Van Heerden and Roberts, 1988), These

mouth bars typically coalesce to form broader depositional

lobes.

Sediments deposited in mouth bars can also be

profoundly reworked by waves, to form shore-parallel beach

and strandplain deposits, or they can be reworked by tides to

form shore-normal tidal bars (Figure D5),

Some distributary channels are fixed in the same position for

long time periods, especially in deep-water mud-dominated

deltas that build into relatively quiet marine basins (low tidal

range, low wave action. Figure D6). The seaward building

forms elongate bar fingers, as in the modern Mississippi

"birdfoot"" delta and in many bayhead tagoonal deltas. By

contrast in siltier or sandier systems, deposited in shallower

water, distributary channels switch more rapidly and coalesce

to form more lobate sediment bodies, as in the Lafourehe and

Atchafalaya deltas in the Gulf of Mexico. Some researchers

have used the term "braid" delta or "braidplain'" delta to refer

fo a sandy or gravelly delta front fed by a braided river and

characterized by a fringe of active mouth bars such as in the

Canterbury Plains of New Zealand or in many glacial outwash

plains (e,g.. McPherson. Shanmugam and Moiola, 1987; see

Reading and Collinson, 1996),

The seaward-dipping slope associated with distributary

mouth bars is referred to as the distal delta front and can

form a relatively continuous sandy fringe in front ofthe active

mouth bar (Figure D4).

The prodelta is the area where fine material settles out of

suspension. It may be burrowed or largely unburrowed.

depending on sedimentation rates. Prodelta muds fend to merge

seaward with fine-grained sediment of the basin floor. The

preservation of silty or sandy lamination is commonly taken to

mark the influence ofthe river, as opposed to total bioturbation

ofthe basin floor sediments in areas away from the active river.

Where the sediments are rhythmically laminated, there may be a

tidal influence. Also, beeause of the abundant suspended

seditnent. certain types of vertical filter feeders and other

organisms that produce open vertical burrows ofthe Skotiihos

iehnofacies tend to be suppressed (e.g.. Gingras etal.. 1998).

Because the delta front and prodelta areas arc characterized

by high levels of suspended sediment and the mixing of fresh

and marine water, they tend to form very stressful environments

for organisms resulting in low biological diversity (see Biotur-

hation and

Trace

Fo.s.sils).

In New Orleans oyster bars, the best

oysters (filter feeders) come f"rom areas away from acfive river

mouths whereas deposit-feeding crawfish favor more muddy

prodelfa and brackish lake environmenfs.

Vertical facies successions

Progradation of a delta lobe will tend to produce a single,

relatively thick eoarsening-upward faeies succession

DELTAS AND ESTUARIES

201

(Figure D7) showing a transition from muddier facies of the

prodelta into the sandier facies of the delta front and mouth

bar environments (see numerous examples in Bhattacharya

and Walker, 1992: and Reading and Collinson, 1996).

Thicknesses may range from a few meters to a hundred meters

depending on the seale of the delta and the water depth.

Continued progradation may result in delta plain f"acies

overlying the delta front sands in a continuous succession.

However, delta front sands may be partially eroded by

progradation of distributary channels or fluvial channels over

its own mouth bar. Commonly, progradational delta lobe

suecessions are truncated by thin transgressive abandonment

facies.

In deltas deposited at the edge ofthe continental shelf, thick

coarsening-upward delta front sueeessions are commonly

completely preserved within thicker deposits of the hanging

wall of growth f\iults

(e.g..

Winker and Fdwards, 1983; see

Bhattacharya and Walker, 1992).

The specific nature of the facies in prograding prodelta and

delta front suecessions will depend on the processes influencing

sediment transport, deposition, and reworking. In addition,

coarsening-upward f\ieies suecessions can be produced by the

progradation of other types of shoreline depositional systems.

F'acies

successions through distributary channels are ero-

sionally based and typically fine upward. Filling commonly

takes place after ehannel switching and lobe abandonment. At

this time, the distributary channel may develop into an

estuary, and the fill may be transgressive. The facies succession

will tend to fine upward, with some preserved fiuvially derived

facies al the base, and a greater proportion of marine facies in

the upper part of the channel

fill.

