Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

Подождите немного. Документ загружается.

characteristics of these features provide information

about glacial processes in the past few tens of thou-

sands of years of Earth history and provide a basis

for understanding the origins of the landscapes

around us. However, most continental glacial deposits

(Fig 7.8) are unlikely to be preserved in the strati-

graphic record in the long term. This is because they

mainly occur in areas that are only regions of deposi-

tion as a consequence of the glacial processes: many

of the modern glacial landscapes are undergoing ero-

sion and over time the continental glacial deposits will

be reworked and removed. Glacial deposits recognized

in pre-Quaternary strata are mostly marine in origin.

7.4.1 Moraines

Accumulations of till formed directly at the margins of

a glacier are known as moraine. Several different

types of moraine can be recognised (Benn & Evans

1998). Terminal or end moraines mark the limit of

glacial advance and are typically ridges that lie across

the valley. Push moraines are formed where a glacier

front acts as a bulldozer scraping sediment from the

valley floor and piling it up at the glacier front. Dump

moraines form at the snout of the glacier where the

melting of the ice keeps pace with glacial advance. If a

glacier retreats the melting releases the detritus that

has accumulated at the sides of the glacier where it is

deposited as a lateral moraine (Figs 7.7 & 7.9).

Lateral moraines form ridges along the sides of gla-

ciated valleys, parallel to the valley walls. Where two

glaciers in tributary valleys converge detritus from

the sides of each is trapped in the centre of the amal-

gamated glacier (Fig. 7.10) and the resulting deposit

upon ice retreat is a medial moraine along the cen-

tre of a glaciated valley. When a cold glacier retreats,

the snout of the glacier is often left with a carapace

of detritus left behind as the glacier front has been

Substrate

Lodgement till.

Thickest deposits

occur close to the

ice front

m - 10s m

Outwash braided

stream deposits

Glacio-lacustrine

deposits with varves

Loess

MUD

clay

silt

SAND

GRAVEL

gran

pebb

cobb

boul

Continental glacial facies

Scale

Lithology

Structures etc

Notes

Fig. 7.8 Graphic sedimentary log illustrating some of the

deposits of continental glaciers.

Fig. 7.9 A lateral moraine left by the retreat of a valley

glacier.

108 Glacial Environments

melting. A few metres thickness of rock debris forms

effective insulation and prevents the ice below it from

melting. These ice-cored moraines (ablation mor-

aines) give the impression of being much larger

volumes of detritus than they really are because

most of their bulk is made up of ice.

7.4.2 Other glacial landforms

Lodgement tills deposited beneath a glacier may form

sheets that can be tens of metres thick, or show

irregular ridges known as ribbed moraines. These

tills also form smoothed mounds known as drumlins,

which are oval-shaped hills tens of metres high and

hundreds of metres to kilometres long, with the elon-

gation in the direction of ice flow. In temperate gla-

ciers partial melting of the ice results in rivers flowing

in tunnels within or beneath the ice, carrying with

them any detritus held by the ice that melted. The

deposits of these rivers form sinuous ridges of material

known as eskers (Fig. 7.7) and they are typically a

few metres to tens of metres high, tens to hundreds of

metres wide and stretch for kilometres across the area

formerly covered by an ice sheet (Warren & Ashley

1994). The deposits are bars of gravel and sand that

form cross-bedded and horizontally stratified lenses

within the esker body. They may be distinguished

from river deposits by the absence of associated over-

bank sediments (9.3 ) and by internal deformation

(slump folds and faults) that forms when the sand

and gravel layers collapse as the ice around the tunnel

melts. Kames and kame terraces are mounds or

ridges of sediment formed by the collapse of crevasse

fills, sediment formed in lakes lying on the top of the

glacier or the products of the collapse of the edge of a

glacier.

