Geology Reference
In-Depth Information
termed “cryostructures.” Intimately related to cryostratigraphy is cryolithology, a sub-
branch of geocryology (see Chapter 1). Cryolithology refers to the relationship between
the lithological characteristics of rocks and their ground ice amounts and distribution.
The standard texts are those by Popov et al. (1985) and Melnikov and Spesivtsev
(2000).
7.4.1. Cryostructures, Cryotextures, and Cryofacies
Cryostructures are determined by the amount and distribution of ice within pores (i.e.
pore or cement ice), and by lenses of segregated ice. The type and arrangement of ice
within frozen material will depend upon the initial water content and the nature of water
migration during freezing. Figure 7.2 illustrates two common cryostructures observed in
northern Canada: (a) a lenticular cryostructure formed in frozen lacustrine clay near
Mayo, Yukon Territory, and (b) a reticulate cryostructure formed in clay diamicton in the
Mackenzie Delta region. Cryotextures must be distinguished from cryostructures. The
former refers to the grain and/or ice crystal size and shape, and the nature of the contacts
between grains and ice crystals in frozen earth materials. Both cryostructures and cryo-
textures are useful in determining the nature of the freezing process and the conditions
under which frozen sediment accumulates.
The fi rst to systematically identify cryotextures and cryostructures were Russian
permafrost scientists (Katasonov, 1969, 1975; Kudryavtsev, 1978). Unfortunately, Russian
classifi cations tend to blur the distinction, made above, between texture (i.e. grain size)
and structure (i.e. aggregate shape). Traditionally, Russian scientists used the term texture
to describe what North Americans denote as structure (see comment, ACGR, 1988,
p. 24). This confusion was perpetuated in one of the few early English-language transla-
tions on this subject (Demek, 1978, pp. 139-153). Russian geocryologists recognize at least
ten “cryogenic textures” (i.e. cryostructures); these are termed massive, massive-porous,
basal, basal-layered, crust-like, porphyry-like, massive agglomerate-lens type, lattice type,
layered, and lattice-block type (Kudryavtsev, 1978, pp. 301-304) This classifi cation of
“cryotextures” is primarily one of cryostructures (Murton and French, 1994).
Problems with Russian cryostructural classifi cations arise primarily from their complex
and unwieldy nature. For example, E. M. Katasonov's (1969) classifi cation involves 18
different cryostructures and A. I. Popov et al.'s (1985) classifi cation has 14, excluding those
that are composite. A second limitation is that these classifi cations apply primarily to
permafrost containing little excess ice, detailing the distribution of ice within sediment
and neglecting that of sediment within ice. Thus, V. A. Kudryavtsev's (1978) classifi cation
contains seven cryostructural terms that describe frozen ground whose ice content is
≤
50% by volume and only one term (basal layered: ataxitic or breccia-like; see Cheng,
1983; Shur, 1988a) that describes permafrost that is very ice-rich.
A simplifi ed North American cryostructural classifi cation encompasses the range of
ice contents found within permafrost (Figure 7.7) (Murton and French, 1994). Several
Russian terms are transliterated but the cryostructures proposed can all be recognized
by the naked ice. A structureless cryostructure refers to frozen sediment in which ice is
not visible and consequently lacks a cryostructure. Lenticular cryostructures, by compar-
ison, can be of several types; they are described by inclination, thickness, length, shape,
and relationship to each other. That shown in Figure 7.2A is best described as “lenticular,
parallel, curved” according to this classifi cation. In the case of lenticular cryostructures,
the orientation of ice lenses refl ects the orientation of freezing fronts and/or the struc-
tural properties of the sediment (e.g. bedding; Smith and Williams, 1990). Layered
