COBISS: 1.01 THE AGE OF KARST RELIEF IN wEST SLOVENIA STAROST KRAŠKEGA RELIEFA V ZAHODNI SLOVENIJI Andrej MIHEVC1 Abstract UDC 551.435.8 (497.4 Kras) Andrej Mihevc: Te age of Karst relief in west Slovenia Te age of a karst can be defned as the time when the karst rocks were uplifed out of the sea. Te other view of the age of karst is to defne the age of certain karst features or assem-blages of karst features. On the Kras plateau there is a variety of forms that were formed at quite diferent times, but due to karst evolution, they coexist in today’s relief. On the plateau, that is slowly rising, the hydrological zones in karst surface are moving downwards. Streams from the side ceased to fow on the karst and former leveled surface that was formed in condi-tions of high ground water is dissected by numerous dolines. Blind valleys are incised at the side and some of them show the infuence of recent tectonics. Te lowering of relief by corro-sion exposes caves that have formed deep beneath the surface and creates unroofed caves that become a part of the surface topography. From the morphological comparison of the un-roofed caves, blind valleys and levelled surfaces and by dating of the sediment and considering the age of tectonic phases we can reconstruct the evolution and estimate the age of the karst landscape. Key words: karst, morphology, age, Kras, Slovenia. Izvleček UDK 551.435.8 (497.4 Kras) Andrej Mihevc: Starost kraškega reliefa v zahodni Sloveniji Starost krasa lahko določimo s trenutkom, ko so bile kraške kamnine dvignjene iz morja. Drugi način opredelitve starosti krasa je z datiranjem reliefnih oblik ali skupin reliefnih oblik. Planoto Kras sestavlja vrsta zelo različnih reliefnih oblik, ki so nastale v različnem času, vendar so se zaradi posebnosti razvoja krasa ohranile in sobivajo v sedanjem reliefu. Na planoti, ki se počasi dviguje se hidrološke cone in kraško površje pomikajo navzdol. Vodotoki s strani so prenehali dotekati na kras in nekdanje v višini talne vode nastalo uravnano površje so razčlenile številne vrtače. Na robu krasa so vrezane slepe doline, nekatere od njih kažejo sledove tudi recentnih tektonskih premikov. Zniževanje reliefa zaradi korozije je razgalilo jame, ki so se oblikovale globoko pod površjem in ustvarilo brezstrope jame, ki so postale del današnje topografje površja. Z morfološko primerjavo brezstropih jam, slepih dolin in uravnav in datiran-jem sedimentov ter upoštevanjem starosti tektonskih faz lahko rekonstruiramo razvoj reliefa in ocenimo starost kraške pokrajine. Ključne besede: kras, morfologija, starost, Kras, Slovenija. INTRODUCTION Te question about time, like velocity of processes or age of karst surfaces and caves is a very important issue in karst studies. Te age and evolution of karst is also im-portant when we study karst as a specifc ecosystem. It can tell us when karst and especially the caves start to form in a given area and how the landscape is changing. The first explanation of geomorphic evolution and the age of the karst in w Slovenia were made by geologists. To estimate the age they used geologic data – the age of last marine sedimentation and the tectonic evolution of Dinaric mountains and the Alps (Grund 1914). 1 Karst Research Institute, ZRC SAZU, Titov trg 2, Sl – 6230 Postojna, Fax: +386 5 7001999, Andrej.Mihevc@guest.arnes.si Received/Prejeto: 01.02.2007 TIME in KARST, POSTOJNA 2007, 35–44 ANDREJ MIHEVC Fig. 1: Te location of the Kras plateau and the study areas. At frst karstologists were focused on understand-ing karst processes and the evolution of karst features like dolines, poljes and corrosion plains. Tey were much infuenced by the ideas of a geographic cycle promoted by Cvijić (1924). Karst evolution was divided into similar steps in the cycle but they also added a pre-karst phase of relief evolution with which they explained some mor-phological elements in karst. Te cyclic explanation of the karst evolution was lat-er modifed with climatic geomorphology (Roglič 1957, Radinja 1972). It emphasised the importance of climate on the morphological processes. Tis meant that some forms of relief, like conical hills and levelled surfaces were explained as a relicts from tropical climate. Because such a climate was present at the end of the Tertiary, these forms were determining the age of that relief features. Another important climatic signal in the morphol-ogy of the Kras they estimate were the cold Pleistocene climates with periglacial processes in lower positions. Scree slopes, collapses in caves, fuvial deposits in con-tact karst areas and some fner sediment were explained as extremes of climate control and not normal karst phe-nomena. Tey were also used for determination of the age of features (Melik 1955, Gospodarič 1985). 