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Eolian Sediments • Eolian Environments Include Those Where Wind Is the Major Agent of Sediment Deposition

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Eolian Sediments • Eolian Environments Include Those Where Wind Is the Major Agent of Sediment Deposition 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 1 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/ Alma, Wisconsin glacial outwash plains 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 2 Qaidam Desert Basin, China • Northern edge of the Tibetan Plateau deflation Courtesy Fletcher and Baylis Depositional Features: windward leeward • Ripples crest • Dunes topset deposits 0.1 to 100 m high (>400 m) slipface Typical geometry - wedge planar cross beds tabular planar interdune • slip face cross-beds deposits • crest • foresets • topsets Texture - - well sorted - well rounded - quartz rich exception: coastal dunes (heavy minerals, unstable rock fragments White Sands, New Mexico 3 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 4 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 5 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 6 Longitudinal dune, Namib Qaidam Basin, China seif dune Star shaped dune, Namib 7 Key Criteria for recognizing dune deposits: • well sorted • pitted, frosted grains • thick cross bed sets • high angle foresets 8 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 9 Sedimentation Processes: • Open lakes - outflow balances inflow 1. stable shorelines 2. 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 10 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 11 Chemical Processes: lake chemistry varies considerably Playa Lake, Mojave • Arid regions: ground water/evap. – 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 Corg clays 2. bioturbation 3. plant remains – high concentrations of terrestrial plant organic matter (low H/C ratios Pleistocene - wet! Pleistocene Lakes Otera, Estancia,Trinity Evaporation wind White Sands, NM 12 Primary Characteristics of Lake Deposits: • Composition: – siliciclastic sediments (derived from rivers) Beaverhill Lake Formation • nearshore - gravels, sands (Middle Devonian), Alberta • offshore - sands (turbidites, grain flows), silts and clays (marl) CaCO3 rich – carbonate sediments C-org rich marl • Fine grain - microscopic shells of algae (also bivalves, ostracodes, gastropods) • organic rich - – Aquatic – Terrestrial detritus Clastic Sediment fluxes: River Plumes 13 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 14 Mecca formation, California Lake Sediments: Hvitarvatn, Iceland 15 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 long- legged fly 16.
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