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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.
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U/Th-dating |
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Surface exposure dating |
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Radiocarbon dating (14C) |
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OSL-dating |
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schematic diagram of a rockslide in a Alpine valley with indicated dating locations
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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.
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U /Th-samples from the Tschirgant
rockslide
U /Th-samples from thePfitsch rockslide |
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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).
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sampling for surface exposure dating at Tschirgant rockslide |
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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.
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sketch of possible 14C sampling locations within a rockslide area |
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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
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a palaeosoil and wood fragments within backwatersedimets at the Obernberg rockslide |
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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.
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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.
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sampling for OSL within the lacustrine backwater sediments at Ridnaun rockslide |
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