36 TIME in KARST – 2007 Geomorphologists have abandoned the cyclic model of relief and are now paying more attention to struc-tural elements in karst morphology like recent tectonic (Habič, 1982), feld measurements and observations on karst denudation (Gams 1963), comparative studies of diferent karst features or types of karst, like contact karst (Gams 1962, Mihevc 1994), the study of dolines and col-lapsed dolines (Mihevc 2001) and new geomorphologic features like unroofed caves (Mihevc 1996, 2001, Slabe 1997) as an important remnants of former landscapes and a source of sediments. Flowstones in the caves were dated (Hajna 1991, Mihevc 2001) and paleomagnetic methods were used in cave and karst sediments (Bosak & al. 1999, 2004). Very important data were provided by latest research on the plate tectonics. Te tectonic evolution of the area is characterized since late Tertiary frst by northward motion of Adria micro plate which caused contraction deformations. Te contraction was exhausted at about 6 Ma ago and was followed by rotation accompanied with uplifs, folding and strike-slip basins formation. Tese events take place in two distinct phases (Vrabec & al. 2006, Fodor & al. 1998). THE AGE OF KARST RELIEF IN w SLOVENIA GEOMORPHIC EVIDENCES ON THE AGE OF KRAS Kras is a low Nw – SE trending longitudinal plateau along Trieste Bay (Adriatic Sea) between fysch Brkini hills on SE, Vipava Valley in NE, and the Soča River low-lands in Nw. Te plateau is about 45 km long and 14 km wide. Te surface of the plateau is slightly tilted from 500 m a.s.l on SE towards NE where it ends at about 100 m above the Soča river. Te central part of Kras is built from highly per-meable Cretaceous carbonate platform shallow marine limestone and less permeable dolomite. Eocene fysch that acts as an important impermeable barrier surrounds the carbonate massif. Te age of the karst of Kras plateau can be defned as the time when the karst rocks were uplifed out of the sea. For the most of Dinaric karst in Slovenia this occurred afer the Eocene, since afer that there is there is no evidence of younger marine sediments. As soon as the carbonate rocks were exposed, we can expect that the karst was formed, but there are no remnants of karst features from that time. Most likely denudation has destroyed them. Te other view on the age of karst is to defne the age of those karst features for which we know how and when they were formed and which evolution was stopped long time ago. Such features are levelled surfaces, which evolve at the level of the karst water and blind valleys that were formed by alogenic rivers. we can compare them with evolution of fuvial relief and unroofed caves, which are caves exposed to surface by denudation. On the Kras plateau there is a variety of forms that were formed at quite diferent conditions and time but due to peculiarities of karst evolution they coexist in to-day’s relief. Tis can make the determination of the one age of a karst landscape difcult or impossible, but it tells us about the genesis of the landscape trough diferent phases. Here we present the study of the part of the Kras, Divaški kras and Matarsko podolje and the edge of Podgorski kras from which there are some evidences about the evolution and age of Kras. THE UNROOFED CAVES OF DIVAŠKI KRAS Te Divaški kras is tilted slightly towards Nw at eleva-tions between 450 and 400 m a.s.l, on the SE part of the Kras plateau. It is built up mostly by Cretaceous and Paleogene limestone. Te karst features here are exceptional; there are the sinking of the Reka river into Škocjanske jame cave via large collapse dolines with and hundreds of dolines. Te largest caves of the area are the 12,500 m long and 275 m deep Kačna jama and the 5800 m long and 250 m deep Škocjanske jame. Te caves were formed by the Reka river which can be reached at a depth of 195 m in Škocjanske jame and 156 m a.s.l. in Kačna jama. Te main morphologic features of the area are col-lapsed dolines and dolines which together cover about 12% of the area. Te collapsed dolines are connected with active water caves. Te solution dolines cover less than 4% of the area. Te rest of the surface (88%) is level. Tese points out the prevailing surface leveling process in the present conditions In this levelled surface there are several large un-roofed caves (Mihevc 1996). As such caves appear on the surface due to denudation, and we may call their remains denuded caves. A cave ceiling will be the frst to be removed by denudation, which is why they are also called unroofed caves. Tey were frst found and described in the Divača Karst. Te unroofed caves share on the surface is small, only about 0.16% of the entire surface. Tree important unroofed caves have been found. Te frst is a 350 m long unroofed cave near Povir village at 400 