Ninth International Conference on Permafrost ... - IARC Research

Pleistocene Sand-Wedge, Composite-Wedge, and Complex-Wedge Growth inFlanders, BelgiumG. GhyselsDepartment of Geography, Ghent University, Krijgslaan 281-S8, Ghent, BelgiumI. HeyseDepartment of Geography, Ghent University, Krijgslaan 281-S8, Ghent, BelgiumJ.-P. BuylaertNordic Laboratory for Luminescence Dating, Department of Earth Sciences, University of Aarhus, Risø National Laboratory,DK-4000, Roskilde, DenmarkA.S. MurrayNordic Laboratory for Luminescence Dating, Department of Earth Sciences, University of Aarhus, Risø National Laboratory,DK-4000, Roskilde, DenmarkD. VandenbergheDepartment of Geology and Soil Science, Ghent University, Krijgslaan 281-S8, Ghent, BelgiumF. De CorteLaboratory of Analytical Chemistry, Institute for Nuclear Sciences, Ghent University, B-9000 Ghent, BelgiumP. Van den hauteDepartment of Geology and Soil Science, Ghent University, Krijgslaan 281-S8, Ghent, BelgiumIntroductionReconstruction of former periglacial environments inEurope heavily relies on the identification of relicts andpseudomorphs of thermal-contraction-cracking wedges insoils (Vandenberghe & Pissart 1993). During the past sevenyears, a large number of these wedge-shaped sedimentarystructures have been identified in lowland Belgium (Ghysels& Heyse 2006, Ghysels 2008). Wedge forming processeshave been carefully reconstructed, providing new informationon Pleistocene palaeoenvironmental and palaeoclimaticconditions of the Flemish sandy lowlands. Investigationmethods included field observations, sedimentologicalanalysis, and optically stimulated luminescence (OSL)dating.DiscussionThe results point to a greater complexity of wedgeformingprocesses and palaeoenvironments compared tothose of previous studies. The latter stressed the dominanceof ice-wedge pseudomorphs (e.g., De Moor et al. 1978,Heyse 1983, 2000, Vandenberghe & Pissart 1993, Huijzer& Vandenberghe 1998). Therefore, former reconstructionsmay have been biased towards conditions suitable for icewedgegrowth. Sparse vegetation and snow covers duringwedge growth may also have favored efficient coolingResultsA key observation includes the identification of compositewedgepseudomorphs, relict sand wedges, and complexwedges. Their appearance was variable. Some structuresshowed a typical wedge shape (Fig. 1), while othersappeared as irregular features with branching sand veins andbundles (Figs. 1, 2). The majority of the wedges compriseda laminated infill, although some showed a massive infill.Besides mature wedge forms, elementary sand-filled veinsand groups of sand-filled veins similar to Murton’s (1996)sand veins in Canadian sand sheets were also identified. Theveins and wedges penetrated Weichselian aeolian and fluvioaeoliansand sheets (Fig. 3), but sometimes extended intounderlying Tertiary sands and clays (Figs. 2, 3).All these structures suggest thermal-contraction-crackingof frozen soils in a former cold periglacial environment.Their windblown infill points to sand transport on a sparselyvegetated ground surface with a limited snow cover duringwedge growth (Murton et al. 2000).Figure 1. Vertical cross-section of a composite-wedge pseudomorphin Belgium (Ghysels & Heyse 2006). Note the upturned host sedimentsand the vertical laminated infill, pointing to the progressiveinfilling of thermal contraction cracks with sand, sand-ice, and ice.Downturned host strata and steeply-dipping tongues of host sedimentlikely formed as ice melted (Murton & French 1993).85