High-precision Schmidt-hammer exposure-age dating of flood berms, Vetlest?lsdalen, alpine southern Norway: First application and some methodological issues

摘要

Schmidt-hammer exposure-age dating (SHD) was applied to three flood berms in upper Vetlest?lsdalen, southern Norway, using a local, high-precision calibration curve that takes account of the colluvial origin of fluvial boulders in the youngest berm, which was deposited during the August 1979 flood. Precision of SHD dating for this berm was estimated as ±210 years, whereas predicted ages of the two older berms were 3195±435 and 3405±340 years, and subsections of the oldest berm yielded age ranges of ～900 years. The results demonstrate the feasibility of high-precision SHD in the context of boulder landforms deposited by high-magnitude floods, the requirement of a large sample of R values in the face of high natural variability, the necessity of an appropriate calibration curve to ensure accuracy, the usefulness of floods of known age for testing and improving calibration curves, and the potential effects of boulders of colluvial origin on R values (especially the susceptibility of young surfaces to roughness variations). The dated berms indicate a return period of ～1000-1500 years for floods of the magnitude of the 1979 flood event in the upper catchment. Thus, the long-term persistence of flood boulder berms in the landscape has potential for reconstructing Holocene flood history and palaeohydrology from the geomorphic legacy of the most extreme Holocene floods.