CONTEMPORARY SAND WEDGE DEVELOPMENT IN EOLIAN SETTINGS OF SEASONALLY FROZEN GROUND

Contemporary sand wedges and sand veins occur within active and stabilized aeolian settings in seasonally frozen ground, in the extensive discontinuous permafrost zone in Northwest Territories, Canada. The region has a subarctic continental climate with 291 mm a-1 precipitation, -4.1°C mean annual air temperature, warm summers (July mean 17.0°C), and cold winters (January mean -26.6°C). Five years of continuous observations indicate that interannual variation in winter air temperature and snow cover conditions control ground thermal regimes. At sandy aeolian sites, thin snow cover and high thermal conductivity promote rapid freezing, high rates of ground cooling, and low near-surface ground temperatures (-15 to -25°C). Thermal contraction cracking occurs to depths of 1.2 m. Cracking potentials are high in sandy soils when air temperatures are <-30°C, mean freezing season air temperatures are ≤-17°C, and snow cover is <0.15 m thick. In contrast, surface conditions in peatlands maintain permafrost, but thermal contraction cracking does not occur because thicker snow cover and the thermal properties of peat prolong freezeback and maintain higher winter ground temperatures. Radiocarbon dating, optical dating, and stratigraphic observations differentiates sand wedge types and formation histories. Thermal contraction cracks beneath bare sandy terrain become infilled with surface and/or host material during the thaw season. Epigenetic sand wedges with available surface sand for infill develop within former beach sediments beneath an active aeolian sand sheet. Narrower and deeper syngenetic wedges developed within actively aggrading aeolian sand sheets, whereas wider and shallower antisyngenetic wedges developed in areas of active erosion. Thermal contraction cracking beneath vegetation-stabilized surfaces leads to crack infilling by host sediments and overlying organic materials, with resultant downturning and subsidence of adjacent strata due to lack of available sediment for infill. Sand wedge development in seasonally frozen ground with limited surface sediment availability can result in stratigraphy similar to ice-wedge and composite-wedge pseudomorphs. Therefore, caution is recommended when interpreting this suite of forms and inferring paleoenvironments.