Geology Reference
In-Depth Information
at depth and separated from the base of seasonal freezing by an unfrozen layer, termed
a residual thaw layer. In this case, the top of permafrost refl ects a paleo-thaw unconform-
ity rather than a paleo-active layer. While the paleo-active layer has paleo-climatic sig-
nifi cance, and a paleo-thaw layer may have regional climatic signifi cance, a paleo-thaw
unconformity may also be of only local signifi cance. For example, it might refl ect the
previous existence of a localized water body or thaw lake. Care is required, therefore, in
the identifi cation of the paleo-permafrost table.
Unfortunately, as explained in Chapter 11, it is also not easy to recognize the paleo-
permafrost table in now-unfrozen sediments. Moreover, it is frequently ignored when
attempts are made to identify past permafrost features and structures.
One of the more reliable methods involves recognition of the mineralogical and weath-
ering differences that might occur above and below a paleo-permafrost table. Both would
refl ect changes associated with the moisture migration that occurs within frozen ground
in response to the temperature gradient (see Chapter 5, Figure 5.13). It can be demon-
strated that, in areas of permafrost today, mineralogical differences may occur at thaw
unconformities. For example, using X-ray diffraction, trends in the peak ratios of mica to
chlorite (10Å : 7.1Å) and mica to quartz (10Å : 4.6Å) are suffi ciently distinctive as to assist
in the recognition of relict (early Holocene) active layers (i.e. paleo-thaw unconformities)
on the Tibet Plateau, China, at both Fenghuo Shan and Wudoaling (Xing et al., 1980),
and in northwestern Arctic Canada at Mayo (Burn et al., 1986) and Garry Island (Burn,
1997). However, if the active layer were developed in soil and rock composed of minerals
that are less susceptible to weathering, if permafrost episodes were short-lived, or if the
ground surface were not stable, mineralogical changes might be less discernable.
If a permafrost table exists for an extended period of time beneath a non-aggrading
ground surface in an arid environment, some fi eld observations suggest that an indurated
layer, or hardpan, may form. However, the nature of the cryo-pedological transformations
that are involved is still unclear. For example, in the western Transbaikalia region of
southern Siberia, where permafrost is not present today, a hard calcrete layer lies beneath
a silt-dust surface horizon that contains faunal remains that indicate cold dry steppe-like
conditions about 750 000-800 000 years ago (Bruhnes-Matayama boundary) (Vogt et al.,
1995). The calcrete layer is broken and displaced and the overlying sand and calcareous
dust contains vertical and oblique cracks, small placations, and folds. The calcrete is
interpreted as having formed at the permafrost table and its dislocation occurred when
underlying sandy layers thawed. In Eastern North America, similar dislocated hardpan
horizons can be observed in Maryland, Delaware, and southern New Jersey (Nikiforoff,
1955). In the Pine Barrens of New Jersey (see Chapter 2), the hardpan preserves sand-
wedge casts that are associated with an old land surface that relates to the penultimate
cold stage (OIS-6) (French et al., 2003).
If an ice-rich layer forms near the top of permafrost and at the base of the active layer
(“aggradational” ice), as can be observed in permafrost environments today (see Chapter
7), this may be refl ected in soil micro-morphology upon thaw. For example, ice segregation
is favored in fi ne-grained sediments and a platey microstructure may result when the ice
lenses thaw. In coherent bedrock, frost action in the active layer and the formation of
ground ice in the near-surface bedrock can lead to brecciation (Murton, 1996a). However,
unless other evidence for previous permafrost is present, brecciation and platey soil micro-
structures can also be the result of seasonal freezing rather than permafrost.
Relatively few studies have convincingly demonstrated the existence of a paleo-perma-
frost table in now-unfrozen sediments. Given the widespread occurrence of other indica-
tors of past permafrost that are described below, the paleo-permafrost table must also
occur widely in the stratigraphic record.
