ACTA CARSOLOGICA	28/2	18	293-305	LJUBLJANA 1999
COBISS: 1.01
AGE AND MAGNETISM OF CAVE SEDIMENTS FROM POSTOJNSKA JAMA CAVE SYSTEM AND PLANINSKA JAMA CAVE, SLOVENIA
STAROST IN MAGNETIZEM JAMSKIH SEDIMENTOV IZ POSTOJNSKEGA JAMSKEGA SISTEMA IN PLANINSKE
JAME, SLOVENIJA
STANKA [EBELA1 & IRA D. SASOWSKY2
1	Karst Research Institute ZRC SAZU, Titov trg 2, SI-6230 POSTOJNA, SLOVENIA
2	University of Akron, Department of Geology, Akron, OHIO 44325-4101, USA
Prejeto / received: 11. 11. 1999
Izvleček	UDK: 550.38:552.54(497.4)
Stanka Šebela & Ira D. Sasowsky: Starost in magnetizem jamskih sedimentov iz Postojnskega jamskega sistema in Planinske jame, Slovenija
Iz 3-eh razli~nih lokacij v Postojnskem jamskem sistemu in iz ene lokacije v Planinski jami smo analizirali 28 vzorcev za paleomagnetizem. To so prvi objavljeni rezultati o paleomagnetizmu jamskih sedimentov iz teh dveh jam. Sedimenti kažejo normalni magnetni obrat saj so bili odloženi v Brunhes normalnem obdobju (mlajše od 0,73 Ma). V zgornjem nivoju sistema Postojnskih jam pa sedimenti kažejo magnetni obrat, ki ga uvrš~amo med 0,73-0,90 Ma.
Ključne besede: paleomagnetne analize, jamski sedimenti, Postojnski jamski sistem, Planinska jama, Slovenija.
Abstract	UDC: 550.38:552.54(497.4)
Stanka Šebela & Ira D. Sasowsky: Age and Magnetism of Cave Sediments from Postojnska jama cave system and Planinska jama cave, Slovenia
We completed paleomagnetic analyses of 28 samples of cave sediments from 3 different locations in Postojnska jama cave system and one location in Planinska jama cave. These are the first published paleomagnetic studies of cave sediments from these caves. These sediments showed normal polarity, and were deposited during the Brunhes Normal Epoch (younger than 0,73 Ma). However, an additional more complex sediment deposit in an upper section of the Postojnska jama cave system, showed reverse polarity, probably indicating an age between 0,73 and 0,90 Ma.
Key words: paleomagnetic analyses, cave sediments, Postojnska jama cave system, Planinska jama cave, Slovenia.
INTRODUCTION
Mineralogical, geochemical, granulometrical, archeological studies on cave sediments from Slovene karst caves have been conducted in the past (Gospodaric 1976, Zupan-Hajna 1992, Istenic 1987, Kranjc 1989, Sušteršič & Mišic 1996), but the first paleomagnetic analyses were completed only recently (Bosak et al. 1998). In order to constrain the age of the caves, and to understand the genetic history of the sediments, we sampled from 3 locations within the caves that have significant sediment accumulations.
For the collection of cave sediments in the field we used methods developed by Schmidt (1982) and Sasowsky et al. (1995), measuring strike, dip direction, dip and tilt of sediment layers. Twenty-eight samples, including duplicates, were collected during the period of 11th to 23ld March 1999. Twelve samples (SLO001-SLO012) are from Planinska jama cave, 8 samples (SLO013-SLO020) are from Otoška jama cave, which is part of Postojnska jama cave system, and 8 samples (SLO021-SLO028) are from Postojnska jama cave.
