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Structures Caused by Repeated Freezing and Thawing in Various Loamy Sediments: a Comparison of Active, Fossil and Experimental Data

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Structures Caused by Repeated Freezing and Thawing in Various Loamy Sediments: a Comparison of Active, Fossil and Experimental Data EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 9,553-565 (1984) STRUCTURES CAUSED BY REPEATED FREEZING AND THAWING IN VARIOUS LOAMY SEDIMENTS: A COMPARISON OF ACTIVE, FOSSIL AND EXPERIMENTAL DATA BRIGITTE VAN VLIET-LANOE AND JEAN-PIERRE COWARD Centre de Giomorphologie du C.N.R.S., Rue des Tilieuis, 14000 Caen, France AND ALBERT PISSART Laboratoire de Giographie Physique et Giofogie du Quarernaire. Universiti de Li2ge, 7 place du XX Aolit. 4000 Liege, Belgique ABSTRACT In this paper, the authors present the results of both macroscopic and microscopic investigations on structure development created by repeated ice lensing in various loamy experiments. Experimental data are compared with observations performed on active forms in High Arctic and Alpine Mountain environments. Those observations are also compared with phenomena observed in fossil periglacial formations of Western Europe. Platy and short prismatic structure formation is bonded to the hydraulic and thermal conditions during ice segr%ation. When a long series of alternating freezing and thawing affects platy structures, the fabric evolves, also being influenced by slope and drainage conditions: cryoturbations, frostcreep, and gelifluction can appear. They are characterized by specific microfabrics which are better developed with an increasing number of cycles: this is clear in experiments where hydraulic and thermal parameters are better controlled. Vesicles are also a prominent characteristic of the surface horizon in experiments and arctic soils. The genesis of vesicles is discussed on the basis of new observations and is related to the mechanical collapse of frost-created aggregates under the mechanical work of soil air escape during soil saturation by water at thaw. KEY WORDS: Ice segregation Loamy textures Structural fabrics Experimental, active and fossil data Micromorphology Single ice lensing structure Sorted platy structure Frostcreep fabric Gelifluction fabric Cryoturbation Vesicles INTRODUCTION For ten years, frost experiments have been performed in both the Caen and Likge laboratories to investigate the structures of various loamy sediments as observed in fossil soils and sediments of Western Europe and to determine their palaeoclimatic significance. These textures are particularly frost susceptible and in massive sediment, it is rather easy to observe the structures created by ice segregation. Field and microscopic observations in High Arctic environments and in Alpine Mountains complete this work. Usually, when a sediment with a minimum content of fines is affected by slow freezing, ice lensing occurs from the surface downwards with a rather regular pattern. In most of the cases, the ice lenses are wider spaced with depth in accordance with the lowering of the cooling rate and the progressive exploitation of the water reserve present in the sediment. Usually the lenses nucleate behind the freezing line (Williamsand Wood, 1982). This feature occurs in non-saturated sediments by capillary feeding at the freezing plane created by a thermal gradient and the subsequent suction. A temporary break in the waterfilm and in its associated thermal contribution (latent heat of crystallization and soil heat transferred by water) favour a sudden displacement of the freezing plane deeper into the soil to more humid material, until the water/thermal equilibrium is restored. In the experiments reported here, permafrost was not reconstructed; most of the hydraulic (drainage) and 0197-9337/84/060553-13$01.30 0 1984 by John Wiley & Sons, Ltd. 554 B. VAN VLIET-LANOP. J-P. COUTARD ET A. PISSART thermal conditions used here were comparable to those occurring in cold temperate areas without permafrost, in the upper horizons of the active layer. In the different experiments from which results are reported here, the lowest temperature recorded reached -7°C; in the field, like in the winter 1982-83 in Northwestern Spitzbergen, temperature reached - 17°C at 20cm of depth. Loams used in experimental works are extracted from different loess-like deposits from Belgium and Normandy. Observations were conducted in the same way in the field and in experiments, at both macroscopical and microscopical level. Macroscopical observations are performed following both microstratigraphical and pedological techniques. Vertical cuts are smoothed and polished with a sharp knife; structures are described from natural faces. All the observations are drawn in close details. Micromorphological observations are performed on thin section: the sample is slowly air dried (two months, woven in a soft paper to avoid shrinkage), and impregnated under vacuum with a polyester resin; sections are 40 to 30 pm thick following the details to be observed. In so far as it is possible, the thin sections are described in language that can be understood by non-specialists. Single ice lensing To