Geology Reference
In-Depth Information
Figure 7.7.
A North American classifi cation of cryostructures. (A) Scheme proposed by Murton
and French (1994). Ice is shown in white and sediment in black. In lenticular and layered cryostruc-
tures, lenses and layers may comprise either ice or sediment. (B) Terms and illustrations used to
describe layered and lenticular cryostructures. Structureless (SI) and reticulate (Rr, Ri) cryostruc-
tures result from epigenetic freezing. Layered (La) and lenticular (Le) cryostructures result from
syngenetic freezing.
cryostructures are continuous bands of ice, sediment, or a combination of both. The
reticulate category, described earlier (see Figure 7.2B), is a three-dimensional net-like
structure of ice veins surrounding mud-rich blocks. The best known, but probably least
common, cryostructure is that of an ice crust or rim around a rock clast. This is termed
crustal; it occurs commonly just beneath the permafrost table where ice crusts up to a
few centimeters thick envelop pebbles and wood fragments, typically within silt-rich
facies. Crustal cryostructures most likely form by localized ice segregation around frost-
susceptible clasts. The suspended category refers to grains, aggregates, and rock clasts
suspended in ice. By defi nition, it refers to icy sediment and massive icy bodies. The
aggregates are typically mud (silt), and range in diameter or length from
≤
1 mm to several
