Sismovalp – WP04 – Generic Alpine Valley Types
Characterization of the geological history of the Upper Soca valley
QUESTIONNAIRE
Please fill the questionnaire inside this file. You can add lines wherever needed. The
questionnaire should be filled as precisely as possible, with all available data.
The questionnaire should then be sent to corinne.lacave@resonance.ch.
Introduction
Upper Soca valley is quite a complex Alpine landform incised in platform carbonate rocks
and clastic sediments of turbiditic origin. It’s course is governed mostly by tectonic zones but
its’ wideness is additionally controlled by the lithology of the bedrock. Upper Soca valley
(North of Most na Soci) is divided into three or four sections where Quaternary sediments
form more or less wide alluvial plains (see geological map). Each of the sections has
different tectonic background affecting Quaternary sedimentation history and dynamics.
Since each of the sections would require separate model and different approach, we decided
to describe in this questionnaire only the first section – i.e. the Bovec basin, but to stress the
necessity of the further analysis concerning the other sections of the valley as well.
The second section of the Soca valley is South of the Bovec basin from Zaga village to
Kobarid. Quaternary sedimentation dynamics is very likely governed by the Idrija fault system
activity in this section. The river is caught in the Idrija fault system beneath steep Polovnik
anticline in this section.
The third section of the Upper Soca valley is between Kobarid and Tolmin. Soca’s course
takes place along the lobes of Southalpine thrust front partly cut by Idrija fault.
Breginj area and a plane West of Kobarid belong only partly to the Soca valley and represent
a fourth section because of it's specific geological characteristics.
1)
Name of the valley : Bovec basin (Upper Soca valley)
Country : Slovenia
2)
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Geography / General information :
General orientation of the valley : ENE - WSW
General shape of the valley : A / B
Length and main width of the valley : L = 6000 m; W = 1500 m
Thickness of Quaternary deposits (min to max) : 0 - 260 m
Elevation (above sea level) of the valley surface (upstream to downstream):
h = 450 m - 350 m
Sismovalp – WP04 – Generic Alpine Valley Types
3)
History :
-
General geological evolution of the valley: repeating glacial, fluvial and lacustrine
environments in the Quaternary
-
Presence of a lake in the past : yes / no ; in the present: yes / no (give location)
-
Historical seismic activity: yes / no ; max Intensity: y.1511 (10 MSC), y.1998 (7- 8 EMS)
max. magnitude: y.1511(M=6.9), y.1998 (ML=5.6)
4)
Over-digging: yes / no ; approximate depth (m) :
Presence of glaciers in the past : yes / no ; number: multiple; surface: ? ; thickness: app.
100 m
Type of source data :
-
Boreholes ? Y…..Gravimetry ? N (only regional map)… Seismic ?..Y (shallow refraction)
-
References :
Ambient vibrations ? Y………
Others: Vertical electrical sounding
Bavec, M., 1999. Quaternary sediments of the Bovec basin. M.Sc. thesis. University of
Ljubljana, 100 pp. (in Slovenian, with English abstract).
Buser, S., 1987. Basic Geologic Map 1 : 100.000. Sheet Tolmin.
Gosar, A., Stopar R., Car, M., Mucciarelli, M., 2001. The earthquake on 12 April 1998 in the
Krn mountains (Slovenia): ground-motion amplification study using microtremors and
modelling based on geophysical data. Journal of Applied Geophysics 47, 153-167.
Jurkovsek, B., 1987. Basic Geologic Map 1 : 100.000. Sheet Beljak.
Kuscer, D., Grad, K., Nosan, A., Ogorelec, B., 1974. Geologic investigation in the Soca
Valley between Bovec and Kobarid. G. Geologija, 17, 425-476 (in Slovenian, with English
summary).
Poljak, M., Toman, M., Bavec, M., 1998. Geologic structure of the Bovec Basin and its’
surroundings. 20 pp. (unpublished report, in Slovenian).
Ribicic, M., 1998. Analysis of the 12. 4. 1998 earthquake effects in Posocje. 22 pp.
(unpublished report, in Slovenian).
Sumi, F., 1963. Report on geophysical investigation for the Trnovo hydro power plant –
accumulation Bovec. 25 pp. (unpublished report, in Slovenian).
