Geology Reference
In-Depth Information
Late Holocene
dune reactivation
(4,000 years old)
Early Holocene
interdune lake deposits
(11,000-4,500 years old)
Modern active dunes
(40 years old)
Early Holocene pedogenic zone
(11,000-4,500 years old)
Late Pleistocene dune core
(20,000-13,000 years old)
Figure 14.7
Amalgamated deposits of linear dunes in the Akchar Sand Sea, Mauritania.
Source:
Adapted from Kocurek
et al.
(1991)
Pleistocene sand laid down 20,000 to 13,000 years
ago. When rainfall increased, from 11,000 to
4,500 years ago, vegetation stabilized the dunes, soil for-
mation altered the dune sediments, and lakes formed
between the dunes. Renewed dune formation after
4,000 years ago cannibalized existing aeolian sediments
on the upwind edge of the sand sea. The active crescentic
dunes that cap the older, linear dunes date from the last
40 years.
It is possible that major dune production episodes
relate to Croll-Milankovitch climatic cycles, which
induce swings from glacial to interglacial climates. Gary
Kocurek (1998, 1999) has presented a model relating
the two (Figure 14.8). The key feature of the model
is the interplay of sediment production, sediment avail-
ability, and transport capacity through a humid-arid
cycle. During the humid period, geomorphic processes
produce sediment, but this becomes available only dur-
ing the arid period. The wind is capable of transporting
sediment throughout the cycle, but its transport capac-
ity is higher during the humid phase. The combined
effects of these changes are complex. The humid phase
sees sediment production and storage, with some sedi-
ment influx limited by availability. As the humid phase
gives way to the arid phase, sediment influx increases as
availability increases. It goes on increasing to the peak of
the arid phases as transporting capacity rises to a max-
imal level. As the arid phase starts to decline, the lack
of sediment production leads to sand-starved conditions.
The dune-fields respond to these changes as follows. Dur-
ing the humid phase, the dunes stabilize. As the arid phase
kicks in, dune-building occurs using sediments released
by increased availability and then increased transport
capacity. Once the sediment supply dries up, the dunes
are destroyed. This plausible model requires detailed
field-testing.
The pattern of dunes within sand seas appears
to involve several factors with a historical dimension
(Box 14.2).
Ancient aridity
The distribution of desert climates has shifted during geo-
logical time. Most modern areas of aridity began during
late Tertiary times, and especially in the Mid- to Late
Miocene, as the climates of subtropical regions took on
a modern aspect.
In the more distant past, the geological record of
aeolian sandstones and evaporite deposits furnishes evi-
dence for extensive deserts at several times in the
Earth's history. The oldest aeolian deposits discovered
so far come from Precambrian rocks in the Northwest
Territories of Canada and from India. In Britain, the
oldest aeolian deposits are of Devonian age and were
formed when Britain lay south of the Equator in an
arid and semi-arid palaeoclimatic belt. Remnants of
