1 Introduction

Karst processes and the formation of karst geomorphological features depend to varying degrees upon the area’s geological structure [1]. Large-scale geological maps, as for example in the case of Slovenia’s basic geological maps (1:100,000), do not show sufficient detail to support researching of karst features at local level. In Slovenia, a few detailed geological maps covering the areas containing important cave systems exist, as for example around Postojna Cave [2], Predjama [3] and the Škocjan Caves [4]. During detailed structural geological mapping of the Škocjan Caves and the surrounding land surface, an interesting stratigraphical characteristic was found within the Mala and Velika collapse dolines. Large collapse dolines form as a result of chemical and mechanical removal of rock material at and below the level of groundwater [5]. The Mala and Velika collapse dolines are “karst windows” [1], though perspectives of the structural geology and lithological relationships within “karst windows” are lacking. Our contribution can support understanding of “karst windows” in terms of their stratigraphy and general geology.

This research focusses on the Škocjan Caves system, which, in 1986, became the only cave in Slovenia to be listed as a UNESCO natural heritage site. The system, in SW Slovenia, is 6474 m long and 254 m deep [6]. Its total volume, from the ponor in Mahorčičeva Jama to Martelova Dvorana (Martel’s Chamber) is 6.13 million m3. Martelova Dvorana itself has a volume of 2.55 million m3, with a maximum length of 314 m, width of 143 m and height of 158 m [7]. Martelova Dvorana is confirmed as being the 11th largest cave chamber in the World and the 2nd largest in Europe, after the Salle de la Verna (3.6 million m3) in France. Škocjan Caves also contain the largest known underground river canyon in Europe [7].

After passing through the Škocjan Caves, the underground Reka River flows northwestwards towards Kačna Jama cave (20 km long and 280 m deep). The two cave systems are linked hydrologically, but have not yet been connected by caving exploration (Fig. 1) and there is still an 870 m unexplored gap. Further to West, the Reka River emerges from its subterranean course about 30 km to the westnorthwest at Timavo, a spring in Italy [6].

Fig. 1
figure 1

Left—Structural geology of the wider area of the Škocjan Caves system, SW Slovenia. Right—Major passages in Škocjan Caves. Extension of karst after [39], active faults after [20]

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A study of speleogenesis [8] and a structural-geological and lithological map of the surface above the cave [9] represented the first published geological research related to the Škocjan Caves. The first geological map of Škocjan Caves did not include the Hankejev Kanal passage [10], and a structural-geological map of Hankejev Kanal was later published by Kranjc et al. [11].

The formation of initial conduits and karstification was studied in the Velika Dolina collapse doline [, 12, 13]. Cave-bearing bedding planes detected in Velika Dolina have been correlated with those in unroofed caves (or denuded caves) on the surface and to those guiding passages within the Škocjan Caves [14]. The structural-geological map of Škocjan Caves [4] is based upon structural-geological field mapping (1:500) of cave passages that was carried out during the periods 1991–1992 (Hankejev Kanal) and 1997–2007 (Tiha and Šumeča Jama). 3D terrestrial laser scanning of Škocjan Caves [7] pointed to a new scientific approach towards understanding the speleological and geological structures of karst caves [15].

Between 2018 and 2022, additional structural-geological mapping was undertaken at 1:500 scale in Marhočičeva and Mariničeva Jama caves (Fig. 1) and the adjacent Velika Dolina and Mala Dolina collapse dolines. This part of the Škocjan Caves system had not undergone detailed field geological research previously [4] and the aim was to enable compilation of a geological map of the entire system. Interaction between massive collapse dolines, the hydrologically active underground Reka River, and cave passages developed within different lithological units resulted in the recognition of particular characteristics that become more evident after the construction of longitudinal cross-sections illustrating the geological structure. The term “karst stratigraphical window” describes a recognizable karst feature that is also characterized by the exposure of a different lithostratigraphical unit that is shown on the geological map. This study underlines the importance of understanding geological structure and lithostratigraphy as well as the karst geomorphological features. The crucial role of detailed (in-situ) geological field mapping of the karst surface and subsurface is emphasized, further demonstrating its fundamental role and instrumental nature in achieving a deeper understanding of karst features.

