Ninth International Conference on Permafrost ... - IARC Research

Thixotropic Wedges or Frost Cracks: A Review from the Pannonian Basin(Hungary, Europe)János KovácsDepartment of Geology, University of Pécs, Pécs, HungarySzabolcs Ákos FábiánDepartment of Physical Geography, University of Pécs, Pécs, HungaryGábor VargaDepartment of Physical Geography, University of Pécs, Pécs, HungaryIstván Péter KovácsDoctoral School of Earth Sciences, University of Pécs, Pécs, HungaryGyörgy VargaDoctoral School of Earth Sciences, University of Pécs, Pécs, HungaryIntroductionThere is a general consensus about the study andinterpretation of soft-sediment deformation structures beinghelpful in paleoenvironmental reconstructions. During theglacial periods of the Pleistocene, Hungary was subjectto a cryogenic environment that produced various relictperiglacial features (Dylik 1963, Pécsi 1964, Tarnocai &Schweitzer 1998, Fábián et al. 2000, Kovács et al. 2007).The reason for the cold climate during these glacial periodsis Hungary’s unique geomorphological setting in thePannonian Basin. The Carpathians, which surround this largebasin, create an almost closed climatic situation, producingclimatic conditions not found elsewhere in Europe. In effect,Dylik seems to imply that the climate in Hungary during theglacial periods of the Pleistocene was somewhat similar tothe recent climate of the dry tundra regions of North Siberia.According to Maarleveld (1976) and Van Vliet-Lanoë et al.(2004), however, the Pannonian Basin was mostly devoid ofcontinuous permafrost during the Quaternary.General SettingIn the Pannonian Basin (Fig. 1), the continental crust isthinner and the stress field more complex, owing to a lateralshift to the east of the basin, which is squeezed between therigid northeast European basement and the Alpine orogene,with transpressional and transtensional phases alternatingfrom the upper Cenozoic to today (Csontos et al. 2002). Thepresent-day seismicity is higher than in northwest Europe,but is still of low magnitude.As a result of the cold climatic conditions (mentionedabove) in the Pleistocene epoch, various types of periglacialfeatures developed and are found in well-preserved relictforms in various deposits. Dylik (1963) summarizes therelict periglacial features of Hungary and discusses theiroriginal development, while Pécsi (1964) provides detaileddescriptions, including diagrams and photographs, of thesefeatures with the locations in which they occur. In the lastdecade, researchers investigated these cryogenic features fortheir implication for past climate (Tarnocai & Schweitzer1998, Fábián et al. 2000, Kovács et al. 2007) and for theirFigure 1. Location of the study sites in Hungary.distinction between tectonic and periglacial deformation(Van Vliet-Lanoë et al. 2004, Horváth et al. 2005, Magyariet al. 2005).MethodsOur interpretation of the periglacial deformationsinvestigated is based on (a) detailed field observations and(b) a synthesis of published data. Polygenetic forms wereanalysed using criteria defined for the periglacial andseismogenic structures. Sections were cleaned and describedusing sedimentological and pedological criteria, with respectto the topographical location and available moisture. Samplesfor grain size analyses and OSL dating were collected fromindividual fill units, and additional samples were collectedfor moisture content and background radiation measurement.The samples were dated at the Geological Institute ofHungary.ResultsThixotropic wedgesThixotropic wedges, developed in fine sandy-to-pebblydeposits, are formed within the sediment but relatively closeto the surface (Montenat et al. 2007). The wedges may be147