Distinction between the Storegga tsunami and the Holocene marine transgression in coastal basin deposits of western Norway

Many coastal lakes were inundated by both the Storegga tsunami (7000 C-14 y r BP) and the mid-Holocene sea-level rise (the Tapes transgression) in west ern Norway. The tsunami eroded lake bottoms and deposited graded and/or mas sive beds of sand, rip-up clasts, and coarse plant material. By contrast, w hen the rising sea entered the lakes, it deposited only gyttja, silt and fi ne sand, without causing much erosion of the underlying lake sediments. Sto regga tsunami deposits in some coastal lakes were interpreted previously as ordinary marine sediments from the Tapes transgression. Our reinterpretati on of these deposits shows that the transgression maximum phase was reached after 6500 yr BP, more than 1000 yr later than previously inferred for the coast of Sunnmore. The new data cannot be combined in a shoreline diagram without showing the 6000 yr BP and 7000 yr BP shorelines as slightly warped . (C) 1998 John Wiley & Sons, Ltd.