Elsevier

Geomorphology

Volume 257, 15 March 2016, Pages 143-154
Geomorphology

Evolution of a karst polje influenced by glaciation: The Gomance piedmont polje (northern Dinaric Alps)

https://doi.org/10.1016/j.geomorph.2016.01.005Get rights and content

Highlights

  • Polje history reconstructed through geomorphology, sedimentology and GPR

  • Karst polje evolution related to glaciation

  • Glacigenic infill of the polje functioning as an aquitard

  • Evidence of Pleistocene glaciation in the northern Dinaric Alps

Abstract

Gomance is a piedmont karst polje in the northern Dinaric Alps presenting geomorphological and sedimentological evidence of past glaciation. During the Pleistocene the polje was situated at the edge of the Snežnik and Gorski Kotar ice fields from where two outlet glaciers reached Gomance. The morphogenesis of the polje was reconstructed by means of geomorphological mapping, sedimentological studies, and ground penetrating radar (GPR) measurements, supported by hand-drillings. With GPR an almost entirely buried moraine system was also imaged and mapped, crucial in reconstructing the polje history. The depression was karstified and well drained without any surface streams before the Last Glaciation. When the glacier front reached the depression, the entire floor became covered by glacial and outwash deposits. Surface runoff dominated over karst drainage in a large part of the polje, particularly where distal outwash deposits with low effective porosity functioned as an aquitard. These deposits diverted surface drainage toward the lowest edge of the polje, which functioned as a ponor front along the entire length. The outwash system of the Gomance polje was active during the Last Glaciation as suggested by radiocarbon-dated outwash deposits.

Introduction

Karst is a landscape where a combination of high rock solubility and well-developed secondary porosity result in distinct hydrology and landforms (Ford and Williams, 2007). Dominance of chemical dissolution of karst rocks results in distinct surface and subsurface morphology. Chemical dissolution is a rather slow process, and consequently the dynamics of geomorphological processes in karst are in general slower than within other geomorphic systems. Dissolution is regularly the dominant process in karst environments, but it can be substantially altered by other geomorphological processes, such as intense mechanical weathering encompassed by mechanical erosion or intense slope processes (Ford and Williams, 2007). Characteristic forms associated with karst areas are a variety of karren, karst depressions, and conical hills as well as large levelled corrosion plains and intramontane basins (Mihevc, 2010). However, a combination of karst and other processes resulted in distinctive surface features and karst functioning. As a consequence, different types of karst environments formed: coastal type along the coasts, contact type alongside hydrological active contacts between carbonate and noncarbonate rocks, fluviokarst in the areas of high mechanical weathering of the surface and glaciokarst in high elevated plateaux affected by glaciations (e.g., Gunn, 2004, Ford and Williams, 2007).

Glaciokarst areas of the temperate zone are situated within higher altitudes, which regularly have the characteristics of plateaux or systems of glacial valleys (e.g., Herak, 1972, Kunaver, 1983, Smart, 1986, Mihevc, 2010, Adamson et al., 2014, Žebre and Stepišnik, 2015). Glaciations shaped the karst surface leaving abundant glacial erosional and depositional forms as well as a variety of closed depressions, which are a typical result of the combination between glacial and karst processes (Smart, 1986). Poljes are the largest enclosed depressions in karst terrains, with a flat floor in rock or in unconsolidated sediments, steeply rising marginal slope at least on one side, and karstic drainage (Gams, 1978). However, in many cases the entire glacial outwash deposits were accumulated in those extensive surface depressions located in the mountains or at their footslopes, forming fans or wide plains within the karst terrain. These types of depressions were studied by Gams (1978), who defined them as piedmont-type poljes according to the hydrological classification system. Many studies are concerned with piedmont-type poljes (e.g., Gams, 1963, Giraudi et al., 2011, Jiménez-Sánchez et al., 2013, Adamson et al., 2014, Stepišnik, 2014) even though they rarely term them so. Based on the study of Velo polje in the Julian Alps in Slovenia (4 in Fig. 1A), Gams (2003) stressed that a piedmont polje formation is limited exclusively to the areas where proglacial and glacial deposits have partially filled earlier karst depressions. During his studies he noticed that glacial and periglacial materials filling those poljes were accompanied with peat and clayey laminated deposits (Gams, 1963). Those fine-grained deposits were interpreted as a result of a lake that inundated the depression in post-glacial time because of the extensive impermeable glacial and periglacial fill (Gams, 1963).The study of the Campo Felice in Italy (5 in Fig. 1A) (Giraudi et al., 2011) provides evidence of several glacial events affecting polje development. During glacial events, the polje was inundated while interglacial stages were characterised by lake recession. The Comeya polje in Picos de Europa in Spain (6 in Fig. 1A) hosts torrential and proglacial lacustrine deposits within the polje floor. The end of lacustrine deposition was followed by the onset of peat deposition, which was established to an age between 14,734 ± 326 and 9,109 ± 74 cal BP (Jiménez-Sánchez et al., 2013). Recent studies of piedmont type poljes surrounding Orjen Mountain in Montenegro (1 in Fig. 1A) (Adamson et al., 2014) revealed that they were filled mainly with outwash deposits during Marine Isotope Stage (MIS) 12, when the largest glaciation covered the mountain with an ice cap and when glacial and fluvial systems were strongly coupled. At that time, very efficient surface meltwater and sediment transfer systems were established. In the following glaciations, the coupling between the glacial and fluvial systems became weaker, creating a much less efficient sediment delivery system and a domination of karst vertical drainage over surface runoff (Adamson et al., 2014). On the other hand, the study of Krasno polje at the footslopes of the Velebit Mountains in Croatia (2 in Fig. 1A) (Stepišnik, 2014) showed that the polje was filled with sediments from at least two different glacial periods. In fact the polje floor is filled not only with outwash deposits but also two parallel moraine ridges preserved on the surface, indicating that the polje floor was covered by a glacier at least twice. In addition, the presence of floodplain deposits was also reported from the lowest sections of the polje (Stepišnik, 2014). These findings indicate that piedmont poljes are of great importance for studying the glacial history of karst terrains. They act as sediment traps, preserving a record of glacial events. Our research is concerned with a piedmont-type karst polje with toponym Gomance, located between Snežnik and Gorski Kotar plateaux in the Dinaric karst (3 in Figs. 1A; 2). The latter is a typical example of a well-developed karst where chemical dissolution acts as the major geomorphological process. However, during the most extensive Quaternary glaciation the Snežnik and Gorski Kotar plateaux hosted ice fields, covering an area of at least 140 km2 (Žebre and Stepišnik, 2015) (Fig. 2A). Several outlet glaciers, one of them also reaching the Gomance polje, were draining the two ice fields.

