Andrej Mihevc

Palaeomagnetic research on karst sediments in Slovenia

Nadja Zupan Hajna1, Andrej Mihevc1, Petr Pruner2, Pavel Bosák2,1 Paper presented during the GSM-03 Symposium “Karst as a global phenomenon a tribute to Derek Ford and Paul Williams” at the 33rd International Geological Congress held at Oslo, August 6-14th 2008 48 Nadja Zupan Hajna, Andrej Mihevc, Petr Pruner, Pavel Bosák International Journal of Speleology, 39(2), 47-60. Bologna (Italy). July 2010 the karst surface and in the subsurface preserve the record of karst evolution and its age. Pannonian Basin and marginal (intermontane tectonic) depressions with Tertiary and Plio–Quaternary fills cover substantial parts of the SE and E parts of Slovenia (belonging both to Southern Alps and External Dinarides). Triassic to Cretaceous carbonate rocks of the Southern Alps were deformed into nappes during Eocene–Middle Oligocene, forming mostly W–Etrending structural pattern. Strong Cenozoic tectonic activity and rotations in both regional geological units affected the accelerated geomorphological evolution and karst processes especially speleogenesis (Zupan Hajna et al., 2008a). The research in the present paper covered all the principal karst regions, from lowlands to high mountains. The sites were located in the Dinaric Karst (32), Julian Alps (2), Isolated Karst of the pre-Alps (1) and Plio–Quaternary fluvial sediments from the tectonic Velenje Basin (1). The low number of non-Dinaric locations is due to the lack of suitable karst sediments there. Sites included both well-known and documented deposits, as well as relatively unknown or newly found locations in caves and on the surface. Karst sediments represent an important source of information on the evolution of tectonic and geomorphological units of different sizes. The territory of Slovenia, with its karst regions, long history of karst evolution, and relatively complete knowledge of the karst sediments, represents an ideal testing ground for comprehensive research on individual infilling processes, their stages and periods. The aim of the research was focused on the time span of karst evolution, age of karst surfaces and speleogenesis, and rates of processes. Fig. 1. Location of studied sites in Slovenia and Italy. Dinaric Karst sites: Kras Plateau and surrounding area (1Črnotiče profiles, 2 – Briščiki, 3 Kozina profile, 4 Divača profile, 5 – Jama pod Kalom, 6 Grofova jama, 7 Divaška jama, 8 Trhlovca, 9 Račiška pečina, 10 Pečina v Borštu), Notranjski kras (11 Križna jama, 12 Planinska jama, 13 Postojnska jama, 14 Zguba jama, 15 Markov spodmol), Dolenjski kras (16 Hrastje profile), Alpine Karst sites: Julian Alps (17 Jama pod Babjim zobom, 18 Jama nad Planino jezero), Kamnik-Savinja Alps (19 Snežna jama), non-karst sediments (20 – Velenje profile) and Isolated Karst: Ponikovski kras (21 Tajna jama). Map source: DMV 25, Geodetska uprava Republike Slovenije. 49 Palaeomagnetic research on karst sediments in Slovenia International Journal of Speleology, 39(2), 47-60. Bologna (Italy). July 2010 The first systematic studies of cave sediments in Slovenia were carried out during the archaeological excavations of sediments in the entrance parts of some caves (Brodar, 1966). More extensive and detailed study of cave sediments was performed by Gospodarič (1976, 1981, 1988). He compared cave sediments from different sites and he used different numerical and other dating methods (Franke & Geyh, 1971; Ikeya et al., 1983; Ford & Gospodarič, 1989) to establish the age of deposits and to distinguish different deposition phases in the subsurface. In the Kras, he linked the karstification of the area with glacioeustatic oscillations of the Adriatic Sea and the global palaeoclimate changes during the Pleistocene. He suspected that the cave sediments were not much older than 350 ka. A better understanding of cave sediments, their age and the chronological sequence of speleologenetic events was achieved by more concentrated dating by the method (Zupan, 1991; Zupan Hajna, 1996; Mihevc & Lauritzen, 1997; Mihevc, 2001a). Data showed that speleothem growth corresponds to warmer periods during the Pleistocene. Nevertheless there are large numbers of speleothems older than the limit of the dating method. The study of cave deposits (Fig. 2) in Alpine caves and in unroofed caves of the Kras (Mihevc & Zupan Hajna, 1996; Mihevc, 2001a) provided entirely new insights into the age of karst sediments and introduced new