Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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lake or sea, where they are reworked by waves or tidal

currents. Documented modern examples are all from

basins where the tidal range is small and wave action

is the main mechanism for distribution of clasts in

shallow water. The energy associated with waves is

strongly depth-dependent (4.4) and so there is a sort-

ing of the sediment into different grain sizes according

to water depth. The largest clasts remain in the shal-

lowest water where the wave action is strongest,

while smaller clasts are carried by waves further off-

shore into slightly deeper water. Across a gently slop-

ing shelf there will be a progressive fining of the clast

size as the water depth increases and hence the

energy of the waves decreases (Fig. 12.7).

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 (Fig. 12.7).

Coarse-grained deltas that display these characteris-

tics have been classified as ‘shelf-type fan deltas’by

Wescott & Ethridge (1990).

12.4.3 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 pro-

grades 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 (Fig. 12.8), 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 (Collinson et al. 1991).

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 (Fig. 12.9). Sandy

and gravelly deposits are therefore likely to be rela-

tively well sorted.

In deeper water , a greater proportion of the sedi-

ment will be depos ited in the lower part of the delta

slope as a thicker coarsening-up succession is gen-

erated during delta progradation (Fig. 12.10). The

area of the delta top will be relatively small, with less

potential for the development of widespread fine-

grained delta-plain facies and mires. Wave -reworked

mouth-bar facies will be limite d in extent because of

the small area of shallow water where wave action

is effective. The delta slope will be extensive and a

potential site for g ravity flows: coarser deposits may

Fig. 12.11 A modern Gilbert-type coarse-

grained delta.

188 Deltas

becom e remobilised to form debris flows and finer

sediment mixed with more water will generate tur-

bidit y currents. The lower part of the delta front

may therefore be a site of deposition of these mass-

flow deposits, which may extend out on to th e

basin plain.

12.4.4 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 (Fig. 12.11). They even have a special

name ‘Gilbert-type deltas ’, named after the Amer-

ican geologist G.K. Gilbert who first described deposits

of this type in 1895. Gilbert-type deltas have a char-

acteristic three-part structure (Figs 12.12 & 12.13).

The topset (the delta top) is a subaerial to shallow-

marine 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

granular 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 (ava-

lanche) process (4.5.3). Slumping (18.1.1) is often

seen on the delta because the steep slopes of the

foresets can become unstable. At the base of the fore-

set slope sediments are finer, comprising mud, sand

and some gravel, which lie approximately horizon-

tally 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

topset: gravelly braided river and beach deposits

foreset: mass flow deposits

Gilbert-type delta

bottomset: turbidites

braided river

topset

foreset

bottomset

Fig. 12.12 Gilbert-type deltas are coarse-grained deltas that prograde into deep water. They display a distinctive pattern of

steeply-dipping foreset beds sandwiched between horizontal topset and bottomset strata.

Variations in Delta Morphology and Facies 189

the form of a sandwich of steeply-dipping conglom-

eratic strata between layers of horizontal beds of

conglomerate and sandstone (Fig. 12.14). The

height of the foreset is determined by the depth of

water the delta is bu ilding into, and ranges from a

few tens of metres to over 500 ms (Fig. 12.14). These

thick packages of steeply dipping strata are unique to

Gilbert-type deltas: the only deposits that even

approach thi s an gle of deposition are on alluvial

fans, and these are at a lower angle than the 308

recorded in Gilbert-type deltas. They are typically

found at the edges of basins that have active faulted

margins such as rift basins (24.2.1), where uplift of

the land at the margin creates steep topography to

supply the gravel and the basin is subsiding to form a

deep, steep-sided basin.

12.4.5 Process controls:

river-dominated deltas

A delta is regarded as river-dominated where the

effects of tides and waves are minor. This requires

a microtidal regime (11.2.2) and a setting where

wave energy is effectively dissipated before the

waves reach the coastline. Under these conditions,

the form of the delta is largely controlled by fluvial

processes of transport and sedimentation. The unidi-

rectional fluvial current at the mouth of the river

continues into the sea or lake as a subaqueous flow.

The channel form is maintained, with well-defined

subaqueous levees and overbank areas (Fig. 12.15).

Bedload and suspended load carried by the river is

deposited on the subaqueous levees, building up to

sea level and extending the front of the delta basin-

wards as thin strips of land either side of the main

channel to form the characteristic ‘bird’s foot’ pattern

of a river-dominated delta (Bhattacharya & Walker

1992). A common feature of fluvially dominated del-

tas is channel instability due to the very low gradient

on the delta plain, resulting in frequent avulsion of

the major and minor channels. The course of the river

changes as one route to the sea becomes abandoned

and a new channel is formed, leaving the former

channel, its levees and overbank deposits abandoned.

