Document Actions

ijege-13_bs-schleier-et-alii.pdf

background image
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
493
DOI: 10.4408/IJEGE.2013-06.B-47
SPATIAL DISTRIBUTION OF ROCKSLIDE DEPOSITS AND THEIR
MORPHOLOGICAL FEATURES SUGGEST TIMING AND
PALAEO-ENVIRONMENTAL CONDITIONS FOR ROCK SLOPE
FAILURES IN INNERDALEN AND INNFJORDDALEN,
MØRE OG ROMSDAL COUNTY, WESTERN NORWAY
M
arkus
SCHLEIER
(*)
,
r
eginald
L. HERMANNS
(**)
&
J
oachiM
ROHN
(*)
(*)
University of Erlangen-Nuremberg, GeoZentrum Nordbayern - Schlossgarten5 - D-91054 Erlangen, Germany
Email: markus.schleier@gzn.uni-erlangen.de / Phone: 004991318522723
(**)
Geological Survey of Norway (NGU) - Trondheim, Norway
K
ey
words
: multiple landslide deposits, rock avalanche,
morphological analyses, palaeo-environmental conditions,
Western Norway
INTRODUCTION
In glacial overprinted mountain areas like in
Western Norway, important hazards are large rock
slope instabilities that might develop into large rock
avalanches. The most severe scenarios are large vol-
umes of rock dropping into a fjord or lake, causing
displacement waves or damming narrow valleys
causing upstream and potential downstream flood-
ing, (h
erManns
et alii, 2004; e
vans
et alii, 2011;
h
erManns
et alii, 2011a; h
erManns
et alii, 2012;
h
erManns
& l
ongva
, 2012).
Furthermore, large rock avalanches and their de-
posits which developed across valleys and built dams
play a major role in late quaternary landscape evolu-
tion (h
ewitt
et alii, 2011). The detailed study of mul-
tiple landslide deposits as for example presented in
(h
erManns
et alii, 2006; w
elkner
et alii, 2010; B
lais
-
s
tevens
et alii, 2011) provides important information
to understand palaeo-environmental conditions, fail-
ure mechanisms, activity of steep rock slopes, and to
estimate recurrence periods for similar events which is
important for hazard assessments.
Data on the temporal distribution of rockslide de-
posits in Storfjord (Møre og Romsdal County, Western
Norway) suggest that catastrophic rockslides were larg-
er and more frequent during the late Pleistocene than in
ABSTRACT
Rock avalanches dropping into a fjord or lake,
initiating displacement waves or damming nar-
row valleys, are severe hazard scenarios in glacial
overprinted mountain areas and have strong influ-
ence on the quaternary valley development. Sev-
eral investigations show, that the collapse of a rock
avalanche increases the probability for prospective
similar events at the same rock slope. Moreover,
data for the Storfjord area in Western Norway in-
dicates that catastrophic rockslides were more fre-
quent during late Pleistocene than in Holocene.
This study is to test these hypotheses, to reconstruct
the palaeo-environmental conditions of multiple
landslide deposits, and to study the effect of rock
avalanches on decaying ice bodies and landforms
influenced by isostatic rebound. For investigation,
the two valleys of Innerdalen and Innfjorddalen in
Western Norway were chosen, where multiple land-
slide deposits have developed on the valley floor.
Its spatial and temporal distribution is studied by
detailed field investigations, including morphologi-
cal and sedimentological analyses. In Innerdalen
glacial and postglacial rock avalanches have oc-
curred, whereas the first was partly deposited on
the decaying ice body. In Innfjorddalen several post
glacial rock avalanches occurred, whereas the first
was deposited on highly water saturated valley fill
sediments. In both valleys distinct morphological
features have developed.
background image
M. SCHLEIER, R.L. HERMANNS & J. ROHN
494
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
STUDY AREA
The two study areas, the valleys of Innerdalen
and Innfjorddalen, are located in Møre og Romsdal
County in Western Norway (Fig 1).
These areas are located in the Western Gneiss Re-
gion of Norway. The exposed rocks are Proterozoic
gneisses, mainly orthogneisses, partly overlain by conti-
nental and oceanic sediments which have been affected
by the Caledonian orogeny (h
acker
et alii, 2010). The
bedrock and the deposits in the study areas are com-
posed out of the typical hard gneisses of this region.
Based on the geological map of Ålesund (t
veten
et alii,
1998), the main rock types can be described as follows:
In the Innerdalen area, the disclosed rocks consist
of three main lithologies, 1) coarse-grained granitic
gneiss, augengneis, gneissic granite, 2) meta-arkose,
quartzite and 3) coarse to fine-grained granitic to dior-
itic, biotite containing gneiss. The additional quartz-
itic rock layers in Innerdalen are located at the border
of a main thrust fault. The multiple landslide deposits
contain all of these lithologies.
The disclosed lithologies in the Innfjorddalen
area are 1) coarse-grained granitic gneiss, augengneis,
gneissic granite, 2) sillimanit containing quartzitic
gneiss, partly containing kyanite and 3) undifferenti-
ated gneiss, mostly quartzdioritic, partly migmatitic.
