Geological Development of Slovenia

German Translation after Prof. Stanko Buser (*1932-02-20; †2006-10-31)

German version by Irena Trebušak und Christian Wolkersdorfer translated into English by DeepL.com

The geological history of Slovenia is very colourful and probably goes back to the Precambrian. That would make it at least 600 million years old. In Pohorje, Kozjak and Strojna and south of Črna na Koroškem are the oldest rocks in Slovenia. These metamorphic rocks were formed from sediments or magmatites. They are gneisses, mica schists, amphibolites, eclogites, marbles and various schists. Because of the lack of fossil content, their age cannot be determined. Some experts classify them as Precambrian and Cambrian, others as Ordovician to Devonian. In the latter case, they would be “only” 500 million years old.

The areas where metamorphic rocks occur are classified as part of the Eastern Alps or Alpides. To the south of these lie the Dinarides. Their boundary is formed by the Periadriatic Seam, which reflects the former contact between the African and European continental plates. The age of the Devonian rocks in the South Karavanke Mountains between Logarska dolina (Logarska valley) and Jezersko has only been established. The Lower Devonian is formed by platy deep-sea limestones in which conodonts occur. In the Middle Devonian, shallow water reef limestones of corals and hydrozoans were deposited. The Upper Devonian is again represented by deep-sea limestones.

During the Lower Carboniferous, the area of the South Karavanke Mountains was covered by deep sea. Today, shaly flysch clays, sandstones and limestones were formed. After a short-lasting continental period, shallow marine quartz sandstones, conglomerates, shaly mudstones and limestones with foraminifera, bivalves and corals were formed in the Upper Carboniferous. Near Litija, these sediments contain lead and zinc ores as well as barite, and in the Karavanke Mountains iron ores.

The subpermian sedimentary rocks correspond to those of the Upper Carboniferous, only in the South Karavanke Mountains limestones predominate. The fossils of the world-famous Dovžanova soteska (The Dovžan Gorge) site near Tržič date from this period. In the Middle Permian, the sea retreated from most of Slovenia. On the mainland area, a different climate prevailed with the sedimentation of typical red sandstones, siltstones, mudstones and colourful breccias. At Žirovski vrh these rocks contain uranium ore. In the Upper Permian, the sea again transgressed over the entire territory of Slovenia. The shallow sea areas were covered with algae and in some places, various bivalves and brachiopods lived. Black limestones and dolomites also date from this period, and granite and granodiorite south of Črna an Koroškem.

Triassic rocks are particularly important for Slovenia. From the Lower to the Middle Triassic shallow marine limestones were deposited, which were later transformed into dolomites. The Upper Middle Triassic (Ladinian) was a revolutionary era for the Slovenian territory. On faults, the area broke into tectonic blocks, some of which came under deep-marine influence, others under continental. Along the large fault zones, magma penetrated and volcanoes produced ashes, which later became tuffs. In addition, deep marine plate limestones with cherts, mudstones and sandstones were deposited. Common fossils in these rocks are bivalves, ammonites, conodonts and radiolarians. Along with the volcanic activity, the important mercury deposit of Idrija was formed.

Until the Upper Cretaceous, central Slovenia was covered by deep sea. Therefore, this area is called the Slovenian Basin or the Inner Dinarides. The area north of it was shallow-marine during the Upper Triassic, and slab limestones with cherts were initially sedimented, later roofstone limestones several 100 metres thick. According to the depositional environment and the rock types found in the Julian and Kamnik-Savinja Alps, they are classified as belonging to the Southern Alps.

In the lower Upper Triassic, shallow water limestones were sedimented south of the Slovenian Basin, which were later mostly transformed into dolomites. Karstification of these ferruginous limestones led to their red colour. Today they are quarried at Hotavlje and Lesno Brdo as building stones for jewellery purposes. The areas of Notranjska and Dolenjska are characterised by deposits of red sandstones, mudstones, breccias and bauxite of the Middle Upper Triassic. Shallow water limestones of the Upper Upper Triassic were later transformed into dolomites there. The entire area south of the Slovenian Basin with shallow-water sedimentation between the Upper Triassic and Upper Cretaceous is called the Outer Dinarides.

In the shallow-marine area of the North Karavanke Mountains, north of the Periadriatic Suture, limestones with lead and zinc ores were formed in the Middle and Upper Triassic, which were mined in Mežica. Interestingly, no traces of volcanic activity corresponding to that in the Dinarides to the south can be found in this area.

Shallow water limestones are known from the lower part of the Lower Jurassic (Lias) in the Southern Alps. Later this area was raised above sea level but soon returned to deep marine influence with the deposition of red nodular limestones.

