Geology Reference
In-Depth Information
7.4.3. Thaw Unconformities
Discontinuities in the nature and distribution of ground ice bodies are the result of either
thawing of frozen material or subsequent refreezing of previously-thawed material. The
signifi cance of these discontinuities, termed “thaw unconformities,” is that they allow
inferences to be made as to past permafrost conditions. This is explained below.
Figure 7.10 shows two typical permafrost conditions that might exist in continuous
permafrost of high latitudes and in the zone of deep seasonal frost overlying relict per-
mafrost in parts of the sub-arctic. In Figure 7.10A the active layer is shown as the near-
surface layer that thaws during the summer. A “relict active layer,” shown as ground
immediately below the modern active layer, was once part of the active layer but is now
perennially frozen. A “paleo-active layer” is also indicated as the horizon between the
ground surface and the base of the relict active layer. In Figure 7.10B, a “residual thaw
layer” is shown as referring to an unfrozen layer, formerly permafrost, lying between the
modern depth of seasonal frost penetration and an underlying (relict) permafrost body.
The base of the active layer, shown in Figure 7.10A, is the simplest and most obvious
example of a thaw unconformity.
It is possible to distinguish between primary (i.e. present-day) and secondary
(i.e. paleo-) thaw unconformities. Both are shown in Figure 7.10A but in Figure 7.10B the
paleo-thaw unconformity overlies relict permafrost that is unrelated to present surface
conditions.
The manner in which permafrost degrades and subsequently forms again, and the cry-
ostratigraphic evidence which it leaves, is illustrated in Figure 7.11. The diagrams depict
an initial permafrost profi le that is subject to thaw-degradation from above, possibly
caused by regional climate warming. As thaw proceeds, a primary thaw unconformity
(A)
(B)
Active layer
Seasonally-
frozen layer
Thaw
unconformity
Paleo-
active
layer
Relict
active layer
Residual thaw
layer
Paleo-thaw
unconformity
Paleo-thaw
unconformity
Relict permafrost
T > 0°C (seasonally-
thawed ground)
T > 0°C (unfrozen
ground)
T < 0°C (perennially-
frozen ground)
T > 0°C (seasonally-
frozen ground)
Figure 7.10.
Types of ice discontinuities commonly found in perennially-frozen sediments.
(A) Arctic regions of continuous permafrost. (B) Sub-arctic regions of discontinuous permafrost or
deep seasonal frost.
