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Tunnel-valley formation in northwest Germany—geology, mechanisms of formation and subglacial bed conditions for the Bornhöved tunnel valley

Sedimentary Geology
Publication Date
DOI: 10.1016/0037-0738(94)90086-8
  • Earth Science


Abstract The Bornhöved tunnel valley consists of a relatively broad, elongated depression filled primarily with tills, and a narrow channel in the axial part filled with outwash, glaciolacustrine sediments and tills. The tunnel valley is at least 13 km long, 222 m deep (bottom at 191 m b.s.l.), and extends parallel to the axis of a peripheral sink between two Permian salt diapirs in the substratum. It is suggested that the initiation of the tunnel valley was controlled by a differential response of the bed materials to ice overriding on top of and between the salt structures. The Bornhöved tunnel valley is a polygenetic feature that resulted from different erosional processes throughout all three main glaciations in northern Germany. During the Elsterian Glaciation glaciotectonic squeezing occurred, to be later replaced by subglacial meltwater erosion. During the ice retreat and the Holsteinian Interglacial, the valley was filled with fine-grained glaciolacustrine and marine sediments, which were subsequently removed and redeposited further down-ice as a 200-m-thick glaciotectonic melange by the first Saalian advance. The second Saalian advance caused only minor changes and the tunnel valley was reactivated as a subglacial channel during the Weichselian Glaciation. It is suggested that at the time of the first Weichselian ice advance a large subglacial water reservoir developed in the area of the Baltic Sea basin and caused a rapid, surge-like ice movement. As the ice sheet advanced out of the Baltic Sea basin, drainage of the water reservoir was prevented by the ice toe overriding the permafrost on the Saalian highlands. During the ice retreat, frozen ground was left beyond the ice margin and subglacial meltwater catastrophically drained through the tunnel valley with a discharge roughly estimated at about 3.75 × 10 3 m 3/s. It seems possible that drainage of the subglacial water collector through the Bornhöved tunnel valley and other similar channels south of the Baltic Sea led to increased ice-sheet basal friction, and was one reason for successively smaller extents of the subsequent Weichselian advances. Evidence against pervasively deforming bed materials in the vicinity of the Bornhöved tunnel valley in the Weichselian Glaciation indicates that the valley served as an efficient drainage path for porewater from the subglacial sediments.

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