The central part of Slovenia was covered by deep sea in the Jurassic and slab limestones with cherts, mudstones and marls were deposited. South of this trench was a large shallow water plateau where only shallow water limestones were sedimented. Dark limestones with long white bivalve shells are particularly interesting. In construction, this rock is known as Podpeč marble, which the architect Jože Plečnik used for his best works of art. Coral and hydrozoic limestones from the Upper Jurassic are also interesting. They were formed from reefs similar to those of the Australian Great Barrier Reef.

The Southern Alps and Central Slovenia were under deep marine influence during the Cretaceous. Initially, green flysch marls and sandstones were formed, later red marls and marl limestones. These were later eroded in the Southern Alps and only in Rdeči rob and Tolmin are they still present as geological features. In central Slovenia, the Cretaceous plate limestones were also called Volčekalkstein after the place Volče near Tolmin. These limestones were beautifully folded by endogenous forces. At the end of the Cretaceous, the Southern Alps were raised above sea level. The southern parts of Slovenia were covered by the shallow sea during the Cretaceous, and thick, layered slab limestones were deposited there. The most beautiful Slovenian karst stones for architectural purposes date from this period and were once quarried in Vrhovlje, Kopriva, Tomaj and Kazlje. Today, only one quarry in Lipica is still working.

At the end of the Cretaceous, most of Slovenian and Croatian Istria was mainland, the limestones weathered, and bauxite was later formed from the red karst soil. Renewed transgression created the Kozina limestones, named after the village of Kozina. Later, foraminifera shells built up the nummulite and alvedine limestones. Before the last sea retreat from Slovenian territory, the sea became deeper again and on its bottom flysch marls and sandstones were formed, which today form the fertile soil of the Vipava and Pivka valleys, the Brkini area and the coastal land.

After a rather long continental period in central Slovenia, larger peat basins were formed in the Middle Tertiary (Oligocene), in which coal and marl layers were deposited. These include the coal deposits between Laško, Trbovlje and Zagorje near Zabukovica and Pečovnik. Later, these areas as well as the Southern Alps were transgressed from the east and underwent shallow water sedimentation. Today, the rocks of this period can be found near Bohinj, in the upper Sava Valley and the Kamnik Alps. Due to the subduction of the African plate under the Eurasian plate, strong volcanic activity began in the Štajerska and Gorenjska in the Oligocene. The ash from the volcanoes formed tuff deposits on the mainland and the sea. The tonalite of Pohorje also intruded during this time. In the western part of Pohorje, the dacite lava cut through the earth’s surface.

At the end of the Oligocene and the beginning of the Miocene, the rivers deposited huge amounts of sand, gravel and clay in the shallow sea area, which later solidified into sandstones, conglomerates and mudstones. In the middle of the Miocene, the rocks of Slovenia were folded and partly overthrust as large blankets. This folding and overthrusting was a result of the subduction of the African plate under the Eurasian plate. After this revolutionary era, seawater once again flooded the Štajerska and Dolenjska areas and the limestones and marls with bivalves, gastropods, corals and sea urchins date from this period. Soon after, the Panonian Sea recedes in an easterly direction. The salt or freshwater basins that remained were filled in by river sediments with quartz sand, gravel and clay. In Prekmurje, oil and gas can be found in these layers, which probably formed as early as the Miocene. The resulting mainland was broken into floes, uplifted and lowered by endogenous forces.

In the Middle Pliocene, large tectonic depressions formed in central Slovenia, which later filled with water. Large Upper Oligocene forests around these lakes were the basis for the formation of thick lignite layers in Velenje and less thick lignite layers near Ilirska Bistrica and between Krmelj and Šentjanž.

In the Pliocene, tectonic activity began again. During this time Slovenia was mostly a plateau. Due to the long-lasting uplift of limestone and dolomite areas, karstification and the relocation of rivers below the surface occurred. At the beginning of the ice ages, large depressions such as those of Ljubljana, Brežice-Krško and Celje and the Ptuj field were formed on long faults. The rivers deposited gravel and sand from the glacial areas into these depressions. Slovenia’s colourful landscape has been formed over the past 10,000 years after the inland glaciers gave way to today’s warm climate.

© of the German translation is by Irena Trebušak and Christian Wolkersdorfer (1995). Using the German translation text needs the explicit permission of the translators.

Literature

Vrabec, M., Šmuc, A., Pleničar, M. & Buser, S. (2009): Geological evolution of Slovenia – An overview. – In: Pleničar, M., Bojan, O., Novak, M. & Pirc, S. (eds): The Geology of Slovenia. – p. 23–40, 17 fig.; Ljubljana (Geološki zavod Slovenije).

Translation remark: This “Introduction to the Geology of Slovenia” from Prof. Buser was written for the geological layperson. In order to preserve the text’s character, the majority of the text was not translated into proper geological language. The text also represents the knowledge of the end of the 20^th century and meanwhile might have changed.