Jame brez stropa so bile stare jame, ki so zaradi znižanja kraškega površja razkrite. Posebno pozornost so nam na to pomembno, tudi površinsko kraško obliko osredotočila zemeljska dela pri gradnji avtocest na Krasu.Unroofed caves are old caves which have appeared due to the lowering of karst relief. Groundwork for motorway construction on Kras has brought our attention to this important surface form of karst relief

Knez, Martin

Slabe, Tadej

English

ZRC SAZU Publishing (Znanstvenoraziskovalni center - Slovenske akademije znanosti in umetnosti)

103COBISS: 1.08Prejeto / received: 15. 7. 1999UNROOFED CAVES AND RECOGNISING THEMIN KARST RELIEF(DISCOVERIES DURING MOTORWAY CONSTRUCTIONAT KOZINA, SOUTH SLOVENIA)JAME BREZ STROPA IN NJIHOVO RAZPOZNAVANJENA KRA[KEM POVR[JU(ODKRITE PRI GRADNJI AVTOCEST PRI KOZINI,JUÆNA SLOVENIJA)MARTIN KNEZ1 & TADEJ SLABE21, 2 Karst research institute ZRC SAZU, Titov trg 2, SI-6230 POSTOJNA, SLOVENIAACTA CARSOLOGICA 28/2 5 103-112 LJUBLJANA 1999104Acta carsologica, 28/2 (1999)Izvle~ek UDK: 551.44:625.7/.8(497.4 Kozina)Martin Knez & Tadej Slabe: Jame brez stropa in njihovo razpoznavanje na kra{kem povr{ju (odkritepri gradnji avtocest pri Kozini, juæna Slovenija)Jame brez stropa so bile stare jame, ki so zaradi zni‘anja kra{kega povr{ja razkrite. Posebno pozornost so namna to pomembno, tudi povr{insko kra{ko obliko osredoto~ila zemeljska dela pri gradnji avtocest na Krasu.Klju~ne besede: krasoslovje, gradnja avtocest, jame brez stropa, Kras, Slovenija.Abstract UDC: 551.44:625.7/.8(497.4 Kozina)Martin Knez & Tadej Slabe: Unroofed caves and recognising them in karst relief (discoveries duringmotorway construction at Kozina, South Slovenia)Unroofed caves are old caves which have appeared due to the lowering of karst relief. Groundwork for motorwayconstruction on Kras has brought our attention to this important surface form of karst relief.Key words: karstology, motorway construction, unroofed caves, Kras, Slovenia.105INTRODUCTIONUnroofed caves are old caves which have appeared due to the lowering of the karst surface. Thecaves are filled and preserved by predominantly fine-grained sediment. Flowstone and rockboundaries are frequently preserved in the caves.Excavation for motorway construction on Kras has brought to our attention this important surfaceform of karst relief (Knez & [ebela 1994; [ebela & Mihevc 1995; Kogov{ek & Slabe & [ebela1997; Mihevc & Zupan Hajna 1996; Slabe 1996, 1997a, 1997b, 1998) and has revealed numerousoccurrences of unroofed caves in karst relief. We also defined the characteristic forms of unroofedcaves in karst relief (Mihevc & Slabe & [ebela 1998).Old caves and shafts open up during motorway construction because of the fluctuation of thekarst relief (Kranjc 1997). Old caves are empty or filled with sediment. They formed as part of acave network while the aquifer was still dammed up to a high level by impermeable rock, causingthe underground water table level within to be high. Due to karst processes, the water table level inthe aquifer declined and at present is at 200 metres or more below the surface, causing the karstrelief to lower.Unroofed caves may therefore be observed as particular surface karst forms partially transformedby surface processes and are an important element of the epikarst.Characteristic forms of unroofed caves were observed during preparation for the constructionof the motorway at Kozina. Single passages and larger cave networks on horizontal or inclinedkarst reliefs were discovered.CAVES DISCOVERED DURING MOTORWAY CONSTRUCTIONWe discovered 56 caves in 6 kilometres on a section of the motorway course between Kozinaand Diva~a. Prevalent are old caves through which water flowed during the transformation of thispart of the aquifer from the phreatic into the epiphreatic zone. Three caves were empty, while therest were filled with fine-grained