Geology Reference
In-Depth Information
In the fourth stage, alas lakes disappear, either by infi lling with thermokarst (mass-
wasted) sediments derived from the sides of the expanding alas or by drainage tapping
towards an adjacent alas fl oor or stream channel. From this stage onwards, permafrost
aggradation and lake-bottom heave occur and young epigenetic ice-wedge systems begin
to form. Sometimes, perennial-frost mounds or pingos form. These are locally termed
“bulgannyakhs.” The larger of these are usually of the open (hydraulic) type since the
unfrozen zone beneath the alas fl oor will often penetrate the permafrost. However,
smaller bulgannyakhs may be hydrostatic (closed) in nature. The end result of this
sequence of alas relief is the development of a depression with gentle slopes and an undu-
lating fl oor.
In central Siberia, alas formation has greatly modifi ed the lowland areas. In one region
near to Yakutsk nearly 40% of the initial land surface has been destroyed by alas forma-
tion. Their coalescence may lead to the formation of depressions often in excess of 25 km
2
.
Complex thermokarst valleys also develop. These are irregular in plan and consist of wide
sections (alas depressions) separated by narrow sections that cut through the intervening
watersheds. Other characteristics of these valleys are right-angle turns, blind spurs, and
a general misfi t relationship with overall topography and drainage.
The rate of alas formation varies considerably since there are reports that some have
developed within historic time, while others are obviously very old features. S. P. Kachurin
(1962) regards the beginning of thermokarst development in Siberia as coinciding with
the early Holocene warm period. However, many of the thermokarst features in the vicin-
ity of Yakutsk are not active today and probably only 10% of the terrain is currently
undergoing thermokarst modifi cation.
Similar thermokarst terrain occurs on a regional scale in the “ice-complex” sediments
that formed during the Late-Pleistocene regression (marine-isotope stages 3-5) on the
drained Laptev Sea shelf and coastal lowlands of northern Siberia (Romanovskii et al.,
2000). More than 50% of the area is, or has been, infl uenced by thermally-induced subsid-
ence (Grosse et al., 2005). During the Late Pleistocene and Early Holocene, thermokarst
processes began to destroy the ice-complex. Then, in the Middle and Late Holocene, and
continuing today, thermokarst lakes and alas basins have been truncated by coastal erosion.
Rising sea level has transformed the coastline into one of shallow headlands, bays, and
lagoons. A fuller discussion of the role of thermokarst in the evolution of coastal topog-
raphy, and the roles of thermal abrasion and coastal processes, is given in Chapter 11.
8.7.2. The Western North American Arctic
Most areas of arctic North America did not experience the same protracted periods
of non-glacial cold-climate conditions that characterized central Siberia during the
Quaternary. As a consequence, it is only in parts of the western North American arctic
that one fi nds similar regional thermokarst terrain to that previously described from
Yakutia. Here, certain aspects of Siberian alas thermokarst relief fi nd North American
analogues.
In central Alaska, W. A. Rockie (1942) fi rst drew attention to the settling of ground
caused by the thawing of ice wedges in recently cleared fi eld systems near Fairbanks.
T. L. Péwé (1954) and others subsequently referred to these hummocks as “thermokarst
mounds.” These are clearly analogous to the baydjarakhii or “graveyard mounds” of
Yakutia. The thaw-sinks, funnels, and “cave-in” lakes reported from the Seward Peninsula
and other areas (Hopkins, 1949; Péwé, 1948; Wallace, 1948) also fi nd a clear counterpart
in the “dujoda” stage. However, the alas depression, with bulgannyakh or pingo growth