The Quaternary filling of Gomance was already mentioned in the early twentieth century (Krebs, 1924, Cumin, 1927, Melik, 1935) with different genetic interpretations. The most detailed research, not only in the Gomance area but also encompassing the entire Snežnik Mountain, was made by Šifrer (1959). He identified large lateral and terminal moraines in the Andrinova draga Valley and outwash fans covering the Gomance polje. The latter were ascribed to the presence of two glaciers related to the Last Glaciation: one flowing from the northeast and the other one from the east. He highlighted the presence of lacustrine deposits in the southwesternmost part of the polje. The same author claimed that up to 2-m-high terraces above the bottom of the polje covered by lacustrine deposits are the result of springtime high waters. He concluded that the outwash deposits are not so widespread in comparison to the large extent of palaeoglaciers, which he explained as owing to the vertical karst drainage below the glaciers. More than 40 years later, a scapula of Bos primigenius was found in the proximal outwash deposits on the Gomance polje in quarry q3 (Fig. 3B), 4 m below the surface. It yielded a radiocarbon age of 17.1 ± 0.4 14C ka BP (Marjanac et al., 2001), which is equivalent to a calendar age of 17.7–19.7 cal ka BP (Reimer et al., 2013).

The present research discusses a glaciokarst environment where glacial and outwash systems influenced the development of the Gomance karst depressions. This article has four key aims: (i) to present sedimentological and geomorphological evidence for glaciation of Gomance and provide a context with the surrounding formerly glaciated area, (ii) to use ground penetrating radar (GPR) data in order to refine the interpretation of sedimentological data, (iii) to discuss the relationship between karst and glacial processes, and (iv) to reconstruct the glacial history of Gomance.

Section snippets

General overview of the study area

The Dinaric karst, the largest continuous carbonate karst area in Europe, is situated within the western Dinaric Alps (Lewin and Woodward, 2009). The study area corresponds to the northwesternmost part of the mountainous Dinaric karst, called Snežnik and Gorski Kotar high karst plateaux. The highest peak is Veliki Snežnik (1796 m asl) (Fig. 2).

The Gomance polje (Fig. 2) is bounded to the northwest by Snežnik Mountain and to the southeast by Gorski Kotar. From a geotectonical point of view, the

Methods

An interdisciplinary approach, combining geomorphological mapping as well as GPR measurements supported by hand-drillings and sedimentological studies, was applied in the research area.

Glacigenic features on the Gomance polje and its vicinity were described and mapped in the field using 1:10.000 (The Surveying and Mapping Authority of the Republic of Slovenia, 1996) and 1:25.000 (National geodetic survey of the Republic of Croatia, 1996–2010) topographic maps. Field mapping was supported by

Geomorphological and sedimentological setting

Almost the entire Gomance polje floor is flattened by deposits, slightly inclined toward the southwest where the depression ends with a more than 21-m-high bedrock ridge. The northeastern part of the polje is covered with two fans (Fig. 3) having an average slope angle of 2–3° in the apical part. Two ridges rise from the plain in the proximal part of the fans. Ridge r1 is made of bedrock and partially covered by scattered boulders, while ridge r2 is ~ 2 m high and ~ 100 m long and consists of loose

The evolution of the Gomance piedmont polje

Poljes are polygenic features that normally develop along regional tectonic structures (Ford and Williams, 2007), but their further evolution is controlled by a complex series of processes. Consequently, different types of karst poljes based on distinct classification systems have been recognised (Gams, 1963, Gams, 1978, Ford and Williams, 2007). The Gomance polje belongs to the piedmont-type polje, defined as those situated at the footslope of a mountain and filled by large quantities of

Conclusions

Gomance polje in the northern Dinaric Alps contains a sedimentological record of past glaciations and thus represents the key area for studying piedmont type poljes. The formation of the Gomance polje was strongly affected by the nearby presence of the Snežnik and Gorski Kotar ice fields. Evidence of two clear sedimentary phases in the area of the polje was found through sedimentological, geomorphological and geophysical methods, accompanied by previous radiocarbon dating. The first phase was

Acknowledgements

We express gratitude to P.D. Hughes for very helpful comments to an early version of the manuscript. We would especially like to thank the Editor, Richard A. Marston, for his time and effort dedicated to improving the quality of this paper, as well as Jesús Ruiz Fernández, David Loibl and other three anonymous referees for their constructive comments. We also acknowledge Stacey Adlard and Iain Rudkin for helping us with the English revision.

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