aljinovic.indd 313 � AB STRA CT Flysch deposits that crop out in the elongated coastal area of the Vinodol Valley (External Dinarides), and particu- larly in the Slani Potok (‘Salty Creek’) catchment, are characterized by excessive erosion. Formation of badlands, creeping and sliding processes have been related to crystallization of thenardite. Unusually intense erosion is caused by the dispersive effect of sodium on clay particles (derived from pelitic fl ysch intervals) in solution, as well as by expansion of thenardite during transformation into a deckahydrate (with a four fold increase in volume). The search for a sodium source required for thenardite crystallization has focused on a particular composition of Slani Potok Eocene fl ysch rock types. Slani Potok fl ysch is predominantly composed of pelitic intervals, with sub- ordinate sandstones and biocalcirudites. Sandstones and biocalcirudites contain appreciable amounts of siliciclastic detritus. Lithic fragments are represented by volcanic rocks revealing porphyry structures - possibly andesite or al- tered volcanic glass fragments, quartzite, schists, shales, and carbonate grains. High amounts of sodium and barium (0.93–1.09 % and 267–276 mg/kg respectively), in marls and sandstones, is essentially ascribed to a particular com- position of fl ysch, refl ecting its area of provenance. A Palaeozoic clastic complex with barite mineralization, (black shales, sandstones and conglomerates), as well as a Mesozoic hornblende andesite located near Fužinski Benkovac in the Gorski Kotar region, are interpreted as fl ysch source rocks. Sodium at least could be partly leached from the lithic and feldspar grains in clastic rocks, due to circulation of pore fl uids. An increased amount of sulphur, barium and strontium in fl ysch can be related to that of a barite ore complex in the source area of the Gorski Kotar region. Transportation and deposition of material eroded from the hinterland (North of Slani Potok), suggests turbidity cur- rent fl ow was transverse to the main axis of the Eocene foreland basin striking NW-SE. Keywords: composition of fl ysch deposits, origin of fl ysch deposits, excessive erosion, Slani Potok, Eocene, External Dinarides, Croatia The origin and composition of fl ysch deposits as an attribute to the excessive erosion of the Slani Potok Valley („Salty Creek“), Croatia � Dunja Aljinović, †Vladimir Jurak, Marta Mileusnić, Dragutin Slovenec and Filip Presečki Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, 10 000 Zagreb, Croatia; (dunja.aljinovic@rgn.hr) doi: 104154/gc.2010.25 Geologia Croatica 63/3 313–322 11 Figs. 1 Tab. Zagreb 2010 Geologia CroaticaGeologia Croatica 1. INTRODUCTION Flysch domains in Croatia occur along the elongated area of the External Dinarides (see overview in MARINČIĆ, (1981)). Although all Eocene fl ysch domains in the External Dinarides are exposed to a high degree of erosion (JURAK et al., 2002, 2003, 2005), erosion is unusually intense in the Slani Potok catchment. This catchment (with an area of 3 km2) is part of the Vinodol Valley (Fig. 1) where erosion is associated with intensive creeping and sliding processes. Ac- cumulation of a colluvial soil-like mass, disposed to fl owing, Geologia Croatica 63/3Geologia Croatica 314 forms badlands type terrain (Figs. 2 & 3). Such erosion rep- resents a unique phenomenon within the Dinaridic fl ysch domain and has been investigated by JURAK et al. (1996, 2005, 2006), MILEUSNIĆ et al. (2004), BENAC et al. (2005). Previous investigations determined the occurrence of the mineral thenardite, (sodium sulfate – Na2SO4) in the Slani Potok catchment. This mineral crystallizes by effl ores- cence at the surface of sedimentary rocks during the dry sea- son (summer – July, August) – Fig. 4 and dissolves during wet seasons. Dissolved thenardite results in the salty taste of the Slani Potok Creek and is the origin of its name. Accord- ing to MILEUSNIĆ et al. (2004) and JURAK et al. (2005, 2006) excessive erosion of the Slani Potok fl ysch complex occurs due to a lithological composition characterized by the prevalence of clay particles in pelitic fl ysch intervals, the presence of swelling minerals, sodium rich pore water, and occurrence of the sodium mineral thenardite. The aim of this study is to determine the source of sodium and sulphur needed for thenardite crystallization. 