Geology Reference
In-Depth Information
in origin. However, some are certainly pits dug by man (Prince, 1961). The possibility has
also been raised that the Breckland Meres of eastern England may be of a thaw-lake origin
(Sparks et al., 1972). In North America, numerous enclosed depressions (“spungs”) in
southern New Jersey were fi rst interpreted as periglacial “frost-thaw” basins (Wolfe,
1953), then as “pingo scars” (Bonfi glio and Cresson, 1982), and more recently as cold-
climate wind-defl ation hollows (French and Demitroff, 2001). Generally speaking, the
separation of thermokarst depressions from the multitude of enclosed depressions of all
sizes and shapes of other origins seems almost impossible.
12.3.2. Paleo-Thaw Layers
Past thermokarst activity may be recognised in the stratigraphic record by the existence
of a paleo-thaw layer (i.e. a horizon corresponding to the depth to which previous thawing
of permafrost had proceeded). In present permafrost regions, paleo-thaw layers often
correspond to secondary thaw unconformities. These have been described in Chapter 6,
in the context of present permafrost environments, in Chapter 11, in the context of relict
permafrost, and earlier in this chapter in the context of the paleo-permafrost table.
Paleo-thaw layers have been described from perennially-frozen sediments in several
areas of the western North American Arctic. Involuted structures (“thermokarst involu-
tions”) are a diagnostic feature of these layers in addition to isotopic differences in the
ground ice that is present both above and below the actual unconformity. If similar to a
paleo-permafrost table (see earlier), mineralogical and other weathering differences
should also be present in and above a paleo-thaw layer.
Late-Pleistocene paleo-thaw layers have been inferred from studies at a number of
localities in the lowlands of western and central Europe (Eissmann, 1978; Maarleveld,
1981; Vandenberghe, 1983; Vandenberghe and Broek, 1982; Vandenberghe and Krook,
1982). Such horizons refl ect the deepening thaw layers that would have existed during
partial permafrost degradation. Typically, the sediments lying above the paleo-thaw layer
are deformed and chaotic. These soft-sediment deformations can be attributed to degrad-
ing ice-rich permafrost at depth.
12.3.3. Thermokarst Involutions and Sediment-Filled Pots
Thermokarst involutions and various other thermokarst structures and sediments that can
be observed in present permafrost environments have been discussed in Chapter 8. Here,
attention focuses upon the different types of disturbed soil horizons (“cryoturbations”)
that are attributed to Pleistocene thermokarst. These structures occur widely in the near-
surface sediments of many mid-latitude areas.
Amorphous deformations appear to be the most widespread (Figure 12.7). In some
instances, the deformed structures extend for several tens of meters in horizontal
extent and affect sediments to depths as great as 3-4m below the ground surface.
Because of this, they cannot be interpreted as the product of traditional cryoturbation
that occurs during the repeated freezing and thawing of a seasonally-frozen, near-
surface layer (see Chapter 6). Even in continental Siberia, where summer temperatures
exceed 30°C, the maximum depth of thaw in unconsolidated sediments rarely exceeds
2.0m. Instead, it is much more likely that, during permafrost degradation, density-
controlled mass displacements in water-saturated sediments would have caused under-
lying sand to ascend and overlying gravel to descend. Smaller deformation structures
of the “bird foot” or “drop soil” type (see Figure 12.8B) are also caused by loading
