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Geomorphology 69 (
Geomorphology and geochronology of sackung features
(uphill-facing scarps) in the Central Spanish Pyrenees
F. Gutierrez-Santolallaa,T, Enrique Acostab, Santiago Rıosb,
Jesus Guerreroa, Pedro Luchaa
aEdificio Geologicas, C/. Pedro Cerbuna, 12, Universidad de Zaragoza, Zaragoza 50009, SpainbInstituto Geologico y Minero de Espana, Oficina de Proyectos de Zaragoza, C/. Fernando El Catoloico, 59, 4 8C, Zaragoza 50006, Spain
Received 12 August 2004; received in revised form 27 January 2005; accepted 29 January 2005
Available online 17 March 2005
Abstract
The paper analyses uphill-facing scarps and associated troughs developed in the oversteepened slopes of two neighbouring
glacial valleys in the central Spanish Pyrenees. Previous studies of sackung landforms in the Pyrenees have argued for deglacial
unloading as the genetic mechanism, but this causal and temporal relationship has not been proved due to the lack of
chronological data. The antislope scarps in the two studied locations, Vallibierna and Estos, are developed in Palaeozoic
metasedimentary rocks (parallel to the contour lines and the structural grain), occur in the intermediate sector of the hillslope,
and are up to 0.5 km long and several meters high. A trench was excavated in a sackung trough fill in each of the valleys in
order to gain information about their chronology and genesis. Charcoal from the lowermost unit in Vallibierna provided an age
of 5.9 cal. ka for the sackung and extrapolation of the three dates obtained in Estos indicates that the trough formed ca. 7.6–7.8
cal. ka. Deglaciation of the studied sectors of the valleys occurred between 16 and 13 ka. The time lag of N5 ka suggests that
glacial erosion and the subsequent debutressing of the oversteepened valley walls created slopes predisposed to sackung
development, but did not initiate the movement. Seismic shaking is proposed as a probable triggering factor. This hypothesis,
although supported by the sudden deformation event recorded by a failure plane exposed in Vallibierna trench, and by the
seismic and neotectonic activity of the area, cannot be proved due to the lack of chronological information about
paleoearthquakes.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Sackung; Uphill-facing scarps; Gravitational spreading; Trenching; Seismic activity; Pyrenees
0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2005.01.012
T Corresponding author. Tel.: +34 979 761090; fax: +34 976
761106.
E-mail address: [email protected] (F. Gutierrez-Santolalla).
1. Introduction
The term sackung, German word for sagging, was
first introduced by Zischinsky (1966, 1969) to
designate the surface manifestations of deep-seated
rock creep in slopes on foliated bedrock. In the
2005) 298–314
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314 299
subsequent literature, sackung (plural sackungen)
generically refers to linear geomorphic features
produced by gravitational spreading in slopes (Varnes
et al., 1989; Crosta, 1996; Ward, 2003). The most
characteristic sackung-type landforms are uphill-fac-
ing scarps (also called antislope scarps, counter
scarps, or antithetic scarps) associated with linear
depressions lying at the upslope side of the scarps
(Radbruch-Hall, 1978; Varnes et al., 1989). These
peculiar landforms commonly form complexes paral-
lel to the contours and ridge tops in the upper sector of
steep slopes. The uphill-facing scarps that split the
ridge tops into two crests give place to double or twin
ridges (dopplegrate) and ridge-top depressions. These
depressions show a graben-like appearance when
scarps with opposite orientations affect the ridge
crests. Some authors note arched uphill-facing scarps
with a downslope convexity (Jahn, 1964; Beck, 1968;
Varnes et al., 1989). The asymmetric troughs asso-
ciated with these scarps frequently host closed
depressions with ephemeral ponds where deposition
of fine-grained material takes place. The presence of
small shallow sinkholes has also been reported along
these depressions despite the absence of soluble rocks
(Bovis, 1982; McCalpin and Irvine, 1995). McCalpin
and Irvine (1995), from a compilation of published
sackung scarp dimensions carried out by McCleary et
al. (1978), give 15–300 m and 1–9 m as typical ranges
of scarp length and height, respectively. Other
frequent geomorphic features found in slopes affected
by sackungen are downhill-facing scarps with accom-
panying linear depressions and a bulge at the toe of
the slope (talzuschub) (Nemcok, 1972; Mahr, 1977;
Radbruch-Hall, 1978; Varnes et al., 1989). In some
cases, the upper sector of the slopes shows a curved
downhill-facing scarp resembling the head scar of a
landslide (Soeters and Rengers, 1983; Bordonau and
Vilaplana, 1986; McCalpin and Irvine, 1995; Agliardi
et al., 2001). Some authors also mention saddles in
secondary ridges (McCalpin and Irvine, 1995) or
different types of mass movements in the lower parts
of the slope (Radbruch-Hall, 1978; Crosta, 1996;
Agliardi et al., 2001).
Examples of sackung-type landforms have been
documented from most of the important mountain
belts (Radbruch-Hall, 1978; Crosta, 1996). They are
particularly frequent in steep-sided ridges flanked by
deep glacial valleys. The sackungen inventory elabo-
rated by McCleary et al. (1978) yields 400–1200 m
and 25–508 as typical ranges of slope height and
gradient. According to Varnes et al. (1989), massive
ridges with rounded crests are more favourable for
sackungen development than narrow ridges. Sackung
scarps have been observed in a wide variety of
lithologies including metamorphic, sedimentary, vol-
canic, and plutonic rocks (Radbruch-Hall, 1978;
McCalpin and Irvine, 1995). The persistent orienta-
tion of the sackung features parallel to the contour
lines indicates that topography is the main factor that
controls their trend. In numerous cases, a clear
parallelism has been found between the direction of
the sackungen and the strike of any set of disconti-
nuity planes (bedding, jointing, cleavage, and folia-
tion) (Jahn, 1964; Tabor, 1971; Bovis, 1982; Soeters
and Rengers, 1983; Varnes et al., 1989; Corominas,
1990; McCalpin and Irvine, 1995; Kellogg, 2001;
Agliardi et al., 2001; Di Luzio et al., 2004).
