Litho‐ and chronostratigraphy of the Late Weichselian in Vendsyssel, northern Denmark, with special emphasis on tunnel‐valley infill in relation to a receding ice margin

Abstract

2009 (November): Litho-and chronostratigraphy of the Late Weichselian in Vendsyssel, northern Denmark, with special emphasis on tunnelvalley infill in relation to a receding ice margin.Lithostratigraphy and chronostratigraphy of samples from 18 deep boreholes in Vendsyssel have resulted in new insight into the Late Weichselian glaciation history of northern Denmark. Prior to the Late Weichselian Main advance c. 23-21 kyr BP, Vendsyssel was part of an ice-dammed lake where the Ribjerg Formation was deposited c. 27-23 kyr BP. The timing of the Late Weichselian deglaciation is well constrained by the Main advance and the Lateglacial marine inundation c. 18 kyr BP, and thus spans only a few millennia. Rapid deposition of more than 200 m of sediments took place mainly in a highly dynamic proglacial and ice-marginal environment during the overall ice recession. Mean retreat rates have been estimated as 45-50 m/yr in Vendsyssel with significantly higher retreat rates between periods of standstill and re-advance. The deglaciation commenced in Vendsyssel c. 20 kyr BP, and the Troldbjerg Formation was deposited c. 20-19 kyr BP in a large ice-dammed lake in front of the receding ice sheet, partly as glaciolacustrine sediments and partly as rapid and focused sedimentation in prominent ice-contact fans, which make up the Jyske Å s and Hammer Bakker moraines. In the northern part of central Vendsyssel, at least four generations of north-south orientated tunnel valleys are identified, each generation related to a recessional ice margin. This initial deglaciation was interrupted by a major re-advance from the east c. 19 kyr BP, which covered most of Vendsyssel. An ice-dammed lake formed in front of the ice sheet as it retreated towards the east; the Morild Formation was deposited here c. 19-18 kyr BP. Related to this stage of deglaciation, eight ice-marginal positions have been identified based on the distribution of large tunnel-valley systems and pronounced recessional moraines. The Morild Formation consists of glaciolacustrine sediments, including the sediment infill of more than 190 m deep tunnel valleys, as well as the sediments in recessional moraines, which were formed as ice-contact sedimentary ridges, possibly in combination with glaciotectonic deformation. The character of the tunnel-valley infill sediments was determined by proximity to the ice margin. During episodes of rapid retreat of the ice margin, tunnel valleys were quickly abandoned and filled with fine-grained sediments in a distal setting. During slow retreat of the ice margin, tunnel valleys were filled in an ice-proximal environment, and the infill consists of alternating layers of fine-to coarse-grained sediments. At c. 18 kyr BP, Vendsyssel was inundated by the sea, when the Norwegian Channel Ice Stream broke up, and a succession of marine sediments (Vendsyssel Formation) was deposited during a forced regression.

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

“…ca 10 kyr after its diversion towards the Bay of Biscay. The strong increase of the Fleuve Manche activity from ca 20 ka which is contemporaneous with the significant retreat of the FIS from its maximum extent (Marks, 2002;Vorren and Plassen, 2002;Dahlgren and Vorren, 2003;Nygard et al, 2004;Lekens et al, 2005;Rinterknecht et al, 2006;Knies et al, 2007;Goehring et al, 2008;Krohn et al, 2009) confirms the close relationship between the Fleuve Manche and the FIS. Moreover, it confirms the southwards flow of the Elbe-Weser system until the end of the last glacial period.…”

“…Moreover, the expected closeness of the BIS and FIS until ca 19-18 ka is corroborated by some significant ice advances in the North Sea before the deglaciation (e.g. Sejrup et al, 2000;Carr et al, 2006;Krohn et al, 2009;Larsen et al, 2009;Sejrup et al, 2009), the most significant being the expansion of the FIS westwards from the Norwegian Channel to the North Sea plateau between ca 22 and 19 ka (Tampen readvance, e.g. Sejrup et al, 2009) (Fig.…”

“…The significant increase of the sediment load at ca 20 ka reflects the main European deglaciation. The FIS retreat reaches a maximum at ca 19-18 ka (Marks, 2002;Dahlgren and Vorren, 2003;Nygard et al, 2004;Lekens et al, 2005;Carr et al, 2006;Rinterknecht et al, 2006;Knies et al, 2007;Goehring et al, 2008;Krohn et al, 2009) and the Fleuve Manche activity is maximal at this time, the ice sheet meltwater strongly increasing the ability of the river and its tributaries to transport sediment to the Bay of Biscay (Fig. 7).…”

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

“…Overdeepened features along the Scandinavian and Laurentian Ice Sheets were likely formed by outbursts of subglacial meltwater within very short periods of time (maximal a few hundred years; Krohn et al 2009). Although subglacial erosional features are also clearly identified in the alpine area, present knowledge does not yet allow a final judgement of whether this process was also the major factor in the formation of overdeepened valleys and basins.…”

“…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.…”