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Dating Rockslides Methods

At the moment there are several methods to estimate the age of a rockslide, each having its pros and cons. Four methods are used within the CRA-project with special focus on U/Th-dating and surface exposure dating.

 
U/Th-dating 
Surface exposure dating
Radiocarbon dating (14C)
OSL-dating

 dating locations

schematic diagram of a rockslide in a Alpine valley with indicated dating locations

234U/230Th dating


A new precise tool to age-date rockslides is the application of the U/Th-method on early diagenetic carbonate precipitations within the rockslide deposits (Ostermann et al. 2006, Prager et al. 2009). These carbonate precipitations (carbonate cements) are a common but less respected feature of many rockslide deposits in the Northern Calcareous Alps and Southern Alps, and presumably in most carbonatic mountain ranges of the world (own investigations and oral impartation of several colleagues). As a afar example for the distal part of the Gohna Tal rockslide (Kumaon Himalayas, India) that consists mainly of carbonate rock clasts, breccias that formed due to cementation by carbonate-rich waters are mentioned, but not described any further (Weidinger, 1998, p. 327). In the rockslide of Flims, Switzerland, locally, breccias that originated by cement precipitation after the sturzstrom event are present, but to date have not been subject to further investigation (U. Haas, pers. comm., 2006). Other already sampled sites provided material suitable for age dating through the U/Th-method and there is a great number of sites waiting for detailed investigations, surely also containing additional material for U/Th-age-dating.
Many breccia deposits originated from rock slides show features of calcite cementation, which was not mentioned by most authors before because missing importance, until we have shown that this can represent a fairly precise proxy of depositional age (Ostermann et al, 2006, Prager et al, 2006). U/Th-age-dating of cemented portions of rockslides by the U-Th method thus represents a new, hitherto unexploited source for proxy age determination of catastrophic mass failures.
For analysis we use a MC-ICP-MS (nu InstrumentsTM) situated in Bern, in which the solutions were introduced by aspiration in an ESI Apex desolvator with ESI microflow nebulizer. U measurements are done in static mode measuring 236U and 234U in separate electron multipliers. Th measurements are done in dynamic mode, measuring 230Th and 229Th alternately in the same multiplier. To control electron multiplier gain in the Th measurements the MOSS (Be Inhouse) standard is used and for gain calibration for U measurements we take the NIST U 050 standard.

                                      U /Th-samples from the Tschirgant rockslide                         U /Th-samples from thePfitsch rockslide

Surface exposure dating

36Cl-surface exposure ages will be determined from samples taken from sliding planes at the scarp area and/or from huge rockslide boulders at the accumulation area. Surface exposure dating with cosmogenic radionuclides can help to determine the event age of rockslides. Up to now this was applied to the crystalline Köfels rockslide (Ivy-Ochs et al., 1998), the calcareous rockslides at Flims (Ivy-Ochs et al., 2004, 2008), Almtal (Ivy-Ochs et al., 2005) and Fernpass (Prager et al. 2006) as well as to a smaller one in the Italian dolomites (Soldati et al., 2004).
36Cl is produced by spallation of 40Ca and 39K, muon-induced reactions on 40Ca and 39K, as well as low-energy (thermal and epithermal) neutron capture on both 35Cl and 39K (Gosse and Phillips, 2001). Sample preparation procedures are given in Ivy-Ochs et al. (2004). Total Cl and 36Cl will be determined using the accelerator set-up at the PSI/ETH Zürich, Switzerland accelerator mass spectrometry facility. Ages will be calculated using the production rates of Stone et al. (1996; 1998). Low-energy neutron contribution to production will be calculated following Liu et al. (1994) and Phillips et al. (2001).

sampling Tschirgantsampnig Tschirgant 2
sampling for surface exposure dating at Tschirgant rockslide

Radiocarbon dating (14C)

Most rockslide ages are based on 14C dating of organic material found within the rockslide deposits or beneath them. Usually these organic materials are tree trunks overwhelmed by the rockslide.

In the past few decades, rockslide deposits commonly were proxy-dated by 14C age determination of organic remnants preserved in:
(a)
(a)
glacial, fluvio-glacial sediments overridden by the rockslide,
(b) (b) within the rockslide mass, or
(c)(c) in rockslide-dammed backwater deposits, or lakes and peat bogs situated atop the rockslide mass.


sketch of  possible 14C sampling locations within a rockslide area

In each case, the 
14C age provides a different constraint on the age of the rockslide event:

in case (a), the 
14C age represents a maximum age of the event; drilling
in case (b), which is quite rare, the 14C age is generally considered as a good proxy of the event age; very seldom good samples
in case (c) the 14C age represents a minimum age for the rockslide event, good proxy, drilling
Obernberg_14Csamples 14C Obernberg
a palaeosoil and wood fragments within backwatersedimets at the Obernberg rockslide

OSL-dating (optical stimulated luminescence)

To substantiate the results of the 230Th/234U-age-dating and the 36Cl-age-dating in the light of certain circumstances two alternative dating methods are possible to ably. Optical dating techniques are applicable to various sedimentary environments (Prescott and Robertson, 1997; Roberts, 1997; Aitken, 1998; Lang et al., 1998, Lang and Zolitschka, 2001). If there are any fine-silt sized clastic lake sediments or sandy backwater sediments, deposited during the phase of river damming through the rock slide, OSL dating of this sands may be feasible. A physical property of silicate minerals (e.g., quartz & feldspar) is the weak emission of light when the minerals are exposed to an external energy source. The Institute for Geology and Palaeontology in Innsbruck has the first laboratory of this kind in Austria.
Proxy-dating of rockslide events by OSL can be applied to silt- to sand-sized silicate minerals present:
(a) (a) directly below,
(b)(b) within, or
(c)(c) above/laterally aside a rockslide mass.
For each case (a) to (c), the determined ages are subject to the same constraints as outlined for radiocarbon dating. Unfortunately, situations allowing for application of OSL to rockslide event dating are comparatively rare, and the resulting ages tend to have a wide error range.
OSL_RidnaunOSL_Ridnaun_2
sampling for OSL within the lacustrine backwater sediments at Ridnaun rockslide