COBISS: 1.01
DOLENJSKA SUBSOIL STONE FORESTS AND OTHER KARST
PHENOMENA DISCOVERED DURING THE CONSTRUCTION OF
THE HRASTJE – LEŠNICA MOTORwAY SECTION (SLOVENIA)
DOLENJSKI PODTALNI KAMNITI GOZDOVI IN DRUGI KRAŠKI POJAVI, ODKRITI PRI GRADNJI AVTOCESTNEGA ODSEKA
HRASTJE – LEŠNICA
Martin KNEZ1 & Tadej SLABE1
Abstract                                           UDC   551.435.8(497.4)
Martin Knez & Tadej Slabe: Dolenjska subsoil stone forests and other karst phenomena discovered during the construc-tion of the Hrastje - Lešnica motorway section
Tis paper explains the investigation of shallow and sediment-covered areas of the Dolenjska karst discovered during the construction of the motorway section between the villages of Hrastje and Lešnica. Research done during construction has again proved to be very useful; we came to numerous and aug-mentative conclusions about the development of karst features typical of the Dolenjska region. Again the importance of the participation of karst researchers in planning major activities concerning the karst and in monitoring the work was demon-strated. we have researched the unique features of the karst surface marked mostly by large areas of stone forests and char-acteristic karstifcation below the thick sediment beds. key words: stone forests, subsoil formation of carbonate rock, Dolenjska karst, Slovenia.
Izvleček                                            UDK   551.435.8(497.4)
Martin Knez & Tadej Slabe: Dolenjski podtalni kamniti gozdovi in drugi kraški pojavi, odkriti pri gradnji avtocestnega odseka Hrastje - Lešnica
članek obravnava raziskave plitvega dolenjskega krasa med gradnjo avtocestnega odseka med vasema Hrastje in Lešnica. Raziskave med gradnjo so se tudi tokrat izkazale kot zelo koristne, saj smo prišli do številnih novih in dopolnjujočih zaključkov o razvoju tega, za Dolenjsko značilnega krasa. Ne nazadnje se je spet izpostavil pomen sodelovanja krasoslovcev pri načrtovanju večjih posegov v kraško površje in spremljava del. Raziskovali smo svojevrstno oblikovanost kraškega površja, ki jo predstavljajo predvsem velike površine kamnitih gozdov ter značilno zakrasevanje pod debelimi plastmi naplavin. ključne besede: kamniti gozdovi, podtalno oblikovanje karbonatnih kamnin, dolenjski kras, Slovenija.
INTRODUCTION
Te monitoring of the construction of the Dolenjska mo-torway by karst researchers has again proved to be of great value in the exploration of our natural heritage through the deepening of our knowledge about the formation and development of this part of the Slovenian karst. Tis characteristic subsoil karst surface was formed under a cover of sediment of varying depth. Subsoil formation of carbonate rock also marks the entire epi-karst and va-dose zone. Te surface is carved into subsoil stone for-
ests (Figs. 1, 2), the surfaces of the karren are smaller and numerous hollow shafs are flled with fne-grained sediment. Te outstanding characteristics of the karst surface are primarily the result of large surfaces of stone forests, which are difcult to detect prior to earthwork or geo-physical research. Most of the surface is above the under-ground water level. Te only exception is the motorway section at Hrastje, which was not fully uncovered due to the construction method and because lower road beds
1Karst Research Institute, ZRC SAZU, Titov trg 2, SI-6230 Postojna, Slovenia, e-mail: knez@zrc-sazu.si, slabe@zrc-sazu.si Prejeto/Received: 05.09.2006
ACTA CARSOLOGICA 35/2, 103–109, LJUBLJANA 2006
MARTIN KNEZ & TADEJ SLABE
fig. 1: Uncovering of subsoil stone forest.
Sl. 1: Razkrivanje podtalnega kamitega gozda.
were put upon a special grounding. For this reason we could only research the karst formations that had been shaped by water percolation through the karst surface. we did not fnd the characteristic subsoil karren as those discovered during the earthwork at Bič, which were also formed by the fuctuation of underground water (Knez et al., 2004). Karst features discovered during construction
fig. 2: Subsoil shaped pillar of stone forest.
