GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits. Lectures 17 & 18: Aeolian Facies

Transcription

1 GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits Lectures 17 & 18: Aeolian Facies

2 Today: Processes air/water differences Deposits bedforms Facies a little on models and controls (Fridays seminar)

3 Aeolian Deposits deserts and coasts Only 20% of modern deserts are sandy eroding mountains (40%) stony deserts (10-20%) desert flats (10-20%) Deserts can be (semi)arid, warm and cold (sub)polar climatic zones: glacial processes dominate (semi)arid climatic zones: aeolian processes dominate We will concentrate on aeolian processes in sandy deserts, also applicable to coastal dunes... Mountney, 2006

4 Sediment transport by WATER and WIND What makes the difference?

5 Principal physical differences Air is less viscous than water Air is less dense than water Buoyancy forces in air are lower than in water Critical shear stress for grain entrainment in air is higher than in water Grain settling velocities in air are higher than in water

6 The principal factors combine to cause distinct differences in sediment transport The boundary layer is much thicker in air than in water Temperature changes are much more frequent in air than in water, and therefore the thickness of the boundary layer is more variable Aeolian sediment can be dry, damp or wet, which affects grain transport The bedload layer is thicker in air than in water, thus increasing the roughness of the flow boundary Grains transported through aeolian bedforms are dominantly sand (and granule) sized Large atmospheric grain settling velocities cause collision effects (and ensuing saltation) to dominate bedload transport Momentum exchange of entrained grains with a bed of grains causes splash-up effects, leading to downwind chain reaction Momentum exchange also causes splash-down effects, causing creep of bed grains, nudged by successive impacts (reptation) Energetic collisions cause abrasion, deflation and rounding of grains (ventifacts) and impact surfaces Suspension transport of sand/granules is more difficult in air than in water

7 Yep, thick boundary layer and active bedload layer High dunes Sangre de Cristo mountains, Colorado, USA Up to 200 m high aeolian dunes

8 Characteristic size of grains in aeolian bedforms and tend to be well-sorted Mountney, 2006

9 Entrainment by grain impact moving grain after impact moving grain before impact saltating grains due to splash-up sediment bed

10 Abrasion Gradual wearing away of lithified material by repetitive grain impacts Ventifacts - Dreikanters Why this shape?

11 Products Deflation Removal of loose surface material by wind Deflation hollows: depressions resulting from the removal of sandand silt-sized grains Desert pavements ( regs ): gravel-rich lag deposit resulting from the preferential removal of sand and finer-grained loose sediment... protects underlying layer from further deflation

12 Structures in dry, damp and wet sediment DRY DAMP WET

13 Suspended load transport Dominantly very fine sand, silt and clay Transport over large distances Process: dust storms Product: loess (often of glacial origin)

14 Summary Grain Transport Mode rarely moving reptation (creep) saltation suspension gravel vc sand coarse sand medium sand fine sand vf sand silt/clay desert pavement bedforms bedforms loess Type of Deposit

15 Aeolian Bedforms and Stratification Wind ripples (aeolian ripples, ballistic ripples) asymmetric outline, O( m) Dunes variable geometry, O(1-100 m) Draa (large compound or complex dunes) variable geometry, O(>100 m)

16 Wind ripples

17 Wind ripples Current ripples relatively flat mostly straight-crested wavelength scales to either saltation or 6-10 x reptation pathlength coarsest particles on ripple crest foreset lamination uncommon relatively high both 2D and 3D crest lines wavelength scales to length of separation vortex coarsest particles in ripple trough foreset lamination common

18 ripple wavelength (m) 1 wind ripples 0.1 current ripples ripple height (m)

19 linguoid current ripples straight-crested wind ripples

20 current ripple & wind ripple (fluid drag-type) flow separation avalanching wind ripple (impact-type) impacts creep saltation settling few impacts

21 Aeolian shelter zone. Mountney, 2006

22 CU (Impact) wind ripple lamination: without foreset laminae (Climbing) current ripple cross-lamination: with foreset laminae

23 Main dune types barchan dunes transverse dunes parabolic dunes longitudinal dunes (seifs) star dunes (rhourds)

24 Factors controlling dune type Sand availability Wind velocity Variability in wind direction Amount of vegetation Obstructions / topography

25 Flow over dunes and stratification Grainflow : avalanching down slipface Grainfall : settling of saltation load onto slipface Wind ripples in dune trough (and on stoss side)

