CHARACTERISTICS AND DATING OF THE ROCK AVALANCHE AT PRAGSER WILDSEE/LAGO DI BRAIES (DOLOMITES, ITALY)

Abstract

Major gravitational slope deformations are widely disseminated in the Dolomite Mountains (NE Italy), one of the world's most conspicuous landscapes and part of the UNESCO world heritage list. In the Prags valley a rock avalanche still dams a backwater lake - the Pragser Wildsee/Lago di Braies. The volume of rock debris accumulations comprise approximately 30-40 106 m³ of mainly dolomitic rock and the area covered by rock debris is about 3.5 km². The run-out distance of about 8.5 km and a maximum vertical drop of 1150 m (H/L-ratio: 0.13) yield a run-out travel angle (Fahrböschungswinkel) of 8°. Especially in the surrounding of Schmieden/Ferrara a hummocky landscape with numerous hills and ridges is developed. Here some classical Toma hills are encountered, i.e. isolated cone- to pyramidal- or roof-shaped hills composed of rock avalanche debris. Mainly because assumptions about the origin of this hummocky landscape has been related to glacial processes, the formation of Pragser Wildsee was previously thought to be of Lateglacial age. We applied cosmogenic 36Cl surface exposure dating of four boulders within the debris accumulations and obtained an early Holocene age of 8.3±0.7 ka for the event. Our findings go along with the results of published radiocarbon dates from sediment-drillcores within the backwater lake, which indicate a recalibrated minimum age for the slope failure of 7410±100 cal years BP (Irmler, 2003). The accumulations at Pragser Wildsee show strong similarities to the Obernberg rock avalanche in geomorphology as well as in age, making them the first examples of early Holocene rock avalanches forming Toma hill landscapes.