Table of Contents

1) Transport and Depositional Process in Deserts: ............................................................................. 2

1.1) Wind as a transporting agent in deserts: .................................................................................... 2

1.1.1) Suspension: ..................................................................................................................... 2

1.1.2) Saltation: ......................................................................................................................... 2

1.1.3) Traction: .......................................................................................................................... 2

1.2) Wind Deposits: ........................................................................................................................ 3

1.2.1) Loess ..................................................................................................................................... 3

1.2.2) Sand Deposits ....................................................................................................................... 3

1.2.3) Lag Deposits .......................................................................................................................... 3

1.1) Bedforms generated by winds: ............................................................................................... 4

Classification of Eolian Environments: .................................................................................................... 4

1.2) Dune Environment: ...................................................................................................................... 4

2.1.1) Composition of Dunes: ......................................................................................................... 6

2.1.2) Characteristics of Dunes: ...................................................................................................... 6

2.2) Interdune: .................................................................................................................................... 6

2.3) Sand Sheets: ................................................................................................................................. 7

2) Kinds of Eolian Systems:.................................................................................................................. 8

3.1) Dry System: .................................................................................................................................. 8

3.2) Wet System .................................................................................................................................. 8

3.3) Stabilized System ......................................................................................................................... 8

3) Bounding Surfaces in Eolian Deposits: ............................................................................................ 8

4) Structures formed By Eolian Processes: ......................................................................................... 9

5.1) Erosional Structures: .................................................................................................................... 9

5.1.1) Deflation ............................................................................................................................... 9

5.1.2) Abrasion .............................................................................................................................. 10

5.2) Depositional Structures: ............................................................................................................ 11

5.2.1) Cross-Stratification ............................................................................................................. 11

5.2.2) Wave Ripples: ..................................................................................................................... 11

6) Characteristics of Eolian Deposits:.................................................................................................... 12

References: ........................................................................................................................................... 13

1 | E o l i a n S y s t e m s

Eolian Systems

Deserts forms where rate of evaporation greatly exceeds precipitation. These areas generally lie within latitude belts of 30o North and 30o South from the equator. Deserts also form in rain shadow zones of large mountains belts and also in Polar Regions (e-g the entire continent of Antarctica is a desert, as are most of Greenland and the northernmost parts of Alaska, Canada, and Siberia.). Generally, areas with annual rain fall less than 25cm are termed as deserts. (Figure A. Figure B.)

Fig. (A) Global air circulation, red arrows show surface winds. Blue arrows show vertical circulation of

air. Air sinks at 30o North and 30o south and at poles.

Figure B. Deserts formed in Rain shadow zones

of larger mountain belts

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Usually it is thought that large areas covered by sand are deserts, but actually a number of sub

environments exists within deserts (e-g Alluvial Fan, Playa Lakes, ephermal lakes, dune, interdune

etc.). The deserts covering larger areas, usually greater than 125 km2 are termed as sand seas or

ergs and those cover lesser areas are known as dune fields.

1) Transport and Depositional Process in Deserts:

Water usually plays fewer roles in sediment transportation in deserts due to occasional rain falls in

these areas. In the days of extensive rain fall in deserts , flash floods creates extensive erosion due to

sparse of lack of vegetation and forms ephermal lakes which drains the desert to low lying areas

where it forms playa lakes which results in the precipitation of carbonates and evaporates. Beside

these, most of the time in a year, deserts faces wind action for erosion , transportation, and

deposition of sediments.

1.1) Wind as a transporting agent in deserts:

Wind is an active transporting agent for finer sediments like clays and silts. But particles larger than

2mm in diameter are less actively transported by wind action. Not only in deserts but wind also plays

an important role in sediment transportation in fluvial, glacial, and marine environments.