Repeated switching of the channels on the delta top

builds up a pattern of overlapping abandoned lobes

(Fig. 12.16).

The deposits of river-dominated deltas have well-

developed delta-top facies, consisting of channel and

Fig. 12.13 A schematic sedimentary log of a Gilbert-type

coarse-grained delta deposit.

190 Deltas

overbank sediments. The characteristics of these

facies will be essentially the same as those of a

similar fluvial system. The overbank areas of a

delta top may be sites of prolific growth of vegeta-

tion, leading to the formation of peat and eventually

coal. The channels build out to form the ‘to es’ of the

‘bird’s foot’, between which there are large inte rdis-

tributary bays. These bays are relatively sheltered

and are sites of fine-grained, subaqueous sedimenta-

tion. Crevasse splays from the distributary channels

supply sediment into these bays and they gradually

fill to sea level to becom e the vegetated part of the

delta plain. The filling of interdistributary bays

results in small scale (a few metre s thick) coarsen-

ing-up successions (Fig. 12.17). In front of the

channels, mouth bars form and are loc alised to

areas in front of the individual delta lobes. Little

redistribution of mouth-bar s ediments by wav e or

tidal processes occurs, so individual mouth-bar

bodies are relatively small.

Fig. 12.14 A Gilbert-type coarse-grained

delta exposed in a cliff over 500 m high. The

exposure is made up mostly of foreset

deposits dipping at around 308: horizontal

topset strata form the top of the cliff and the

toes of the foreset beds pass into gently

dipping bottomset facies.

delta plain

river

sandy mouth bars

prodelta deposits

distributary

channel and levees

interdistributary bay

sea

Fig. 12.15 A river-dominated delta with the distributary channels building out as extensive lobes due to the absence of

reworking by wave and tide processes. Low-energy, interdistributary bays are a characteristic of river-dominated deltas.

Variations in Delta Morphology and Facies 191

12.4.6 Process controls:

wave-dominated deltas

Waves driven by strong winds have the capacity to

rework and redistribute any sediment deposited in

shallow water, especially under storm conditions.

The river mouth and mouth-bar areas of a delta are

susceptible to the action of waves, resulting in a mod-

ification of the patterns seen in river-dominated

deltas (Bhattacharya & Giosan 2003) (Fig. 12.18).

Progradation of the channel outwards is limited

because the subaqueous levees do not form and bed-

load is acted upon by waves as quickly as it is depos-

ited. Any obliquity between the wind direction and

the delta front causes a lateral migration of sediment

as the waves wash material along the coast to form

beach spits and mouth bars that build up as elongate

bodies parallel to the coastline. Wave action is effec-

tive at sorting the bedload into different grain sizes

and the mouth-bar deposits of a wave-influenced delta

may be expected to be better sorted than those of a

river-dominated delta.

Progradation of a wave-dominated delta occurs

because the wave action does not transport all the

material away from the region of the river mouth.

A net supply of bedload by the river results in a series

of shore-parallel sand ridges forming as mouth bars

build up and out to form a new beach (Fig. 12.19).

Wave-dominated delta deposits display well-developed

mouth bar and beach sediments, occurring as

elongate coarse sediment bodies approximately

perpendicular to the orientation of the delta river

channel. This is in contrast to the river-dominated

delta deposits, which would be expected to show

less continuous mouth bars, and a higher proportion

of channel and overbank deposits forming the

delta lobes. Delta-front and prodelta deposits may not

significantly differ between these two delta types

(Figs 12.17 & 12.20).

12.4.7 Process controls:

tide-dominated deltas

Coastlines with high tidal ranges experience onshore

and offshore tidal currents that move both bedload

and suspended load. A delta building out into a region

with strong tides will be modified into a pattern that is

different to both river- and wave-dominated deltas

(Fig. 12.21). First, the delta-top channel(s) are subject

to tidal influence with reverses of flow and/or periods

of stagnation as a flood tide balances the fluvial dis-

charge. This may be seen in strata as reversals of

palaeocurrent indicated by cross-stratification, and

sea

delta plain

delta channel

switches

course

delta lobes

switch position

new lobe (F)

on top of old

lobe (A)

river

Fig. 12.16 When a delta channel avulses a new lobe starts to build out at the new location of the channel mouth. The

abandoned lobe subsides by dewatering until completely submerged. Through time the channel will eventually switch back to a

position overlapping the former delta lobe. This results in a series of delta-lobe successions, each coarsening-up.

