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

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 conditions resulting from the bedrock acting as aquitards, the narrow reverse sloping outlet and a large catchment area. While stratigraphic and lithological evidence suggests that erosion and the onset of basal sedimentation occurred during the same glaciation, different scenarios for the relative timing of infilling in relation to formation and glaciation of the bedrock trough are discussed. Overlying deltaic and glaciolacustrine sediments suggest deposition during subsequent deglaciation of the bedrock trough. The basal sediment characteristics are in agreement with previous reports in hydrogeological and seismic exploration and suggest the occurrence of similar basal successions in other subglacially overdeepened basins in the Alps and elsewhere.

High‐resolution reflection seismics reveal the structure and the evolution of the Quaternary glacial Tannwald Basin

“…; Buechi et al . ; Pomper et al . ), only a few geophysical data are available from over‐deepened basins in the German part of the Alpine foreland (Ehlers and Gibbard ; Ellwanger et al .…”

“…In contrast to studies of proximal basins (Pfiffner et al 1997;van Rensbergen et al 1998van Rensbergen et al , 1999de Franco et al 2009; Barnaba et al 2010;Brückl et al 2010;Reitner et al 2010;Dehnert et al 2012;Buechi et al 2017;Pomper et al 2017), only a few geophysical data are available from over-deepened basins in the German part of the Alpine foreland (Ehlers and Gibbard 2004;Ellwanger et al 2011). These basins are typically located distally to the corresponding catchment areas.…”

Overdeepened glacial basins as archives for the Quaternary landscape evolution of the Alps

“…The macro-and micro-sedimentology suggest that these sediments are emplaced subglacially and reflect deposition, re-working, and deformation in response to repeated coupling and decoupling of the ice-bed-interface promoted by high basal water pressures (Fig. 2, see Buechi et al 2017a). These results therefore give insights into the subglacial conditions that existed in these otherwise inaccessible bedrock troughs.…”

“…In a first step, the characteristics and origin of the infilled sediments were studied using core descriptions, micro-sedimentology in thin-sections, and compositional analysis of different grain-size fractions. The study by Buechi et al (2017a) focusses on the sedimentology of coarse-grained diamictons with sorted interbeds, which overlie bedrock in the deepest depressions and mark the onset of deposition in many other glacial bedrock troughs. The macro-and micro-sedimentology suggest that these sediments are emplaced subglacially and reflect deposition, re-working, and deformation in response to repeated coupling and decoupling of the ice-bed-interface promoted by high basal water pressures (Fig.…”

Ice‐margin fluctuation sequences and grounding zone wedges: The record of the Late Palaeozoic Ice Age in the eastern Karoo Basin (Dwyka Group, South Africa)

“…The boulder conglomerate capping this sedimentary sequence is interpreted as the most dynamic deposit, most probably deposited in an ice‐contact environment by the deconfinement of meltwater flows exiting subglacial tunnels (Russell et al , ; Alexander and Cooker, ; Aquino et al , ). Altogether, the boulder pavement, the undulating geometry of the sandbeds making up the top surface of SU2, as well as the microscopic grain bands that could be interpreted as boudins are evidence of overriding ice (Visser and Hall, ; Buechi et al , ; Busfield and Le Heron, and references therein). The rhythmic climbing ripples seem to indicate a tidal influence on deposition and might have resulted from the interaction of tides with glaciofluvial inputs to generate and/or support tide‐influenced sediment gravity flows (Smith et al , ; Cowan et al , ; Dietrich et al , ).…”