Geology Reference
In-Depth Information
Segregated ice has been discussed earlier in the context of downward freezing (see
Chapter 4). However, upward freezing may also occur when, for example, the upper surface
of permafrost rises in response to sediment accretion, the accumulation of organic mate-
rial, or climate change. Under these conditions so-called “aggradational” ice is formed
(Mackay, 1972a, p. 10). A second sub-type of segregated ice is reticulated-vein ice (Figure
7. 2B) . T his develops in fi ne-grained sediment in response to desiccation and shrinkage
upon freezing with water being derived from sediment in a semi-closed freezing system
rather than from an upward migration of water in an open system (Mackay, 1974b).
7.2.3. Intrusive Ice
Intrusive ice is formed by water intrusion, usually under pressure, into the seasonally- or
perennially-frozen zone. Sill and pingo ice are two types of intrusive ice which are usually
identifi ed. Pingo ice, formed by pore-water expulsion, has been discussed in Chapter 6.
Sill ice forms when water is intruded under pressure into a confi ning material and then
freezes in a tabular mass. Sill ice may be stratigraphically controlled, as in sandstone and
coal bedrock on the Fosheim Peninsula of Ellesmere Island (Pollard, 2000b; Robinson
and Pollard, 1998), or it may cross-cut sediment boundaries to form ice “dikes” in softer
sediments (Figure 7.3) (Mackay, 1975b; Mackay and Dallimore, 1992a, p. 1237). Typically,
sill ice forms a sharp contact with enclosing sediment, is relatively clear, and fractures
conchoidally, indicative of its formation under pressure. Although groundwater is usually
the source of sill ice, it may also form adjacent to river banks by fl ood water being injected
laterally along the base of the active layer and parallel to the permafrost table (Lewis,
1962; Mackay, 1972a).
Figure 7.3.
Injection (i.e. intrusive) ice, Pullen Island, Pleistocene Mackenzie Delta, Canada. The
ice bed continues for more than 100 m to the left of the photo (see Mackay, 1975c). The hypsithermal
thaw unconformity is parallel to the top of the snowbank. Photo is supplied courtesy of Professor
J. R. Mackay.