2. GEOLOGICAL SETTING The Slani Potok catchment is part of the larger Mala Dubračina catchment in the elongated and irregularly shaped Vinodol Valley which has a NW-SE axis. In the Vinodol Valley, the oldest rocks are Cretaceous platform carbonates. Tectonic movements at the end of Cre- taceous formed the thrust fold belt of the External Dinarides (MARINČIĆ, 1981; BERGANT, 2003). In front of the up- lifted thrusts, in a deep foreland basin, Early-Middle Eocene foraminiferal limestone and Middle Eocene fl ysch were de- posited (ŠUŠNJAR et al., 1970; GRIMANI et al., 1973). Late Eocene limestone, fossiliferous conglomerates and sandstones, (equivalents of Late Eocene/Oligocene Promina Figure 1: Location of investigated area with air photograph of Vinodol Valley and Slani Potok catchment area (light gray). Figure 3: Creeping and sliding processes in Slani Potok Valley. Arrows are pointing margins of slided rock mass. Figure 2: Coluvial mass forming badland. Aljinović et al.: The origin and composition of fl ysch deposits as an attribute to the excessive erosion of the Slani Potok Valley ... Geologia Croatica 315 formation), breccias (equivalent to the Late Eocene/Oli- gocene Jelar formation), coal-bearing late Pliocene brackish clastics, and Quaternary talus breccias have also been deter- mined in the Vinodol Valley (ŠUŠNJAR et al. 1970; GRIM- ANI et al., 1973 and BLAŠKOVIĆ & TIŠLJAR, 1983). BLAŠKOVIĆ (1999), describes the Vinodol Valley as a syncline in which the axis is parallel to the valley strike. The syncline is mostly composed of Middle Eocene fl ysch deposits. Marginal rocks of the Vinodol Valley are repre- sented by Upper Cretaceous limestones and Early-Middle Eocene foraminiferal limestone (ŠUŠNJAR et al., 1970) (Fig. 5). The Late Eocene limestone breccia and Quaternary deposits have a patchy occurrence in the Valley. Quaternary talus breccias occurred as a consequence of tectonic activity and were deposited on a foothill of steep cliffs (faults). The fault systems, that controlled the evolution of the Vinodol Valley morphology, have variable kinematic features that change along their strike and dip, merge into reverse or even over-thrust structures (BLAŠKOVIĆ, 1997). The Middle Eocene fl ysch deposits constitute the central area of the Slani Potok catchment. Flysch is mostly repre- sented by thick pelitic intervals interlayered with minor sand- stones and biocalcirudites (‘nummulite breccias’). As a result of intensive processes of sliding, creep and formation of a colluvial soil mass (badlands), the fl ysch deposits are not currently in their original position, but instead exhibit a rather chaotic appearance with overturned, fragmented and deformed beds (Fig. 6). The mineral thenardite occurs on the surface of all rock types in the Slani Potok area (MILEUSNIĆ et al., 2004; JURAK et al., 2005) – (Fig. 4). It has been documented as the fi rst occurrence of this mineral in Croatia (MILEUSNIĆ et al., 2004). As there is no occurrence of salts in any of the fl ysch terrains, the appearance of thenardite seems to be a unique phenomenon of the Slani Potok fl ysch in the whole Figure 4: Thenardite crystals (white) precipitated at the surfaces of fl ysch sedimentary rocks. Fi gu re 5: Schematized geo- logic map of the Vinodol val- ley with Slani Potok catch- ment area. of the External Dinarides. Therefore we investigated a pos- sible source of sodium and sulphur required for the crystal- Geologia Croatica 63/3Geologia Croatica 316 lization of thenardite. The petrographic and geochemical composition of all fl ysch rock types in Slani Potok has been studied