Eolian Sediments
• Eolian environments include those where wind is the major
agent of sediment deposition.
• Eolian sedimentary rocks are usually well-sorted, fine
grained sandstones and siltstones
Sand Dunes, White Sands, NM
Desert Systems
25% of the earth surface
• DEPOSITIONAL ENVIRONMENT
–
DRY < 25 cm/yr
•
•
•
•
leeward side of mountain ranges (rain shadow)
20 to 30°N & S, descending limbs of the Hadley cells
episodic precipitation
sparse vegetation
1. Dunes ~ 20% of desert surfaces
–
–
Sand seas (ergs) - areal coverage > 125 km2
Dune fields - smaller areas
2. Eroding mountains, rocky areas, desert flats Remaining 80%
Sand Dunes, White Sands, NM
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Desert Systems
• DEPOSITIONAL PROCESSES
Temperature, Wind are both highly variable
– water
rain episodic - debris and mud flows
– wind is the main transport agent
• Ineffective erosion force
• effective transport sand size & smaller grains
Modes of wind transport
• suspension (< 0.07 mm )
• saltation (0.07 to 1 mm)
• traction (>0.5 mm) impact of saltating grains
Sand Dunes, White Sands, NM
Major Aeolian
Depositional Features:
1. Loess
• silt deposits accumulating far
from source
thickest deposits associated w/
glacial outwash plains
Alma, Wisconsin
2. Sand Deposits
• well sorted
• texturally mature particles
• well rounded, pitted &
frosted, dominated by quartz
3. Deflation pavement - lag
deposits
gravel size particles too large to
be transported
Desert Pavement, Mojave Desert
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Qaidam Desert Basin, China
• Northern edge of the Tibetan
Plateau
deflation
Courtesy Fletcher and Baylis
Depositional Features:
• Ripples
• Dunes
0.1 to 100 m high (>400 m)
Typical geometry • slip face
• crest
• foresets
• topsets
leeward
windward
crest
topset deposits
slipface
wedge planar
tabular planar
cross beds
cross-beds
interdune
deposits
Texture - well sorted
- well rounded
- quartz rich
exception: coastal dunes
(heavy minerals,
unstable rock fragments
White Sands, New Mexico
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Transport and Deposition of Sand in Eolian Deposits
•Sediment accumulates at Crest
•Avalanche (Gravity Transport) down Slip face
Saltation Zone -10 cm
Depositional Features:
Structure - enormous cross bedding with laminae
• medium to large scale cross strata
• foresets dip at high angles, up to 34° (i.e., angle of repose)
• tabular cross-strata -meters thick
• individual beds of cross strata are long (10’s m)
• bounding planes between individual sets of cross-strata (horizontal or slightly
dipping leeward)
Cross Strata
Navajo Sandstone, UTAH
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Eolian Facies: possible
bedding structures;
• Successions mostly random
Dune Morphology
• Unidirectional Winds:
single slip faces
– Barchans
– Barchanoid ridges
– Transverse
Transition reflects increasing
sand supply
White Sands, NM
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cresent-shaped barchan dune,
Qaidam Basin, China
Barchanoid Dunes & Transverse
Ridge, White Sands, NM
Transverse dune, Namib
Dune Morphology
• Multi-directional Winds:
– Linear
– Longitudinal
– Reversing
– Star
multiple slip faces
White Sands, NM
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Longitudinal dune, Namib
Qaidam Basin, China seif dune
Star shaped dune, Namib
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Key Criteria for recognizing
dune deposits:
• well sorted
• pitted, frosted grains
• thick cross bed sets
• high angle foresets
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LACUSTRINE
SYSTEMS
• Minor component of
sedimentary rocks
– Economic importance
~ oil shale
– Archive of terrestrial
paleoenvironments
• Chemistry
• Mineralogy
• Pollen
LACUSTRINE SYSTEMS
• Environmental Setting:
– form in any depression: downwarped basins, fault grabens
– all latitudes and geologic settings (glacial, mountains, plains)
– Dry (Salt Lake) and wet climates
Impact Crater Lake Bosumtwi, Ghana
Hvitarvatn, Iceland
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Sedimentation Processes:
• Open lakes - outflow balances inflow
1.
2.
stable shorelines
siliciclastic sediments
Sedimentation Processes:
• Closed lakes - no major outflow
1. fluctuating shorelines
2. evaporation (alkaline, chemical precipitates)
• Climate Influence:
1. water level
2. chemical sedimentation
3. sediment input - vegetation and aridity
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Physical processes:
• wind - small waves
• river inflow - sediment supply, density currents
atmospheric heating - density stratification, currents
Physical Processes:
depositional processes:
• similarities w/ marine environments
– waves (beaches)
– currents
– mass transport - turbidity currents, slumps
Suspension
- varve sedimentation - seasonal effects
• differences w/ marine environments
–
–
Limited surface area (fetch) - small waves, low energy
ratio of drainage area to lake area is high
•- high sedimentation rates (10 x marine)
–
lakes are tideless - tidal currents are negligible
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Chemical Processes:
lake chemistry varies considerably
• Arid regions: ground water/evap.
Playa Lake, Mojave
– inorganic carbonate sediments -CO2
outgassing - precipitation of calcite
(finely laminated beds)
– sulfates - gypsum/anhydrite
– chlorides - halite
• Humid regions:
– carbonates (both inorganic and organic)
– Silicates
Organic Processes:
1. Production of shells
2. bioturbation
3. plant remains
– high concentrations of terrestrial plant
organic matter (low H/C ratios
Corg clays
Pleistocene - wet!
Pleistocene Lakes Otera,
Estancia,Trinity
Evaporation
wind
White Sands,
NM
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Primary Characteristics of Lake Deposits:
• Composition:
– siliciclastic sediments (derived from
rivers)
• nearshore - gravels, sands
• offshore - sands (turbidites, grain flows),
silts and clays (marl)
– carbonate sediments
Beaverhill Lake Formation
(Middle Devonian), Alberta
CaCO3 rich
C-org rich marl
• Fine grain - microscopic shells of algae
(also bivalves, ostracodes, gastropods)
• organic rich – Aquatic
– Terrestrial detritus
Clastic Sediment fluxes: River Plumes
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Primary Characteristics of Lake Deposits:
• Structures:
–laminated bedding and varves
–thin, laterally continuous beds (as opposed to fluvial
sediments)
–other features are similar to those in marine sediments
Las Vegas Bay, Lake Mead
Salton Trough
• 3 to 6 km late Cenozoic continental
sediments (Mecca formation)
–
–
–
–
Alluvial fan
Braided rivers
Lacustrine
Aeolian
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Mecca formation, California
Lake Sediments:
Hvitarvatn, Iceland
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Eocene Green River Formation,
Colorado, Wyoming, Utah
• 3 large lakes (Uinta, Gosiute)
• Cyclical bedding
-orbital, annual
• Lithology (fine grain, organic rich)
– Oolititic grindstones
– Oil shales
– Mudstones
Laminated, Corg rich Shale, WY
Bedded Claystones, Green River, WY
Eocene Green River Formation,
Colorado, Wyoming, Utah
• Superb fossil preservation - anoxia?
gar Lepisosteus, predatory fish
Gryllidae - a cricket
Knightia
Dolichopodidae - a longlegged fly
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