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

Povzetek

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 kraškem vodonosniku (Julijske Alpe, SZ Slovenija)

Julijske Alpe skoraj v celoti sestavljajo triasni in jurski karbonati, kar se odraža v morfologiji kraške visokogorske pokrajine. V tovrstnih razmerah ni pričakovati večjih površinskih pojavov vode, saj padavinska voda pronica vertikalno in se akumulira v obsežnih globokih vodonosnikih, iz teh pa izteka v izdatnih kraških izvirih, ki so v globoko vrezanih dolinah. Kljub temu v alpskem visokogorju obstaja nekaj manjših jezer. Večina jih je nastala nad neprepustnimi ledeniškimi sedimenti in jih lahko opredelimo kot stoječe vode. Jezero v Ledvicah, ki je eno izmed sedmih jezer v dolini Triglavskih jezer, je izjema, saj ima močno podzemno napajanje in iztok (je pretočno) ter se pojavlja med močno zakraselimi in prepustnimi karbonati. Na podlagi predhodnih raziskav, izdelave nove geološke karte okolice jezera in sedimentoloških raziskav spodnjejurskih plasti predlagamo strukturno-geološki in hidrogeološki model, da bi razložili nastanek in lokacijo jezera. Naše ugotovitve kažejo, da je A) Jezero v Ledvicah del žlebu podobnega vodonosnika z visečo podzemno vodo, ki se pojavlja v jurskih plasteh pod dnom doline Triglavskih jezer, B) da so hidrogeološke bariere vodonosnika strukturne (prelomi in nariv) in stratigrafske (plasti gline v spodnjejurskem apnencu), C) da je jezero nastalo v strukturni, hidrogeološki in morfološki depresiji znotraj vodonosnika, D) da se podzemna voda ne napaja izključno iz padavin, ki padejo neposredno na površino vodonosnika, ampak dodatno s podzemnim napajanjem iz višje ležečega 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.

Ključne besede: visokogorsko jezero, kraški vodonosnik, Julijske Alpe, dolina Triglavskih jezer, jurski apnenec, Južne Alpe.