The paper is composed of five sections starting with general description of the study hypothesis and aims of the research included in introduction Sect. 1. In the next Sect. 2 we describe general geological setting. Section 3 shows research methods and Sect. 4 results with discussions. In the final Sect. 5 we present conclusions with major findings.

2 Geological setting of the Škocjan Caves area

According to the first, fundamental, geological studies of the Škocjan Caves, the system has developed in bedded Turonian (K22), massive Senonian (K23) and thin-bedded Paleocene (K24 + Pc) limestones. The strata dip generally towards the SE, and are cut by fault systems that trend approximately NW–SE and NNE–SSW [9].

More-recent geological mapping of the surface and subsurface of Škocjan Caves at 1:50,000 scale, has revealed that the stratigraphical sequence of the researched area is composed of three lithostratigraphical units [, 16, 17]. The oldest rocks, belonging to the Sežana Formation (K22−4), are 400–500 m thick and mostly comprise bedded limestones with rare rudist biostromes [, 16, 17]. Overlying the Sežana Formation conformably, the Lipica Formation (K24−5) is 250–400 m in thickness [, 16, 17] and is represented by bedded and massive limestone with rudist biostromes and bioherms [16]. The youngest lithostratigraphical unit seen in the Škocjan Caves is the Liburnian Formation (K-Pc), comprising bedded limestones (containing the foraminifera genus Alveolinae) with a thickness of 50–300 m. The boundary between the older Lipica Formation and the Liburnian Formation is a disconformity, representing a regional discordance [, 16, 17].

Thus, the passages of the Škocjan Caves system are developed within components of a 300 m-thick sequence of Cretaceous and Paleocene limestones [4]. The underground River Reka in Šumeča Jama and Hankejev Kanal flows mostly within a 130 m-thick segment of the Lipica Formation (K24−5) and follows the direction of the bedding-plane dip. Bedding planes affected by interbedding slips [, , 4, 12, 14] appear to have acted as inception horizons [18], which were especially favourable in guiding the initial development of the earliest cave passages.

Strata in the Škocjan Caves area have a Dinaric strike orientation trending NW–SE and generally dipping towards the SW. The area is part of the geotectonic unit known as the Trieste–Komen Plateau; more specifically it lies on the SW wing of a regional anticlinal fold structure [, 8, 17]. Many faults run parallel to the bedding strike (NW–SE), with some also cutting across the strike (NE–SW) [8].

The area of Škocjan Caves belongs to the Adria Microplate, which was overthrust by the External Dinaric thrust belt at the end of the Eocene [19]. During the Miocene the Adria Microplate underwent segmentation and rotation in a counter-clockwise sense, with underthrusting beneath the Dinarides. Degradation of the Adriatic–Dinaric carbonate platform and deposition of clastic flysch rocks occurred during the Eocene [19].

The most important regional faults in the area are the Dinaric-oriented (NW–SE) Raša and Divača faults (Fig. 1). According to Atanackov [20] both faults remain active. The Raša Fault shows evidence of a multiphase kinematic development. During the first phase it was a reverse fault with its overthrusting tendency directed towards the SW. During subsequent relaxation of regional pressure, individual parts of the fault took on a gravitational character, and in its final phase it evolved into a strike-slip fault [16].

The surface trace of the Divača Fault passes about 1 km north of the Škocjan Caves (Fig. 1). Examination of its wider deformation zone indicates that it is a shear zone. Evidence of overthrusting, normal subsidence, and horizontal fault movement are all present locally [16].

