Subglacial emplacement of tills and meltwater deposits at the base of overdeepened bedrock troughs

作者：Marius W. Buechi, Stephan M. Frank, Hans Rudolf Graf, John Menzies, Flavio S. Anselmetti · 发表于：Sedimentology · 年份：2016 · DOI：10.1111/sed.12319 · 被引用次数：43 · 研究领域：Geology and Paleoclimatology Research、Geological formations and processes、Landslides and related hazards

Abstract The sedimentary infills of subglacially eroded bedrock troughs in the Alps are underexplored archives for the timing, extent and character of Pleistocene glaciations but may contain excellent records of the Quaternary landscape evolution over several glacial–interglacial cycles. The onset of sedimentation in these bedrock troughs is often reflected by diamicts and gravels directly overlying bedrock in the deepest basin segments. Subglacial or proglacial depositional environments have been proposed for these coarse‐grained basal units but their characteristics and origin remain controversial. This article presents results from drill cores that recovered a coarse‐grained basal unit in a major buried bedrock‐trough system in the Lower Glatt Valley, northern Switzerland. The excellent core recovery allowed a detailed study combining macroscopic, microscopic and geochemical methods and gives unprecedented insights into the transition from erosion to deposition in overdeepened bedrock troughs. These results show that the basal infill comprises diamicts, interpreted as subglacial tills, separated by thin sorted interbeds, originating from subglacial cavity deposition. The stacking of these units is interpreted to represent repeated switching between a coupled and decoupled ice–bed‐interface indicating an ever‐transforming mosaic of subglacial bed conditions. Decoupling in response to high basal water pressures is probably promoted by the confined subglacial hydraulic condit...