flysch sediment, pebbles and gravel. The passage diameters rangedup to five metres. The greater part of the old caves were unroofed caves, caused by the lowering ofthe karst relief. The largest cave network, located at the beginning of the motorway course, wascomposed of empty passages; one of these was discovered before excavation had been carried out,and the passages were filled with sediment and had a more or less thin ceiling, or were withoutceilings. Vertical shafts through which water from the permeable surface percolates were mainlydiscovered at the bottom of dolines beneath the soil and sediment, but their openings were smalland did not allow access.UNROOFED CAVES AND KARST RELIEFKarst relief surrounding the course of the motorway between Kozina and Klanec is distinct inits variegated geologic structure. The road more or less traverses the different lithological andstratigraphical belts of Cretaceous and Paleogene carbonates and Eocene non-carbonates.In the north-to-south direction at Kozina we found Turonian and Senonian Cretaceous limestonesM. Knez & T. Slabe: Unroofed caves and recognising them in karst relief (discoveries during motorway construction...106Acta carsologica, 28/2 (1999)followed by beds of Liburnian formation. Among them in the lower part are Cretaceous Vremebeds followed by Palaeocene Kozina beds, Thanetian milliolid limestone and Eocene alveolinid,nummulitid and alveolinid-nummulitid limestones. The motorway crosses a narrow impermeablebelt of flysch rocks (marlstone with crustaceans) for the first time to the west of Klanec. After lessthan 100 metres its course returns onto alveolinid-nummulitid limestone. The final section of thecourse does not cross karst landscape, as there marlstone with crustaceans and sandstones (flysh)are exposed.The surface karst morphology and morphogenesis is most likely the reflection of thelithostratigraphic foundation in this part of the karst. Karst relief is generally flat on the southernsection of the course, where Paleogene limestones prevail. It is only slightly wavy and withoutsharp karst forms indicating surface erosion. By contrast, karst relief on the northern part of thecourse, north and east of Kozina, has intensively eroded the lower Cretaceous rock surface withFig. 1: Caves filled with flowstone and sediment.1. road cut, 2. paleogene limestone, 3. caves filled with sediment and flowstone.Sl. 1: Jame zapolnjene s sigo in naplavino.1. cestni usek, 2. paleogenski apnenec, 3. jame zapolnjene z naplavino in sigo.107numerous karren and various surface depression.During the motorway planning stage we defined the depressions that cut through the slopes ofthe dolines, then continued away from them for several tens of metres as unroofed passage. Roadconstruction confirmed most of these suppositions.A narrow and shallow depression was found in the slope of a large doline behind a gas station atKozina; we knew of its location from observing it in the field as well as from the map. Excavationrevealed fine-grained sediment filling the cave. Part of the passage was also uncovered at the boundaryof the bottom of the doline; on the opposite slope a passage was cross-sectioned. Both were filledwith fine-grained sediment. The dolines seem to have developed in the middle of a cave network.On the relatively steep hillside overlooking Klanec, the beginning of a horizontal passage filledwith fine-grained sediment appeared on the surface as a small half-doline (Fig. 1). Excavation todig a passage revealed a higher and a lower lying passage as part of a cave network.Fig. 2: Unroofed caves and dolines.1. unroofed caves, 2. dolines,  3. flowstone.Sl. 2: Jame brez stropa in vrta~e.1. jame brez stropa, 2. vrta~i, 3. siga.M. Knez & T. Slabe: Unroofed caves and recognising them in karst relief (discoveries during motorway construction...108Acta carsologica, 28/2 (1999)The