Geology Reference
In-Depth Information
Regional-scale thermokarst activity is best developed in lowland environments under-
lain by ice-rich unconsolidated sediments or soft bedrock, and which often have extended
histories of cold, non-glacial, conditions. Two such environments are described below.
8.7.1. The Lowlands of Central and Northern Siberia
Highly distinctive thermokarst terrain occurs in Siberia, where thermokarst phenomena
are thought to develop in a predictable and sequential fashion. The type area is the ter-
races of the Lena and Aldan rivers of Central Yakutia (Soloviev, 1973a, b), but similar
terrain occurs in much of the coastal lowlands of the Laptev and Kara Seas (Romanovskii
et al., 2000).
As with almost any type area, a number of regional site-specifi c factors favor the dis-
tinctive nature of the thermokarst topography in Central Yakutia. First, the terraces are
constructed from thick sequences of fi ne-grained alluvial sediments in which segregated
ice constitutes up to 50-80% by volume. In addition, large syngenetic ice wedges, reported
to exceed 50-60 m in vertical extent in some areas, underlie between 30% and 60% of
the terrace surfaces. In terms of ground ice, therefore, the area is exceptionally well-suited
for thermokarst development. Second, the geomorphic history which enabled such condi-
tions to develop is uncommon. The central Yakutian lowland remained unglaciated for
much, if not all, of the Quaternary and acted as a stable aggradational region in which
alluvial sediments were deposited under cold-climate conditions over a long period of
time. Third, the present climate is continental in nature; for example, the annual air tem-
perature range at Yakutsk is 62 °C and summer temperatures commonly exceed 30 °C (see
Chapter 3). As a result, the typical active-layer thickness may exceed 1.5-2.0 m. In general
terms, therefore, one must regard the thermokarst terrain of central Yakutia as unusual,
and it is unlikely that exactly similar conditions will be found elsewhere.
The range of thermokarst relief features in central Yakutia, initially described by P. A.
Soloviev (1962), was subsequently expanded into a progressive (cyclic) sequence of land-
scape evolution (Czudek and Demek, 1970; Katasonov and Ivanov, 1973; Popov et al.,
1966; Shumskii and Vtyurin, 1966; Soloviev, 1973a, b). It is summarized in Figure 8.12.
The fi rst stage follows upon an increase in annual thaw depth that leads to preferential
thaw-subsidence along large ice-wedge polygon systems. Continued ground subsidence
leads to slumping and disruption of the vegetation cover and to the isolation of distinct
conical mounds. These are termed “baydjarakhii,” a local Yakut term used to describe
silty or peaty mounds, and are commonly between 3 m and 4 m high and between 3 m and
15m wide. From the air, baydjarakhii resemble a cobblestone surface, distributed in a
checkerboard or rectilinear pattern. Similar features occur in the Tamyr Peninsula, near
Dickson, and in many other parts of Northern Siberia. In the discontinuous permafrost
zone of southern Siberia, where the active layer is thicker, and in areas where ice wedges
have been consumed (thawed) by thermokarst-induced subsidence, baydjarakhii are more
subdued in form and locally referred to as “graveyard mounds.”
The second stage is characterized by the progressive collapse and decay of baydjarakhii
as depressions develop in the center of the thermokarst terrain, often with a central hollow
or sinkhole. With the linkage of these sinkholes, continuous depressions form with steep
slopes and uneven bottoms. This hummocky depressed topography is locally called a
“dujoda.”
By stage three, a distinct depression with steep sides and a fl at bottom has developed
through the continued collapse of baydjarakhii on the side of the dujoda. This depression
is locally called an “alas,” a Yakut term that denotes a circular or oval depression with
