2.3 Eolian Sediments
Specific internal structures are often indistinct or
even absent in these dunes. However, dunes growing
episodically may become vegetated and then develop
soils, calcrete or gypcrete in times of stabilization.
Some examples show carbonate nodules originating
from dissolution and reprecipitation of near-surface
carbonate (cf. Sect. 9.1.2).
Clay dunes have not received much attention by
sedimentologists. This and their limited size appears to be
the reason why only a limited number of these deposits
have been identified in the ancient record. Young clay
dunes have been described, e.g., by Bowler (1973), Pye
(1987), Lees and Cook (1991), and Holliday (1997). The
last author deals with late Pleistocene examples from Texas
and New Mexico where the lunettes experienced significant climatic variation between dryer and wetter periods
and therefore grow irregularly. The clay dunes are 4-8 m
high and 200-300 m wide. Their internal stratification
mainly reflects pedogenic horizons and is either
subhorizontal, oblique, or irregular due to phases of partial
erosion and rebuilding. This dunes have been stable in the
late Holocene.
Some ancient, massive, red mudstones, for example in
the Keuper (Upper Triassic) of central Europe, are believed
to represent ancient clay dunes formed in a slowly subsiding, serni-arid basin with frequently switching shallow
ephemeral lakes. In addition, the deposition of eolian dust
(see below) may have contributed to the formation of such
poorly defined deposits.
2.3.7 Eolian Dust, Loess
General Aspects
Eolian dust, termed loess, covers large areas (5-10%)
of the present-day land surface of Asia, Europe,
North and South America, and it contributes significantly to the mass of other, non-eolian sediment
types, including those of the deep sea.
The various aspects of the source areas, transport and deposition, texture, and composition of eolian dust are summarized, e.g., by Yaalon and Dan (1974), Morales (1979), Pye
(1987), and Nickling (1994).
Similarly to sand-size particles, dust particles are entrained into the air by drag and aerodynamic uplift
exerted by turbulent wind currents. The threshold
wind velocities to move both fine sand and silt-sized
dust are on the order of 0.1 to 0.3 mls. Storms capable of entraining and transporting dust commonly
occur several times each year. The deflation surface,
however, must be dry and the dust partieies should
not adhere to each other by cohesion. Even low moisture contents or low concentrations of cementing
salts can significantly raise this threshold velocity
and thus prevent dust transport. Furthermore, the
roughness of the surface plays an important role. A
dense desert pavement resulting from the removal of
sand and dust finally terminates further deflation.
61
While there is little difference in the threshold
velocities for the entrainment of fine sand and dust,
the modes of particle transport and distance of migration of these grain size fractions differ significantly
(cf. Fig. 2.21 b). The finer the dust partic1es are, the
higher and longer can they be transported in the atmosphere. Global wind systems may carry finegrained dust over distances of several thousands of
kilometers and distribute it widely.
Dust mainly consists of silt-size partieies smaller
than about 20 flm; larger grains settle quickly back to
the ground when the turbulence of strong wind decreases. Far-traveled dust particles are srnaller than
10 flm and many are even smaller than 2 flm. Such
dust may remain in the atmosphere for weeks and
then largely accumulate in the oceans. Loess which is
commonly transported shorter distances is composed
mainly of partic1es in the range from 10 to 50 flill.
The dust partic1es may be derived from several
different sources. Mechanical and chemical weathering provide silt and c1ay-sized material. Grinding of
solid rocks by glacial action and fluvial transport can
produce substantial amounts of silt-sized particles. In
desert regions, silt is generated by eolian abrasion of
rocks during saltation of sand grains (cf. Fig. 2.21a).
Finally, volcanoes can eject large amounts of siltsized material (Sect. 2.4). The relative importance of
these various mechanisrns to produce dust partic1es
differs from area to area, reflecting the effects of c1imate, relief, lithology, and geomorphic history. In
regions of cold c1imate, glacial and fluvial abrasion
as well as frost action are most important. Mountain
regions with their high rate of mechanical erosion
(cf. Sect. 9.3) generally provide more silt-sized material than do low-relief areas.
The mineralogical composition of dust also varies
in relation to the source area. Many, relatively
coarse-grained, local dusts are rich in quartz, feldspar, and carbonate minerals. They reflect the composition of nearby source rocks, such as quartz sandstones and carbonate sequences. With increasing distance from the source, however, textural and mineralogical sorting of the dust particles takes place. Consequently, far-traveled dusts tend to be enriched in
fine-grained micas and clay minerals and may contain organic matter such as pollen grains, fungal
spores, seeds, etc. With the aid of such components,
the source of the dust can be determined (e.g.,
Sirocko and Sarnthein 1989).
Dust Deposition
Eolian dust is deposited when the velocity and turbulence of the dust storm wane or the dust partic1es are
washed out from the atmosphere by precipitation.