The study area exhibits two broad groups of tectonic deformations. The first were imposed by Cretaceous–Paleogene compression in a NE–SW direction. They are represented by Dinaric thrusting structures, regional folds with NW–SE-oriented axes and reverse faults with essentially the same strike orientation. Within phases of relaxation some of the original reverse faults were reactivated as normal faults, and at the same time cross-Dinaric normal faults also developed. During the Neogene and Quaternary, the second set of deformations resulted from regional compression in a generally N–S direction, which produced strike-slip faults with a NW–SE orientation [16].

3 Methods

Detailed structural-geological mapping of the karst surface and subsurface of the Škocjan Caves system was carried out in the field according to a technique first developed and used in Slovenian karst areas by Čar [21]. This technique distinguishes between the degrees of brittle fault-rock deformations. Three types of tectonic zone are defined (from the least deformed to the most degraded), comprising fissured, broken, and crushed zones [21]. The crushed zone is the inner part of the fault zone where original depositional structures are highly disrupted or obliterated, whereas in the broken zone the bedding is generally deformed, and the fissured zone contains parallel fissures between which the original bedding planes are still preserved [21].

Geological mapping of the surface and subsurface in the Škocjan Caves area, at a scale of 1:5000, with longitudinal profiles, was carried out during different periods between 1991 and 2022. Field measurements of geological structures present in the carbonate lithologies at outcrop included angle and direction of dip for bedding planes and faults, and—in line with the technique described above—the strike and dip angles of fissured, broken and crushed zones [, 3, 21].

Additional information derived from the results of earlier studies of the Škocjan Caves [, , 4, 11, 22] was the base for the observations from the most recent field geological mapping (2016–2022) of Mala Dolina, Velika Dolina, Mahorčičeva Jama and Mariničeva Jama at a scale of 1:500 that have not been published before.

Names of the lithostratigraphical units in the karstified area of Škocjan Caves are adopted from the geological maps of the Karst region at scales of 1:50,000 [16] and 1: 100,000 [17].

To enhance the correlation of the karst morphologies within the studied area with the geological information, a hill-shade DEM model of the topography [23] was overlain on the geological map (Fig. 2).

Fig. 2
figure 2

Geological map of the surface above the Škocjan Caves system, hill-shade morphology (ARSO 2022), and Škocjan Caves map with the River Reka. 1—reference crossing point number of longitudinal profiles on ground-plan and longitudinal profiles, 2—course of longitudinal profile, 3—Liburnian Formation (K-Pc), 4—Lipica Formation (K24−5), 5—Sežana Formation (K22−4), 6—supposed fault trace at the surface, 7—vertical and horizontal displacements along faults, 8—dip direction and dip angle of bedding planes, 9—crushed zone, 10—broken zone, 11—fissured zone

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4 Results and discussion

4.1 Interpretation of the tectonics and stratigraphy of the Škocjan Caves area

On the surface above the Škocjan Caves bedding planes dip towards the S and SW, with an angle of 10–40°. Also, there are less-common examples of bedding planes that dip towards the SE. In Mahorčičeva Jama bedding-plane slips indicate reverse fault-plane movements, with small displacements (< 1 cm). The bedding-plane slips are related to compressional regional deformations associated with folding and thrusting. North of Tominčeva Jama, the bedding dip reaches 40° towards the SW (210/40), a higher angle than in the area of Mahorčičeva Jama and Mariničeva Jama and also higher than at Velika Dolina and Mala Dolina, where it is 20–30°.

Surface geological mapping and the geological interpretation of longitudinal cross-sections showed vertical movements along faults with a general N–S orientation. Predominantly, western blocks of N–S-trending faults are displaced downwards relative to the eastern blocks (Fig. 3E–F). In contrast, in the ceiling of Šumeča Jama, a fault zone (Fig. 3G–H) on a generally N–S trend exhibits uplift of the western block, relative to the eastern one. The possibility that N–S-trending faults are still active cannot be disregarded.

