2.3 Eolian Sediments
c=3001H (mla) , where H is the height of the dunes measured in meters (mean value of H=9 m, Samthein and
Walger 1974). In a cross-section perpendicular to the dominating wind direction, a sand volume of approximately
200 m1/a per 1 km width is transported in the form ofbarchan dunes seaward to the West African coast. However,
sand drift on flat bottom is significantly more effective for
transporting sand (on the order of 50 000 m l/a per km
width) in this region.
These values indicate that large volumes of sand reach
the coast !ine and the sea in this region. The giant draas of
large sand seas, on the other hand, mi grate at rates as slow
as 1 to 2 cmla.
2.3.3 Bed Forms and Sedimentary Structures
of Eolian Dunes
Inland Dunes
Accumulating eolian sand generates specific bed
forms which are found more or less in all deserts. As
long as the climate is sufficiently arid (precipitation
less than 100 to 150 mmla), vegetation on sand is
virtually absent and the eolian bed forms are mobile.
Dry, sandy bed forms move, but more or less maintain their shape. The barchan dunes with their two
horns pointing downwind (Fig. 2.22a) represent a
typical example of this behavior. They lose sand on
the windward side and add about the same amount of
sand on the leeside. The dune migrates by this process, but only slowly changes its shape and volume.
Eolian bed forms occur in various shapes and
sizes. Here, only the most important types can be
mentioned. The smallest features are wind ripples on
flat ground (Fig. 2.2Ia). They display a high
lengthlheight ratio (ripple index ~ 15) and commonly
consist of medium to coarse sand, because the finer
sand is sorted out and accumulated in larger dunes.
The largest grains of wind ripples, possibly including
pebble-size grains and heavy minerals, are concentrated along ripple crests. Crossbedding is frequentIy
indistinct. These ripples may alternate with plane or
low-angle bedded sand forming thin, sheet-like sand
bodies around the margins of dune fields or
interdune areas (see below). Climbing ripples which
occur on the inclined surfaces of all types of large
dunes are widespread (Fig. 2.22a). They consist of
fine sand, and their direction of migration (perpendicular to the ripple crests) frequentIy deviates from
the dominant wind direction controlling the movement of larger dunes.
The large dunes tend to be oriented either parallel
(longitudinal) or transverse to the prevailing wind
direction (Fig. 2.22a). Many forms, such as the barchan dunes, however, exhibit transverse (the center
of the barchans) and longitudinal components (the
horns). The less elevated horns migrate faster than
the higher center of the dune. Isolated barchans de55
velop in areas of limited sand supply at the edge of
larger sand seas. They may be arranged in longitudinal or oblique chains. As a result of growing size and
diminishing space between the barchan dunes, these
may pass into more straight-crested transverse dunes.
Parabolic dunes are mainly known from coastal regions and periglacial environments. In contrast to the
barchans, the lower arms of the parabolic dunes tend
to stick to the moist andlor vegetated ground, while
the higher, central part of the dune migrates or is partially blown away.
Longitudinal sand ridges or seif dunes are the
most widespread form in the present-day deserts, for
example in Australia, but their preservation potential
in the ancient record is presumably limited. Seif
dunes develop parallel or subparallel to the dominant
wind or the resultant of two wind directions (Fig.
2.22a). They grow downwind and tend to be spaced
at equal intervals (often 100 to 300 m). Some oftheir
sand may be derived from their interdune areas from
where it is transported by side winds or so-called
helicoidal flow processes to the dune. Seif dunes typically reach several tens of kilometers in length and
are 10 to 30 m high.
The largest eolian sand bodies are draas, i.e., elongate sandy hills of considerable height (up to several
hundreds of meters) and wide spacing (some kilometers). They are typical representatives of giant sand
seas. Because draas eonstitute huge volumes of sand,
they move very slowly and rcquire long time periods
to form (on the order of at least 0.1 Ma). Draas may
be regarded as complex dunes resulting from the coalescence and growing together of other dune types.
A special feature of draas are the so-called star dunes
or rhourds (Fig. 2.22a) with high central peaks and
radiating arms reflecting multidirectional wind patterns.
Sedimentary Structures of Inland Dunes
Eolian sands display various large and small-scale
sedimentary structures. The most conspieuous features are large-scale planar cross beds which characterize transverse dunes and thc central parts of barchans (Fig. 2.22b), but also occur in the other dune
types. The cross beds develop on the lee-side of the
dune crests and dip at angles around 30 degrees. In
the case of trans verse and barchan dunes, their strike
is approximately perpendicular to the dominant wind
direction. The fore sets of the planar cross beds result
from two different processes (Fig. 2.22b):
- Grain fall of sand particles blown over the dune
crest blankets the lee slope. The angle of dip of these
mostly thin, grain-falliaminae is 28 degrees or less.