Our goal was to determine the period of deposition of cave sediments in Planinska jama cave and Postojnska jama cave system and thus understand in which period the cave passages already existed in a shape and size very similar to their present one. To select the best sampling points in the caves, the literature of Gospodaric (1976) was studied. In 1987 he had collected some cave sediments from Planinska jama cave (Rudolfov rov) and Postojnska jama cave system, but unfortunately died before the paleomagnetic analyses were accomplished (personal information Drame, Mihevc and Sušteršič 1999). According to Gospodaric (1981) the oldest known cave sediment in the SW part of Slovenia is a colored chert gravel which is believed to be from the lower or upper-middle Pleistocene, Mindel glacial (between 0,35 Ma-0,59 Ma). Those gravels can be found also on the surface outside the caves in the Pivka flysch basin. Above the colored chert gravels Gospodaric (1976) described an "older laminated loam" (older in relation to other loams in the cave). Our purpose was to sample that loam and determine the time of its deposition.
GEOLOGICAL AND SPELEOLOGICAL DATA
Postojnska jama cave system (20 km) and Planinska jama cave (6 km) (Fig. 1) both belong to the same hydrological system, although passages have not been connected yet through diving exploration (Cave Cadastre IZRK ZRC SAZU). Both caves are developed in Cretaceous limestone, which in tectonic sense belongs to Javorniki-Snežnik tectonic sheet. At the entrance of Postojnska jama cave the river Pivka sinks from the Eocene flysch into the cave's underground, and the entrance to Planinska jama cave is one of the Unica river springs. Both rivers belong to the Black Sea watershed, although the Adriatic Sea is much closer.
Postojnska jama cave system and Planinska jama cave are situated between two Dinaric oriented (NW-SE) regional faults, Predjama fault in the South and Idrija fault in the North.
According to Gospodaric (1976) the colored chert gravel can not be found in all passages of Postojnska jama cave system. The preserved sediments are found in still-active water passages such as Podzemeljska Pivka, Martel's collapse chamber, Otoška jama. They were probably deposited also in higher, presently inactive. passages where younger sediments overlay them. In Planinska jama cave the colored chert gravels are preserved in Pivka and Rak passages.
Fig. 1: Geographical and basic geological position of sampling points with the lithological column of cave fills in Planinska jama cave (after Gospodaric, 1976).
Sl. 1: Geografski in osnovni geološki položaj vzorcev z litološkim stolpcem jamskih zapolnitev v Planinski jami (po Gospodaricu, 1976).
SAMPLING POINTS
Gospodaric (1976) described the column of cave fills from Postojnska jama cave system and Planinska jama cave. The oldest is the layer of colored chert gravels, above which is an "older laminated loam" (Fig. 1). Overlying this are limestone gravels, and above them white chert gravels. Following this are a younger laminated loam, and above that younger and older concretions and boulder rocks. The complete column of cave fills is not preserved in all sampling points.
In our field examinations we were guided by Gospodaric's (1976) ideas about the deposition and correlation of cave fills in different cave passages. Using these, we sought to sample the oldest cave sediments.
Fig. 2: Demagnetization diagram for sediment sample SLO013 from Oto{ka jama cave. Sl. 2: Diagram demagnetizacije za sedimentni vzorec SLO013 iz Oto{ke jame.
MAGNETISM OF SEDIMENTS
Sediments were collected in oriented plastic cubes, and were analyzed at the University of Pittsburgh Paleomagnetic Laboratory (Pennsylvania, USA). Samples were measured for natural remanent magnetization (NRM) in a cryogenic (SQUID) magnetometer. They were then subjected to alternating-field (AF) demagnetization to reveal underlying magnetic components. Both the magnetometer and AF demagnetization device were housed in a magnetically shielded room that reduced the present field strength to 1500 gammas (about 10 gammas in the magnetometer).
Although the 28 samples measured for this study all exhibited normal polarity, there was some variability in demagnetization behavior. Initial sample strength ranged from 6 x 10-7 kAm-1 to 1 x 10-5 kAm-1. Some samples, such as SLO013 from Oto{ka jama (Fig. 2), showed very clean demagnetograms, with decay of the magnetic vector following directly from the expected NRM towards the origin. Other samples showed less direct paths, some with NRM slightly East and inclination slightly greater than that expected. We interpret the samples to contain a detrital remanent magnetization, with a similarly oriented viscous component.
