Geology Reference
In-Depth Information
8.3.3. Other Processes
Numerous other processes that operate in cold-climate environments are related to
thermokarst activity. For example, thermal abrasion or thermo-erosional “wash” are
considered by some to be important components of periglacial slopewash (“ruiselle-
ment”) (Czudek and Demek, 1973; Dylik, 1971, 1972; Kachurin, 1962, p. 52). In fact,
thermal erosion, in all its forms, is intimately linked to the whole question of slope and
valley development in periglacial environments. This topic is discussed in Chapter 10.
Likewise, thermal abrasion and fl uvio-thermal erosion are important causes of rapid
coastal erosion and river-bank retreat wherever ice-rich unconsolidated sediments are
exposed (Are, 1988; Romanovskii et al. 2000; Walker and Arnborg, 1966). These aspects
are discussed more fully in the context of cold-climate fl uvial and coastal processes
(Chapter 11).
8.4. THERMOKARST SEDIMENTS AND STRUCTURES
Thermokarst sediments and structures are little studied, yet they are widespread in many
areas of permafrost terrain and, almost by defi nition, are ubiquitous in areas in which
Pleistocene-age permafrost no longer exists.
Thermokarst sediments form when ice-rich permafrost degrades. By defi nition,
excess ice is involved. Typically, thermokarst sediments mantle slopes and infi ll valley
bottoms and depressions. They are colluvial in nature and consist of a range of locally
redeposited and heterogeneous materials, or diamictons, which often incorporate
clumps of organic materials. When refrozen, thermokarst sediments typically contain
cryostructures indicative of epigenetic freezing (see Chapter 7). Where thaw lakes and
basins form, re-sedimentation may occur and where the original permafrost was
exceptionally icy, debris-fl ow deposits associated with retrogressive-thaw-slumps may
result.
Frequently, thermokarst sediments are stratigraphically confused and often indicate
differential loading and density readjustment in water-saturated sediment (Murton and
French, 1993c). Ice-wedge and composite-wedge casts must also be regarded as thermokarst
structures. Likewise, thermokarst-cave ice (“pool” ice), and other ice, silt, and gravel
pseudomorphs, all associated with erosion, re-sedimentation and refreezing of material
within permafrost, are especially complex thermokarst structures (Bray et al., 2006; Shur
et al., 2004).
8.4.1. Involuted Sediments
Thermokarst involutions take the form of load casts, pseudo-nodules, ball-and-pillow
structures, and diapirs. The usual mechanism is loading, buoyancy, and water-escape
although fl uidization may also be involved. In dimensions, thermokarst involutions vary
from a few to 90 cm in width and from a few centimeters to several meters in depth. There
is nothing sedimentologically unusual about these involutions, similar structures having
been observed in other depositional environments throughout the geologic record (Murton
and French, 1993c).