Wind transports sediments in three ways i-e

(i) Suspension

(ii) Saltation

(iii) Traction

1.1.1) Suspension:

Small particles may be held in the atmosphere in suspension. Upward currents of air support the

weight of suspended particles and hold them indefinitely in the surrounding air. Typical winds near

Earth's surface suspend particles less than 0.2 millimeters in diameter and scatter them aloft

as dust or haze. (Figure 1.1)

1.1.2) Saltation:

Saltation is downwind movement of particles in a series of jumps or skips. Saltation normally lifts

sand-size particles no more than one centimeter above the ground, and precedes at one-half to one-

third the speed of the wind. A saltating grain may hit other grains that jump up to continue the

saltation. The grain may also hit larger grains that are too heavy to hop, but that slowly creep

forward as they are pushed by saltating grains. (Figure 1.1)

1.1.3) Traction:

Relatively larger grains tend to creep along the ground both by wind itself and due to interaction of

saltating grains with the larger diameter grains. Surface creep accounts for the 25% of sediment

transport in deserts. (Figure 1.1)

3 | E o l i a n S y s t e m s

1.2) Wind Deposits:

The transporting and sorting action of winds tends to produce three types of deposits.

1.2.1) Loess

Deposits of windblown dust, known as loess, which usually accumulates away from the source.

Loess is a deposit of wind-blown silt and clay composed of unweathered, angular grains of

quartz, feldspar, and other minerals weakly cemented by calcite. Loess has a high porosity,

typically near 60%. Deposits of loess may blanket hills and valleys downwind of a source of fine

sediment, such as a desert or a region of glacial outwash. China has extensive loess deposits,

more than 100 meters thick in places. Wind from the Gobi Desert carried the silt and clay that

formed these deposits. Loess is easy to dig into and has the peculiar ability to stand as a vertical

cliff without slumping, perhaps because of its cement or perhaps because the fine, angular,

sediment grains interlock with one another.

1.2.2) Sand Deposits

Sand deposits consists of sand sized particles (>2mm), usually quartz grains , which are transported

by wind and are well sorted.

1.2.3) Lag Deposits

Lag Deposits, consisting of gravel size particles that are too large to be transported by wind and

hence formed deflation pavement.

Figure 1.1 shows suspension, saltation, traction, and solution

transportation in deserts.

Figure 1.2 Desert pavement

4 | E o l i a n S y s t e m s

1.1) Bedforms generated by winds:

Wind sorting and transport generates following bedforms.

Ripples

Dunes

Draas

The bedforms range in size from as small as ripples i-e 0.01mm long and few millimeters in height to

dunes i-e 500-600m long and 100m in height. Some gigantic dunes have wavelength measured in

kilometers and up to 400m high, they are termed as draas.

The height and length of ripples, dunes and draas in the function of wind velocity and sediment grain

size. Large grain size results in relatively high bedforms and high wind velocities create long

bedforms.

Classification of Eolian Environments:

The Eolian environments are divided in to three main sun environments enlisted here.

(i) Dune Environment

(ii) Interdune Environment

(iii) Sand Sheet Environment

1.2) Dune Environment:

Many types of dunes formed in deserts. A simple dune form is shown (Figure 2.1)

Figure 2.1 shows a cross-section of dune. Dune migration and formation

of Cross-Stratification is clearly displayed.

Figure 1.3 Shows relationship between bedforms and Grain size of sediments

5 | E o l i a n S y s t e m s

Dunes with various number of slip faces are found in nature e-g bachan dunes, longitudinal dunes,

star dunes etc.

Morphology of sand dunes is largely dependent on sand supplies, variability in wind direction , and

wind intensity. These factors results in various forms of dunes as illustrated (Figure 2.2)

Figure 2.2. Each of the major type of dunes represents a unique balance

between sand supplies, direction and intensity of winds. Arrows shows the

wind direction.

6 | E o l i a n S y s t e m s

2.1.1) Composition of Dunes:

Dunes usually contains well sorted, well rounded quartz grains, but it may also contain carbonates,

gypsum, heavy metals, unstable rock fragments, skeleton fragments , oolites in coastal areas.

2.1.2) Characteristics of Dunes:

Cross-bedding is usually considered to be the characteristic large scale feature of sand dunes. But

they possess some internal structures as well which reflects their dune nature such as plane-bed

laminae, ripple form laminae, grain flow laminae etc which developed in vertical succession during

migration of dunes. (Figure 2.3)

Paleocurrent analysis of dunes declares them polymodal.

2.2) Interdune:

Interdune areas are formed surrounded by sand dunes or other sand deposits such as sand sheets.

Interdune are either deflationary (erosional) or depositional. Interdune deposits are characterized by

low angle of stratification (<10o ). They are deposited by the process other than dune migration, their

structure may be lost due to bioturbation. (Figure 2.4)

Figure 2.3 illustrates different types

of sedimentary structures in Eolian

Dune deposits.

