58
gions, the interdune and backdune areas are covered
by vegetation and the inland migrating sand is soon
trapped and stabilized by moist soils and plants. In
arid regions, the coastal sand can travel far inland,
produce a broad dune belt, and contribute to the formation of large, interior sand seas.
The dune types of coastal sands are principally the
same as those ofthe interior deserts. Many coasts are
characterized by a narrow longitudinal sand ridge of
considerable thickness (mostly up to several tens of
meters). Where the coastal sand ridge is blown away,
transverse, barchanoid, and parabolic dunes may follow farther inland, but it is hardly possible to establish a general rule for the succession of special dune
types. In the more arid zones, barchan dune fields
originating from the coastal sands may pass landward
into large transverse and longitudinal dunes.
The internal structures of coastal dunes resemble
those of inland desert dunes, but tend to be more
complex. They may be affected by storm-wave erosion and flooding of their interdune areas, and locally
they interfinger with fluvial, lake and lagoonal deposits. Dunes along arid co asts often alternate with
coastal sabkhas and may prograde over supratidal
algal mats and salt crusts. In more humid climates,
root horizons and generation of soils may alter the
primary structures and facies of the sands.
Coastal dune sands frequently contain considerable amounts of bioclastic carbonate, or they consist
entirely of carbonate sand derived from various marine organisms. Such sands are readily stabilized due
to the dissolution and reprecipitation of carbonate
near their surface.
Early lithification of carbonate-bearing eolian dunes is
known, for example, from the southem coast of the Mediterranean, from Florida and the Bahamas, or from southem
Australia, where several chains of Quatemary dunes are
aligned parallel to a prograding coast. The older the dunes,
the more they are cemented by carbonate and thus gain a
high preservation potential and protection against truncation, as shown in Fig. 2.22d (see also, e.g., McKee and
Ward 1983).
2.3.4 Large-Scale Facies Associations
with Eolian Sands
Interaction with Fluvial and Playa Sediments
Continental eolian sands may alternate laterally and
vertically with deposits of marginal desert and fluvial
environments. Sand dunes often overlie alluvial fans,
fluvial deposits, or playa sediments and frequently
pass laterally into such sediments deposited in the
interdune or marginal dune areas (Fig. 2.23a; cf.
Sect. 7.7.3). Fluvial flow in interdune areas erodes
dunes, but mayaiso contribute to the preservation of
basal portions of the sand dunes when both fluvial
Chapter 2 Continental Sediments
and eolian aggradation occur successively. Then,
fluvial channel and overbank-interdune deposits may
form elongate, lenticular bodies in between eolian
dunes.
Flooding of interdune areas can create ephemeral
lakes. The lateral interfingering of eolian sands and
interdune calcareous or siliceous lake sediments in
Pleistocene and older sediments is mostly ascribed to
a change to a wetter climate (Fig. 2.23a). Then the
mobile sand dunes are stabilized to some extent by
vegetation and soil formation. During the subsequent
geologic development, however, the higher portions
of the dunes te nd to become eroded by fluvial action.
Thus, in a normally subsiding continental basin, the
thickness ofthe eolian sands preserved is limited.
In some regions, volcanic ash falls are intercalated in
eolian sandstones without any indication of fluvial transport. This situation was observed, for example, in Permian
eolian sandstones of Argentina (Limarino and Spaletti
1986).
Thickness of Eolian Sands
Eolian sands of the modem large sand seas commonly attain thicknesses of several tens to a few hundreds of meters. The average sand thicknesses of
widely extended longitudinal dune fields is usually in
the order of 10 to 20 m.
This is the case, for example, in Australia. One has to consider, however, that these dunes accumulated in the relatively short time period of approximately 30 ka (Wasson
1986; Wopfuer and Twidale 1988). These and many other
modem occurrences of eolian sand are affected significantly by the shift of climatic belts during the Pleistocene.
These shifts diminished rather than enhanced the accumulation of large volumes of eolian sand, because great proportions of the older sands were repeatedly redistributed by
fluvial processes and deposited in other environments, including coastal areas.
Some ancient examples of eolian sediments exhibit
very thick sequences of wind-blown sand. In Europe,
Permian sandstones (Rotliegend) mostly have a
thickness up to a few hundreds of meters, but locally
they reach more than 1000 m, probably due to accentuated synsedimentary subsidence. The Mesozoic
Navajo sandstone ofNorth America and eolian sandstones in China attain several hundred meters in
thickness.
