Geology Reference
In-Depth Information
Miocene period. It has subsequently been lowered by
deflation (p. 85), but is partly a relict karst feature
(Albritton
et al.
1990).
To complicate matters even more, buried karst is
sometimes re-exposed through the erosion of the cov-
ering strata to form
exhumed karst
. Near Madoc,
Ontario, Canada, pure dolostones dating from the
Grenville Orogeny, some 977 million years ago, today
form a hilly terrain that is being exhumed from the Late
Cambrian-Lower Ordovician cover rocks. Cone karst
and cockpits, together with lesser dolines and grikes,
have been identified in the exhumed surface (Springer
1983). If the environmental conditions on re-exposure
are favourable, renewed karstification may proceed and
create
rejuvenated karst
. The present upland surface
of the Mendip Hills of Somerset, England, is the
rejuvenated surface of a Triassic island, and some of
the fissures on the Mendips may have been dolines
or cenotes (Ford 1989). Similarly, the Yunnan Stone
Forest (p. 195) started as a rugged tor-and-pediment
topography that was buried by Tertiary sands and clays.
Smooth and rounded pinnacles developed while the
cover was present. Recent re-exposure is sharpening
the pinnacles over an area of 35,000 ha.
are removed by erosion. It is debatable how the exhumed
erosion surface relates to landscape evolution. If a thin
cover has been stripped, then the old erosion surface plays
a large part in the modern topography, but where hun-
dreds or thousands of metres of overlying strata have been
removed the exhumed erosion surface is all but a chance
component part of the modern landscape, much like any
other structural surface (Ollier 1991, 97). The Kimberley
Plateau of Western Australia bears an erosion surface car-
rying striations produced by the Sturtian glaciation some
700 million years ago and then covered by a glacial till.
The thin till was later stripped to reveal the Kimberley
surface, the modern topography of which closely matches
the Precambrian topography and displays the exhumed
striations (Ollier 1991, 24).
The relief differentiation on the Baltic Shield, once
thought to result primarily from glacial erosion, is
considered now to depend on basement-surface expo-
sure time during the Phanerozoic aeon (Figure 15.7;
Plate 15.2a-d). Three basic relief types occur on the
Fennoscandian Shield (Lidmar-Bergström 1999). The
first is the exhumed and extremely flat sub-Cambrian
peneplain, which with sub-Vendian and sub-Ordovician
facets has been the starting surface for all relief upon
the shield (Figure 15.8a). The second is the exhumed
sub-Mesozoic etchplains, which possess an undulating
and hilly relief and vestiges of a kaolinitic saprolite and
Mesozoic cover rocks (Figure 15.8b, c). The third is
a set of plains with residual hills that are the end
result of surface denudation during the Tertiary period
(Figure 15.8). Figure 15.9 depicts the likely evolution
of bedrock relief in southern Sweden. In the late Pre-
cambrian era, denudation reduced the surface of the
Precambrian bedrock to an extremely flat surface,
the sub-Cambrian peneplain, with residual hills only
occurring as exceptions (Figure 15.9a). Starting in the
Cambrian period, the sea transgressed the peneplain
and Cambrian rocks were deposited on the flat surface
and were succeeded by Ordovician-Carboniferous strata
(Figure 15.9b). These cover rocks protected the Precam-
brian basement in southern Sweden from further erosion
for a long time. In the Kattegat area, a thick Palaeozoic
cover accumulated in the Caledonian foreland basin. In
the Permian period, inversion of the Caledonian fore-
land basin removed the Palaeozoic cover rocks in parts of
the Kattegat area and probably in most of south-western
Sweden (Figure 15.9c). Through the Triassic to the early
Cretaceous periods, uplift and erosion continued and
the climate became humid. Kaolinitic weathering pen-
etrated deep into the basement along fracture zones
(etching), producing thick kaolinitic weathering man-
tles (saprolites). By alternating etching and erosion of the
saprolites (stripping), the landscape developed an undu-
lating hilly relief with Palaeozoic remnants still occurring
locally on down-faulted blocks (Figure 15.9d). From the
late Cretaceous to the Mid-Miocene periods, the sea
transgressed across Denmark and large parts of southern
Sweden, covering the area with sediments (Figure 15.9e).