192 Deltas

the formation of mud drapes (11.2.4). Overbank areas

on the delta top may be partially tidal flats, and all of

the delta top will be susceptible to flooding during

periods of high fluvial discharge coupled with high

tides. The tidal currents rework sediments at the river

mouth into elongate bars that are perpendicular to the

shoreline. These are modified mouth bars, which may

show bidirectional cross-stratification and mud drapes

on the cross-bed foresets due to the reversing nature of

the ebb and flood tidal currents (Willis et al. 1999)

(11.2.4).

The deposits of a tidally influenced delta can be

distinguished from other deltas by the presence of

sedimentary structures and facies associations which

indicate that tidal processes were active (reversals of

palaeoflow, mud drapes, and so on), and subaqueous

mouth bars will be elongate parallel to the river

channels. The overall succession of strata will

display the characteristic coarsening-up of a delta

(Fig. 12.22), a feature that allows it to be distin-

guished from other tidally influenced environments

such as estuaries, which have much in common in

terms of depositional processes. The main distinguish-

ing feature is that a delta is always a progradational

feature, whereas an estuary commonly forms as

part of a retrogradational, or transgressive, succession

(23.1.6).

12.5 DELTAIC CYCLES AND

STRATIGRAPHY

When the channel on the delta top changes course,

the former lobe is abandoned as a new site of

deposition is occupied. River-dominated deltas tend

to have the most frequent changes in position of the

active lobe, but avulsion of channel course also

occurs in other delta types. The deposits of an aban-

doned lobe will gradually compact as water depos-

ited with the fine-grained sediment escapes from the

pore spaces and the bulk density increases. This

compaction occurs without any add itional load,

and results in the abandoned lobe subsiding below

sea level. The fall below sea level of the abandoned

lobe will be accelerated if the delta is located in a

region of overall subsidence or if there is a eustatic

rise in sea level.

The beds that mark the end of sedimentation on a

delta lobe are known as the abandonment facies

(Reading & Collinson 1996). In the upper part of the

Fig. 12.17 A schematic graphic sedimentary log of river-

dominated delta deposits.

Deltaic Cycles and Stratigraphy 193

delta plain these will be peats or palaeosols, which

represent a low clastic supply to this part of the plain

now that active lobe progradation has moved else-

where on the delta. The fringes of the delta lobe will

be areas of slow, fine-grained deposition in shallow

water, while further offshore, carbonate facies may

form over the toe of the delta. Abandonment facies

may show intense bioturbation because of the slow

sedimentation rate.

After a number of changes in channel position the

active delta lobe may reoccupy an earlier position and

prograde over an older, compacted and submerged

delta-lobe succession. In cross-section the result is

one coarsening-up delta-lobe succession built up on

top of another. Repetition of this pattern has been

recognised in the stratigraphic record and are referred

to as delta cycles, each ‘cycle’ representing the pro-

gradation of an individual delta lobe. The thickness of

sea

delta plain

river

wave fronts

sandy mouth bars

coastal sands

prodelta deposits

Fig. 12.18 A wave-dominated delta formed where wave activity reworks the sediment brought to the delta front to form

coastal sand bars and extensive mouth-bar deposits.

Fig. 12.19 Sand bars at the mouth of a wave-

dominated delta.

194 Deltas

a delta ‘cycle’ will be controlled by the depth of water

in the receiving basin (see above) and may range from

a few metres to tens or hundreds of metres in thick-

ness (Elliott 1986).

Variants on the idealised delta cycle are frequently

encountered (Fig. 12.23). A complete succession from

offshore fine-grained deposits up to the delta channel

fill will be seen only at the point where the axis of the

lobe has built out basinward. In other positions, the

top of the cycle will vary from delta-plain carbona-

ceous mudstones, to interdistributary bay deposits or

mouth-bar sands. In a hinterland direction, subsi-

dence will not be great enough for fully marine con-

ditions to develop at the base of each delta cycle, and

only the upper parts of the typical succession may be

seen (Elliott 1986).

In addition to the trends that represent the progra-

dation of delta lobes, smaller scale grain-size patterns

are also present. The filling of an interdistributary

bay results in a coarsening-up succession, but this

will normally be on a scale that is an order of magni-

tude smaller than the main delta cycle. Small-scale

fining-up trends are formed by the filling of distribu-

tary channels when they are abandoned.