The quaternary landscape evolution is also impor-
tant for the presented study of rock avalanches onto dif-
ferent substrates. In Western Norway the present distri-
bution of marine sediments at elevations up to 220 m
a.s.l. show that the land mass near the shoreline partly
raised up for several hundred meters since the Last Gla-
cial Maximum (h
erManns
et alii, 2012). This develop-
ment is part of the dome-like postglacial uplift of Fen-
noscandia, which is explained generally as movements
of solid earth originated in glacioisostatic rebound due
to melting of the thick ice sheets and attended unload-
ing of the crust (F
Jeldskaar
et alii, 2000). This uplift
is a still ongoing process and following the neotectonic
map of Norway (d
ehls
et alii, 2000), the present ap-
parent uplift rates are up to 8 mm/yr, showing values of
2 to 3 mm/yr for the study areas (Fig 1).
Therefore, the glaciation during the last ice age
and the following deglaciation, the crustal uplift and
sea level changes have enormous influence on the
landscape evolution due to deep valley incisions,
changes in sedimentation, erosion, and stress fields in
rock slopes as well as induced seismicity.
the Holocene (h
erManns
& l
ongva
, 2012). With this
study we want to test if similar temporal pattern can also
be expected for other valleys in Norway. In addition we
aim to study the effect of rock avalanching on decaying
ice bodies and landforms subjected to isostatic rebound.
We selected the Innerdalen and Innfjordalen in
Møre og Romsdal County in western Norway, where
multiple rock avalanche deposits occur. During field
investigations the spatial distribution of the multiple
rockslide deposits, soft sediments and the morpholog-
ical structures were mapped. The different sediments
were described by plotting grain size and grain shape.
Furthermore, samples for dating the different deposits
with in-situ terrestrial cosmogenic nuclides were tak-
en to determine the exact chronological distribution.
The main aims of this publication are:
1) to distinguish the multiple rock avalanche depo-
sits spatially and stratigraphically,
2) to reconstruct the palaeo-environmental condi-
tions under which all events formed,
3) to describe the facies of rock avalanches onto dif-
ferent type of substrate.
Fig. 1 - Map of Norway showing the locations of the study
areas (red dots) and the current apparent uplift
of Fennoscandia. Values are uplift rates in mm/yr
following D
ehls
et alii (2000)
background image
SPATIAL DISTRIBUTION OF ROCKSLIDE DEPOSITS AND THEIR MORPHOLOGICAL FEATURES SUGGEST TIMING
AND PALAEO-ENVIRONMENTAL CONDITIONS FOR ROCK SLOPE FAILURES IN INNERDALEN AND INNFJORDDALEN,
MØRE OG ROMSDAL COUNTY, WESTERN NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
495
The Fahrböschung angle (angle between the hori-
zontal and the connecting line of main scarp’s upper
limit and the deposit’s most distal part) was calculated
for the rock avalanche deposits, showing the apparent
friction angle (h
eiM
, 1932).
In order to determine the absolute ages of the mul-
tiple deposits and therefore to reconstruct the valley
history and to estimate the recurrent period, surface
exposure dating with in-situ terrestrial cosmogenic
nuclides using
10
Be were taken in both valleys. Results
are not yet available.
RESULTS AND DISCUSSION
DEPOSITS WITHIN THE INNERDALEN
In Innerdalen multiple deposits composed of rock
boulders several meter in diameter occur. They can be
divided in two groups: a) continuous rock avalanche
deposits with typical lobate shapes with boulders sev-
eral meters to tens of meters in diameter that cover the
entire surface of the deposits, b) suspicious discon-
tinuous boulder deposits that form various morpho-
logical units, and that are separated from each other
by several km, and that are covered by almost identi-
Both, the Innerdalen and the Innfjorddalen
are steep glacial affected valleys which show high
topographic relief with mean elevation differences
of around 1,000 m between valley bottom and sur-
rounded mountain crests.
METHODS
In order to determine and describe the different
multiple landslide deposits in the two study areas and
to reconstruct the palaeo-environmental conditions an
area of ~16 km
2
was mapped in the valley of Innerd-
alen and ~4 km
2
in Innfjorddalen (Fig. 2, Fig. 5).
We mapped the spatial distribution of Late Pleis-
tocene and Holocene deposits and their geomorpho-
logical features and, described their grain size and
shape by statistically sampling of 100 randomly con-
nected boulders. We used the six roundness classes af-
ter Folk as described in (B
oggs
, 2001). For Innerdalen
such data was collected at 75 sample sites, distributed
in the different landslide and moraine deposits and 13
sample sites for the multiple deposits in Innfjorddalen.
We also assessed the mean thickness of the depos-
its to estimate volumes of different events.
Fig. 2 - Map of multiple landslide deposits in Innerdalen. A-F mark the main deposits described in the text. The source area
of the multiple landslides is marked with red rectangle. The runup area is marked with dotted rectangle
background image
M. SCHLEIER, R.L. HERMANNS & J. ROHN
496
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
multiple landslide deposits are summarized in Tab. 1.
