Geology Reference
In-Depth Information
Table 7. 3.
Excess ice content (%) of the upper 50 cm of permafrost beneath point-bar willow
and alder communities, spruce forests, and lake-side alder and sedge communities, southern,
central, and northern Mackenzie Delta, Canada. From Kokelj and Burn (2005, table 1).
Reproduced by permission of John Wiley & Sons Ltd.
Willow
Alder
Spruce/alder-
Spruce/
Spruce/
Lakeside
bearberry
feathermoss
crowberry-lichen
alder/sedge
Cores (N)
14
10
10
18
10
11
Mean
0.2
6.6
6.0
27.3
41.1
33.6
Median
0.0
7.3
5.6
26.8
40.9
32.8
Minimum
0.0
0.0
0.0
14.8
35.0
19.6
Maximum
3.2
11.8
12.5
43.2
48.6
46.9
Our understanding of moisture migration within freezing and frozen soils now permits
explanation of this ice-rich zone in the upper 0.5-2.0 m of the permafrost profi le. In
Chapter 4, it was explained how unfrozen water moves in the direction along which the
ground temperature decreases in response to an imposed thermal gradient. In summer,
moisture migrates downwards in the near-surface permafrost. Later, in Chapter 5, the
concept of the transient layer, an ice-rich zone at the base of the active layer and at the
top of permafrost, was explained. In many ways, the transient-layer concept is the modern
equivalent of the ice-rind concept of Büdel, but based upon a better understanding of
ground-ice dynamics.
7.3.3. Ice in Bedrock
Relatively little information is available concerning ice in bedrock. Theoretically, ice
amounts should vary in accordance with rock porosity and permeability, and the presence
or absence of discontinuities such as faults, joints, and bedding planes. The Russian litera-
ture recognizes a number of distinct ice-distribution patterns (“cryogenic textures”; see
below) that characterize solid or semi-solid rock (Figure 7.5).
As a generalization, fi ne-grained rocks that are both permeable and porous contain
higher ground ice amounts than coarse-grained rocks. For example, on Melville Island,
fi ne-grained shale of Mesozoic age is more ice-rich that older and more consolidated
sandstone and siltstone of Paleozoic age (French et al., 1986) (Figure 7.6A, B). Likewise,
on the Fosheim Peninsula of Ellesmere Island, areas underlain by well-lithifi ed sandstone
and siltstone have a mean ice content of 30%, while fi ne-grained (disaggregated) shale
and siltstone have a mean ice content of 44% (Hodgson and Nixon, 1998). The presence
of joints, bedding planes, faults, and other structural discontinuities in bedrock complicates
any assessment of ground ice. For example, bodies of intrusive ice and ice-expanded joints
are common in sedimentary bedrock sequences (Figure 7.6C, D) (French, 1981, p. 12;
Christiansen et al., 2005; Robinson and Pollard, 1998; Wang, 1990; Wernecke, 1932).
7.3.4. Ice in Unconsolidated Sediments
The occurrence of ground ice in unconsolidated sediments is much better understood than
in bedrock. First, many regional surfi cial geology mapping programs in northern Canada
demonstrate that the majority of heterogeneous, fi ne-grained surfi cial materials, be they
of alluvial, fl uvial, glacial, lacustrine, or marine origin, typically possess relatively high
