Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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aeolianites, and these may be locally important com-

ponents of coastal deposition (McKee & Ward 1983).

8.4 AEOLIAN BEDFORMS

The processes of transport and deposition by wind

produce bedforms that are in some ways similar to

subaqueous bedforms (4.3), but with some important

differences that can be used to help distinguish aeo-

lian from subaqueous sands. Three groups can be

separated on the basis of their size: aeolian ripples,

dunes and draas. Each appears to be a distinct class of

bedform with no transitional forms and a plot of the

range of sizes for each (Fig. 8.3) shows that they fall

into three distinct fields (Wilson 1972).

8.4.1 Aeolian ripple bedforms

As wind blows across a bed of sand, grains will move

by saltation forming a thin carpet of moving sand

grains. The grains are only in temporary suspension,

and as each grain lands, it has sufficient energy to

knock impacted grains up into the free stream of air,

continuing the process of saltation. Irregularities in

the surface of the sand and the turbulence of the air

flow will create patches where the grains are slightly

more piled up. Grains in these piles will be more

susceptible to being picked up by the flow and at a

constant wind velocity all medium sand grains will

move about the same distance each time they saltate.

The result is a series of piles of grains aligned perpen-

dicular to the wind and spaced equal distances apart.

These are the crests of aeolian ripples (Figs 8.4 &

8.5). The troughs in between are shadow zones where

grains will not be picked up by the air flow and where

few saltating grains land.

Aeolian ripples have extremely variable wave-

lengths (crest to crest distance) ranging from a few

centimetres to several metres. Ripple heights (bottom

of the trough to the top of a crest) range from less than

a centimetre to more than ten centimetres. Coarser

grains tend to be concentrated at the crests, where

the finer grains are winnowed away by the wind, and

as aeolian ripples migrate they may form a layer of

inversely graded sand. Where a crest becomes well

developed grains may avalanche down into the adja-

cent trough forming cross-lamination, but this is less

common in aeolian ripples than in their subaqueous

counterparts.

8.4.2 Aeolian dune bedforms

Aeolian dunes are bedforms that range from 3 m to

600 m in wavelength and are between 10 cm and

100 m high. They migrate by the saltation of sand

up the stoss (upwind) side of the dune to the crest

(Fig. 8.6). This saltation may result in the formation

of aeolian ripples which are commonly seen on the

stoss sides of dunes (Fig. 8.7). Sand accumulating at

the crest of the dune is unstable and will cascade

down the lee slope as an avalanche or grain flow

(Fig. 8.8) (4.5.3) to form an inclined layer of sand

(Fig. 8.6). Repeated avalanches build up a set of cross-

beds that may be preserved if there is a net accumula-

tion of sand. At high wind speeds some sand grains

are in temporary suspension and are blown directly

over the crest of the dune and fall out onto the

lee slope. These grain fall deposits accumulate on

Bedform wavelength

2.0

0.5

1.0

1cm 4cm 16cm 40m

Grain size (mm)

1.4

0.3

0.7

10m2.5m64cm 160m 640m 2.5km

0.2

dune

field

h =10cm

to 10m

draa

field

ripple

field

h = mm to cm

Aeolian bedform sizes

100m

Fig. 8.3 Aeolian ripples, dunes

and draas are three distinct types of

aeolian bedform.

118 Aeolian Environments

the lee slope, but they will usually be reworked

from the upper slope by avalanching: some may

be preserved at the toe bedded with grain flow

deposits.

The orientation and form (planar or trough) of the

cross-bedding will depend on the type of dune

(Figs 8.9 & 8.10) (McKee 1979; Wasson & Hyde

1983). Planar cross-beds will form by the migration

of transverse dunes, straight-crested forms aligned

perpendicular to the prevailing wind direction. Trans-

verse dunes form where there is an abundant supply

of sand and as the sand supply decreases there is a

transition to barchan dunes, which are lunate struc-

tures with arcuate slip faces forming trough cross-

bedding. Under circumstances where there are two

prominent wind directions at approximately 908 to

each other, linear or seif dunes form. The deposits

of these linear dunes are characterised by cross-bedding

reflecting avalanching down both sides of the dune and

hence oriented in different directions. In areas of multi-

ple wind directions star dunes have slip faces in many

orientations and hence the cross-bedding directions

display a similar variability.

