Early Holocene (8.6ka) rock avalanche deposits, Obernberg valley (Eastern Alps): Landform interpretation and kinematics of rapid mass movement

Abstract

In the Obernberg valley, the Eastern Alps, landforms recently interpreted as moraines are re-interpreted as rock avalanche deposits. The catastrophic slope failure involved an initial rock volume of about 45 million m³, with a runout of 7.2 km over a total vertical distance of 1330 m (fahrböschung 10°). 36Cl surface-exposure dating of boulders of the avalanche mass indicates an event age of 8.6 ± 0.6 ka. A 14C age of 7785 ± 190 cal yr BP of a palaeosoil within an alluvial fan downlapping the rock avalanche is consistent with the event age. The distal 2 km of the rock-avalanche deposit is characterized by a highly regular array of transverse ridges that were previously interpreted as terminal moraines of Late-Glacial. ‘Jigsaw-puzzle structure’ of gravel to boulder-size clasts in the ridges and a matrix of cataclastic gouge indicate a rock avalanche origin. For a wide altitude range the avalanche deposit is preserved, and the event age of mass-wasting precludes both runout over glacial ice and subsequent glacial overprint. The regularly arrayed transverse ridges thus were formed during freezing of the rock avalanche deposits.

Two early Holocene rock avalanches in the Bernese Alps (Rinderhorn, Switzerland)

“…However, a growing number of dated post-glacial landslides show lag times (i.e., time between deglaciation and slope failure) of thousands of years (e.g., Ivy-Ochs et al, 2009a;Prager et al, 2009;McColl, 2012;Ostermann et al, 2012;Ballantyne et al, 2014a;2014b), contradicting this hypothesis.…”

Post-glacial rock avalanches in the Obersee Valley, Glarner Alps, Switzerland

“…While the recurrence interval of these massive landslides is long, numerous historical examples and geological evidence of prehistoric events provide proof of their consequence to people and the environment (e.g. Heim, 1932;Grater, 1945;Shreve, 1968;Sosio et al, 2008;Guthrie et al, 2012;Ostermann et al, 2012;Pankow et al, 2014). Further research probing the geological record of past events is crucial in order to supply reliable information on the causes of failures, their expected recurrence intervals, and runout behavior.…”

“…Concurrently, advances in dating of rock avalanche deposits using in-situ produced cosmogenic radionuclides has generated a wealth of data on event timing (e.g. Ivy-Ochs et al, 1998, 2009aHormes et al, 2008;Prager et al, 2009;Delunel et al, 2010;Stock and Uhrhammer, 2010;Hewitt et al, 2011;Ostermann et al, 2012). Cosmogenic exposure age dating has been used successfully in several cases where independent age control exists from complementary methods (e.g.…”

Diagnostic characteristics and paleodynamics of supraglacial rock avalanches, Innerdalen, Western Norway

“…Glacial and landslide deposits are often diamictons, which makes them difficult to distinguish in many alpine environments, particularly if a landslide fell onto a glacier (Evenson et al, 2009). This has led to misinterpretations of landforms associated with rock-slope failures as moraine deposits (Hewitt et al, 2011;Ostermann et al, 2012). It is even more difficult when a rock avalanche runs over a glacier but continues into an ice-free terrain and entrains moraine material (Gosse & Evenson, 1994;Hermanns et al, 2014).…”