Pavel Bosák

Magnetostratigraphy of Cave Sediments: Application and Limits

The application of the magnetostratigraphy for dating of clastic and chemogenic cave sediments has been limited by the complex conditions underground and the lack of age constraints on these deposits for correlation with geomagnetic polarity timescale (GPTS). Without age constraints any correlation of obtained results cannot be explicit. Additionally, the dynamic character of cave infilling, exhumation and fossilisation expressed by unconformities within preserved sedimentary profiles can hide a substantial part of the geological time. The detailed internal division of deposits (breaks in deposition and related processes) and scarcity of fossils make the correlation of obtained magnetostratigraphic log with geomagnetic polarity timescales sometimes problematic. The analytical results confirmed that the complete step/field procedure offered by demagnetisation methods must be applied. The application of complete analysis only to pilot samples and shortened, selected field/step approach, to other samples did not offer sufficient data set for reliable interpretation.

Early diagenetic origin and persistence of gamma-ray and magnetosusceptibility patterns in platform carbonates: comparison of Devonian and Quaternary sections

Abstract Gamma-ray logs from boreholes in the Middle–Upper Devonian carbonate platform of Moravia display tripartite anomalies at locations, where lithological and biostratigraphic data suggest the occurrence of 4th order sedimentary cycle boundaries. Further, where sedimentary boundaries have been changed by later development of caves (usually phreatic caves changed to unroofed caves––erosion), the carbonate infillings in these corroded cycle boundaries are marked by another pattern that consists of a smooth symmetrical peak on gamma-ray activity in vertical section. The details procured using gamma-ray spectrometric and magnetosusceptibility methods suggest that the upper peak of the tripartite pattern corresponds solely to uranium concentration (flooding surface). The middle peak is marked by a thorium signal and a magnetosusceptibility response from paramagnetic minerals (paleosols). The lower peak corresponds to trapped uranium and microbial magnetite in cemented rock pores (originally dysoxic microenvironments in calcite). The boundaries marked with filled caves display only one broad and symmetrical uranium-related peak, and the thorium peak that is roughly similar to that seen at normal boundaries, but it is shifted slightly downward. At boundaries with caves the magnetosusceptibility peaks are shifted downward considerably, and may even occur within the underlying host rock. The question of whether these patterns are a primary imprint of early diagenetic influences or a much later redistribution that originated during pressure solution and cementation, was answered by study of Late Quaternary sections on San Salvador Island, Bahamas. This pragmatic test on young carbonate sediments confirmed the early origin and fixation of these geophysical patterns.

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.

A pragmatic test of the early origin and fixation of gamma-ray spectrometric (U, Th) and magneto-susceptibility (Fe) patterns related to sedimentary cycle boundaries in pure platform limestones

In a pragmatic test conducted on vertical stratigraphic sections in Quaternary platform limestones of San Salvador Island, The Bahamas, gamma-ray spectrometric (GRS) and magnetosusceptibility (κ) data confirmed that characteristic geophysical patterns are coupled with depositional cycle boundaries. These geophysical patterns appear to develop in the early stages of diagenesis and are long lasting, because similar patterns are found both in the very young Bahamian limestones and in very old Devonian (Givetian-Frasnian) platform limestones of Moravia, Czech Republic. Because the Devonian limestones retain gamma ray and magnetic signatures similar to those seen in the Bahamian rocks, these signals are apparently resistant to changes that occur in later diagenetic alteration, including deep-burial diagenesis and 380 million years of rock-fluid interactions. Each sedimentary cycle on the Bahamian carbonate platform is marked by a terra rossa paleosol horizon that represents a lowstand emergent surface. The paleosol is typically characterized by a GRS-spike related to increased Th concentration. There is only a subtle downward infiltration of that GRS signal, but the Th signal may diffuse upward via sediment recycling. Two U-related GRS maxima are regularly developed within short distances below and above the cycle boundary. The lower anomaly reflects U enrichment in the sub-soil cementation zone, whereas the upper anomaly is related to increased U-content in the flooding beds of the next cycle. Such a combination of one Th-spike between two U-anomalies forms a distinctive tripartite GRS pattern.The κ-anomalies form a bimodal signal that consists of a narrow but extraordinarily strong positive κ-anomaly that is coincident with the Th-spike, and another spike that is developed in the sub-soil cementation zone. In cases where a buried cycle boundary forms the truncated floor of a horizontal cave that is filled with carbonate sediment, both U and Th GRS peaks are broadened. The κ-curves also display elevated but strongly oscillating values across the cave fill. The spikes are arranged asymmetrically downward and the strongest spike corresponds to infiltration/cementation of the cave floor. The evidence from the Quaternary limestones suggests that these two patterns (the tripartite Th and U pattern of the standard cycle boundary, and the broadened pattern related to filled caves) have an early origin. In spite of large inhomogeneities on cycle-boundary surfaces, the above geophysical patterns appear to be distinctive, differ from the normal GRS-and κ-backgrounds of platform carbonates, and appear stabile over the long term. This test indicates that these two patterns may be useful for recognition of exposure surfaces/cycle boundaries via routine pattern searches in GR, GRS and κ well-logs from platform limestone sequences of a wide range of ages and paleoenvironments.

Agriculture and nature conservation in the Moravian karst (Czech Republic)

Moravian Karst is a narrow strip of limestone with long history of settlement, agricultural use and man impact to karst. It is naturally divided into smaller units karst plateaus separated by deep valleys (glens). Each plateau has different proportion of land use, i.e. the percentage of agricultural land, forests, etc. The agricultural land constitutes now up to 70% in the north and max. 30% in the centre and south of the total area of plateaus. Intensive agricultural use of the arable land since 60ties of this Century caused great impact to quality of soils and groundwater by overdoses of fertilisers and other artificial chemical substances. Detailed research in 1980 to 1997 resulted in a plan of care based on the zonation of land. There were defined zones with different degree of restriction of land use, agricultural activities and application of fertilisers and biocides. Arable lands has been gradually changed to meadows and pastures by introduction of grass since 1987 in the most strictly protected zone to protect especially subsurface karst forms.

Magnetic Record in Cave Sediments: A Review

Dating cave sediments by the application of the palaeomagnetic method – magnetostratigraphy – is a difficult and sometimes risky task, as the method is comparative in its principles and does not provide numerical ages. For dating clastic cave sediments and speleothems it is limited by the complex conditions occurring underground so that it is often necessary to combine it with other methods that offer supplementary absolute-, calibrate-, relative- or correlate-ages. Interpretation of magnetostratigraphic results faces other serious problems that may endanger palaeomagnetic studies in given caves if they are not detected. The sedimentary fills of a number of profiles are separated into individual sequences and cycles, divided by breaks in deposition (unconformities). The dynamic character of cave fill deposition is reflected in the start or termination of individual magnetozones at unconformities in a number of profiles. The general character of cave depositional environments with their numbers of post-depositional changes, hiatuses, reworking and re-deposition does not allow precise calculation of the temporal duration of individual interpreted magnetozones. All these factors contribute to the fact that exact calibration of the geometric characteristics of the magnetostratigraphic logs with the GPTS cannot be attained at all or only with problems, if it is not adjusted using results of other dating methods.

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.