Rapid tunnel‐valley formation beneath the receding Late Weichselian ice sheet in Vendsyssel, Denmark

Abstract

Interpretation of Transient ElectroMagnetic (TEM) data and wire-line logs has led to the delineation of an intricate pattern of buried tunnel valleys, along with new evidence of glaciotectonically dislocated layers in recessional moraines in the central part of Vendsyssel, Denmark. The TEM data have been compared with recent results of stratigraphical investigations based on lithological and biostratigraphical analyses of borehole samples and dating with Optically Stimulated Luminescence (OSL) and radiocarbon. This has provided an overview of the spatial distribution of the late Quaternary lithostratigraphical formations, and the age of the tunnel valleys has been estimated. The tunnel valleys are typically 5-10 km long, 1 km wide and are locally eroded to depths of more than 180 m b.s.l. The valleys are interpreted to have been formed by subglacial meltwater erosion beneath the outermost part of the ice sheet during temporary standstills and minor re-advances during the overall Late Weichselian recession of the Scandinavian Ice Sheet. The formation of the tunnel valleys occurred after the retreat of the Main ice advance c. 20 kyr BP and before the Lateglacial marine inundation c. 18 kyr BP. Based on the occurrence of the tunnel valleys and the topography, four ice-marginal positions related to the recession of the northeastern Main advance and seven ice-marginal positions related to the recession from the following eastern re-advance across Vendsyssel are delineated. All the tunnel valleys were formed within a time interval of a few thousand years, giving only a few hundred years or less for the formation of the tunnel valleys at each ice-marginal position.

Distribution, geometry, age and origin of overdeepened valleys and basins in the Alps and their foreland

“…During the ice retreat, frozen ground was left beyond the ice margin and subglacial meltwater catastrophically drained through the tunnel valleys. Sandersen et al (2009) demonstrate that the incision of tunnel valleys in Vendsyssel, Denmark, that reach 180 m below sea level, was related to the main Weichselian advance and a later re-advance of the Scandinavian Ice Sheet. Luminescence dating of sediment in which the valley was eroded and of its sedimentary fill (Krohn et al 2009) shows that nine generations of individual tunnel valleys formed between c. 20 and 18 ka ago.…”

“…Thus, the formation of each individual tunnel valley must have occurred in less than a few hundred years. Sandersen et al (2009) suggest that the process behind tunnel valley formation are repeated out-bursts of meltwater that eroded narrow subglacial channels. With the decrease of water pressure after each outburst, the channels were closed by ice or re-filled with glacial sediments and gradually broadened and widened by glacial erosion.…”

The first estimation of Fleuve Manche palaeoriver discharge during the last deglaciation: Evidence for Fennoscandian ice sheet meltwater flow in the English Channel ca 20–18 ka ago

“…7). The concomitant rapid tunnel-valley formation beneath the receding south-western FIS (Sandersen et al, 2009), which displays strong erosional processes and sediment transport through large quantities of meltwater to the southern North Sea, could also have enhanced the contemporaneous Fleuve Manche sediment load, thus showing that the expected proglacial lake between the FIS and the Dover Strait, if it existed, did not act as a significant sediment trap. Anyway, the minimum mean sediment load is estimated at ca t yr -1 around 18 ka but the seasonal behaviour of the Fleuve Manche runoff in response to the seasonal variability of the meltwater production from the ice sheet at this time (Eynaud et al, 2007;Toucanne et al, 2009a) probably implies a strong sediment load variability around the estimated value.…”

Compilation of a Resistivity Atlas of Danish lithologies based on direct resistivity measurements and wireline logging data

“…Only some studies based on borehole lithology and wireline resistivity logging related to specific surveys, covering smaller regions, are found (Jørgensen et al 2005, Sandersen et al 2009). Now, the use of densely sampled large-scale airborne electromagnetic (AEM) surveys in groundwater mapping call for procedures to integrate the geophysical information in the geological and hydrological modelling in an objectively and efficient manner way.…”