GEOMORPHOLOGICAL FEATURES OF THE BAŠKE OŠTARIJE KARST POLJE

The Baške Oštarije karst polje is stretching out in the direction E-W at length of 3.75 km, while the width is varying from 0.25 to 2.0 km. It divides the central from the south part of Velebit mountain (Croatia) and it is formed at the contact of two orographic wholes. Besides karst and fluvio-karst processes, the derasion and periglacial processes have been most expressed at the relief formation. The water which flows down from edge parts of polje has great importance in relief formation because of its corrosional aggressivity.


Introduction
Oštarijsko polje divides the middle and southern part of Velebit Mt. representing at the same time one of the important ridges.It extends east-west for 3.75 km and its width varies from 0.25 to 2.0 km (Fig. 1).It was developed on the contact of two orographic units which in morphostructural context represent conformal denudationaltectonical units.The influence of exogenic processes on the relief formation becomes distinct primarily by selective denudation.On steeper parts of the flanks of Oštarijsko polje, where clastics and dolomites predominate, the most prominent relief forming processes are slope (derasional) destructive and accumulation processes.
Regarding that this polje was formed on the contact of more permeable and less permeable rocks, it can be considered as a marginal field in the karst according to Gams (1974Gams ( , 1978)).The main role in shaping of Oštarijsko polje is attributed to the water which comes down from the marginal noncarbonate areas and periglacial processes during glacials.Namely, the parts that were formed in Permian and Triassic clastics formed faster because those rocks were an easier subject to mechanical erosion Fig. 1 The view from the west toward eastern part of polje Sl. 1. Pogled sa zapada prema istočnom dijelu polja

Methods of research
Field investigation was the basic method.Only direct observations in the field could enable identification of particular geomorphological processes and relief types.The field work also yielded data on mutual relationship of petrographic composition of rocks, hydrological characteristics and climatic conditions within the processes of relief formation of Oštarijsko polje.

General morphological characteristics
Oštarijsko polje, in fact, represents a ridge that divides Middle from Southern Velebit Mt.It was formed on the contact of two orographic units which are conform morphostructures of denudational-tectonical type (BOGNAR, 1994(BOGNAR, , 1999(BOGNAR, -2001)).Oštarijsko polje extends east-west for 3.75 km and its width varies from 250 m on the east side (till the Linići village) to 2 km on the west side.Polje is located 900 m a.s.l. at the west end, while it raises up to 980 m on the north in the area of Stupačinovo (Fig. 2, 3, 4 and 5).The origin of polje was preconditioned by tectonics, namely Brušane fault striking WNW-ESE along the marginal northern part of the polje, and Oštarije fault of E-W strike along the southern margin.Regarding the fact that Brušane fault cuts a fundamental monoclinal structure of Velebit Mt., exactly in the area of Oštarijsko polje, the zonal arrangement of lithostratigraphic units or members is therefore disturbed by horizontal movement and distorsion of Brušane-Oštarije anticline (SOKAČ ET AL, 1970, 1976).It can be supposed that the crest of the anticline was fractured by horizontal strikeslip movement, which enhanced the effect of selective denudation (mechanical erosion, derasion, corrosion and fluvial erosion).In eastern part of the polje, along the Brušane fault, impermeable and low permeable sediments of Carbon, Permian and Lower Triassic crop out (PAVIČIĆ, 1995).Water that springs there (Ljubica Creek) runs towards the lower west part of the polje where approaching the fractured zone of partly impermeable rocks (Anisian limestone and dolomite) gradually sinks.

. Poprečni i uzdužni profili Oštarijskog polja
Interpretation of the origin of the highest, northern part of the polje represents a special problem.It is most probable that the bend visible from Prpići village (on the west side) eastwards, was conditioned by a reverse fault.At that place the most intensive bending of Oštarije-Brušane anticlinal structure, which is, at that place, also fractured along the transverse fault striking N-S.Sink holes of the Ljubica Creek are connected to this fault (Fig. 6).