largest cave network was uncovered at the beginning of the new section of the route andappeared on the surface as a network of more or less distinct depressions (Fig. 2, Fig. 3) that weremore discernible on the slope of the doline, as a connection among them, and as smaller, relativelyshallow dolines. The cave network was, besides the parts without ceilings (Fig. 4), composed of asmall hollow passage — its existence was known before excavation revealed it — and of passagewith ceilings (Fig. 5). Most of the cave was therefore filled with fine-grained sediment. The bottomof the south-western part was almost fully covered with flowstone, stalactites and stalagmites. Theflowstone was also covered with fine-grained sediment. Sharp-edged debris covered the surface orfilled caves in some locations; frequently it was found among old flow sediments or filling thehollow below shelves in the beginnings of passages. The debris had formed by the rock surfacebreaking up during the cold Pleistocene epoch. The bottoms of the larger dolines (Fig. 2, Fig. 3) thatformed at the edge of an old cave network were covered with layers of brown and red soil a fewmetres deep. Rock formations located on their edges indicate distinct downward flow of water.Entrances into narrow vertical shafts were also located on the bottoms. Unroofed caves developinto more distinct depressions if the flow of water is able to transport sediment from the surface,which is frequently enabled by the vicinity of a doline.On a section of the route between Kozina and Klanec the dolines are positioned in a line. Wefound pieces of flowstone and sediment at the bottom of these dolines. At the edge of a doline nextto a belt of flowstone, we dug into a passage with a diameter of over five metres. It was filled withsharp-edged gravel, stalactites and stalagmites. During drilling we also found flowstone in one ofthe depressions cut into the slope of one of the dolines.Flowstone was found at many locations on the route. The flowstone discovered on the rocksurface following the removal of soil layers, was one of the first “warnings” to observe furtherdigging. We expected to find other sediment in the vicinity, vertically and laterally. In thisenvironment we most frequently discovered cave sediment.In the northern part of the route, on Cretaceous rock, we discovered the largest heaps of flowstonein terms of surface and thickness. After it was dug out, it was mostly revealed as variously beddedflowstone. Stalactites and stalagmites were hardly present, as they were probably removed togetherwith the sediment or had been crushed and removed earlier. Flowstone in this area is mainly orange-brown in colour, with much less frequent occurrence of white, meat-red and grey-yellow colouredFig. 3: Cross-section through unroofed caves and dolines with shafts at their bottom.Sl. 3: Pre~ni prerez ~ez jame brez stropa in vrta~e, ki imajo na dnu brezna.109Fig. 5: Cave filled by fine-grained sediment and gravel.Sl. 5: Jama, ki je zapolnjena z drobnozrnato naplavino in gru{~em.Fig. 4: Unroofed cave.Sl. 4: Jama brez stropa.M. Knez & T. Slabe: Unroofed caves and recognising them in karst relief (discoveries during motorway construction...110Acta carsologica, 28/2 (1999)flowstone. The flowstone alters a great deal in short distances in terms of morphology, colour andstate of preservation. Despite this, the flowstone is relatively well preserved as the process ofweathering has not yet reached it due to good protection from the surface effects. Most frequentlyfound were column-shaped calcite crystals up to a few centimetres in size, in rare cases over 10centimetres.We discovered completely black flowstone of smaller dimensions on the P11 profile (approachKozina road); this was partially weathered and non-homogenous. It contained a large amount ofnon-carbonate insertions. A large amount