7 | E o l i a n S y s t e m s

Fewer types of sediment accumulate in deflationary interdune and results in lag deposits of gravel

sized particles which show rippled surfaces and inverse grading.

Depositional Interdunes are characterized whether they are deposited in wet or dry interdune or in

evaporite interdune.

Dry Interdunes are occasionally wetted and results in deposition of poorly sorted, coarse

grained sediments. These sediments are deposited due to first accumulation of sediments at

top of dunes and the grain fall towards the wind shadow zone or leeward site.

Wet Interdunes are the standing bodies of water or lakes where silt and clay sized particles

settles down which can’t be deflated. These environments may contain some freshwater

species of organisms such as pelecypods, gastropods, ostracods, or diatoms etc. Sediments

in such environments are generally bioturbated and may contain foot prints of some

vertebrates.

Evaporite Interdunes occurs where drying up of ephermal lakes results in deposition of

carbonates, gypsum, or anhydrites. These are generally characterized by presence of

desiccation cracks, rain drop imprints , evaporite layers , or pseudomorphs etc.

2.3) Sand Sheets:

Sand sheets are flat, gently undulating plots of sand surfaced by grains that may be too large

for saltation. They form approximately 40 % of eolian depositional surfaces. Sand sheets exist where

grain size is too large, or wind velocities too low, for dunes to form. These sheets may contain cross-

stratification and interbeds of ephermal stream deposits with common occurrence of bioturbation

(any kind). (Figure 2.5)

Figure 2.4 Shows Interdune Environment.

Figure 2.5 represents whole Eolian Environment

including sand sheets.

8 | E o l i a n S y s t e m s

2) Kinds of Eolian Systems:

Not all the sand, silt or clay sized particles that accumulates in deserts become the part of geologic

record. Before classifying Eolian systems in to various categories one should be familiar with these

two terms i-e Accumulation space & Preservation Space.

(i) Accumulation Space:

The vertical space in which sediments accumulates is known as accumulation space.

(ii) Preservation Space:

The space where sediments are preserved which lies below the baseline of erosion is known as

preservation space.

Eolian Systems are classified as:

3.1) Dry System

3.2) Wet System

3.3) Stabilized System

3.1) Dry System:

Dry systems are those in which water table and its capillary fringes lie below the depositional

surfaces. This shows water table has no influence on the stabilizing surface or near surface

sediments. The shape or aerodynamic configuration of deposit depicts how it is formed. The

preservation of sediments in any eolian system is the factor of both depth of water table and

baseline of erosion. The baseline is mainly affected by subsidence which may be the result of

tectonic activity, compaction, and loading. The subsidence of baseline in dry systems close to water

table causes water to rise in sediments to preserve them.

3.2) Wet System:

Wet systems are those in which water table and its capillary fringes lie at or near the depositional

surfaces. In these systems deposition is influenced by moisture content in subsurface. Sediments in

such systems are less eroded when first accumulated than in dry systems. In wet systems , the

accumulation space is also the preservation space due to at or near surface water table which allows

preservation without any need of subsidence of baseline of erosion.

3.3) Stabilized System:

In stabilized systems, factors such as vegetation cover, surface cementation , and mud drapes play a

significant role in stabilizing sediments. In stabilized systems, some preservation may occur above

the regional baseline of erosion. Examples of all these environments are found in Sahara Desert.

3) Bounding Surfaces in Eolian Deposits:

Bounding surfaces are surfaces separating distinct sets of strata. Bounding surfaces are known to be created by the following mechanisms

An interdune migration surface (or first-order bounding surface) is the interdune surface (left behind by stoss-side erosion of a passing dune, and possibly further eroded as an interdune

9 | E o l i a n S y s t e m s

surface). Moist sand is more resistant to erosion than dry sand, so sometimes the water table controls the extent of interdune erosion. Interdune migration surfaces tend to be near horizontal.(Figure 4.1)

A superimposition surface (or second-order bounding surface) is created by the migration of one dune (usually a secondary smaller dune) over another dune. This can happen without erosion of the underlying dune. (Figure 4.1)

A reactivation surface (or third-order bounding surface) is created when a deposition process is temporarily interrupted by a change of wind direction causing erosion. The erosion surface becomes a reactivation surface if later deposition resumes in the original direction, laying down more deposits in the original orientation. (Figure 4.1)

4) Structures formed By Eolian Processes:

There are two mainly two kind of structures formed in Rocks.