Several mechanisms have been proposed to explain
the origin of the uphill-facing scarps. The earliest
researchers (e.g., Paschinger, 1928) and some other
authors quoted by Tabor (1971) ascribed the sackung
scarps to structurally controlled differential erosion,
primarily due to frost action, nivation processes, and
aeolian deflation. Subsequent theories attribute sack-
ung features to deformational processes that involve
different modes of lateral spreading in rock masses
(Fig. 1). Jahn (1964), for example, suggested that the
uphill-facing scarps result from the combination of
gravity-induced spreading of fractured ridges and
erosion in the uphill sides of the resulting tension
cracks (Fig. 1A). That author stresses the downward
removal of particles through the cracks (ravelling) in
the development of the trenches. Other authors
suggest that the uphill-facing scarps and troughs are
produced by the differential downslope bend of rocks
affected by steeply dipping discontinuity planes (Ter-
Stepanian, 1966; Zischinsky, 1966, 1969; Tabor,
1971) (Fig. 1B). In this model, the troughs correspond
to linear depressions developed between the rocks
rotated valleyward and the undisturbed rocks. Tabor
(1971) believes that the zone of creep may extend as
much as 200 m below the surface. Bovis (1982)
proposes that the sackung features are produced by a
combination of flexural slip toppling of block slabs
defined by joints steeply dipping into the slope and
erosion of the upslope side of the resulting linear
A
Spreading of fracturesand erosion in uphill sides
Downslope bending ofrocks with steeply dippingplanes
Flexural toppling anderosion in uphill sides
Normal "fault" steeplydipping into the slope.
B C D
Fig. 1. Diagrams illustrating the mechanisms involving lateral spreading proposed by several authors for the generation of uphill-facing scarps.
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314300
trenches (Fig. 1C). The most widely accepted
interpretation attributes the uphill-facing scarps to
the dip-slip displacement of failure planes steeply
dipping into the slope (Beck, 1968; Mollard, 1977;
Radbruch-Hall et al., 1976; Radbruch-Hall, 1978;
Varnes et al., 1989; McCalpin and Irvine, 1995;
Pasuto and Soldati, 1996; Agliardi et al., 2001) (Fig.
1D). These tensional structures are related to the
lateral spreading of the ridge flanks and the conse-
quent subsidence of the ridge crest (Radbruch-Hall,
1978; Varnes et al., 1989). In this model, the slope
troughs correspond to half-graben depressions devel-
oped on the downthrown uphill side of the fault scarp
(hanging wall). The ridge-top depressions bounded by
fault scarps with opposite orientations are the topo-
graphic manifestation of graben structures. The lateral
spreading of the rock mass perpendicular to the ridge
axis explains the bulges frequently observed in the
lower sector of the slopes affected by sackungen. The
existence of nearly vertical failure planes at the toe of
uphill-facing scarps has been corroborated at some
locations by trenching (McCleary et al., 1978;
McCalpin and Irvine, 1995; Tibaldi et al., 2004) and
in natural exposures of sackung trough fills (Beget,
1985). Very little is known about the depth and
geometry of the sackung failure planes due to the
difficulty of obtaining subsurface data. Some authors
suggest that these brittle structures are accommodated
downwards by a broad zone of continuous deforma-
tion (Mahr, 1977; Mahr and Nemcok, 1977; Rad-
bruch-Hall, 1978), whereas others believe that these
planes are linked to deep-seated discrete and contin-
uous sliding surfaces (Beck, 1968; Agliardi et al.,
2001; Tibaldi et al., 2004; Di Luzio et al., 2004). A
complete spectrum may exist between both extreme
situations. In spite of the abundant literature, numer-
ous authors consider that the mechanisms that produce
sackung features are not well understood (Radbruch-
Hall, 1978; Varnes et al., 1989; Pasuto and Soldati,
1996; Agliardi et al., 2001). Possibly the sackung
geomorphic features, although they look remarkably
similar, are a case of morphologic convergence or
equifinality.
Concerning the dynamics, several sources of
stresses have been proposed to explain the lateral
spreading involved in the generation of sackungen. (1)
The majority of the investigators attribute spreading to
the removal of the load and lateral support in glacially
oversteepened slopes by the retreat of valley glaciers.
This hypothesis has been substantiated in a particular
example of the Italian Alps with geomechanical
numerical simulations (Agliardi et al., 2001). Some
authors like Kellogg (2001) or Agliardi et al. (2001)
indicate that the unloading-induced lateral spreading
may be favoured by increased pore water pressures
during deglaciation periods. (2) Tensional stresses
caused by gravity forces acting on the upper sectors of
steep-sided ridges are considered to be capable of
producing sackung scarps (Radbruch-Hall, 1978).
This theory is supported by analytical modelling of
stresses in symmetric linear ridges and elongated
shield volcanoes (Radbruch-Hall, 1978; Savage and
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314 301
Swolfs, 1986; Varnes et al., 1989; Pan and Amadei,
1994). (3) Earthquake shaking is proposed as a likely
cause of sackungen generation in active seismic areas
(Beck, 1968; McCleary et al., 1978; Radbruch-Hall,
1978; Solonenko, 1977; McCalpin, 1999). The
dynamic loading produced by earthquakes may trigger
discrete episodes of rapid movement (Beck, 1968) or
may accelerate the long-term and low-rate creep
movement of the sackung structures (Pasuto and
Soldati, 1996). (4) Uphill-facing scarps have also
been interpreted as the result of the coseismic
displacement of deep-seated tectonic faults (fault
scarps) (Cotton, 1950; Plafker, 1967; Forcella and
Orombelli, 1984; McCalpin, 1999). (5) Subsidence
due to evaporite dissolution is also a possible cause
for the generation of sackung scarps. In Teruel
Neogene Graben (Iberian Range, Spain), the localized
interstratal karstification of Triassic evaporites has
produced a monocline structure concordant with the
topography in the overlying Neogene sediments. The
conspicuous uphill-facing scarps affecting the inclined
limb of the monocline have been related to the
tensional forces caused by the dissolution-induced
subsidence phenomena (Gutierrez, 1998; Calvo et al.,
1999).