ROCK AVALANCHE DEPOSIT ALONG VALLEY
BOTTOM THAT IMPOUNDS A LAKE (A)
The rock avalanche deposit (A) stretches on the val-
ley floor over a length of ~1700 m and a width of
1100 m in the upper part and 270 m (mean 800 m)
in the most distal part, covering an area of around
1,303,000 m
2
. That represents a volume of ~39
x
10
6
m³ (Tab. 1). The deposit shows lateral levees and
frontal rims. It covers the valley on its whole width
and indicates a runup on the opposite valley slope
of around 65 m. The deposit forms a natural dam
that impounds a lake. Furthermore the deposit part-
cal boulders. We divided all deposits in six units, A-F
(Fig. 2, Fig. 3).
A) This is a typical lobate rock avalanche deposit
spanning the valley bottom and impounding a lake, B)
three prominent frontal moraine ridges in the outer val-
ley, C) scattered isolated hills composed of boulders
on the valley floor in the lower part of Innerdalen, D)
isolated patches of boulders on the slope several 100
m above the valley floor, E) distinctive valley parallel
ridges along and F) flat boulder patches in the upper In-
nerdalen. In total an area of around 2.4 km
2
is covered
by these deposits and their total estimated volume is
~64
x
10
6
m
3
. The main geometrical characteristics of the
Fig. 3 - Oblique view of Innerdalen valley showing the main part of the multiple landslide deposits. View direction to WSW. A - F
mark the main deposits described in text. The rock face of the source area is marked with red rectangle, the runup with
dotted rectangle. Orange lines mark lateral moraines, red lines mark the frontal moraines
background image
SPATIAL DISTRIBUTION OF ROCKSLIDE DEPOSITS AND THEIR MORPHOLOGICAL FEATURES SUGGEST TIMING
AND PALAEO-ENVIRONMENTAL CONDITIONS FOR ROCK SLOPE FAILURES IN INNERDALEN AND INNFJORDDALEN,
MØRE OG ROMSDAL COUNTY, WESTERN NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
497
SCATTERED ISOLATED HILLS ON VALLEY FLOOR
(C)
Within the area between deposit (A) and (B) the val-
ley floor is characterized by scattered single isolated
hills (C) of boulder accumulations only few meters
high and several meters in diameter which occur
scattered on the normal valley sediments. These
boulders are surrounded by valley fill material.
ISOLATED BOULDER PATCH ON THE SLOPE
ABOVE THE VALLEY FLOOR (D)
Within the slope and ~270 m to 370 m above the
valley floor several-100-m
2
-large boulder fields oc-
cur on both sides of the valley that are separated
from each other. These patches lower towards the
outer valley and terminate in the frontal moraines.
The most prominent boulder patch is located on the
northern valley slope. It covers an area of 147,000
m
2
. Its estimated volume is 2.2
x
10
6
m
3
(Tab. 1).
Another isolated boulder patch is located on the
opposite southern valley slope at similar elevation
but it has a smaller extent (Fig. 2). It is obvious that
there exists no connection to any possible scarp
upslope at any of the localities (Fig. 3). The grain
size distribution of these deposits shows a mean
diameter of ~1 m with maximum clast size rea-
ching >10 m. The deposit shows a relatively wide
and not distinct grain roundness distribution with
angular (46%) and subangular (43%) boulders with
a tendency towards very angular boulders (12%).
PROMINENT VALLEY PARALLEL RIDGES
ALONG THE LAKE (E)
There are two prominent morphological ridges
parallel to the valley along the southern shore of
the lake. The lower one (E1), has a length of 870
ly covers moraine deposits. The source area of this
event is located at the steep northern rock face of
the “Skarfjellet” south of the lake (Fig. 3). The ho-
rizontal distance of the top of the scarp to the end
of the runup deposit is ~3270 m and the elevation
difference is ~1290 m. Therefore the Fahrböschung
angle is 21.5°. The distal part of the deposit has a
distance from the source of 3320 m and an elevation
difference of 1460 m, therefore is the Fahrböschung
angle 23.7°. The mean grain size of this rock-ava-
lanche deposit is 1.5 m. The distribution shows a
concentration between 0.5 and 2 m with maxi-
mum size boulders >10 m in diameter. The mean
roundness is subangular (57%) to angular (37%).
PROMINENT FRONTAL MORAINE RIDGES IN
THE OUTER VALLEY (B)
In the outer part of the valley three distinct frontal
moraines (B) occur that are closely spaced. These
are one km long and have elevations of 12 to 15 m
above the current valley floor. The mean width of
the deposits ranges from 100 to 140 m. These mo-
raine deposits cover an area of around 395,000 m
2
and contain an estimated volume of 7.3
x
10
6
m
3
.
The sedimentological features of the three ridges
are identical. They are entirely covered by boul-
ders several meters to tens of meters in size with
no finer grained material (matrix) visible. Howe-
ver in fluvial cuts the core is visible that contains
a sandy matrix with angular but also well rounded
grains (Fig. 4). The deposits have a mean grain
size of ~1 m with clasts that exceed 5 m in diame-
ter. According to the grain roundness they show
normal distribution with a peak for subangular
boulders (54%). Angular (25%) and subrounded
(21%) boulders are almost equal distributed.
Tab. 1 - Geometrical characteristics of multiple landslide deposits in Innerdalen
background image
M. SCHLEIER, R.L. HERMANNS & J. ROHN
498
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
m and a mean width of 110 m and a mean height
of 30 m above the lake (water level 395 m a.s.l.).
