Geology Reference
In-Depth Information
20
Organic unit
: 1 - 13.5 m, colluvial and in-place
Colluvium
: pebble to cobble diamicton
Erosional
18
Silt
Thaw
16
Colluviated silt + organics, 3 to 12 m thick; abundant grass
and twigs
Erosional
14
Grassy silt, 0 - 10 m
Tephra: 0 - 20 cm thick
12
Erosional
Cryoturbated bed: up to 75 cm
10
Thaw
Silt, 1m
Erosional
8
Creek gravel: gold-bearing pebble cobble gravel
Graphitic schist (bedrock)
6
Ice: wedges and lenses
Mammal bones
Grass beds and rhizomes
Tree twigs, branches
4
Figure 11.10.
Generalized cryostratigraphy of frozen unconsolidated sediments (“muck deposits”)
in the Klondike, Yukon Territory, Canada. From Fraser and Burn (1997), Kotler and Burn
(2000).
11.5. PLEISTOCENE PERIGLACIAL ENVIRONMENTS
OF MID-LATITUDES
This section discusses the nature and extent of the mid-latitude periglacial environments
of the Pleistocene. There is lack of a satisfactory modern analogue (French, 2000). Other
accounts are also available (Dawson, 1992, pp. 109-125; West, 1977, pp. 71-110) and more
advanced reading is indicated at the end of this chapter.
11.5.1. General Considerations
The mid-latitude periglacial environments of the Pleistocene were different from those of
high latitude. The major difference related to solar radiation. In mid-latitudes, there is no
equivalent of the “arctic night” or “arctic day.” Therefore, the annual ground-thermal
regime would have been dominated by diurnal freeze-thaw cycles of shallow depth and
short duration. Permafrost may, or may not, have formed. Given the number of freeze-
thaw cycles that occur in both high-latitude and alpine mid-latitude environments today
(see Chapter 4), an average annual frequency of between 50 and 100 cycles might be a
realistic assumption for the Pleistocene mid-latitudes. This would have contributed to
mechanical rock disintegration and to the mass-wasting of any frost-shattered debris.
Second, the effects of orientation, with respect to solar-insolation, were more marked than
in high latitudes. The effect of orientation is complex, however, because climate fl uctua-
tions promote a space-time change. For example, with the onset of cooling following upon
