Contemporary sand wedge development in seasonally frozen ground and paleoenvironmental implications

摘要

Contemporary sand wedges and sand veins are active in seasonally frozen ground within the extensive discontinuous permafrost zone in Northwest Territories, Canada. The region has a subarctic continental climate with 291 mm a(-1) precipitation, -4.1 degrees C mean annual air temperature, warm summers (July mean 17.0 degrees C), and cold winters (January mean -26.6 degrees C). Five years of continuous observations indicate that interannual variation of the ground thermal regime is dominantly controlled by winter air temperature and snow cover conditions. At sandy 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 degrees C), resulting in thermal contraction cracking to depths of 12 m. Cracking potentials are high in sandy soils when air temperatures are -30 degrees C on successive days, mean freezing season air temperatures are =-17 degrees 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. A combination of radiocarbon dating, optical dating, and stratigraphic observations were used to differentiate sand wedge types and formation histories. Thermal contraction cracks that develop in the sandy terrain are filled by surface (allochthonous) and/or host (autochthonous) material during the thaw season. Epigenetic sand wedges infilled with allochthonous sand develop within former beach sediments beneath an active eolian sand sheet. Narrower and deeper syngenetic wedges developed within aggrading eolian 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 autochthonous host and overlying organic material, with resultant downtuming and subsidence of adjacent strata. Sand wedge development in seasonally frozen ground with limited surface sediment supply can result in stratigraphy similar to ice-wedge and composite-wedge pseudo morphs. Therefore, caution must be exercised when interpreting this suite of forms and inferring paleoenvironments. Crown Copyright (C) 2018 Published by Elsevier B.V. All rights reserved.