Geology Reference
In-Depth Information
12.2.2. Frost-Fissure Pseudomorphs and Casts
Wedge-shaped sedimentary structures, interpreted as the result of thermal-contraction
cracking of perennially-frozen ground, are convincing evidence for the previous existence
of permafrost. These structures are frequently reported from mid-latitudes. Although
there is general agreement as to their paleo-environmental signifi cance, their specifi c rel-
evance to air temperature is far less clear. As discussed earlier in Chapter 6, cracking
appears to be controlled not only by ground temperature but also by site-specifi c condi-
tions such as lithology and associated thermal conductivity, antecedent conditions, and
the duration and thickness of the snow cover. The fact that certain structures may result
from seasonal frost (soil or “ground wedges”) further complicates the situation (see
Chapter 13). Finally, one must distinguish thermal-contraction cracks from dilation cracks,
desiccation cracks, sand dykes, and various other soft-sediment deformations (Burbidge
et al., 1988; Butrym et al., 1964). Pleistocene frost fi ssures may also be epigenetic, syn-
genetic, or anti-syngenetic in nature (see Chapter 7).
When discussing frost fi ssures in non-frozen sediments as opposed to those present in
perennially-frozen sediments, slightly different terminology must be used in order to take
account of the changes that occur as permafrost degrades (Harry and Gozdzik, 1988;
Kudryavtsev, 1978; Murton and French, 1993b). This is essential for correct interpretation.
For example, following N. N. Romanovskii (in Dylikowa et al., 1978), one can recognize
both primary and secondary wedges. Primary wedges can be fi lled with ice, ice and
mineral soil, eolian sand and ice, and eolian sand. They can also be divided into
those that form in the seasonally-frozen ground (initially-ground wedges) and those
that penetrate permafrost (ice wedges, sand-ice wedges, and primary-sand wedges).
Secondary wedges result when frozen ground, either seasonal or perennial, thaws.
The resulting structures are ice-wedge pseudomorphs and sand-wedge casts. The
presence of Pleistocene permafrost bodies further complicates the Romanovskii scheme
because one must also recognize not only active and inactive wedges but also ancient
wedges.
The nature of the wedge depends upon the degree of thaw-deformation that the struc-
ture has experienced. This means the resulting form can be categorized as being either a
cast (i.e. bears some resemblance to the original form) or a pseudomorph (i.e. bears little
resemblance to the original form). The degree of deformation largely depends upon a
number of factors: the nature of the enclosing sediments, the amount of ice originally
contained within the fi ssure, the rapidity of thaw, and the degree to which water or other
processes have eroded the fi ssure.
Thaw-modifi cation results in the selective preservation of pseudomorphs and casts.
Most ice-wedge pseudomorphs and sand-wedge casts are found in sand and gravel. This
is because these coarse-grained sediments are usually ice-poor, while few pseudomorphs
or casts are recognizable in silt and clay, which are usually ice-rich. Thus, sand wedges
are also more likely to be preserved as casts than are ice wedges to be preserved as pseu-
domorphs. Furthermore, whereas ice wedges preferentially develop in ice-rich, fi ne-
grained sediments (thaw-sensitive), their pseudomorphs are selectively preserved in
ice-poor, coarse-grained sediments (thaw-stable).
The fi ll of a frost-fi ssure cast or pseudomorph is either primary (i.e. the initial sand/
mineral soil remains, as in a sand-wedge cast) or secondary (i.e. the initial icy fi ll thaws
and the void left is fi lled with different material, as in an ice-wedge pseudomorph). Where
fi ll is both primary and secondary, in varying amounts, the cast or pseudomorph is deemed
“composite” in nature. The range of structures that ice-wedge pseudomorphs can assume
is large (Figure 12.1).
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