7.4.3 Outwash plains

As the front of a glacier or ice sheet melts it releases

large volumes of water along with any detritus being

carried by the ice (Fig. 7.10). Rivers flow away from

the ice front over the broad area of the outwash

plain, also known by their Icelandic name sandur

(plural sandar). The rivers transport and deposit in

the same manner as a braided river (9.2.1) (Boot-

hroyd & Ashley 1975; Boothroyd & Nummedal

1978; Maizels 1993; Russell & Knudsen 2002). The

large volumes of water and detritus associated with

the melting of a glacier mean that the outwash plain

is a very active region with river channels depositing

sediment rapidly to form a thick, extensive braid plain

deposit (9.2.1). Outwash plain deposits (Fig 7.8) can

be distinguished from other braided river deposits by

their association with other glacial features such as

moraines.

The most spectacular events associated with glacial

sedimentation are sudden glacial outburst events

known by their Icelandic name as jo¨kulhlaups. The

outburst can be either the result of the failure of a

natural dam holding back a lake at the front of the

glacier or a consequence of melting associated with a

subglacial volcanic eruption. Very large volumes of

meltwater create a dramatic surge of water and sedi-

ment, which may include some very large blocks onto

the outwash plain. Deposits of glacial outbursts are

thick beds of sand and gravel that are massive and

poorly sorted or cross-bedded and stratified (Maizels

1989; Russell & Knudsen 2002). Reworking of this

material by ‘normal’ fluvial processes on the outwash

plain may occur.

The absence of widespread vegetation under the

cold climatic conditions means that fine-grained sedi-

ment on the outwash plain remains exposed and is

subject to aeolian reworking. Sand may be blown into

accumulations within and marginal to the outwash

plain, forming deposits with the characteristics of

wind-blown sediment (8.3). Silt- and clay-sized grains

may be transported long distances and be widely

distributed: accumulations of wind-blown silt of

Fig. 7.10 A dark ridge of material within a valley glacier

that will form a medial moraine when the ice retreats

(viewed from the air).

Continental Glacial Deposition 109

Quaternary age (loess – 8.6.2) are thought to be

sourced from periglacial environments. Clasts exposed

on the outwash plain may be abraded by wind-blown

sediment to form ventifacts (8.2).

7.4.4 Periglacial areas

In polar regions the areas that lie adjacent to ice

masses are referred to as the periglacial zone

(6.6.2). In this area the temperature is below zero

for much of the year and the ground is largely frozen

to create a region of permafrost. Only the soil and

sediment near the surface thaws during the summer,

and to a depth of only a few tens of centimetres, below

which the ground remains perennially frozen. The

thin layer of thawed material is often waterlogged

because the water cannot drain away into the frozen

subsurface. This upper mobile layer can be unstable

on slopes and will slump or flow downslope. Other

features of regions of permafrost are patterned

ground (Fig. 7.11), which is composed of polygons

of gravelly deposits formed by repeated freezing and

thawing of the upper mobile layer, and ice wedges,

which are cracks in the ground formed by ice that

subsequently become filled with sediment.

7.5 MARINE GLACIAL ENVIRONMENTS

Where a continental ice sheet reaches the shoreline the

ice may extend out to sea as an ice shelf (Figs 7.12 &

7.13). Modern ice shelves around the Antarctic con-

tinent extend hundreds of kilometres out to sea form-

ing areas of floating ice which cover several hundred

thousand square kilometres (Drewry 1986). These ice

shelves partially act to buffer the seaward flow of

continental ice: melting of the floating ice of an ice

shelf does not add any volume to the oceans, but if

they are removed then more continental ice will flow

into the sea and this will cause sea level rise. Ice

shelves such as those around the Antarctic contain

relatively small amounts of sediment because there is

little exposed rock to provide supraglacial detritus, so

the main source is basal debris. Ice shelves break up at

the edges to form icebergs and melt at the base in

contact with seawater. Ice in a marine setting also

occurs where temperate or poythermal valley glaciers

flow down to sea level: these tidewater glaciers can

contain large amounts of both supraglacial and basal

debris. Sea ice is frozen seawater and does not contain

any sedimentary material except for wind-blown dust.

