Geology

Simplified geological map of Idrija UGG (after Buser and Draksler 1993)

The region of the Idrija hills (western Slovenia) has a complex geological composition, but from the geological aspect this is also one of the most investigated areas in Slovenia. The oldest rocks are more than 300 million year-old dark grey shales and sandstones of the Carboniferous Era. Slightly younger are the quartz sandstones, dolomites and fossil-rich limestones of the Permian Era. These are followed by variegated Scythian rocks of the Lower Triassic. Dolomites, marlstones and various limestones are the most commonly found rocks of this period, which cover extensive parts of the Idrija region. Poorly bedded Anisian dolomite and conglomerate are also very common. Steep slopes covered with a shallow layer of poorly fertile soil are typical for this type of rock. This was followed by a highly dramatic period in the geological history of the Idrija region. At the beginning of Upper Anisian, extensive movements began during Idrija's Middle Triassic normal tectonic phase. Several tectonic trenches (aulacogens) extending East-West were formed in the Idrija region (Čar – in print). In one of these, i.e. the so-called Ore deposit trench, the Idrija mercury ore deposit was formed. In this zone of strong normal faults with a vertical displacement of 600 to 900 m (Čar 1990), individual blocks were pushed up, while others were subsided. Around 750 m of strata were eroded of the upthrown blocks (Čar 1990). The Middle Traissic (Anisian), Lower Triassic (Scythian), Permian and partly also Carboniferous rocks were removed, and various Middle Triassic (Ladinian) rocks are lying on other blocks in the ore deposit, i.e. directly on the eroded Carboniferous clastites. These dramatic changes were accompanied also by volcanic activity. Lower Ladinian conglomerates, sandstones, claystones and marlstones with tuff commonly occur in the area. In the final phase of development of the Idrija Middle Triassic trench, the entire region was covered with marshland in which Upper Ladinian rocks rich in organic materials, locally known as “Skonca beds”, began to form (Mlakar 1969; Čar, in print). Hydrothermal solutions rich in mercury began to penetrate along the deep faults towards the surface. On their way, they permeated all the older layers, from Carboniferous to older Ladinian rocks. This resulted in the formation of highly diversified, rich epigenetic cinnabar ores. Part of the rich cinnabar solutions and cinnabar gels discharged directly into the marshland, forming exceptionally rich (up to 78% Hg), unusual syngenetic sedimentary cinnabar ores (Mlakar & Drovenik 1971). The final period of Ladinian tectonic events was accompanied by extensive volcanic activity with outpourings of diabase and keratophyre.

Epigenetic cinnabar ore  Syngenetic cinnabar ore

After the Middle Triassic tectonic phase stabilized, Carnian layers containing varieties of quartz conglomerates and sandstones began to be deposited throughout the entire Idrija region, including large quantities of limestones and white dolomite. In the Norian and Rhaetian stages, stromatolitic and oncolitic dolomites known as the ”Hauptdolomite” (Lipold 1874; Kossmat 1898, 1900; Mlakar 1969) with rare limestone inclusions appeared in many areas. Jurassic limestones and dolomites can only be found in the Trnovo Forest, south of the Idrija region. These were followed by more or less stratified organogenous Lower and Upper Cretaceous limestones. The sedimentation process in the Idrija region was concluded with Palaeocene-Eocene flysch, which are »only« around 60 million years old.

Onkolitic dolomite Rudist limestone

 