There are circumstances in which the whole dune

bedform is preserved but more commonly the upper

part of the dune is removed as more aeolian sand is

deposited in an accumulating succession. The size of

the set of cross-beds formed by the migration of aeolian

dunes can vary from around a metre to ten or twenty

metres (Fig. 8.11). Such large scale cross-bedding is

common in aeolian deposits but is seen less frequently

in subaqueous sands, which are typically cross-bedded

in sets a few tens of centimetres to metres thick.

8.4.3 Draa bedforms

When an erg is viewed from high altitudes in aerial

photographs or satellite images, it is possible to see a

finer grains sheltered

in ripple troughs

winnowing of finer grains

leaves coarser grains

on ripple crests

wind

sand grains saltate

inversely graded laminae result

from accumulation on ripples

mm-cm

Fig. 8.4 Aeolian ripples form by sand grains saltating: finer grains are winnowed from the crests creating a slight inverse

grading between the trough and the crest of the ripple that may be preserved in laminae.

Fig. 8.5 Aeolian ripples in modern desert sands:

the pen is 18 cm long.

Fig. 8.6 Aeolian dunes migrate as sand

blown up the stoss (upwind) side is either

blown off the crest to fall as grainfall on

the lee side or moves by grain flow down

the lee slope.

wind

sand grains migrate up

windward side of the dune

by rolling, saltation and

ripple migration

grains blown off crest of

dune fall on to lee slope

accumulation of grains

in upper part of slope

causes oversteepening

and results in grain flow

Aeolian Bedforms 119

pattern of structures that are an order of magnitude

larger than dunes. The surface of the erg shows an

undulation on a scale of hundreds of metres to kilo-

metres in wavelength and tens to hundreds of metres

in amplitude. These structures are known as draas

and there is again evidence that they are a distinct,

larger bedform separate from the dunes that may be

superimposed on them (Wilson 1972). Draas are

usually made up of dunes on the stoss and lee sides,

but a single slip face may develop on some lee slopes.

They show a similar variability of shape to dunes with

star, linear and transverse forms.

8.4.4 Palaeowind directions

The slip faces of aeolian dunes generally face down-

wind so by measuring the direction of dip of cross-

beds formed by the migration of aeolian dunes it is

possible to determine the direction of the prevailing

wind at the time of deposition (Fig. 8.9). Results can

be presented as a rose diagram (5.3.3). The variability

of the readings obtained from cross-beds will depend

upon the type of dune (McKee 1979). Transverse

dunes generate cross-beds with little variability in

orientation, whereas the curved faces of barchan

dunes produce cross-beds that may vary by as much

as 458 from the actual downwind direction. Multiple

directions of cross-bedding result from the numerous

slip faces of a star dune. In all cases the confidence

with which the palaeowind direction can be inferred

from cross-bedding orientations is improved with the

more readings that are taken.

Wind directions are normally expressed in terms of

the direction the wind blows from, that is, a south-

westerly wind is one that is blowing from the south-

west towards the northeast and will generate dune

cross-bedding which dips towards the northeast. Note

that this form of expression of direction is different from

that of water currents that are normally presented in

terms of the direction the flow is towards.

8.5 DESERT ENVIRONMENTS

Aeolian sands form one part of an arid zone facies

association that also includes ephemeral lake deposits

and alluvial fan and/or ephemeral river sediments

(Figs 8.12 & 8.13). In these dry areas, sediment is

brought into the basin by rivers that bring weathered

detritus from the surrounding catchment areas and

deposit poorly sorted mixtures of sediment on alluvial

Fig. 8.7 Aeolian ripples superimposed on an aeolian dune.

Fig. 8.8 Grain flow on the lee slope of an aeolian dune.

120 Aeolian Environments

fans (9.5) or associated with the channels of ephem-

eral rivers (9.2.3). The sandy component of these

deposits is subsequently reworked by the wind and

redeposited in other parts of the basin in aeolian dune

complexes. Water from these rivers and fans ponds in

the basin to form ephemeral lakes and these tempo-

rary lakes dry up to leave deposits of mud and evapo-

rite minerals (10.4). Through time the positions of the

ephemeral lakes, sand dunes and the alluvial fans will

change, and the deposits of these three subenviron-

ments will be preserved as intercalated beds in the

succession of strata (5.6.3).