in order to defi ne the provenance of the detrital ma- terial. Therefore the geological framework of the hinterland has been taken into consideration as a potential source area. The hinterland, located north of the Slani Potok area, consists of a Palaeozoic clastic complex (black shales, sand- stones and quartz rich conglomerates), with barite and pyrite ore mineralization (JURKOVIĆ, 1959; ŠIFTAR, 1981; ALJI NOVIĆ et al., 2000), and a Mesozoic carbonate com- plex, with hornblende andesite (in the vicinity of Fužinski Benkovac (VRAGOVIĆ & GOLUB, 1969)). The hornblende andesite was attributed to intense low temperature (hydrother- mal) alteration manifested as propylitized and albitized rock minerals with andesine, containing 33 vol. % of anorthite as well as pure albite as a common constituent (VRAGOVIĆ & GOLUB, 1969). 3. METHODS The lithological, mineralogical and geochemical character- istics of the Flysch sedimentary rocks have been analyzed in order to determine the origin of sodium and sulphur ions necessary for the crystallization of thenardite. Several sam- ples of sandstones and biocalcirudite have been studied mi- cropetrographically and chemically. Some samples were taken randomly (due to the restricted occurrence of fl ysch in the original position) while some are from the 8 m thick sec- tion that has been measured and sampled (Fig. 7). Samples of pelitic fl ysch intervals from the section were analyzed for carbonate content, mineral and chemical composition. In or- der to compare the surface and internal chemical composi- tion, two marl samples were taken from the same bed, one from the surface (sample number SPP 5a), and another from 0.5 m depth (SPP 5b). The mineral and chemical composi- tion of calcitic platelets, accumulated at the surface of fl ysch rocks, has also been analyzed. Petrographic investigation suggested further micropetro- graphic analyses of the samples and determination of the main constituents in sandstones using semi quantitative vi- sual estimation (TERRY & CHILINGAR, 1955). The chemi- cal composition (main macro- and microelements) has been determined in four rock samples (from 2 sandstones and 2 marls) as well as one sample of a calcitic platelet. Prior to analysis, samples were ground in an agate mortar followed by fusion with LiBO4/Li2B4O7. Their chemical composition was determined by inductively coupled plasma - emission spectroscopy (ICP-ES) in the ACME Analytical Laboratory, Canada. The carbonate content was determined according to Scheibler’s method (ISO 10693). 4. RESULTS 4.1. Petrological and mineralogical characteristics of fl ysch deposits Flysch outcrops in the Slani Potok catchment area are rep- resented by the prevalence of pelitic fl ysch intervals deter- mined mainly as marls and silty marls, calcareous clayey siltstones, siltstones, and clayey siltstones (MILEUSNIĆ et al., 2004; JURAK et al., 2005). Subordinate fl ysch sequences include sandstones. Very restricted occurrences of biocal- cirudite have also been observed within fl ysch deposits. A graphic log of a short fl ysch succession in its primary posi- tion is shown in Figure 7. Sandstones and marls of this sec- tion are arranged in fi ning upward sequences containing fi - ne-grained sandstones at the base and marls at the top. Fi ne -grained basal sandstones exhibit small scale cross-lam- ination which can be attributed to the Tc intervals of turbidite sequences. Sandstones of Tc intervals are overlain by paral- lel laminated very fi ne-grained sandstones (Td interval), that gradually pass upwards into silty marl or marl of Te inter- vals. Six Tc-Te and two Td–Te sequences have been logged and are shown in Figure 7. Their recurrence in the vertical succession can be interpreted as deposition from several dis- tal turbidity currents. Sandstones have been determined as fi ne-grained lithic grey wackes (Tc and Td intervals) with dominantly silici- clastic detritus (Fig. 8). Quartz grains are present with ca. 50 vol. %, followed