Exogenic relief
The influence of exogenic processes on relief formation stands out primarily by selective denudation.Triassic and Permian dolomites show strong physical erosion, and even corrosion during wet periods of spring and fall.In the areas with Paleozoic and Lower Triassic clastics processes of flushing and trenching predominate, as well as fluvial erosion and accumulation.As a consequence of lower permeability of Middle and Upper Triassic dolomites, these deposits are shaped by fluvial erosion.In the most northwestward and south-eastward parts of the studied area, where Jurassic limestones predominate, the corrosion processes predominate in relief formation while mechanical disintegration and flushing become distinct only there where dolomites occur, or they alternate with limstones.In general, besides lithology, tectonics had a great impact on intensity and trend of exogenic processes.
Slope processes and forms.On the steeper slopes of Oštarijsko polje, where clastics and dolomites predominate, the most prominent are slope destruction and accumulation processes (derasion).Especially noticeable are the processes of rock-fall, collapsing, washout, trenching and creeping.
The processes of rock-fall and collapsing are mainly preconditioned by temperature oscillations between day and night.They are connected to tectonic escarpments of Dabarski Kukovi and Filipov Kuk.On the steep slopes of Dabarski Kukovi smaller gullies formed with talus beneath, composed of fairly sorted angular debris.
Processes of gullying are most intensive on the slopes of Badanj and Basača uplands, northwest from the polje bottom (near Prpići village) where Triassic dolomite prevails.Intensive gullying processes are also visible on hillsides of the eastern and middle part of the polje where Permian Lower Triassic clastics and dolomites occur.The length of gullies varies from few tens to few hundreds of meters.They commonly end with smaller proluvial (torrent) fan and occasionally form derasion valleys.
They were formed by widening of older gullies (probably of Pleistocene age) with rainfall waters.In the foothills of Badanj and Basač uplands the gullies are partly incised in colluvial cones.Intensive flushing by rainfall waters produced gullies and colluvial cones.Colluvial cones are also visible in the foothills of Dabarski Kukovi, Ljubačko brdo, and on southern slopes of Debela Kosa, Jelar, Ivić Vršak and Glavičica uplands.Their composition is not easy to observe because they are covered with grass.An exception is the area of Ljubica Creek, next to the ski lift, where in open profiles coarse sand/gravel predominates.
Fluvio-denudational and fluviokarst relief.Predominance of impermeable rocks in the most eastern part of the polje, and partly permeable rocks in the middle of it, favoured the occurrence of intermittent or permanent springs of low capacity as a result of unfavourable pluviometric regime.Springs are productive in fall and winter, namely more humid part of the year due to heavier rainfalls, snow melt and lower evaporation.The springs feed smaller creeks from 250 to 500 m long.An exception is the Ljubica Creek running from eastern to central part of the polje.Other brooks are tributaries of Ljubica.In the central area of the polje Ljubica Creek flows into the area of microtectonically fractured carbonates, and sinks in a line of sieve-type.Open swallowholes lie down beneath Linić vršak upland.The main characteristic of the Ljubica Creek, as well as that of its tributaries, is the oscillation of discharge.Therefore, Ljubica Creek commonly floods the central polje during cold periods (near Linići village) and dries out during summer.Fig. 7 The Ripište with spring part of the Ljubica stream Sl. 7. Ripište s izvorišnim dijelom potoka Ljubice Differential tectonical movements conditioned a complex morphostructural development of the Ljubica Creek valley.Intensive microtectonical crushing resulted in formation of wider part of polje in Ripište area (Fig. 7).The central section of the valley became narrower, only 250 m wide, due to compression during the uplift of Linić Vršak and Sladovača blocks (Fig. 8).
Fig. 8 The narrowest part of the polje with the doline of the Ljubica stream Sl. 8. Najuži dio polja s dolinom potoka Ljubice Waterflow is tectonically preconditioned and turns from the E-W direction to N-NW direction and sinks beneath the west hillside of Linić vršak (Fig. 9.).Higher, eastern and northern section of polje bottom (up to 915 m a.s.l.) is mainly covered with residuum, slope material and alluvial deposits.Thickness of those deposits in the valley of the Ljubica Creek is up to a few meters.The most intensive sedimentation was during Pleistocene, as it was also in Rujansko Polje.
Fig. 9 The swallow-holes of the Ljubica stream Sl. 9. Ponori potoka Ljubice Karst relief.Northern, southern and western rims of the polje is characterized by the occurrence of carbonate rocks and Jelar breccias.Their microtectonical fractures enabled intensive corrosion processes and formation of karst features.The area of the north rim, precisely the escarpments and top parts of Dabarski Kukovi, is characterized by the occurrence of extreme karst with various karren types, while slope parts of lower gradient and west rim of the polje are characterized by the occurrence of rocky karst.Other parts of polje, like central and west part of the polje bottom, are characterized by occurrence of covered and semi-covered karst.Some parts above 1200 m that are exposed to strong winds, are characterized by crionival karst occurrence.Namely, lack of snow (the wind bora blows it away) as thermal isolator enables cryogenic processes and rock fracturation by freezing.
On the west and lowest side of the polje (the area of Šikić stanovi) where Lower Jurassic limestones predominate, many funnel-shaped sink-holes (ponikvas) occur.In the neighbouring, more eastwards part, where Upper Triassic dolomites predominate there are many pan-shaped sink-holes (ponikvas).In fact, due to a very low dip angle (2-5 °) a mechanical erosion of dolomites is distinct.In some parts, mechanical erosion goes even 5 m deep (Fig. 10).Fig. 10 The surface mechanical weathering of the dolomites Sl. 10.Površinsko mehaničko trošenje dolomita One of the polje characteristics is the lack of speleo-features, except the swallow-holes of the Ljubica Creek.The Ljubica swallow-holes are very narrow probably due to dolomite rock characteristics where they were formed.Meanwhile, it must be pointed out that speleo-features could have been buried with earth, rock debris and gravel during colder periods of Pleistocene when periglacial processes were dominant in relief shaping.Today, speleo-objects occur in higher areas, especially in the area of Brušane fault (GARAŠIĆ, 1981, LUKIĆ, 1992).

Conclusion
Oštarijsko Polje is a part of the main ridge and at the same time it orographically divides central from southern Velebit.It extends in the E-W direction and it is 3,75 km long and 0,25-2,0 km wide.
The origin of the polje is tectonically preconditioned by Brušane fault (north section) and Oštarije fault (south section).The most eastern section of the polje is built of impermeable rocks of Permian age which enabled occurrence of water on the surface.Further westwards partly permeable Triassic rocks occur, while the most westwards part comprises mainly highly permeable Jurassic deposits (PAVIČIĆ, 1995).Higher, eastern part of polje is filled with residuum, slope clays and alluvial deposits even several meters thick in the valley of Ljubica Creek.In the most west section where Jurassic limestones predominate, there are mainly semi-covered karst with numerous funnel-shaped sinkholes (ponikvas).
Regarding that this polje developed on the contact of more permeable and less permeable rocks, it can be considered as a marginal karst polje.Waters coming from marginal noncarbonate areas were very important for shaping the Oštarijsko polje because of corrosion.Flushed slope clays, residuum and other fine-grained accumulated material that fills the polje, also have great importance in increasing the corrosive activities.Water and higher CO 2 content (humic acids) increase the lateral widening of polje.Periglacial processes during glacials had also great influence.Namely, noncarbonate areas, because of stronger exposure to mechanical erosion, developed more quickly.