2.3 Eolian Sediments
occasional rain falls providing an additional load to
the normally dry sand. Due to its moisture content,
the avalanching sand gains some cohesion and may
break up into small blocks or display contortions.
These features are distinctive for eolian sands, because loose sand under water cannot develop such
structures. The slip planes of the sand flows are inclined up to about 35 degrees.
- Several minor features may be superimposed on the
dominant, large-scale cross beds (Fig. 2.22b). Climbing ripples caused by various wind directions migrate
with the dominant wind to the dune crest and provide
sand for grain fall lamination and sand flow foresets.
Additional climbing ripples result from aerodynamic
eddies behind the dune crest or from wind reversals.
They move upward on the lower leeside slope of the
dune and create a characteristic type of cross-bedding
which is frequently preserved in ancient eolian sandstones. Further climbing ripples of differing orientation can develop on both the windward and leeside
slope of the dune. Overall, climbing ripple stratification can distinctly contribute to the structures of
large-scale bed forms. Strong wind reversals may
blow away sand on the lee-slope and allow sand accumulation on the gentIer windward side of the dune.
An interdune area which is overridden by trans verse
or barchan dunes thus typically exhibits the following vertical succession of structures (from top to bottom):
Climbing ripple larnination and plane bed lamination near the dune crest.
Laige-scale fore sets of grain-fall lamination and
sand flow.
Wind-ripple bedding and parallel to sub- parallel
laminae sticking to the flat ground due to adhesion
caused by soil moisture and sparse vegetation (adhesion ripples, adhesion laminae).
In places: lag sediment (desert pavement) where
sand and dust is blown away.
This dune sequence is commonly incomplete in the
ancient record for two reasons. (1) A migrating dune
can truncate its own deposits such that only the lowermost part of the dune is preserved. (2) Large dune
fields may be eroded by wind action down to the capillary fringe of the (climatically fluctuating) groundFig. 2.22. a Most important dune forms in relation to
prevailing wind directions. From right to left increase
in sand supply and volume of sand accumulation, but
decrease in the velocity of dune migration. Playa lake
provides sand-size clay aggregates, carbonate, and
evaporites (e.g. gypsum) which make up part of the
neighboring dunes. (Based on various sources, e.g.,
Cooke and Warren 1973; McKee 1979). b Large
transverse or barchan dune with unidirectional grain57
water table, where the sand is stabilized by adhesion
(Fig. 2.22d). In this case, widely extended, approximately horizontal truncation surfaces are generated
("Stokes" surfaces, cf. Sect. 7.7.3). Particularly the
dip of the preserved large-scale cross beds is used to
deterrnine paleo-wind directions and to identify the
former dune type. Transverse dunes show the least
scatter of wind directions, whereas longitudinal
dunes tend to display a bimodal distribution of paleowind pattern (see below).
The sedimentary structures of longitudinal dunes
and draas are complex as a result of more than one
dominant wind direction and the coalescence of different basic dune types to these sand bodies. Our
knowledge about the structures of compound dunes
is limited, because it is difficult to study these phenomena in recent examples consisting of dry loose
sand. Longitudinal dunes commonly exhibit two or
three large-scale cross beds dipping laterally from the
dune crest (Fig. 2.22b). Frequently, the upper part of
these foresets is truncated by subsequent lateral wind
action. In this manner, typical wedge-shaped planar
cross beds are generated. In addition, these dunes
may develop all the minor features previously mentioned for barchan and transverse dunes. However,
only the lowermost portions of the dune, if any, are
normally preserved. The huge draas and star dunes
are assumed to produce particularly thick sets of
cross beds (up to several tens of meters in height)
which are also known from ancient eolian sandstones.
Finally it should be mentioned that eolian sands
may display the imprints of rain drops and various
biogenic structures. Burrows and traces of small animals, particularly arthro- pods, and molds of plant
roots an: common in present-day examples, and they
were frequently found in ancient eolian sandstones.
Even footprints of large animals were found in such
rocks.
Coastal Dunes
Coastal sand dunes normally do not reach the same
large extent as many inland sand seas. They develop
along both arid and humid coast lines. Their sand is
derived from the beach zone and blown inland by
prevailing onshore winds (Fig. 2.23a). In humid refall lamination and sand-flow cross bedding, superimposed by small-scale climbing ripple lamination
caused by various subordinate, wind directions.
(Modified from Hunter 1981). c Hypothetical cross
section of longitudinal dune. (After McKee 1979).
d Truncation of older dune at capillary fringe of
groundwater table and prograding new dune over
erosion surface
occasional rain falls providing an additional load to
the normally dry sand. Due to its moisture content,
the avalanching sand gains some cohesion and may
break up into small blocks or display contortions.
