Deserts

Question 1

rain or groundwater fed are…

Answer 1

interdunes: have evaporites, microbialite crusts preserved. thin mudstone, deflation pans (saline muds)

Question 2

how are interdunes made?

Answer 2

theres a depression because of the deflation. the sand will be removed from depression because of wind and will be deposited as dunes. water sweeps out of the aquifer and forms swamps/oasis. salt weathering will occur after deflation which will create accommodation space and provides sediment

Question 3

desert pavements

Answer 3

• regs, serirs, gibber

- may become basal conglomerates

Question 4

ventifacts

Answer 4

-pepples have 2 or 3 faces. paleocurrent indicator

Question 5

yardings

Answer 5

ridges of bedrock or indurated (lake) sediment made by sand abrasion. teardrop shaped with steep uwind slope. paleo wind indicator

Question 6

textures of eolian particles

Answer 6

well sorted: fine coastal dunes
mod sorted: fine to med inland dunes
poor sort: interdune and reg (stony desert) sands

Question 7

what are some eolian surface structures?

Answer 7

• adhesion warts: wart/blister. made by wind blowing dry sand over damp surface
• adhesion ripples: subparallel ridges formed -perpendicular to wind blowing dry sand over a moist surface
• invertebrate trail

Question 8

climbing ripple lamination

Answer 8

cant recognize easily bc of low relief, have inverse grading

Question 9

grainfall lamination

Answer 9

• deposited from suspension on lee side of dunes
• poor grain size segregation and weak lamination
• grading from falling wind velocity
• they thin downwards
• variable in thicknesses

Question 10

grainflow lamination

Answer 10

• avalanche cx beds
• slumping and grainflow down slipface of dunes
• interfingers with grainfall laminae
• may see deformed laminae

Question 11

eolian bedforms

Answer 11

• ripples
• dunes
• draas

Question 12

eolian ripples

Answer 12

• grains move by saltation (carpet of sand moving)
• surface irregularities and turbulence create patches where grains are slightly elevated
• when theres constant velocity: medium grains move same distance during saltation, which leads to equidistant piles of grains aligned 90 degrees to wind flow
• finer grains will be sheltered in ripple troughs
• coarser grains will be on crests because of winnowing
• inverse grading caused by accommodation on ripples
• oversteepening and avalanching creates cx lamination

Question 13

eolian dunes

Answer 13

• at least 3 m-600m in wavelength
• saltation on stoss side (rippled there),
• grainflow/fall on lee side, which produce cx beds

Question 14

seif dunes

Answer 14

• longitudinal: two wind directions at 90 degrees

- avalanche cx beds are orientated in diff direction

Question 15

draas

Answer 15

-made of dunes on stoss and lee side which have star, linear, and transverse forms

Question 16

transverse bedform model

Answer 16

• dune types: barchan, transverse, interdune, eolian lamination, 1st, 2nd order surfaces.
• at least 25 m
• sediments may interfinger with fluvial and lake deposits
• SEQUENCE: left to right above
• draa during max aridity
• alluvial fans supply the sand
• wind direction is unidirectional

Question 17

longitudinal bedform model

Answer 17

• at lease 100m
• draa, interdraa, draa sequence
• has similar development to transverse but early sand patches and barchans are followed by linear dunes/draa. or star draa when its complex

Question 18

differences between arid and hyper arid conditions for eolian systems

Answer 18

HYPER ARID

• alluvial fans are abandoned and cut off. the fans will cause deflation which will be the sand supply for the system
• abandoned braided system
• badland topography
• basin bounded fault
• evaporitic playa lake
• climbing dunes with dry interdune depressions

ARID

• alluvial fans will be reactivated because there is precipitation
• will have ephermal streams between interdunes
• not a lot of vegetation
• the ephermal system will replenish playa lake
• enhanced badland development
• playa lake expansion, permanently there
• salt precipitates will dissolve again
• wedge bodies of alluvial fan strata

Question 19

semi arid climate regimes

Answer 19

• alluvial fans will overlap
• periodic lobe switching
• deltas form
• damp water table is controlled by sand sheet
• alluvial fans pass into subaqueous deltas at lake margins
• partial stability bc of vegetation
• dunes do not climb
• not a lot of sediment available