No abstract

On the Fern Pass rockslide (Eastern Alps, Austria), projecting boulders collected surface runoV and delayed percolation of water into the rockslide mass, leading to decimetre-scale, Xuctuating, phreatic/ vadose diagenetic systems along their contact. In these systems, aragonite and calcite precipitation were nourished mainly by dissolution of carbonate-rock Xour. Cement precipitation was limited to southern-and eastern-exposed "runoV haloes" of boulders and mainly resulted in cemented breccias. Aragonite precipitation was related to dissolved Mg 2+ and/or to high CaCO 3 supersaturation in evaporative-concentrated pore waters. Early aragonite cement yielded a 234 U/ 230 Th age of 4,150 § 100 years. Relative to other radiometric ages ( 36 Cl, 14 C; by other authors) for the rockslide event, the U-Th age of the aragonite is the most precise proxy of depositional age. Carbonate cements are present in other rockslide and rockfall deposits also. U-Th dating of such cements is thus a comparatively rapid and inexpensive method of minimum-age dating catastrophic mass movements.

Quaternary carbonate-lithic talus slope successions of the Eastern Alps record an overall correlation between prevalent sedimentary facies, depositional geometry, and geomorphic maturity of the slope. After exposure of high cliffs by deglaciation or rocksliding, a low-dipping immature talus dominated by unsorted rockfalls initially accumulates. With progressive talus buildup, slope segments of different dips develop. Concomitantly, prevalent depositional processes change to grain flows and sorted rockfalls in the proximal, steep-dipping (35°-30°) slope segment, while deposits of cohesive debris-flows, ephemeral fluid flows and larger rockfalls prevail in the distal, lower-dipping slope segment. In mature talus deposystems, the proximal slope succession overlies the lower-dipping package of the distal slope along a thin 'downlap interval'. Immediately after cliff exposure by deglaciation or rocksliding, talus may aggrade at rates of up to a few tens of meters per 1,000 years, but the accumulation rate slows strongly with progressive maturity of slopes.

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.

No abstract