7.5.1 Erosional features associated

with marine glaciers

Where continental ice from an ice sheet or valley

glacier reaches the shoreline of a shallow shelf the

ice may be grounded on the sea floor. The movement

of the ice mass and drifting icebergs may locally scour

the sea floor, resulting in grooves in soft sediment that

may be metres deep and hundreds of metres long.

Meltwater flowing subglacially may be under consid-

erable pressure and can erode channels into the sea-

floor sediment beneath the ice, forming tunnel val-

leys that subsequently may be filled with deposits

from the flowing water. The tunnel-valley deposits

and the glacial scours features are preserved within

shallow-marine strata in places such as continental

shelves that have been covered with ice.

7.5.2 Marine glacial deposits

The terms till and tillite are also used to describe

unconsolidated and lithifed marine glacial, glacio-

marine, deposits (Fig. 7.14). The primary character-

istics of the material are the same as the glacial

sediment associated with continental glaciation. The

detritus released from the bottom of an ice shelf forms

till sheets (Fig. 7.12), which may be thick and exten-

sive beneath a long-lived shelf (Miller 1996). These

Fig. 7.11 In periglacial areas, freeze–thaw processes in the

surface of the permafrost form polygonal patterns.

110 Glacial Environments

deposits may be divided into those deposited close to

the ice front (ice-proximal glaciomarine sedi-

ments), which are typically poorly sorted diamictons

with little or no stratification or other sedimentary

structures, and ice-distal glaciomarine sediments,

which are composed mainly of sediment released from

icebergs. The more distal glaciomarine deposits are

subject to reworking by shallow marine processes

(Chapter 11): waves and currents produce a grain-

size sorting of material, sand may be reworked to form

wave and current bedforms and the finer-grained

material may be transported in suspension to be

deposited as laminated mud. Mixing of the glacially

derived material with other sediment, such as bio-

genic material, can also occur.

The edges of ice shelves break up to form icebergs

that can travel many hundreds of kilometres out into

the open sea, driven by wind and ocean currents, but

they often carry relatively little detritus. Icebergs

formed at the front of tidewater glaciers are generally

small, but may be laden with sediment. As an iceberg

melts, this debris will gradually be released and depos-

ited as dropstones in open marine sediments. Drop-

stones can be anything up to boulder size and their

size is in marked contrast to surrounding fine-grained,

pelagic deposits (16.5.1). Although rarely found in

deep marine strata, dropstones are important indica-

tors of the presence of ice shelves and hence provide

evidence of past global climate conditions. However,

floating ice sheet

till sheet

laminated

mud

dropstones

icebergs

continental

margin

continental ice

Fig. 7.12 At continental margins in polar areas, continental ice feeds floating ice sheets that eventually melt releasing detritus

to form a till sheet and calve to form icebergs, which may carry and deposit dropstones.

Fig. 7.13 An ice shelf at the edge of a continental glaciated

area.

Marine Glacial Environments 111

similar deposits can also result from sediment released

from floating vegetation.

7.6 DISTRIBUTION OF GLACIAL

DEPOSITS

Quaternary valley and piedmont glaciers form distinc-

tive moraines but are largely confined to upland areas

that are presently undergoing erosion. In these

upland areas glacial and periglacial deposits such as

moraines, eskers, kames, and so on have a very poor

preservation potential in the long term. Of more inter-

est from the point of view of the stratigraphic record

are the tills formed in lowland continental areas and

in marine environments as these are much more

likely to lie in regions of net accumulation in a sedi-

mentary basin. The volume of material deposited by

ice sheets and ice shelves is also considerably greater

than that associated with upland glaciation.

Extensive ice sheets are today confined to the polar

regions within the Arctic and Antarctic circles. Dur-

ing the glacial episodes of the Quaternary the polar ice

caps extended further into lower latitudes. The sea

level was lower during glacial periods and many

parts of the continental shelves were under ice.