PLANINSKA JAMA CAVE RUDOLFOV ROV PASSAGE
Rudolfov rov is an approximately 150 m long passage in the southern part of Rakov rokav in Planinska jama cave (Fig. 1). There is a smaller stream in the passage flowing from south to the
north. In Rudolfov rov the thickness of the older laminated loam is at least 5 m. We did not find colored chert gravels beneath the loam. Gospodaric (1976) suggested that the older laminated loam lies on the passage bedrock of Rudolfov rov. On top of the older laminated loam, white chert gravels can be found. The southern part of Rudolfov rov passage where we took 12 samples is developed in a fault dipping towards the SW at about 70°.
Fig. 3: Planinska jama cave. Column of clay for paleomagnetic samples + detail. Sl. 3: Planinska jama. Stolpec gline za paleomagnetne vzorce + detajl.
In Rudolfov rov the sampled profile of older laminated loam is 3,94 m high. The lowest samples (SLO001-SLO002) were taken at 460 m a.s.l. (Fig. 3). The color of the sampled sediments was determined after Munsell Soil Charts:
-SLO001-SLO002 (10YR 5/6-yellowish brown) -SLO003-SLO004 (10YR 5/6-yellowish brown) -SLO005-SLO006 (10YR 6/8-brownish yellow) -SLO007-SLO008 (10YR 5/4-yellowish brown) -SLO009-SLO010 (10YR 5/8-yellowish brown) -SLO011-SLO012 (10YR 5/6-yellowish brown)
Zupan-Hajna (1992) analyzed some sand and loam samples from Rakov rokav. Its mineral composition shows the origin from flysch with prevailing quartz (about 70%), plagioclase and muscovite (10%). Other minerals such as chlorite, kaolinite and goethite are only found in trace amounts. In some samples calcite and dolomite can be found due to the erosion and corrosion of cave walls.
The samples for paleomagnetic analyses were taken from the same profile as Gospodari~ used in 1987. All 12 samples have normal polarity, which we interpret to be younger than 0,73 Ma. Paleomagnetic analyses support the interpretation of Gospodari~ (1982), that the older laminated loam from Planinska jama cave is probably from the Mindel glacial (0,35-0,59 Ma).
POSTOJNSKA JAMA CAVE SYSTEM
Zupan-Hajna (1992) conducted mineralogical analysis of cave sediments from Postojnska jama cave by X-ray difractometry. The prevailing minerals are quartz, calcite, illite, kaolinite, muscovite, chlorite, feldspar and goethite in traces. On average, the composition of cave sediments is quartz (75%), then microcline (8%) and calcite (5%). There is also some chlorite and muscovite (together 7%). Other minerals are in traces. Such mineral composition shows that the source for cave sediments is the Eocene flysch from Pivka valley.
OTOŠKA JAMA
Oto{ka jama is part of Postojnska jama cave system (Fig. 1). All together 8 samples were taken from 2 different sampling points (distance between them is 25 m). Samples SLO013-SLO016 (Fig. 4 a) were at 529-528 m a.s.l., and samples SLO017-SLO020 were taken at 524 m a.s.l. (Fig. 4 b). According to Gospodari~ (1976) both sampling points belong to the older laminated loam. Samples SLO013-SLO016 are from higher parts of the layer and samples SLO017-SLO020 are just above the colored chert gravels. The color of the sampled sediments was determined after Munsell Soil Charts:
-SLO013-SLO014 (10YR 5/4-yellowish brown) -SLO015-SLO016 (10YR 5/4-yellowish brown) -SLO017-SLO018 (10YR 5/8-yellowish brown) -SLO019-SLO020 (10YR 5/4-yellowish brown)
Fig. 4 a: Oto{ka jama cave. Column for paleomagnetic samples with prevailing clay lithology. Sl. 4 a: Oto{ka jama. Stolpec paleomagnetnih vzorcev. Spodnja plast in plast, ki je debela 0,4 cm je pe{~ena glina, preostali stolpec je glina.
