The spatial fades of a group of pingo remnants on the southeast Frisian till plateau (the Netherlands)

Abstract

A group of Weichselian pingo remnants on the southeast Frisian till plateau has been investigated in 2 m deep ditch exposures and in borings. Pingo geometry comprises a more or less continuous outer till rim, an inner till terrace, and a central depression, which runs down through the 3.5-5 m thick till. The rim developed in a peripheral stressfield, the inner terrace consists of sagged pingo skin, and the central depression approximately represents the former pingo crater. Sand, gyttja, and peat cover the inner terrace and fill the central depression, which possibly contains some till interlayers in the basal parts. The pingos originated at the flanks of a winding erosion valley (80 m wide), a tributary to one of the main valleys of the till plateau. This setting and the minor relief of the area indicate a closed-system pingo origin. However, the interpreted positions of the former pingo ice lenses at the base of the till layer, for pingos which are strongly different in size, may indicate some artesian influence (open-system processes). The pingos and sand wedges (2 m long, several decimetres wide) originated under an arid, arctic climate which succeeded a wet, Pleniglacial phase. Pingo degradation with some climatic amelioration was accompanied by minor lateral transport of the pingo skin, which explains the low relief of the rampart. Aeolian sand accumulated in the remnants, where furthermore loess became mixed with organic material (gyttja). Under Holocene climates aeolian transport ceased and peat growth started, together with the formation of podzolic soil profiles.