Geology Reference
In-Depth Information
Table 7.4 .
Cryofacies types applicable to ice-rich sediments in the Pleistocene Mackenzie Delta,
Canada.
Cryofacies type
Volumetric
Cryofacies
Code
Cryostructures
Ice Content (%)
Pure ice
100
Pure ice
I
Le, Le
Sediment-poor ice
>
75
Sand-poor ice
SPI
Le, La, Su
Aggregate-poor ice
API
Sediment-rich ice
>
50 to
≤
75
Sand-rich ice
SRI
Aggregate-rich ice
ARI
Le, La, Su
Ice-rich sediment
25 to
≤
50
Ice-rich sand
IRS
SI, Le, La
Ice-rich mud
IRM
Le, La, Rr, Ri, Cr
Ice-rich diamicton
IRD
Ice-poor sediment
≤
25
Ice-poor mud
IPM
Sl; various non-ice
Ice-poor sand
IPS
sedimentary structures
Ice-poor gravel
IPG
Ice-poor diamicton
IPD
Ice-poor peat
IPP
Source: Murton and French (1994).
centimeters or more. In segregated ice, angular mud aggregates are abundant in the ice-
rich layer commonly observed at the top of permafrost (e.g. Cheng, 1983; Burn, 1988;
Mackay, 1972a; Shur, 1988b). Rounded mud aggregates have been reported from debris-
rich glacier ice (Boulton, 1970; French and Harry, 1988).
Many cryostructures are composite or transitional, either merging into adjacent catego-
ries or mixing two categories. They may also be hierarchical, although such hierarchies
do not necessarily imply genesis.
Cryostratigraphy adopts many of the principles of modern sedimentology. For example,
“cryofacies” are defi ned according to volumetric ice content and ice-crystal size, and then
subdivided according to cryostructure. One such system is illustrated in Table 7.4, where
fi ve cryofacies are distinguished according to arbitrarily-defi ned volumetric-ice-content
values. Although this particular facies classifi cation was developed specifi cally for ice-rich
sediments, facies can be modifi ed to meet other conditions.
Cryofacies can also be described according to cryostructures. Where a number of cryo-
facies form a distinctive cryostratigraphic unit, these are termed a cryofacies assemblage.
Two examples illustrate the application of cryostructural analysis. First, Figure 7.8A
summarizes the ground ice conditions observed in shale bedrock on Eastern Melville
Island. The cryostratigraphy is indicated together with ice types according to the Pihlainen
and Johnston (1963) classifi cation (see earlier) and the Murton and French (1994) cryo-
structural classifi cation. A more sophisticated example (Figure 7.8B) describes an 8 m high
exposure of massive ice overlain by sandy diamicton. Two cryofacies assemblages are
recognized together with a secondary thaw contact (see below).
7.4.2. Epigenetic and Syngenetic Cryostructures
Certain cryostructures are diagnostic of certain freezing conditions. Here, we ignore the
details of the various Russian classifi cations (Melnikov and Spesitvsev, 2000) and attempt
to generalize in terms of the cryostructures listed in Figure 7.7A.