12.6 SYNDEPOSITIONAL

DEFORMATIONINDELTAS

The delta front is a slope that can vary from about 18

in mud-rich settings to over 308 in coarse-grained

deltas. Even the very low angle slopes are potentially

unstable and mass movement of loose, soft sediment

on the delta slope is common. Debris flows, slumps

and slides (6.5.1) that consist of remobilised delta-

front deposits reworked and remobilised occur and

may be seen as part of the succession in deltaic facies.

The slumps and slides can be large-scale, involving

the movement of bodies of sediment tens of metres

thick and hundreds of metres across. The surfaces on

which the slides move are like faults, and these

features are often regarded as growth faults,

synsedimentary deformation structures (18.1.1)

(Bhattacharya & Davies 2001). Further instabilities

also arise as a result of the relatively rapid accumula-

tion of sediment on a delta: coarser, and relatively

denser sediment of the delta top is built up on top of

muddy, wet and less dense delta-front facies and the

result is the formation of mud diapirs (Hiscott 2003)

(18.1.4).

Fig. 12.20 A schematic graphic sedimentary log of wave-

dominated delta deposits.

Syndepositional Deformation in Deltas 195

These deformation processes occurring during the

formation of the deltaic deposit give rise to some

quite complicated sedimentary features within a

delta succession. Beds may be deformed on a scale

ranging from slump folds a few centimetres across to

synsedimentary faults that rotate and displace

packages of strata tens of metres thick (18.1.1).

Similar features can occur in other depositional envi-

ronments, but they are probably most common in

deltaic facies, especially if the succession contains a

high proportion of muddy sediments that deform rela-

tively easily.

12.7 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 coarsening-

upwards succession from lower energy, finer grained

deposits of the prodelta to the higher energy condi-

tions of the delta mouth bar where coarser sediment

accumulates.

The delta top contains both relatively coarse sedi-

ment of the distributary channel as well as finer

grained material in overbank areas and interdistribu-

tary 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 cur-

rents. The delta top will show signs of subaerial con-

ditions, 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.

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 ge-

ometry and extent of the mouth-bar sand bodies will

be determined by the relative importance of river, tidal

and wave processes. 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. How-

ever, 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 pro-

cesses: the term ‘non-alluvial delta’ may be applied to

these deposits (Nemec 1990a). Limestone beds are

intertidal zone

coastal and delta plain

river

sea

tidal sand bars

tidal currents

Fig. 12.21 A tide-dominated delta in a macrotidal regime will show extensive reworking of the delta front by tidal currents

and the delta top will have a region of intertidal deposition.

196 Deltas

only rarely associated with deltas, occurring as accu-

mulations on delta fronts where the supply of clastic

detritus is low: examples in modern settings and from

the stratigraphic record indicate that carbonates form

on deltas in arid and semi-arid environments, where

the supply of clastic sediment to the delta is highly

ephemeral (Bosence 2005).

Palaeontological evidence from fauna and flora

can be important in the recognition of the marine

and continental subenvironments of a delta. A dis-

tinct fauna tolerant of brackish water may be found

near the mouths of channels and in the interdistri-

butary 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.

A final point to emphasise is that the various

models of different types of delta presented in this

chapter are just a few examples of the possible com-

binations of the controls that determine the form and

facies of a delta. Any modern or ancient delta should

be considered in terms of the evidence for the effects

of different factors – sediment grain size, basin water

depth, the relative importance of river, tide and wave

influences – and should not be expected to exactly

match any of the models presented here or any other

text books.

Characteristics of deltaic deposits

. lithologies – conglomerate, sandstone and mudstone

. mineralogy – variable, delta-front facies may be

compositionally mature

. texture – moderately mature in delta-top sands and

gravels, mature in wave-reworked delta-front deposits

. bed geometry – lens-shaped delta channels, mouth-

bar lenses variably elongate, prodelta deposits thin

bedded

. sedimentary structures – cross-bedding and lamina-

tion in delta-top and mouth-bar facies

. palaeocurrents – topset facies indicate direction of

progradation, wave and tidal reworking variable on

delta front

. fossils – association of terrestrial plants and animals

of the delta top with marine fauna of the delta front

. colour – not diagnostic, delta-top deposits may be

oxidised

Fig. 12.22 A schematic graphic sedimentary log of tide-

dominated delta deposits.

Recognition of Deltaic Deposits 197