by lithic fragments (ca. 30 vol. %), and feldspar grains (plagioclase, orthoclase and microcline) for ca. 20 vol. %. Quartz grains are mainly represented by mo- no crystalline undulose grains. Polycrystalline quartz grains are the dominant component among the lithic fragments. Subordinate grains with faintly preserved porphyry textures with plagioclase phenocrysts surrounded by microcrystalline Figure 6: Chaotic appearance of fl ysch strata. Aljinović et al.: The origin and composition of fl ysch deposits as an attribute to the excessive erosion of the Slani Potok Valley ... Geologia Croatica 317 Figure 7: Investigated section. Figure 8: Composition of sandstone. Note prevalence of siliciclastic mate- rial (lithic particles and feldspars are marked by arrow). Stained with Alizarin and K-ferricyanide. Microphotograph; crossed nicols. Figure 9: Devitrifi ed volcanic particles in the sandstone showing faint por- phyric structure (arows) and altered feldspars. Stained with Alizarin and K-ferricyanide. Microphotograph; plane polarised light. quartz or volcanic glass occur (Fig. 9). They are usually in- tensively altered and exhibit a brown clayey patina. Other lithic grain types correspond to quartzite, schists, shales, limestones and dolostones. Fossils are rarely present in all the examined samples. A clayey carbonate matrix fi lls the intergranular pore spaces. The primary matrix is partially recrystallized to chlorite. Sandstone grains with porphyry or faint altered porphyry textures suggest a volcanic origin for these grains. Further- more, grains showing a microcrystalline quartz matrix may Geologia Croatica 63/3Geologia Croatica 318 be interpreted as altered volcanic glass particles. The pres- ence of volcanic, metamorphic and sedimentary rocks sug- gests a complex provenance for the sediments. Acidic or in- termediate magmatic rocks are assumed as the main source of lithic particles. The main constituent of the rather restricted biocalciru- dite is fossil detritus, among which nummulites fragments are most common. In addition, fragments of echinoids and lithothamnium algae occur similarly as MAGDALENIĆ (1972) reported for similar rock types from the Istrian pe- ninsula. Nevertheless, with the exception of fossil detritus in the biocalcirudite from Slani Potok, they contain an ap- preciable amount of coarse grained siliciclastic detrital ma- terial. It is worth mentioning that within the siliciclastic de- tritus, lithic fragments are the most common constituent, followed by quartz and feldspar grains. Lithic fragments clearly show a porphyry texture, whereas plagioclase phen- ocrysts are surrounded by altered microcrystalline quartz and a chlorite matrix (Fig. 10). Volcanic particles have been al- tered in a similar manner to the lithic grains described in sandstones. Metamorphic grains have also been discerned. Pelitic fl ysch intervals in the Slani Potok area have been characterized mainly as marls, silty marls, calcareous clayey siltstones, siltstones, and clayey siltstones (MILEUSNIĆ et al., 2004; JURAK et al., 2005). The amount of clay in all the analyzed pelitic lithotypes varies from 2.5–36.9 wt. % where particles less than 2 μm prevail. Silty grains comprise 28.7–87.8 wt. %. The carbonate component varies from 3.8–60.0 wt. %. A small amount of sand grains (< 10 wt. %) is usually present in the pelitic rocks. Semiquantitative XRD analyses of the mineral composition showed that the main constituents of pelitic rocks are: muscovite and illitic mate- rial (<30 wt. %), quartz (<25 wt. %), calcite (<15 wt. %) and feldspar (<10 wt. %). Minor constituents include chlorite, kaolinite, smectite and pyrite (JURAK et al., 2005). Thenardite, documented earlier by MILEUSNIĆ et al. (2004) and JURAK et al. (2005), seems to be the most char- acteristic feature of the Slani Potok fl ysch deposits. The big- gest crystals observed by SEM have diameters of up to 2 μm (MILEUSNIĆ et al., 2004). Crystallization of thenardite (Fig. 4) has