Little is known about the regime (continuous or
episodic) and rate of movement of the sackung
features. In the Southern Rocky Mountains, from a
sackung trough fill exposed by an artificial trench,
McCalpin and Irvine (1995) deduce a slow and
continuous displacement for a bfaultQ plane overlain
by fine-grained deposits affected by synsedimentary
ductile deformation and devoid of colluvial wedges.
In the North Cascades, Beget (1985) infers three to
four episodes of displacement from dated tephra
layers cut by sackung failure planes. Tibaldi et al.
(2004) infer two episodes of deformation from
trenches excavated across sackung features developed
in low relief slopes (190 m) in the Western Alps. The
episodic displacement of sackung scarps is also
claimed by those who support a causal link between
sackung features and earthquake activity (i.e., Beck,
1968). Regarding the rates of movement, in Bald
Eagle Mountain (Rocky Mountains of Colorado),
accurate geodetic measurements carried out during
more than two decades yield maximum rates of 4 mm/
year of horizontal displacement (Varnes et al., 1990,
2000). Bovis and Evans (1995) have measured geo-
detically continuous slip rates of up to 10 mm/year in
antislope scarps in the British Columbia. McCalpin
and Irvine (1995), from the stratigraphy of a sackung
trough fill, calculate mean rates of 0.14–0.75 mm/year
and 0.43 mm/year of vertical and horizontal displace-
ment, respectively.
The sackung geomorphic features have some
implications from the applied point of view. The
uphill-facing scarps may be easily misinterpreted as
recent tectonic fault scarps and as evidences of large
earthquakes, since gravitational movements com-
monly takes place along pre-existing brittle tectonic
structures (Radbruch-Hall, 1978; McCalpin, 1996).
As Beck (1968) and Varnes et al. (1989) point out, a
diagnostic criterion is that sackungen orientation is
generally determined by the local topography. McCal-
pin (1999) reviews several criteria and methods to
differentiate between nontectonic sackung scarps and
tectonic fault scarps. Despite that, in active seismic
areas, the sackung scarps may undergo episodes of
rapid movement triggered by earthquakes. Although
the probability of these slope movements to turn into
dangerous catastrophic failures is believed to be low
(Crosta, 1996), examples of catastrophic failures from
slopes affected by sackung scarps have been reported
in Japan (Chigira and Kiho, 1994) and Canada (Evans
and Couture, 2002). On the other hand, the continuous
(Varnes et al., 1990, 2000) or intermittent (Beget,
1985) differential movements in the slopes affected by
sackung pose a constraint to engineering structures
(Mahr, 1977; Radbruch-Hall, 1978; McCalpin and
Irvine, 1995).
Data about the age of sackung scarps are very
scarce. Most of the information is based on relative
chronological criteria. Beget (1985) dated three to
four episodes of spreading movement in Washington
State by means of tephra layers cut by sackung failure
planes in natural exposures. Agliardi et al. (2001)
dated an uphill-facing scarp that offsets a rock glacier
in the Alps based on the minimum age estimated for
this periglacial accumulation (5000–770 years BP). In
Bristish Columbia, Bovis (1982), applying lichen-
ometry, gives an age of 110 years for a 1 m high
sackung scarp affecting a Neoglacial moraine. Beck
(1968) reports scarps crossing active scree slopes in
the Southern Alps of New Zealand. The only radio-
metric age of an uphill-facing scarp known by the
authors has been provided by McCalpin and Irvine
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314302
(1995) from the Southern Rocky Mountains of
Colorado. These authors date the base of a sackung
trough fill (11,000–11,500 years BP) exposed by a
backhoe trench extrapolating to the base of the fill the
radiocarbon ages of three paleosoils.
This paper analyses the sackung features found in
two neighbouring glacial valleys of the central
Spanish Pyrenees. The main aspects addressed in this
work are: (1) the geomorphology of the sackung-type
features, (2) the chronology of the uphill-facing scarps
and their kinematics by means of the study of trough
fills exposed in artificial trenches, and (3) the
temporal and causal relationships between the degla-
ciation of the valleys and the gravitational spreading
phenomena.
2. General geological and geomorphological setting
The two studied locations with sackung scarps are
located in the glacial valleys of Estos and Vallibierna,
both tributaries of the Esera River Valley, a major
transverse drainage of the central Spanish Pyrenees
(Fig. 2). These areas are situated in the so-called Axial
Pyrenean Zone, a structural unit formed by strongly
deformed Paleozoic sedimentary and metasedimen-
Benasque
F R A N C E
Mt Aneto (3 404 m)
Sackungen
Kilometers
N
242o36'0''N
0o37'48''E0o32'48''E0o27'0''E
42o36'0''N
42o39'36''N
0o27'0''E 0o32'24''E 0o37'48''E
42o39'36''N
Fig. 2. Topography of the Benasque Valley headwaters and location of the studied sackung features.