- A valanching sand flows on steeper slopes create
thicker, irregular foresets. This process is favored by
c=3001H (mla) , where H is the height of the dunes measured in meters (mean value of H=9 m, Samthein and
Walger 1974). In a cross-section perpendicular to the dominating wind direction, a sand volume of approximately
200 m1/a per 1 km width is transported in the form ofbarchan dunes seaward to the West African coast. However,
sand drift on flat bottom is significantly more effective for
transporting sand (on the order of 50 000 m l/a per km
width) in this region.
These values indicate that large volumes of sand reach
the coast !ine and the sea in this region. The giant draas of
large sand seas, on the other hand, mi grate at rates as slow
as 1 to 2 cmla.
2.3.3 Bed Forms and Sedimentary Structures
of Eolian Dunes
Inland Dunes
Accumulating eolian sand generates specific bed
forms which are found more or less in all deserts. As
long as the climate is sufficiently arid (precipitation
less than 100 to 150 mmla), vegetation on sand is
virtually absent and the eolian bed forms are mobile.
Dry, sandy bed forms move, but more or less maintain their shape. The barchan dunes with their two
horns pointing downwind (Fig. 2.22a) represent a
typical example of this behavior. They lose sand on
the windward side and add about the same amount of
sand on the leeside. The dune migrates by this process, but only slowly changes its shape and volume.
Eolian bed forms occur in various shapes and
sizes. Here, only the most important types can be
mentioned. The smallest features are wind ripples on
flat ground (Fig. 2.2Ia). They display a high
lengthlheight ratio (ripple index ~ 15) and commonly
consist of medium to coarse sand, because the finer
sand is sorted out and accumulated in larger dunes.
The largest grains of wind ripples, possibly including
pebble-size grains and heavy minerals, are concentrated along ripple crests. Crossbedding is frequentIy
indistinct. These ripples may alternate with plane or
low-angle bedded sand forming thin, sheet-like sand
bodies around the margins of dune fields or
interdune areas (see below). Climbing ripples which
occur on the inclined surfaces of all types of large
dunes are widespread (Fig. 2.22a). They consist of
fine sand, and their direction of migration (perpendicular to the ripple crests) frequentIy deviates from
the dominant wind direction controlling the movement of larger dunes.
The large dunes tend to be oriented either parallel
(longitudinal) or transverse to the prevailing wind
direction (Fig. 2.22a). Many forms, such as the barchan dunes, however, exhibit transverse (the center
of the barchans) and longitudinal components (the
horns). The less elevated horns migrate faster than
the higher center of the dune. Isolated barchans de55
velop in areas of limited sand supply at the edge of
larger sand seas. They may be arranged in longitudinal or oblique chains. As a result of growing size and
diminishing space between the barchan dunes, these
may pass into more straight-crested transverse dunes.
Parabolic dunes are mainly known from coastal regions and periglacial environments. In contrast to the
barchans, the lower arms of the parabolic dunes tend
to stick to the moist andlor vegetated ground, while
the higher, central part of the dune migrates or is partially blown away.
Longitudinal sand ridges or seif dunes are the
most widespread form in the present-day deserts, for
example in Australia, but their preservation potential
in the ancient record is presumably limited. Seif
dunes develop parallel or subparallel to the dominant
wind or the resultant of two wind directions (Fig.
2.22a). They grow downwind and tend to be spaced
at equal intervals (often 100 to 300 m). Some oftheir
sand may be derived from their interdune areas from
where it is transported by side winds or so-called
helicoidal flow processes to the dune. Seif dunes typically reach several tens of kilometers in length and
are 10 to 30 m high.
The largest eolian sand bodies are draas, i.e., elongate sandy hills of considerable height (up to several
hundreds of meters) and wide spacing (some kilometers). They are typical representatives of giant sand
seas. Because draas eonstitute huge volumes of sand,
they move very slowly and rcquire long time periods
to form (on the order of at least 0.1 Ma). Draas may
be regarded as complex dunes resulting from the coalescence and growing together of other dune types.
A special feature of draas are the so-called star dunes
or rhourds (Fig. 2.22a) with high central peaks and
radiating arms reflecting multidirectional wind patterns.
Sedimentary Structures of Inland Dunes
Eolian sands display various large and small-scale
sedimentary structures. The most conspieuous features are large-scale planar cross beds which characterize transverse dunes and thc central parts of barchans (Fig. 2.22b), but also occur in the other dune
types. The cross beds develop on the lee-side of the
dune crests and dip at angles around 30 degrees. In
the case of trans verse and barchan dunes, their strike
is approximately perpendicular to the dominant wind
direction. The fore sets of the planar cross beds result
from two different processes (Fig. 2.22b):
- Grain fall of sand particles blown over the dune
crest blankets the lee slope. The angle of dip of these
mostly thin, grain-falliaminae is 28 degrees or less.
- A valanching sand flows on steeper slopes create
thicker, irregular foresets. This process is favored by