247 Quaternary Geology (Posters)

Wednesday, 21 October 2009
9:00 AM - 6:00 PM
Hall A

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

Paper number: 247-2
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

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.