The extent of marine facies

development will depend on the degree of fluvial dominance.

Numerous examples of these different types are presented in

Bhattacharya and Walker (1992) and Reading and Collinson

(1996).

The overall proportion of distributary channel faeies is a

function of the type of delta. In general, the more wave-

dominated the delta, the greater will be the proportion of lobe

sediment with more limited amounts of interlobe and

distribulary channel facies.

Infcrdisfributary and interlobe areas tend to be less sandy,

and commonly eontain a series of relatively

thin,

stacked

coarsening- and fining-upward facies successions. These are

usually less than ten meters thick, and much more irregular

than the successions found in prograding deltaic lobes (see

examples in Reading and Collinson, 1996). The proportion of

lobe versus interlobe successions will depend on the nature

and type of delta system and will tend to be greater in more

river- or tide-infiuenced systems and less in wave-dominated

deltas, although wave-influenced systems, like the Danube

Delta in the Black Sea, ean contain significant lagoonal and

bay niudstones in regions downdrift of the river mouth

(Bhattacharya and Giosan.

inpre.-i.s).

Lateral facies variability

The lateral facies variability of many depositional systems

requires a detailed understanding of bedding geometry. Bed-

ding geometry and lateral facies variability can be addressed

by the use of seismic data. Ground Penetrating Radar.

continuous outcrop data

(e.g..

Willis ('/ ul.. 1999), and

interpolation of well data. Analysis of lateral facies variability

by tracing facies and bedding is termed Facies .4rchitectural

Analysis, in which discrete sediment bodies are analyzed on

the basis of bounding surfaces and bedding geometry (Miall,

1985). Facies variations associated with bounding surfaees ean

have signilicant impact on fluid fiow in subsurface reservoirs.

Considerable research in the petroleum industry has thus been

dedicated to investigating the importance of bounding surfaces

at a variety of scales. Much of this research emphasis has been

on fluvial systems (see examples in Miall and Tyler. 1991; Flint

and Bryant, 1993) but more recent studies of deltaic systems

are beeoming available

(e.g..

Ainsworth et ai.. 1999; Willis

etal..

1999; Tye (Vc//.. 1999),

Dip-variability

In eross-seclional view, deltas show the distinet topset. foreset

and bottomset geometry, with strata typically organized into

an ofllapping pattern, formed as the delta progrades

(Figures D4 and D7), Strata associated with progradation of

the entire continental shelf-slope and basins also form larger-

scale "elinoforms"'. Delta-front foreset-dips range from a few

degrees up to the angle of repose in coarse-grained Gilbert-

type deltas. Prodelta bottomsets typically dip at less that T'

whereas nonmarine topset facies are typically fiat to undulat-

ing.

The seaward migration of these geomorphic areas of

deposition builds the vertical facies successions as shown in

Figure D7.

Berg (1982, see Bhattacharya and Walker. 1992) discussed

fypical seismic facies in deltaic depositional systems and

suggested that sandy wave-dominated systems tend fo be

eharaefenzed by a more shingled pattern. The muddier river-

dominated delta lypes tend to show more of an oblique-

sigmoidal pattern. The steeply dipping, sigmoid-shaped por-

tions are probably characteristic of the mud-dominated

prodelta faeies whereas the more flat-lying upper reflectors

represent the sandier delta front and delta plain faeies. Frazier

(1974.

see Bhattacharya and Walker, 1992) showed a similar

geometry based on geological studies of the Mississippi delta

plain,

Offiapping clinoform geometries have also been

recognized in the Late Quaternary wave-dominated sediments

ofthe Rhone shelf (Tesson

etal..

1990: see Bhattacharya and

Walker, 1992) and in many other studies

(e.g.,

Hart and Long,

1996).

Strike variability

Along strike, facies relationships may be less predietable and

depositional surfaces may dip In different directions

(F^igure D4). This is particularly so in more river-dominated

deltas where along-strike reworking is not significant and

abrupt facies transitions may occur between distributaries and

interdistributary areas. Overlapping delta lobes result in lens-

shaped stratigraphie units that exhibit a mounded appearanee

on seismic lines (Figure D4). 3D bedding geometries have been

simulated in computer-based models developed by Tetzlaff

and Harbaugh (1989).

