10BE SURFACE EXPOSURE AGES FROM RELICT TALUS-DERIVED ROCK GLACIER LOBES AT ØYBERGET, SOUTHERN NORWAY

Lobate, coarse rock-debris accumulations are found at 500-550 m a.s.l. beneath the south-facing wall of Øyberget (1225 m a.s.l.), upper Ottadalen, southern Norway. The area is located inside the Younger Dryas margin of the Scandinavian Ice Sheet.

Matthews et al. (2013, Permafrost and Periglacial Processes 24, 336-346) discussed whether these landforms are relict talus-derived rock glaciers or rock-slope failure accumulations, and concluded that the morphology is more consistent with the former. Three of the lobes were dated using Schmidt hammer surface exposure dating, yielding 10,340 ±1280, 9920 ±1385, and 8965 ±1700 years, lobes 1, 2 and 3, respectively. The numerical ages are not compatible with the known, non-permafrost climatic conditions at the time. Two scenarios were suggested to explain the landform ages: (1) rapid early-Holocene paraglacial formation where residual glacial ice was buried by debris; or (2) a slower formation under permafrost conditions during an earlier interstadial with subsequent preservation beneath cold-based ice.

¹⁰Be surface exposure dating has been performed to test these two scenarios. Although both dating approaches rely on subaerial exposure, their measures of time rely on fundamentally different parameters affecting the rock surfaces – i.e. rate of chemical weathering versus accumulation of in situ cosmogenic nuclides. ¹⁰Be surface exposure dating of lobe 2 and 3 yields arithmetic mean ages of 10,142 ±663 and 10,072 ±1090 years (propagated 1-sigma analytical uncertainties). ¹⁰Be surface exposure dating further shows that the summit of Øyberget was ice free at 10,159 ±599 years, whereas bedrock surfaces just up-valley of the lobes implies valley-floor deglaciation at 9042 ±741 years.

The similarity of the ¹⁰Be surface exposure ages from all four sites indicates that lobe formation and stabilisation must have been very rapid, via paraglacial formation in the early Holocene, explicable by burial of residual ice in the valley by enhanced debris supply during deglaciation, possibly involving rock slope failure as well as talus material. The previous suggestion that the rock glaciers may have formed during the Mid-Weichselian, and survived beneath a cold-based ice sheet until renewed exposure during the early Holocene, is rejected.