Geology and radiometric 14C-, 36Cl- and Th-/U-dating of the Fernpass rockslide (Tyrol, Austria)

One of the largest mass movements in the Alps, the fossil Fernpass rockslide in the Northern Calcareous Alps (Tyrol, Austria), was dated absolutely for the first time. Three independent radiometric dating methods were applied to geologically individual sample sites and enabled a cross-checking of the results. Close to the scarp-front, rockslide-dammed torrent deposits indicate a 14C minimum age of at least 3380–3080 cal. yrs BP. The chronostratigraphic base of this backwater sequence has not been dated yet, but is assumed to date somewhat older into the middle Holocene. However, this coincides with two cosmogenic radionuclide 36Cl ages of sliding planes at the steep and rugged scarp. Exposure data thus obtained are 3400 ± 900 and 4800 ± 1200 yrs respectively with an arithmetic mean of 4100 ± 1300 yrs. Additional age data were acquired from post-depositional calcareous cements, which locally lithified the rockslide debris. These meteoric cements contain remarkably high Uranium total contents and yielded a 230Th/234U minimum age of about 4150 ± 100 yrs for the accumulation of the rockslide deposits. All dating data agree well and indicate that the Fernpass rockslide is of Subboreal age. Thus the intensively structured present morphology, characterised by the well-known cone-shaped Toma-hills and several lakes in the depressions, is indicative of primary rockslide morphology and clearly not shaped by late-glacial ice as assumed formerly.