Geology Reference
In-Depth Information
Massive icy bodies of both segregated and buried glacier-ice origin may appear similar
in the fi eld. Both may contain considerable quantities of sediment, often stratifi ed (Klassen
and Shilts, 1987, pp. 15-22). Moreover, basal glacier ice that has experienced regelation
is essentially segregation ice. A further complication arises because, in both cases, the ice
may have existed beneath an overburden for several thousands of years and may have
experienced major post-formational or post-burial alteration (moisture migration, thermal
and geochemical changes, and loading/unloading).
Several observations may assist in differentiating between intra-sedimental and buried
basal glacier ice. First, gradational contacts should rarely exist with buried ice, and, in the
case of intrusive ice, suspended fragments might occur just below the contact. Second, if
the contact between overlying material and underlying ice is of a thaw or erosional nature,
this indicates the ice predates burial and must be buried. Third, features suggestive of ice
origin may be found within the ice itself. For example, matched soil fragments indicate
segregated and/or intrusive ice, ice coatings beneath clasts within the ice indicate segrega-
tion, striated clasts and pods of diamicton within the ice suggest glacier ice, and lumps of
clear ice frozen (i.e. suspended) within silty bubbly ice suggest regelation ice. Finally, ice
dykes formed from water intruded upwards under pressure into permafrost along fi ssures
are associated with segregated and/or segregated-intrusive ice (Mackay, 1989a; Mackay
and Dallimore, 1992).
7.6.2. Intra-Sedimental Ice
Undoubtedly, the strongest evidence in favor of an intra-sedimental (ice segregation)
origin for massive icy bodies is provided by over 600 drill hole records from both the
western Canadian Arctic (Mackay, 1973b) and northern Siberia (Dubikov, 1982, 2002).
These are summarized in Mackay and Dallimore (1992, fi gure 3). They indicate two
salient facts about the occurrence of massive ice bodies: (1) in the majority of instances
where massive ice is encountered, it is overlain by clay-grade sediment (diamicton) and
underlain by sand-grade sediment; and (2) a signifi cant number of drill holes encounter
massive ice at depths in excess of 30 m, sometimes at depths of 100-200 m; in some holes
two or more layers of massive ice, usually separated by sand, are penetrated by the same
drill hole.
These observations support a segregation or segregation-injection origin and fi t a model
of ice growth with fi ne-grained sediment underlain by coarse-grained sediment. Support-
ing evidence includes ice dykes which penetrate the overlying diamicton and possess iso-
topic signatures similar to the massive ice. These structures are proof of the high water
pressures that must have been involved. Moreover, the contact between the massive ice
and the overlying diamicton is always conformable and the continuity of
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profi les from the top of the massive ice downwards into underlying sand indicate a
common water source for the massive ice and the interstitial ice in the underlying sand.
Finally, a network of reticulate ice veins in the overlying diamicton and bubble trains
which originate at the upper ice-sediment contact are evidence of downward freezing.
More than 40 years ago, J. R. Mackay concluded “The ice-sheets are believed to have
formed during progressive downward aggradation of permafrost in fi ne-grained soils in
an open system, where fresh water moved to the bottom of the freezing plane, as new ice
formed.” He also added “some ice sheets may be sill-like injection features, formed similar
to pingo ice” (Mackay 1963, p. 63). These statements must remain the standard against
which other interpretations are judged.
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