Fig. 8.9 Four of the main aeolian dune types, their forms determined by the direction of the prevailing wind(s) and the

availability of sand. The small ‘rose diagrams’ indicate the likely distribution of palaeowind indicators if the dunes resulted in

cross-bedded sandstone.

linear

star

barchan

transverse

variable

uniform

abundant

wind direction

sparse

sand

supply

Fig. 8.10 Sand supply and the variability of prevailing wind

directions control the types of dunes formed.

Desert Environments 121

The dominant factor in determining the distribu-

tion and extent of sandy deserts is climate. Arid con-

ditions are necessary to inhibit the development of

plants and a soil that would stabilise loose sediment,

and an absence of abundant surface water prevents

sediment from being reworked and removed by fluvial

processes. Sand accumulates to form an erg where

there are local or regional depressions: these may

range from small build-ups of sand adjacent to topo-

graphy to broad areas of the continent covering many

thousands of square kilometres.

8.5.1 Water table

The land surface in sandy deserts is mainly dry, but if

the substrate is porous sediment or rock there will be

groundwater below the surface. The level below the

surface of this groundwater, the water table, is deter-

mined by the amount of water that is charging the

water-bearing strata, the aquifer, and the relative

level of the nearest lake or sea (Fig. 8.14). Charge to

the aquifer is from areas around the desert that

receive rainfall, and direct precipitation on the desert

itself. The level of a lake in these settings will be

largely determined by the climate and, if the erg

borders the ocean, the sea level will be controlled by

a number of local and global factors (23.8). A rise in

the water table will affect aeolian processes in the erg

if it comes up to the level of the interdune areas: wet

interdune sediment will not be picked up by the wind,

so it becomes stable and not available for aeolian

reworking (Fig. 8.14). A rise in water table therefore

tends to promote the accumulation of sediment

within the erg. Conversely, a fall in water table from

Fig. 8.11 Aeolian dune cross-bedding in sands deposited in

a desert: the view is approximately 5 m high.

ephemeral lake

alluvial fans

dry alluvial plain

sand dunes

Fig. 8.12 Depositional environments in arid regions: coarse material is deposited on alluvial fans, sand accumulates to form

aeolian dunes and occasional rainfall feeds ephemeral lakes where mud and evaporite minerals are deposited.

122 Aeolian Environments

the level of the interdunes will make more sediment

available to be transported by the wind and this

material may be removed from the area of the erg,

that is, there will be net erosion. The relationship

between sea level and sediment accumulation in

other environments is considered in Chapter 23.

8.5.2 Global climate variations

The formation of ergs requires an appropriate config-

uration of topography and wind patterns within a

suitable climate belt. Modern sandy deserts are in the

warm subtropical regions, which have predominantly

dry, offshore wind patterns: most, in fact, lie to the

western sides of continents in belts of westerly winds

that have lost all of their moisture while crossing the

eastern side of the continent (Fig. 8.15). Although

similar conditions are likely to have existed at many

times and places through Earth history, the number

and extent of sandy desert areas are likely to have

varied as plate movements rearranged the continents.

Global climate is also known to have changed

through time. There have been periods of ‘green-

house’ conditions when the temperature worldwide

was warmer, and ‘icehouse’ periods when the whole

world was cooler. During ice ages large ice sheets

formed on one or both of the polar regions. The

increased areas of ice created larger areas of high

pressure, and there would have been steep pressure

gradients between the expanded polar regions and the

lower pressure equatorial belt. These conditions

resulted in belts of strong winds in the subtropical

regions, and hence increased potential for aeolian

transport and deposition (Fig. 8.16). The large ergs

of some modern deserts may be relics from the Pleis-

tocene when they were very active, but have since

become largely immobile. It is also notable that there

are extensive aeolian deposits in the Permian of

northern Europe, a time of Gondwana glaciation in

the southern hemisphere.

8.5.3 Colour in desert sediments

The sands in modern deserts such as the Sahara are

generally yellow. This colour is due to the presence of

iron minerals, which occur as very fine coatings to

the sand grains, particularly the iron hydroxide

goethite (3.5.1) (Fe(OH)

), which is a dull yellow

mineral. Oxidation of goethite forms the common

iron oxide mineral haematite, Fe

, and this very

common mineral has a strong red colour when it is

very finely disseminated as a coating on sand grains.

Ephemeral lake

deposits. Thinly

bedded couplets of

mudstone and

evaporites

10s metres

Alluvial fan

deposits.

