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Titel |
Investigations on alluvial deposits through borehole stratigraphy,
radiocarbon dating and passive seismic technique (Carnic Alps, NE Italy) |
VerfasserIn |
Alessia Viero, Lorenzo Marchi, Marco Cavalli, Stefano Crema, Alessandro Fontana, Paolo Mozzi, Corrado Venturini |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250132189
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Publikation (Nr.) |
EGU/EGU2016-12672.pdf |
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Zusammenfassung |
Alluvial sediment investigations provide fundamental tools to infer the processes that control
geomorphological evolution of mountain environments. By analyzing sediment stratigraphy
in depth, it is possible to retrieve the source, the geology, the time of deposition, the relative
distance travelled by material as well as to distinguish among different type of transport (i.e.,
gravitational, fluvial or glacial).
In this work, we present a combination of log stratigraphy, radiocarbon dating and
geophysical surveys carried out on the valley floor of the But River (Carnic Alps, North East
Italy). The But River basin drains an area of 326 km2 with a range in elevation from 2769 to
323 m a.s.l.; the bedrock mainly consists of carbonates and quartz arenites with minor
inclusions of effusive rocks. After Pleistocene the gravitational deposits from mountain
slopes have impounded the But River several times. In particular, we analyzed a sector of the
upper portion of the But valley close to the confluence of the Moscardo Torrent, frequently
affected by debris flows.
A borehole was drilled in the But River floodplain, at the intersection with the Moscardo
Torrent alluvial fan, down to a depth of 80 m. The analysis of the core samples allowed
discerning three sedimentary levels rich in clay and organic materials, which testify the
presence of small dam lakes, originated from the Moscardo debris-flow deposits. Three
samples of wood and plant debris were collected from 13, 14 and 23 m of depth, respectively.
They were analyzed through radiocarbon dating in order to determine the age of the lakes
and, thus, to infer the activity of the debris flows building the Moscardo cone. The
calibrated ages of the 3 samples are close to the younger limit of the radiocarbon
method indicating a fast aggradation of the valley floor, starting from a period ranging
between 1450 - 1632 AD. Historical maps and documents confirm the presence of the
lakes until 19th century and they permit to assess their extent and the maximum
depths.
Two passive seismic campaigns were carried out near the borehole site and along the But
valley at different elevations. The aim was to investigate the depth of the buried bedrock and
therefore to indirectly characterize the thickness of alluvial deposits. We calibrated the
fundamental frequency of each site by constraining average shear velocity of the alluvial
sediments close to the borehole site with known stratigraphy. Eight HVSR (Horizontal to
Vertical Spectral Ratio, Nakamura, 1989) were carried out, and thus a first sketch of the
buried valley floor along a longitudinal profile of about 5 km was depicted. The values of the
derived bedrock depth allow to quantify the differences in thickness between the alluvial
deposits and the Moscardo Torrent fan deposits. This information helps to address the
contribution of the debris-flow processes in damming the upper But River during the last five
centuries.
The results confirm the role of debris-flow deposits from the Moscardo Torrent in
shaping the morphology of the valley floor of But River and show suitability of
an integrated approach, encompassing log stratigraphy, geophysical surveys and
analysis of historical documents, for gaining insights on the evolution of alpine
valleys.
Reference
Nakamura, Y., 1989. A method for dynamic characteristic estimation of subsurface using
microtremor on the ground surface. Quarterly Report of Railway Technical Research
Institute, 30(1): 25-33. |
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