tary rocks (affected by the Hercynian and Alpine
orogenies), and Late Hercynian syntectonic grano-
dioritic plutons (Garcıa-Sansegundo, 1991, 1992;
Barnolas and Pujalte, 2004). The glaciers sculptured
the main topographic features in the Esera Valley
headwaters in Pleistocene times. The area shows the
typical inherited erosional and depositional landforms
of glaciated alpine mountain environments including
horns, aretes, cirques, overdeepened basins, deep
glacial valleys flanked by steep and high-relief slopes,
moraines, and moraine-dammed glaciolacutrine
basins (Martınez de Pison, 1989; Garcıa-Ruiz et al.,
1992) (Fig. 2). Several authors have established a
general chronology for the Late Pleistocene–Holocene
glacial phases in the Pyrenees based on absolute
datings and the morpho-topographic correlation of
glacial deposits (Bordonau, 1992a; Bordonau et al.,
1992). According to this regional chronology, the
Pyrenean glaciers reached the maximum extent
between 50,000 and 45,000 years BP, long before
the Last Glacial Maximum recorded in the majority of
mountain ranges (Garcıa-Ruiz et al., 2003). At that
time, the Esera glacier was around 36 km long, from
3400 to 900 m in elevation, and reached more than
900 m in thickness in Benasque area (Martınez de
Pison, 1989; Bordonau, 1992b, 1993). Subsequent to
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314 303
the Post-Maximum Stabilization Phase (31,000–
45,000 years BP) and the Valley Glaciers Phase
(N26,000 years BP), the Phase of High Altitude
Glaciers is divided into the Episode of High Altitude
Valley Glaciers and the Episode of Cirque Glaciers.
During the first episode, dated at 16,000–15,000 years
BP, the sectors of Vallibierna and Estos valleys, where
the studied sackungen are located, were still occupied
by glaciers (Copons and Bordonau, 1997). In the
Episode of Cirque Glaciers, situated at 14,000–13,000
years BP, the front of the glaciers were upstream of
these sectors. Thus, the two studied sackung slopes
were deglaciated between 16,000 and 13,000 years
BP. The present-day glaciers in the Esera watershed
are restricted to some cirques carved in the highest
granodioritic massifs (Martınez de Pison and Arenil-
las, 1988). As the glaciers have retreated towards
topographically higher positions, the profile of the
oversteepened slopes carved by the glaciers has been
partially attenuated by periglacial, fluvial, and mass
wasting processes creating talus slopes and cones,
alluvial fans, and a wide variety of slope movements
including sackungen.
The authors of this work know of four areas in the
Pyrenees where sackung type landforms have been
documented, all of them developed in outcrops of
Paleozoic metasediments of the Axial Pyrenean Zone:
(a) Bohı area (Soeters and Rengers, 1983), (b) Vielha
area (Bordonau and Vilaplana, 1986), both in the
headwaters of the Noguera Ribagorzana watershed;
(c) the Valira d’Orient Valley in Andorra (Corominas,
1990); and (d) Vallibierna Valley, one of the locations
studied in this work which was briefly described in a
previous work by Lampre (1998). In all of these cases,
the sackung features have been attributed to gravita-
tional spreading phenomena caused by deglacial
unloading in oversteepened slopes. However, this
causal and temporal relationship has not been proved
due to the lack of geochronological data (Moya et al.,
1992).
3. Sackungen in Vallibierna Valley
3.1. Geology and geomorphology
The sackung landforms are located in the north-
eastern flank of the Sierra Negra Range, which forms
the southern slopes of Vallibierna glacial valley (Fig.
2). The orientation of this relief is parallel to a WNW–
ESE trending anticlinorium composed of SW verging
folds. The metasedimentary rocks that form the slope
affected by the sackung scarps strike parallel to the
topographic grain and show a general dip towards the
valley (Rıos et al., 1997). Most of the slope is
underlain by tightly folded and densely jointed
Silurian black slates. These rocks have a high content
of carbonaceous material and pyrite. The lowermost
part of the slope and a relatively small patch located in
the middle–upper sector of the slope are formed by a
sequence of dark limestone and black slates Devonian
in age (Garcıa-Sansegundo, 1991, 1992; Rıos et al.,
1997).
The slopes in Vallibierna Valley show a composite
profile with a relatively gentle upper segment and a
steep lower segment. The upper segments reflect the
preglacial topography of the valley, whereas the lower
segments delineate the trough excavated in glacial
times. This geomorphic evidence indicates that the ice
reached more than 300 m thick in this sector of the
valley during the Last Glacial Maximum (LGM).
Considering that the density of the glacial ice is
around 0.9 g/cm3 (Benn and Evans, 1998), a stress
release in excess of 27 kg/cm2 (2.7 MPa) can be
estimated for the rocks located in the valley bottom
since the LGM (50,000–45,000 years BP). Terminal
moraines have been mapped in the valley about 1 km
downstream and 2–3 km upstream of the slope
affected by sackungen (Martınez de Pison, 1989;
Garcıa-Ruiz et al., 1992; Copons and Bordonau,
1997; Lampre, 1998). Copons and Bordonau (1997),
based on morpho-topographic criteria, correlate these
deposits to the Episode of High Altitude Valley
Glaciers (16,000–15,000 years BP) and the Episode
of Cirque Glaciers (14,000–13,000 years BP), respec-
tively. Thus, according to this chronology, the sector
of the valley where the sackungen are located was
deglaciated sometime between 16,000 and 13,000
years BP.
The slope with sackung scarps has a local relief of
750 m, from 2691 to 1940 m, and an average gradient
of 19.58 (Figs. 3 and 4). A concave ridge defines the
crest of the slope with a conspicuous downhill facing
scarp (Lampre, 1998) up to 50 m high that resembles
the head scar of a landslide. A similar situation has
been described by several authors in slopes with
Fig. 3. Geomorphological sketch of the slopes affected by antislope scarps in Vallibierna and Estos valleys and location of the trenches
excavated in the sackung trough fills.