These features are distinctive for eolian sands, because loose sand under water cannot develop such
structures. The slip planes of the sand flows are inclined up to about 35 degrees.
- Several minor features may be superimposed on the
dominant, large-scale cross beds (Fig. 2.22b). Climbing ripples caused by various wind directions migrate
with the dominant wind to the dune crest and provide
sand for grain fall lamination and sand flow foresets.
Additional climbing ripples result from aerodynamic
eddies behind the dune crest or from wind reversals.
They move upward on the lower leeside slope of the
dune and create a characteristic type of cross-bedding
which is frequently preserved in ancient eolian sandstones. Further climbing ripples of differing orientation can develop on both the windward and leeside
slope of the dune. Overall, climbing ripple stratification can distinctly contribute to the structures of
large-scale bed forms. Strong wind reversals may
blow away sand on the lee-slope and allow sand accumulation on the gentIer windward side of the dune.
An interdune area which is overridden by trans verse
or barchan dunes thus typically exhibits the following vertical succession of structures (from top to bottom):
Climbing ripple larnination and plane bed lamination near the dune crest.
Laige-scale fore sets of grain-fall lamination and
sand flow.
Wind-ripple bedding and parallel to sub- parallel
laminae sticking to the flat ground due to adhesion
caused by soil moisture and sparse vegetation (adhesion ripples, adhesion laminae).
In places: lag sediment (desert pavement) where
sand and dust is blown away.
This dune sequence is commonly incomplete in the
ancient record for two reasons. (1) A migrating dune
can truncate its own deposits such that only the lowermost part of the dune is preserved. (2) Large dune
fields may be eroded by wind action down to the capillary fringe of the (climatically fluctuating) groundFig. 2.22. a Most important dune forms in relation to
prevailing wind directions. From right to left increase
in sand supply and volume of sand accumulation, but
decrease in the velocity of dune migration. Playa lake
provides sand-size clay aggregates, carbonate, and
evaporites (e.g. gypsum) which make up part of the
neighboring dunes. (Based on various sources, e.g.,
Cooke and Warren 1973; McKee 1979). b Large
transverse or barchan dune with unidirectional grain57
water table, where the sand is stabilized by adhesion
(Fig. 2.22d). In this case, widely extended, approximately horizontal truncation surfaces are generated
("Stokes" surfaces, cf. Sect. 7.7.3). Particularly the
dip of the preserved large-scale cross beds is used to
deterrnine paleo-wind directions and to identify the
former dune type. Transverse dunes show the least
scatter of wind directions, whereas longitudinal
dunes tend to display a bimodal distribution of paleowind pattern (see below).
The sedimentary structures of longitudinal dunes
and draas are complex as a result of more than one
dominant wind direction and the coalescence of different basic dune types to these sand bodies. Our
knowledge about the structures of compound dunes
is limited, because it is difficult to study these phenomena in recent examples consisting of dry loose
sand. Longitudinal dunes commonly exhibit two or
three large-scale cross beds dipping laterally from the
dune crest (Fig. 2.22b). Frequently, the upper part of
these foresets is truncated by subsequent lateral wind
action. In this manner, typical wedge-shaped planar
cross beds are generated. In addition, these dunes
may develop all the minor features previously mentioned for barchan and transverse dunes. However,
only the lowermost portions of the dune, if any, are
normally preserved. The huge draas and star dunes
are assumed to produce particularly thick sets of
cross beds (up to several tens of meters in height)
which are also known from ancient eolian sandstones.
Finally it should be mentioned that eolian sands
may display the imprints of rain drops and various
biogenic structures. Burrows and traces of small animals, particularly arthro- pods, and molds of plant
roots an: common in present-day examples, and they
were frequently found in ancient eolian sandstones.
Even footprints of large animals were found in such
rocks.
Coastal Dunes
Coastal sand dunes normally do not reach the same
large extent as many inland sand seas. They develop
along both arid and humid coast lines. Their sand is
derived from the beach zone and blown inland by
prevailing onshore winds (Fig. 2.23a). In humid refall lamination and sand-flow cross bedding, superimposed by small-scale climbing ripple lamination
caused by various subordinate, wind directions.
(Modified from Hunter 1981). c Hypothetical cross
section of longitudinal dune. (After McKee 1979).
d Truncation of older dune at capillary fringe of
groundwater table and prograding new dune over
erosion surface