STRATIGRAPHY AND SEDIMENTATION (DESERT) Flashcards
Definition of Dry based on precipitation
25 cm per (10 inches) per year
Dryness
any situation where there is water deficiency
Dry Climate
Climate in which yearly precipitation is less thanevaporation
A marginal more humid variant of desert that represents a transition zone
Steppe or semiarid
In which parts of the world are desert and steppe regions occur
Subtropics nd middle latitudes
These are zone of high air pressure found usually in the dry regions of lower latitudes
Subtropical highs or Horse Latitude
Describe the air movement in subtropical highs?
Air currents are subsiding and when air sinks, it becomes compressed and warmed
These type of deserts are the ones situated in the deep interiors of large land masses or continents
Mid Latitude deserts and steppes
What are the foremost barrier to moisture from the pacific in N.America?
1)Coast ranges
2) Sierra Nevada
3) Cascades
What type of weathering is dominant in humid regions and provie evidence?
Chemical Weathering, slopes and rock edges are rounded and soil are well developed
What type of weathering prevails in derserts and provide example
Mechanical Weathering, unaltered rock and mineral fragments
What produces the rust-colored stain that tints some of the desert landscapes?
Oxidation of Iron Bearing Silicates
Why is weathering in dry lands greatly reduced?
1) lack of moisture
2) lack of organic acids from decaying plants(carbonic acid)
Streams that carry water only in response to specific episodes of rainfall
Ephemeral Streams
American names for Ephemeral Streams
Wash or Arroyo
Dry desert stream in arabia and north africa
Wadi
Dry desert stream in South America
Donga
Dry desert stream in India
Nullah
This does most of the erosional work in deserts
Running Water
Permament streams that do cross arid regions
Nile and Colorado
Desert that is being traversed by Nile without having any single tributary
Sahara
What is the main role of wind in deserts?
Transportation and deposition of sediment
ridges and mounds of sediments formed primarily by wind action
Dunes
This means discontinuous pattern of ephemeral streams that do not flow our of the desert to the ocean
Interior Drainage
This pertains to the difference in elevation between high and low points in an area
Relief
This forms when water heavily loaded with sediments emerged from the confines of the canyon and the runoff spread over the base of the mountains and loses velocity creating a cone of debris
Alluvial Fan
Coalescing alluvial fans from adjacent canyons producing an apron of sediment
Bajada
When streams flow across alluvial fans to the center of the basin converting the basin floor into a shallow lakes which lasts only few days or weeks
Playa lakes
The dry flat bed left after evaporation of such lakes
Playa
Playas that are encrusted with salts
Salt Flats
Large bedrock knobs projecting above the sediment-filled basin
Inselberg
Difference of wind from water erosion
1) Wind has lower capacity
2) Can spread sediment over larger areas as well as high into atmosphere
Largest dust source in the world
N.Africa
A disaster which happened in the 1930s which caused severe dust storm
Dust Bowl
This is the lifting and removal of loose fine grained materials by wind
Deflation
Shallow depressions which are the most noticeable results of deflation
Blowouts
A stony veneer composed of coarse pebbels and cobbles thata re too large to be moved by the wind
Desert Pavement
The process of cutting and polishing exposed rocks surfaces by windblown sand
Abrasion
What is the limit of the vertical extend of wind abrasion
1m
Extensive blankets of silts which were once carried in suspension
Loess
mounds and ridges of sand from the winds bed load
Dunes
Two primary sources of sediments that form loess
Stratified drift, deserts
The most extensive deposits of loess on Earth
Noethern and western china
River which got its name from the buff colored loess
Huang Ho or Yellow River
Which areas of the world have loess formed from stratified drift
US and Europe
Formation of dunes
1) Moving air encounters barrier
2) Wind sweeps around and over it leaving a shadow of more slowly moving air behind the obstacle an smaller zone of quieter air infront of the obstacle
3) Salting and sand grains rest in these ind shadows
The steep side of a dune
Leeward
the gently incline side
Windward
In which side does sand move by saltation
Windward
the leeward side of the dune
Slip face
what is the maintaining angle of repose of the leeward side?
34 deg
This forms when sand deposition forms layer inclined in the direction the wind is blowing
Cross Bedding
A an example of cross bedding
Zion Canyon in Utah
Factors affecting sand dune form and size
1) Wind Direction
2) Velocity
3) Availability of sand
4) Amount of vegetation
Solitrary dunes shaped like crescents and with their tips pointing downwards
Barchan Dunes
In what environment does it form?
Hard and flat ground
Lack vegetation
limited sediment supply
Max annual migration rate of Barcham
15 m (50ft)
Dunes that form a long rige that are separate by troughs and oriented at right angles to the prevailing wind
Transverse Dunes
In what environment does it form?
Steady winds
plenty sand supply
sparse or lack vegetation
Transverse dunes that form in arid regions where there is extensive amount of wavy sand
Sand sea
Where do Transverse usually form
Coastal Dunes
a relatively common form intermediate between isolated 61 and extensive waves of 64 which form scalloped rows of sand oriented at right angles to the wind which resembles series of brachans which have been positioned side by side
Barchanoid Dunes
Example of Barchanoid
Gypsum Dunes, white sands, NM
Long ridges of sand that form more or less parallel to the prevailing wind and where sand supplies are moderate
Longitudinal Dunes
This dunes form where vegetation partially covers the sand and shape resembles that of barchan but tips are pointing into the wind direction
Parabolic dunes
Envi in which 71 usually forms
Coastal where there are strong winds and abundant sand
Usually three or four shar crested ridges diverge from a central high poing which are developed due to variable wind directions
Star Dunes
Deserts where star dunes can be found
Sahara and Arabian