Interaction with Shallow-Marine Sediments
Eolian sands mayaiso interact with coastal and
shallow-marine sediments. In the case of dominant
offshore winds as, for example, along the western
margin of the Sahara desert in Africa, sand dunes,
gions, the interdune and backdune areas are covered
by vegetation and the inland migrating sand is soon
trapped and stabilized by moist soils and plants. In
arid regions, the coastal sand can travel far inland,
produce a broad dune belt, and contribute to the formation of large, interior sand seas.
The dune types of coastal sands are principally the
same as those ofthe interior deserts. Many coasts are
characterized by a narrow longitudinal sand ridge of
considerable thickness (mostly up to several tens of
meters). Where the coastal sand ridge is blown away,
transverse, barchanoid, and parabolic dunes may follow farther inland, but it is hardly possible to establish a general rule for the succession of special dune
types. In the more arid zones, barchan dune fields
originating from the coastal sands may pass landward
into large transverse and longitudinal dunes.
The internal structures of coastal dunes resemble
those of inland desert dunes, but tend to be more
complex. They may be affected by storm-wave erosion and flooding of their interdune areas, and locally
they interfinger with fluvial, lake and lagoonal deposits. Dunes along arid co asts often alternate with
coastal sabkhas and may prograde over supratidal
algal mats and salt crusts. In more humid climates,
root horizons and generation of soils may alter the
primary structures and facies of the sands.
Coastal dune sands frequently contain considerable amounts of bioclastic carbonate, or they consist
entirely of carbonate sand derived from various marine organisms. Such sands are readily stabilized due
to the dissolution and reprecipitation of carbonate
near their surface.
Early lithification of carbonate-bearing eolian dunes is
known, for example, from the southem coast of the Mediterranean, from Florida and the Bahamas, or from southem
Australia, where several chains of Quatemary dunes are
aligned parallel to a prograding coast. The older the dunes,
the more they are cemented by carbonate and thus gain a
high preservation potential and protection against truncation, as shown in Fig. 2.22d (see also, e.g., McKee and
Ward 1983).
2.3.4 Large-Scale Facies Associations
with Eolian Sands
Interaction with Fluvial and Playa Sediments
Continental eolian sands may alternate laterally and
vertically with deposits of marginal desert and fluvial
environments. Sand dunes often overlie alluvial fans,
fluvial deposits, or playa sediments and frequently
pass laterally into such sediments deposited in the
interdune or marginal dune areas (Fig. 2.23a; cf.
Sect. 7.7.3). Fluvial flow in interdune areas erodes
dunes, but mayaiso contribute to the preservation of
basal portions of the sand dunes when both fluvial
Chapter 2 Continental Sediments
and eolian aggradation occur successively. Then,
fluvial channel and overbank-interdune deposits may
form elongate, lenticular bodies in between eolian
dunes.
Flooding of interdune areas can create ephemeral
lakes. The lateral interfingering of eolian sands and
interdune calcareous or siliceous lake sediments in
Pleistocene and older sediments is mostly ascribed to
a change to a wetter climate (Fig. 2.23a). Then the
mobile sand dunes are stabilized to some extent by
vegetation and soil formation. During the subsequent
geologic development, however, the higher portions
of the dunes te nd to become eroded by fluvial action.
Thus, in a normally subsiding continental basin, the
thickness ofthe eolian sands preserved is limited.
In some regions, volcanic ash falls are intercalated in
eolian sandstones without any indication of fluvial transport. This situation was observed, for example, in Permian
eolian sandstones of Argentina (Limarino and Spaletti
1986).
Thickness of Eolian Sands
Eolian sands of the modem large sand seas commonly attain thicknesses of several tens to a few hundreds of meters. The average sand thicknesses of
widely extended longitudinal dune fields is usually in
the order of 10 to 20 m.
This is the case, for example, in Australia. One has to consider, however, that these dunes accumulated in the relatively short time period of approximately 30 ka (Wasson
1986; Wopfuer and Twidale 1988). These and many other
modem occurrences of eolian sand are affected significantly by the shift of climatic belts during the Pleistocene.
These shifts diminished rather than enhanced the accumulation of large volumes of eolian sand, because great proportions of the older sands were repeatedly redistributed by
fluvial processes and deposited in other environments, including coastal areas.
Some ancient examples of eolian sediments exhibit
very thick sequences of wind-blown sand. In Europe,
Permian sandstones (Rotliegend) mostly have a
thickness up to a few hundreds of meters, but locally
they reach more than 1000 m, probably due to accentuated synsedimentary subsidence. The Mesozoic
Navajo sandstone ofNorth America and eolian sandstones in China attain several hundred meters in
thickness.
Interaction with Shallow-Marine Sediments
Eolian sands mayaiso interact with coastal and
shallow-marine sediments. In the case of dominant
offshore winds as, for example, along the western
margin of the Sahara desert in Africa, sand dunes,