Sl. 2: Podtalno oblikovan steber kamnitega gozda.
work give us insight into the characteristics and manner of formation of the karst in the wider area of southern Slovenia; they thus unveil yet another characteristic of our karstic natural heritage that is hidden from view, but which also provides guidelines for planning activities on the surface.
THE MOTORwAY ROUTE AND THE MAIN CHARACTERISTICS OF THE LANDSCAPE
Te north-eastern part of the motorway section starts in the vicinity of a swallow hole of the Igmanca stream near the villages of Hrastje, Dolenja vas and Šentjurij and pass the village of Selo along characteristic Dolenjska lowland. Lateral, changing and relatively thick beds of sediment and soil cover the land. Te underground water is close to the surface of the predominant fuviokarst. Here we fnd individual karstic features, among them minor swal-low holes, esatavellas and, to a lesser extent, an outcrop of carbonate rock Te terrain rises slightly towards the Strmec and Dobrava hills, where the road climbs more steeply and soon afer the pass drops towards the Krka River. we have detected thinner sediment beds and fre-quent outcropping of karstifed rock, which mostly disin-tegrates into small fragments. Tere is less surface water at that location since it fows into the subsoil relatively quickly due to the inclination of the terrain, the thin sediment beds and fragmentary cover of disintegrated rock Te stone forests and karren, which reach various depths, are composed of compact and also tectonically very crushed rock where the rock is not crushed and where compact blocks of limestone occur between the cracks, we noticed stone teeth on the surface. During the
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earthwork, these had in many cases revealed themselves as real stone pillars. where the rock was tectonicalb-cracked or crushed, we did not fnd karren on the sur face; however, at some locations rudimentary stone teeth hidden beneath the soil immediately disintegrated during earthwork. Between the Brezovica and Lešnica hills we again fnd typical shallow Dolenjska karst with its underground level close to the surface, and its characteristic collapses and sinkholes. A rare network of streams is formed on the surface, but a substantial part of the source and side channels do not have permanent fows. Minor springs of underground water are frequent and fow along narrow, corroded cracks in the rock Surface and subsoil karst features are rare. Te surrounding valleys are dry for most of the year, and streams and foods only occur afer downpours. Percolating water feeds small but permanent sources. Fluctuation of the fow from these sources is minimal and rarely more abundant, which re-fects greater permeability and cavernosity Underground water fow is close to the surface, for which reason karst formations such as this are called “shallow karst”.
Tick beds of Plioquaternary sediments on carbonate rock, especially on a moist surface, are usually acid. It
DOLENJSKA SUBSOIL STONE FORESTS AND OTHER KARST PHENOMENA DISCOVERED DURING ...
is not yet fully understood whether these sediments are autochthonous or brought from the nearby dolomite sur-roundings (Gams 2004). Afer comparison with the cir-cumstances in tropical karst, the opinion developed that
in these valleys the thick layers of disintegrated material could have been preserved only because of the high un-derground water level and poor erosion, and thus developed into subsoil karst.
GEOLOGICAL CHARACTERISTICS OF THE AREA
Te motorway route between Hrastje and Lešnica runs, for the most part, on Jurassic (Lower Malm) rock In some cases it also crosses Plioquaternary sediment and alluvial river sediment. From the geo-tectonic aspect it belongs to the Outer Dinarids, characterised by its block structure and the Dinaric orientation of the faults and alignment of the folds. A covered fault runs along the western side of the road section. Te anticline fold of the Upper Jurassic beds run in the Dinaric sense from Dolenja Nemška vas on the northwest to Novo mesto on the southeast. Te road mostly runs along its northern fank Te dip of the beds changes, but the general direction is towards the northeast. we fnd various micro and macro fauna in the rock along the route as well as macro fora in its central part.
Te Upper Jurassic beds to the north border on Upper Cretaceous brown and green marl, sandy marl, marly limestone and grey and red platy limestone with intru-sions of breccia. To the south they border on Plioqua-ternary brown clay sediments. North of Prečna we fnd a minor area of upper Triassic stratifed and un-stratifed grey dolomite.
