Geology Reference
In-Depth Information
Two common examples of epigenetic freezing are the structureless (Sl) and reticulate
(Rr, Ri) cryostructures. The structureless cryostructure refl ects in-situ freezing of pore
water. It is termed “massive” in the Russian transliterated literature. The reticulate
(regular, irregular, or “chaotic') cryostructure, (see Figure 7.2B) refl ects either the migra-
tion of water into shrinkage cracks in a semi-closed freezing system (Mackay, 1974b) or
hydraulic fracturing in unfrozen soil ahead of the advancing freezing front (McRoberts
and Nixon, 1975).
Layered, lenticular-layered and micro-lenticular cryostructures indicate syngenetic
freezing. The various sub-types recognized in the Russian literature are shown in Figure
7.9. It also shows how the inclined cryostructures shown in Figure 7.2A would have formed
during the freezing of lake-bottom sediments. Based upon observations in the CRREL
permafrost tunnel, Alaska, layered and lenticular cryostructures are regarded as diagnos-
tic indicators of syngenetic permafrost (Shur et al., 2004; Bray et al., 2006).
Figure 7.9.
The different cryostructures that form during syngenetic freezing of subaqueous
lake-bottom sediments. (A) Those associated with freezing of the seasonally-thawed layer from
below (i and ii), and in subaqueous situation (iii). (B) Schematic illustration of cryostructures that
form during freezing of lake sediments. Legend: 1, thawed material beneath original lake; 2/3, lake-
bottom sediments; 4, seasonally-thawed layer; 5, reticulate ice cryostructure; 6, permafrost bound-
ary at different stages of lake infi lling and water-level lowering. From Kudryavtsev (1978) and
Melnikov and Spesivtsev (2000).
