A Holocene lacustrine rock platform around Storavatnet, Osterøy, western Norway

Abstract

A recently discovered lacustrine rock platform at low altitude exists along a 1Okm2 lake on the Osterøy Island near Bergen, western Norway. The lake was converted to a reservoir in 1920 and therefore is subject to frequent changes in level above and below the previous natural level of 151.5 m a.s.l. The rock platform, up to 20 m wide, is developed in strongly foliated metamorphic Proterozoic and Palaeozoic berock. Overhanging notches and small caves are developed along bedrock fractures in some places along the platform, which typically tilts 5 to 10 degrees towards the lake and is veneered with angular debris from the cliffs. The study area was deglaciated at the Pleistocene/Holocene transition 10 000 14C yr BP, and the tilting of the glacial rebound was complete about 6000 14C yr BP. The west-coast climate in Norway duing the latter part of the Holocene, with high precipitation and frequent freeze-thaw cycles during winter, resulted in highly fluctuating lake levels. These conditions are consistent with the conclusion that the plaform was formed by frost weathering. Mass movement from the steep slopes, together with ice-push duing ice breakup, was responsible for transportation of the debris. Because the platform around the lake is essentially at the same level, it must have been formed between the mid-Holocene and AD 1920.