Time-transgressive deposits of repeated depositional sequences within interlobate glaciofluvial (esker) sediments in Köyliö, SW Finland

ABSTRACT Evidence of conspicuous repeated seasonal to annual deposition of glaciofluvial and glaciolacustrine sequences within a structurally complex interlobate esker segment in SW Finland is presented. The time‐transgressive, overlapping depositional sequences consist of deposits from two successive melt seasons, including three vertically stacked lithofacies associations: (1) massive to stratified coarse gravels = summer deposits; (2) trough and ripple cross‐stratified fine‐grained deposits = autumn to winter deposits; and (3) sandy stratified beds = spring deposits. The depositional environment of each lithofacies association involves a transition from subglacial or submarginal tunnel to a subaqueous re‐entrant environment, which then passes to a proglacial glaciolacustrine environment. The study also presents evidence of headward extension of subglacial tunnel deposits, related to the rapid shifting of a tunnel expansion point during the increasing spring discharge, which occupied the old tunnel exit: this mode of annual deposition has not been reported previously in esker studies. The good preservation of the rhythmic lithofacies associations is suggested as resulting from interlobate depositional conditions associated with rapidly decaying icestreams. Therefore, the depositional model may provide a key to recognizing time‐transgressive interlobate eskers that form an important geomorphological and sedimentological record of meltwater activity during the last deglaciation of the Fennoscandian and Laurentide ice sheets. The identification of time‐transgressive interlobate eskers and associated palaeo‐icestream behaviour is an essential step forward for more accurate models of ice sheet behaviour and palaeoclimatic reconstructions.