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314304
counter scarps (Soeters and Rengers, 1983; Bordonau
and Vilaplana, 1986; McCalpin and Irvine, 1995;
Agliardi et al., 2001). An elongated depression with
an ephemeral pond has been mapped at the foot of this
scarp north of Estibafreda Peak. The depression
hosting the Ardones Pond constitutes a NW–SE
ridge-top graben-like morphostructure flanked by the
main downhill-facing scarp and two uphill-facing
scarps (Fig. 5C). On the other hand, the profile of the
upper sector of the slope shows an undulating
topography. All these features give the slope the
appearance of being affected by a landslide; however
no obvious geomorphic features allow identifying the
margins and toe of such type of slope movement. In
addition, the lower portion of the slope and the trace
of Vallibierna River do not show any evidence of
postglacial outward bulging.
The main complex of uphill-facing scarps is
located in the intermediate sector of the slope,
between 2500 and 2100 m (Figs. 3, 4, and 5A).
This distribution contrasts with the majority of the
documented sackung scarps, commonly located in
the crestal area of the slopes (Radbruch-Hall, 1978;
Varnes et al., 1989). These geomorphic features
follow a WNW–ESE trend parallel to the structural
and topographic grain (Lampre, 1998) and some of
them show a slight valleyward convexity. Very
likely, the failures have occurred along pre-existing
discontinuity planes. The length of the antislope
scarps ranges from a few tens of meters to around
500 m. Some of the uphill-facing scarps are
accompanied by linear depressions with a flat and
turf-covered aggradational surface underlain by fine-
grained deposits (Fig. 5B). The scarps, with heights
varying form 0.4 to 3.6 m, do not show a fresh
appearance, indicating that they are not active or that
the degradation rate exceeds the vertical component
of the deformation rate. Regarding the hydrology of
Fig. 5. Photographs of the sackung features in Vallibierna Valley. (A) General view of the antislope scarps. (B) Excavated sackung trough. (C)
Ridge-top depressions flanked by downhill and uphill-facing scarps in Ardones Pond area. (D) Trench in a trough fill.
Fig. 4. Stereoscopic images of the sackung area in Vallibierna Valley.
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314 305
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314306
the slope, it shows well-defined creeks which dissect
some sackung scarps. In the lower portion of the
slope, there is also a permanent spring of Fe-rich
Bedrock slope
ColluviumTrough Sackung ridge
Bedrock slopeTrench
ENEWSW
0
2
4
6
8
10
12
100 20 30 40 50 60 70 m
20°
11°
Bedrockslope
Trough
Sackung ridge
Trench
NESW
0
2
4
6
8
10
12
100 20 30 40 50 60 m
23°
Bedrock slope
20,5°
29°
Weatheredbedrock
Angulargravel
Marls withclasts
Organic soilhorizon
Oxidationhorizon
Sackung "fault" N 116 E 76 STrench limitWeathered bedrock
Weathered bedrock
Weatheredbedrock
Angularclasts
Clay andscattered clasts
Organic soilhorizon
WSW ENEVALLIBIERNA
c2
b2c1
b1
a2
a15850 (5890) 5900cal yr BP
5580 (5640) 5750cal yr BP
2700 (2750) 2850cal yr BP
m 6 5 4 3 2 1 0
0 m
1
6400 (6650) 6790cal yr BP
5860 (5890) 5910cal yr BP
3460 (3480) 3570cal yr BP
Weathered bedrock
ESTÓSSW NE
Trench limit
m 6 5 4 2 1
0 m
1
2
z
y
x
03
ig. 6. Diagrams of the trenches dug in the sackung trough fills of Vallibierna and Estos and longitudinal profiles of the slopes showing the
cation of the trenches. The datings cited are calendar-corrected ages (see Table 1 for conventional radiocarbon ages).
F
lo
waters that give place to an actively forming
accumulation ferruginous sinter (Fig. 3). Probably,
the Fe content of the water is derived from the
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314 307
alteration of the pyrite of the Silurian slates under
oxidizing conditions. This spring reveals that a
relatively large quantity of water flows through the
rock mass forming the slope.
3.2. Trench stratigraphy and geochronology
The trench of Vallibierna was excavated in a
sackung trough at about 2260 m situated next to the
tree line (Figs. 3, 4, and 5). The reasons for selecting
this location include the existence of an area affected
by frequent ponding in the closed depressions and the
presence of trees in the vicinity. Both circumstances
suggested that datable material like charcoal and/or
organic horizons could be found in the trough fill. The
trench was oriented perpendicular to the sackung
ridge and trough, and its downslope end was located
at the slope break between the ridge and the
depression (Figs. 5D and 6). The excavation, 1.5 m
deep and 5.5 m long, was performed with pick, hoe,
and shovel. The longitudinal profile of the slope
across the sackung ridge and trough is shown in Fig.
6. It shows that the sackung ridge does not have an
obvious scarp next to the flat-bottomed depression,
which grades into a colluvium slope. In addition, the
bedrock hillslope below the sackung is ca. 98 steeperthan that above the sackung, suggesting that the ridge
has experienced an outward toppling (Jahn, 1964;
Bovis, 1982; McCalpin and Irvine, 1995).
