Geology Reference
In-Depth Information
If one assumes typical average surface movement rate of between 0.5 and 5.0 cm/year,
and that movement in the upper 50 cm is, on average, one-quarter of this amount, it follows
that gelifl uction can transport between 6.0 and 60.0 cm 3 cm −1 year −1 of material (French,
1974a, 1976b). In plug-like fl ow the vertical velocity profi le is convex downwards. Also,
movement usually occurs in late summer when the active layer is at its thickest. Rates of
plug fl ow appear similar to those of gelifl uction. For example, J. R. Mackay (1981a) meas-
ured rates of movement that ranged between 0.2 and 1.0 cm/year, which indicate volumet-
ric transport of between 7 and 52 cm 3 cm −1 year −1 . Small mudbursts and mudfl ows, sometimes
observed in mid- and late summer on lower slopes (Egginton and French, 1985), are
related to plug-like fl ow. They probably result from liquefaction associated with positive
differential pore-water pressures that are released along shearing zones.
9.4.4. Solifl uction Deposits and Phenomena
Solifl uction deposits are heterogeneous, largely unstratifi ed, sediments that have under-
gone transport and subsequent deposition by frost creep and gelifl uction processes. They
are commonly matrix-supported, the matrix consisting of fi ne-grained sediment that, typi-
cally, has low liquid limits and plasticity indices (Figure 9.10). A characteristic of solifl uc-
tion deposits is that the larger clasts tend to become oriented with the longer axis parallel
to the direction of movement (Harris, 1981, 1987). Referring specifi cally to solifl uction
deposits on Baffi n Island, NWT, A. Cailleux and G. Taylor (1954) suggest that at least
two-thirds of all elongate clasts in solifl uction deposits lie parallel to the slope direction.
Platy microfabrics are typical of solifl uction deposits and refl ect the previous existence of
segregated ice lenses that promote localized slip during thaw (Harris, 1985; van Vliet-
Lanoë et al., 1984). Where plug-like fl ow has occurred, soil pods may possess slicken-sided
surfaces due to small-scale shears (Reanier and Ugolini, 1983).
Geomorphic features produced by solifl uction include uniform sheets of locally-derived
surfi cial materials and tongue-shaped lobes (Figure 9.11). Solifl uction sheets are probably
best developed where the absence of vegetation enables solifl uction to operate uniformly.
Tundra and forest-tundra favor more localized lobate movement. The correlation between
Figure 9.11.
A turf-banked solifl uction lobe, Holman, western Victoria Island, NWT, Canada.
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