ideas concerning the development of karst and caves. The application and interpretation of palaeomagnetic analyses and magnetostratigraphy of cave sediments, both clastic and chemogenic, which began on the Kras in 1997, suggested substantial changes of time span in which deposition took part in caves (e.g., Bosák et al., 1998, 1999, 2000, 2004; Šebela & Sasowsky 1999, 2000; Audra, 2000; Mihevc et al. 2002; Zupan Hajna et al. 2008a, b). Magnetostratigraphy data and the arrangement of obtained magnetozones often indicated ages of the cave fill from 1.77 Ma up to over 5 Ma. METHODS The present paper summarizes our results from the period of 1997 to 2008; full details are available elsewhere (Zupan Hajna et al., 2008a). Our palaeomagnetic research included a total of 21 sites (19 in Slovenia and 2 in Italy) with 36 profiles; all except one were cave or karst surface sediments. During the last ten years we did complex research of karst sediments applying a number of geologic methods: palaeomagnetism and magnetostratigraphy, stratigraphy (numerical and correlated dating methods including, palaeontology – fauna, pollen), sedimentology, and mineralogy (X-ray diffraction). Palaeomagnetic studies conducted in caves have been applied to determine the age of sediments (principally fine-grained deposits – fine-grained sands, silts, clays – and speleothems) based on magnetic polarity (magnetostratigraphy) and/or palaeo-secular variations, and on palaeoenvironmental applications of mass-specific magnetic susceptibility (MS). Palaeomagnetic analyses were completed in the Laboratory of Palaeomagnetism, IG AS CR, v. v. i. in Praha–Průhonice. Procedures were selected to allow the separation of respective components of the remanent magnetization (RM) and the determination of their geological origin. Oriented hand samples from consolidated rocks and speleothems were cut into cubes of 20 x 20 x 20 mm and subjected to alternating field demagnetization (AF) and/or thermal demagnetization (TD). Samples from unconsolidated sediments were demagnetized only by AF. The laboratory procedures yielded results about (see Zupan Hajna et al., 2008a): mean palaeomagnetic directions, directions of C-components (with normal and reverse polarity), mean palaeomagnetic values and standard deviations (Jn, kn). Basic magnetic and palaeomagnetic properties were compiled in the logs. Dating of cave sediments by the application of the palaeomagnetic method is a difficult and sometimes risky task, as the method is comparative in its principles and does not provide numerical ages. There exist two principal rules to obtain data for reliable interpretations: (1) to apply only dense sampling in the field (high-resolution approach with sampling distance of 2–4 cm; Zupan Hajna et al., 2008a), and (2) to apply both complete step and/or field procedures offered by both demagnetization methods; the application of complete analysis only to pilot samples and shortened, selected field/step approach, to other samples did not offer sufficient data set (Bosák et al., 2003). Correlation of the magnetostratigraphic results we obtained, and the interpretations tentatively placed upon them, has shown that in the majority of cases, application of an additional dating method is needed to either reinforce the palaeomagnetic data or to help to match them with the geomagnetic polarity timescale. RESULTS Cave deposits (both clastic and chemogenic) provide a record of processes (Ford & Williams, 2007) and evidence which has not been preserved on the surface in most of karst regions of Slovenia. They can help to decipher the younger geological and tectonic history. About 2,000 samples have been sampled and processed by standard palaeomagnetic analyses, and biostratigraphic dating, mineralogical, petrological and geochemical analyses, etc. Palaeomagnetic and magnetostratigraphy studies, combined with other dating and analytical methods, offer a surprisingly new time frame for cave depositional processes – they showed that most of analyzed sediments can be up to several millions of years old; which is in accordance to the idea of Sasowsky (2007). Sites with dated cave and surface karst sediments are presented on Figure 1. Sites were located along the Dinaric Karst (Kras Plateau and surrounding area, Notranjski kras and Dolenjski kras). There were also samples from 3 sites in the Alpine Karst, one from Isolated Karst, and for comparison of the results, one from non-karst area.