26 Flow over dunes and stratification Grainfall Grainflows

27 Flow over dunes and stratification Grainflows (ancient)

28 Flow over dunes and stratification Characteristic properties of grainfall lamination: 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) Characteristic properties of grainflow lamination: lens-shaped geometry, of limited horizontal and vertical extent deposition at angle of repose inversely graded loosely packed often erodes underlying grainfall lamina Other types of lamination: plane-parallel lamination (aggrading upper stage plane bed) climbing ripple lamination (closely packed)

29 Flow over dunes and stratification 29

30 Flow over dunes and stratification grainflow laminae grainfall laminae downdip terminating grainflow laminae wind ripple cross-laminae

31 Barchan dunes Crescent shaped ridges, with arms pointing downwind Typically m high Low variability in wind direction Limited sand supply or incomplete sand cover due to bypassing Dune spacing increases with decreasing sand cover 31

32 Longitudinal section through dune centre stoss lee Complex, mainly downwind stratification with erosive and irregular internal bounding surfaces Transverse section through right-lateral arm of dune outward Foresets and bounding surfaces dipping outwards from the dune core, so at angle with dominant wind direction

33 Foreset orientations of barchan dunes not preserved 40º 30º 20º 10º predominant wind direction rare common

34 Large-scale trough x-bedding in barchan dune

35 Transverse dunes Straight-crested or sinuous ridges (aklé dunes), oriented perpendicular to wind direction Length < 100 km, height < m, width < 1-3 km Low variability in wind direction In areas of complete sand cover and scarce vegetation Great Sand Dunes National Park, Colorado

36 Longitudinal section Relatively simple stratification with parallel foresets dipping at 35º Reactivation surfaces, signifying periods of strong reversed or transverse wind stoss lee Transverse section Trough-shaped cross-sets in sinuous dunes

37 Foreset orientations of transverse dunes 40º 30º 20º 10º predominant wind direction

38 Longitudinal dunes (Seifs) (sinuous crestlines) Egypt

39 Longitudinal dunes (Seifs) (straight crestlines) bifurcations Groups of straight or sinuous longitudinal ridges, oriented parallel to prevailing wind direction, and bifurcating in upwind direction Length < 100 km, height < 100 m, typically ±60 m wide and ±5 m high Moderate variability in wind direction, possibly bimodal at acute angles Moderate sand supply Sinuous seifs have slip faces on alternating sides Simpson Desert, Australia

40 Two models of seif dynamics 1 3D pattern of paired spiral vortex flow Ground flow lines High-level sand-poor eddies moving into interdune Seifs Convergence of flow Erosion Ground-level sand-laden eddy moving onto dune faces Deposition

41 Two models of seif dynamics 2 Two effective winds at acute angle to one another (ideally º) Oblique flow over dune crest Flow separation on lee of dune with strong along-dune component Dune migration by avalanching, largely in longitudinal direction

42 Stratification wind High angle of incidence (but not perpendicular!) to local crest direction Vertical stack of bimodal large-scale x-stratification... representing avalanche accretion on alternate sides of dune under (seasonally?) changing wind direction

43 Foreset orientations of longitudinal dunes 40º 30º 20º 10º predominant wind direction

44 Fields of unconnected starshaped ridges Length m, height m Highly variable in wind direction ð complicated flow patterns Internal organisation is poorly known, but most probably complex Star dunes

45 Typical distribution of wind directions over star dunes Libya resultant drift direction Is distribution of foreset orientations similar?

46 Ventifacts and desert pavements between barchan dunes and longitudinal dunes (i.e., sediment bypassing-types) Dry conditions: small-scale bedforms/stratification, e.g. wind ripples & small dunes Damp conditions: adhesion warts/ripples Wet conditions: ponds with wave/current ripples, small delta lobes, algal mats Evaporites in ephemeral ponds (inland sabkhas) Plants remains and rootlets may disturb lamination Interdune areas (dune troughs and fringes)

47 Bounding Surfaces An erosional surface within or between sets of cross-strata Simple dune Compound dune (draa with superimposed dunes) interdune surfaces superposition surfaces reactivation surfaces 47

48 Accumulation in A) Dry and B) Wet Aeolian Systems

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50 Mountney, 2006

51 Summary Air vs- H 2 0 Bedform types Stratification Interdune Wet vs- dry Facies Reading: B&D: Chapter 16 Nichols: Chapter 8 Boggs; Eolian sections, Chapter 8 Mountney, New Facies Models Revisited Reading, Chapter 5 Leeder, Chapter 16