Another possibility of depositing dust is the coalescence of smaller particles to larger aggregates which
Specific internal structures are often indistinct or
even absent in these dunes. However, dunes growing
episodically may become vegetated and then develop
soils, calcrete or gypcrete in times of stabilization.
Some examples show carbonate nodules originating
from dissolution and reprecipitation of near-surface
carbonate (cf. Sect. 9.1.2).
Clay dunes have not received much attention by
sedimentologists. This and their limited size appears to be
the reason why only a limited number of these deposits
have been identified in the ancient record. Young clay
dunes have been described, e.g., by Bowler (1973), Pye
(1987), Lees and Cook (1991), and Holliday (1997). The
last author deals with late Pleistocene examples from Texas
and New Mexico where the lunettes experienced significant climatic variation between dryer and wetter periods
and therefore grow irregularly. The clay dunes are 4-8 m
high and 200-300 m wide. Their internal stratification
mainly reflects pedogenic horizons and is either
subhorizontal, oblique, or irregular due to phases of partial
erosion and rebuilding. This dunes have been stable in the
late Holocene.
Some ancient, massive, red mudstones, for example in
the Keuper (Upper Triassic) of central Europe, are believed
to represent ancient clay dunes formed in a slowly subsiding, serni-arid basin with frequently switching shallow
ephemeral lakes. In addition, the deposition of eolian dust
(see below) may have contributed to the formation of such
poorly defined deposits.
2.3.7 Eolian Dust, Loess
General Aspects
Eolian dust, termed loess, covers large areas (5-10%)
of the present-day land surface of Asia, Europe,
North and South America, and it contributes significantly to the mass of other, non-eolian sediment
types, including those of the deep sea.
The various aspects of the source areas, transport and deposition, texture, and composition of eolian dust are summarized, e.g., by Yaalon and Dan (1974), Morales (1979), Pye
(1987), and Nickling (1994).
Similarly to sand-size particles, dust particles are entrained into the air by drag and aerodynamic uplift
exerted by turbulent wind currents. The threshold
wind velocities to move both fine sand and silt-sized
dust are on the order of 0.1 to 0.3 mls. Storms capable of entraining and transporting dust commonly
occur several times each year. The deflation surface,
however, must be dry and the dust partieies should
not adhere to each other by cohesion. Even low moisture contents or low concentrations of cementing
salts can significantly raise this threshold velocity
and thus prevent dust transport. Furthermore, the
roughness of the surface plays an important role. A
dense desert pavement resulting from the removal of
sand and dust finally terminates further deflation.
61
While there is little difference in the threshold
velocities for the entrainment of fine sand and dust,
the modes of particle transport and distance of migration of these grain size fractions differ significantly
(cf. Fig. 2.21 b). The finer the dust partic1es are, the
higher and longer can they be transported in the atmosphere. Global wind systems may carry finegrained dust over distances of several thousands of
kilometers and distribute it widely.
Dust mainly consists of silt-size partieies smaller
than about 20 flm; larger grains settle quickly back to
the ground when the turbulence of strong wind decreases. Far-traveled dust particles are srnaller than
10 flm and many are even smaller than 2 flm. Such
dust may remain in the atmosphere for weeks and
then largely accumulate in the oceans. Loess which is
commonly transported shorter distances is composed
mainly of partic1es in the range from 10 to 50 flill.
The dust partic1es may be derived from several
different sources. Mechanical and chemical weathering provide silt and c1ay-sized material. Grinding of
solid rocks by glacial action and fluvial transport can
produce substantial amounts of silt-sized particles. In
desert regions, silt is generated by eolian abrasion of
rocks during saltation of sand grains (cf. Fig. 2.21a).
Finally, volcanoes can eject large amounts of siltsized material (Sect. 2.4). The relative importance of
these various mechanisrns to produce dust partic1es
differs from area to area, reflecting the effects of c1imate, relief, lithology, and geomorphic history. In
regions of cold c1imate, glacial and fluvial abrasion
as well as frost action are most important. Mountain
regions with their high rate of mechanical erosion
(cf. Sect. 9.3) generally provide more silt-sized material than do low-relief areas.
The mineralogical composition of dust also varies
in relation to the source area. Many, relatively
coarse-grained, local dusts are rich in quartz, feldspar, and carbonate minerals. They reflect the composition of nearby source rocks, such as quartz sandstones and carbonate sequences. With increasing distance from the source, however, textural and mineralogical sorting of the dust particles takes place. Consequently, far-traveled dusts tend to be enriched in
fine-grained micas and clay minerals and may contain organic matter such as pollen grains, fungal
spores, seeds, etc. With the aid of such components,
the source of the dust can be determined (e.g.,
Sirocko and Sarnthein 1989).
Dust Deposition
Eolian dust is deposited when the velocity and turbulence of the dust storm wane or the dust partic1es are
washed out from the atmosphere by precipitation.
Another possibility of depositing dust is the coalescence of smaller particles to larger aggregates which