Aeolian Environments and Controls on Sedimentation Alex Bryk, Ron Cash, Jacob Wikle, Rebecca Alberts
Aeolian dunes develop in desert systems where there is an abundance of sand-grade material available for sediment transport, and where to transport capacity of the wind is sufficient to move that material. -Mountney, N.P. 2004 Aeolian Desert Environments (external controlling forces): :Hyper-Arid :Arid :Semi-Arid Control sub-environments: :Aeolian dunes :Inter-dunes :Sand sheets :Salt flats :Playa lakes :Ephemeral fluvial systems :Alluvial fans With intrinsic Aeolian behavior: :Amount of available sediment :Wind velocity By understanding how external controlling forces interact with intrinsic aeolian behaviors to form sub-environments, we can focus, then, on the preservation of bed forms and additional influences on sedimentation. :Why is preservation significant and what more can it tell us about the interactions of/individual environments?
http://www.dur.ac.uk/d.a.jerram/research1.html
Aeolian Dunes: Develop from the interaction of wind and sand-grade material. Migrate downwind due to sediment movement over the stoss side avalanching over the lee sides, decelerating and falling out of suspension. Interdunes: Form the flat areas between dunes, and depending on water table height, can be damp or dry. http://www.geo2all.com/vb/showthread.php?168 4-geology-of-sand-dunes Sandsheets: Develop in areas with an inadequate amount of sediment to cause the formation of dunes. Rippled sands are the most common feature, can contain plant life if water table is high enough. Salt Flats: Occur in flat areas dominated by evaporite precipitation near the water table. http://picasaweb.google.com/lh/photo/eynhalumhzqjuzay http://bikerscafeblog.com/ 2010/06/21/burt-munroofferings-to-the-god-ofspeed/bikefastsaltflats/
Hyper-arid setting: Extremely low water table, small playa lakes and brine pools, large aeolian dunes, sandsheets, and inactive alluvial fans, with very rare flash flooding. Arid Setting: Expansion of playa lakes, salt flats, flash floods restart alluvial fan lobes, lower supply of sand due to damp surface from water table proximity, smaller dunes, limited vegetation. Semi-arid Setting: Further playa lake and salt flats expansion, ephemeral fluvial system now causes the formation of a delta at the lake entry. Flash floods create active alluvial fans, increasing lobe switching, majority of sediment is blown into the playa lake area, creating a sediment sink. Remaining dunes are covered with vegetation and stabilized.
D.A. Jerram, N.P. Moutney, J.A. Howell, D. Long, H. Stollhofen
Current site in NW Namibia Large scale tectonic forcings during the breakup of Gondwana cause large flood basalts. These flood basalts buried and preserved an active aeolian sand sea.
Key Points (sediment transport): Critical shear stress for grain entrainment in air is higher than in water Grain settling velocities in air are higher than in water Temperature is more variable in air than in water, and therefore the thickness of the boundary layer is also more variable The bedload layer is thicker in air than in water, thus increasing the roughness of the flow boundary
Grainfall: Settling of Saltation load onto slipface wedge-shaped geometry with greatest thickness near the top of the slipface and gradual thinning down the slipface no vertical grading more closely packed than grainflow laminae deposit thickens until it oversteepens and remobilisation occurs (grainflow event) Grainflow : avalanching down slipface lens-shaped geometry, of limited horizontal and vertical extent deposition at angle of repose inversely graded loosely packed often erodes underlying grainfall lamina
Bounding Surfaces: An erosional surface within or between sets of crossstrata. Active vs. Inactive slipfaces
Aggradation rate is generally low compared to lateral translation rate (Generally < 1 ) The majority of aeolian deposits in the rock record only preserve the lowest 10-20% of the large-scale bedforms in the system
And this is important because: By looking at each snapshot of the active aeollian environment preserved we can learn about: -paleo-climates (and how we can predict the future) -migration and direction in/of aeolian environments -reservoir rocks (Hydrocarbon Exploration)
Resources: Mountney, N.P. 2004. The sedimentary signature of deserts and their response to environmental change, Geology Today, 20, 101-106. Mountney, N.P., Howell, J.A., Flint, S.S. and Jerram, D.A., 1999. Relating aeolian bounding surface geometries to the bed forms that generated them: Etjo Sandstone Formation, Cretaceous, Namibia. Geology, 27, 159-162. Jerram, D.A., Mountney, N.P., Howell, J.A., Long, D. and Stollhofen, H. 2000. Death of a Sand Sea: An Active Erg Systematically Buried by the Etendeka Flood Basalts of NW Namibia. Journal of the Geological Society, London, 157, 513-516.