Matrix-supported

conglomerates

deposited by debris

flows

Aeolian sand

dunes. Well-sorted

cross-bedded

sands

MUD

clay

silt

SAND

GRAVEL

gran

pebb

cobb

boul

Arid zone

Scale

Lithology

Structures etc

Notes

Fig. 8.13 Graphic sedimentary log of the arid-zone envi-

ronments shown in Fig. 8.12.

Desert Environments 123

wind

fall in water

table

rise in water

table

sediment in

interdune areas

stabilised

sediment in

interdune areas

eroded

dune migration

net accumulation of sediment

net erosion of sediment

Fig. 8.14 The preservation of aeolian dune deposits is influenced by the level of the groundwater table: if the water table is high the interdune areas are wet and the

sand is not reworked by the wind.

124 Aeolian Environments

Some modern desert sands are strongly oxidized, with

haematite coatings on the grains giving them a vivid

orange-red colour. Green, grey and black sediments

are unlikely to be found in desert environments

because these colours are due to reduced iron oxide

(FeO, which is green) and the presence of dark organic

material preserved in the sediment: preservation of

these materials is unlikely in an environment that is

mainly dry and exposed to the air, and therefore

strongly oxidising.

Sedimentary successions composed of strongly red-

dened sandstone and mudstone are sometimes referred

to as red beds (Turner 1980). It is tempting to use the

presence of haematite as evidence that the sediments

were deposited in an oxidising continental environ-

ment. However, some caution is needed when using

the colour of the beds as an indicator of depositional

environment. First, not all desert deposits are red in

the first place, and second, the colour may be the

result of oxidation after deposition (a diagenetic pro-

cess – 18.2.4). There are also cases of sediments that

are deposited in other environments which are also

shades of red: for example, mud deposited in deep

oceans (16.5) may contain aeolian dust that includes

haematite, giving the sediment a reddish colour.

8.5.4 Life in deserts and fossils

in aeolian deposits

The absence of regular supplies of water in deserts

makes them harsh environments for most plants and

animals. A few specialised plants are able to survive

long periods of drought and these form the bottom of

the food chain for insects, reptiles, birds and mam-

mals, but none occur in large quantities. The inter-

dune areas are the most favourable places to support

life because these can be places where water tempo-

rarily ponds and are the closest points to any ground-

water if the water table is relatively close to the

surface. In terms of fossil preservation, the paucity of

organisms in deserts is compounded by the fact that

Fig. 8.15 The global distribution of modern deserts: most lie within 408 of the Equator.

Desert Environments 125

they are also strongly oxidising environments, so

plants and animals are likely to completely decompose

and leave no fossil material. Only the most resistant

animal remains, such as the bones of large animals

such as dinosaurs, have much potential to be pre-

served in aeolian environments. Trace fossils (11.7)

are also rare because few animals live on active sand

dunes, but there is the possibility of walking traces

being preserved in fine sediment in wet interdune

areas. Strata deposited in desert environments are

therefore likely to be barren of any fossils.

8.6 AEOLIAN DEPOSITS OUTSIDE

DESERTS

8.6.1 Aeolian dust deposits

There are deposits of Quaternary age in eastern Eur-

ope, North America and China that are interpreted as

accumulations of wind-blown dust (Pye 1987). These

deposits, known as loess, locally occur in beds several

metres thick made up predominantly of well-sorted

silt-sized material, with little clay or sand-sized mate-

rial present. The origin of loess is related to episodes of

retreat of ice sheets, as large amounts of loose detritus

carried in the ice were released. In the cold periglacial

environment in front of the receding ice colonisation

by plants and stabilisation of the soil would have been

slow, so the glacial debris was exposed on the out-

wash plains, where wind picked up and transported

the silt-sized dust. This dust was probably transported

over large parts of the globe but accumulated as loess

deposits in some places. Similar processes probably

occurred during other glacial episodes in Earth his-

tory, but pre-Quaternary loess deposits have not been

recognised. The preservation potential of loess is likely

to be quite low because it is soft, loose material that is

easily reworked and mixed with other sediment.

