MIDDLE PLEISTOCENE MEGAFLOODS IN NW GERMANY:DEPOSITIONAL SETTING AND FLOOD-RELATED LANDFORMS

Lake outburst related landforms have been studied in numerous examples from the last glaciation. However, examples from older Pleistocene glaciations are rare. In this study we will present a Middle Pleistocene field example from northern Germany. During the Middle Pleistocene glaciation ice-damming of the Upper Weser Valley led to the formation of glacial Lake Weser. The lake drained catastrophically as an ice dam failed, probably releasing up to 70 km³ of water within a few days with a peak discharge of up to 150.000 m³/s.

Geological maps, well logs and digital elevation models were integrated into a 3D-subsurface model (GOCAD) to reconstruct lake-outburst related landforms, which are partly buried by younger Late Pleistocene and Holocene sediments.

The drainage routes are characterized by the occurrence of deep plunge pools, streamlined hills and trench-like channels, cut into bed rock and/or Quaternary deposits.

Plunge pools are developed in front of the main overspill channels and are incised into Mesozoic basement rocks. They are up to 780 m long, 400 m wide and 35 m deep. Downflow fan-shaped arrays of hills occur, covering an area of approximately 250 km². These streamlined hills are 2-13 m high, 550-4000 m long and 150-3000 m wide and consist of meltwater sand, overlain by relics of diamicton. The hills partly resemble features produced by currents around obstacles. The “obstacle” is a plug of more resistant diamicton and the tail is composed of sand, representing sand bars formed in the lee of streamlined hills during flood. The hills are separated by 50-400 m wide and 5-9 m deep channels, which merge downflow into 1-2 km wide, 5-7 km long and 10-20 m deep trench-like channels, now occupied by small underfed streams. The catastrophic drainage probably modified the existing drainage ways and subsequently became the new site of fluvial systems.