IN SITU 10BE AGES AND CLAY MINERAL ASSEMBLAGES CONSTRAINING THE ICE THICKNESS DURING THE LGM IN THE ANDØYA-SKÅNLAND REGION, NORTHERN NORWAY

The surface geometry of the last ice sheet in the Andøya-Skånland region has been reconstructed by mapping of blockfields and weathering boundaries along a latitudinal transect. The reconstruction is supported by in situ cosmogenic 10Be ages from geomorphic features, by evaluation of the degree of rock surface weathering, and by analysis of clay mineral assemblages from sites above and below glacial trimlines.

10Be ages from surfaces above the suggested Late Weichselian glacial maximum trimline range from 23 to 75 ka, whereas one date from below the trimline gives 15 ka. From the northern tip of Andøya, 10Be ages obtained on boulders and bedrock range from 28 to 53 ka. This is in agreement with previously presented evidence from lake sediment records indicating that the area was not covered by the inland ice sheet during the Late Weichselian glacial maximum.

Gibbsite is found in the clay mineral assemblages at nearly all tested sites above the trimline and is taken as an indication of absence of glacial erosion during the Late Weichselian maximum. The trimline goes from the present sea level at northern Andøya to about 950 m a.s.l. at Skånland, suggesting an average ice sheet surface gradient of 9-10 m/km.