develop this type of formation, the material must undergo at least one but generally less than ten freezethaw cycles; in other words, it is produced by a small number of freeze-thaw cycles. Platy structure After melting of the segregated ice lenses, also called thaw consolidation, a platy structure is observed, as commonly described in the literature (e.g. Kokkonen, 1926; Taber, 1930; Pissart, 1970). Like the ice lenses themselves, the size of the platy aggregates (structure from a pedological viewpoint) is controlled by the thermal gradient, in other words by the depth and the water content. It ranges from a tenth of millimetre to several centimetres in thickness. In sediments on loamy sediments, structure development ranges from ‘thinly bladed‘ (0.5 to 1.0 mm) 1-3 cm from the soil surface to ‘fine platy’ at 25 cm depth (2 to 4 mm in thickness). Microscopically, platy aggregates are separated by smooth fissures, gently undulating and with un- conformable walls, as a main contrast with desiccation cracks (Van Vliet-Lanoe, 1976). This unconformity of the walls is often associated with a ‘gaping’character of the fissure (Figure 1). Ice segregation usually develops on a previous desiccation crack appearing below the freezing front (Beskow, 1935; Van Vliet-Lanoe, 1976)and smooths the roughness of its walls. Close to the soil surface, ice segregation is less intense than at depth and the fissure left by the ice after thawing at this level is very close to the shape of the desiccation cracks. The length/thickness ratio of the ice-created plates (Van Vliet-Lanoe, 1976) is generally close to 3 or lower, but in such a surface horizon, it can be a little higher. The bulk density of these aggregates, as seen in thin sections, is also less close to the surface (Figure 2) than at a greater depth, resulting in a more effective ice segregation. Those observations are very important in order to explain the relative low firmness of the frost-formed aggregates of the soil surface compared to their extreme firmness at depth. During the experiments performed in Caen, the water content reached after thawing was only 20 per cent by volume. Here the platy aggregates were relatively close to those created by desiccation, with nearly complementary fissure walls. In contemporary lacustrine mud (silty loam) of a temporary lake (lac du Gouffre, La Mortice, southern French Alps), the water content by volume was after thawing higher than 30 per cent (thixotropic);the thickness of the aggregate was 2-3 mm at a depth of lOcm which is similar to the experiments. A discontinuous fissure separating the aggregates is clearly visible, ranging from 0.1 to 0.5 mm in width. In a marine loamy clay (near Fort Chimo, Ungava Bay, Canada), the platy structure in a non-permafrost area ranges from thinly bladed close to the surface to coarse platy at a depth of 140cm; the discrete ‘gaping’ of the fissures ranges from 0.1 to 0.2 mm in well-drained sites and up to 1.0 mm in poorly-drained ones. In a close area (Ramah Bay, Canada) another glaciomarine loamy clay presents a well-developed platy structure at a depth of 60cm, just below the top of a permafrost. The discrete ‘gaping’ of the fissure has a width of 1.5mm after slowly thawing the segregated ice (thermokarst) (Figure 1). The unconformity of the walls and the width of the fissure left by ice lensing is the expression of a local more accentuated growth of ice, as a result of an abundant water supply. It is in agreement with the drainage conditions and also with the possibility of water migration bonded to the texture at temperatures lower than 0°C (Burt and Williams, 1976) which may occur at temperatures as low as - 4°C in loams and - 20°C in loamy clays. These features can also be accentuated by the refreezing at ICE LENSING STRUCTURES 555 Figure 1. Traces of ice lenses in the top horizon of permafrost developed in a glaciomarine clay loam (Bay of Ramah, Labrador, Canada). The fissures left by the slowly thawed ice lenses are wide open (F). Notice the small lateral displacement (arrows) bonded to thaw settlement at the level of an older bioturbation (more sandy area) Figure 2. Frostcreep fabric produced experimentally in Caen after six cycles of alternating freezing and thawing. The interaggregate fissures are rather rough like desiccation cracks and the aggregates are weakly compacted. Notice the presence of a slipping plane associated with an incipient skeleton infilling (arrow) 5 56 B. VAN VLIET-LANOE. J-P. COUTARD ET A. PISSAR’I depth of thaw water as observed by Hoekstra (1969) in the top layer of permafrost, and by Mackay (1980) in hummocky soils. Aggregates created in sediments by ice lensing are generally very stable because of their mechanical compaction (Van Vliet-Lanoe, 1976) and their ultradesiccation (Pissart, 1970; Journaux and Coutard, 1972). They are responsible for the characteristics of a ‘periglacial’ type of fragipan horizon which indicates a former permafrost top horizon (Van Vliet-Lanoe and Langohr, 1981a). In the field and in thin sections, small lateral displacements of the aggregates can be observed resulting in irregular settlement after the melting of the entire segregated ice (Figure 1).
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