Geology Reference
In-Depth Information
THERMOKARST
8.4.4. Ice, Silt, Sand, and Gravel Pseudomorphs
Ice wedges and composite wedges are often partially thawed because thermal erosion
operates preferentially along ice-wedge troughs. As a result, piping and tunnels are
common (see Figure 8.6C). Cast structures of partially-thawed wedges may refl ect tunnel
shape and the material that has either fallen from the roof or sides of the tunnel or been
transported along the tunnel. If the initial wedge were of a composite nature, the infi ll
may be a highly variable and chaotic mix of ice and mineral material.
Sometimes, pool ice (“thermokarst-cave ice”) replaces wedge ice that has been eroded
during a thermokarst, or underground erosion, episode. The result is the formation of an
“ice cast,” that is, an “ice pseudomorph” (Rozenbaum et al., 1978; Solomatin, 1986).
Where sediment fi lls the void, formed either in ice or adjacent sediment, the result is a
“soil pseudomorph” (Murton and French, 1993b, pp. 186-188). These are especially dif-
fi cult to recognize because of the similarity of the fi ll material with enclosing material.
Ice pseudomorphs are also diffi cult to recognize because they are often intersected by ice
veins related to the continued growth of the wedge that they replace. Examples of various
sand pseudomorphs that form above partially-thawed ice wedges are illustrated in Figure
8.6B and C. Ice pseudomorphs, usually composed of “pool” ice (see Chapter 7), are sig-
nifi cant because the ice may extend laterally within an erosional niche beyond the extent
of the original wedge, thereby producing an ice body that bears little or no relationship
to the original wedge shape.
The manner by which ice wedges can be thaw-modifi ed is illustrated schematically in
Figure 8.7. The schematic diagram initially considers undisturbed permafrost that is pen-
etrated by large ice wedges. The permafrost contains micro-lenticular cryostructures
typical of syngenetic permafrost growth. Thaw-modifi cation of the ice wedges could result
in an ice-wedge pseudomorph (left), a partially-thawed (truncated) ice wedge (center), or
an ice pseudomorph (thermokarst-cave ice) fi lling a tunnel void in the wedge (right). The
extent of secondary (thaw-modifi ed) deposits is indicated schematically. Expanded images
are intended to represent micro-lenticular cryostructure in the permafrost, indicative of
syngenetic growth and a reticulate-chaotic cryostructure adjacent to the ice pseudomorph
indicative of epigenetic growth.
A fuller discussion of syngenetic and epigenetic permafrost and their associated cry-
ostructures was given earlier in Chapter 7.
8.5. ICE-WEDGE THERMOKARST RELIEF
The surface morphology created by ice-wedge polygons is a distinctive characteristic of
much lowland tundra terrain (Carson and Hussey, 1962; Hussey and Michelson, 1966;
Mackay, 1963). However, the genetic relationship between this topography and thermokarst
processes needs to be emphasized. Two types of polygon relief can be recognized. First,
high-centered polygons develop by preferential fl uvio-thermal erosion along ice wedges.
Second, low- centered polygons result from the growth of ramparts adjacent to the thermal-
contraction crack.
8.5.1. Low-Centered Polygons
Low-centered polygons are characteristic of poorly-drained tundra. They commonly
possess a double raised rim, or rampart, often in excess of 50 cm in height, on either side
of the ice-wedge trough. The depressed wet center contains sedges and grass. The raised