Upland glacial regions were also more extensive,

with ice reaching beyond the immediate vicinity of

the mountain glaciers. The growth of polar ice caps is

known to be related to global changes in climate, with

the ice at its most extensive when the globe was

several degrees cooler. Other glacial episodes are

known from the stratigraphic record to have occurred

in the late Carboniferous and Permian (the Gond-

wana glaciation in the southern hemisphere), in the

early Palaeozoic and in the Proterozoic.

7.7 ICE, CLIMATE AND TECTONICS

7.7.1 Glaciation and global climate

The continental ice caps at and near the poles contain

the vast majority of the ice on the planet. The Ant-

arctic ice cap covers almost all of the continent and

has a fringe of floating ice shelves; much of the ice in

the Arctic is sea ice in the Arctic Ocean, but Green-

land also has a large ice cap. Compared with these

polar regions, the ice in the mountain glaciers is of

little global significance, although individual conti-

nental ice masses are important parts of local envi-

ronments.

Evidence from the distribution of glacial sediments

and sedimentary rocks indicates that there have been

a number of periods during Earth history when the

polar ice caps covered much larger areas than at

present. The best documented glacial periods are

from the Quaternary, a time of fluctuating global

temperatures that has experienced advances and

retreats of the polar ice caps a number of times over

the past few hundred thousand years. The causes of

the global changes in climate that lead to the ice caps

growing and shrinking are complex and are consid-

ered further in Chapter 23. When the polar ice melts

the water released adds to the volume of water in the

oceans, and the sea level rises worldwide: it is esti-

mated that complete melting of the Antarctic ice sheet

would result in a global sea level rise of over 50 m,

while the Greenland ice cap would add 7 m to world

sea levels (Hambrey & Glasser 2003). The effects of

10s metres

Laminated muds

with ice-rafted sands

and gravels as beds

and isolated

dropstones

MUD

clay

silt

SAND

GRAVEL

gran

pebb

cobb

boul

Marine glacial facies

Scale

Lithology

Structures etc

Notes

Fig. 7.14 Glaciomarine deposits are typically laminated

mudrocks with sparse coarser debris derived from icebergs.

112 Glacial Environments

sea level changes on sedimentation are covered in

Chapter 23.

7.7.2 Glacial rebound – isostasy

During periods of glaciation the ice layer on the con-

tinents may be hundreds to thousands of metres thick.

This mass of ice creates an extra load on the crust that

forces the base of the crust down into the mantle.

When the ice melts and the ice is removed, there is

an isostatic uplift of the crust (6.7). The rate of melt-

ing is typically much faster than the isostatic uplift

and consequently the crust continues to go up for

thousands of years after the ice has melted. This effect

is seen in many areas, such as Scandinavia, which

were covered during the last ice age and are still

undergoing uplift of a few millimetres a year. The

effects of this so called glacial rebound are most

clearly seen around coasts, where raised beaches

provide evidence of the position of the land relative

to the sea thousands of years ago, prior to uplift of

the land.

7.8 SUMMARY OF GLACIAL

ENVIRONMENTS

Glacial deposits are compositionally immature and

tills are typically composed of detritus that simply

represents broken up and powdered bedrock from

beneath the glacier. Reworked glacial deposits on out-

wash plains may show a slightly higher compositional

and textural maturity. There is a paucity of clay

minerals in the fine-grained fraction because of the

absence of chemical weathering processes in cold

regions. Continental glacial deposits have a relatively

low preservation potential in the stratigraphic record,

but erosion by ice in mountainous areas is an impor-

tant process in supplying detritus to other deposi-

tional environments. Glaciomarine deposits are more

commonly preserved, including dropstones which

may provide a record of periods of glaciation in the

past. The volume of continental ice in polar areas is

closely linked to global sea level, so the history of past

glaciations is an important key to understanding var-

iations in the global climate.

