Forskning ved Københavns Universitet - Københavns Universitet


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

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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 WeichselianMain advance c. 23-21 kyrBP, Vendsyssel was part of an ice-dammed lake where the Ribjerg Formation was deposited c. 27-23 kyrBP. The timing of the Late Weichselian deglaciation is well constrained by the Main advance and the Lateglacialmarine inundation c. 18 kyr BP, and thus spans only a fewmillennia. Rapid deposition ofmore than 200m 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 45-50m/yr in Vendsyssel with significantly higher retreat rates between periods of standstill and re-advance. The deglaciation commenced in Vendsyssel c. 20 kyrBP, and the Troldbjerg Formation was deposited c. 20-19 kyrBP 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 A 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 kyrBP, 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 kyrBP. 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 190m 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 icemargin. 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 icemargin, 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 kyrBP, 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.

Udgave nummer4
Sider (fra-til)811-833
Antal sider23
StatusUdgivet - 1 dec. 2009
Eksternt udgivetJa

ID: 235141796