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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 Systems25% 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 areas2. 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 AeolianDepositional Features:

1. Loess• silt deposits accumulating far

from sourcethickest deposits associated w/

glacial outwash plains2. Sand Deposits

• well sorted• texturally mature particles• well rounded, pitted &

frosted, dominated by quartz3. Deflation pavement - lag

depositsgravel size particles too large to

be transported

Desert Pavement, Mojave Desert

Alma, Wisconsin

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Qaidam Desert Basin, China• Northern edge of the Tibetan

Plateau

Courtesy Fletcher and Baylis

deflation

Depositional Features:• Ripples• Dunes

0.1 to 100 m high (>400 m)Typical geometry -• slip face• crest• foresets• topsets

White Sands, New Mexico

topset depositsslipface

tabular planarcross-beds

wedge planarcross beds interdune

deposits

crest

leewardwindward

Texture -- well sorted- well rounded- quartz rich

exception: coastal dunes(heavy minerals,unstable rock fragments

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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

Navajo Sandstone, UTAH

Cross Strata

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)

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Eolian Facies: possiblebedding structures;

• Successions mostly random

White Sands, NM

Dune Morphology

• Unidirectional Winds:single slip faces– Barchans– Barchanoid ridges– TransverseTransition reflects increasing

sand supply

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cresent-shaped barchan dune,Qaidam Basin, China

Barchanoid Dunes & TransverseRidge, White Sands, NM

Transverse dune, Namib

White Sands, NM

Dune Morphology

• Multi-directional Winds:– Linear– Longitudinal– Reversing– Starmultiple slip faces

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Star shaped dune, Namib

Qaidam Basin, China seif dune

Longitudinal dune, Namib

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Key Criteria for recognizingdune deposits:

• well sorted• pitted, frosted grains• thick cross bed sets• high angle foresets

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LACUSTRINESYSTEMS

• Minor component ofsedimentary rocks

– Economic importance~ oil shale

– Archive of terrestrialpaleoenvironments• 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

Hvitarvatn, Iceland Impact Crater Lake Bosumtwi, Ghana

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• Open lakes - outflow balances inflow1. stable shorelines2. siliciclastic sediments

Sedimentation Processes:

Sedimentation Processes:• Closed lakes - no major outflow

1. fluctuating shorelines2. evaporation (alkaline, chemical precipitates)

• Climate Influence:1. water level2. chemical sedimentation3. 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

depositional processes:• similarities w/ marine environments

– waves (beaches)– currents– mass transport - turbidity currents, slumpsSuspension- 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

Physical Processes:

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Playa Lake, MojaveChemical Processes:lake chemistry varies considerably• Arid regions: ground water/evap.

– inorganic carbonate sediments -CO2outgassing - precipitation of calcite(finely laminated beds)

– sulfates - gypsum/anhydrite– chlorides - halite

• Humid regions:– carbonates (both inorganic and organic)– Silicates

Organic Processes:1. Production of shells2. bioturbation3. plant remains– high concentrations of terrestrial plant

organic matter (low H/C ratios

Corg clays

Evaporation

wind

White Sands,NM

Pleistocene - wet!

Pleistocene Lakes Otera,Estancia,Trinity

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Primary Characteristics of Lake Deposits:• Composition:

– siliciclastic sediments (derived fromrivers)• nearshore - gravels, sands• offshore - sands (turbidites, grain flows),

silts and clays (marl)– carbonate sediments

• Fine grain - microscopic shells of algae(also bivalves, ostracodes, gastropods)

• organic rich -– Aquatic– Terrestrial detritus

Beaverhill Lake Formation(Middle Devonian), Alberta

CaCO3 richC-org rich marl

Clastic Sediment fluxes: River Plumes

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Las Vegas Bay, Lake Mead

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

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

Bedded Claystones, Green River, WYLaminated, Corg rich Shale, WY

Eocene Green River Formation,Colorado, Wyoming, Utah

• Superb fossil preservation - anoxia?

Gryllidae - a cricket

Dolichopodidae - a long-legged fly

Knightia

gar Lepisosteus, predatory fish