of yellow-grey cave sediment was also contained in alarger flowstone deposit near the P335 profile. The basic structure of the flowstone was not weathered.Cave sediment later filled the cavities in the otherwise very porous flowstone.North of the motel at Kozina, still in the northern section of the route, an extremely homogenousand compact bed of orange-brown flowstone was found by drilling. Based on the presence of agroup of active dolines in the area, we correctly presumed that an old cave network appeared in thisarea.Flowstone appears in considerably smaller amounts in the middle and southern parts of theroute. Individual locations are less frequent, although not to be disregarded. Because of the attentiongiven to locating flowstone on the surface, we were able to describe some underground karstphenomena in a road cutting over 20 metres deep.Flowstone composition and form are also quite various in the middle and southern parts of theroute. We mention only the discovery of a large heap of yellow-white bedded flowstone in alveolinic-nummulitic limestone. The flowstone stands out in its colour, the way of crystallisation of the calcitecrystals, and its state of preservation. It was discovered only a few metres below the surface, almoston the top of the hill near profile P522.CONCLUSIONUnroofed caves are an important karst formation, a part of the karst surface and epikarst, aswell as significant evidence of the development of the karst. Their forms are a consequence of thecross-section of lowering karst relief with cave networks. Water from the surface also sweeps awaycave sediment, which is why unroofed caves frequently hollow out karst relief. In many cases theyare complex karst formations. Dolines develop in the middle of old passages or on their boundaries.In these cases the unroofed caves have forms similar to dolines. A number of them can be connectedinto a series. Open semicircular forms similar to dolines form when the slope is cut into by an oldsediment-filled passage.There can also be old depressions on flat terrain or located on the slopes of dolines. In mostcases there is no rock on them and, in cases where one is unable to find evidence of cave sedimentand flowstone, they may be distinguished from the rest of the surface by the vegetation coveringthem. Besides the lithostratigraphical composition of the epikarst, which is a major influencingfactor on surface morphogenesis, the appearance of flowstone on the surface and on rock relief canin many cases indicate the existence of an old (and present) underground morphology.Unroofed caves become clear and explainable surface karst phenomena through observation ofcharacteristic forms, and ever more familiar forms: oblong depressions and doline-like forms onflat or inclined terrain, series of dolines and flowstone belts.111REFERENCESKnez, M. & S. [ebela, 1994: Novo odkriti kra{ki pojavi na trasi avtomobilske ceste pri Diva~i.-Na{e jame, 36, 102, Ljubljana.Kranjc, A. (ed.), 1997: Slovene Classical Karst.- ZRC SAZU, pp 254, Ljubljana.Kogov{ek, J. & T. Slabe & S. [ebela, 1997: Motorways in Karst (Slovenia).- Proceedings & aFieldtrip excursion guide, 48th higway geology symposium, 49-55.Mihevc A. & N. Zupan Hajna, 1996: Clastic sediments from dolines and caves found during theconstruction of the motorway near Diva~a, on the Classical karst.- Acta carsologica 25,169-191, Ljubljana.Mihevc, A. & T. Slabe & S. [ebela, 1998: Denuded caves.- Acta carsologica 27/1, 165-174, Ljubljana.Slabe, T., 1996: Karst features in the motorway section beetwen ^ebulovica and Dane.- Actacarsologica 13, 221-240, Ljubljana.Slabe, T., 1997a: Karst features discovered during motorway construction in Slovenia.-Environmental Geology 32/3, 186-190, Springer-Verlag.Slabe, T., 1997b: The caves in the motorway Dane-Ferneti~i.