Hypogenic origin of Provalata Cave, Republic of Macedonia: a distinct case of successive thermal carbonic and sulfuric acid speleogenesis

temperature, which leads to a progressive increase in CaCO3 solubility, and also a drastic drop of solubility near the water table due to the loss of CO2. As a result a geochemical zone of carbonate dissolution and zone of carbonate precipitation appear (Dublyansky, 2000a). Where H2S rich waters mix with shallower oxygen rich waters, sulfuric acid forms at or near water table, which rapidly dissolves the carbonate rocks (Egemeier, 1981). This process is known as sulfuric INTRODUCTION

Concentration and stable carbon isotopic composition of CO2 in cave air of Postojnska jama, Slovenia

13 C airCO2 lower than in the outside atmosphere. Strong seasonal fluctuations in both parameters were observed at locations deeper in the cave, which are isolated from the cave air circulation. By using a binary mixing model of two sources of CO2, one of them being the atmospheric CO2, we show that the excess of CO 2 in the cave air has a δ 13 C value of -23.3 ± 0.7 ‰, in reasonable agreement with the previously measured soil-CO 2 δ 13 C values. The stable isotope data suggest that soil CO 2 is brought to the cave by drip water. Abstract:

Source assessment of deposited particles in a Slovenian show cave (Postojnska jama): evidence of long-lasting anthropogenic impact

Postojnska jama (Postojna Cave) is one of the most famous karst caves in the world and has been a well-known tourist attraction for nearly 200 years. It is particularly famous for its unique double-track railway. Eight heavy metals – aluminium (Al), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), manganese (Mn), strontium (Sr), and zinc (Zn) – were determined in dust deposits by ICP-MS in order to assess sources of deposited particles on the cave walls. The samples were collected along the main passage in the cave, at different horizontal and vertical levels, in order to test horizontal homogeneity and study vertical distribution of the particles. It seems that the railway is an important anthropogenic source of particles, reflected in increased concentrations of Cu, Pb, and Zn, as well as of Fe and Mn in dust deposits at individual sampling sites. The maximum concentrations of Cu (217 µg g -1 ), Pb (4,940 µg g -1 ), and Zn (1,060 µg g -1 ) considerably exceeded their natural abundance and were explained by anthropogenic impact. The three heavy metals are markers for vehicles, engine oil and brake wear. On the other hand, mixed sources could prevail for Fe and Mn. The maximum concentrations of Fe (85,900 µg g -1 ) and Mn (682 µg g -1 ) in dust deposits were similar to the concentrations determined in fragments of the railway tracks (97,100 µg g -1 for Fe and 821 µg g -1 for Mn) and were explained by track wear and/or corrosion. In most other parts of the cave, Fe and Mn concentrations were, however, below the concentration of their natural abundance. Al, Sr, and Cr seem to be predominantly of natural origin. They generally exhibited concentrations lower than their natural abundance.

Impact of Motorways on Karst Waters

Impervious surfaces of road networks accumulate contaminants and pollutants, which are washed off during rain and snowstorm events into nearby waters and lands. Especially in areas with a developed traffic network, runoff from motorways is a significant source of pollution. Various protection and remediation measures were developed and implemented for preventing or at least diminishing its negative influences. Three major sources of pollution are associated with the road network: vehicles (emissions of motor vehicles, spilled and released oil, tires’ particles, de-icing agents), road characteristics and paint markers, and atmospheric depositions influenced by the adjacent land use. Besides pollutants directly associated with traffic, the impermeable surfaces can collect and drain a negligible quantity of organic waste, nitrogen and phosphorous, herbicides, pesticides and faecal pathogens. Soil infiltration treatment, building of collection basins and constructed wetlands are used as protection and remediation measures. In karst areas, motorway runoff has an amplified impact on ground water, compared to other types of landscapes. The soil layer is commonly thin or non-existent and thus soil infiltration treatment is practically inexistent. Stormwater runoff may immediately flow into the aquifer through subsurface conduit networks, fractures, sinkholes and sinking streams, which makes karst groundwater especially vulnerable to pollution. Accidents that cause greater quantities of hazardous substances to flow into the karst, for whatever reason, endanger our environment, karst waters, and even in smaller quantities also the quality of the karst springs that have been captured for drinking water supply. Especially dangerous are spills of oil derivatives, since we know too little about the flow of such substances that do not dissolve in water and are lighter than it. Based on observations of Globocec spring following the accident near Ortnek and of the Rižana Spring following the spillage near Obrov and other similar cases, we anticipate a longer retention time and washing away of oil derivatives due to the possibility of adsorption on sediments and retention in siphons.

