Geology Reference
In-Depth Information
from the gobi (cold) deserts of central Asia following uplift of the Tibet Plateau during
late Tertiary and Quaternary times (see earlier).
One of the best documented records of Quaternary loess deposition comes from Gold
Hill, near Fairbanks, Central Alaska, where large sections are regularly exposed during
placer gold-mining operations. Originally, the loess that mantled the upland surfaces
was termed “upland silt” (Péwé, 1955). Following initial deposition, the loess was re-
transported by mass-wasting processes towards lower elevations. Typically, the thickest
deposits of these loess-like materials now occur on valley-side slopes and in valley bottoms
(Figure 11.9A) (Péwé, 1975). At Gold Hill, their magnetostratigraphy provides a near-
complete record of deposition from approximately 3 Ma to the present (Preece et al., 1999)
(Figure 11.9B). The silty sediments are characterized by large syngenetic ice wedges (see
Figure 7.16A) and by numerous faunal and organic remains. Several major periods of
global climate warming are recorded, with times of permafrost thawing and great erosion
of loess alternating with periods of loess deposition and permafrost aggradation. A promi-
nent marker horizon is the Eva Forest Bed, a layer of now-frozen organic material that is
the remnant of an interglacial boreal forest that fl ourished approximately 125 000 years
ago (oxygen-isotope stage 5e) (Péwé et al., 1997). It is clear that the last interglacial period
in central Alaska was a period of major erosion of loess and deep and rapid thawing of
permafrost. Subsequently, during the last 100 000 years, the treeless steppe environment
returned and thawed loess deposits were refrozen.
11.4.5. Mass Wasting and “Muck” Deposits
The organic-rich and perennially-frozen silty sediments derived from reworked loess were
fi rst encountered by placer miners in the Kolyma, Klondike, and Alaska (Yukon) regions
at the end of the 19th Century. They were colloquially termed “muck” deposits, a term
now widely accepted in the English-language scientifi c literature. In central Siberia,
similar materials are termed “Ye do m a Su it e.”
Muck deposits are highly variable in composition yet, because of their ice-rich and
organic-rich nature, provide useful cryostratigraphic and paleo-environmental informa-
tion. For example, a typical cryostratigraphic section of Klondike muck deposits shows
silty material, containing large ice wedges and numerous organic and faunal remains,
overlying Late Tertiary/Early Quaternary gold-bearing creek gravels (Figure 11.10).
Although the Early Quaternary record is often incomplete, several periods of thaw and/or
erosion are usually indicated in the Late-Pleistocene sediments (Fraser and Burn, 1997;
Kotler and Burn, 2000). They are also clearly cold-climate in origin because ice wedges
are preserved in the basal unit while the main unit contains ice that has an isotopic com-
position indicative of full-glacial conditions (
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Evidence that the permafrost is syngenetic in nature is provided by aggradational ice (see
Chapters 5 and 7). Finally, thermokarst-cave ice (“pool ice”) indicates fl uvio-thermal
erosion and localized secondary modifi cation (see Chapter 8). In some places, ground-
water intrusion, probably along the base of the active layer, has resulted in massive
icy beds.
These typical characteristics of muck deposits provide useful information for paleo-
environmental reconstruction. They can also be reconciled with our understanding of
permafrost conditions today (see Part II). Fuller accounts of the numerous faunal and
organic remains that are contained within these perennially-frozen muck deposits can be
found in Guthrie (1990, 2001), Harington (2003), Schweger (1997), Zazula et al. (2003),
and others.
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