Characteristics of glacial deposits

. lithologies – conglomerate, sandstone and mud-

stone

. mineralogy – variable, compositionally immature

. texture – extremely poorly sorted in till to poorly

sorted in fluvio-glacial facies

. bed geometry – bedding absent to indistinct in

many continental deposits, glaciomarine deposits

may be laminated

. sedimentary structures – usually none in tills, cross-

bedding in fluvio-glacial facies

. palaeocurrents – orientation of clasts can indicate

ice flow direction

. fossils – normally absent in continental deposits,

may be present in glaciomarine facies

. colour – variable, but deposits are not usually

oxidised

. facies associations – may be associated with fluvial

facies or with shallow-marine deposits

Further Reading 113

Aeolian Environments

Aeolian sedimentary processes are those involving transport and deposition of material

by the wind. The whole of the surface of the globe is affected by the wind to varying

degrees, but aeolian deposits are only dominant in a relatively restricted range of

settings. The most obvious aeolian environments are the large sandy deserts in hot,

dry areas of continents, but there are significant accumulations of wind-borne material

associated with sandy beaches and periglacial sand flats. Almost all depositional envi-

ronments include a component of material that has been blown in as airborne dust,

including the deep marine environments, and thick accumulations of wind-blown dust

are known from Quaternary strata. Aeolian sands deposited in desert environments have

distinctive characteristics that range from the microscopic grain morphology to the scale

of cross-stratification. Recognition of these features provides important palaeoenviron-

mental information that can be used in subsurface exploration because aeolian sand-

stones are good hydrocarbon reservoirs and aquifers.

8.1 AEOLIAN TRANSPORT

The term aeolian (or eolian in North American

usage) is used to describe the processes of transport

of fine sediment up to sand size by the wind, and

aeolian environments are those in which the depos-

its are made up mainly of wind-blown material.

8.1.1 Global wind patterns

The wind is a movement of air from one part of the

Earth’s surface to another and is driven by differences

in air pressure between two places. Air masses move

from areas of high pressure towards areas of low pres-

sure, and the speed at which the air moves will be

determined by the pressure difference. The circulation

of air in the atmosphere is ultimately driven by tem-

perature differences. The main contrast in temperature

is between the Equator, which receives the most

energy from the Sun, and the poles, which receive

the least. Heat is transferred between these regions by

air movement (as well as oceanic circulation). Hot air

at the Equator rises, while cold air at the poles sinks, so

the overall pattern is for a circulation cell to be set up

with the warm air from the Equator travelling at high

altitudes towards the poles and a complementary

movement of cold air back to the Equator closer to

ground level. This simple pattern is, however, compli-

cated by two other factors. First, the circulation pattern

breaks up into smaller cells, three in each hemisphere.

Second, the Coriolis force (6.3) deflects the pathway of

the air mass from simple north–south directions. The

result is the pattern of winds shown in Fig. 8.1,

although these patterns are modified and influenced

by local topographic effects. Air masses blowing over

mountain ranges are forced upwards and are cooled,

and similarly the air is chilled when winds blow over

ice caps: this results in katabatic winds, which are

strong, cold air masses moving down mountain slopes

or off the edges of ice masses.

8.1.2 Aeolian transport processes

A flow of air over a loose grain of sand exerts a lift force

on the particle (4.2.3) and with increasing velocity the

force may increase to the point where the grain rolls

or saltates (4.2.2). The strength of the lift force is

proportional to both the velocity of the flow and the

density of the medium. Air has a density of 1.3 kg

3

, which is three orders of magnitude less than

that of water (1000 kg m

3

) so, whereas water flows

of only a few tens of centimetres a second can cause

movement of sand grains, much higher velocities are

required for the wind to move the same grains. Winds

of 55 m s

1

or more are recorded during hurricanes,

but strong winds over land areas are typically around

30 m s

1

, and at these velocities the upper limit on

the size of quartz grains moved by the wind is around

a half a millimetre in diameter, that is, medium sand

size storms (Pye 1987; Nickling 1994). This provides

an important criterion for the recognition of aeolian

deposits in the stratigraphic record: deposits consist-

ing of grains coarser than coarse sand are unlikely to

be aeolian deposits.