Caves and Hydrology of the Contact Karst of Beka and Ocizla

The Beka-Ocizla cave system extends at the contact between Palaeocene limestone and Eocene flysch at an altitude of 350 m in a shallow depression called Loke. Six known caves are connected to this cave system: Ocizeljska jama (Ocizla cave), Blažev spodmol (Blaž’s rock shelter), Maletova jama s slapom (Maletova jama cave with waterfall), Jama z naravnim mostom (Cave with the natural bridge), Jurjeva jama v Lokah (Jurjeva jama cave in Loke) and S-4/Socerb. Three larger streams and a smaller one flow into the depression from flysch and sink into marly limestone. Together, they drain the flysch surface of approximately 3.5 km2. Some of the cave entrances function as periodic sinks of the surface waters that flow in from beneath the villages of Beka, Ocizla and Petrinje. The water from the entire cave system flows towards the Boljunec spring. Hydrological observations conducted in the cave include mapping of traces of the past flood events and continuous monitoring of the water level and temperature at five different locations in the cave system. A few flood events happened in past years: an extreme flood event between 17 and 19 September 2010, and flood events between October 2012 and March 2013. Special hydrological conditions during these events are also described.

Unroofed Caves Provide Important Clues to the Karst Development

Unroofed caves are old caves that were revealed on account of the lowering of the karst surface. They are preserved by their infill—mostly alluvium and flowstone. It became clear during the motorway construction undertaking in Slovenia that unroofed caves constitute a relatively common karst landform. In fact, more common than karstologists had imagined before the karst surface was uncovered through earthworks. The various types of notches occurring on the surface have long been interpreted as types of dolines or as the result of the lithological properties of rock and its fracturing. 75 km long and, on average, 25 m wide stretch of the motorway across the classical Karst gave up 350 caves, of which 90 are unroofed caves. Some of them make up the same cave system. New findings prompted karstologists to become more aware of these unique surface karst forms. In the process, numerous unroofed caves filled with all types of alluvium were discovered. There were several attempts at typification of the characteristic shapes of unroofed caves and to design partial models to explain their typical formation processes. The surface and subcutaneous dissolution of carbonate rock and its disintegration from back in the Ice Age, brought about the lowering of the karst surface. Old caves, which were formed by erstwhile water flows and are partly intersected by shafts which drain water from the permeable karst surface, pop up as either empty or filled with alluvium. The caves were formed as a part of a system of cavities in a period when impermeable rocks had enclosed the aquifer higher up, causing the underground water in the aquifer to be at a higher level. The hypothesis was that the karst topography and its remarkable systems of valleys can be traced back to former surface water throughflow. However, revealing the surface did not provide the evidence to support this hypothesis, instead obvious signs of former water throughflow in carbonate rock were identified—manifested as open and cut through old caves.

