60
drifting sand, and dust are blown into the Atlantic by
trade winds (Fig. 2.23a). The eolian sands are redistributed by waves, longshore and tidal currents, and
generate a seaward prograding sandy coastal belt.
The interdune area of seaward migrating, longitudinal dunes may be occupied for some time by
hypersaline marine bays or lagoons.
Tidal and supratidal flats bordering coastal sabkhas are
characterized by sediments containing high proportions of
eolian sand, inciuding quartz-sandy algal mats and
stromatolites (Schwarz et al. 1975). The present-day shelf
sediments off west Africa, which accumulated since the
Holocene transgression, contain high proportions of eolian
sand and dust (Einseie ct al. 1977). A situation similar to
the Pleistocene marine sediments on the west African shelf
is assumed for Permian marine sandstones and siltstones in
Texas and New Mexico (Fischer and Samthein 1988).
Short-period, high sea levels, flooding the interdune
areas, may lead to the deposition of thin, shallowmarine, ca1careous sands in between large dunes
(Fig. 2.23a). A particularly high, long-persisting sea
level, however, will cause dune erosion and liquefied
sand flows, and shift the entire coastline landward.
Then the basal marine bed may consist of a massive,
fine grained sands tone derived from the pre-existing,
planated eolian dunes, as for example observed in the
upper Jurassic of Utah, North America (Eschner and
Kocurek 1986). The prcservation of the dune topography may be better with a very rapid, low-energy
transgression, when the dunes were already cemented
by evaporites or carbonate.
During sea level fall, the coastline moves toward
the shelf edge, enabling the eolian sand to migrate
over the emerged shelf. If the sand reaches the head
of submarine canyons, it is trapped, easily liquefied,
and transported by turbidity currents into deeper water.
Deep-sea turbidites consisting of eolian sand have been
described from the eastem Atlantic off the Saharan desert
(Samthein and Diester-Haass 1977).
2.3.5 Distinctive Features of Eolian Sandstones
(Summary)
The characteristic features of modem and ancient
eolian sands and sandstones are summarized as follows:
(1) Features observed in single outcrops.
- Most striking are large-scale sets of steep, truncated tabular cross beds and planar-wedge cross
beds, I to > 10m thick. The cross bed sets are either
vertically stacked without muddy intercalations (large
dunes), or they alternate with thin, flat-bedded or
rippled interdune deposits. The dips of cross beds
Chapter 2 Continental Sediments
frequently indicate varying paleo-wind directions and
wind reversals.
- Minor distinctive features include avalanching and
brecciation of individual cross laminae, and an abundance of climbing ripples with a high length/height
ratio, imprints of rain drops, desert pavement, faceted
gravel, and other ventifacts.
- Occurrence of plant roots, burrows and tracks of
animals living in the desert or in coastal dune environments.
(2) Lateral and vertical facies transitions.
- Occurrence of broad, widely extended deflation
surfaces.
- Alternation with and lateral transition into playa
and sabkha deposits, fluvial and alluvial fan systems.
- Great areal extent of interior sand seas, but more
elongate geometry of coastal dunes.
- Lateral transition of coastal dunes or seaward migrating inland dunes to the shore zone and shaHowmarine environments.
(3) Texture and composition of eolian sands.
- Mostly fine to medium-grained sand, frequently
well sorted, fairly to weH rounded, frosted grain surfaces of modem eolian sands.
- MineralogicaHy immature to fairly mature, depending on the sand source and recycling of sand.
- Scarcety of mica; silt and clay are found only in the
interdune area.
- Yellowish, brown, or red staining.
In spite of this considerable number of criteria, thin
eolian sand layers of limited extent, intercalated in
fluvial or other deposits, are frequently overlooked
and sometimes difficult to identify.
2.3.6 Clay Dunes
Clay dunes or lunettes occur along the margin of seasonally drying shallow lakes in semi-arid climates,
for example in North Africa (Tunesia), North Amcrica, and South Australia. Mud from the lake bed is
either entrained by waves and redeposited in emergent shoals or, more frequently, the drying mud and
salt in the lake crinkles and forms silt and sand-sized
pellets by aggregation. These consist of clay, silt,
some sand, and carbonate and/or gypsum and other
salts. The pellets are blown away from the lake surface to the downwind lake margin. The larger clay
pellets usually accumulate close to the lake in the
form of flat, parabolic dunes; finer pellets are dispersed more widely. After rainfall, the clay dunes
become stabilized as a result of cohesion between the
wet clay and silt particles. Hence, no further migration of these dunes can take place, but they may grow
larger and higher in subsequent dry seasons or after
longer periods of non-deposition.
drifting sand, and dust are blown into the Atlantic by
trade winds (Fig. 2.23a). The eolian sands are redistributed by waves, longshore and tidal currents, and
generate a seaward prograding sandy coastal belt.
