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channel through deflation is not impeded. The third condition is a regime of strong,
unidirectional winds to allow the mobilisation and transport of sand from the dry river
channel to form linear dunes downwind of the channel. In regard to the first condi-
tion, for the dunes to develop and continue to extend downwind, the alluvial sand
supply needs to be regularly replenished. We discuss the relationship between Qua-
ternary fluvial activity, climate and source-bordering dune formation in greater detail
in
Section 8.15
.
8.7 Lunettes and clay dunes
Lunettes are another form of source-bordering dune. However, they are invariably
associated with desert lakes rather than river channels, and they occur as transverse
dunes located on the downwind margin of playa lakes. They owe their original French
name to their crescent shape, with the concave margin facing upwind, in contrast to
barchan dunes in which the concave margin lies to leeward. Initially widely recog-
nised in north-west Africa (Boulaine,
1954
), they are very common in the drier
regions of south-east and south-west Australia, as well as the area adjoining the
desert pans in southern Africa (Shaw and Thomas,
1989
). The lithology of lunettes
can be highly variable, ranging from sand to clay to gypsum (Coque,
1962
;Bowler,
1973
; Benazzouz,
1986
; Shaw and Thomas,
1989
; Williams et al.,
1991
). The clay is
present in pelletal form, and the sand may contain wind-blown charcoal and even the
calcareous oogonia of charophytes (Williams et al.,
1991
).
Bowler (
1973
) proposed a simple model to account for polygenic lunettes, such as
those characteristic of late Pleistocene Lake Mungo in semi-arid western New South
Wales, in which a basal unit of sand is capped by finely laminated pelletal clay. The
sand is blown from beach sands laid down during times of high lake level. Once the
lake level drops, the fine-grained lake floor silts and clays become exposed to seasonal
deflation during the hot dry summers, and the loose aggregates, formed as the saline
mud dries out, are transported as sand-sized particles to form thin sheets of clay
(
Figure 8.7
). Lunettes can therefore provide a detailed history of transgressions and
regressions in their parent lake (Williams et al.,
1991
; Dutkiewicz and von der Borch,
1995
;Bowler,
1998
; Bowler and Price,
1998
; Dutkiewicz and von der Borch,
2002
)
and are especially amenable to dating using luminescence techniques (Dutkiewicz
and Prescott,
1997
; Dutkiewicz et al.,
2002
).
8.8 Paleomagnetic and luminescence dating of eolian sands and silts
There have been a number of attempts to date times of dune activity using thermolu-
minescence (TL) (Stokes et al.,
1997
; Huntley and Prescott,
2001
; Singhvi et al.,
2010
)
and, more recently, optically stimulated luminescence (OSL) dating methods (Stokes
et al.,
1998
; Teeuw and Rhodes,
2004
; Tchakerian,
2009
; Williams et al.,
2010b
).
