Sedimentological traces of ice-raft grounding in a Weichselian glacial lake near Dukuli (NE Latvia)

Abstract

A clearly erosional, asymmetrical structure with a large concentration of unsorted clasts (ranging from gravel to boulder size) in its deepest point is present in Weichselian glaciolacustrine, mainly fine-grained sediments exposed in an outcrop near Dukuli (NE Latvia). No traces of currents that were sufficiently strong to be capable to erode the sediments significantly have been encountered in the sedimentary succession under study, and such currents would certainly not have been capable to transport boulder-sized clasts. Neither are traces present of mass movements that could deposit the boulder-sized clasts. The glaciolacustrine setting of the succession, the deep scouring and the high concentration of large clasts must therefore be ascribed to erosion of the lake bottom by the keel of an ice raft that became grounded and gradually melted; the debris that was carried along by the ice raft was released and concentrated at the deepest point of the depression that had been eroded by it.