Geology Reference
In-Depth Information
monly termed “niveo-eolian” (Cailleux, 1974, 1978; Koster and Dijkmans, 1988). They
occur not only in high latitudes but also in the snow environments of mid-latitudes (Bal-
lantyne and Whittington, 1987; Jahn, 1972; Rochette and Cailleux, 1971).
Evidence for niveo-eolian deposition is frequently observed during spring snowmelt,
when it is not uncommon for the snow surface to assume a gray, almost black color, and
for pitted, small-scale thermokarst-like relief to form (Figure 10.9C). The sediment veneer,
sometimes 1-2cm thick, lowers the snow albedo and leads to earlier melt (Woo et al.,
1991). Snow banks may also possess distinct sediment layers, each refl ecting an event of
wind deposition. Sediment concentrations in snow banks typically range from zero to as
much as 0.2-2.0 g/l (Czeppe, 1965; Lewkowicz and Young, 1991, pp. 201-206).
As snow progressively ablates, wind-blown sediment is reworked and locally redepos-
ited. The term “denivation” is sometimes used to refer to the sedimentological and mor-
phological disturbances that occur. Because the sediments are predominantly silty in
nature and susceptible to frost heave and cryoturbation activity, they are sometimes
thought to promote the formation of slope hummocks (Lewkowicz and Gudjonsson, 1992).
Niveo-eolian activity is closely related to slopewash activity (see Chapter 9). Facies analy-
ses of niveo-eolian sediments are relatively few, and the processes by which these wind-
derived sediments are reworked by snowmelt are still poorly understood. Experimental
studies (Dijkmans and Mucher, 1989) suggest that the intercalation of snow during
sedimentation does not, in itself, induce a laminated structure. Further investigation is
required into the interactions between permafrost, moisture (snow) and eolian transport
(sediment) in present periglacial environments.
10.3.4. Loess-Like Silt
Sequences of either buff or gray-colored silt-sized material are globally-widespread sedi-
ments attributed to wind transport and deposition. Not all are related to periglacial condi-
tions since much is found in and around the margins of hot deserts of the world. Presumably,
in periglacial environments, the silt is entrained from unvegetated fl oodplains, braided
channels, glacial outwash plains, till plains, and lake shores.
Wind-blown silt deposits of cold-climate origin have been described from Alaska
(Péwé, 1955, 1975), where they are called “upland silt,” from central Siberia (Péwé and
Journaux, 1983), and from Tibet (Péwé et al., 1995). They are largely Pleistocene in age
(see Chapter 11) and analogous to the “loess” of mid-latitudes (see Chapter 13). In detail,
wind-blown silts are well-sorted, homogeneous, and unstratifi ed. Loosely coherent grains
of between 0.01 mm and 0.05 mm in diameter often exceed 50-60% of the deposit. The
typical grain-size distribution of wind-blown silt is illustrated in Figure 10.10.
Silt entrainment requires a dry soil surface. This probably explains why loess-like silt
is rarely encountered in damp oceanic periglacial environments. In cold and arid environ-
ments, however, suitable conditions for entrainment from the ground surface occur during
summer by evaporation and during the cold winters through sublimation. Both processes
promote desiccation of the near-surface materials.
In Arctic North America, wind-blown silt of Holocene-age mantles upland surfaces in
both central Alaska and parts of Yukon Territory. There is evidence of deposition from
the Middle Quaternary onwards through to present day. Tephra (volcanic ash) layers and
relict ice wedges indicate these sediments have been perennially frozen (i.e. permafrost)
for several hundred thousand years. Many of the valley bottoms contain re-transported
silt mixed with organic debris that is locally termed “muck.” These predominantly
Pleistocene-age deposits are discussed more fully in Chapter 11.