Fig. 4 b: Oto{ka jama cave. Column for paleomagnetic samples. Between colored chert gravels and collapsed blocks of Cretaceous limestone is sandy clay.
Sl. 4 b: Oto{ka jama. Stolpecpaleomagnetnih vzorcev. Med zgornjo plastjo podornih blokov krednega apnenca in spodnjo plastjo pisanega ro'enca je pe{cena glina.
In Oto{ka jama there are more examples of old collapses in cave bottom. One such collapse is an 18 m deep shaft that in the upper part exposes about a 2 m thick layer of red loam with bones of cave bears (Ursus Speleous). This species was extinct before 15.000 years. The collapsed shaft is therefore younger than 15.000 years, and proves that beneath some passages of Oto{ka jama there are younger, presently active passages.
Samples SLO017-SLO020 were taken on the margin of a very large collapse block. These sediments were difficult to sample, due to induration with more calcite, and were also sandier than the loam.
All analyzed samples from Oto{ka jama are normal polarity (younger than 0,73 Ma). Based upon morphology, the large collapse block was emplaced before the deposition of the cave sediments sampled as SLO017-SLO020.
POSTOJNSKA JAMA, MALE JAME
In Male jame 8 samples (SLO021-SLO028) were taken from a 2,35 m thick profile (Fig. 5). Lower sediments (SLO021-SLO022) are from 525 m a.s.l.. The color of the loam is almost the same as the one from Oto{ka jama. Above the sample SLO028, the sediment is more sandy and harder to sample than the loam. We can find colored flysch gravels, but it is difficult to determine their real position of sedimentation. We believe that the samples belong to the upper part of the older laminated loam. The paleomagnetic analysis showed the deposition time of the sediments to be one of normal polarity (younger than 0,73 Ma).
Fig. 5: Postojnska jama cave system, Male jame. Column of clay and sandy clay (under samples 025-026) for paleomagnetic samples + detail.
Sl. 5: Postojnski jamski sistem, Male jame. Stolpec paleomagnetnih vzorcev + detajl. V stolpcu prevladuje glina, pod vzorcema 025-026 je plast pe{~ene gline. V detajlu je plast med vzorcema 026 in 028 glina temne barve, verjetno bogata z Manganom.
The color of the sampled sediments was determined after Munsell Soil Charts: -SLO021-SLO022 (10YR 5/6-yellowish brown) -SLO023-SLO024 (10YR 5/8-yellowish brown) -SLO025-SLO026 (10YR 5/6-yellowish brown) -SLO027-SLO028 (10YR 5/4-5/8-yellowish brown)
CONCLUSIONS
Paleomagnetic analyses of these cave sediments from Slovenia showed that all of the 28 samples have normal polarity. The origin for all analyzed cave sediments was the Eocene flysch. For the older laminated loam, which is deposited directly above colored chert gravels in Postojnska jama cave system and Planinska jama cave, the paleomagnetic results indicate that it is younger than 0,73 Ma. This is in concordance with the ideas of Gospodari~ (1976, 1981, 1982) for deposition of colored chert gravels in the Mindel glacial period (0,35-0,59 Ma). Gospodari~ (1976, 1981) interpreted the deposition periods of cave sediments based on 14-C, U/Th methods, and correlation of altitudes of cave sediments from different profiles.
However, Gospodari~'s (1981) theory that the oldest known cave sediment in the SW part of Slovenia is the colored chert gravel is not proved with our paleomagnetic studies. Samples from a small natural passage which is accessible from artificial passage (Fig. 1) between Postojnska and
Črna jama cave (SLO029-SLO032) showed reversed polarity and are at least 0,73-0,90 Ma old (Sebela & Sasowsky, in review), because they belong to Matuyama Reversed Epoch (0,73-2,48 Ma). Flysch deposits of sands (and just a thin layer of clay) are the oldest cave deposits found in Postojnska jama cave system, and their deposition time is older than the one of the colored chert gravels and older laminated loam.
