5
Interreg III B: SISMOVALP
WP 4: Generic Alpine Valley Types
Geologic cross sections across the Bovec basin – Upper Soca valley (Slovenia)
Prepared by:
Igor Riznar
Andrej Gosar
Environmental Agency of the Republic of Slovenia
Seismology Office 21 May, 2004
Geologic cross sections across the Bovec basin – Upper Soca valley
The presented model is based on three geological cross sections across the Soca valley at its' broadest part in the Bovec basin. The two roughly N – S oriented cross sections (A-A' and B–B') were chosen according to the available data distribution. Central part of the eastern (A–A') cross section is actually identical with the geoelectrical profile (Gosar et al., 2001), whereas the southern and the northern parts of the cross section are constructed on the basis of other available data. The most recent geological data of the Bovec area (Bavec, 1999; Poljak et al., 1998; Ribicic, 1998) were combined with results of other geophysical and geological investigations (Kuscer et al., 1974; Sumi, 1963) and the basic geological map in 1:100.000 scale (Jurkovsek, 1987; Buser, 1987). The E-W oriented cross section (C-C' ) was chosen to run in part roughly parallel with the geoelectrical profile Mala vas-Cezsoca (Stopar & Car, 1998) and the Bovec-Trnovo geologic cross section (Kuscer et al., 1974). In its' eastern part the C-C' section is crossing the Kal-Koritnica village where the microtremor analysis were performed (Gosar et al., 2001).
The geology of the Bovec basin and its surroundings is very complex and still not resolved in details as far as the structural geology is concerned. There are CCW rotated Palaeogene Dinaric structures (Mt. Polovnik anticline and related reverse faults) meeting steep Alpine reverse faults in the area.
The lithology of the area is well known. For the purpose the lithologic column can be divided into three sections according to their physical properties. The basement consists of the platform carbonates. The Upper Triassic Main dolomite and Dachstein limestone are overlain with micritic and oölitic limestone of Liassic age. Atectonic (normal erosional) contacts with the succession of deep water clastites (commonly referred to flysch) of Jurassic and Cretaceous age are found all around the steep flanks of the Bovec basin. In the eastern part the succession of deep water sediments starts with the carbonate breccia (wild flysch) whereas on other locations platform carbonates are overlain with micritic and marly limestones (type scaglia) and interchanging calcarenites, shales, marls and conglomerates.
Quaternary sediments represent the third section. Thick sequence of lacustrine chalk is covering more and less lithified glacial and fluvial sediments in the central part of the basin (Bavec, 1999; Kuscer et al., 1974). The dating of the chalk with 14C method has shown the age of 12490 ± 70 years. In Holocene the chalk was eroded and in places covered with gravel.
Glaciofluvial sediments below the lacustrine chalk are up to 80 m thick. Up to 65 m of this sequence is lithified. The lacustrine sediments are up to 200 m thick and are covered with unconsolidated fluvial deposits (Bavec, 1999).
The aim of the model (cross sections) is to asses the approximate form of the valley in terms of lithologic boundaries of significant difference in the shear waves velocity. In the case of the Bovec basin there are two such discontinuities. The one mentioned above is the boundary between the deep sea clastites and platform carbonates. The two alternatives (fig. 2 and 3) are presented due to the lack of deeper geophysical information or drilling and ambiguity of the geophysical data in some parts. The northern section of the geoelectrical profile Cezsoca - Mala vas (Gosar et al., 2001) is ambiguous as the two completely different electrical soundings are located very close to each other. Two alternative models are given for this reason in order not to loose the valuable geophysical data as there is no physical proof of its' reliability yet.
Due to lack of the deep boreholes it was purely conceptual issue to define the boundary between the deep sea clastites and platform carbonates and this is the other reason for two alternative solutions.
