Geology Reference
In-Depth Information
climatic change during the Quaternary period, involv-
ing swings from glacial to interglacial conditions,
have played a key role in influencing sediment sup-
ply, availability, and mobility. Furthermore, different
sand seas may react differently to sea-level and cli-
matic changes. A crucial factor appears to be the
size of the sand source. Where the sand supply is
small, as in the Simpson Desert and the Akchar
sand sea of Mauritania, the chief control on aeolian
accumulation is sediment availability, and sand seas
suffer multiple episodes of dune reworking. Where
sand supply is plentiful, as in the Gran Desierto,
Namib, and Wahiba sand seas, the accumulation of
sand is effectively unlimited and multiple dune gener-
ations are likely to develop. A third possibility, which
applies to the Australian Desert, is where sand accu-
mulation is limited by the transporting capacity of
the wind.
large star dunes have been identified in Devonian
sandstones of Scotland and fossil sand seas in Ireland.
The best-known aeolian sandstones in Britain and north-
ern Europe occur in Permo-Triassic rocks deposited when
Britain had moved north of the Equator and into another
arid climatic zone. The Rotliegendes (Early Permian)
sandstones of the North Sea basin trap oil and gas.
Quarry sections at Durham, England, reveal large linear
mega-dunes with smaller features superimposed. Some
of the Triassic sandstones of Cheshire and Lancashire are
also aeolian deposits.
within oceans, alter it (Box 14.3). Tectonics is the ulti-
mate control of sea level. In the case of tectono-eustasy,
the control is direct. In the case of glacio-eustasy, the con-
trol is indirect: tectonics (and other factors) alter climate
and climate alters sea level.
Sea level fluctuates over all timescales. Medium-term
and long-term changes are recorded in sedimentary
rocks and revealed by the technique of seismic stratig-
raphy (e.g. Vail
et al
. 1977, 1991). This technique
offers a precise means of subdividing, correlating, and
mapping sedimentary rocks. It uses primary seismic
reflections. The reflections come from geological dis-
continuities between stratigraphic units that result from
relative changes of sea level. The discontinuities are litho-
logical transitions caused by abrupt changes in sediment
delivery, and they can be correlated worldwide. They dis-
play six superimposed orders of cyclical sea-level change
during the Phanerozoic aeon (Table 14.5). Each cycle
COASTAL CHANGES
Sea-level change
Sea level seldom remains unchanged for long as changes
in ocean volume, or changes of the distribution of mass
Box 14.3
CAUSES OF SEA-LEVEL CHANGE
Volumetric and mass distribution changes in the oceans
cause sea-level change (Table 14.4). Ocean volume
changes are eustatic or steric.
Eustatic change
results
from water additions or extractions from the oceans
(glacio-eustatic change), and from changes in ocean-
basin volume (tectono-eustatic change).
Steric change
results from temperature or density changes in sea-
water. Much of the predicted sea-level rise during the
twenty-first century will result from the thermal expan-
sion of seawater. Ocean thermal expansion is about
20 cm/
◦
C/1,000 m (Mörner 1994).
Glacio-eustatic change
Glacio-eustatic change is tightly bound to climatic
change. Globally, inputs from precipitation and runoff