m above the sea level. Tere is a remnant of a cave passage that was 6 m wide and over 5 m high. Te entire volume of the passage has been flled by allochtonous fuvial sediments of clay, silicate sands and gravel with pebbles up to 25 cm in diameter. Te second is an unroofed cave near Divača on the slopes of doline Radvanj at the altitude of 390 - 415 m above sea level. It is exposed on the slope that dissects large cave passage, which is entirely flled with sediments. Similar sediments can be seen in the Divaška jama cave. Tis is a 600 m long cave, whose continua-tion towards 250 m distant unroofed cave is completely flled. Te cave was also flled, but the sediment was later washed from it by the seepage water. Here we can see that a part of the unroofed cave that still exists as an underground cave. Te longest roofess cave is 1.800 m long remnant of caves whose passages were about 20 m large, and therein few a great underground river. Te cave was flled with fuvial sediments and massive fowstone. It is located TIME in KARST – 2007 37 ANDREJ MIHEVC Fig. 2: Te map of the divača karst. On the levelled surface the large collapse dolines are dominating features, solution dolines are frequent, but they represent only small proportion of the surface. Te outlines of the main caves and the main unroofed caves are marked. On the map made of dEm with 12.5 m grid the road cuts or causeways are also seen. Legend: 1. Outline of the active river caves, 2. divaška jama cave, 3. Unroofed cave, 4. Unroofed caves mentioned in the text: A: Unroofed cave near Povir, b: Unroofed cave in doline Radvanj, continuation of divaška jama, C: Unroofed cave above škocjanske jame, 5. height of the surface, 6. height of the water level in caves, 7. Reka river and ponors, 8. Te supposed direction of water fow, 9. Outline of the town divača. partly above the Škocjanske jame, where the actual river bed in the cave is 230 m below the unroofed cave. On the basis of the shape of walls and sediments we may reconstruct some evolution of the caves and later the surface. Te caves are remnants of larger cave sys- tems, which conducted waters from diferent sinking streams. Growth of speleo-thems in them was frequent-ly interrupted by phases of erosion or backfll. Te caves were aferwards flled up with fuvial sediments. Te large pebbles in the caves testify the great gradient of the sur-face streams. Later all caves were flled with fner sediment, which could mean the lowering of the gradient in karst and aplanation. Later, the surface was tilted and up-lifed which caused lowering of the karst water level. Te age of the unroofed caves can be established by comparative methods ac-cording the denudation rate of the surface. If we pre-sume, that it is about 50 m/ Ma (Gams 1962) and there was some 100 m - 200 m of rock removed from above the caves that they are at least 2 – 4 Ma old, and probably older (Mihevc 1996, 2001). Similar time frames 1.6 – 1.8 Ma or/and 3.8 to 5 Ma were given also by paleomag-netic datation of clastic sediments (Bosak & al. 1998) and by the timing with tectonic phase that started at 6 Ma (Vrabec & al. 2006). Te age of the roofess cave can also be illustrated by the time, in which the water table in Kras lowered for 240 m, from about 400 m to 160 m a.s.l. 38 TIME in KARST – 2007 THE AGE OF KARST RELIEF IN w SLOVENIA Fig. 3: Formation of the unroofed cave. Te idealised drawing is representing actual cases of unroofed or partly denuded caves from the divača karst, where probably more than hundred meters of the rock above unroofed caves were removed. Te transformation of cave to the unroofed cave is here presented in three stages: a: Epiphreatic cave passage was formed deep below the surface, some fowstone was deposited afer the cave became inactive; b: Surface approached the cave. At one side the slope cut the cave and made the entrance into the passage; from the horizontal surface former vadose shafs transformed into vertical entrance. trough both entrances piles of boulders and scree deposited. c: Great deal of the ceiling dissolved, some collapsed and formed relief oblong depression of the unroofed cave ending in front of the entrance to the cave. Fig. 4: Formation of the unroofed cave. Te idealised drawings are representing the actual cases of unroofed or partly unroofed caves from the divača karst which were completely flled with allogenic fuvial sediment. Te transformation is here presented in three stages: a: Cave passage was formed deep below the surface. Tere was alternation of the sedimentation of fowstone and allogenic sediments of the underground river. towards the top of the profle sediments became fner. b: Surface approached the cave. At the side the slope cuts passage and exposed the cave sediments on the surface. c: Afer disintegration