Conclusions

A good case can be made for regarding deltas as the single

most important elastic sedimentary environment (Leeder.

1999,

p. 383) but because of their complexity deltas eannot

be classified according to any one simplistic model. Parameters

now considered to be essential in understanding facies

202

[:)ELIAS

AND ESTUARIES

distribution in deltas include: feeder type, river discharge,

sediment caliber, water depfh. basin physiography, storms,

waves and tides, sea level, physical position in the basin, and

degree of soff-sedimenf deformation. Clearly, combination of

these parameters results in a nearly infinite number of

possible delta types reflecting a chaotic, nonlinear, dynamic

sedimentary confinuum.

Many deposifional systems (e.g,, barrier-Islands, deltas,

estuaries) are not mutually exclusive and components of one

ean be found in another. Also, because of changes in

parameters through time, one type of depositional environ-

ment can evolve into another. There are now too many end-

member delta models. Future facies models must take a more

quantitative, dynamic, predictive, parametric approach, such

as used by Tetzlaff and Harbaugh

(\9S9)

in simulafing deltaie

deposition. These approaches will necessarily focus on the

mechanics of delfa formation and resulting facies distribution,

informed by fbeused field studies, rather than merely classify-

ing the type of delta observed in an outcrop or core.

Janok Bhattacharya

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Cross-references

Angle of Repose

Barrier Islands

Biogenic Sedimentary Structures

Coastal Sedimentary Faeies

C"yclic Sedimentation

Deformation Structures and Growth f aults

Gravity-Driven Mass Flows

Lacustrine Sedimentation

Neritic Carbonate Deposilional Lnvironments

Ripple. Ripple Mark. Ripple Structure

Ri\ers and ,Mluvial Fans

Sediment Transport by Tides

Sediment Transport by Waves

Tidal Flats

Tidal Inlets and Deltas

DEPOSITIONAL FABRIC OF MUDSTONES

Figure D8 (Al example of band specimen with alternafing

carbonaceous layers (blackl and silt/clay couplets (lightl. Note

lenticular and

ross-laminated silt layer. (B) thin section of same shale

as in A, Note graded siit/clay couplets (stitrm layersl, and wavy crinkly

laminae in carbonaceous layers Imicrobial mat deposit; Schieber,

19861.

sttidy features in the millimeter ratige. such as small-scale

sedimentary structures, Petrographic thin sectiotis should

initially be examined at low tnagtiificatioti (binocular tiiicro-

scope).

Further study by petrographic microscope at higher

magnification can reveal additional details (e.g., nature of

lamina contacts), as well as providing information on mineral

cotnposition.

For the investigation of the textural ditnension ol'mudstone

fabrics (relations between particles) high magnifications arc

required to see individual grains and their contact relations.

The scanning electron microscope is the too! of choice here

(Trewin. 1988). operating iti either secondary (SFM) or

backscattered (BSE) mode (Figure D9), Observation in SFM

mode is popular because sample preparation is easy (a freshly

broken surface will suffice), but BSE itnaging has gained

popularity because it provides compositional information in

addition to much clearer contact relations. The main drawback

of the BSE mode is the need for a polished surface.

Definition

The fabric ofa rock is the total of all textural and structural

features (Whitten and Brooks. 1972). Texture is defined as the

relationship between the particles that fortn a rock, and

structure has to do with discontinuities and tnajor inhotno-

geneities. Both are intrinsic properties of the rock that serve as

guides to its origins (Pettijohn and Potter, 1964), as well as

providing a basic rationale for the study of mudstone fabrics.

Studying mudstone fabrics

The structural dimension of mudstone fabrics (discontinuities

and major inhomogencities) is best examined in hand speci-

mens (preferably cut and polished surfaces) and in petro-

graphie thin sections (Figut"e D8). For hand specimens, one

starts with visual inspection, followed by closer examination

tinder a binocular microscope. This approach is suitable to

Fabric elements

Frotn a seditnentologisi's perspective, small-scale sedimentary

structures tend to be the most informative fabric elements,

Incotispicuous or even invisible in outcrop (they range iti size

from a few centimeters to less than a millimeter), they can

nonetheless reveal a wealth of information about depositional

conditions and history of a mudstone. When studied in

polished slabs and petrographic thin sections, even seemingly

drab shales can show a large range and variability of

sedimentary features (Figure DIO), If density contrasts are

sufficient, small-scale seditnentary features may also be

enhanced through X-radiography.

