Multi-method (C-14, (3)6Cl, U-234/Th-230) age bracketing of the Tschirgant rock avalanche (Eastern Alps): implications for absolute dating of catastrophic mass-wasting

作者：Ostermann Marc^*; Ivy Ochs Susan; Sanders Diethard; Prager Christoph

来源：Earth Surface Processes and Landforms, 2017, 42(7): 1110-1118.

DOI：10.1002/esp.4077

• landslide
• rock avalanche
• cosmic ray exposure dating
• U/Th-dating
• radiocarbon

Correct and precise age determination of prehistorical catastrophic rock-slope failures prerequisites any hypotheses relating this type of mass wasting to past climatic regimes or palaeo-seismic records. Despite good exposure, easy accessibility and a long tradition of absolute dating, the age of the 230 million m(3) carbonate-lithic Tschirgant rock avalanche event of the Eastern Alps (Austria) still is relatively poorly constrained. We herein review the age of mass-wasting based on a total of 17 absolute ages produced with three different methods (C-14, Cl-36, U-234/Th-230). Chlorine-36 (Cl-36) cosmogenic surface exposure dating of five boulders of the rock avalanche deposit indicates a mean event age of 3.06 +/- 0.62 ka. Uranium-234/thorium-230 (U-234/Th-230) dating of soda-straw stalactites formed in microcaves beneath boulders indicate mean precipitation ages of three individual soda straws at 3.20 +/- 0.26 ka, 3.04 +/- 0.10 ka and 2.81 +/- 0.15 ka; notwithstanding potential internal errors, these ages provide an 'older-than' (ante quam) proxy for mass-wasting.
Based on radiocarbon ages (nine sites) only, it was previously suggested that the present rock avalanche deposit represents two successive failures (3.75 +/- 0.19 ka BP, 3.15 +/- 0.19 ka BP). There is, however, no evidence for two events neither in surface outcrops nor in LiDAR derived imagery and drill logs. The temporal distribution of all absolute ages (C-14, Cl-36, U-234/Th-230) also does not necessarily indicate two successive events but suggest that a single catastrophic mass-wasting took place between 3.4 and 2.4 ka BP. Taking into account the maximum age boundary given by reinterpreted radiocarbon datings and the minimum U/Th-ages of calcite precipitations within the rock avalanche deposits, a most probable event age of 3.01 +/- 0.10 ka BP can be proposed. Our results underscore the difficulty to accurately date catastrophic rock slope failures, but also the potential to increase the accuracy of age determination by combining methods.

• 出版日期2017-6-15