The artificial trench exposes three fining-upward
sequences, each made of two units (a, b, and c) lying
on weathered bedrock (Fig. 6). The lower units (a1,
b1, and c1), dark brown-grey in colour, are composed
of pebble-sized angular clasts with a clay matrix. The
upper units (a2, b2, and c2) consist of black, dark grey
marly clay with scattered pebble-sized clasts. In the
Table 1
Summary of radiocarbon ages provided by Beta Analytic, from charcoal s
Laboratory number
(Beta Analytic)
Method Unit (location
h-171813 Radiometric b2 (Vallibierna
h-171812 Radiometric a2 (Vallibierna
h-171811 AMS a1 (Vallibierna
h-171810 AMS z (Estos)
h-171809 AMS y (Estos)
h-171808 Radiometric x (Estos)
Flanking calendar ages represent 1r limits.
upper sequence (c2), two stacked organic soil
horizons have been identified. The lower units of
each of the sequences, up to 20 cm thick and with no
distinctive bedding planes, are interpreted as sheet-
flow deposits derived from the upper slope and
accumulated in single rapid events. The upper units
represent low energy and slow sheetwash deposition
in a closed depression subjected to ponding. The
presence of only two soil horizons at the top of the
trough fill suggests a continuous aggradation with no
significant depositional hiatus. Radiocarbon ages have
been obtained from small pieces of charcoal found in
three stratigraphic units (a1, a2, and b2) of the
exposed trough fill. A summary of the conventional
radiocarbon ages and the calendar ages is shown in
Table 1. Units a1 and a2 yield 5890 and 5640 cal.
years BP, respectively, and unit b2 dates at 2750 cal.
years BP. The date of unit a1 (5890 cal. years BP),
lying directly on weathered bedrock, may be consid-
ered the age of the sackung trough and scarp since it
represents an instantaneous depositional event that
took place just after the uphill-facing scarp formed the
depression.
Regarding the structure, the lower sequence (a) is
affected by a normal fault that dips 768 into the slope
and has a N116E strike (Fig. 6). The fault shows 22
cm of dip slip displacement and about 23 cm of
vertical offset subsequent to deposition of the lower
sequence. The movement of the fault has produced the
bending (drag folds) of the lower unit on both sides of
the structure. The upper unit of this pre-fault sequence
is only preserved on the hanging wall. This sackung
fault is truncated by an erosional surface and
fossilized by the two upper sequences showing an
onlap contact with the upthrown block. This geo-
metrical configuration indicates that the fault was
amples
) Conventional
age (14C years BP)
Calendar age
(cal. years BP)
) 2620F150 2700 (2750) 2860
) 4920F130 5580 (5640) 5750
) 5080F40 5850 (5890) 5900
3290F40 3460 (3480) 3570
5090F40 5860 (5890) 5910
5830F160 6440 (6650) 6790
Fig. 7. Sketch showing the inferred evolution of the sackung scarp
and trough excavated in Vallibierna.
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314308
generated later than the deposition of unit a2 and just
before the accumulation of unit b1. With the available
datings, this deformation can be situated between
Fig. 8. Stereoscopic images of the
5640 and 2750 cal. years BP. The fact that the bedrock
in the footwall is overlain by deposits of the lower
sequence suggests that the depression was initially
formed by a master sackung scarp located beyond the
trench towards the downslope side (Fig. 7). This scarp
was formed soon before the deposition of unit a1
(5890 cal. years BP). Subsequent to deposition of unit
a2, a new sackung scarp formed within the depression
between 5640 and 2750 cal. years BP, causing the
erosion of unit a2 in the upthrown side. Once this
scarp was eliminated, the accumulation of the two
upper sequences took place fossilizing the fault and
onlapping the previously uplifted block.
4. Sackungen in Estos Valley
4.1. Geology and geomorphology
The sackung scarps of Estos are situated on a slope
on the northeastern flank of the valley with a NW–SE
orientation (Fig. 2). In this case, the topographic grain
is also controlled by the structure, a NW–SE trending
synclinorium composed of south verging folds
affected by north dipping thrusts. The axis of the
synclinorium runs roughly along the main ridge and
the strata show a general NE dip into the slope (Rıos
et al., 1997). The upper half of the slope is formed by
Devonian slates with intercalated arenaceous beds and
sackung area in Estos Valley.
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314 309
the lower half is underlain by Devonian slates and
limestone (Garcıa-Sansegundo, 1991, 1992; Rıos et
al., 1997). These formations are affected by a dense
network of discontinuity planes. Several geomorphic
features like the composite profile of the valley slopes
and truncated bedrock spurs indicate that the thickness
of the Estos glacier was more than 350 m during the
Last Glacial Maximum (50,000–40,000 years BP).
Thus deglaciation resulted in unloading in excess of
31.5 kg/cm2 (3.15 MPa) for the rocks located in the
valley bottom. Although there are no specific studies
about the glacial chronology in the Estos Valley, it can
be assumed that deglaciation took place approxi-
mately at the same time as in Vallibierna Valley
(16,000–13,000 years BP), given the marked topo-
graphic similarity and proximity between both valleys
(Martınez de Pison, 1989) (Fig. 2).
The slope affected by the uphill-facing scarps has
an average gradient of 228 and a local relief of about
900 m, between 2420 and 1500 m. The upper sector
of the slope is carved by a shallow glacial cirque
(Garcıa-Ruiz et al., 1992) that hosted an ice body once
linked to the main Estos glacier (Figs. 3 and 8). To the
west of Estos Peak there are two tongue-shaped talus
rock glaciers in direct connection with active talus
Fig. 9. Photographs of the sackung-type landforms in Estos Valley. (A an
Excavated sackung trough. (D) Close-up view of trench.
slopes (Barsch, 1996). Several characteristics, such as
the subdued and subsided topography of the inner part
of the rock glaciers and the dense turf vegetation of
the front and outer sides, suggest that these are relict
accumulations free of internal ice (Barsch, 1996).
Only one antislope scarp has been identified in the
crestal area. As in Vallibierna, most of the uphill-
facing scarps are located in the middle sector of the
slope, between 2070 and 1800 m (Figs. 3 and 8). The
upper ones clearly disrupt the glacial erosional
topography in the cirque bottom. These morphostruc-
tures and the accompanying troughs show a NW–SE
direction parallel to the topographic contours and the
structural grain. This is also the strike of a penetrative
set of cleavage planes dipping into the slope measured
in some outcrops located close to the front of the
largest rock glacier. Although probable, it is not
possible to ascertain whether the sackung features
correspond to pre-existing bedding or cleavage planes
since the dip of the failure surfaces is unknown. The
uphill-facing scarps range in length from 80 to 500 m
and the height varies from 0.6 to 2.75 m (Fig. 9).
