Origin of the Jezero v Ledvicah lake; a depression in a gutter-shaped karstic aquifer (Julian Alps, NW Slovenia)

Abstract

The Julian Alps are composed almost exclusively of Triassic to Lower Jurassic carbonates, which results in a karstified high-al­pine landscape. In such settings, large water accumulations are not expected and precipitated water drains vertically, gathers in deep, large-scale aquifers, and outflows in large karstic springs located in deeply incised valleys. Some small lakes, however, exist in high alpine areas. Most commonly, they formed above impermeable glacial sediments and are generally characterized by stagnant waters. Jezero v Ledvicah lake, which is one of the seven lakes in the Triglav Lakes Valley, is an exception, because it shows high subaqueous water inflow and outflow and occurs among highly karstified and permeable carbonates. Combining previous research with our new, detailed geological mapping of the lake surroundings and sedimentary research on the Lower Jurassic strata, we propose a hydrogeological model with the aim of explaining the extraordinary behaviour of the lake. We propose that Jezero v Ledvicah lake: A) is part of the “gutter-shaped” aquifer with perched groundwater that is situated below the floor of the Triglav Lakes Valley; B) barriers of the aquifer are structural (faults and thrust) and stratigraphic (clay interlayers in Lower Jurassic limestone); C) the lake formed in a structural, hydrogeological and morphological depression within this aquifer; D) the groundwater of the aquifer is re­charged not solely from the surface directly above the aquifer but additionally by subterraneous inflow from the overlying Slatna Nappe aquifer; and E) groundwater outflows from the aquifer at the southern end of the Triglav Lakes Valley, where the Lower Jurassic limestone pinches out.Key words: high-alpine lake, karstic aquifer, Julian Alps, Triglav Lakes Valley, Jurassic limestone, Southern Alps.Nastanek Jezera v Ledvicah – globel v žlebu podobnem kraskem vodonosniku (Julijske Alpe, SZ Slovenija)Julijske Alpe skoraj v celoti sestavljajo triasni in jurski karbonati, kar se odraža v morfologiji kraske visokogorske pokrajine. V tovrstnih razmerah ni pricakovati vecjih povrsinskih pojavov vode, saj padavinska voda pronica vertikalno in se akumulira v obsežnih globokih vodonosnikih, iz teh pa izteka v izdatnih kraskih izvirih, ki so v globoko vrezanih dolinah. Kljub temu v alpskem visokogorju obstaja nekaj manjsih jezer. Vecina jih je nastala nad neprepustnimi ledeniskimi sedimenti in jih lahko opredelimo kot stojece vode. Jezero v Ledvicah, ki je eno izmed sedmih jezer v dolini Triglavskih jezer, je izjema, saj ima mocno podzemno napajanje in iztok (je pretocno) ter se pojavlja med mocno zakraselimi in prepustnimi karbonati. Na podlagi predhodnih raziskav, izdelave nove geoloske karte okolice jezera in sedimentoloskih raziskav spodnjejurskih plasti predlagamo strukturno-geoloski in hidrogeoloski model, da bi razložili nastanek in lokacijo jezera. Nase ugotovitve kažejo, da je A) Jezero v Ledvicah del žlebu podobnega vodonosnika z viseco podzemno vodo, ki se pojavlja v jurskih plasteh pod dnom doline Triglavskih jezer, B) da so hidrogeoloske bariere vodonosnika strukturne (prelomi in nariv) in stratigrafske (plasti gline v spodnjejurskem apnencu), C) da je jezero nastalo v strukturni, hidrogeoloski in morfoloski depresiji znotraj vodonosnika, D) da se podzemna voda ne napaja izkljucno iz padavin, ki padejo neposredno na povrsino vodonosnika, ampak dodatno s podzemnim napajanjem iz visje ležecega vodonosnika Slatenske tektonske krpe in E) da podzemna voda izteka iz vodonosnika na južnem delu doline Triglavskih jezer, kjer se plastnati spodnjejurski apnenci (in s tem vodonosnik) izklinjajo.Kljucne besede: visokogorsko jezero, kraski vodonosnik, Julijske Alpe, dolina Triglavskih jezer, jurski apnenec, Južne Alpe.