At high wind velocities silt- and clay-sized particles

are carried as suspended load. This aeolian dust can

become entrained in the wind in large quantities

in dry areas to create dust storms that can carry

airborne sediment large distances away from its ori-

gin. The dust will remain in suspension until the wind

speed drops and the fine sediment starts to fall to

the ground or onto a water surface. Significant

Fig. 8.1 The distribution of high- and

low-pressure belts at different latitudes

creates wind patterns that are deflected by the

Coriolis force.

Subtropical high

Intertropical convergence zone

Subpolar low

Polar high

Prevailing

westerlies

Prevailing

westerlies

Tropical

easterlies

Tropical

easterlies

Polar

easterlies

Polar

easterlies

Aeolian Transport 115

accumulations of aeolian dust are relatively uncom-

mon (8.6.2), but airborne material can be literally

carried around the world by winds and be deposited

in all depositional environments.

8.2 DESERTS AND ERGS

A desert is a continental area that receives little

precipitation: they are arid areas that receive less

than 250 mm yr

1

precipitation. (Areas that receive

average precipitation of between 250 and 500 mm

1

are defined as semi-arid and are not usually

considered to be true deserts.) This definition of a

desert does consider temperature to be a factor, for,

although the ‘classic’ deserts of the world today, such

as the Sahara, are hot as well as dry places, there are

also many dry areas that are also cold, including

‘polar deserts’ of high latitudes. The shortage of

water limits the quantity and diversity of life in a

desert: only a relatively limited range of plants and

animals have adapted to live under these dry condi-

tions and large parts of a desert surface are devoid of

vegetation. The lack of vegetation is an important

influence on surface processes because without a

plant cover detritus lies loose on the surface where it

is subject to aeolian activity.

An erg is an area where sand has accumulated as

a result of aeolian processes (Brookfield 1992): these

regions are also sometimes inappropriately referred to

as a ‘sand sea’. Ergs are prominent features of some

deserts, but in fact most deserts are not sandy but are

large barren areas known as rocky deserts. Rocky

deserts are areas of deflation, that is, removal of

material, and as such are not depositional environ-

ments. However, pebbles, cobbles and boulders that

lie on the surface may subsequently be preserved if

they are covered by other sediment, and these clasts

may show evidence of their history in a rocky desert.

Rocks in a desert are subject to a sand-blasting effect

as sand and dust particles are blown against the sur-

face by the wind: this erosive effect on the faces pro-

duces a characteristic clast shape, which is called a

zweikanter if two faces are polished smooth, or drei-

kanter if there are three polished faces, with angled

edges between each face. Long exposure of a rock

surface in the oxidising conditions of a desert also

results in the development of a dark, surface patina

of iron and manganese oxides known as a desert

varnish (Fig. 8.2).

8.3 CHARACTERISTICS OF

WIND-BLOWN PARTICLES

8.3.1 Texture of aeolian particles

When two grains collide in the air they do so with

greater impact than they would experience under

water because air, being a much lower density med-

ium than water, does not cushion the impact to the

same extent. The collisions are hence relatively high

energy and one or both of the grains may be damaged

in the process. The most vulnerable parts of a grain

are angular edges, which will tend to get chipped

off, and with multiple impacts the grains gradually

become more rounded as more of the edges are

smoothed off. Sand grains that have undergone a

sustained period of aeolian transport therefore

Fig. 8.2 Pebbles in a stony desert with a shiny desert

varnish of iron and manganese oxides.

116 Aeolian Environments

become well-rounded, or even very well-rounded.

Grain roundness is therefore a characteristic that

can easily be seen in hand specimen using a hand

lens, or will be evident under the microscope if a

thin-section is cut of an aeolian sandstone. Inspection

using a hand lens reveals another feature, which is

more obvious if the grains are examined by scanning

electron microscopy (SEM) (2.4.4): the grain surfaces

will have a dull, matt appearance that under high

magnification is a frosting of the rounded surface.

