eolian environment
DESCRIPTION
Macam-macam pegendapanTRANSCRIPT
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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