Its deposit covers an area of 102,000 m
2
. The se-
cond higher ridge (E2) is 1060 m long, 150 m wide
and 50 m higher than the shoreline. The deposit
spans an area of around 168,000 m
2
. The depth of
the lake is at least 18 m in the front part (west).
Therefore the mean thickness of the deposit of the
lower ridge can be estimated to be 40 m and 50 m
for the higher ridge. This results in a volume of
4
x
10
6
m
3
for the lower and 8.4
x
10
6
m
3
for the upper
ridge (Tab. 1). No deposits of similar composition
can be found on the opposite (northern) shore of
the lake. The material of the ridges (deposit E) is
similar. They show a mean grain size of surficial
boulders of around 1.5 m. The roundness of boul-
ders for both ridges shows a distinct peak for su-
bangular boulders (76% for E1 and 60% for E2).
In addition also angular boulders occur whereas its
amount is higher in E2 (34%) and there also few
subrounded boulders exist.
FLAT BOULDER PATCH SE OF LAKE (F)
At the SE boarder of the lake patches of boul-
der deposits occur (F). Morphologically these
patches are relatively flat (no ridges). The depo-
sit covers an area of ~265,000 m
2
and its mean
thickness is estimated with 10m, therefore it is
containing a volume of ~2.6
x
10
6
m
3
. The mean
grain size of deposit (F) is around 1 m with no
distinct concentration. This deposit shows a
widely distribution of grain roundness with a
peak for subangular (58%) and angular boulders
(30%), but also subrounded (10%) and less very
angular (2%) boulders occur.
DISCUSSION OF DEPOSITS WITHIN THE IN-
NERDDALEN
In this glacial affected, U-shaped valley, multiple
landslide deposits occur that have similar sedimentolog-
ical features of their bouldery carapace (grain size, grain
roundness). However, they show strong differences in
the morphology of the deposits thus strongly differing
palaeo-environmental conditions of their formation.
The post glacial event is a very distinct rock ava-
lanche deposit (A). It shows the typical morphological
characters of a rock avalanche with the carapace com-
posed of large rock boulders of subangular (57%) and
angular (37%) forms. This indicates that the deposit was
not reworked after deposition. In addition, the lateral
levees and frontal rim (d
uFresne
& d
avies
, 2009) with
a run-up of 65 meters indicate the typical high mobility
Fig. 4 - Morphological end moraine ridge in Innerdalen, around 12 meter high (a). Four meter high fluvial cut (b) shows fine
grained sandy matrix in subsurface. For scale consider mean diameter of visible boulders marked in (a) and (b)
background image
SPATIAL DISTRIBUTION OF ROCKSLIDE DEPOSITS AND THEIR MORPHOLOGICAL FEATURES SUGGEST TIMING
AND PALAEO-ENVIRONMENTAL CONDITIONS FOR ROCK SLOPE FAILURES IN INNERDALEN AND INNFJORDDALEN,
MØRE OG ROMSDAL COUNTY, WESTERN NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
499
normal supra- and subglacial load into the three closely
spaced frontal moraine ridges. According to (h
ewitt
,
2009; s
hugar
& c
lague
, 2011) rock avalanche mate-
rial that covers the glacier protects the ice from melt-
ing. In addition fine grained material is washed out by
supraglacial processes and therefore rock boulders are
concentrated. Following our model, the formation of the
two morphological ridges (D) is related to that process.
A decaying ice body as at the end of the Younger Dryas
is rather stagnant. Insulation on the N side and hence S-
facing side of a valley would cause such a stagnant ice
body decay asymmetrically leading to the concentration
of boulders on the S (and thus N facing) valley side.
We see the boulder patch (deposit F) east and
up-valley disconnected to this process, although com-
posed of similar material (grain size and grain round-
ness). Due to its relatively flat surface, and its coin-
cidence with moraine deposits, we suspect that this
material is related to retransported rock fall and rock
avalanche material from higher up in the catchment as
catastrophic landslides are also very abundant in the
upper catchment (h
erManns
et alii, 2013).
DEPOSITS WITHIN THE INNFJORDDALEN
In the lower Innfjorddalen a succession of multi-
ple rock avalanche deposits with lobate forms overly
each other. These are covered with rock boulders, me-
ter to several meters in diameter. The second of those
deposits is damming a lake, however also the older
deposits span the entire width of the valley suggesting
that similar dams also existed before (Fig. 5).
Several distinguishable morphological features
can be mapped: a marine terrace deposit (A), a first
stratigraphically lowest rock avalanche deposit that
can be divided into a continuous deposit (B) in the
upper valley and a distal part (E). In between there are
an area of deformed (C) and an area of undeformed
(D) valley fill deposits. The proximal part of the strati-
graphically lowest rock avalanche deposit is overlain
by a second rock avalanche deposit with a smaller ex-
tension (F) and these are overlain by a third rock fall
deposit with much smaller extension (G).
