aeolian and barrier island system.pptx

8/14/2019 Aeolian and Barrier Island System.pptx

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Group Members :

Mustahsan ShuaibMohtasim Niazi

Ehsan Haroon

Syed Muhammad Ali Shah

Ahsan Khursheed

Rehamt Ali

*Aeolian and Barrier Island

Depositional System


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*There are various depositional environmentsin which the processes of sedimentationoccurs.

*Here in this presentation two importantdepositional systems Aeolianand BarrierIsland Systems will be discussed.

*Aeolian depositional system is a very diversesystem which includes a lot of geological

features and structures so it will be discussedin detail as compared to Barrier Island system


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*

*Aeolian sedimentary processes are those

involving transport and deposition of material

by the wind

*The most obvious aeolian environments arethe large sandy deserts in hot dry areas of

continents

*Aeolian sands deposited in desert

environments have distinctive characteristicsthat range from the microscopic grain

morphology to the scale of cross-

stratification

*Aeolian sandstones are good hydrocarbon

reservoirs and aquifers.

Introduction


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*


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Two forms of wind erosion

Deflation

Removal of material by wind transport

Abrasion

Mechanical weathering of rock by wind

blown sand

Wind Erosion


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The blowing away ofloose sand, silt, and claysized particles

Larger particlesremain

Only takes place indesert areas, or in theabsence of vegetation

Roots tend to holdsoil in place


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*

Desert pavement is formed when sand and

silt are blown away

Deflation may excavate depressions in excess

of 50 m


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*

*Natural sandblasting

*Kinetic energy of wind is transferred to wind-driven

particles

*Particles impact rock surfaces & knock off small pieces


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Abraded and polishedrock surface producedby wind erosion

At least one polishedsurface facing thewind

Prevailing winddirection may bedetermined

Uncommon comparedto stream erosion


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*Following are the main features and structures

that are present in a typical Aeolian System,

o

Rippleso Dunes

o Cross Beddings

o Mud Cracks


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*Ripples form perpendicular to wind from saltatinggrains

*Ripples occur at regularly spaced intervals

*Ripples may form on dune surfaces

*Crest to crest distance ranging from a few centimetres

to several metres

*Ripple heights range from less than a centimetre to

more than ten centimetres


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*

A hill or ridge of sand deposited by wind form fromsmall irregularities in the land surface

Air flow is disrupted

A pocket of low wind velocity is formed

Sediment accumulates in the pocket

Growing mound further disrupts air flow

Crest becomes unstable, sand avalanches down the

slip face


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*

Dune geometry

Asymmetrical shape

Shallow upwind slope,

10-12O

Steep downwind slope,

33-34O, angle of repose

There are many types of

the sand dunes

depending upon thewind patterns and flows.


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*Cross beddings are the features which are

strongly related to the Aeolian system

*Cross-bedding is the bedding in which adjacent

beds may dip in different directions

*Each bed shows the paleo-current of the winds

*In deserts the wind direction is changing

constantly thats why we find alternating beds

dipping in different directions


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*Cross beddings showing wind flowing pattern

in paleo-environment.


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* In the deserts temporary lakes are formed known

as Playa lakes which are formed in the rainy

season

*Especially when flash floods occur

*These playa lakes are the dump site for the

minerals deposits and the ores also*Along with the minerals fine sediments like clay

and evaporites also deposit

*These fine grain sediments form a plain covering a

large area

*Mud cracks are produces in these areas when

water evaporates

*Sometimes these cracks are preserved hence can

be used for interpretation of paleo-environment.


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* Lithologiessand and silt only

*Mineralogymainly quartz, with rare examples ofcarbonate or other grains

* Texturewell- to very well-sorted silt to medium sand

* Fossilsrare in desert dune deposits, occasionalvertebrate bones

* Bed geometry sheets or lenses of sand

* Sedimentary structures large-scale dune crossbeddingand parallel stratification in sands

* Palaeocurrents dune orientations reconstructed fromcross-bedding indicate wind direction

* Colouryellow to red due to iron hydroxides and oxides

* Faciesassociations occur with alluvial fans, playa lakes,and lake facies in deserts,


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*

*Mainland beaches are long, narrow accumulationsof sand aligned parallel to the shoreline andattached to land

*Barriers range in size from less than 100m wide toseveral kilometers and their length ranges from afew hundred meters to many tens of kilometers

*They are similar to mainland beaches but areseparated from land by a shallow lagoon, estuary,

or marsh*Barrier-island systems are generated

predominantly by marine processes

Introduction


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*

*Beach and barrier-island are best developed onwave-dominated coasts with small to moderatetidal range

*Types of coasts with respect to tidal range:

Microtidal (0-2 m tidal range)

Mesotidal (2-4 m tidal range)

Macrotidal (>4 m tidal range)

*Barriers are generally absent on Macrotidal

coasts


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*The barrier-island setting is not a singleenvironment but a composite of three separate

*environments

*They may be partially attached to the land,forming a beach spit,

*or wholly attached as a welded barrier thatcompletely encloses a lagoon,

*or can be isolated as a barrier island in front ofa lagoon


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*

*Beach deposits form on the beach face or

foreshore, which is the intertidal zone extending

from mean low-tide level to mean high-tide level,

corresponding to the zone of wave swash

*Shore facedepositsform in an environment thatextends from mean low tide level on the beach

down to the lower limit of fair-weather wave base

*Back-barrier sediments are deposited in several

sub environments in the back-barrier lagoon

landward of barrier beaches.


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*Lithologysand and conglomerate

*Mineralogymature quartz sands and shelly sands

*Texturewell sorted, well rounded clasts

*Bed geometry elongate lenses

*Sedimentary structures low-angle stratification andwave reworking

*Palaeo currentsmainly wave-formed structures

*Fossils shelly debris

*Colornot diagnostic ( depends on local lithology)

*Facies associations may be associated with coastalplain, lagoonal or shallow-marine facies


37/37THATS ALL!

aeolian and barrier island system.pptx

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