Thermal-contraction-crack polygons are the most widespread, most visible, and most characteristic feature of permafrost terrain. Thermal-contraction cracks can be filled with ice, mineral soils (sand, loess, etc.), or a combination of both .
In cold aeolian environments, the progressive primary infilling of thermal contraction cracks with sand forms distinctive sedimentary structures known as sand wedges . Wedge-shaped bodies of sand can also form by secondary infilling of voids resulting from thaw of ice veins and ice wedges, producing ice-wedge casts; and by processes unrelated to thermal contraction cracking [14, 41, 43].
Thermal contraction cracking occurs when reduced frozen ground temperatures generate tensile stresses greater than the tensile strength of the ground. Such stresses are favored by both rapid cooling and low temperature , but the controls on cracking are complicated by such factors as snow thickness and creep of frozen ground . Open cracks can be 1-100 mm wide, 2.0-5.0 m or more deep and 1 m to several tens of meters long [35, 36]. In plan view, the cracks form orthogonal or non-orthogonal networks, according to the kind of angles where cracks or their projections intersect; and complete or incomplete, depending on whether the cracks join . Polygon diameter varies from a few meters to > 100 m.
The local character of the aeolian infill is indicated by the close correspondence between the grain-size parameters of the overlying sands and the crack infill material (R2=0.96). Contraction fissures were filled from the surface with allochthonous, mostly sandy sediments. Infilling usually was (probably) carried out by wind action. Aeolian activity during the development of the sand-wedge polygons resulted in a desert pavement on the surface. The sand in these wedges shows sub-vertical lamination, suggesting that frost cracking occurred a number of times. Upturned strata of the host probably resulted from deformation caused by the addition of wedge material to permafrost and ground expansion in summer . The irregular margin of the cracks (in plan view) may were formed upon thawing of the permafrost.
Numerous, mainly horizontal but some vertical, cracks 2-5 mm wide filled with CaCO3 are present in the red paleosol. The origin of these cracks is most likely associated with vein ice development in permafrost environment .
It was expected that the wedges had developed during the Late Pleniglacial (Würm) glacial period, based on other western and central European information [2, 28, 42, 48, 74]. Both the samples gave ages within the last part of the Late Pleniglacial. As expected from the structure of the wedge, sample M 5/4 provided a younger age (20.75 ± 2.3 ka B.P.) and sample M 5/5 gave an older age (22.66 ± 2.86 ka B.P.) . This period (22-20 ka B.P. MIS 2) could be related with North Atlantic cold Dansgaard-Oeschger stadials (DO 1-2) associated with Heinrich-events 1-2 [52, 76].
Ice-Wedge Pseudomorphs (Casts)
Ice-wedge networks are patterns in permafrost that form by filling cooling-derived tension fractures with ice. They form in a broad range of stable, aggrading or sloped surfaces, with mean annual temperatures below -4°C to -6°C, and winter temperature extremes ranging from -15°C to -35°C . Individual wedges are tens to hundreds of meters long, commonly terminating orthogonally at another wedge. The overall orientation of the network usually is random, but sometimes is dictated by the trace of a river or a shoreline. Wedges usually penetrate 3-6 m where set in non-aggrading surfaces. Enclosed regions between wedges are polygonal, mostly squares to hexagons, with diameters commonly 10-30 m. Despite numerous commonalities, ice-wedge networks also differ amongst and within sites, displaying complicated behaviors that include variable frequency of fracture and growth in ice wedges, differing spacing and relative orientation of wedges in the network pattern, and varying modes of deformation of frozen ground around expanding wedges.
Casts, or pseudomorphs, represent the previous shape of a structure and can be formed in materials other than that which formed the original structure. Ice-wedge pseudomorphs are wedges of secondary mineral infilling and are thermokarst structures because they result from the thaw of excess ice . Ice-wedge pseudomorphs form when the ice in the wedge slowly melts, usually as permafrost degrades. As this happens, there is a general collapse of sediment into the void that is created. These kinds of wedges are from mid-latitude regions where permafrost no longer exists. Most ice-wedge pseudomorphs are found in gravel. Ice wedges preferentially develop in ice-rich, fine-grained sediments (thaw-sensitive); their pseudomorphs are selectively preserved in ice-poor, coarse-grained sediments (thaw-stable) .
Most patterned-ground phenomena form within the active layer. Circles, earth hummocks, and mudboils are undoubtedly the most common. Some patterned ground may also form in seasonally frozen ground .
Low-centered polygons are characteristics of poorly drained tundra. They commonly possess a double raised rim, or rampart, often in excess of 50 cm in height, on either side of the ice-wedge trough. The depressed wet center contains sedges and grass. The raised rims are the results of thermal expansion within the active layer moving material from the polygon center to the periphery.
Thermokarst Involutions (Cryoturbations)
Past thermokarst activity may be recognized in the stratigraphic record by the existence of paleo-thaw layer. In present permafrost regions, paleo-thaw layers often correspond to the secondary thaw unconformities . Paleo-thaw layers have been described from perennially-frozen sediments in several areas of the western North American Arctic. Late-Pleistocene paleo-thaw layers have been inferred from studies at a number of localities in the lowland of western and central Europe [9, 10, 20, 34, 65, 71, 72]. These structures occur widely in the near-surface sediments of mid-latitude areas.
Smaller deformation structures of the bird-foot or drop soil type are also caused by loading and density differences in water-saturated sediments, probably during the degradation of underlying permafrost.
Soft sediment deformations observed at Atkar site are similar but more strongly expressed than the ones at Visonta site. Traces of ice segregation on the top of the eroded Late Miocene alluvial formations suggest that frost action played a significant role in the formation of these involutions. Other observations of cryogenic features in the area [8, 40, 47] and the comparison of other cryogenic features presented by Van Vliet-Lanoe [67, 68] support the cryogenic origin. Compact aggregates of micritic carbonate and clay organized into horizontal, subhorizontal layers and occurring in association with the platy breccia structure. On the margins of these aggregates forming the platy, lenticular structure, Fe-oxide hypo-coatings are clearly discernible (cf. Plate III. Fig. 10. in ).
The platy, lenticular structure of paleosols could be correlated with segregated ice of ice lenses growing parallel to the thermal gradient in the sediment, usually roughly parallel to the soil surface. Aggregates created by ice lenses are generally very stable [67, 70].
In Visonta site, the flames are built up by powdery calcrete and are interpreted as water escape structures. The formation of water escape structures was accompanied by the development of drops, pillows or isolated patches (irregular pseudonodules) and involutions. The general presence of the platy, lenticular structure indicating frost action on the uppermost part of the Late Miocene sediments suggest that the involutions in the Post Miocene soil-sedimentary complex were formed as a result of cryoturbation.
All the soil features from Paks site appear to result from cryogenic processes. They probably developed in a cold environment, when cryogenic processes were active and permafrost conditions existed at this site.
Similar cryogenic features are found in contemporary sandy permafrost soils throughout the Canadian north. The active layers of two such soils, both having permafrost 60 to 80 cm beneath the soil surface, are shown in Figure 5. The brownish B-horizons and the dark-colored organic horizons of this soil are contorted, broken and displaced as a result of cryoturbation. The grayish sand in the middle of the photograph has been pushed up from the underlying parent material by cryoturbation. It has high organic matter content and shows similar effects of cryoturbation.
The Stirling Bend paleosol developed during the early Pleistocene epoch under a cold, probably glacial, climate . As a result of cryogenic processes, it has contorted organic and mineral horizons and a hummocky microtopography.
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