MARINE TERRACE DEPOSITS (A)
A distinct terrace occurs in the inner part of the
valley. This deposit (A) is composed out of fine
grained material, chiefly rounded and well rounded
sand and gravel and shows typical graded and with
of a rock avalanche. The source area can be located at
the steep northern rock face of the ”Skarfjellet” moun-
tain. Evidence for its postglacial origin is that the deposit
partly covers some moraine deposits and that it fills the
valley floor over its whole width. This deposit dams a
lake that is evidently stable since formation. Following
the description of rockslide dams of (h
erManns
et alii,
2011b) this dam can be classified as an IIa ii 2 event.
Grain sizes of the carapaces of other landforms
as well as grain roundness suggest a further rock ava-
lanche event in the valley. For example are the three
closely spaced distinct morphological moraine ridges
(B) also covered by a carapace of rock boulders sev-
eral meters in diameter. These deposits (B) show the
smallest mean grain size of boulders (chiefly <0.5 to 1
m) and mainly subangular (54%) forms however with
a trend towards subrounded forms (21%). This sug-
gests more reworking and mixture of material during
transport has taken place. In fluvial cuts it gets visible
that the moraine material is not pure rock avalanche
material but that the boulders are imbedded in fine
grained, sandy matrix. Suspicious are also the isolated
hills composed of rock boulders surrounded by fluvial
sediments (C) that are scattered in the whole valley
bottom between the post glacial rock avalanche de-
posit (A) and the moraine ridges (B).
The isolated boulder patches on both sides of the
valley (D) suggest that these landforms formed when
a landform occupied that valley that does not exist
anymore. The mean elevation of around 350 m above
valley suggests the thickness of that landform at that
time. The high amount of angular (46%) and even
very angular boulders shows less reworking of those
boulder patches.
Both of the valley parallel ridges (E) are also en-
tirely covered by subangular boulders (76%) and no
subrounded boulders could be found, that indicates no
significant transportation. Their location at only one
side of the lake suggests a process that leads to con-
centration of sediments into these ridges.
Therefore we interpret that a rock avalanche oc-
curred in this valley, when it was filled by a glacier at the
end of the Younger Dryas. This rock avalanche would
have traveled over the glacier and deposited the bouldery
patches above the valley floor on unglaciated ground,
thus marking the thickness of the ice body at that time.
However most of the material would have been depos-
ited onto the glacier where it was redeposited with the
background image
M. SCHLEIER, R.L. HERMANNS & J. ROHN
500
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
Fig. 5 - Map of multiple Holocene deposits in Innforddalen. A - G mark the areas which are described in the text. The source
area of the multiple rock avalanches and rock falls is marked with red rectangle. The runup area (~110 m above valley
floor) is marked with a dotted rectangle. Values showing mean elevation of deposits above sea level
background image
SPATIAL DISTRIBUTION OF ROCKSLIDE DEPOSITS AND THEIR MORPHOLOGICAL FEATURES SUGGEST TIMING
AND PALAEO-ENVIRONMENTAL CONDITIONS FOR ROCK SLOPE FAILURES IN INNERDALEN AND INNFJORDDALEN,
MØRE OG ROMSDAL COUNTY, WESTERN NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
501
a Fahrböschung angle of 22.3° (only including the
continuous deposit B), which is smaller than tho-
se of the other events. The elevations above sea
level of the deposits in the inner valley (proximal
part) are 130 m and only 35 m in the outer valley
(most distal part).
The intermediate rock avalanche deposit (F) is the
second largest with a volume of 5.4
x
10
6
m
3
covering
an area of 542,500 m
2
. The Fahrböschung angle of this
deposit is 25.1°. Altitude of this deposit above sea level
is 80 m down to 50 m. This deposit is damming a lake.
The uppermost deposit (G) that partly covers the
intermediate is much smaller with an area of 39,000 m
2
,
and a volume of 0.19
x
10
6
m
3
. This deposit is thus a rock
fall deposit. This is also indicated by the Fahrböschung
angle of 28.8° and therefore that of a rock fall. It is lo-
Fig. 6 - Marine terrace deposit in Innfjorddalen valley. Rock avalanche material is deposited on top of the terrace at
120 m a.s.l. (upper left corner) (a). Typical bedding structures of a propagating delta are visible in (b). View
towards SSE. For scale consider Geologist in lower right of (a)
Tab. 2 - Geometrical characteristics of multiple landslide deposits in Innfjorddalen
25 degrees inclined bedding structures of a propa-
gating marine delta (Fig. 6). This terrace surface
lies at an altitude of 120 m a.s.l., marking an iso-
static rebound of that area of at least 120 m. All
rock-avalanche deposits lie on top of this terrace.
SUCCESSION OF MULTIPLE ROCK AVALANCHE
AND ROCK FALL DEPOSITS (B, F, G)
The deposits of the lowest (B) and intermediate
(F) rock avalanche and the upper (G) rock fall de-
posit are composed of rock boulders, several me-
ters in diameter building a typical carapace. The
deposit (B) covers an area of 1,407,000 m
2
and
has an estimated volume of 21
x
10
6
m
3
(Tab. 2).