The enlarged part of the cross sections featuring the distribution of the Quaternary deposits and their relation to the bedrock (Mesozoic basement) depict two types of the valley profile. One of typical glacial origin (A-A') and the other much more narrow. The V shaped valley in the fig. 5 appears as such because of the triple vertical exaggeration, but the ratio in breadth of the valley in cross section A-A' (fig. 4) and B-B' (fig. 5) suggests that the confluence of the two rivers (Soca and Koritnica) was right between booth cross sections.
Physical parameters of difeferent lithological units
To investigate the geological structure and to ascertain the physical properties of nearsurface rocks shallow seismic refraction and geoelectrical investigations were performed in the Bovec basin. Additionally, seismic velocity measurements in 6 selected geomechanical boreholes were performed, the depths of which were up to 20 metres (Gosar et al., 2001).
By using two complementary methods, we were able to identify a geophysically heterogeneous structure within the same geological unit (inside glacigenic and glaciofluvial deposits) through the determination of different physical parameters of the sediments, i.e. their elastic properties and resistivities. The observed and calculated physical parameters of the different lithological units are compiled in Table 1. The only possible means of distinguishing between some of these units (for example, between till and tillite or between flysch and "wild" flysch) was by crossexamining their measured parameters. Namely, different characteristics of material often do not appear as a contrast in all physical properties. While resistivity is generally more sensitive to mineral changes of the material, seismic velocities better reflect the compactness of the material. As evident from Table 1, a single lithological unit shows much more diversity in resistivity than in elastic properties.
Table 1. Overview of the physical parameters of different lithological units observed in the Upper Soca valley (after Gosar et al., 2001).
Physical parameter / Velocity ofP-waves
(m/s) / Velocity of
S-waves
(m/s) / Poisson's ratio / Density
(g/cm3) / Electrical resistivity
(different layers inside the same litholog. unit)
(ohmm)
Lithological unit
Glaciofluvial deposits
(sand, gravel, conglomerate) / 600 - 1200 / 250 - 550 / 0.39 / 2.0 - 2.1 / 23 - 50
110 - 180
320 - 800
1080 - 1600
Till-
unconsolidatedglacial moraine
(clay, debris) / 600 - 1300 / 300 - 600 / 0.33 - 0.37 / 2.0 - 2.2 / 45 - 75
100 - 300
400 - 800
>1000
Tillite -
consolidatedglacial moraine / 1000 - 1500 / 600 - 800 / 0.33 / 2.2 - 2.4 / 20
45 - 90
250 - 850
3300
"Wild" flysch
(conglomerate, sandstone, lower clay component) / 2500 - 3000 / 1000 - 1400 / 0.35 - 0.40 / 2.1 - 2.3 / 100 - 280
Flysch
(sandstone, marl, calcareous breccia,
higher clay component) / 2700 - 3700 / 1000 - 1600 / 0.35 - 0.39 / 2.5 - 2.6 / 12 - 17
30 - 85
105 - 130
5
References
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-sheet Tolmin and Videm 1:100.000. Geological Survey, Beograd.
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-sheet Beljak and Ponteba1:100.000. Geological Survey, Beograd.
Kuscer, D., Grad, K., Nosan, A., Ogorelec, B., 1974. Geology of the Soca valley between Bovec and Kobarid. Geologija, 17, 425-476 (in Slovenian, with English summary).
Poljak, M., Toman, M., Bavec, M., 1998. Geological structure of the Bovec basin. 20 pp., IGGG (unpublished report, in Slovenian).
Ribicic, M., 1998. Analysis of the effects of the April 12,1998 earthquake in the Upper Soca valley. 22 pp., GI-ZRMK (unpublished report, in Slovenian).
Stopar, R., Car, M. 1998. Geophysical investigations in the Soca valley. 16 pp., IGGG (unpublished report, in Slovenian).
Sumi, F., 1963. Report on geophysical investigations for HE Trnovo- Bovec reservoir in years 1961-1962. 25 pp., Geological Survey of Slovenia (unpublished report, in Slovenian).