Although the scarps do not show a fresh appearance,
they are more conspicuous than the scarps of
Vallibierna. This is probably due to the higher
d B) General view of a sackung antislope scarps and troughs. (C)
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314310
resistance to erosion of the bedrock in Estos.
Concerning the hydrology, the upper half of the slope
underlain by slates is devoid of gullies, suggesting
that most of the surficial water infiltrates in the slope.
The lower and steeper half of the slope is dissected by
two gully systems, which show some stretches
controlled by the sackung troughs.
4.2. Trench stratigraphy and geochronology
The sackung trough excavated in Estos Valley is
located at about 2000 m in elevation, close to the tree
line, although in this sector the upper limit of the
forest has been lowered for grazing. The flat bottom of
this closed depression is covered by grass and shows
evidence of frequent ponding. As in Vallibierna, the
trench was aligned perpendicular to the antislope
scarp with the downslope end located at the foot of the
scarp face (Fig. 9C and D). The excavation, dug with
manual tools, reached weathered bedrock at a depth of
2.1 m (Fig. 6). Similarly to the studied sackung in
Vallibierna, the longitudinal profile of the slope shows
that the bedrock hillslope located below the trough is
steeper than the slope segment above the trough,
probably due to outward toppling of the sackung ridge
(Jahn, 1964; Bovis, 1982; McCalpin and Irvine, 1995)
(Fig. 6). In this case, the sackung shows a well-
defined 1.1 m high uphill-facing scarp (Fig. 9C).
The 2.1 m thick trough fill exposed in the trench
shows a horizontal structure with no evidence of
synsedimentary deformation. It is dominantly com-
posed of massive beds of dark brown-grey marls with
Fig. 10. Plot representing the calendar age and the depth of the dated beds
fast and slow rates of deposition to the bottom of the trough provides an
scattered granule and pebble-sized subangular clasts
(Fig. 6). Some organic soil horizons and two
oxidation horizons have been identified within the
marl beds. Two layers of angular clasts are interca-
lated in the marl sequence. The lower bed, between
1.53 and 1.75 m below the ground surface, is made up
of angular cobbles with a clast-supported texture and a
marly matrix. The upper coarse-grained bed, between
0.85 and 1.01 m, consists of clast-supported fine
angular boulders with a marly matrix, following Blair
and McPherson’s (1999) grain size terminology. The
lack of well-developed paleosoils suggests a contin-
uous deposition with no significant hiatus. The
massive dark marl beds represent deposition in a
ponded area under reducing conditions. The angular
clasts embedded with the marls were probably trans-
ported by sheet flow or dry ravel from the adjacent
slope. The thin organic soil horizons record short
sedimentary interruptions and the oxidation horizons
periods of desiccation. The angular gravel beds are
interpreted as the product of rapid high-energy sheet
flow events probably caused by high intensity storm
events.
Small pieces of charcoal have been collected from
three layers of the trough fill (x, y, and z) located at
1.75–1.9, 1.53–1.75, and 0.85–1.01 m below the
ground surface. Layer x is composed of marls with
scattered clasts, and y and z layers correspond to the
two gravel beds. The calibrated ages provided for x, y,
and z units are 6650, 5890, and 3480 cal. years BP,
respectively. The conventional radiocarbon ages are
supplied in Table 1. The graph representing the
in the sackung trough fill of Estos. The downward projection of the
estimate of the age of the sackung.
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314 311
calendar age of the dated beds versus the depth below
the ground surface provides two rates of deposition:
0.29 mm/year between 3480 and 5890 cal. years BP
and 0.24 mm/year between 5890 and 6650 cal. years
BP (Fig. 10). The projection of the plot using both
rates yield an age range of 7795–7589 years BP for
the bottom of the sackung, situated 2.1 m below the
ground surface. Considering that the trough fill shows
a relatively continuous and homogenous deposition
and that the lowest dated bed is very close to the
bottom of the trough, this temporal range can be
considered as an acceptable estimate of the age of the
sackung trough and scarp.
5. Discussion and conclusions
The available data suggest that the studied uphill-
facing scarps and the adjacent depressions were
generated by the combination of at least two types
of movement. On the one hand, dip-slip displacement
occurred along failure planes steeply dipping into the
slope. The sackung fault identified in the trench of
Vallibierna proves the existence of this type of brittle
structure. On the other hand, the higher inclination of
the bedrock slopes located below the sackung troughs
compared to that of the hillslopes above the troughs
suggests that blocks defined by the sackung faults
have undergone a forward toppling with the conse-
quent horizontal extension. Although the obtained
dating provides the age of an individual antislope
scarp and depression for each location, it is likely that
the whole complex of sackung features in each slope
formed during a relatively short time span, given their
remarkably similar appearance.
Regarding the chronology, the Estos and Valli-
bierna uphill-facing scarps formed at about 7.6–7.8
and 5.9 cal. ka BP, respectively, at least 5000–6000
years after the deglaciation of the valleys that occurred
between 16 and 13 ka BP (1 ka=1000 years). A
considerable lag between deglaciation (14–15 ka BP)
and the onset of sackung generation (11–11.5 ka BP)
was also found by McCalpin and Irvine (1995) in the
Southern Rocky Mountains. The temporal distance
(N5 ka) between deglaciation and the age of the
studied sackungen indicates that the unloading of the
oversteepened slopes produced by withdrawal of the
valley glaciers was not the direct cause of the uphill-
facing scarps in Estos and Vallibierna, as has been
suggested for some sackung features studied else-
where in the Pyrenees (Soeters and Rengers, 1983;
Bordonau and Vilaplana, 1986; Corominas, 1990). In
our case, it seems that glacial erosion and the
subsequent debutressing of the valley sides generated
slopes predisposed to sackung development but did
not initiate the lateral spreading phenomena. There-
fore, a different triggering factor must be invoked to
explain the temporal occurrence of the antislope
scarps. Two possible options are climate change and
earthquake shaking.
