 |
| Titel |
Age-dating of rockslides: Methods and limitations |
| VerfasserIn |
M. Ostermann, D. Sanders, C. Prager |
| Konferenz |
EGU General Assembly 2009
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250021340
|
|
|
|
|
|
| Zusammenfassung |
| Age-dating of deposits of catastrophic rockslides is prerequisite to unravel the potential
relation between the frequency of mass-wasting events with climatic change or earthquakes.
In the Alps, about 250 rockslides exceeding 106 m3 in volume are known, but the
age as yet is determined only for a comparatively small number of events. For age
determination of rockslide events, different methods are available (e. g. Lang et al.,
1999).
Radiocarbon Dating
In the past few decades, rockslide deposits commonly were proxy-dated by 14C age
determination of organic remnants preserved (a) in glacial, fluvio-glacial sediments
overridden by the rockslide, (b) within the rockslide mass, or (c) in rockslide-dammed
backwater deposits or lakes situated atop the rockslide mass. 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; in case (b), which is quite rare, the 14C age is
generally considered as a good proxy of the event age; in case (c) the 14C age represents a
minimum age for the rockslide event. Unfortunately, radiocarbon dating often cannot be
applied because of absence of suited deposits or exposures thereof, lack of organic remnants
or of remnants suited for age-dating, and/or because determined 14C ages are substantially
biased.
Optically Stimulated Luminescence (OSL)
Proxy-dating of rockslide events by OSL can be applied to silt- to sand-sized quartzose
sediments present (a) directly below, (b) within, or (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.
Surface Exposure Dating with cosmogenic radionuclides
Surface exposure ages can be determined for rock samples taken from the sliding planes
at the rockslide scarp, and/or taken from boulders accumulated at the surface of rockslide
deposits. Surface exposure dating is the only ’direct’ approach to determine the age of a
rockslide event. To date, however, exposure dates are fraught with comparatively large error
ranges.
234U/230Th Dating
U/Th dating of diagenetic carbonate cements formed within rockslide masses represents
a new method for proxy age determination. Breccias formed by precipitation of
carbonate cements within rockslide deposits are fairly common. U/Th dating of the
diagenetic cements can provide a good proxy of rockslide event age. U/Th ages are
cementation ages, not event ages; to minimize the error, it is thus important to detect
petrographically early cement, and (if necessary) to produce multiple U/Th ages
from different samples. A major advantage of U/Th dating of cement is rapid, easy
extraction of numerous samples of comparatively small size. Combined with other
methods of numerical age determination, U/Th dating of cements in lithified rockslide
deposits thus provides an indepedent check of correctness, and may enhance the
overall precision of determination of event age (Ostermann et al., 2007, Prager et al.,
2009).
Lang, A., Moya, J., Corominas, J., Schrott, L. & Dikau, R., 1999: Classic and new dating
methods for assessing the temporal occurrence of mass movements. Geomorphology, 30, 1,
33-52.
Ostermann, M., Sanders, D., Prager, C. & Kramers, J. 2007: Aragonite and calcite
cementation in ’boulder-controlled’ meteoric environments on the Fern Pass rockslide
(Austria): implications for radiometric age-dating of catastrophic mass movements. Facies,
53, 189-208.
Prager, C., Ivy-Ochs, S., Ostermann, M., Synal, H.-A. & Patzelt, G. 2009: Geology and
radiometric 14C-, 36Cl- and Th-/U-dating of the Fernpass rockslide (Tyrol, Austria).
Geomorphology, 103, 1, 93-103. Please fill in your abstract text. |
|
|
|
|
|
|