Geology Reference
In-Depth Information
Table 14.3 Evidence used in reconstructing dune palaeo-environments
Evidence
Explanation
Biological evidence
Dune vegetation
Presence of dune vegetation indicates reduced aeolian activity and dune stabilization
Geomorphological evidence
Dune form
Degraded or wholly vegetated dunes in areas not presently subject to aeolian
activity (with mean annual rainfall less than 250 mm) indicate relict dunes
Dune mobility
A 'dune mobility index' (Lancaster 1988) indicates whether dunes are active or
inactive
a
Dune size
Mega-dunes may form only during sustained high winds, as blew in the tropical
deserts around the peak of the last ice age around 18,000 years ago
Dune dating
Relative or absolute dating techniques may be used to fix the age of a dune,
luminescence dating being a promising approach in environments where
organic remains are very limited
Sedimentological evidence
Granulometric analysis
Standard granulometric measures - mean grain size, sorting (standard deviation),
skewness, and kurtosis - may sometimes be used to distinguish ancient from
modern dunes
Sedimentary structures
Primary structures may be altered or destroyed by processes after deposition, but
may help in identifying past aeolian beds
Grain roundness
Active aeolian sand grains tend to be sub-rounded to sub-angular; ancient sand
grains tend to be more rounded, but roundness also varies with dune type
Palaeosols and carbonate
horizons
When found in aeolian accumulations, these suggest periods of geomorphic
stability and act as useful dating markers
Silt and clay particles
Ancient dunes tend to contain a higher proportion of silt and clay particles than
active dunes
Dune reddening
Ancient dune sediments tend to be redder than modern dune sediments, though
factors determining the redness of sediments are complex and ambiguous
Quartz surface
microfeatures
Scanning electron microscope analysis of sand grains may help to identity aeolian
sediments and to distinguish between different depositional environments
Aeolian dust
May be found in alluvial fans, soils, and marine sediments
Note:
a
The dune mobility index,
M
, is defined as the length of time the wind blows above the threshold velocity for sand
transport (5 m/s),
W
, multiplied by the precipitation-potential evapotranspiration ratio,
P/PE: M
=
W/(P/PE)
. Lancaster
(1988) suggests four classes of dune activity: (1) inactive dunes (
M
<
50
); (2) dune crests only active (
50
<
M
<
100
); (3) dune
crests active, lower windward and slip faces and interdune depressions vegetated (
100
<
M
<
200
); (4) fully active dunes
(
M
200
)
Source:
Adapted from discussion in Tchakerian (1999)
>
years ago. The arid, dune-building phases lasted some
5,000 to 20,000 years, while the intervening humid peri-
ods lasted longer - between 20,000 and 40,000 years.
Figure 14.7 shows the compounded nature of large,
complex, linear dunes in the Akchar Erg, Mauritania
(Kocurek
et al
.
two dune-building (arid) episodes occurred, one between
27,000 and 23,000 years ago and the other between 17
and 10 million years ago. In the north-eastern portion,
four dune-building episodes occurred at the follow-
ing times: 115,000-95,000 years ago, 46,000-41,000
years ago, 26,000-22,000 years ago, and 16,000-9,000
1991). The
dune
core
consists
of