of the ceiling from the top oblong depression formed. In it there are alochtonous sediments and few blocks of limestone and some fowstone. Te unroofed cave ends with steep limestone wall or slope from where the karst surface continues. TIME in KARST – 2007 39 ANDREJ MIHEVC THE BLIND VALLEyS OF THE MATARSKO PODOLJE CONTACT KARST Alogene rivers fowing to karst enhance the karstifcation process and form particular relief features. Phenomena and forms that develop at the contact of fuvial and karst relief are the result of the interaction of both morpho-logical systems. Te Matarsko Podolje is a 25 km long and 2-5 km wide tilted karst surface. In longitudinal section it gen-tly raises from about 490 m on Nw to 650 m on SE side. Te karst surface continues towards SE but from the highest point there is an abrupt change and relief lowers over the distance of 2 km for 200 m to Brgudsko podolje karst surface. Fig. 5: blind valleys brezovica (br) and Odolina (O) on the NW part of the matarsko podolje karst. blind valleys cut for about 50 m into the edge of the levelled karst surface where dolines and larger collapse dolines prevail. Tere are no traces of dry valleys or dry blind valleys. Legend: 1. Sinking streams, 2. boundary fysch – limestone. From the fysch Brkini hills that are NE of podolje there are 17 sinking streams that formed a row of large blind valleys in the edge of the Matarsko Podolje. Te bottoms of these valleys are situated between 490 to 510 m. As the valleys are incised in the border of the karst, 40 TIME in KARST – 2007 uplifed towards SE, the blind valleys lying more to the south are deeper. Te most Nw lying, Brezovica and Od-olina blind valley are cut for about 50 m only while the deepest is the last one, Brdanska dana on SE, deepened into limestone for 250 m. Te blind valleys started to cut into the corrosion plain with small transverse and longitudinal gradient as in the other case the fuvial valleys should develop in them. Tey should be preserved on karst as dry valleys. Te corrosion plains along the ponors were controlled by the piezometric level this is why they are all at same altitude. In the SE part where the uplif was stronger, the blind valleys show the disturbances caused by fast tectonic uplif and are preserved on the karst surface. Above the Račiška Dana blind valley there is a fossil one, on the bottom of which are some old sediment from fysch. Tis is now higher than the fysch hills where the sediment came from. Te other case is the most SE blind valley Brdan-ska Dana. It developed in the SE edge of the Matarsko Po-dolje. Te tectonic structure along which the Matarsko Podolje ends caused also the asymmetric development of the blind valley. Te w side of the blind valley was up-lifed and developed two fossil higher levels in the side of the blind valley. Te Brkini series of blind valleys ofer enough data to follow the sequence of the morphological events and dominant factors which were decisive for the formation of the actual relief forms. Te former shape along the ponors on the border of impermeable hills was karst corrosion plain. Te water fow-ing on it had a modest gradient in karst and was capable of the aplanation of the surface only. Te lowering of the piezometric level in the karst enabled the development of the relief depressions along the ponors. Te deepen- THE AGE OF KARST RELIEF IN w SLOVENIA ing and the contemporane-ous widening of the valleys followed the lowering of the karst water to the altitudes about 500 m. Te incision of blind valleys into the leveled sur-face probably started and continued trough the last tec-tonic phase that is 6 Ma be-fore present. Tis is also ac-cordance with the age of the cave sediments from Račiška pečina which were dated by paleomagnetic method and correlated with palaeonto-logic data to 3.5 Ma (Pruner & al. 2003). Fig. 6: blind valleys Račiška dana (R) and brdanska dana (b) with fossil blind valleys (f1, f2). Tese valleys developed in SE part of matarsko podolje during the tectonic uplif. Uplif deformed older corrosion plain and created height diference between matarsko and brgudsko podolje. Further SE there is another blind valley (š) which developed at the edge of brgudsko podolje that was not uplifed. Račiška pečina cave that was once formed by sinking streams is at elevation about 600 m high above the recent ponors. Legend: 1. Sinking streams, 2. boundary fysch – limestone, 3. Cave Račiška pečina. THE UNROOFED CAVES OF THE EDGE OF THE PODGORSKI KRAS Podgora karst is small 5 km wide and long karst plateau, Sw continuation of the Kras. Its surface is located at 500 to 450 m a.s.l. Te plateau surface is leveled and dismem-bered only by numerous dolines. Tere is a sharp edge of the plateau and towards w in less than 1 km relief drops for 400 