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Thenardite-efflorescence indicating cause of the excessive flysch erosion, Slani potok, Croatia

THENARDITE-EFFLORESCENCE INDICATING CAUSE OF THE EXCESSIVE FLYSCH EROSION, SLANI POTOK, CROATIA MILEUSNIĆ, M., SLOVENEC, D. & JURAK, V. (Faculty of Mining, Geology and Petroleum Engineering, Pierottijeva 6, HR-10 000 Zagreb, Croatia) mcovic@rgn.hr Significant amount of Mid-Eocene flysch is present in the costal parts of Croatia. In the whole region flysch is subjected to the erosion to lesser or greater extent. However, there is an excessive flysch erosion in Slani potok ("Salt creek") catchment (in the area of Vinodol) which, together with accompanying landslides, covers the area of approx. 3 km2. At this location, the total site degradation takes place forming terrains of "badlands" type. Slani potok flysch, as also the one from surrounded area without excessive erosion, is represented mainly with calcareous clayey siltstone and calcareous silted claystone and to lesser extent with marls and silted sandstones. There are no significant differences in the mineral content and grain size distribution of clayey siltstone and silted claystone in the whole area of Vinodol. These rocks contain muscovite and illitic material (up to 30 wt. %), quartz (up to 25 wt. %), calcite (up to 15 wt.%), feldspars (up to 10 wt. %), chlorite, kaolinite and smectite, and in some samples small quantity of pyrite. There is a great amount of small particles. The particle size fraction smaller than 2 mm and fraction 2-4 mm sum up to 45 wt. % and up to 15 wt. %, respectively. However, phenomenon of efflorescent salt crust on flysch in Slani potok during dry period of the year is unique in the area. The mineral content of white salt crust determined by XRD is represented by thenardite (sodium sulphate) and negligible amount of gypsum. The largest thenardite crystallites observed by means of a scanning electron microscope (SEM) have a diameter of 2 mm. Occurrence of this water soluble mineral, indicates that some components of flysch encompass sodium, which accelerates desintegration followed by intensive erosion. A series of physical and chemical tests are carried out in an attempt to verify mentioned assumption. Flysch samples are submited to physical tests of stability (pinhole test and cyclic dry-wet repeat treatment) indicating high soil erodibility. Determination of soluble salts in flysch pore water was carried out following modified procedure used by the International Soil Reference and Information Centre (VAN REEUWIJK, 2002). Based on criteria established by SHERARD et al., 1976, analysed flysch fall in the group of dispersive, i.e. erodible soils. The analysis of stream water for soluble salts was carried out as well. Obtained concentrations of Na+ and SO42-are manyfold than average in running surficial waters. Illustrated appearance of thenardite is the first finding of this mineral in Croatia. References SHERARD, J.L., DUNNINGAN, L.P. & DECKER, R.S. (1976): Journal of Geotechnical Engineering Division, 102: 69-85. VAN REEUWIJK, L.P. (ed.) (2002): Procedures for soil analysis.- International Soil Reference and Information Centre, technical paper, sixth edition, Wageningen.

thenardite-efflorescence; flysch; erosion

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