Geology Reference
In-Depth Information
Loess is important as a Quaternary paleo-environmental indicator because a near-
complete sequence of loess deposition occurs on the Loess Plateau of Central China. By
its nature, loess is highly suitable to thermo-luminescence (TL), optically-stimulated
luminescence (OSL), and amino-acid and paleo-magnetic dating techniques (Forman et
al., 1995; Oches and McCoy, 1995; Whintle, 1990). One of the most complete records of
Quaternary environmental change in North America comes from the loess of Central
Alaska (see Chapter 11, Figure 11.9).
It is generally accepted that most mid-latitude loess formed in glacial rather than inter-
glacial times. The environments must have been cold and dry. This is indicated by three
things. First, if wetted, loess experiences shrinkage and compaction, proving that it was
not soaked by water or deposited by it. Second, much loess is calcareous and shows no
signs of leaching by water (precipitation). Third, loess often contains faunal remains,
especially land snails (mollusca) suited to cold steppe-like conditions.
In Europe, loess is thickest in the east, where it mantles both upland plateaus and
lowland plains and reaches a thickness of over 30 m. In southwestern Poland, at least fi ve
Late-Pleistocene fossil soils and several horizons containing cryogenic structures (sand-
wedge casts, involutions) are preserved within the loess (Jersak, 1973, 1977). In Belgium,
northern France and southern England, the loess is not so extensive or as thick but still
provides good evidence of cyclic environmental changes throughout much of the late
Pleistocene (Balescu et al., 1988; Haesaerts, 1983; Haesaerts and van Vliet-Lanoë, 1981;
Vandenberghe and Nugteren, 2001). In southern England, many surfi cial deposits that
mantle the Chalk and other upland surfaces clearly possess a loess fraction (Bateman,
1998; Loveday, 1962). Loess is also reported from the head deposits of southwest England
(Mottershead, 1977; Roberts, 1985).
The source area for the loess of Western Europe was probably the exposed fl oor of the
North Sea when sea level dropped during the glacial periods (see Chapter 11). This is
certainly the case for The Netherlands (Kasse, 1997). Typically, successive episodes of
loess deposition in Western Europe have left a widespread veneer of silty sediment that
has been reworked by water and incorporated into underlying materials through frost
action and solifl uction.
In North America, extensive loess deposits occur in the Missouri-Mississippi drainage
basins of north-central United States (Smith, 1964). Here, the Late-Pleistocene loess
depositional record is closely related to fl uctuations in the Laurentide ice (Forman and
Pierson, 2002). In all probability, the loess was derived from the large braided alluvial
channels formed by rivers draining from the ice margins. This is because loess thickness
progressively decreases with increasing distance from these channels. As in Europe,
various episodes of loess deposition are recognized. Faunal remains usually suggest a
forest/grassland cover rather than the steppe/tundra cover inferred for Europe.
The Loess Plateau of western China, where loess thicknesses can exceed 200-300 m,
has attracted attention from both Chinese and western scientists. Proximity to the deserts
of central Asia, combined with the progressive uplift of the Tibet Plateau during Quater-
nary times, may explain its occurrence, but a complete understanding of the origin of
much Chinese loess is still lacking. The extensive literature on the loess of central China
is not discussed here.
13.3.3. Cold-Climate Eolian Sand
Eolian sand is coarser than loess and usually in the size range 0.06-1.00 mm in diameter.
The term “cover-sand” is used to describe these sediments when a cold-climate deposi-
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