Unroofed Caves Near Kozina and Their Identification

Unroofed caves are an important karst form that makes up a part of the karst surface and epikarst, and provides us with valuable evidence of the karst development. They are old caves that became exposed by the lowering of the karst surface. In fact they are preserved by their fill—mostly fine-grained alluvium. It is also often that they feature preserved flowstone and an intact rock rim. During earthworks preceding the motorway construction, this important karst feature, also characterizing the surface, attracted special attention. Earthworks revealed that the karst surface is scattered with several distinct types of unroofed caves, which in itself are not an uncommon phenomena. The typical shapes of unroofed caves found on karst terrain could be singled out, i.e. individual doline-like forms that occur in strings, and oblong notches. Because the surface of the Karst region has lowered so dramatically, there are old caves and shafts opening up all the time in the course of the construction of the motorways. Old caves are either void or filled with alluvium. The caves were formed as a part of a system of cavities in a period when impermeable rocks had enclosed the aquifer higher up, causing the ground water in the aquifer to be at a higher level. But karstification gave rise to a drop in the water table in the aquifer—today it is 200 m and more below ground, and the karst surface is still lowering. Unroofed caves are therefore regarded as distinct surface karst forms which were in part reshaped by surface processes that make up an important part of epikarst. In the course of the earthworks for the construction of the Kozina motorway, the typical shapes of unroofed caves, passages and large cave systems carved in horizontal or inclined karst surfaces were discovered.

Palaeomagnetic Research of an Unroofed Cave Near Kozina

The unroofed cave from which samples for the palaeomagnetic analysis were taken, is located to the northeast of Kozina, near the existing Ljubljana–Koper main road, in the cutting which was formed during the construction of the Divaca–Klanec motorway. The sampled profile consists of more than 5 m of alluvium in two main sequences. The lower sequence comprised ochre-coloured sandy to clayey sediments that were app. 3 m thick. These sediments were sampled to undergo the palaeomagnetic method. The lower sequence was covered with collapsed breccia and limestone blocks, and no samples were taken from it. The profile’s lithology clearly manifests a two-phase depositing in the past. The lower sequence underwent erosion after having been deposited. Subsequently, during the collapse, the empty space in the cave filled with rubble ranging in size from rocks to blocks mixed with brown karst soil. The ochre-coloured intercalations in the upper part of the upper sequence may indicate the presence of eroded sediment comparable with the lower sequence. The thinning of cave ceilings through erosion and karst denudation triggered collapsing. The sediment originates most probably from the weathered Eocene flysch. The Kozina profile is older than the Brunhes/Matuyama boundary (0.78 million years). The distribution of individual magnetozones supports the claim that the sediment is older than the end of the Olduvai epoch (1.77 million years) as the magnetostratigraphic profile concludes with the inverse polarity magnetozone and contains two normal polarity zones.

The Karst in the Breccia of Rebrnice in the Vipava Valley

The road runs in the NW–SE direction across two landscape units: across the bottom of the Vipava Valley and the southwestern slopes of Mount Nanos, Breg and Rebrnice. The geological, geomorphological, speleological, and hydrological diversity of the Slovene karst has been demonstrated also by the study of karstification of breccia that have been forming beneath the western slope of Mount Nanos. Water, in most cases percolating diffusely through the permeable surface of rubble or breccia and into the more or less impermeable flysch bedrock, is forming young karst phenomena. The percolating water accumulates where breccia is most consolidated. Earthworks have revealed the early stages in the formation of unique dolines. Since the motorway runs diagonally to the slopes, at the same time it cuts many and various relief forms in the upper and lower sections of the slopes. Road construction has also indirectly affected many, particularly fluvial relief forms. Due to the changes in the outflow from the slopes, changes in the old and the formation of the new relief forms are likely. Characteristic types of cavities developed in the young and very porous breccia which is consolidated only in places that lie on the more or less slanting flysch, i.e. on an impermeable bedrock. The true karst cavities are small and their development was influenced by the alluvium that has been depositing on their bottom and mainly fills them. They formed in a locally and periodically flooded zone or they were paragenetically enlarged. The largest cavities formed above the contact with the impermeable flysch bedrock where the big streams joined together. Their shape reflects the varying degrees of consolidation of breccia. In areas where breccia is less solid and along fissures they rise into domes. Along fissures which emerged due to the sliding of breccia and rubble down the slanting bedrock of frequently soggy flysch, fissure caves formed diagonally to the dip direction of the slope; some of them are very long and wide enough in places to make them accessible. Their walls are mainly covered with flowstone.