According to a geo-tectonic survey map (Pleničar & Premru 1977) the area of the motor way section lies on the Novo mesto block. Its northern part is a transition between the Sava folds and the Dolenjsko-Notranjska block. Te oldest rock here is Middle and Upper Triassic dolomite over which Jurassic limestone had been discor-dantly deposited and over Cretaceous pelagic rock Te characteristics of the terrain are synclines and anticlines running in the Dinaric direction, which at places diverge from their characteristic orientation.
Te Jurassic beds in the Novo mesto surroundings are generally composed of light grey limestone that lies
on Cordevol Upper Triassic dolomite and forms the base for their discordantly deposited Upper Cretaceous pelagic sediments (Pleničar & Premru 1977).
Te Lower Malm rocks are strongly varied along the motorway section north of Novo mesto (Pleničar et al., 1976). In the northern biostratigraphic zone and south-ern biostratigraphic zone we fnd alternating white and grey limestone, oolitic limestone, reef limestone with hy-drozoans and bedded limestone with chert.
Te northern biostratigraphic zone can in a broader sense be found on the motorway section between Poljane and Mali Slatnik, east of Novo mesto (Pleničar & Premru 1977), which mostly runs north-east of this motorway section. Here light-grey, un-stratifed reef limestone and large-grained reef breccia are predominant. Here and there we can fnd dark grey and almost un-stratifed limestone between these beds. Rich hydrozoan fauna can be found in the un-stratifed limestone. Occasionally we also found platy limestone with chert.
Along the south-eastern parts we are most probably following a middle biostratigraphic zone, which is litho-logically similar to the northern biostratigraphic zone; hydrozoans are not as present in the rock Oolitic limestone can also be found in the reef limestone (Pleničar & Premru 1977). Corals appear in some places. According to the geological map and its commentary (Pleničar et al., 1976; Pleničar & Premru 1977), southern biostratigraphic rock characterised by grey, dense and oolitic bedded limestone, does not exist along the route.
In the Plioquaternary sediments southwest of Mačkovec we fnd an outcrop of bentonite clay Te clay lies in the pockets of Triassic dolomite and in the Jurassic limestone. At places it is deposited in beds of a total thickness of up to 12 m.
KARREN SURFACE
Most of the surface of the higher-lying land is karren.      by sediment, but with thin layer of soil. Tey cover the
Te bottoms of dales are covered by sediment beds. Two      major part of the surface. Te carbonate rock is dissected
types of karren can be clearly distinguished. Karren in      along the cracks, and on the rock we can detect the traces
their original meaning occur in areas that are not covered      of its former a) subsoil formation (Slabe 1999; Slabe &
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MARTIN KNEZ & TADEJ SLABE
Knez 2004) - these are relatively few, and b) indirectly transformed by precipitation, the surface was overgrown and c) fnely shaped by bio-corrosive factors. Te surface was mostly forested. Such karren existed on the surface of the cone in the dale at the beginning of the motorway section.
Especially in the area of the Strmec and Dobrava hills we observed well-expressed bio-corrosive activities on numerous karren outcrops of carbonate rock (Fig. 3). Te rock was more diluted by bio-corrosion on the shadier sides. Moss mostly grows there while lichen can also be found on areas exposed to the sun. Bio-corrosive processes do not take place equally on the entire surface of the rock, but selectively. Most probably lithologically slightly diferent clasts in crushed and then cemented rock are diluted to various depths or else there are different organisms in the various neighbouring clasts. Te contact areas between the various clasts are especially corroded, in some spots up to several mm deep.
Te major part of the forest-covered surface was dissected mainly by individual rocks of various sizes with partially similar traces of formation to the karren described above. Tey are frequently defned by fun-
fig. 3: Bio-corrosively etched surface of karren. Sl. 3: Biokorozisko razjedena površina škrapelj.
nel-shaped mouths of larger subsoil channels. Te rocks reach up to one to two metres in height, with a narrow-ing on top and with large areas of soil between them. Te earthwork uncovered them as the tips of the larger areas of stone forests.