Numerous investigations demonstrate a good corre-
lation between the concentration in time of landslides,
including large deep-seated slope movements, and
climatic changes. The available data from several
European regions concerning these temporal relation-
ships have been recently reviewed in several papers
(Borgatti et al., 2001; Borgatti and Soldati, 2002;
Soldati et al., 2004). Obviously, in our case, the
possible causal relationship between the lateral spread-
ing movements and climate is merely speculative since
we only have two dates. On the other hand, the
triggering effect of climatic factors on these type of
deep-seated large movements is much more limited
than on shallow and small mass wasting processes.
However, it may be interesting to find out if the
sackungen of Vallibierna and Estos were formed during
periods of high landslide frequency. The initial gravita-
tional spreading in both valleys took place during the
Atlantic period, characterized by a relatively warm and
dry climate (Lamb, 1977). In the Pyrenees, the scarce
number of datings does not permit identification of
clustering periods of landslides (Moya et al., 1992). In
the Cantabrian Mountains, the prolongation of the
Pyrenees to the west, a period with a relatively high
frequency of landslides, has been identified from the
Preboreal to the beginning of the Atlantic periods
(10.2–7 ka BP) (Gonzalez Dıez et al., 1996, 1999),
which partially overlaps the time interval when our
sackung were generated (7.8–7.6 and 5.9 ka).
Concerning seismic activity, Beck (1968) proposed
that the sackung scarps he studied in New Zealand were
formed by discrete episodes of rapid movement
triggered by earthquakes. Radbruch-Hall (1978)
reports that the generation of fresh antislope scarps in
Alaska and California has been attributed to historic
earthquakes. McCalpin (1999) quotes several studies
F. Gutierrez-Santolalla et al. / Geomorphology 69 (2005) 298–314312
that document the generation or rejuvenation of
sackung-like landforms during historic earthquakes.
Several arguments support the hypothesis that earth-
quake-induced dynamic loading was involved in the
development of the sackung scarps. The studied slopes
are located in a seismically and neotectonically active
area. In 1373, an earthquake with a MSK intensity of
VIII–IX destroyed buildings in this area of the
Pyrenees (Olivera et al., 1994). According to the IGN
data base, five earthquakes with maximum MSK
intensities higher than V have been felt during the last
century in areas located at a distance less than 25 km
from the study area. The 1923 earthquake of Viella, a
village located 14 km to the NE of the study area,
reached a maximum MSK intensity of VIII (IGN,
1982) and caused severe damage to structures includ-
ing Romanasque churches. Susagna et al. (1994), based
on the analysis of macroseismic and instrumental data,
attribute a magnitude ML=5.6 and MW=5.3 to this
event. Ortuno et al. (2004) propose the Northern
Maladeta Fault as the most likely source of this
earthquake. This ENE–WSW structure crosses the
headwaters of Benasque Valleys. Ortuno et al. (2004)
identify a planation surface offset up to 475 m on both
sides of this fault and estimate a slip rate between 0.04
and 0.07 mm/year for the last 10 Ma based on the
elevation of the base of lacustrine sediments of
probable Upper Miocene age deposited in a tectonic
basin generated in the downthrown block. According to
the INQUA intensity scale based on seismically
induced ground effects (Michetti et al., 2003), earth-
quakes with intensities of VII and IX may trigger large
mass movements (105–106 m3). The largest instrumen-
tally recorded earthquake in Benasque Valley reached
4.1 in Richter magnitude. The epicentre of this event,
which occurred in 1982, was located less than 10 km to
the south of the studied sackung features. Several
earthquakes with magnitudes higher than 5 have been
recorded in the eastern sector of the Pyrenees (IGN,
2004). Some other geomorphic evidences of neo-
tectonic activity have been documented in the area. In
Barrancs and Escaleta Valleys, at about 9 km from the
studied sackung features, Moya and Vilaplana (1992)
reported conspicuous normal faults offsetting glacial
erosional landforms. Evidence of post-glacial faulting
(b30 ka BP) has also been documented in the adjacent
Noguera Ribagorzana Valley (Bordonau andVilaplana,
1986). The hypothetical participation of the seismic
activity is also supported by the sudden deformation
event recorded by the fault identified in Vallibierna
trench. Once the sackungen are initiated by an earth-
quake, the subsequent development could result from
earthquake-triggered incremental movements, or from
a continuous slow creep interrupted by coseismic
episodes of rapid displacement. However, although
the seismic hypothesis seems to be more probable than
the climate change option, the lack of chronological
information about palaeoearthquakes precludes the
corroboration of this interpretation (Jibson, 1996;
McCalpin, 1999).
Acknowledgements
The authors would like to thank Drs. James
McCalpin (Crestone Science Center, Colorado) and
Jose Marıa Garcıa-Ruiz (Pyrenean Institute of Ecology,
Spain) for the their corrections and suggestions, which
have helped to substantially improve the original
manuscript. The paper has also been benefited from
the thorough review of Drs. Pablo Silva and Martin
Thorp. We are also grateful to Mr. Inocencio Altuna
(Head of Posest-Maladeta Natural Park) for providing
the permit to dig the trenches in the Posets-Maladeta
protected area. The investigation has been financially
supported by the DAMOCLES project (N.EVG1-CT-
1999-00007) founded by the European Union.
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