This is a further consequence of the impacts suffered

during transport and grain surface frosting is also a

characteristic of aeolian processes. Aeolian dust

shows similar grain characteristics but features on

these sizes of grains can be recognised only if viewed

under the high magnifications of an SEM.

A wind blowing at a relatively steady velocity can

transport grains only up to a particular size threshold

(Nickling 1994), and large, heavier grains are left

behind. Grains close to the threshold for transport

are carried as bedload and deposited as ripples and

dunes (4.3.1 & 4.3.2), whereas finer grains remain in

suspension and are carried away. This effective and

selective separation of grains during transport means

that aeolian deposits are typically well-sorted (2.5).

Sands in dunes are normally fine to medium grained,

with no coarser grains present and most of the finer

grains winnowed away by the wind. This winnowing

effect, the selective removal of finer grains from the

sediment in a flow, also occurs in water flows, but is

more effective in the lower density and viscosity med-

ium of air.

A clastic deposit that consists of only sand-sized

material, which is well sorted and with well-rounded

grains, is considered to be texturally mature (2.5.3).

Aeolian sandstones are, in fact, one of very few

instances where granulometric analysis (2.5.1) pro-

vides useful information about the depositional envi-

ronment of the deposit. There is, however, a need for

caution when using petrographic characteristics

alone as an indicator of environment of deposition.

Consider an area of bedrock made up of sandstone

deposited in a desert tens or hundreds of millions of

years ago. After deposition it was buried and lithified,

then uplifted and eroded. The sand that is being

weathered off this bedrock will have the characteris-

tics of the deposits of an aeolian environment, but is

presently being transported and deposited by streams

and rivers in a very different climatic and depositional

setting. In these circumstances the sands have fea-

tures that have been inherited from the earlier stage,

or cycle, of deposition (2.5.4).

8.3.2 Composition of aeolian deposits

The abrasive effect of grain impacts during aeolian

transport also has an effect on the grain types found

in wind-blown deposits. When a relatively hard

mineral, such as quartz, collides with a less robust

mineral, for example mica, the latter will tend to

suffer more damage. Abrasion during transport by

wind therefore selectively breaks down the more labile

grains, that is, the ones more susceptible to change. A

mixture of different grain types becomes reduced to a

grain assemblage that consists of very resistant

minerals such as quartz and similarly robust lithic

fragments such as chert. Other common minerals,

for example feldspar, are likely to be less common in

aeolian sandstones, and weak grains such as mica are

very rare. A deposit with this grain assemblage is

considered to be compositionally mature (2.5.3), and

this is a common characteristic of aeolian sandstone.

Most modern and ancient wind-deposited sands are

quartz arenites.

In places where loose carbonate material is exposed

on beaches, the sand-sized and finer sediment can be

transported and redeposited by the wind. If the wind

direction is onshore, wind-blown carbonate sands can

accumulate and build up dune bedforms. Dunes built

up of carbonate detritus have many of the same char-

acteristics as a quartz-sand dune, and are several

metres high with slip faces dipping at around 308

creating large-scale cross-bedding. The clasts may be

ooids, bioclasts or pellets, depending upon what is

available on the beach, and are well-rounded and

well-sorted; if the clasts are bioclastic they will com-

monly have a relatively low density, so wind-blown

grains may be very coarse sand or granule size. Wind-

blown carbonates may accumulate in temperate as

well as tropical settings: they are most commonly

found near to coasts, but may also occur tens of kilo-

metres inland. Loose carbonate grains on land are

subject to wetting by the rain and subsequent drying

in the s un; this leads to loc al dissolution and re-

precipitation of carbonate, which results in rapid

formation of cements and lithification of the sedi-

ment. Aeolian carbonate deposits are therefore more

stable features than dunes made of quartz sand.

Lithified wind-blown carbonate deposits are termed

Characteristics of Wind-blown Particles 117