Geology Reference
In-Depth Information
Figure
8.5.
A retrogressive-thaw-slump, eastern Banks Island, Canada. Rates of headwall retreat
averaged 3.0 m per year between 1972 and 1974.
valleys and are triggered by lateral stream erosion undercutting the steeper bank. Others
are found adjacent to lakes or the coast, and probably owe their initiation to wave action
or ice push. Finally, some appear unrelated to any obvious trigger mechanism and appear
to occur randomly. These may have developed initially from simple active-layer slope
failures. Such failures are not uncommon in permafrost regions (Bird, 1967, pp. 217-220;
Harris and Lewkowicz, 1993a, 2000; Lewkowicz, 1992b) and are associated with the
development of high pore-water pressures after either prolonged summer rain or during
years of exceptionally high winter snowfall and/or rapid and late thawing. The nature of
thaw consolidation, the stability of thawing slopes, and active-layer detachments are dis-
cussed in more detail in Chapter 9.
8.4.3. Ice-Wedge Pseudomorphs and Composite-Wedge Casts
Casts, or pseudomorphs, represent the previous shape of a structure and can be formed
in material other than that which formed the original structure. Ice-wedge pseudomorphs
are wedges of secondary mineral infi lling and are thermokarst structures because they
result from the thaw of excess ice. Composite wedges are slightly different because they
comprise variable proportions and distributions of both ice and clastic sediment (Gozdzik,
1973; Romanovskii, 1973). Thus, they may or may not contain excess ice, and only certain
forms can also be regarded as thermokarst structures.
Ice-wedge pseudomorphs form when the ice in the wedge slowly melts, usually as per-
mafrost degrades. As this happens, there is a general collapse of sediment into the void
that is created. The majority of pseudomorphs described in the literature are from mid-
latitude regions where permafrost no longer exists. These are discussed in detail in Chapter
12. However, ice-wedge pseudomorphs also exist in present-day permafrost environments
(Murton and French, 1993b). The latter provide the best indication of the range of proc-
esses that are involved in their thaw-transformation. These include thermal erosion, col-
lapse, subsidence, refreezing, loading, buoyancy, spreading, folding, and shearing. Typical
pseudomorphs above partially-thawed ice wedges in northern Canada are illustrated in
Figure 8.6. Noteworthy is the bent, U-shaped involution, 60-70 cm deep and
∼
20 cm wide,
