engleski

Karst lanforms on the Island of Krk

In this paper karstic relief, with an emphasis on distribution of dolines and linear forms will be presented. The Island of Krk is one of two biggest Adriatic islands (405 km2). It is located in the north-eastern part of the Adriatic Sea channel area, between the Istrian peninsula and the Vinodol –Velebit coast. This island consists mainly of carbonate rocks: Cretaceous limestones, dolomitic limestones and dolomitic breccias, Palaeogene limestones and carbonate breccias. Outcrops of siliciclastic rocks (marls and flysch) are restricted in relatively narrow, compressed and isolated zones. This island is part of the External Dinarides, and its major orographic axis and geological structures have a Dinaric strike (NW–SE to NNW–SSE). This dominant strike of tectonic structures has been disturbed by younger diagonal and transverse strike-slip faults during the Pliocene and Quaternary under the influence of re-oriented, a neotectonic regional stress orientated approximately N–S (VLAHOVIĆ et al., 2005). Due to this, carbonate rock mass are partially extremely deformed and fissured. Present landscape of the Island of Krk is a consequence of simultaneous influence of tectonic movements, and climatic and sea-level changes during Pliocene and Pleistocene. Due to complex tectonic fabric karst forms that occur on the Island of Krk are numerous and various. Diversity of karst forms has been previously noticed by BENAC et al. (2013). Using a combination of topographic maps (scale 1:5.000), geometrically corrected aerial photography (in resolution 0.5 m, dating in 2004) and field work all together 5.610 depressions and more than 742 km of linear forms, both of various types, have been detected on carbonate rocks. Depressions are mainly dolines of accelerated dissolution type, partly or completely filled with sediments. They reach the density of 61 dolines/km², with average distance 〖dist〗_avg = 108 (with simmilar values in entire area) and median diameter d_m = 48 m. With decrease of their size the dispersion of dolines distribution over entire area markedly increases. It is evident that dolines developed quite independently of lithology. The main factor of their distribution seems to be recent relief configuration ; however, they have been detected even under Quaternary deposits. On spatially opposite locations, mainly on slopes with higher inclination, appear valley-like forms of tectonic, paleofluvial, tecto-karstic, karstic and not yet completely defined origin (Fig.1). Linear forms of tectonic origin are young surface forms, formed mainly by gravitational processes. Morphometrically they are short and unbranched, and developed on steep slopes. They clearly show recent tectonic activity of the area on north-eastern edge of the island and in the south. Tecto-karstic linear forms can seem fluvial in appearance, but have developed by accelerated corrosion along geologic structures with increased permeability. The most obvious tecto-karstic forms developed along strata with high dip. The majority of linear forms on carbonates however follow principles of fluvial erosion: some well-branched systems with direction towards local baselevel reach a length of even 15 km. These old well branched paleofluvial networks with complex drainage patterns (2nd or 3rd order) developed in recently tectonically inactive areas, some of them nowadays being located on high karst plateaus. These forms are now inactive in fluvial sense but are still subject to processes of karstification. They developed characteristic smooth relief with lack of accumulated fluvial and gravitational material. Some of them were strongly tectonically disturbed and have adapted to the drop of groundwater level by cutting canyons in their lower parts of the streams. Clear evidences of contact karst (blind valleys) are rare. Denuded caves which also appear as shallow irregular linear forms with traces of sinter were located in the south of the area. The majority of valley forms on carbonates are recently dry. Partly and periodically active are solely valleys which cross zones of non-carbonates. Findings of sandstone remnants in the upper parts of valleys proved that non-carbonates covered much wider areas, or that compressed zones of non-carbonates were much more numerous than it was thought. This might have partly contributed to development of complex drainage networks in this bare-rock landscape of today. The distribution of dolines and valley forms clearly express the competitive relationship between point and linear geomorphic processes. However, the origin of all types of linear forms has not been satisfactory explained yet. Even though relief forms follow the principle of surface runoff, deposit sediments and form complex drainage networks, there have been clear traces of depth karstification found in the same regions as well.

karstic valley; spatial analysis; Adriatic Sea

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