Sedimentological and GPR Studies of Subglacial Deposits in the Joux Valley (Vaud, Switzerland): Backset Accretion in an Esker Followed by an Erosive Jökulhlaup

Abstract

During the Würmian glaciation, the Jura ice sheet covered the Joux Valley (Vaud, Switzerland). A geomorphological study reveals many drumlins in this valley. Some are composed of gravels and sand, others of till. Outcrops show that the surface of the sandy-gravel drumlins is a major and sharp erosion surface. Given the lack of shearing structures in sediments below this erosion level, its origin cannot be linked to ice action of the glacier. Very high-energy subglacial meltwater floods (jökulhlaups), probably due to the drainage of subglacial or supra-glacial lakes, are the more likely cause of the erosion. Results of a ground penetrating radar (GPR) survey show the internal structure of one of these sandy-gravel drumlins to depth of 15 m. These GPR data, together with sedimentological observations, indicate that prior to erosion, subglacial sedimentation occurred in closed conduits (eskers) with strong and rapid flow variations. The sediments contain large chute-and-pool structures (high flow energy backset accretion) with dimensions comparable to the conduit width. Therefore, we interpret these sandy-gravel drumlins as portions of eskers, their present drumlin shape being the result of erosion by one or many jökulhlaups.The good preservation of the subglacial meltwater deposits is the result of the closed-basin geometry of the Joux Valley, which limited movement at the base of the glacier. This new contribution to the interpretation of the Joux Valley glacial features underlines the importance of meltwater in sedimentological processes under the Jura ice sheet.Durant la glaciation würmienne, la calotte glaciaire jurassienne recouvrait la vallée de Joux (Vaud, Suisse). Une étude géomorphologique du fond de cette vallée révèle la présence de nombreux drumlins composés de sables et de graviers stratifiés ou de till. Les affleurements montrent que la surface des drumlins gravelo-sableux correspond à une surface d’érosion importante. Étant donné l’absence de structures de cisaillement dans les sédiments sous-jacents, l’origine de cette érosion ne peut être liée au mouvement du glacier et serait due à un écoulement sous-glaciaire important (jökulhlaup) résultant du drainage rapide d’un ou plusieurs lacs supra-glaciaires ou sous-glaciaires. Une campagne géoradar sur un de ces drumlins gravelo-sableux montre sa structure interne jusqu’à 15 m de profondeur. Ces données géoradar, associées à des observations sédimentologiques, montrent qu’avant l’érosion, la sédimentation sous-glaciaire avait lieu dans des conduits fermés (eskers) avec de fortes et rapides variations de courant. Les sédiments présentent de grandes structures de chute-and-pool (accrétion en backsets liée à un fort courant) aux dimensions similaires à la largeur du conduit. Nous interprétons ces drumlins gravelo-sableux comme des portions d’eskers ayant acquis leur forme actuelle à la suite d’un ou plusieurs jökulhlaups. La bonne préservation des dépôts sous-glaciaires est attribuable à la géométrie de bassin fermé de la vallée de Joux, qui limitait le mou...

Complex kame belt morphology, stratigraphy and architecture

“…We interpret this as possible evidence for post‐depositional deformation resulting from collapse due to ice melt out/removal of ice walls during deglaciation (e.g. Flint, ; Holmes, ; McDonald and Shilts, ; Brennand, ; Fiore et al, ; Livingstone et al, ). This is consistent with the geomorphological context, as RF7 is located close to a small (30 m wide) kettle on top of the ridge (Figure A).…”

“…The entire section has been heavily deformed, with the prevalence of normal faulting indicative of gravitational failure, possibly due to the removal of supporting ice walls (e.g. Flint, ; McDonald and Shilts, ; Brennand, ; Fiore et al, ), and (sub‐)vertical clastic dykes (Figure F) recording hydrofracture during periods of high water content and hydrostatic pressure (e.g. Rijsdijk et al, ; van der Meer et al, ; Phillips and Hughes, ).…”

“…RF7 in Figure C) that are interpreted as potential evidence of deformation due to removal of lateral ice support leading to sediment collapse (e.g. Holmes, ; Fiore et al, ; Johnson and Clayton, ; Clayton et al, ; Burke et al, ).…”

Stratigraphy and infill history of the glacially eroded Matane River Valley, eastern Quebec, Canada

“…In flow with shear velocities well above critical values, large clasts have higher velocities than small clasts, which leads to strong sorting and creates an openwork texture (Shaw and Gorrell 1991). Graded bimodal gravel has been reported in glaciofluvial and subglacial deposits (Sandgren 1983;Veillette 1986;Fiore et al 2002). Several other observations could indicate intense flows near glacier ice.…”

“…Also, the up-valley orientation of stratification could indicate backset deposition. Backset deposition implies that the current was under high hydraulic pressure, like in bedrock-confined flow, and is reported from subglacial channel deposits (Fiore et al 2002). Finally, basal contact with the polished valley bedrock suggests intense erosion prior to deposition.…”