5.1) Erosional Structures

5.2) Depositional Structures

5.1) Erosional Structures:

Wind erosion acts in two ways i-e Deflation and Abrasion.

5.1.1) Deflation

The most important type of wind erosion is deflation. In deflation loose particles (sand and silt sized)

are lifted and blown away. This type of erosion creates certain features:

Deflation Basins commonly occurs in semiarid regions where the protective cover of grass and shrubs has been removed by the activity of humans and animals. The results are broad shallow depressions called deflation basins. Deflation basins also commonly develop where calcium carbonate cement, in sandstone formations, is dissolved by groundwater, leaving loose sand grains that are picked up and transported by the wind.

Figure 4.1 shows Bounding Surfaces in Eolian Deposits

10 | E o l i a n S y s t e m s

Desert Pavement In general, wind can move only sand and dust-sized particles, so deflation leaves concentrations of coarser material known as lag deposits, or desert pavements. These striking desert features of erosion stand out in contrast to deposits in dune fields and playa lakes. Deflation occurs only where unconsolidated material is exposed at the surface. It does not occur where there are thick covers of vegetation or layers of gravel. The process is therefore limited to areas such as deserts, beaches, and barren fields. (See Figure 1.2)

5.1.2) Abrasion

Abrasion by sand results in somewhat similar to as sandblasting techniques. The desert areas where

weakly consolidated rocks are exposed are affected by wind abrasion. Wind carries sediments which

Strike with the exposed rock surface and knock off grains which are loosely gathered in rock. This

knocking off results in:

Polishing of Rocks occurs when wind carrying sediments rubbed with rock for long time.

(Figure 5.2)

Grooves Marks are linear depressions created in rocks due to wind action (Figure 5.3)

Yardangs are distinct linear ridges formed by wind erosion. (Figure 5.4)

Figure 5.1 shows small deflation basins

in sandstone formation in the Colorado

Plateau.

Figure 5.3 Grooves Marks

developed on bedrock near

palm spring, California

Figure 5.2 Shows Polished

surface on rock due to

grains impact

11 | E o l i a n S y s t e m s

5.2) Depositional Structures:

Those structures which formed during deposition of Eolian deposits are termed as depositional

structures which includes:

5.2.1) Cross-Stratification:

Cross-Stratification results when depositional layers of sediments are inclined to each other.

Different types of cross bedding develops in Sand Deposits , which are:

5.2.1.1) Planar Cross Stratification:

This structure formed through the migration of straight crested ripples, producing planar cross

stratification.(Figure 5.5a)

5.2.1.2) Trough Cross Stratification:

Trough cross-beds have lower surfaces which are curved or scoop shaped and truncate the

underlying beds. The foreset beds are also curved and merge tangentially with the lower surface.

They are associated with sand dune migration. (Figure 5.5b)

5.2.2) Wave Ripples:

Wind action produces undulating surfaces in sand deposits known as wavy ripples. The shape of

ripples is dependent on the direction of wind. Ripples such as current ripples, climbing ripples etc.

5.2.2.1) Climbing Ripples:

Where there is rapid deposition, ripples builds up as well as move forward. This results in Climbing

ripples as shown in Figure 5.6a.

Figure 5.4 shows Yardang in Desert

Figure 5.5a shows Planar cross

stratification Figure 5.5b shows Trough cross

stratification

12 | E o l i a n S y s t e m s

:

5.2.2.1) Current Ripples:

Asymmetrical ripples formed due to one way flow of currents (wind direction) are known as current

ripples. (Figure 5.6b)

6) Characteristics of Eolian 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.

color – 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

Figure 5.6a shows Climbing Ripples

Figure 5.6b shows Current Ripples

13 | E o l i a n S y s t e m s

References:

i. Principles of Sedimentology and Stratigraphy, Sam Boggs, Jr.

ii. Sedimentology and Stratigraphy, Gary Nichols.

iii. Physical Geology Earth Revealed , Diane H. Carlson, Charles C. Plummer, Lisa Hammersely

iv. Dynamic Earth , Eric H. Christiansen, W.K. Hamblin