Nadja Zupan Hajna

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.

Karstic morphology in northern Sinus Meridiani, Mars

This work describes karst landforms observed in the northern Sinus Meridiani region located between 1°18′N to 2°30′N latitude and 2°30′W to 0°13′W longitude, which covers approximately 9,100 square kilometres, characterized by spectral signatures of evaporite minerals. An integrated analysis of the Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE), High Resolution Stereo Camera (HRSC), and Mars Orbiter Camera (MOC) imagery were used to perform a morphological and morphometric investigation of the study area. Results resolved three specific morpho-units, characterized by different doline features, which displayed distinct types and degrees of karstification in the study area. The landforms indicated different solutional properties of the units, and suggested enough liquid water in the recent past was present to form the karst landforms, emphasizing climatic changes during the Amazonian period occurred in the Sinus Meridiani area.

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.

Bat urea-derived minerals in arid environment. First identification of allantoin, C4H6N4O3, in Kahf Kharrat Najem Cave, United Arab Emirates

*audra@unice.fr Citation:

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.

The Section Between Divača and Kozina Revealed Many Characteristics of Karst Development

The motorway section from Divaca to Kozina, stretching 7.5 km, revealed 50 old caves, most of them filled with alluvium, many of which unroofed, and six of them qualifying as shafts. The old caves were once, in a time when the underground water table was still high enough, characterized by through-flowing water. At that time, the aquifer was encased in flysch high up and partly covered with it. The runoff from the flysch introduced loam and sand into the caves. After they had been dry for a while and featured flowstone deposits, flood waters filled the caves with fine-grained sediments in one of the last cave development phases. The alluvium sealed the cracks and withstood even the long periods of the aquifer development during which its surface lowered for several tens of metres. It thus appears that old unroofed caves are an important component of the karst surface, providing clues to the development of the aquifer. Seeing as today the underground water level is 200 m and more below the surface, the traces of ancient water flow through the aquifer are now preserved only in caves and their respective alluvium. There was no indication of surface water flows that could have reshaped the aquifer’s surface. Old caves therefore contain the oldest geological traces dating back to the very early periods of the karst aquifer development. The more significant caves were able to be preserved. Today, these are either hidden below the road or accessible via man-made entrances.

Great Cavernosity Between Dane and Fernetiči Points to Diverse Karst Formation

The motorway route between Dane and Fernetici penetrates the karst ridge near Sežana, running over the karst plain to the border with Italy. The karst developed in Cretaceous limestone, which is intersected by smaller faults; this was best visible during the tunnel excavation under the Tabor Hill. It is quite remarkable the extent to which the surface of this part of the karst aquifer is dotted by old caves which are filled up by fine-grained alluvium and rubble and that had, at one point or another, lost their roofs. The higher parts of the aquifer have been subject to reshaping by long-term percolating water. Old caves reveal glimpses into a time when subsurface water was close to the actual surface of today. The continuous lowering of the karst regions led to the situation where today’s surface is lower than the original karst surface was. It appears that great cavernosity came about as the result of the high permeability of this part of the aquifer. The size of the passages suggests that they were passed by larger water flows which had originally completely filled the passages. It is assumed that a remnant of a flysch nappe was somewhere in the vicinity, feeding the aquifer with surface runoff. This is also suggested by the ponor properties of some caves. The cave bottoms display meandering patterns carved out by small-scale water flows. Even after the subsurface water table had dropped, flysch layers still lingered over the limestone. Despite the water drop, high waters occasionally still reached the caves, eventually filling them up with fine-grained sediments. In the cooler periods of the Pleistocene, some of the old caves were filled up by rubble.