SUBSOIL STONE FORESTS
Relatively large areas of subsoil stone forests illustrate the manner and long-lasting subsoil formation of this part of the karst surface, covered by fne-grained sediment and soil. Te stone pillars are completely covered by the sedi-ment and soil, or their tips protruding on the surface.
Te confguration of the surface includes smaller or larger dolinas (Fig. 4). Te largest have a diameter of several tens of metres. Some are flled with grey clay, the
origins of which we are still investigating. Both are dissected by subsoil stone forests.
Te subsoil stone forests are composed of a dense network of more or less thickset and pointed pillars, which reach up to 8, sometimes even 10 metres, although most are lower. Te narrower pillars with a diameter of one to two metres have sharp or rounded spire (Fig. 5), while the thickest ones reaching up to ten metres (Fig. 6)
fig. 4: filling of dolina with gravel. Sl. 4: Zapolnjevanje vrtače z gruščem.
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*;J   '
fig. 5: Pointed tops of narrower pillars. Sl. 5: Kničasti vrhovi ožjih stebrov.
DOLENJSKA SUBSOIL STONE FORESTS AND OTHER KARST PHENOMENA DISCOVERED DURING ...
fig. 6: wider stone pillars.
Sl. 6: Obsežnejši podtalni stebri.
in width have one or more spires or their tops are com-posed of more or less curved crests. Among them in most parts are funnel mouths of the perpendicular subsoil channels or horizontal subsoil channels.
Subsoil rock features predominate in the rock relief of the pillars (Slabe 1999; Slabe & Knez 2004), which indicates gravitational fow of water from the surface. Tese are mainly subsoil channels. Te most typical are the vertical ones (Fig. 7) with diameters that can reach up to one metre, the largest of which, as we shall ex-plain below, can also be called subsoil shafs, which at the top develop into funnel-shaped mouths. Surface wa-ter that fows from the soil along the rocks collects in them. In cross-section the funnel-shaped mouths can take various shapes. Tey can be open, semicircular or nearly round. Teir shapes are ofen the result of the permeability of the rock and the sediment contact along which the water fows downwards. Long, drawn-out formations of the mouth at less permeable connections causes the rock features to erode deeper into the rock Te water found its way through the rock less frequently and veritable funnels were shaped. Smaller and curv-ing channels are formed at less permeable connections or when minor quantities of water collect on the sur-
KARST
Uncovered hollows are generally the result of vertical water percolation through the epi-karst and the vadose parts of the aquifer. Te shafs can be classifed as hol-low and those flled with sediment. Te latter are termed subsoil due to the similarity of their features with subsoil rock, mainly with vertical subsoil channels.
fig. 7: Subsoil channels.
Sl. 7: Podtalni žlebovi.
face. Tis is characteristic for smaller pillars with tops that project out of the earth. Subsoil scallops are rare and generally indicate a well-permeable rock connec-tion with the sediment that surrounds it, and moreover, we fnd elongated notches on the pillar walls, which are traces of water accumulation at the less permeable part of the connection and accelerated corrosion of the rock next to it.
Close below the surface where the rock is covered with soil the rock surface of the stone pillars is relativeb-smooth, while deeper, at the connection with the sedi ment that covers the surface, it is coarse and ofen has a confguration of rounded pendants. Te rock there is weathered. Te thickness of the weathered layer mea-sures up to 1 cm. It is sof when moist, but as it dries out afer being exposed on the surface for a longer period and the water evaporates from it, it hardens. Te state of weathering of the top layer of the rock is the result of the connection with the sediment, which is moist most of the time. Te connection is relatively less permeable, and the water that does permeate it only slowly washes the solu-tion. Te connection with more permeable soil is corre-spondingly also more permeable.