Fig. 3
figure 3

Longitudinal sections A–B, C–D, E–F, G–H and I–J. Legend as for Fig. 2

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Continuations of two NW–SE-oriented faults—the Risnik (RF) and Krgunca (KF) faults above the Kačna Jama Cave [22] have been confirmed towards the SE of the Škocjan Caves area (Fig. 2). The main fault zone of the regionally important Divača Fault (Fig. 1) is identified on the basis of the faulted lithological contact between the Sežana Formation (K22−4) and the Liburnian Formation (K-Pc), by interpretation of the DEM topography, and by the position of the crushed zone (internal fault zone) recognized in the field by the current authors.

The Velika Dolina collapse doline has a volume of up to 2,000,000 m3 compared to the 560,000 m3 of the Mala Dolina; each of them has a mean surface area of about 15,500 m2. Field mapping of the geological structure of the Velika Dolina and Mala Dolina collapse dolines (1:500) revealed several distinctive fault planes that are not traceable over long distances because they are displaced by other tectonic structures. Sinistral and dextral horizontal movements, as well as vertical movements, suggest that there has been multiphase tectonic activity. Principal fault-strike directions are NW–SE and WSW–ENE, with secondary fault-strike directions close to W–E and N–S (Fig. 2).

Extending from the general land surface to the Mariničeva Jama cave in Mala Dolina there is a well-expressed fault 170/85 (Fig. 4), which cuts the 70/85 fault. Along the 170/85 fault, which was previously described by Pavlovec [24] and Šebela [4], karst morphological dissolution features suggest vertical movement (< 2 m), where the southern block has lowered relative to the northern block.

Fig. 4
figure 4

Mala Dolina: an almost vertical fault with structural elements (dip direction/dip angle) 170/85 is traced from the surface to Mariničeva Jama. Photo S. Šebela

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Geological field-mapping employing Čar’s [21] classification led to recognition of numerous fissured and broken zones. One example is well exposed at the bottom of Velika Dolina (Fig. 5), where the fissured zone has a dip direction of 100° and is 30 m wide and 100 m long.

Fig. 5
figure 5

Bedding planes (dip direction towards the SW and S) and fissured zone (general direction N–S) near the bottom of Velika Dolina. Photo S. Šebela

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Longitudinal cross-sections A–B, C–D, E–F, I–J and K–L (Fig. 3) reveal a geological peculiarity within Mala Dolina and Velika Dolina that had not been remarked by previous researchers [, , , , 4, 10, 12, 16, 25]. Both Mala Dolina and Velika Dolina have developed through two different lithostratigraphical units.

Considering the bedding dip direction of the Sežana (K22−4) and Lipica (K24−5) formations, the lithostratigraphical boundary between the two formations [, 9, 16] and the spatial geometry between surface and the Velika and Mala collapse dolines (Fig. 3) it was apparent that on the structural-geological map of the surface (Fig. 2) limestones of the Sežana Formation appear in Velika Dolina and Mala Dolina. Karst erosion and corrosion resulted in the formation of two collapse dolines (120–165 m deep) that led to the older Sežana Formation being exposed as small inliers beneath the younger Lipica Formation.

4.2 Tectonic versus stratigraphical versus karst windows

Karst stratigraphical windows are also called “karst windows” or “karst fenster” [1], but these terms are not ideal in the cases of the Velika Dolina and Mala Dolina collapse dolines. Karst windows are essentially hydrological and geomorphological features, whereas in the Škocjan Caves example there are additional issues related to stratigraphy and structural geology.

Karst windows are a geomorphic feature found in karst landscapes, where the course of an underground river (active or relict) is visible from the surface within a sinkhole, and the term is used broadly to describe an unroofed cave that reveals a segment of a subterranean river course. Generally, formation of a karst window is caused by a caving-in of portions of the roof of a subterranean stream passage, thus making a part of the underground stream route visible from the surface. Theories relating to the creation of karst topography and karst windows involve vadose water above the water table, and deep-circulating phreatic water transporting subsurface rock away. Karst windows can also form because of the effects of weathering from above [26]. Synonyms for karst window are: (French) fenetre karstique; (German) Karstfenster; (Greek) karstikon parathyron; (Italian) finestra carsica; (Russian) karstovoe okno; (Spanish) dolina en ventana; (Turkish) karst penceresi; (Slovene) kraško okno.

