Geology Reference
In-Depth Information
Table 15.2 Lester King's global planation cycles and their recognition
Cycle
Old name
New name
Recognition
I
Gondwana
Gondwana planation
Jurassic, only rarely preserved
II
Post-Gondwana
'Kretacic' planation
Early mid-Cretaceous
III
African
Moorland planation
Late Cretaceous to mid Cenozoic. Planed
uplands with no trees and poor soils
IV
Rolling land-surface
Mostly Miocene. Undulating country above
incised valleys
V
Post-African
Widespread landscape
Pliocene. The most widespread global
planation cycle. Found mainly in basins,
lowlands, and coastal plains, and not in
uplifted mountain regions
VI
Congo
Youngest cycle
Quaternary. Represented in deep valleys
and gorges of the main rivers
Source:
Adapted from Ollier (1991, 92)
leaving deep ferrallitic soils and weathering profiles on
the continents. Rhexistatic conditions are triggered by
bouts of tectonic uplift and lead to the stripping of the
ferrallitic soil cover, the headward erosion of streams,
and the flushing out of residual quartz during entrench-
ment. Intervening plateaux become desiccated owing to
a falling water table, and duricrusts form. In the oceans,
red beds and quartz sands are deposited.
Later reincarnations of the stability-instability model
take account of regolith, tectonics, sedimentation, and
sea-level change. A
cratonic regime model
, based on
studies carried out on the stable craton of Western
Australia, envisaged alternating planation and transgres-
sion occurring without major disturbance for periods
of up to a billion years (Fairbridge and Finkl 1980).
During this long time, a
thalassocratic regime
(corre-
sponding to Erhart's biostasy and associated with high
sea levels) is interrupted by short intervals dominated
by an
epeirocratic regime
(corresponding to Erhart's
rhexistasy and associated with low sea levels). The alter-
nations between thalassocratic and epeirocratic regimes
may occur every 10-100 million years. However, more
frequent alternations have been reported. A careful study
of the Koidu etchplain in Sierra Leone has shown that
interruptions mirror environmental changes and occur
approximately every 1,000-10,000 years (Thomas and
Thorp 1985).
A variant of the cratonic regime model explains the
evolution of many Australian landscape features (Twidale
1991, 1994). An Early Cretaceous marine transgression
flooded large depressed basins on the Australian land
mass (Figure 15.12). The transgression covered about
45 per cent of the present continent. The new submarine
basins subsided under the weight of water and sediment.
Huge tracts of the Gondwanan landscape were preserved
beneath the unconformity. Hinge lines (or fulcra) would
have formed near shorelines. Adjacent land areas
would have been uplifted, raising the Gondwanan
palaeoplain, and basin margins warped and faulted. Parts
of this plain were preserved on divides as palaeoplain
remnants. Other parts were dissected and reduced to low
relief by rivers graded to Cretaceous shorelines. Subse-
quent erosion of the Cretaceous marine sequence margins
has exhumed parts of the Gondwanan surface, which is
an integral part of the present Australian landscape.
Evolutionary geomorphology
The non-actualistic system of land-surface history known
as
evolutionary geomorphology
(Ollier 1981, 1992)
makes explicit directional change in landscape devel-
opment. The argument runs that the land surface has
changed in a definite direction through time, and has
not suffered the 'endless' progression of erosion cycles