Te ffeen shafs (Fig. 8), vertical and mostly sim-ple with only one in level, the deepest measuring 24 m, three others deeper than 10 m, while the rest were less deep, with diameters reaching up to 5 metres, but in most cases less, indicate here and there, greater perme-ability, allowing dense vertical water percolation due to
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MARTIN KNEZ & TADEJ SLABE
fig. 8: discovering of shaf. Sl. 8: Odkrivanje brezna.
their expressed vertical cracks and vertical rock strata (Figs. 9, 10). Te shafs occur among the subsoil karren and forests. Tey do not reach the surface. Teir walls are carved with larger or minor vertical channels and of-ten covered with a thin layer of sediment, which causes their thiny dissection (Slabe & Knez 2004). Te foors of
Hrastje-Lešnica P95
(profile)
Hrastje-Lešnica P95
(ground plan)
Scale: /.700
I       1       i       4       Sni
$urveyins: M PrelovSek, F. Drole DrawhiL: M. Preli^v^ek I7.RK ZRC SAZU
fig. 9: Shaf on profle 95. Sl. 9: Brezno na profle 95.
108     ACTA CARSOLOGICA 35/2 – 2006
fig. 10: Shaf on profle 208. Sl. 10: Brezno na proflu 208.
the shafs are ofen covered by sediment or sediment flls their lower part.
Subsoil shafs are more or less vertical hollows, sim-ilar to ordinary shafs, through which water also perco-lates from the karst surface, but they are almost entirely flled with sediment, with only individual vertical sections
fig. 11: Subsoil shaf along fault.
Sl. 11: Podtalno brezno vzdolž preloma.
DOLENJSKA SUBSOIL STONE FORESTS AND OTHER KARST PHENOMENA DISCOVERED DURING ...
hollow. Te water that fows through them deposits the sediment that covers the surface. Teir cross-sections are more or less round or extended at the cracks and bedding planes. Teir diameters reach two metres. Sediment fll-ing facilitates the shaping of their periphery, and notches appear at less permeable connections. Subsoil shafs are formed at local dense fow of larger quantities of water. Tey can develop from subsoil channels. Teir walls are carved with along-sediment rock features, which are the
traces of formation at the connection with fne-grained sediment. with greater permeability in the karst interior, the subsoil shafs can be emptied.
Above-sediment channels ofen occur on horizontal bedding planes and in cracks, or networks of anastomo-ses, the traces of paragenetic stratifcation. Tus, tempo-rarily fooded areas occur locally and the water, which carries fne-grained sediment and deposits, cuts its way upwards.
CONCLUSION
More and more, the unique natural heritage and devel-opment of the Dolenjska karst reveal themselves. Te importance of the participation of karst researchers in planning major activities in the environment and moni-toring the work has been demonstrated once again. Co-operation with the road constructors has set an excellent example for many years.
Tis time we were given the opportunity to follow the water precipitating into the epi-karst and the upper
part of the vadose zone, which were shaped under a rela-tively thick cover of sediment and soil and where stone forests, shafs and subsoil shafs have been formed over large areas.
Te scarcity of stone forests and special geomorpho-logical karst features characteristic of this part of the karst (Knez et al., 2003) demand that we prepare guidelines for further planning of activities in the karst landscape.
ACKNOwLEDGEMENT
we would like to thank Mitja Prelovšek to draw fgures 9 and 10.
REFERENCES
Knez, M., Slabe, T. & Šebela, S. 2004: Karst uncovered during the Bič-Korenitka motorway construction (Dolenjska, Slovenia).- Acta carsologica 33/2, 75-89, Ljubljana.
Knez, M., Otoničar, B. & Slabe, T. 2003: Subcutaneous stone forest (Trebnje, Central Slovenia).- Acta car-sologica 32/1, 29-38, Ljubljana.
Gams, I. 2004: Kras v Sloveniji v prostoru in času.-Založba ZRC, 515 p., Ljubljana.
Pleničar, M., Premru, U. & Herak, M. 1976: Osnovna geološka karta SFRJ 1:100000, list Novo mesto. Zvezni geološki zavod, Beograd.
Pleničar, M. & Premru, U. 1977: Tolmač za list Novo mesto.- 61, Zvezni geološki zavod, Beograd.
Slabe, T., 1999: Subcutaneous rock forms.- Acta carso-logica 28/2, 255-269, Ljubljana.
Slabe, T. & Knez, M. 2004: Kraške podtalne skalne oblike.- Annales 14/2, 259-266, Koper.
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