Sediment charater

1. SEDIMENT CHARACTERSTICES IN DESERTIC
ENVIRONMNET
Prepared by:-
Samyog Khanal (13/077)
M.Sc. Engineering Geology
(1St Semester)
Central Department of Geology
Tribhuvan University
Email:- Samyogkhanal2017@gmail.com
Submitted to:-
Institute of Science and Technology
Tribhuvan University
Central Department of Geology
Engineering Geology
Kirtipur, Kathmandu, Nepal

2. What Are Deserts?
• I might define a desert as a land area
characterized by sparse and infrequent rainfall.
I wonder what images pop into your mind
when I mention deserts: vast expanses of dry,
rocky, rugged land? enormous shifting sand
dunes? Both of these images are good
representations of many deserts, but I think it’s
important to point out that
1) Most desert have at least some vegetation
2) Sand dunes constitute only a small percentage
of the area of most desert.

3. • Arid regions generally described as desert receive <25
cm rainfall annually. Deserts are dry, have high rate of
evaporation, poorly developed soil, and are mostly or
completely devoid vegetation.
• Average summer temperature for low-altitude desert is
32˚ to 38˚C. Daytime winter temperature average 10˚ to
18˚C.
• Most desert support
at least Sparse plant
Cover. Desert plants
are small, widely
Spaced and slow growing.
Marshak (2002)

4. • Erosion, transport, and deposition by wind is
commonly associated with deserts.
• Deserts are characterized by unique landforms.
Understanding the process by which the wind
erodes, transport, and deposits sediment helps
explain the distribution and origin of mostly
desert landforms.
• The geographic distribution of deserts is
largely controlled by global and regional
atmospheric circulation pattern.

5. Where Do Desert Occur?
• The distribution of dry climates (semiarid and
arid) and deserts is controlled primarily by the
general atmospheric circulation pattern which
produces arid climates and deserts (such as the
Sahara and Arabian) along belts centered
roughly on 30˚north & south latitude. The
remaining deserts of the mid-latitudes are
rainshadow deserts (Such as the Great Basin of
North America).

6. Photograph:- Photograph showing the Distribution of Dry Climate ( Semiarid and
arid) and Deserts.

7. Four Basic Desert Region
• Polar deserts:- Permanent Snow cover, Low
precipitation and intense cold, e.g. Antarctica
• Subtropical deserts:- Largest deserts, occur in
regions of subsiding high pressure air masses, e.g.
Sahara, Kalahari, Australia
• Mild-Latitude Deserts:- Located deep within
continental interiors e.g. The Gobi Desert
• Coastal desert:- Lie on the coastal side of large land
masses, tempered y cold, upwelling ocean currents,
e.g. the Atacama Desert, Chile.

8. Characteristics of Deserts
• Most deserts are not covered by
sand, but rather consist of vast
areas of exposed rock and desert
pavement.
• Most erosion in deserts is due to
running water. In deserts, most
precipitation falls during relief,
heavy, localized cloudbursts.
Considerable erosion takes place
because the ground cannot absorb
all the rainfall.

9. Characteristics of Deserts
• Mechanical weathering via
temperature fluctuation and frost
wedging is dominant processes in
desert. The scarcity of water greatly
reduces chemical weathering.
• Rock varnish is a thin, shiny red,
brown, or black coating on many
rock surfaces. The coating consists
of iron and manganese oxides
precipitated by microorganisms or
carried as windblown dust.
• Deseret soil are usually thin and
patchy because the limited rainfall
and vegetation restrict chemical
weathering and soil formation.

10. Wind Transport
• Wind is turbulent fluid and transports sediment in
much the same way as running water.
• Wind can transport silt- and Clay size grain as
suspended load.
• Sand and longer grains are transported ass bedload.
• Larger grains slide or roll, but sand gains generally
move by saltation, intermittent bouncing or hopping.
• Salting grains seldom rise more than a meter above
the surface.

11. Figure:- Figure shows the process of wind transport.

12. How does wind Erode Landforms?
• Wind action produces
many distinctive
erosional features.
Largely because of its
low density, wind is an
extremely efficient
sorting agent. Wind
erodes material in two
ways: Abrasion and
deflation.
Photograph:- Sliding stones of the Playa
Racetrack, California. Marshak (2002)

13. Abrasion
• Abrasion is caused by the impact of saltating
grains and is similar to sandblasting. Abrasion
etches, pits, smoothes, and polishes, but abrasion
of rock of varying durability can produce features
with bizarre shapes.
• In deserts, wind abrasion shapes the rocks and
boulders. In areas where strong winds consistently
pick up sand and carry it, rocks and boulders in
the wind's path can be impacted by wind abrasion.
If the wind blows over low-lying rocks, it will
cause them to become flattened on the upwind
side

14. Figure:- Figure shows the process
of abrasion.

15. • Yardangs are elongated, streamlined ridges that look
like the hull of an overturned ship. They are often
found in clusters aligned parallel to prevailing wind
direction. Yardangs are larger than ventifacts and
formed by differential wind erosion and abrasion.
Photograph :- Kharga Depression Egypt

16. Deflation
• Deflation is a mechanism of wind erosion that
removes loose surface sediments. Deflation
hollows or blowouts from in arid and semiarid
regions where deflation forms removes loose
surface sediment to form depression ranging
from kilometers in width and meters in depth
to small depressions only a few meters wide
and less than meter deep.

