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| Titel |
Three rockslides shaped the valleys around Sterzing/Vipiteno, Northern Italy |
| VerfasserIn |
Marc Ostermann, Diethard Sanders, Helena Rodnight |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034392
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| Zusammenfassung |
| The post-Glacial development of mountain valleys near the city of Sterzing/Vipiteno (Italy)
was profoundly influenced by instantaneous local base-level rise caused by rockslides. Each
of the mass-wasting events had blocked the valley it descended into, resulting in
rockslide-dammed lakes that became filled, or largely so, by lacustrine and fluvial
deposits.
Three rockslides have been investigated. (1) The Pfitsch-valley rockslide, situated in the
tributary Pfitsch valley east of Sterzing, has an estimated volume of 0.38 km3 and covers an
area of about 0.92 km2. This rockslide consists of calcareous phyllites and, locally, is lithified
into a rockslide breccia (Sanders et al., 2010). The backwater lake of the rockslide mass
shows well-developed shoreline terraces. Uranium/Thorium age-dating of post-rockslide
calcite cements, found within the rockslide deposit, yielded a minimum age of 2686 ± 50 a
respectively 11290 a ± 500. Additionally, the shoreline terraces and lack of glacial overprint
of the rockslide indicate that the valley bottom was free of ice when the Pfitsch rockslide
occurred.
(2) The Ridnaun-valley rockslide descended into a tributary valley west of Sterzing. The
Ridnaun rockslide is of an estimated volume of 0.6 km3, and covers an area of about 2.4 km2.
The corresponding rockslide-dammed lake has been breached and run out an unknown
interval of time ago; again, however, well-developed shoreline terraces are present that
can be traced laterally over a few kilometres. The rockslide mass of Ridnaun is
composed of paragneiss, phyllithic mica schist and, subordinately, of amphibolite and
marble. Radiocarbon dating of organic remnants from a sandy deltaic succession
shed into the former rockslide lake yielded an age of 8865 ± 50 a BP; this implies
that the rockslide event is older. Again, however, there is no evidence for glacial
overprint of the rockslide mass and its associated backwater deposits. Samples for
optically-stimulated luminescence dating of post-rockslide deltaic sands are in process at
present.
(3) The Stilfes rockslide, in turn, descended into the trunk valley a few kilometres to the
south (=downstream) of Sterzing. This rockslide mass has an estimated preserved volume of
about 0.32 km3, and covers an area of about 2.1 km2. The rockslide mass consists mainly of
calcareous phyllites, phyllites, and marble. The rockslide had dammed up a lake some
15.5 km2 in area that became completely filled, and turned into a swampy area.
Pollen analysis of lacustrine sediments by Sarnthein (1946) suggested that lake
sedimentation started during the ’first beech-fir-time’ (corresponding to the Subboreal
5000-2700 BP, in present terminology). Radiocarbon samples of a layer rich in
organic debris intercalated into the lacustrine sediments yield an age of 4405 ± 45 a
BP.
The longitudinal profiles of the streams draining the tributary Pfitsch and Ridnaun valleys,
respectively, are knicked at the rockslide masses. Conversely, the trunk valley with the Stilfes
rockslide does not show a knick in longitudinal profile, despite significant aggradation
within the former rockslide-dammed lake. At present, the reasons for this are not
fully clear to us. Perhaps, in re-grading its profile, the rate of incision by the trunk
river (and its capacity for sediment redistribution down stream) were so high that
the disturbance of profile exerted by the rockslide mass is already eliminated. In
summary, in the investigated area, aside of glacial erosion, the rockslides and their
associated geomorphic effects were most significant in shaping the valleys since the last
glaciation.
Sanders, D., Ostermann, M., Brandner, R., Prager, C., 2010. Meteoric lithification of
catastrophic rockslide deposits: diagenesis and significance. Sedimentary Geology 223,
150-161. doi:10.1016/j.sedgeo.2009.11.007
Sarnthein, R. v., 1946: Moor- und Seeablagerungen aus den Tiroler Alpen in
ihrer waldgeschichtlichen Bedeutung. Österr. Botan. Zeitschrift, 95, 1, 605- 611. |
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