This deposit shows a runup of 110 m on the op-
op-
posite valley slope. It is the largest deposit and has
background image
M. SCHLEIER, R.L. HERMANNS & J. ROHN
502
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
were opened to describe the deformation of valley
fill deposit (see Fig. 7 as an example). The excava-
tion shows five different units which are composed
of silt, sand, pebbles and gravel. Layered silt and
sand (E1) is covering the top of most of the exca-
vation including a channel (E2) that occurs only in
one part. Most of the excavation is characterized
by well homogenized deposits without any sedi-
mentological structures such as layering. Within
this matrix there float two islands of gravelly de-
posits E3 and E5 also without any layering. The
homogenized deposit and the islands of gravel de-
posits are underlain by layered sand and silt E4.
The lower part of the valley fill deposit (15 m a.s.l.)
is undeformed (D). Again no rock boulders are
present.
The mean diameter of the boulders imbedded in this
undulated surface (C) is 1m. Its grain size distri-
bution shows a characteristic peak for grain sizes
<0.5 to 1 m. Biggest boulders found are 6m in
diameter. Boulders are subrounded (58%) and su-
bangular (33%).
ISOLATED ISLANDS OF ROCK AVALANCHE
MATERIAL (E)
Isolated hills composed of rock avalanche boul-
ders (E) lie in the outer part of the valley at 15
m a.s.l.. This deposit covers an area of 50,000 m
2
and has a volume of 0.25
x
10
6
m
3
(Tab. 2). The de-
posit shows rather isolated centric hills, which are
composed out of rock boulders of meters to seve-
ral meter diameter embedded in fine grained ma-
terial. These hills are up to three meters high and
cated at 75 m a.s.l.
Deposit (B) has a mean grain size of surficial boul-
ders of 1.5 m in diameter with a spread from <0.5 to
10 m. The grain roundness is subangular with (60%) to
angular and subrounded boulders with 20% each. De-
posit (F) has a mean grain size of 2 m. Boulders also
range from <0.5 to 10 m. These boulders are similarly
to deposit (B) subangular (around 65%) with 26% of
angular and 5% of subrounded clasts. In contrast to
deposits (B) and (F) the rock fall deposit (G) shows
a different distribution. The mean grain size of large
boulders is 4.5 m. Although there is a concentration of
boulders between 2-5 m in this deposit all of the grain
sizes are well distributed in the range between <0.5 up
to 10 m. Compared to the other deposits they show a
relatively high amount of boulders >5 m. Angularity
of boulders is more pronounced with 68% angular and
23% subangular boulders.
DEFORMED (C) AND (D) UNDEFORMED VAL-
LEY FILL DEPOSITS
The stratigraphically lowest rock avalanche depo-
sit is split into two parts by valley fill deposits that
are deformed valley deposits (C) between 30 and
20 m a.s.l. and undeformed below 20 m a.s.l. In the
deformed area (C) no connected boulder deposits
occur, however isolated boulders could be found.
The surface is undulated with small “valleys and
ridges” that are oriented perpendicular to the axis
of the valley. Within those deposits and along its
frontal part where the undulated valley floor depo-
sits limit towards the undeformed valley fill depo-
sits (D), two excavations (see Fig. 5 for location)
Fig. 7 - Exemplary excavation of the deformed valley fill deposits at the border between deformed and undeformed valley
sediments. The blue raster shows 6 cells (1 m lengh, 0.5 m width). (E1, E2: layered silt and sand; E3, E5: islands of
gravelly deposits without layering; E4: layered sand and silt)
background image
SPATIAL DISTRIBUTION OF ROCKSLIDE DEPOSITS AND THEIR MORPHOLOGICAL FEATURES SUGGEST TIMING
AND PALAEO-ENVIRONMENTAL CONDITIONS FOR ROCK SLOPE FAILURES IN INNERDALEN AND INNFJORDDALEN,
MØRE OG ROMSDAL COUNTY, WESTERN NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
503
its mean thickness is estimated in the field to be 5
m (Fig. 8). The grain size distribution of deposit
(E) is similar to (B) and (F). The mean grain size
is 1m, with a peak at 0.5-2 m. Boulders >3.5 m are
less common but exceptional boulders are 10 m
large. The boulders are subrounded (57%) to su-
bangular (29%) with also some rounded boulders
(14%) present.
DISCUSSION OF DEPOSITS WITHIN THE IN-
NFORDDALEN
The multiple landslide deposits in Innfjorddalen
show a simple succession where the different rock
avalanche deposits and the rock falls are deposited on
top of each other. The timing of the events is poorly
understood. However a historical rock failure in 1665
is descibed in historical records from that valley in
the church books as the event caused the death of
farmers, not by the direct impact but due to suffoca-
tion due to the enormous dust (F
urseth
, 2006). We
interpret that this age is the age of the youngest rock
slope failure that forms deposit (G).
