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Table of Contents
1) Transport and Depositional Process in Deserts: ............................................................................. 2
1.1) Wind as a transporting agent in deserts: .................................................................................... 2
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
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
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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
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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
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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
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:
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
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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