A window into development of a complex ice-marginal lake prior to the Late Glacial Maximum (LGM) in Austria

Abstract

Although the retreat process of glaciers from the Late Glacial Maximum (LGM) is well documented, high-resolution insight into conditions prior to the maximum is lacking in the eastern European Alps, resulting in a gap in our understanding about the processes associated with this important climatic tipping point. We describe an outstanding sand and gravel outcrop at Gröbming in the Enns Valley (Ennstal), Austria, that represents the development of a delta complex that debouched into a large body of water that we name paleo-Lake Gröbming, fed by a major valley glacier. The succession consists of sands bearing climbing ripples, parallel laminations, and supercritical bed forms (bottomsets) overlain by meter-scale gravelly foresets. Topsets comprise gravels. We argue that sustained accumulation of supercritical bed forms required a jet efflux feeder mechanism best explained by a direct glacial meltwater source. Complex vertical and lateral repetition of this tripartite succession is observed, with sequence stratigraphic analysis permitting us to argue that stratal complexity is attributable to base-level changes in paleo-Lake Gröbming resulting from dam breaches of the lake. Thus, application of well-established sequence stratigraphic approaches to Quaternary ice-marginal successions in the Alps has significant potential to yield unprecedented insights into conditions prior to the LGM.