Geology Reference
In-Depth Information
Figure 15.11
Cross-section across the Saint Elzéar region. The palaeosurface, which has been exhumed by erosion, can be
seen to the south-south-east of the Garin Scarp cutting across Ordovician and Silurian rocks.
Source:
Adapted from Jutras and Schroeder (1999)
years ago, a time spanning the Permian and Jurassic peri-
ods. The planation surface, which cuts horizontally across
all geological structures, has suffered little dissection
(Figure 15.11). The exhumation of the surface must also
have begun by the Jurassic period following the
en bloc
uplift of the evolving Atlantic Ocean's passive margins.
Some geomorphic features on the exhumed palaeosurface
are guides to Carboniferous palaeoenvironments and tec-
tonics in the area. The Saint Elzéar planation surface is
separated from the uplands of the Gaspesian Plateau -
a higher planation surface formed in the same forma-
tions - by the 200-300-m-high Garin Scarp. So far as
is known, four processes could have produced an ero-
sion surface bounded by a scarp: faulting, etching and
double planation, rock pedimentation controlled by dif-
ferential erosion, and coastal erosion by a transgressive
sea. Pierre Jutras and Jacques Schroeder (1999) favour
the latter process and interpret the erosion surface as
a wide wave-cut platform produced by the Windsor
transgression. They interpret the Garin Scarp as an old
sea-cliff.
but it looks to be a not insignificant figure. In Australia,
Gondwanan land surfaces constitute 10-20 per cent of
the contemporary cratonic landscape (Twidale 1994).
An important implication of all this work is that
some landforms and their associated soils can survive
through various climatic changes when tectonic con-
ditions permit. A problem arises in accounting for
the survival of these
palaeoforms
. Most modern geo-
morphological theory would dictate that denudational
processes should have destroyed them long ago. It is
possible that they have survived under the exceptional
circumstance of a very long-lasting arid climate, under
which the erosional cycle takes a vast stretch of time to
run its course (Twidale 1976, 1998, 1999). A contro-
versial explanation is that much of the Earth's surface
is, in geomorphic terms, rather inactive: the ancient
landscape of south-eastern Australia, rather than being
an exceptional case, may be typical of Africa and, to a
lesser extent, Eurasia and the Americas (e.g. Young 1983;
Twidale 1998).
Two related mechanisms might explain stagnant parts
of landscapes (cf. Twidale 1999). The first mechanism is
unequal erosion
. Some parts of landscapes are more sus-
ceptible of erosion than are other parts (cf. Brunsden and
Thornes 1979). Mobile, fast-responding parts (rivers,
some soils, and beaches) erode readily. They quickly
Stagnant landscapes
Just what proportion of the Earth's land surface pre-
dates the Pleistocene epoch has yet to be ascertained,
