Eolian Deposits

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Eolian Deposits

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• Eolian depositional environment
• Eolian bedforms
• Empirically- and physically-derived
  relationships between bedforms and
  environmental conditions
• Use of loess for paleoclimatology

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Eolian

• Sources of sand
• Alluvial fans and ephemeral streams
• Playas
• Sandy deserts (ergs)

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Global Distribution of Eolian Deposits
Dr. Michael Pidwirny, Department of Geography,
Okanagan University College
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Principal bed forms

• Sheet sands
• Streaks
• Dome
• Barchan
• Transverse
• Seif
• Star
• Parabolic

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Bed forms and their geometry

• Name Wavelength Height Orientation
• (m) (m)
• Draa 300-5500 20-450 Longitudinal or transverse
• Dunes 3-600 0.1-100 Longitudinal or transverse
• Ripples 0.15-2.5 2-5 cm Longitudinal or
  transverse

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Nebraska sand hills
(NASA)
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Dune Formation
a. Ripple formation by saltating sand grains. b
(i) The formation of longitudinal dunes by pairs
of vortices the bedding results from migration
of transverse dunes alone the flanks. b(ii) A
Y-junction develops where one pair of vortices
lifts off the ground. c(i, ii) Hypothetical
stages in the development of transverse dunes-
c(iii) A simple transverse pattern tike that of
c(ii) is transformed into a network- of sinuous
ridges by vortices which distort successive
duties. d(1) Airflow around an isolated harchan.
Note that the wave creates a roller in front of
the slip face and that this carries sand to the
arms of the dune- d(ii) The theory of Bagnold
(1941 that seif dunes are formed by elongation
of one arm of a barcban which is extended in the
direction of a stronger wind (these figures are
based on the work of P. Knott and A. Warren.
1979).
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Physics similar to streams

• Water and air are fluids
• But air is less dense (so higher velocities
  required)
• Suspension and saltation are key
• Consider impact threshold

V
diameter
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Dune Factors
Relationships between dune form and the factors
of sand supply, wind strength and vegetation in
the Great Plains ,Unitd States . Llongitudinal,
Ttransverse. and P parabolic forms. After
Hack.1941.
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Grain sizedependence
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Loess

• Definition
• Yellowish-buff unstratified silt, sometimes
  calcareous, well sorted, with a modal grain size
  in the range 0.02-0.5 mm, and with pronounced
  vertical structure

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Global Distribution of Loess Deposits
Dr. Michael Pidwirny, Department of Geography,
Okanagan University College
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Discussion regarding loess

• Chinese loess
• Principal (continuous) sources are the Tibetan
  plateau and Chinese deserts
• Thick deposits that continue over 3 Myrs
• Soil units are developed during times when loess
  flux decreased (but not stopped)
• Chronology
• Luminescence, 14C, ash
• 10Be and magnetic susceptibility
• Atmospheric 10Be and a measurement of the
  magnetization
• Climate
• 10Be and mag. Susceptibility correlate with 18O
  record

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Lithology They are essentially homogeneous, and
the dominance of sand-grade (skewness) sediments
which is generally well sorted, matrix free, and
well rounded. Fossils Eolian sediments to be
palaeontologically barren. Geometry Irregular
plan and cross-section geometries. Sedimentary
structures Large scale cross-bedding of both
tabular planer and less frequent trough are up to
30m. heights and 20-30 dip, and ripple indices (R
I gt 15).
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Paleocurrents Dip directions of cross-bedding
show wide scatters within one out-crops
Sequence and cycles In eolian no, pebbles, clays,
sign of an aqueous biota of either marine or
non-marine, and conglomerate-lined channels
suggestive of running water are absent
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