been interpreted as being due to effl orescence processes in the dry summer season. During the wet season, thenardite dissolves and therefore surface water is enriched in sodium and sulphate ions (MILEUSNIĆ et al., 2004; JURAK et al., 2005). The surface of pelitic fl ysch intervals in Slani Potok has often been covered by calcitic platelets as loose detritus (Fig. 11). It is assumed that calcitic platelets were formerly a thin crust at the surface of sediments or soils. The calcite crust breaks due to creeping or sliding processes and forms angular calcitic platelets. The occurrence of calcitic platelets is a diagenetic feature. XRD analyses indicate calcite as the only constituent of such platelets which are composed of elongated prismatic crystals. Calcite veins of similar appear- ance have not been found within the marls and sandstones. 4.2. Geochemical characteristics of fl ysch deposits Results of macro- and microelement analysis of sandstones, marls and calcitic platelet samples are listed in Table 1. All analyzed rock types have a high content of sodium and barium. Sandstones contain higher amounts of silicon and lower amounts of aluminium compared to marls. Amounts of magnesium and potassium are higher in marls, while the content of sodium is similar in both rock types. Calcium is higher in the sandstones, while trace elements are enriched in the marls. It can be clearly seen that the composition of macro- and microelements of the marl sampled at the surface (SPP 5a) Figure 10: Microphotograph of biocalcrudite (‘nummulite breccia’). Note abundance of siliciclastic particles and a large lithic fragment on the right, clearly showing prophyric structure, with plagioclase fenocrysts and al- tered microcrystalline quartz/chlorite matrix. Stained with Alizarin and K-ferricyanide. Plane polarized light. Figure 11: Calcite platelets covering a surface of pelitic fl ysch rocks. Aljinović et al.: The origin and composition of fl ysch deposits as an attribute to the excessive erosion of the Slani Potok Valley ... Geologia Croatica 319 and the marl sampled at a depth of 0.5 m (SPP 5b), is very similar (Table 1). The surface marl sample has slightly de- pleted Mn and Sr compared to the marl sample at depth. Chemical analysis of calcite platelets showed appreci- able contents of barium and strontium (Table 1). XRD anal- yses of calcitic platelets did not show any barium or stron- tium minerals, but high contents of Ba and Sr can be in ter preted as isomorphic substitution of calcium ions by Ba and Sr in calcite. 5. DISCUSSION Marls and pelitic rocks predominate in fl ysch deposits of the Slani Potok area. They represent the pelitic intervals of tur- bidite sequences. Sandstones and biocalcirudite (‘nummulite breccia’) occur as a much thinner Tc and Td turbiditic inter- vals. These rock units forming the Slani Potok catchment area have been subjected to specifi c types of erosion that are characteristic of excessive erosion. (JURAK et al., 2002, 2005; MILEUSNIĆ et al., 2004). According to JURAK et al. (2005), and MILEUSNIĆ et al. (2004), excessive erosion can be related to the occurrence of thenardite (sodium sul- phate). Thenardite crystallizes by effl orescence on the sur- face of sedimentary rocks and soils during the dry and warm summer period and dissolves during the wet seasons. The interrelationship of thenardite and the erosion of fl ysch rocks is two fold: 1) Thenardite transforms into mirabilite (sodium sulphate decahydrate) on contact with water, increasing its volume at least four-fold. This transformation can intensify erosion forming pressure in the host rock and 2) available sodium in solution acts as a dispersive agent to the clay par- ticles of the pelitic fl ysch intervals. Both effects may be driv- ers for the creeping and sliding processes as well as the for- mation of a colluvial clay rich substratum subjected to mass fl ow. The signifi cant excessive erosion that has been inter- preted as the infl uence of thenardite