5)
Human activity :
-
Urbanised surface : Yes, population: Municipality of Bovec: 3350, Upper Soca valley:
20.000, during the tourist season 2 or 3 times larger
-
Industrial activity : yes / no ; type: small: textile, wood, chemical, electrical
Sismovalp – WP04 – Generic Alpine Valley Types
6)
Geology :
6.1) Bedrock :
Bedrock type: Both sides of the Soca valley in the Bovec depression are built of platform
carbonates. These are covered by pelagic and turbiditic sediments that are preserved only at
the base of the slopes and in the valley itself. (see geologic cross sections)
Average slope: Left side: 45 - 50º considering tangent at the base of the slope, as it
decreases uphill. Right side: 30 - 45º (see geologic cross sections)
Outcrop of bedrock inside the valley : yes / no
6.2) Quaternary deposits (valley filling and lateral deposits) :
-
Description of the Quaternary deposits constitutive materials :
- alluvial : gravel, sandy gravel, sand
- lacustrine : chalk
- torrential : gravel
- morainic : till, tillite
- others : slope scree, diamiktit, diamikton (mass movement deposits)
-
General dip direction of the deposits : ESE
7)
Mud flow channel: yes / no
Fan delta: yes / no
Lacustrine delta: yes / no
Marsh / peat bog : yes / no
Collapse zone : yes / no, probable on the NE slope N of Bovec
Landslide, creeping : yes / no (not known but possible)
Rockfall activity : yes / no
Scree deposits on the sides : yes / no
Lateral water stream, torrent, secondary valley : yes / no ; type : fluvial and glacial
Artificial fills : yes / no
Others :………
Attached documents :
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Geological map of the Upper Soca valley (with structural information)
Geologic map of the extended area bears only the principal structural features. Structural
features concerning the Bovec basin, shown on an basic geologic map (in 1 : 100 000
scale) do not reflect the nature of the Bovec basin as a tight and quite deformed syncline.
-
Geological map of the Bovec basin
Cross-sections
Sismovalp – WP04 – Generic Alpine Valley Types
Brief description of the geological setting in the Bovec basin
Bovec basin and it’s surroundings is built of Upper Triassic and Liassic platform carbonates.
Upper Liassic to Lower Cretaceous pelagic and slope sediments of turbiditic origin
progressively cover platform carbonates in ENE direction. All these are uniformly covered by
Upper Cretaceous fine grained turbiditic sediments (Scaglia type) and subsequently by
flysch. As the pelagic and turbiditic sediments are less resistant to erosion, they are
preserved within the Bovec basin and eroded from surrounding mountains.
Bovec area, Bavscica and Soca area (ENE of Bovec) were folded (and partly thrusted) in
Palaeogene, forming NW striking synforms and antiforms. As a part of the Southern Alps,
Julian Alps were thrusted to the South in form of several nappes (Tolmin nappe, Julian
nappe). Western part of the Julian Alps was subsequently folded and thrusted along steep
reverse faults in the NNE direction determining deep ENE trending valleys including Bovec
basin, Upper Soca valley ENE of Bovec, Moznica valley, Vrata, Krma and some others.
Bovec Basin is therefore considered a remnant of two interfering synformes, the Palaeogene
and the Alpine one. Complex structural evolution of the area is additionally complicated by
Dinaric strike slip faulting e.g. Idrija and Ravne faults. Possible pull-apart and upthrust
structures of various sizes are expected along these faults as they are considered active in
Quaternary.
The southern part of the Soca valley (S of Zaga) appears to have different tectonic
background. Here the valley has a NW strike and roughly follows the Idrija fault. Most of the
valley between Zaga and Kobarid runs along the NE side of the fault, following the
lithological boundary between platform carbonates of Mt. Polovnik anticline and the flyschoid
sediments. The valley turns South and crosses the trace of the Idrija fault some 3 km North
of Kobarid. Alluvium has been eroded and the river bed is cut in the bedrock along its’ route
across the fault system. South of Kobarid and all the way down to Volarje village, Soca’s
alluvial plane is following the structures that appear to be roughly parallel to the Southalpine
Front lobe. Between Volarje village and Tolmin Soca follows the Idrija fault trace again.
Lacustrine sediments under Soca’s recent alluvium found North of Kobarid indicate that the
Idrija fault system might have blocked the river at Kobarid forming a lake that extended up to
Bovec. The highest level of the lacustrine sediments is approximately 360 m. This must also
have been the level up to which the river has been blocked at the time.
The same mechanism and distribution of the sediments is expected between Tolmin and
Kobarid too, as lacustrine sediments are reported from wells at Kamno village SE of Kobarid.