The interdune area of seaward migrating, longitudinal dunes may be occupied for some time by
hypersaline marine bays or lagoons.
Tidal and supratidal flats bordering coastal sabkhas are
characterized by sediments containing high proportions of
eolian sand, inciuding quartz-sandy algal mats and
stromatolites (Schwarz et al. 1975). The present-day shelf
sediments off west Africa, which accumulated since the
Holocene transgression, contain high proportions of eolian
sand and dust (Einseie ct al. 1977). A situation similar to
the Pleistocene marine sediments on the west African shelf
is assumed for Permian marine sandstones and siltstones in
Texas and New Mexico (Fischer and Samthein 1988).
Short-period, high sea levels, flooding the interdune
areas, may lead to the deposition of thin, shallowmarine, ca1careous sands in between large dunes
(Fig. 2.23a). A particularly high, long-persisting sea
level, however, will cause dune erosion and liquefied
sand flows, and shift the entire coastline landward.
Then the basal marine bed may consist of a massive,
fine grained sands tone derived from the pre-existing,
planated eolian dunes, as for example observed in the
upper Jurassic of Utah, North America (Eschner and
Kocurek 1986). The prcservation of the dune topography may be better with a very rapid, low-energy
transgression, when the dunes were already cemented
by evaporites or carbonate.
During sea level fall, the coastline moves toward
the shelf edge, enabling the eolian sand to migrate
over the emerged shelf. If the sand reaches the head
of submarine canyons, it is trapped, easily liquefied,
and transported by turbidity currents into deeper water.
Deep-sea turbidites consisting of eolian sand have been
described from the eastem Atlantic off the Saharan desert
(Samthein and Diester-Haass 1977).
2.3.5 Distinctive Features of Eolian Sandstones
(Summary)
The characteristic features of modem and ancient
eolian sands and sandstones are summarized as follows:
(1) Features observed in single outcrops.
- Most striking are large-scale sets of steep, truncated tabular cross beds and planar-wedge cross
beds, I to > 10m thick. The cross bed sets are either
vertically stacked without muddy intercalations (large
dunes), or they alternate with thin, flat-bedded or
rippled interdune deposits. The dips of cross beds
Chapter 2 Continental Sediments
frequently indicate varying paleo-wind directions and
wind reversals.
- Minor distinctive features include avalanching and
brecciation of individual cross laminae, and an abundance of climbing ripples with a high length/height
ratio, imprints of rain drops, desert pavement, faceted
gravel, and other ventifacts.
- Occurrence of plant roots, burrows and tracks of
animals living in the desert or in coastal dune environments.
(2) Lateral and vertical facies transitions.
- Occurrence of broad, widely extended deflation
surfaces.
- Alternation with and lateral transition into playa
and sabkha deposits, fluvial and alluvial fan systems.
- Great areal extent of interior sand seas, but more
elongate geometry of coastal dunes.
- Lateral transition of coastal dunes or seaward migrating inland dunes to the shore zone and shaHowmarine environments.
(3) Texture and composition of eolian sands.
- Mostly fine to medium-grained sand, frequently
well sorted, fairly to weH rounded, frosted grain surfaces of modem eolian sands.
- MineralogicaHy immature to fairly mature, depending on the sand source and recycling of sand.
- Scarcety of mica; silt and clay are found only in the
interdune area.
- Yellowish, brown, or red staining.
In spite of this considerable number of criteria, thin
eolian sand layers of limited extent, intercalated in
fluvial or other deposits, are frequently overlooked
and sometimes difficult to identify.
2.3.6 Clay Dunes
Clay dunes or lunettes occur along the margin of seasonally drying shallow lakes in semi-arid climates,
for example in North Africa (Tunesia), North Amcrica, and South Australia. Mud from the lake bed is
either entrained by waves and redeposited in emergent shoals or, more frequently, the drying mud and
salt in the lake crinkles and forms silt and sand-sized
pellets by aggregation. These consist of clay, silt,
some sand, and carbonate and/or gypsum and other
salts. The pellets are blown away from the lake surface to the downwind lake margin. The larger clay
pellets usually accumulate close to the lake in the
form of flat, parabolic dunes; finer pellets are dispersed more widely. After rainfall, the clay dunes
become stabilized as a result of cohesion between the
wet clay and silt particles. Hence, no further migration of these dunes can take place, but they may grow
larger and higher in subsequent dry seasons or after
longer periods of non-deposition.