We think that the deposition of the oldest flysch sands found in Postojnska jama cave system belong to earlier stages of cave development, probably to the period when the present-day small river Nanoscica was an important tributary to the underground drainage. According to Gams (1965), Nanoscica previously sunk into the present dry blind valley at Črna jama.
There is also a very obvious difference in the granulometric properties of the "younger" older laminated loam and the "older", mostly sandy cave sediments. Both were deposited not only in different time periods, but also under different climatic conditions and at different stages of the cave passage development. The color of the "younger"older laminated loam is mostly yellowish-brown, and the color of the clay layer inside the "older" mostly sandy cave sediments is reddish-brown.
Brodar (1966) determined that in Čarobni vrt of Postojnska jama cave there is a substantial accumulation of sediment, which is older than the Atlantic interstadial or interglacial flowstone. The passage and its larger extension should already have been developed in the older Pleistocene (younger than 1,8 million years) or even in the younger Pliocene (3-1,8 million years). Based upon the presence of cave bear bones, the flowstone is attributed to the Riss-Würm (74.000-125.000 years).
With studies of cave sediments, Gospodaric (1976) determined 10 principal development stages of the Postojnska jama cave system. They belong to the period between first alohtone sedimentation of cave sediments until the present. The stages have been determined by absolute dating of flowstone, and according to the relative age of cave sediments. The older laminated loam belongs to the 2nd stage, which represents a typical accumulative period (middle Quaternary).
According to Gospodaric (1976), the older laminated loam and colored chert gravel could both originate from the interglacial period (Mindel-Riss), or could be from different climatic events. The older laminated loam could be from the interglacial accumulative period, and the colored chert gravel could be from the older glacial (Mindel) period.
Gospodaric (1976) doesn't agree with Brodar's ideas (1966) that the oldest cave sediments in Postojnska jama cave system are as old as lower Pleistocene.
ACKNOWLEDGMENTS
We would like to thank William Harbert (Department of Earth and Planetary Sciences, University of Pittsburgh, Pennsylvania, USA) who is responsible for the Paleomagnetism Laboratory for his help during our analyses on April 26-27th 1999.
Stanka Sebela would like to thank Ira D. Sasowsky for his invitation to come to University of Akron, Ohio, USA as a research scholar in geology from 14th April until 14th May 1999. The grant for Stanka Sebela was provided by Research Fund of ZRC SAZU (Raziskovalni Sklad ZRC SAZU). For help on the field work with collecting samples for paleomagnetic analyses, we thank Andrej Mihevc and Franjo Drole, both from Karst Research Institute ZRC SAZU. Jure Hajna and Franjo
Drole (Karst Research Institute ZRC SAZU) provided figure 1. Selena Walko (University of Akron)
helped with the preparation of several figures.
Thanks to the head of Karst Research Institute ZRC SAZU, Tadej Slabe, for supporting the
decision of Stanka Sebela to expand her scientific expertise at the University of Akron, Ohio, USA.
REFERENCES
Bosak, P., Pruner, P. & Zupan Hajna, N., 1998: Palaeomagnetic Research of cave sediments in SW Slovenia.- Acta carsologica SAZU, XXVII/2, 151-179, Ljubljana.
Brodar, S., 1966: Pleistocenski sedimenti in paleolitska najdi{~a v Postojnski jami.- Acta Carsologica SAZU, 4, 57-138, Ljubljana.
Cave Cadastre IZRK ZRC SAZU
Gams, I., 1965: H kvartarni geomorfogenezi ozemlja med Postojnskim, Planinskim in Cerkni{kim poljem.- Geografski vestnik 37, 61-101, Ljubljana.
Gospodari~, R., 1976: Razvoj jam med Piv{ko kotlino in Planinskim poljem v kvartaiju.- Acta Carsologica SAZU, 7, 8-135, Ljubljana.