Another example of a “karst window” outside of Slovenia is provided by Cedar Sink (Mammoth Cave National Park, Kentucky, USA), which is a sinkhole where a subterranean river emerges, providing a view into a far larger hydrological system. The concept of a “karst window” at Cedar Sink contributes an important element to the understanding of the groundwater system [27]. Also in Kentucky (USA), Short Creek in Pulaski County is a small river that emerges and disappears within a distance of less than 100 m. Short Creek has been described as a karst window [28].

Recognized examples of stratigraphical windows are rare in the published reference sources. An example is described where Lower Permian carbonate rocks are exposed in a 2 km2 stratigraphical window beneath Cretaceous and Tertiary volcanic rocks near Rancho La Cueva, northern Sonora, Mexico [29].

In Iran, the Aghdarband erosional window (15 × 25 km) represents a thick succession of Triassic rocks resting upon Late Palaeozoic units exposed SW of the village of Aghdarband that is surrounded by a more than 6 km-thick post-Cimmerian Middle Jurassic to Miocene sedimentary rock succession forming the Kopeh-Dag fold-and-thrust belt [, 30, 31]. The formation of the Aghdarband erosional window [, 32, 33] is related to tectonic factors and it cannot be compared to the examples of the Velika Dolina and Mala Dolina collapse dolines because it is a far larger (regional) feature and represents a tectonic window.

As discussed above and shown in Fig. 6, tectonic windows differ from karst stratigraphical windows. Tectonic windows are geological structures formed by erosion or normal faulting interacting with a thrust system. When erosion or normal faulting produces a hole in the thrust’s hanging wall (normally a nappe), where the underlying autochthonous (i.e. un-transported) rocks crop out, the resulting phenomenon is termed a tectonic window. Size of tectonic windows ranges from a few metres to hundreds of kilometres. Well-known examples of tectonic windows are the Hohe Tauern [34] and Engadin windows in the Austrian and Swiss Alps [35]. Between many examples of tectonic windows, we can mention stratigraphy and tectonics of the tectonic window described in Polish Outer Carpathians [36] and complex nappe structure with erosion processes that resulted in formation of planation surface in Western Carpathians (Slovakia) [37].

Fig. 6
figure 6

Schematic picture of: A. tectonic window (A1—older rocks thrusted over younger rocks, A2—younger rocks thrusted over older rocks); B karst stratigraphical window; C the area around Mala Dolina and Velika Dolina (Photo Matej Blatnik)

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4.3 The singularity of the Škocjan Caves area as a karst stratigraphical window

This unusual geological relationship and geomorphological feature is described here as a karst stratigraphical window, where older rocks within a conformable stratigraphical succession crop out as inliers beneath younger strata that have been penetrated, in this case because of the effects of karstification processes in deepening collapse dolines.

Stratigraphical windows are not the same as tectonic windows (Fig. 6) where, due to the effects of erosion (and corrosion in the case of karst areas), the rocks beneath a thrust fault contact crop out. Although bedding planes with interbed slips are present in Tiha Jama, Mahorčičeva Jama and Mariničeva Jama, such interbed movements represent small displacements (< 1 cm) and these cannot be equated with thrust fault movements and tectonic windows. This demonstrates additionally that Velika Dolina and Mala Dolina cannot be considered as being tectonic windows.

The case of the Velika Dolina and Mala Dolina collapse dolines is more closely related to the geological term “karst stratigraphical window” (Fig. 7), which has not yet been described in the karst literature. To confirm the identification of such features it is necessary to check detailed geological maps of the surface for the exposure of different formations in the area of karst stratigraphical windows, collapse dolines and unroofed caves.