in the Slani Potok area resulted in a search for the source of sodium and sulphur needed for thenardite crystallization. The composition of fl y- sch rocks was examined in an attempt to defi ne the prove- nance of the fl ysch and a possible source of sodium and sul- phur. Flysch rocks were deposited by turbidity currents in the Middle Eocene in a deep water environment. Tectonic move- ments at the end of the Cretaceous form a complex foreland basin on the south-western side of the fault thrusts of the Ex- ternal Dinarides (MARINČIĆ, 1981; BERGANT, 2003). It is accepted that activation of the numerous Dinaridic (NW- SE) faults triggered immense gravitational fl ows and that fl ysch was deposited by turbidity currents. Models for palaeo-transport of the turbidity currents have been discussed by numerous authors including MAG- DALENIĆ (1972), MARINČIĆ (1981), TUNIS & VEN- TURINI (1992), and BABIĆ et al. (2007). As the fl ysch do- main represents an elongated trough with NW-SE stri ke, MAGDALENIĆ (1972), proposed a longitudinal tran sport direction toward the ESE, which derived material from the Alps, and a transverse component toward the SW where the source area was the rising Dinarides in Palaeogene times. A similar pattern was proposed by MARINČIĆ (1981), but as- sumed the main transport paths were transverse toward the SW (from the Bosnian ridge), and defl ection of the currents to the ESE in the deeper part of the basin. According to TU- NIS & VENTURINI (1992) the main transport direction was transverse to the NW-SE strike of the uplifted fault thrust belt, while BABIĆ et al. (2007) explicitly reported transverse transport but with the direction toward the NE. Considering the particular composition of the fl ysch de- posits in Slani Potok (prevalence of siliciclastic particles versus carbonate grains and presence of thenardite), it was necessary to determine a potential provenance area and pos- sible source of sodium and sulphur. The difference in fl ysch composition stems from the composition of adjacent terrains and distances from the source area. Flysch sequences derived dominantly from carbonate platforms contained mainly car- bonate detritus (MAGDALENIĆ, 1972; TUNIS & VEN- TURINI, 1992; BABIĆ et al., 2007) which is not the case for the Slani Potok fl ysch units. The hinterland (at the north of the Slani Potok area) con- sists of Palaeozoic shales, sandstones, and quartz rich con- glomerates, with barite and pyrite ore mineralization (JUR- Table 1: Chemical analyses of fl ysch samples. Sample SPP 3 SP 0 SPP 5a SPP 5b - Type of material sand- stone sand- stone marl marl calcitic plate Si (%) 32.58 31.87 26.12 24.43 0.67 Al (%) 4.10 3.39 8.61 8.59 0.21 Fe (%) 2.13 1.70 4.88 4.94 0.95 Mg (%) 0.68 0.80 1.60 1.60 0.37 Ca (%) 4.8 6.25 2.45 3.42 36.04 Na (%) 1.09 0.93 0.99 0.98 0.02 K (%) 1.07 0.93 2.37 2.33 0.06 Ti (%) 0.29 0.23 0.49 0.47 0.01 P (%) 0.038 0.031 0.057 0.053 0.002 Mn (%) 0.06 0.08 0.06 0.1 0.38 Cr (%) 0.021 0.023 0.018 0.016 <0.001 Ba (ppm) 181 169 267 273 7793 Ni (ppm) 62 36 120 104 <20 Sr (ppm) 123.6 119.4 149.3 170.5 590.9 Zr (ppm) 138 114 166 152 6 Y (ppm) 16 16 28 29 6 Nb (ppm) 14 16 22 18 10 Sc (ppm) 7 6 18 18 <1 LOI (ppm) 7.3 9.2 9.1 11.1 42.5 C/TOT (ppm) 1.58 2.18 1.17 1.42 11.99 S/TOT (ppm) 0.15 0.12 0.33 0.16 0.18 Geologia Croatica 63/3Geologia Croatica 320 KOVIĆ, 1959, ŠIFTAR, 1981; ALJINOVIĆ et al., 2000), and a Mesozoic carbonate complex, with hornblende andes- ite in the vicinity of Fužinski Benkovac. It was assumed that the Gorski Kotar might be the source area of the Slani Potok fl ysch rock units. Prior to deposition of the fl ysch by turbid- ity currents, Dinaridic land masses were subjected to inten- sive erosion. A high amount of pelitic detritus, forming thick Td–e fl ysch intervals, was possibly derived from the erosion of Palaeozoic shales. The detrital siliciclastic component of the fl ysch sandstones and biocalcirudites may also have orig- inated from Palaeozoic sandstones or conglomerates