Page 132 - Sedimentology and Stratigraphy

Aeolian Bedforms 119

$
$
%
%
%
%
% &
%
%

% %

$
%
% & % '
& &

%

% $

%
Fig. 8.4 Aeolian ripples form by sand grains saltating: finer grains are winnowed from the crests creating a slight inverse
grading between the trough and the crest of the ripple that may be preserved in laminae.

of sand and as the sand supply decreases there is a
transition to barchan dunes, which are lunate struc-
tures with arcuate slip faces forming trough cross-
bedding. Under circumstances where there are two
prominent wind directions at approximately 908 to
each other, linear or seif dunes form. The deposits
of these linear dunes are characterised by cross-bedding
reflecting avalanching down both sides of the dune and
hence oriented in different directions. In areas of multi-
ple wind directions star dunes have slip faces in many
orientations and hence the cross-bedding directions
display a similar variability.
There are circumstances in which the whole dune
bedform is preserved but more commonly the upper
Fig. 8.5 Aeolian ripples in modern desert sands: part of the dune is removed as more aeolian sand is
the pen is 18 cm long.
deposited in an accumulating succession. The size of
the set of cross-beds formed by the migration of aeolian
the lee slope, but they will usually be reworked dunes can vary from around a metre to ten or twenty
from the upper slope by avalanching: some may metres (Fig. 8.11). Such large scale cross-bedding is
be preserved at the toe bedded with grain flow common in aeolian deposits but is seen less frequently
deposits. in subaqueous sands, which are typically cross-bedded
The orientation and form (planar or trough) of the in sets a few tens of centimetres to metres thick.
cross-bedding will depend on the type of dune
(Figs 8.9 & 8.10) (McKee 1979; Wasson & Hyde
1983). Planar cross-beds will form by the migration 8.4.3 Draa bedforms
of transverse dunes, straight-crested forms aligned
perpendicular to the prevailing wind direction. Trans- When an erg is viewed from high altitudes in aerial
verse dunes form where there is an abundant supply photographs or satellite images, it is possible to see a

%

$$
%
$
&
$

%
% &
%

$
%
Fig. 8.6 Aeolian dunes migrate as sand & &
% &
$

&

$
%
'
(
%
&
blown up the stoss (upwind) side is either
% %
%

blown off the crest to fall as grainfall on &
% %

$
the lee side or moves by grain flow down
the lee slope.