Further constrain on the age of the oldest de-
posits can be derived from the valley fill deposits
and their elevation a.s.l. This deposit split into two
parts. While the proximal deposit is continuous the
distal part occurs in form of separated island hills of
rock avalanche material. Following this interpreta-
tion the Fahrböschung angle of the whole event is
19.6°. Such island hills have been also described in
the distal part of the Flims rock avalanche depos-
its in the Swiss Alps and called there “Toma-hills”
(
von
P
oschinger
, 2002). The Flims rock-avalanche
is interpreted to have dropped into a lake (
von
P
o
-
schinger
et alii, 2006) and also at that site the fron-
tal hills (Toma-hills) are separated from the main
body of the rock avalanche by valley fill deposits
that have no sedimentary structure and are strongly
homogenized (Bonaduz gravel). Similarly to Flims
the deformed valley fill deposits in Innfjorddalen are
strongly homogenized and leak sedimentary struc-
ture. Therefore, we interpret that similar to the Flims
rock avalanche also the oldest rock avalanche in the
Innfjorddalen dropped in its distal part into a water
body. As no morphological landforms as moraine
dams or rockslide dams exist in the lower Innfjord-
dalen valley the only water body could have been
the fjord itself. The lower limit of deformation struc-
tures in the valley fill lie at 20 m a.s.l. This line is
interpreted to represent the sea level at the time of
the oldest rock avalanche. Following this interpreta-
tion, this rock avalanche would have dropped into
Fig. 8 - Isolated island hill deposits of rock avalanche material in Innfjorddalen at the border to the undeformed valley fill
deposits (view towards NNE). For scale consider Geologist in the center
background image
M. SCHLEIER, R.L. HERMANNS & J. ROHN
504
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
INNERDALEN
1) Several rock avalanches occurred in late Pleisto-
cene and Holocene age.
2) At least one large rock avalanche occurred in late
glacial age onto the decaying ice body, creating
distinct morphological features (isolated boulder
patches above the valley, moraine ridges compo-
sed of rock avalanche material, hills on the valley
floor composed of rock avalanche material sur-
rounded by valley fill deposits, and valley parallel
ridges of rock avalanche material).
INNFJORDDALEN
1) Several rock avalanches occurred after deglacia-
tion towards the end of the Holocene, sourcing
from the same area.
2) The oldest event was deposited onto highly water
saturated valley fill sediments above, partly below
and close to sea level, that also created isolated
island hills of rock avalanche material.
ACKNOWLEDGEMENTS
The authors gratefully thank the Norges ge-
ologiske undersøkelse (NGU) for great financial and
scientific support and acknowledge the help from col-
leagues and friends during field work and discussion.
We thank the anonymous reviewer for the suggestions.
the valley and made a 90 degree turn after a run-up
of 110 m at the opposite valley site. The remaining
mass would have run down the valley strongly lique-
fying and deforming the valley fill deposits in those
areas where they have strongly been water saturated
close to the former shore line of the fjord. Here the
rock avalanche may have split into two bodies, one
coming to rest onshore and another running out into
the shallow fjord. An alternative interpretation is that
both bodies are connected below the surface and that
the undeformed valley fill deposits have been depos-
ited later under water level covering the connection
of both parts.
In Innfjorddalen the sea level drop due to isostatic
rebound of Scandinavia is not well dated throughout
the Holocene, however in other areas of western Nor-
way the sea level was at ca. 25 m around 3000 years
ago (M
aiForth
, 2010). Therefore the limit between
undeformed and deformed deposits suggests that this
event occurred ca. 3000 years ago.
CONCLUDING REMARKS
Although isolated hills made up of rock avalanche
material are similar in the Innerdalen and Innfjordalen
valley, the interactions with other landforms allow to
interpret the genesis of these landforms differently:
REFERENCES
B
lais
-s
tevens
a., h
erManns
r.l. & J
erMyn
c. (2011) - A
36
Cl age determination for Mystery Creek rock avalanche and its
implications in the context of hazard assessment, British Columbia, Canada. Landslides, 8: 407-416.
B
oggs
s.J
r
. (2001 Ed) - Principles of sedimentology and stratigraphy. Prentice Hall.
d
ehls
J.F., o
lesen
o., B
unguM
h., h
icks
e.c., l
indholM
c.d. & r
iis
F. (2000) - Neotectonic map: Norway and adjacent areas.
Geological Survey of Norway, Trondheim.
d
uFresne
a. & d
avies
t.r. (2009) - Longitudinal ridges in mass movement deposits. Geomorphology, 105 (3-4): 171-181.
e
vans
s.g., h
erManns
r.l., s
troM
a. & s
carascia
-M
ugnozza
g. (2011 e
ds
.) - Natural and artificial rockslide dams. Lecture
Notes in Earth Sciences 133, Springer, Berlin-Heidelberg.
F
urseth
a. (2006) - Skredulykker i Norge. Tun Forlag, Oslo.
h
acker
B.r., a
ndersen
t.B., J
ohnston
s., k
ylander
-c
lark
a.r.c., P
eterMan
e.M., w
alsh
e.o. & y
oung
d. (2010) - High-
temperature deformation during continental-margin subduction and exhumation: The ultrahigh-pressure Western Gneiss
Region of Norway.