17. Figure:- Figure Shows the process of Deflation.

18. Desert Pavement
Figure:- Figure shows the
process of formation of Desert
Pavement.

19. Where deflation removes sand size and smaller grains, a
surface armored with pebbles, cobbles, and boulder is formed.
Photograph:- Photograph showing surface
armored with pebble, cobble and boulderd.

20. Wind deposits
• Wind is the major agent of deposition, producing deposits
of two general type:- Dunes and loess.
• Dunes are mounds or ridges composed of wind-blown
sand size grains usually deposited near their source.
• There are several types of dunes: barchan, longitudinal,
transverse, parabolic, and star. The size, shape , and
arrangement of dunes depends on sand supply, direction
and velocity of the prevailing wind, and the amount of
vegetation.
• Loess consist of layer of windblown silt and clay
composed of angular grains of quartz, feldspar, mica and
calcite. Loess is deposited over large areas downwind of
and far from its source.

21. Desert Landform Created by water
Fig:- Figure Shows the desert landform created by water.

22. Figure:- Figure shows the desert different type of landforms.

23. Bare rock surfaces
• Naturally sparsely vegetated or non-vegetated areas
where at least 90 % of the land surface is covered by
rocks.
• stable rocks with limestone pavements, block litter
and mountain-top-debris.
• areas with loss of vegetation due to erosion, recent
mud flows, landslides, rock avalanches.
• Non-vegetated abandoned mineral extraction sites.
• Bare rock, large mineral fragments (boulders, scree,
lapilli) occupying at least 90% of the area;
• Scattered vegetation occupying < 10% of area.

24. Photograph:- Photograph shows bear rock surface.

25. Alluvial Fans
• An alluvial fan is a fan-shaped or cone-shaped body
of loose sediment deposited at the foot of a steep
stream valley.
• Alluvial fans usually consist mostly of sand and
gravel.
• Fans are deposited because the slope of the stream,
and therefore the sediment-transporting ability of the
stream, decreases abruptly where the stream leaves it
mountain channel and spreads into a valley.
• At the head of the fan, the stream passes from being
laterally confined to laterally unconfined, so on
average it becomes wider and shallower.

26. • The contrast in slope between the confined mountain
stream and the unconfined fan surface is greatest in the
very early stages of fan development, when there’s only
a small fan at the base of a steep mountain front, and
much less in the later stages, when the fan has built to
greater size and the stream has had a chance to cut a
deep canyon and extend its headwaters far into the
mountain mass.
• Once on the fan, the stream progrades the fan locally,
and in doing so the elevation of the stream bed is raised
somewhat above the level of the adjacent fan surface.
• At some point the stream shifts its position to a lower
area of the fan. In this way, over a long time period the
stream sweeps irregularly across the entire fan, building
it in a symmetrical, conical shape.

27. Photograph:- Alluvial fan, Taklimakan Desert, XinJiang Province, China,
NASA, ASTER

28. Intermittent Stream Deposits
• Intermittent streams means a stream which flows only
at certain times when it receives water from springs
or from some other source, such as melting snow or
rain.
• Intermittent streams means any non-permanently
flowing drainage feature having a definable channel
and evidence of annual scour of deposition.
• Ephemeral rivers are developed by intermittent
stream deposits.

29. Photograph:- Ephemeral river present in desert.

30. Eolian Deposits
• Aeolian processes, also spelled Eolian, pertain
to wind activity in the study
of geology and weather and specifically to the wind's
ability to shape the surface of the Earth (or
other planets).
• Winds may erode, transport, and deposit materials
and are effective agents in regions with
sparse vegetation, a lack of soil moisture and a large
supply of unconsolidated sediments.
• Although water is a much more powerful eroding
force than wind, Aeolian processes are important in
arid environments such as deserts.

31. • A deposit that forms as a result of the accumulation of
wind-driven products of the weathering of solid
bedrock or unconsolidated alluvial, Lacusterine,
marine, or other deposits.
• Eolian deposits are found primarily in arid regions
(Sands, loesses), but they also occur in other natural
zones.
• Eolian sand deposits are found along seas and lakes
and on river terraces, in addition to deserts.
• Eolian deposits are usually formed from deposits of
sand of marine, delta, alluvial, lacustrine or
glaciofluvial origin.