located in the Gorski Kotar area. Analysed sandstones and biocalcirudites from the Slani Potok catchment contain a signifi cant amount of lithic frag- ments of volcanic origin, probably andesite, as well as altered volcanic glass particles. The hornblende andesite located in the Gorski kotar (vicinity of Fužinski Benkovac) north of the studied area, is considered as a source rock. This volcanic rock was attributed to the intense low temperature (hydro- thermal) alteration manifested as propilitized and albitized rock minerals (VRAGOVIĆ & GOLUB, 1969). These au- thors also determined pure albite as a common constituent of andesite. Thus andesitic complexes from Fužinski Benk- ovac contain even more sodium content than usual. It is likely and possible that feldspar and volcanic particles de- rived from sodium rich andesite near Fužinski Benkovac could increase the sodium content in the sedimentary rocks of the Slani Potok area. Intermediate plagioclase as well as diagentic albite and propylite from andesite can produce ap- preciable amounts of sodium during weathering. Thus the increased sodium content in the analyzed samples can stem from the specifi c composition of the fl ysch deposits. The composition of macro- and microelements has been compared to the soil geochemical map data for the fl ysch area in Istria (PEH et al., 2003). Flysch samples from Slani Potok do not differ signifi cantly from the fl ysch deposits in Istria, except for the sodium and barium contents. Mean con- centrations of sodium in Istrian fl ysch are 0.55 wt. %, while in the Slani potok fl ysch it is double this value. The mean concentration of barium in Istrian fl ysch is 233 mg/kg, while in Slani potok pelitic fl ysch intervals the mean concentration is higher (267–276 mg/kg). It is worth pointing out that stream water has an appre- ciable amount of dissolved sodium while water at the spring of Slani potok creek has average sodium content, indicating a sodium source from fl ysch and not from the groundwater. Sulphur is present in the pelitic fl ysch deposits in the form of pyrite (MAGDALENIĆ, 1972). Oxidation of pyrite by weathering processes at and near the surface can lead to the production of sulphate minerals. The uplifted Palaeozoic ore complex from the Gorski Kotar region might have been infl uencing the composition of fl ysch deposits of the Slani Potok area. It is characterised by complex barite ore paragenesis where barite is the main ore mineral, accompanied by pyrite, quartz and celestine (JURKOVIĆ, 1959; ŠIFTAR, 1981; ALJINOVIĆ et al., 2000). ŠIFTAR (1981) indicated bacteriogenic dissolution of barite, and therefore also dissolution of barium, sulphur and strontium contained in barite. This evidence suggests that barium, sulphur and strontium could have been derived from the deteriorated ore complex in Gorski Kotar. Complex recent diagenetic processes also infl uenced deposition in Slani Potok. A vast quantity of calcite crystal- lizes from solution saturated with carbonates as a thin crust at the surface of sediments or soils. The calcite crust breaks and forms angular calcitic platelets. High values of barium and strontium in calcitic platelets, not typical for the primary sedimentary rocks, is possibly related to deterioration of the ore complex in the hinterland (Gorski Kotar region) and has a diagenetic origin. If the Gorski Kotar region, located north of the Slani Potok valley, is considered the source area, it means that tur- bidity currents transported material from the uplifted hinter- land (uplifted cordillera belt) and that palaeotransport was transverse to the strike of the foreland basin axis. Our obser- vations show that the most likely transport directions are from the N-NE to the S-SW. 6. CONCLUSION Flysch deposits that crop out in the Slani Potok catchment area, (as a part of Vinodol Valley), have been subjected to excessive erosion. A colluvial soil-like clayey mass attrib- uted to creeping and sliding processes forms