m. At the foot of the plateau there are recent karst springs of the rivers Rižana and Osapska reka at altitudes of about 50 m a.s.l. In the Črnotiče quarry, that is located on the edge of the plateau, several caves were opened. Shafs with stalagmites and stalactites on the walls were flled by gravel as well as numerous bones of large Pleistocene mammals felt down to shafs. Tere are also large remnants of horizontal caves. Te largest, 150 m long partly unroofed passage with the diameter of more than 10 m was opened in the western part of the quarry. Te passage was entirely flled by massive fowstones deposited over the fuvial sediments, lay-ers of gravel and conglomerate mixed up with sand and clay layers. Sedimentary fll was 17 m thick at least. In the cave calcareous tubes a serpulids were found both in sediments and still attached to the scalloped wall. Tey match the morphology of extant serpulid tubes of marifugia cavatica (Mihevc 2000; Mihevc et al., 2001a). Marifugia cavatica Absolon and Hrabe, 1930 is the only fresh-water species of the Serpulidae family and the only TIME in KARST – 2007 41 ANDREJ MIHEVC Fig. 7: dEm of the Podgorski kras. Levelled karst surface of Paleocene limestone and some intercalated fysch is in sharp contrast with fuvial relief that developed on Eocene fysch. At the foot of the karst there are the major karst springs where marifugia cavatica still lives today. Te fossil tubes were found in the large cave exposed in the Črnotiče quarry. Legend: 1. Unroofed cave, 2: Flysch, 3: Limestone. known tube worm inhabiting continental caves. Stable isotope analysis (Mihevc et al., 2002) of fossil tubes from Črnotiče quarry site is comparable with stable isotope compositions of recent fresh-water species and greatly difers from those of marine serpulids. marifugia cava- tica is flter feeder with free-swimming larvae (Matjašič & Sket 1966). It is widely dis-tributed within the Dinaric Karst and lives in springs of rivers Rižana and Osapska reka which are only few km and 370 m apart from the quarry. Two profles were anal-ysed within the cave and dat-ed back to 1.76 Ma (Črnotiče I) and 2.5–3.6 Ma (Črnotiče II site) (Bosak & al. 1999, Bo-sak & al. 2004). Geomorphologic evolution of the plateau shows similarities to those of Kras and Matarsko podolje. Epi-phreatic caves of the sinking rivers were flled with sediments; the surface was levelled and uplifed to present altitude. In the quarry there are several unroofed caves or remains old caves. Te evolution of vertical shafs with dominance of later autochthonous fll resulted from younger vadose speleogenesis and Pleistocene sedimentation. Fig. 8: Te view of the unroofed cave in a quarry face. Lower part of the cave passage was flled with mostly laminated yellowish brown fuvial sediments. Upper part is flled with fowstone. Te karst denudation already unroofed the cave, so that the fowstone is exposed to the surface. tubes of marifugia cavatica are on the scalloped walls in the lower part of the cave profle, which were protected by fne fuvial sediments. 42 TIME in KARST – 2007 THE AGE OF KARST RELIEF IN w SLOVENIA CONCLUSIONS Tree diferent relief settings on the Kras, Matarsko podolje and Podgorski kras plateau show quite similar evolution. Tere are old caves present everywhere, which are now exposed by denudation. Tey were epiphreatic caves that were formed by sinking rivers, bringing allo-genic sediments to caves. At the end of the morphogenet-ic phase all these caves were completely flled with fuvial sediments. Tis indicates the diminishing of the gradient in the whole area. Afer the caves were flled the three ar-eas were levelled. Planation occurred in the similar con-ditions, most likely close to the level of the karst water. Diminishing of the gradient which ended with pla-nation could mean the same tectonic phase which ended at about 6 Ma ago. Afer that a new tectonic phase started. Tree areas faced uplif and tilting for several hundreds meters. Te uplif was stronger in the SE part of the area. Karst denudation was evenly lowering the surface, so the surface remained well preserved, dissected on central parts of karst with dolines, which represent few percent of total area only. Te even denudation exposed former caves to the surface. Some of them are flled with sediments, from some sediments were washed away or were never flled. On the edges of Matarsko podolje there were several sinking streams shaping blind valleys. Teir incision was controlled by the piezometric level of the water in karst or the Matarsko podolje and by the tectonic uplif, they are getting deeper towards SE. Tilt of planation surface, diferent depth and asymmetric or fossil blind valleys are clear indicators of the recent tectonics. 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