Fig. 7
figure 7

3D model of Velika and Mala Dolina with stratigraphic boundary (red plane) between older Lipica and younger Sežana Formations, DEM after [23]

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Another important question is that of when the Velika and Mala collapse dolines were opened to the surface? Results of numerical model calculations of the rate of dissolutional bedrock removal where a heavily fractured zone is intersected by a karst conduit in the phreatic zone suggest that development of large, closed depressions could take in the order of 1 million years [5]. For Velika Dolina and Mala Dolina, Gospodarič [9] suggested a shorter timescale, e.g. with the formation at the height of the Würm (W3) cold stage or in immediate postglacial times—which fits within a period between about 115,000–11,700 years ago.

Study of a DEM-based doline map of Slovenia [38] showed that most collapse dolines are situated near ponors, near springs or near the courses of major underground rivers. Velika Dolina and Mala Dolina are near the Reka River ponor and near the underground course of the Reka River. Consideration of the morphological characteristics of collapse dolines in Slovenia showed that only 20 of them are deeper than 100 m [38]. Velika Dolina and Mala Dolina are among Slovenia’s deeper collapse dolines, with 120 m depth (Mala Dolina) and 165 m depth (Velika Dolina) respectively. Nationally the distribution of dolines can reach 500 dolines per km2, and the Škocjan Caves are part of the Karst area, where the average number of dolines is 60 per km2 and cumulatively they cover 12% of the surface [38].

Directly above the passages of the Škocjan Caves system (Fig. 2) there are 8 collapse dolines in an area of 4 km2. In general, the distribution of collapse dolines in karst areas can indicate the presence of “karst stratigraphical windows” with possibly special lithostratigraphical relationships that must be shown clearly on detailed geological maps where inliers of older rocks are exposed beneath surrounding younger rocks due to the erosional effects of karstification.

5 Conclusion

Existing basic geological maps are generally performed in scales that are not precise enough for karst areas. For our study new detailed (in-situ) geological mapping of karst surface (1:5,000) and Škocjan Caves (1:500) was used to find connections between karst features development and geological structure.

Particular karst-geological characteristics, described here as karst stratigraphical windows, where older Sežana Formation rocks crop out through the base of the younger Lipica Formation as a result of karst collapse doline formation, were detected and reported for the first time. The Velika Dolina and Mala Dolina karst stratigraphical windows, in the Škocjan Caves area, are not only “typical” karst geomorphological features but also distinctive lithostratigraphical windows.

The study showed that Velika and Mala collapse dolines are not tectonic windows. Even if there are small (< 1 cm) displacements of bedding planes related to fold flexures affecting the wider area, the rocks exposed are within a normal stratigraphic succession. Karst dissolution, erosion and subsidence resulted in the formation of the Mala and Velika collapse dolines.

The most important added value of this research is that while engaged in the geological mapping of collapse dolines it is crucial to look for outcrops of different stratigraphical units. If two or more rock formations that are in a normal stratigraphical succession are recognized, the limits of the different lithostratigraphical units in the area of the collapse doline should be shown on the geological map of the surface, thus transforming a typical collapse doline into a karst stratigraphical window. While present geological science includes important modern data proccesing approaches, basic geological mapping cannot be fulfilled without field (in-situ) inspection and hard work geology.

This might seem a “minor” issue in the context of regional geological mapping at 1:50,000 or 1:100,000 scale, but when producing detailed geological maps (1:5000 scale or less) of karst areas such characteristics must be taken in account. Considering that many karst areas contain numerous collapse dolines and other types of doline, we believe that examples of karst stratigraphical windows (which are not tectonic windows) are ubiquitous in situations where stratigraphically older rocks crop out as stratigraphical inliers where they are exposed beneath younger rocks due to the effects of karstification.

We strongly recommend that karst areas are studied with the help of detailed geological maps, what is the important basis for understanding karst speleogenesis. There are some scientific facts that can be discovered only by field work.