‘Dancing’ load casts and pseudonodules as characteristic features of glaciolacustrine kames

Abstract

Kames (ice-walled lake plains) are common forms in glacigenic landscapes, widely recognised as indicators of areal deglaciation. In northeastern Poland and southern Lithuania, fine-grained kames are characterised by similar morphology and sedimentological features. This study examines three of these, which characterise a particular style of deformation. The kames are characterized by a distinctive sediment grain size, comprising an admixture of clay, a full range of silt fractions, and fine-grained sand. Based on this granulometric composition, all of the studied kames are classified as glaciolacustrine (glaciolimnic, limnoglacial), subsequently infilled by meltwaters. A characteristic feature of all the kame sediments is the presence of extraordinarily rich soft-sediment deformation structures (SSDS), such as load casts, pseudonodules, flame structures, clastic dykes, dish structures, and fragments of broken-up set of laminae which occur in very thick layers. The largest load casts and pseudonodules, with diameter reaching up to 0.7–2 m, can enclose two, three, or even four generations of smaller load casts and pseudonodules. This complexity results from multiple stages of kame sediment deformation processes, influenced by a large volume of water present during sedimentation and deformation, as well as by variations in sediment density and the melting of ice walls surrounding the glacial lake. The specific water regime played an essential role in SSDS formation. Sedimentation must have occurred before the melting of the depressions’ ice walls, within which the kame sediments were accumulated.

We suggest that the large size and variable reorientation of the load casts and pseudonodules (described as ‘dancing’) are significant criteria for classifying kames as glaciolacustrine, deposited between blocks of dead ice under fully saturated conditions. We suggest currents of water, ice cracking along with the dumping of ice blocks and density gradients within the sediments, as the three main trigger mechanisms responsible for the deformation processes.