- Acta carsologica 26/2, 361-372,Ljubljana.Slabe, T., 1998: Karst features discovered during motorway construction between Diva~a andKozina.- Acta carsologica 27/2, 105-113, Ljubljana.[ebela, S. & A. Mihevc, 1995: The problems of construction on karst - the examples from Slovenia.-In: Beck B.F., Pearson F.M. (eds) Karst geohazards, enginnering and environmental problemsin karst terrane. Proceedings of the Fifth Multidisciplinary Conference on Sinkholes andthe Engineering and Environmental Impacts on Karst. A.A. Balkema, Rotterdam, TheNetherlands, 475-479.JAME BREZ STROPA IN NJIHOVO RAZPOZNAVANJE NAKRA[KEM POVR[JU(ODKRITE PRI GRADNJI AVTOCEST PRI KOZINI, JUÆNA SLOVENIJA)PovzetekJame brez stropa so bile stare jame, ki so zaradi zni‘anja kra{kega povr{ja razkrite. Ohranjajopa jih naplavine, predvsem drobnozrnate, s katerimi so zapolnjene. V njih sta pogosto ohranjenatudi siga in skalni obod.Posebno pozornost so nam na to pomembno, tudi povr{insko kra{ko obliko osredoto~ilazemeljska dela pri gradnji avtocest (Knez & [ebela 1994; Slabe 1996, 1997a, 1997b, 1998; Mihevc& Zupan 1996; Kogov{ek & Slabe & [ebela 1997), ki so razkrila {tevilne razli~ice pogostegapojavljanja jam brez stropa na kra{kem povr{ju. Izlu{~ili smo tudi zna~ilne oblike jam brez stropana kra{kem povr{ju: posamezne vrta~am podobne oblike, vrta~am podobne oblike, ki so povezanev nize in jarke (Mihevc & Slabe & [ebela 1998).Zaradi ni‘anja povr{ja Krasa (Kranjc 1997) se pri gradnji avtocest odpirajo stare jame in brezna.Stare jame so prazne ali pa zapolnjene z naplavinami. Nastale so kot del spleta votlin, ko je bilvodonosnik {e visoko zajezen z neprepustnimi kamninami in zato je bila vi{je tudi gladinaM. Knez & T. Slabe: Unroofed caves and recognising them in karst relief (discoveries during motorway construction...112Acta carsologica, 28/2 (1999)podzemeljske vode v njem. Zaradi zakrasevanja se je zni‘ala gladina vode v vodonosniku, zdaj 200in ve~ metrov globoko pod povr{jem, in zni‘uje se kra{ko povr{je.Jame brez stropa lahko torej razbiramo tudi kot svojevrstne povr{inske kra{ke oblike, delomaso preoblikovane s povr{inskimi procesi in so pomemben del epikrasa.Zna~ilne oblike jam brez stropa smo razbrali tudi pri zemeljskih delih ob gradnji avtoceste priKozini. Odkriti so bili posamezni rovi in ve~ji jamski spleti na vodoravnem ali nagnjenem kra{kempovr{ju.Jame brez stropa so pomembna kra{ka oblika, del kra{kega povr{ja in epikrasa in pomembnasled razvoja krasa. Njihova oblika je posledica prereza zni‘ujo~ega kra{kega povr{ja z jamskimispleti. Voda s povr{ja odna{a tudi jamsko naplavino in jame brez stropa so zato pogosto vdolbljenev kra{ko povr{je. Ve~krat so sestavljene kra{ke oblike. Sredi starih rovov ali pa na njihovem robuso nastale vrta~e. V tem primeru imajo jame brez stropa obliko podobno vrta~am.Ve~ jih je lahkopovezanih v nize. Odprte polkrogelne, vrta~am podobne oblike nastajajo, ko pobo~je preseka starrov, ki je zapolnjen z naplavinami. Lahko pa so tudi samostojne zajede na ravnem povr{ju ali pa~lenijo boke vrta~. Na njih najve~krat ni skalovja in od ostalega povr{ja se lo~ijo po rastju, seveda,~e na povr{ju ni razbrati tudi jamskih naplavin in sige. Poleg litostratigrafske sestave epikrasa, kibistveno vpliva na povr{insko morfogenezo nam tudi pojavljanje sige na povr{ini lahko nakazujemo‘nosti nekdanje (pa tudi sedanje) podzemeljske morfologije.Po zna~ilnih, vse bolj znanih oblikah: zajede, vrta~am podobne oblike na ravnem ali nagnjenempovr{ju in nizi vrta~ ter proge sige, postajajo jame brez stropa vse bolj razlo~ljiv in berljiv povr{inskikra{ki pojav.

Unroofed Caves and Recognising them in Karst Relief (Discoveries during Motorway Construction at Kozina, South Slovenia)

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Unroofed Caves and Recognising them in Karst Relief (Discoveries during Motorway Construction at Kozina, South Slovenia)

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