Due to the far-reaching consequences of the Tertiary tectonic activity, starting at the end of the Eocen or beginning of the Oligocen (Placer 1981, 1999; Vrabec & Fodor 2006), no normal stratigraphic sequences of rocks can be found anywhere in the Idrija region. During the convergence of the Eurasia and Africa plates and the subsequent moving of the Adriatic microplate towards the Eurasian continent in Tertiary, two large geotectonic units were formed in the territory of Slovenia (Placer 1999, 2008). The western and central parts of Slovenia belong to the External Dinarides, which to the east gradually pass into the Internal Dinarides, while the northwestern Slovenia is part of the Southern Alps, which are thrusted southwards from the north on the External Dinarides (Placer 1999, 2008). Both geotectonic units are built of extensive thrust units with complex lithostratigraphy.
The Idrija region forms part of the External Dinarides, and for this reason has a typical thrust structure. It developed from a huge recumbent fold (Mlakar 1969; Placer 1973, 1981). The strata of the upper limb of the fold are thus in an inverse position. Owing to further displacements, the fold was cut up into individual large nappes and intermediate thrust slices, which moved in south-west direction along the thrust fault plane for even more than ten kilometres (Mlakar 1969; Placer 1973, 1981). In a deeper basement of the entire Idrija region lies the Hrušica nappe comprised of Carboniferous to Palaeocene-Eocene flysch in normal position. Thrust onto the Hrušica nappe is the Koševnik thrust slice, built of Lower and Upper Cretaceous rocks also in normal position. These are followed by partly normal and partly inverse strata of Upper Triassic (Norian-Rhaetian) dolomite in the Čekovnik thrust slice. These are covered by inverse Carboniferous, Permian, Scythian, Ladinian and Carnian rocks of the Kanomlja thrust slice. The highest lying thrust unit is the Trnovo nappe comprised of two internal thrust blocks. The lower block is called Idrija internal thrust block. It is composed of layers of Carboniferous to Carnian rocks in normal and partly inverse position. The mercury ore deposit is located within this structure. The upper internal thrust block is called Tičnica internal thrust block. It is built of Carboniferous shales to Eocene flysch in normal position (Mlakar 1969; Placer 1973, 1981).

The consequences of thrusting and the present-day erosion conditions are, alongside the extensive levellings, also huge tectonic windows lying in deeply cut gorges. Lying in the Idrijca gorge is a single tectonic window, Strug, and nestled in the Nikova ravine is another single tectonic window, Bevk. A rare phenomenon in the Southern Alps is a triple tectonic window in the Bratuš ravine in Zgornja Kanomlja (Mlakar 1969; Placer 1973, 1981). Here the strata of the Čekovnik and Koševnik thrust slice and the rocks of the Hrušica nappe are exposed on the surface below the Kanomlja thrust slice.
Of the numerous faults, let us mention only the most important Idrija fault, which runs through the entire Idrija region.

Simplified geological map of Idrija UNESCO GLobal Geopark (after Buser and Draksler 1993)

Idrija ore deposit

A complex geological phenomenon with mercury mineralisation, in which the main ore mineral is cinnabar. It is built of Carboniferous, Permian, Scythian, Anisian, Ladinian and Carnian rocks. A particularity of the Idrija ore deposit are the rich sedimentary ores and native mercury.

Synsediment cinnabar ore Native mercury

Anthony's Main Road

Anthony's Main Road is the oldest preserved part of the ore deposit that has remained accessible after shutdown of the mine. Preserved in this site are extraordinary geological phenomena such as Carboniferous rocks mineralised with mercury, Middle Triassic erosional unconformity, sedimentary and coral ores. The ore extraction methods used through history
are presented.

Antonijev rov 1 Antonijev rov 2

Wild lake

Wild lake is a complex geological, geomorphological and hydrological natural phenomenon, as it is simultaneously a cave, spring and lake. Alpine plants (Slovenian endemite (Primula carniolica) thrive on its steep and cold slopes, and its cave is home to the only European cave vertebrate - the human fish (Proteus anguinus). Its international historical significance is linked to such famous scientists as Scopoli and Hacquet, broad-thinking researchers who worked as physicians in the Idrija Mercury Mine. It is arranged as a museum in nature.

 

 

Idrija fault from Kanomeljsko Razpotje

 

Idrija fault

The Idrija fault is one of the largest faults in the Southern Alps, which cuts across the entire
territory of Slovenia, continuing into Italy on the northeast side, and into Croatia on the southeast side. The events occurring along the Idrija fault are also connected to the Idrija ore deposit, which was cut off and displaced from the fault. Numerous earthquakes occurred in the broader fault zone. Remarkable views of the fault zone open up at Kanomeljsko Razpotje and Hudournik on the Vojsko plateau.

Zala fault, Podroteja

A distinctive and morphologically prominent fault zone which separates a variety of rocks. The fault zone is directly visible at several locations. It has an important role in forming the morphology of the terrain in the surroundings of Idrija.

Bevk tectonic window

Lying in the base of this single tectonic window are Upper Cretaceous rudist limestones, which are confined on top by erosional unconformity overlain by Eocene-Paleocene flysch rocks.