Gospodari~, R., 1981: Generacije sig v Klasi~nem krasu Slovenije (Sinter Generations in Classical Karst of Slovenia).- Acta Carsologica SAZU, 9, (1980), 87-110, Ljubljana.
Gospodari~, R., 1982: Stratigrafija jamskih sedimentov v Najdeni jami ob Planinskem polju (Stratigraphy of cave sediments in the Najdena jama at Planina polje).- Acta Carsologica SAZU, 10, (1981), 173-195, Ljubljana.
Isteni~, M., 1987: Geokemi~na primerjava ilovic v Taborski jami s tlemi v okolici.- Diplomsko delo, 1-85,Univerza v Ljubljani, Ljubljana.
Kranjc, A., 1989: Recent fluvial cave sediments, Their origin and role in speleogenesis.- SAZU, Dela Opera 27, 167 pp., Ljubljana.
Munsell Soil Color Charts, 1975, Macbeth Division of Kollmorgen Corporation, Baltimore, Maryland.
Sasowsky, I.D., White, W.B., and Schmidt, V.A., 1995, Determination of stream incision rate in the Appalachian plateaus by using cave-sediment magnetostratigraphy: Geology, v. 23, p. 415-418.
Schmidt, V.A., 1982, Magnetostratigraphy of sediments in Mammoth Cave, Kentucky: Science, v. 217, p. 827-829.
[u{ter{i~, F. & Mi{i~, M., 1996: Some minerals from Najdena jama.- Acta Carsologica SAZU, 25, 291-305, Ljubljana.
Zupan-Hajna, N., 1992: Mineralna sestava mehanskih sedimentov iz nekaterih delov slovenskega krasa.- Acta Carsologica SAZU XXI (1992), 115-130, Ljubljana.
STAROST IN MAGNETIZEM JAMSKIH SEDIMENTOV IZ POSTOJNSKEGA JAMSKEGA SISTEMA IN PLANINSKE JAME, SLOVENIJA
Povzetek
Iz Postojnskega jamskega sistema in Planinske jame smo vzeli 28 vzorcev jamskih sedimentov (Sl. 1) za paleomagnetne analize, ki smo jih opravljali na Department of Earth and Planetary Sciences, University of Pittsburg, Pennsylvania, USA. Pri jemanju vzorcev smo se opirali na predhodne raziskave Gospodari~a (1976, 1981, 1982). Po Gospodari~u je najstarej{i sediment prod pisanega roženca ~ez katerega je odložena starej{a pasovita ilovica. Vzorci SLO001-SLO012 so bili vzeti v Rudolfovem rovu Planinske jame (Sl. 3), vzorci SLO013-SLO020 so bili vzeti iz dveh lo~enih profilov v Oto{ki jami (Sl. 4 a in b), ki sta med seboj oddaljena 25 m. Vzorce SLO021-SLO028 (Sl. 5) smo vzorcevali v Postojnski jami, in sicer v Malih jamah.
V ve~ini primerov gre za glinaste vzorce, ki pa ponekod vsebujejo ve~ pe{~ene frakcije. Glede na {tudije Gospodari~a (1976, 1981, 1982) smo vzorcevali predvsem starej{o pasovito ilovico. V Oto{ki jami smo z vzorci SLO017-SLO020 zajeli plast tik nad prodom pisanega roženca. Analiza vseh 28 vzorcev je pokazala normalni obrat, zato jih uvr{~amo obdobje Bruhnes (mlaj{e kot 0,73 Ma).
Gospodari~eva trditev, da je prod pisanega roženca najstarej{i jamski sediment v Postojnskem jamskem sistemu in Planinski jami, pa ne drži, saj smo v manj{i dvorani v umetnem tunelu med Postojnsko in Oto{ko jamo v bolj pe{~enih sedimentih dolo~ili negativni magnetni obrat (0,73 -0,90 Ma).