Geology Reference
In-Depth Information
(A)
(B)
(C)
Refreezing
Thawing
Paleo-active
layer
Thaw
T-U
1
T-U
1
Residual
thaw
layer
Relict active
layer
T-U
2
T-U
1
Active
ice wedge
Active
ice
wedge
Inactive
(truncated)
wedge
Permafrost
-massive cryostructure
Active layer
Permafrost
-lenticular/layered
cryostructure
Seasonally-frozen
layer
Top of permafrost
Base of seasonal frost
Residual thaw
layer
Figure 7.11.
Schematic diagram illustrating the cryostratigraphic evidence associated with the
degradation and subsequent aggradation of permafrost. In (A) and (B) an initial permafrost
sequence is subject to downwards thawing, possibly as the result of climate warming. The cryostruc-
ture of the permafrost is indicated as being lenticular-layered. As thaw proceeds, a primary thaw
unconformity (T-U
1
) forms at depth below a residual thaw layer. In (C), the climate subsequently
deteriorates, permafrost aggrades, and the base of the active layer again becomes the primary thaw
unconformity. In (C) a massive cryostructure is illustrated.
(T-U
1
) forms at depth below a residual-thaw layer. At this time, the ground surface experi-
ences only seasonal freezing and thawing. In the process, an ice wedge is truncated and
is no longer active. When climate subsequently deteriorates, permafrost aggrades and the
base of the active layer again becomes the primary thaw unconformity. Renewed thermal-
contraction cracking at the ground surface permits a new ice wedge to form. During this
process, the original thaw unconformity at depth becomes a secondary (i.e. paleo-) thaw
unconformity (T-U
2
). The latter can be recognized by both the truncated ice wedge and
by different ice structures (cryostructures) in sediment above and below.
Besides the truncation of ice bodies and differences in ice contents, a thaw unconform-
ity might be recognized by differences in stable isotope values (see Figure 7.8B), heavy
mineral and pollen assemblages above and below the unconformity (Burn et al., 1986),
and horizons of enhanced micro-organisms (Gilichinsky and Wagener, 1995). The recog-
nition of thaw unconformities is a valuable line of evidence in reconstructing past perma-
frost histories. In currently-unfrozen Pleistocene sediments of middle-latitudes, thaw