Strug single tectonic window

Exposed in the base of the window are Upper Cretaceous limestones and Paleocene-Eocene flysch rocks of the Hrušica nappe overlain by an overthrust plate of Lower and Upper Cretaceous limestone. This is a site of the Carniolan Primrose (Primula carniolica).

Kanomlja triple tectonic window

A triple tectonic window with three folding lines that structurally separate four overthrust
units. A morphologically and tectonically complex phenomenon of regional importance, covering an area of 1.8 km2.

Bratuš triple tectonic window

The Bratuš tectonic window reveals three overthrust packages of rocks lying one above the other. The deepest lying are the youngest Eocene flysch rocks, and the highest lying are an outcrop of the oldest Carboniferous shales.

Waterfalls and pools in Kramaršca gorge

Kramaršca gorge

An approx. 200 m-long gorge in the upper Idrijca River valley with a very narrow entrance, partly impassable. Water has formed several waterfalls and pools in the gorge. The high walls are built of Upper Triassic, unstratified, grained dolomite. The area is cut by several faults running N-S. Site of the Bear's Ear (P. auricula), Carniolan Primrose (P. carniolica), and the Idrija Primrose (P. x venusta).

Bedrova grapa gorge

A scenic gorge with waterfalls and rapids, also an exceptional geological locality. The entire profile of Upper Triassic Carnian beds and part of Norian-Rhaetian (main) dolomite are visible. The clastic rocks contain numerous sediment textures and remains of Carnian shells (Myophoria and Pachycardia). A site of endangered and protected plant species.

Tratnik landslides

Exposed clastic Julian beds passing upwards into Cordevolian dolomite. Green and red sandstones frequently contain Jasper, fossils of Pachycardia rugosa HAUER, Myophoria kefersteini BITTNER and Lopha montiscaprilis KLIPSTEIN shells. The well visible, exposed, folded, inversely lying beds represent a rare geological phenomenon in Slovenia.

Tratnik landslides Landslides in Gačnik

Wetlands in Gačnik

Carnian sandstones with fossils in Gačnik

Carnian sandstones pass in some spots into conglomerate with red jasper and intercalations
of nodular limestone. The limestones contain the fossil remains of shells, ostracos, corals, spines of sea urchins, algae, and rare remains of land plants. Wetlands can be found below the barren sandy hills and ridges.

Ladinian beds

Alteration of tuffaceous claystones and marlstones
as well as nodular silicified limestones. Several well-preserved ammonites have been found here.

Diabase

Basalt outcrop with calcite almonds and basalt breccia (red and green coloured), which bear witness to volcanic activity 230 million years ago.

Basalt with calcite almonds Ladinian beds

Volcanic and pyroclastic rocks, Pisance

The surroundings of the Pisance farm are built of purple, red and grassy green tuffs and tuffites with intercalations of diabase and diabase breccia.

Pisance

Footprints of upper triassic reptiles, Medvedje Brdo

Fossil footprints of an unknown reptile. The footprints are on an almost horizontal, bedded
surface of Upper Triassic Norian-Rhaetian 'main' dolomite. A sequence of five footprints is visible.

Megalodontid shells, Knipajz

Profile of Carnian beds (claystones and sandstones) from Julian-Tuvalian, an intermediate horizon between Julian and Tuvalian, with numerous remains of Triadomegalodon idrianus
megalontid shells.

Megalodontide limestone

Upper cretaceaus rudist limestone, Vojsko memorial plate in Strug

Fossil remains of rudist shells in bedded rudist limestone of Upper Cretaceous age.

Moraine, Hudo polje

Distinct terminal moraine, 5 to 10 m high, believed to date from the peak of Würmian glaciation. In the terminal moraine is a large outcrop revealing the structure of the moraine hill. Fine gravel mixed with large blocks and fine mushy materials are predominant.

Trohova ravan

An extensive, dry valley with periglacial debris, a typical alluvial doline. A botanical locality (Gentiana Clusii, Lilium bulbiferum).

Uvala and solitary boulders

Solitary boulders of Anisian dolomite in a karst depression sunken by several meters.

Babji zob in the valley of the Belca stream

A solitary boulder in the form of a several meter-high dolomite column in the valley of the Belca stream.

Tomaž's table

A huge "rock table" comprised of a thick rock plate situated on a narrow rock pillar. A product
of tectonic activity and erosion.

Tomaž's table

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