Volcanism is an important source of dust in the

atmosphere. Explosive eruptions can send plumes of

volcanic ash high up into the atmosphere where it is

distributed by wind. Coarser ash tends to be deposited

close to the volcano (although in very large eruptions

this can be hundreds of kilometres away – 17.6.2),

while the silt-sized ash particles can be transported

around the world. Large amounts of atmospheric dust

from eruptions can darken the sky, and it will gradu-

ally fall as fine sediment. A further source of atmo-

spheric dust is from fires that propel soot (fine carbon)

up into the air, where it can be redistributed by the

wind. Despite the fine grain size, soot, volcanic and

Subtropical high

Intertropical convergence zone

Subpolar low

Glacial high

pressure

Glacial high pressure

Intertropical convergence zone - low pressure

(a) Glacial periods -

strong winds due

to pressure

gradients

(b) Interglacial periods -

weak wind systems

Fig. 8.16 During glacial periods the regions of polar high pressure are larger, creating stronger pressure gradients and hence

stronger winds. In the absence of large high pressure areas at the poles in interglacial periods the pressure gradients are

weaker and winds are consequently less strong.

126 Aeolian Environments

terrigenous dust can all be distinguished by geochem-

ical analysis.

Aeolian dust is dispersed worldwide, but most of it

ends up in other marine and continental depositional

environments where it mixes with other sediment and

its origin cannot easily be determined. In most places

the proportion of aeolian dust is very low compared

with other sediment being deposited, but there are

some environments where terrigenous clastic deposi-

tion is very low, and the main source of silt and clay

can be aeolian dust. Limestones formed in carbonate-

forming environments can usually be shown to con-

tain a residue of dust if the calcium carbonate is dis-

solved, and dust settled on ice sheets and glaciers may

be seen as layers within the ice. The parts of the deep

oceans that are distant from any continental margin

receive very little sediment (16.5): airborne dust that

settles through the water column can therefore be an

important component of deep ocean deposits.

8.6.2 Aeolian sands in other environments

Beach dunes

Sand dunes built up by aeolian action can form adja-

cent to beaches in any climatic setting. In the inter-

tidal zone of a foreshore loose sediment is subaerially

exposed at low tide, and as it dries out it is available to

be picked up and redeposited by the wind. Beach dune

ridges form where the foreshore sediments are mainly

sandy, exposed at low tide and subject to removal by

onshore winds. The sand then accumulates at the

head of the beach, either as a simple narrow ridge or

sometimes extending for hundreds of metres inland.

In humid climates the dunes become colonised by

grasses, shrubs and trees that stabilise the sand and

allow the ridges to build up metres to tens of metres

thickness. The roots of these plants and burrowing

animals disrupt any depositional stratification, so the

cross-bedding characteristic of desert dunes may not

be preserved in beach dune ridges. The association of

beach dune ridges with other coastal facies is dis-

cussed in 13.2.1.

Periglacial deposits

Glacial outwash areas (7.4.3) are places where loose

detritus that has been released from melting ice

remains exposed on the surface for long periods of

time because plant growth and soil formation is slow

in periglacial regions. Wind blowing over the out-

wash plain can pick up sand and redeposit it locally,

usually against topographic features such as the side

of a valley. These patches of aeolian sand may there-

fore occur intercalated with fluvio-glacial facies

(7.4.3), but rarely form large deposits.

8.7 SUMMARY

Aeolian deposits occur mainly in arid environments

where surface water is intermittent and there is little

plant cover. Sands deposited in these desert areas are

characteristically both compositionally and mineralo-

gically mature with large-scale cross-bedding formed

by the migration of dune bedforms. Oxidising condi-

tions in deserts preclude the preservation of much fossil

material, and sediments are typically red–yellow col-

ours. Associated facies in arid regions are mud and

evaporites deposited in ephemeral lakes and poorly

sorted fluvial and alluvial fan deposits. Aeolian depos-

its are less common outside of desert environments,

occurring as local sandy facies associated with beaches

and glaciers, and as dust distributed over large dis-

tances into many different environments, but, apart

from Quaternary loess, rarely in significant quantities.

Characteristics of aeolian deposits

. lithologies – sand and silt only

. mineralogy – mainly quartz, with rare examples of

carbonate or other grains

. texture – well- to very well-sorted silt to medium sand

. fossils – rare in desert dune deposits, occasional

vertebrate bones

. bed geometry – sheets or lenses of sand

. sedimentary structures – large-scale dune cross-

bedding and parallel stratification in sands

. palaeocurrents – dune orientations reconstructed

from cross-bedding indicate wind direction

. colour – yellow to red due to iron hydroxides and

oxides

. facies associations – occur with alluvial fans,

ephemeral river and lake facies in deserts, also with

beach deposits or glacial outwash facies