54
2.3.2 Source, Migration, and Specific Properties
of Eolian Sands
Sand Sources
Various sources supply wind-blown sand:
- Fluvial sands of alluvial fans, ephemeral streams
and flood plains which fall dry part of the year or
longer periods.
- Coastal sands of mixed origin inc1uding biogenic
components.
- Weathering of older sandstones inc1uding former
dune sands.
- Weathering of plutonic and metamorphic rocks.
- Sand-sized silt-c1ay aggregates inc1uding salt particles from deflation pans and playa lakes.
Composition of Sand
Because eolian sands may be derived from different
sources, their composition varies considerably. Many
present-day sand dunes are, however, minera~ogica~ly
fairly mature, i.e., they represent quartz aremtes wlth
minor amounts of feldspar and other weatherable
silicate phases. The thin, platy minerals of mica are
usually rare in eolian sands, because they behave differently during air transport than the other, more
compact mineral grains. At first glance, the predominance of quartz is somewhat surprising because
chemical weathering in arid regions is limited (cf.
Sect. 9.1) and therefore cannot account for the advanced maturity of most of the desert sands. The reason for the minor importance of immature sands is
that many dune sands are not derived from nearby
sources of the desert region itself, but are transported
by rivers from areas of more intensive weathering
into the desert. In other cases, the sands are recyc1ed
from older, more or less mature sandstones. Coastal
sand dunes composed of carbonate grains or special
dune fields consisting of gypsum grains are fed by
nearby primary sources. Their potential for being
preserved in the geologic record or to become recyc1ed is limited.
Color of Eolian Sands
Many recent sand dunes are yellow, light-brown or
reddish in color which is caused by iron hydroxide
impurities, for example limonite on the surface of the
sand grains. Ancient eolian sandstones are commonly
red-colored. The hematite pigmentation of these
rocks usually results from the decomposition of unstable, iron-bearing minerals during diagenesis (cf.
Sect. 6.3). The reddening commonly increases with
Chapter 2 Continental Sediments
the age of the sands. In the Australian dune fields,
the dunes farthest away from the source area exhibit
the most intensive reddening.
Transport and Texture of Eolian Sands
The capacity of winds to transport solid particles of.a
density similar to that of quartz (2.65 g/cm 3 ) IS
mostly limited to fine and medium sand (grain diam~ter 0.125 to 0.5 mm). These size fractions predormnate in coastal sand dunes (mainly fine sand) and
inland sand seas (fine to medium sands). Only very
strong storms (wind velo city on the order of 20 to 30
m/s) can slowly move pebbles and gravel by creep.
These particles are too coarse to be lifted and therefore roll along the surface where they suffer impacts
by saltating sand grains. Silt size partic1es, on the
other hand, are transported very fast (Fig. 2.21; cf.
Sect. 2.3.7) and thus sorted out from sand. Hence,
wind is principally very effective in producing and
accumulating well-sorted sands.
Sorting of sand in eolian dunes and particularly in
interdune areas may, however, be also poor. Each
individual storm event can move a certain population
of grain sizes. Thus, sand beds originating from a
number of storms of varying intensity tend to be less
sorted than individual sand laminae. Relatively finegrained and very well sorted sand is typically found
in coastal dunes, consisting of pre-sorted beach sand,
and on the crests of large inland dunes. Coarser and
less sorted sand is characteristic of dune flanks and
interdune areas.
Due to saltation transport and continued grain impacts, eolian sands become better rounded th~n sands
accumulating in any other environment, but In many
sand dunes not all of the grains are well rounded.
Generally, smaller grains remain angular longer than
larger ones. The grain surface of young eolian sands
commonly displays frosting and small pits cau~ed ?y
grain impacts. Older eolian sands often lose thls dlstinctive criterion due to diagenetic overgrowth and
cementation ofthe pore space (cf. Sect. 13.3).
Wind-blown sands travel with varying velocities.
Sand blown over a flat and smooth surface migrates
fast. As soon as the sand has to overcome obstac1es
of some size and elevation, it tends to settle on the
leeside. The migration of total dune bodies is much
slower than that of single grains. It is a function of
the dune size and sand volume. Small dunes such as
isolated barchans (see below) move relatively fast
(several tens of meters per year), while large sand
dunes and sand seas are fairly stable.
