Formy pierścieniowe w rejonie Wejherowa jako prawdopodobne pozostałości po-pingo i ich znaczenie dla paleogeografii późnego glacjału w północnej Polsce = Ring forms in the area of Wejherowo as likely remnants of pingos, and their significance for Late-Glacial paleogeography in Northern Poland

Abstract

Several potential Polish locations for the occurrence of fossil pingos were determined on the basis of analyses of a digital terrain model. Subsequent field reconnaissance connected with drilling into the geological structure, confirmed that one form located NW of Gdańsk, was indeed a fossil pingo. The aforementioned forms occur in a moraine plateau area related to the last ice-sheet retreat towards the Gardno phase moraine. This surface of the plateau is noticeably inclined south-north, at elevations of between 170 and 110 metres. It in fact proved possible to identify more than 80 very well-developed fossil pingos in the area investigated, with each found to consist of a central depression of average diameter 60‒80 m, as surrounded by a rampart 3–7 m high. By drilling into the central parts of the fossil pingos, we found them to be filled with organic sediments up to 6 or 7 m deep. The bottom layer of infill has carbonate and mineral-carbonate gyttjas up to 2 m thick. These are overlain by a peat layer up to 4 m thick, while these organic sediments are underlain by gley till sand. The ramparts are of sandy till frequently intercalated with silty sand. The established sequence of infilling of the central parts of the fossil pingos indicates that, in the immediate aftermath of ice-core melting, these played host to small ponds in which the accumulation of gyttja was able to take place. The gradual accumulation of lake-bottom sediments resulted in a shallowing of the ponds and the development of peat bogs. The morphological image of the above forms and initial drilling in the studied area suggest an association between their genesis and the presence of an ice-cored mound of the pingo type, experiencing subsequent degradation in the direction of the current, fossil pingo, form. Besides the classical, literal morphology of these forms, a decisive argument for acceptance of the above concept is provided by rampart lithology indicating how essential slow processes were in their accumulation. The nature and thickness of the organic infilling in the central part of a post-pingo prove equally important, suggesting an extended period of lake and peat-bog accumulation, probably lasting for the entire Holocene. The aforementioned arguments allow for the precluding of any origin linked with direct human activity (ground construction, bomb craters). The high density and close proximity and morphological similarity of the forms are likewise inimical to an identification as craters caused by above-ground meteorite explosions. Likewise, comparative analysis of the studied forms and kettle holes (usually larger irregularly-shaped larger forms of varied bottom topography) fails to indicate that the ring forms under study here have somehow arisen through the melting of buried dead ice. Analysis of deep boreholes made previously may support a geological structure of the analysed area consisting of a sand layer over 90 metres thick covered by a discontinuous till moraine several metres thick. The thick sand layer in question consists of differently-aged glaciofluvial sediments. This is a hydrogeological window connecting three main Quaternary aquifers and offering a perfect location for the ascension of groundwater. In conditions of developing discontinuous permafrost, this movement led to the creation of pingo forms in open systems on the surface. It is clear that investigation work is not currently at a stage allowing for about as to age to be made, or all details regarding evolution provided. However, the results of planned geomorphological, hydrogeological and geochronological studies should provide for both the recognition and detailed definition of the forms, thereby prompting discussion as to the evolution of permafrost during the late Weichselian transgression and recession in Central Europe.