Relict Talus-Foot Rock Glaciers at 0yberget, Upper Ottadalen, Southern Norway: Schmidt Hammer Exposure Ages and Palaeoenvironmental Implications

摘要

Three relict talus-foot rock glaciers at 0yberget, upper Ottadalen, were dated using high-precision Schmidt hammer exposure-age dating (SHD). A SHD calibration curve was constructed for the local banded gneiss using two control points: (1) fresh bedrock in a road cutting; and (2) glacially scoured bedrock surfaces deglaciated c. 9.7 ka. Two alternative hypotheses are suggested to explain SHD ages (± 95% confidence intervals) of 10 340 ±1280, 9920± 1385 and 8965± 1700 years obtained for the three rock glaciers located- 1000 m below the present lower limit of mountain permafrost, at a site covered by glacier ice throughout the Late Weichselian. First, the rock glaciers may be paraglacial features, which formed rapidly during deglaciation when enhanced debris supply may have buried residual ice bodies. If this was the case, the SHD ages are a true reflection of rock glacier age. Second, the rock glaciers may have formed during several thousand years of permafrost conditions in the Mid-Weichselian (possibly during the Alesund Interstadial, c. 34.5-38.2 ka) and then been preserved beneath a cold-based ice sheet until renewed exposure during Holocene deglaciation. If the second hypothesis is true, then the true age of the rock glaciers is much older than their SHD ages, the latter failing to record the interval of burial when rock weathering would have been zero. Although evidence is currently insufficient to distinguish between these two interpretations, the paper demonstrates both the potential and limitations of high-precision SHD in the context of rock glaciers.