hrvatski

Pretjerana erozija fliša - Slani potok

Srednjoeocenski fliš značajno je zastupljen u geološkoj građi primorskog dijela Hrvatske. U svim područjima taj je litološki kompleks više ili manje izložen eroziji. Međutim, u slivu Slanog potoka u području Vinodola erozija fliša je prekomjerna. Zajedno s pratećim klizištima, razmatrana površina iznosi oko 3 km2 (sl. 1). Na njoj dolazi do potpune degradacije okoliša pri čemu nastaju tereni tipa &laquo ; badlands&raquo ; . Fliš na toj lokaciji, kao i na okolnom području koje nije izloženo pretjeranoj eroziji, zastupljen je uglavnom kalcitičnim glinovitim prahovnjakom i kalcitičnom prašastom glinom, manjim dijelom laporima i prašastim pješčenjacima (sl. 2). Mineralni i granulometrijski sastav glinovitih prahovnjaka i prašastih glina također je u cijelom području Vinodola sličan. Ove stijene sadrže muskovit i ilitični materijal (do 30 mas. %), kvarc (do 25 mas. %), kalcit (do 15 mas.%), feldspate (do 10 mas. %), klorit, kaolinit i smektit, a u nekoliko uzoraka određene su i male količine pirita (sl. 3). Udio sitnih čestica je velik ; frakcija čestica < od 2 mm iznosi do 45 mas. %, a onih promjera 2-4 mm do 15 mas. % (sl. 4). U inženjerskom smislu, litološke komponente fliša većim dijelom predstavljaju prijelazno područje: čvrsto tlo &#8211; mekana stijena (HS &#8211; SR) (sl. 5). Trošenjem toga kompleksa nastaje tlo izrazito sklono klizanju i plastičnom tečenju. Pojava eflorescencije u obliku bijeloga praha za vrijeme sušnog ljetnog perioda na nekim litološkim članovima je jedinstvena. Primjenom XRD određeno je da se bijeli prah sastoji od tenardita (natrijevog sulfata) i neznatnih količina gipsa (sl. 6). Najveći kristaliti tenardita opaženi pomoću SEM promjera su do 2 mm (sl.7). Pojave toga, u vodi topivog minerala, upućivale su da neke komponente fliša Slanog potoka sadrže u vodi topivi natrij, koji u kišnom periodu pospješuje dezagregaciju za kojom slijedi intenzivna erozija. Ove pretpostavke potvrđene su eksperimentalno nizom fizičkih i kemijskih testova. Uzorci fliša podvrgnuti su pokusima stabilnosti (pinhole test i postupak opetovanog vlaženja i sušenja ; IGH, Zagreb) koji su ukazali na visoku erodibilnost tla. Stupanj ekspanzivnosti inženjerskih tala procijenjen je na slici 8. Topive soli u pornoj vodi fliša određene su prema modificiranoj proceduri opisanoj u VAN REEUWIJK (2002). Na temelju kriterija određenih u SHERARD et al. (1976) analizirani fliš spada u grupu disperznih, a time i erodibilnih tala (sl. 9). Određene su i koncentracije iona u potočnoj vodi. Dobivene vrijednosti za Na+ i SO42- višestruko su veće od prosječnih vrijednosti za ove ione u slatkoj vodi. Opisana pojava tenardita prvi je nalaz toga minerala u Hrvatskoj. Pretjerana erozija flišnih naslaga Slanog potoka događa se zbog: litološkog sastava s pretežito sitnozrnatim članovima, pa čak i česticama nanometarskih dimenzija ; skupine materijala poznate kao tvrdo tlo &#8211; mekana stijena ; minerala sklonih bubrenju ; kemizma porne vode koja je obogaćena natrijevim ionom ; prisutnosti natrijeva minerala &#8211; tenardita, kao indikatora uzroka pretjerane erozije. 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 (&laquo ; Salt creek&raquo ; ) catchment (in the area of Vinodol) which, together with accompanying landslides, covers the area of approx. 3 km2 (Fig. 1). At this location, the total site degradation takes place forming terrains of &laquo ; badlands&raquo ; type. Slani potok flysch, as also the one from surrounded area without excessive erosion, is represented mainly with calcareous clayey siltstone and calcareous silty claystone and to lesser extent with marls and silty sandstones (Fig. 2). There are no significant differences in the mineral content and grain size distribution of clayey siltstone and silty 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 (Fig. 3). 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 (Fig. 4). In terms of ingeneering geology, lithological components of flysch are in transition between hard soil &#8211; soft rock (HS &#8211; SR) (Fig. 5). Soil produced by weathering of this complex tends to slide and flow. 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 (Fig. 6). The largest thenardite crystallites observed by means of a scanning electron microscope (SEM) have a diameter of 2 mm (Fig. 7). Occurrence of this water soluble mineral, indicates that some components of flysch encompass sodium, which accelerates disintegration 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 ; Civil Engineering Institute of Croatia, Zagreb) indicating high soil erodibility. Degree of expansiveness of engineering soils are described by Fig. 8. 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 (Fig. 9). The analysis of stream water for soluble salts was carried out as well. Obtained concentrations of Na+ and SO42-are manyfold than average in fresh water. Illustrated appearance of thenardite is the first finding of this mineral in Croatia. Excessive erosion of Slani potok flysch complex occurs due to: lithological composition dominantly presented with fines including nanometer particles ; group of materials known as HS &#8211; SR ; presence of swelling minerals ; pore water rich in sodium ; presence of sodium mineral &#8211; thenardite, indicating cause of the excessive flysch erosion. Literatura References SHERARD, J.L., DUNNINGAN, L.P. & DECKER, R.S. (1976): Identification and Nature of Dispersive Soils.- Journal of the Geotechnical Engineering Division. ASCE, 102, GT 4, 287-301. VAN REEUWIJK, L.P. (ed.) (2002): Procedures for soil analysis.- International Soil Reference and Information Centre, technical paper, sixth edition, Wageningen. WILLIAMS, A.A.B. & DONALDSON, G. (1980): Building on expansive soils in South Africa.- Proc. 4th Int. Conf. on Expansive Soils, Denver, 2, 834-838.

pretjerana erozija; fliš; eflorescencija; tenardit; Slani potok; Hrvatska

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