Laminae are the mosl typically observed sedimentary

feature in shales, and exhibit a large range in thickness and

lamination styles (even, di.seoniinuous, lenticular, wrinkled.

etc.),

which can represent quiet settling, sculpting of the

sediment surface by bottom currents, and growth of microbial

204

DEPOSITIONAL FABRIC OF MLJDSTONES

mats respectively (Schieber, 1986). Internal lamina

include: (a) grading; (b) random clay orientation; (c) preferred

clay orientation; (d) sharp basal contacts; and (e) sharp

top contacts. They may be interpreted as indicative of:

(a) event-sedimentation (e.g.. floods, storms, turbidites);

(b) llocciikttion or sediment trapping by microbial mats;

Figure D9 lAI SFM piclure ot biolurb.iled mudstone fabrir: (Bl SFM

piclure of Limin^ited mudstone; (C) BSF pitlure ol tarbonatenjus shale.

Minerals with high molecular weight (py = pyrite) appejr brighter than

those of lower molecular weight (qtz =quartz). Gray streaks consist of

clay minerals, dark-black streaks are kerogen. Note horizontal

alignment of sediment grains, clay streaks, and kerogen stringers.

prior to deposition; and (e) eurrent flow and erosion/reworking

after deposition (Schieber. 1990). Silt laminae are usually the

most easily observed lamina type in shales because of their

somewhat larger grain size. They also imply somewhat more

energetic eonditions, and may. for example, indicate deposi-

tion by density currents (grading, fading ripples), storm

reworking and transport (graded rhythmites). wave winnowing

(fine even laminae with seonred bases), and bottom currents

(silt layers with sharp bottom and top). Gradual eompositional

changes between elay and silt dominated laminae are another

commonly observed feature, and are suggestive of continuous

(although slow) deposition, possibly frotn deltaie sediment

plumes and shifting nepheloid flows.

Other small seale sedimentary features that ean be observed

in shales are muderaeks. load easts, flame structures,

bioturbation, graded rhythmites (Reineek and Singh, 1980),

fossil concentrations and lags, cross-lamination, and loop

struetures (Cole and Pieard, 1975). all of whieh earry

inforrnation about conditions of sedimentation. Shales may

also reveal elay-lilled mud cracks, breceiation due to desieea-

tion, and sands or conglomerates that consist entirely of shale

particles (Sehieber etal., 1998). Beeause in the latter features

the flils in eraeks and between shale particles consist of the

same components as the eraeked substrate or the shale

particles, there is little compositional contrast to reveal them

on broken surfaees in outcrop. Polished slabs or petrographie

thin seetions are typieally required to deteet shale-filled eraeks

and shale sands and conglomerates. The latter, for example,

can form as a result of soil erosion (pedogenic particles; Rust

and Nanson, 1989), erosion of eraeked mud crusts, and

submarine seouring of mud substrates by strong currents.

Biologic agents may produee mierobial laminae and

protection of inud surfaees from erosion (Sehieber. 1986;

O'Brien and Slatt, 1990), or may manifest themselves as

bioturbation and destruetion of primary fabrics. In many

instanees, however, suffleient primary features survive biotur-

bation, and observation of bioturbation features can thus

provide additional information about substrate firmness, event

deposits (eseape traees). and rates of deposition. Feeal pellets

and pelleial fabrics are another by-produet of organie activity.

clay

matrix

0.05mm

graded silt/clay couplet

(has moderate fissility)

parallel to x-laminated silt

gray mudstone, moderately

to strongly bioturbated

clay layer (homogeneous)

silt layer at base of massive clay layer (graded top)

Figure D10 Line drawing thai summarizes features observed in a mudstone from the Late Devonian Sonyea Group in New York

(Schieber, 1999bl. M —mica; Q = quartz; py = pyrite.