Relict Sorted Patterned Ground in Rostu, Northernmost Sweden

ABSTRACT

Relict sorted patterned ground (sorted circles and polygons) is a characteristic periglacial feature of Rostu in northernmost Sweden. In an attempt to assess the age of the formation and the time of activity, the distribution and relationship to glacial morphology were analysed with detailed cross-sections in the fine-grained centres.

Relict sorted patterned ground located on narrow, well- drained plateaus between glaciofluvial channels with truncated sorted circles and large boulders on top of the fine-grained centres suggest a pre-Late Weichselian age. This is further supported by the fact that relict sorted circles occur in a relatively older, superimposed, marginal channel system but rarely in a younger system.

Cross-sections in relict sorted circles revealed multiple soil injections, interstratified with organic horizons. This stratigraphy was interpreted to be a result of several phases of reactivation with diapiric soil injections from below, increasing the height of the fine-grained centre up to a metre. The main mechanism of the soil injections is suggested to be translocation of silt and clay, causing an increased frost susceptibility at deeper layers and differential frost heave. Radiocarbon datings suggest a minimum circle age of 5 200 BP.

Keywords:

• Relict sorted patterned ground
• spatial distribution
• stratigraphy
• reactivation
• radiocarbon dating