In the trade wind belt of the western Sahara, for example,
the position ofbarchan dunes was repeatedly photographed
from the air, because a railway transporting iron ore from
inland to the coast was endangered by sand dunes. Isolated
dunes migrate with a mean velocity, c, of approximately
2.3.2 Source, Migration, and Specific Properties
of Eolian Sands
Sand Sources
Various sources supply wind-blown sand:
- Fluvial sands of alluvial fans, ephemeral streams
and flood plains which fall dry part of the year or
longer periods.
- Coastal sands of mixed origin inc1uding biogenic
components.
- Weathering of older sandstones inc1uding former
dune sands.
- Weathering of plutonic and metamorphic rocks.
- Sand-sized silt-c1ay aggregates inc1uding salt particles from deflation pans and playa lakes.
Composition of Sand
Because eolian sands may be derived from different
sources, their composition varies considerably. Many
present-day sand dunes are, however, minera~ogica~ly
fairly mature, i.e., they represent quartz aremtes wlth
minor amounts of feldspar and other weatherable
silicate phases. The thin, platy minerals of mica are
usually rare in eolian sands, because they behave differently during air transport than the other, more
compact mineral grains. At first glance, the predominance of quartz is somewhat surprising because
chemical weathering in arid regions is limited (cf.
Sect. 9.1) and therefore cannot account for the advanced maturity of most of the desert sands. The reason for the minor importance of immature sands is
that many dune sands are not derived from nearby
sources of the desert region itself, but are transported
by rivers from areas of more intensive weathering
into the desert. In other cases, the sands are recyc1ed
from older, more or less mature sandstones. Coastal
sand dunes composed of carbonate grains or special
dune fields consisting of gypsum grains are fed by
nearby primary sources. Their potential for being
preserved in the geologic record or to become recyc1ed is limited.
Color of Eolian Sands
Many recent sand dunes are yellow, light-brown or
reddish in color which is caused by iron hydroxide
impurities, for example limonite on the surface of the
sand grains. Ancient eolian sandstones are commonly
red-colored. The hematite pigmentation of these
rocks usually results from the decomposition of unstable, iron-bearing minerals during diagenesis (cf.
Sect. 6.3). The reddening commonly increases with
Chapter 2 Continental Sediments
the age of the sands. In the Australian dune fields,
the dunes farthest away from the source area exhibit
the most intensive reddening.
Transport and Texture of Eolian Sands
The capacity of winds to transport solid particles of.a
density similar to that of quartz (2.65 g/cm 3 ) IS
mostly limited to fine and medium sand (grain diam~ter 0.125 to 0.5 mm). These size fractions predormnate in coastal sand dunes (mainly fine sand) and
inland sand seas (fine to medium sands). Only very
strong storms (wind velo city on the order of 20 to 30
m/s) can slowly move pebbles and gravel by creep.
These particles are too coarse to be lifted and therefore roll along the surface where they suffer impacts
by saltating sand grains. Silt size partic1es, on the
other hand, are transported very fast (Fig. 2.21; cf.
Sect. 2.3.7) and thus sorted out from sand. Hence,
wind is principally very effective in producing and
accumulating well-sorted sands.
Sorting of sand in eolian dunes and particularly in
interdune areas may, however, be also poor. Each
individual storm event can move a certain population
of grain sizes. Thus, sand beds originating from a
number of storms of varying intensity tend to be less
sorted than individual sand laminae. Relatively finegrained and very well sorted sand is typically found
in coastal dunes, consisting of pre-sorted beach sand,
and on the crests of large inland dunes. Coarser and
less sorted sand is characteristic of dune flanks and
interdune areas.
Due to saltation transport and continued grain impacts, eolian sands become better rounded th~n sands
accumulating in any other environment, but In many
sand dunes not all of the grains are well rounded.
Generally, smaller grains remain angular longer than
larger ones. The grain surface of young eolian sands
commonly displays frosting and small pits cau~ed ?y
grain impacts. Older eolian sands often lose thls dlstinctive criterion due to diagenetic overgrowth and
cementation ofthe pore space (cf. Sect. 13.3).
Wind-blown sands travel with varying velocities.
Sand blown over a flat and smooth surface migrates
fast. As soon as the sand has to overcome obstac1es
of some size and elevation, it tends to settle on the
leeside. The migration of total dune bodies is much
slower than that of single grains. It is a function of
the dune size and sand volume. Small dunes such as
isolated barchans (see below) move relatively fast
(several tens of meters per year), while large sand
dunes and sand seas are fairly stable.
In the trade wind belt of the western Sahara, for example,
the position ofbarchan dunes was repeatedly photographed
from the air, because a railway transporting iron ore from
inland to the coast was endangered by sand dunes. Isolated
dunes migrate with a mean velocity, c, of approximately