32. • Prolonged eolian processing forms deposits of sand
consisting of well-rounded, polished grains of the
most stable minerals only such as quartz, zircon,
garnet and less frequently, magnetite and others.
• Eolian sandy deposits usually have diagonal-wavy or
oblique-wedge stratification.
• Their thickness ranges from a few meters to few
dozen meters, occasionally reaching several hundred
meters, corresponding to the height of major
aggradational eolian landform.
• Sandy deposits are subdivided into stationary (fixed)
and migrating (drifting) sands on the basis of the
degree and nature of eolian processing.

33. Figure:- figure shows the process of formation of eolian deposits.

34. P
Photograph :- Photograph shows the Eolian deposits.

35. Inland Sabkha
• A term typically used by Earth scientists, a Sabkha is
a coastal, supratidal mudflat or sandflat in which
evaporite-saline minerals accumulate as the result of
semiarid to arid climate.
• Sabkha are gradational between land and intertidal
zone within restricted coastal plains just above
normal high-tide level.
• Within a Sabkha, evaporite-saline minerals sediment
typically accumulate below the surface of mudflats
or sandflats.

36. • Evaporite-saline minerals, tidal-flood, and
aeolian deposits characterize many sabkhas
found along modern coastlines.
• Continental sabkha is used for environments
found within desert.
• The climate is one of the main factors in
sabkha development. Rainfall in this arid
region usually occurs as thunderstorms and
average 4cm/year. Temperature can range in
excess of 50˚C to as low as 0˚C.

37. Photograph:- Sebkhat (or Sebkha) El Melah, Tunisia in 2001, mostly dry.
Note rectangular industrial evaporite pans, probably for sea-salt
production, upper right. Landsat 7 image.

38. Playa Deposits
• Playa, (Spanish: shore or beach) , also
called pan, flat, or dry lake, flat-bottom depression found
in interior desert basins and adjacent to coasts within arid
and semiarid regions, periodically covered by water that
slowly filtrates into the ground water system or
evaporates into the atmosphere, causing the deposition of
salt, sand, and mud along the bottom and around the
edges of the depression.
• Playas are among the flattest known landforms. Their
slopes are generally less than 0.2 meter per kilometer.
When filled with only a few centimeters of water, many
kilometers of surface may be inundated. It is the process
of inundation that develops and maintains the near-perfect
flatness so characteristic of these arid-region landforms.

39. • Playas occupy the flat central basins of desert plains.
They require interior drainage to a zone where
evaporation greatly exceeds inflow. When flooded, a
playa lake forms where fine-grained sediment and
salts concentrate.
• A saline playa may be called a salt flat, salt marsh,
salada, salar, salt pan, alkali flat, or salina. A salt-free
playa may be termed a clay pan, hardpan, dry lake
bed, or alkali flat.
• Saline flats are specialized forms located adjacent to
large bodies of water, as, for example, along coasts,
lakeshores, and deltas.

40. • Two broad classes of playas may be defined on the
basis of past histories.
• One type develops from the desiccation of a former
lake. Sediments in such a playa are
primarily lacustrine, rather than derived from modern
depositional processes.
• The second type of playa has no paleo lacustrine
heritage.
• The supply of material, basin depth, and duration of
accumulation all contribute to variations in the
thickness of playa deposits.
• Very thick playa sequences may have alternating
layers of lacustrine clays and salt beds.

41. Photograph:- Playa in southwest Idaho.

42. Photograph:- Playa Lakes

43. Measas
• Measas are broad, flat-
topped erosional
remnants bounded on all
sides by steep slopes.

44. Buttes
• Buttes are pillar-like
erosional remnants.
Both mesas and buttes
consist of easily
weathered sedimentary
rock capped by more
resistant rock.

45. Pediments
• Pediments are erosional
bedrock surfaces of low
relief that slope gently
away from mountain
bases.
• Most pediments are
covered by thin layer of
debris, alluvial fans,
and/or bajadas.

46. Cross Bedding

47. Loess
• Loess is easily eroded, due to its
unconsolidated nature. Areas of
eroded loess are characterized by
steep cliffs and rapid lateral and
headward stream erosion. Loess-
derived soils are among the
world’s most fertile. Indeed the
world’s major grain-producing
regions corresponds to areas with
large loess deposits, such as the
Great planed of North America.

48. Ventifacts
• Rigid solid surfaces tend to be abraded by the impact
of flying sand grains.
• The upwind-facing surfaces of outcropping bedrock
or pieces of gravel are abraded to smooth and
sometimes fluted surfaces. Such rocks are called
ventifacts, or wind-worn stones.
• Loose pieces of gravel sometimes show a faceted
shape formed by abrasion from two or three different
directions—either by variability of wind direction or
shifting of the position of the clast relative to the
prevailing sand-transporting wind direction.

49. Photograph:- ventifact from the Mojave Desert near Barstow, California.

50. Thank you