Tectonophysics, 480 (1-4): 149-171.
h
eiM
A. (1932) - Bergsturz und Menschenleben. Beiblatt zur Vierteljahrsschrift der Naturforschenden Gesellschaft in Zürich,
77: 218.
h
erManns
r.l.,B
likra
l.h., a
nda
e., s
aintot
a., d
ahle
h., o
PPikoFer
t., F
ischer
l., B
unkholt
h., B
öhMe
M., d
ehls
J.F.,
l
auknes
t.r., r
edField
t.F., o
sMundsen
P.t. & e
iken
t. (2011a) - Systematic mapping of large unstable rock slopes in
Norway. Proceedings of the 2
nd
World Landslide Forum, Rome.
h
erManns
r.l., B
likra
l.h., n
auMann
M., n
ilsen
B., P
anthi
k.k., s
troMeyer
d. & l
ongva
o. (2006) - Examples of multiple
rock-slope collapses from Köfels (Ötz valley, Austria) and western Norway. Engineering Geology, 83 (1-3): 94-108.
h
erManns
r.l., d
ahle
h., B
Jerke
P.l., c
rosta
g.B., a
nda
e., B
likra
l.h., s
aintot
a., l
ongva
o.& e
iken
t. (2013) - Rock
background image
SPATIAL DISTRIBUTION OF ROCKSLIDE DEPOSITS AND THEIR MORPHOLOGICAL FEATURES SUGGEST TIMING
AND PALAEO-ENVIRONMENTAL CONDITIONS FOR ROCK SLOPE FAILURES IN INNERDALEN AND INNFJORDDALEN,
MØRE OG ROMSDAL COUNTY, WESTERN NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
505
slide dams in Møre og Romsdal county, Norway: examples for the hazard and potential of rock slide dams. In: M
argottini
c., c
anuti
P. & s
assa
k. (e
ds
.). Landslide science and practise. Springer, Berlin. in press.
h
erManns
r.l., h
ansen
l., s
letten
k., B
öhMe
M., B
unkholt
h.s.s., d
ehls
J.F., e
ilertsen
r.s., F
ischer
l., l'h
eureux
J.s., h
øgaas
F., n
ordahl
B., o
PPikoFer
t., r
uBensdotter
l., s
olBerg
i.-l., s
talsBerg
k. & y
ugsi
M
olina
F.x. (2012) -
Systematic geological mapping for landslide understanding in the Norwegian context. In: e
Berhardt
e., F
roese
c., t
urner
k. & l
eroueil
s. (e
ds
.). Landslides and engineered slopes: protecting society through improved understanding: 265-271,
Taylor & Francis Group, London.
h
erManns
r.l. h
ewitt
k. s
troM
a. e
vans
s.g. d
unning
, s.a. & s
carascia
-M
ugnozza
g. (2011b) - The Classification of
Rockslide Dams. In: e
vans
s.g., h
erManns
r.l., s
troM
a. & s
carascia
-M
ugnozza
g. (e
ds
.). Natural and artificial
rockslide dams. Lecture Notes in Earth Sciences, 133: 581-593, Springer, Berlin-Heidelberg.
h
erManns
r.l. & l
ongva
o. (2012) - Rapid rock-slope failures. In: c
lague
J.J. & s
tead
d. (e
ds
.) - Landslides: types,
mechanisms and modeling: 59-70, Cambridge University Press.
h
erManns
r.l., n
iederMann
s., i
vy
-o
chs
s. & k
uBik
P.w. (2004) - Rock avalanching into a landslide-dammed lake causing
multiple dam failure in Las Conchas valley (NW Argentina) - Evidence from surface exposure dating and stratigraphic
analyses
. Landslides, 1 (2): 113-122.
h
ewitt
K. (2009) - Rock avalanches that travel onto glaciers and related developments, Karakoram Himalaya, Inner Asia.
Geomorphology, 103 (1): 66-79.
h
ewitt
k., g
osse
J. & c
lague
J.J. (2011) - Rock avalanches and the pace of late Quaternary development of river valleys in the
Karakoram Himalaya. Geological Society of America Bulletin, 123 (9/10): 1836-1850.
M
aiForth
J. (2010) - Kulturminner på Flakk. Master Thesis, pp. 23. NTNU, Trondheim
s
hugar
d.h. & c
lague
J.J. (2011) - The sedimentology and geomorphology of rock avalanche deposits on glaciers.
Sedimentology, 58 (7): 1762-1783.
t
veten
e., l
utro
o. & t
horsnes
t. (1998) - Geologisk kart over Norge, berggrunnskart Ålesund, 1:250,000. Geological Survey
of Norway, Trondheim.
von
P
oschinger
a. (2002) - Large rockslides in the Alps: A commentary on the contribution of G.Abele (1937-1994) and a
review of some recent developments. In: e
vans
s.g. & d
e
g
raFF
J.V. (e
ds
.). Catastrophic landslides: effects, occurrence and
mechanisms. Geological Society of America Reviews in Engineering Geology, XV: 237-255, Boulder, Colorado.
von
P
oschinger
a., w
assMer
P. & M
aisch
M. (2006) - The Flims Rockslide: History of interpretation and new insights. In: e
vans
s.g., s
carascia
M
ugnozza
g., s
troM
a. & h
erManns
r.l. (2006). Landslides from massive rock slope failure. NATO
Science Series IV: Earth and Environmental Sciences, 49: 329-356, Springer, Netherlands.
w
elkner
d., e
Berhardt
e. & h
erManns
r.l. (2010) - Hazard investigation of the Portillo Rock Avalanche site, central Andes,
Chile, using an integrated field mapping and numerical modelling approach. Engineering Geology, 114 (3-4): 278-297.
background image
Statistics