Geology Reference
In-Depth Information
(A)
(B)
Figure 12.8.
Late-Pleistocene thermokarst structures. (A) Typical “sediment-fi lled pot” contain-
ing heterogeneous locally-derived materials developed in Tertiary-age gravel, Atlantic County,
Southern New Jersey, eastern USA. (B) Bird-foot involutions in fl uvial silt and sand with organic
layers, caused by loading and density differences in water-saturated sediments during degradation
of Pleistocene-age permafrost northern Belgium.
have occurred during the thaw-degradation of near-surface permafrost bodies in mid-
latitudes during the Pleistocene.
12.3.4. Large-Scale Soft-Sediment Deformations
Much Plei sto c ene -age per ma f ro st wou ld have for med i n fi ne-grained consolidated bedrock
such as clay and shale, and in unconsolidated sand, silt, and gravel. All these sediments,
when frozen, would have probably been ice-rich because this is certainly the case for most
of these lithologies in modern-day permafrost terrains (see Part II and especially Chapter
7). It is also likely that many Pleistocene permafrost bodies were several tens of meters
thick. Therefore, as these bodies progressively thawed at the end of each cold stage, any
ice layers or massive icy bodies at depth within the permafrost would have melted. Excess
water would have resulted in super-saturation, high pore-water pressures, and deformation
through either gravitation (i.e. collapse), loading (i.e. upturning or injection), water-
escape (i.e. high pore-water pressures and low inter-particle cohesion), or a combination
of the above.
Undoubtedly, the most impressive cryogenic-induced deformations that have been
observed anywhere in the mid-latitudes are exposed in the thick sequences of unconsoli-
dated deposits in the opencast lignite workings of central Germany. They reveal large,
numerous, and complex plastic-deformation structures that indicate a near-continuous
record of changing permafrost conditions throughout the Quaternary (Eissmann, 1978,
1994, 2002). Figure 12.9 schematically illustrates the range of structures. It should be
noted that they are not all drawn at the same scale. Many (categories 1-3, Figure 12.9)
are glacigenic in nature and relate to ice movements in the North German Plain. These
are beyond the scope of this topic. Others (category 5, Figure 12.9) relate to ice-wedge