terrains of the badlands type. According to JURAK et al. (2002, 2005) and MILEUSNIĆ et al. (2004), the phenomenon of excessive erosion can be related to the occurrence of the mineral the- nardite (Na2SO4). Increased concentrations of sodium and sulphate ions in surface water lead to the crystallization of thenardite in the dry and warm season. Water evaporating from the surface of sediment/soil in the dry season increases saturation of sodium and sulfate ions and thenardite occurs as effl orescence crusts on the surface of rocks/soils. When thenardite is later dissolved, sodium solutions act as a dis- persive agent on the clay particles from fl ysch pelitic rocks types. As thenardite transforms into mirabilite (sodium sul- phate decahydrate), there is a four fold increase in volume of the mineral. Frequent crystallization and dissolution of thenardite becomes the main driver for the erosion and this process results in mass wasting and disintegration of rock masses. Surface waters in Slani Potok are enriched in sodium and sulphate, while the ground water does not have any ap- preciable amount of these ions. Geochemical analysis of marls and sandstones indicate a high amount of sodium and barium (0.93–1.09 % and 267–276 mg/kg, respectively). To- gether this suggests that increased sodium and sulphate con- centrations can be ascribed to the particular composition of fl ysch rock units. The composition of Slani Potok fl ysch rocks differs from other fl ysch areas in the External Dinar- ides. The explanation for this was discovered in the complex composition of the provenance area – the Gorski Kotar re- gion located in the hinterland of the Slani Potok. In general, in the External Dinarides, fl ysch have been deposited in a deep foreland basin formed in front of the Di- Aljinović et al.: The origin and composition of fl ysch deposits as an attribute to the excessive erosion of the Slani Potok Valley ... Geologia Croatica 321 naridic thrust belt that has a longitudinal axis striking NW-SE. Turbidity currents transported flysch from the source area to the North of Slani Potok, in a direction trans- verse to the main axis of the foreland basin, (from the N-NE to the S-SW), depositing sediments in the study area. The particular composition of fl ysch deposits – prevalence of pe- litic rocks, appreciable amounts of siliciclastic material in sandstones and biocalcirudites, with signifi cant amounts of volcanic particles and feldspar grains, stems from the geo- logical framework of the source area. It was assumed that a Palaeozoic clastic rock complex (black shales, sandstones and quartz rich conglomerates with barite ore mineraliza- tion), and Mesozoic carbonates with hornblende andesite cropping out in the Gorski Kotar region, hinterland of Slani Potok, was the provenance from where the clastic detritus has been derived. Thick pelitic fl ysch intervals are related possibly to derivation of vast quantities of fi ne grained ma- terial from black shales as well as sandy particles from Gor- ski Kotars’ Palaeozoic and Mesozoic sandstones, conglome- rates, carbonates and andesite. Some sodium could be lea ched from the lithic and feldspar grains due to circula- tion of pore fl uids while the increased amount of sulphur, barium and strontium in the fl ysch can be related to the bar- ite ore complex in the source area of the Gorski Kotar re- gion. ACKNOWLEDGEMENT This paper is dedicated to our dear colleague and professor dr. sc. Vladi- mir JURAK who passed away in February 2010. He worked in Slani Potok for years and introduced all of us to the interesting erosion phe- nomenon. The authors kindly thanks dr. sc. Marta CRNJAKOVIĆ for her thor- ough review, comments and suggestions that have signifi cantly im- proved this manuscript. We would also like to express our gratitude to dr. sc. 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Manuscript received December 21, 2009 Revised manuscript accepted June 25, 2010 Available online November 11, 2010