dating speleothems from the silberen cave system and surrounding areas: speleogenesis in the muota...

307Dating speleothems

Dating speleothems from the Silberen Cave System and surrounding areas:Speleogenesis in the Muota Valley (Central Switzerland)

Andres Wildberger, Mebus A. Geyh, Urs Groner, Philipp Häuselmann,Friedrich Heller and Michael Ploetze

with 5 fi gures and 4 tables

Abstract. Inactive stalagmites from the Silberen cave system and its surroundings (Muota Valley, Central Switzerland) have been dated by the 230Th /U method (alpha spectrometry and TIMS). Several samples yielded ages of approximately 200 ka (MIS 7) and 320 ka (MIS 9), others were beyond the upper limit of the method (about >350 ka) or might have behaved as open systems with respect to uranium.

Th e following complementary dating methods were applied: 234U/238U, paleomagnetism, palynology and electron spin resonance. Paleomagnetism yielded positive inclination of the natural remanent magnetization of two stalagmite samples, corresponding to the Brunhes-epoch (<780 ka) or older epochs with positive inclination (Jamarillo 970–900 ka, Olduvai 1870–1670 ka). Th e paly-nological investigations of four samples indicated no deviation to the recent vegetation; especially Pliocene and Early Pleistocene pollen and spores were absent. Th e combination of 230Th /U ages, palynological analysis and paleomagnetic records provides evidence of stalagmite ages between 780 and 200 ka. 234U/238U and ESR dating did not yield conclusive results.

Th e uplift rate of the mountains, the corrosion rate of the karstifi ed surface and the incision of the valley bottom, suggest that karstifi cation started 4 to 3 Ma (Middle to Upper Pliocene). Th e dated speleothems are therefore substantially younger than the beginning of speleogenesis.Key words. Dating of speleothems, speleogenesis, Axen nappe (Helvetic realm), Silberen cave system, Hölloch cave system, Muota valley

Zusammenfassung. Datierung von Tropfsteinen aus dem Silberen-Höhlensystem und dessen Umgebung: Speleogenese im Muotatal (Zentralschweiz). Inaktive Stalagmiten aus dem Silberen-Höhlensystem und dessen Umgebung (Zentralschweiz) sind mittels der 230Th /U-Methode (Alpha-Spektrometrie bzw. TIMS) datiert worden. Einige Proben ergaben 230Th /U-Alter zwischen von 320 ka (MIS 9) und 200 ka (MIS 7), andere lagen ausserhalb des Zeitfensters der 230Th /U-Methode (>350 ka), wieder andere könnten off ene Uran-Systeme gebildet haben.

Ergänzend zu den numerischen Datierungen wurden versuchsweise weitere Datierungsmethoden eingesetzt: 234U/238U, Paläomagnetik, Palynologie und Elektronen-Spin-Resonanz. Die Paläomagnetik zeigte bei den zwei untersuchten Tropfsteinen eine positive Inklination der natürlichen remanenten Magnetisierung, was der Brunhes-Epoche (<780 ka) oder einer älteren Epochen mit positiver Inkli-nation entspricht (Jamarillo 970–900 ka, Olduvai 1870–1670 ka). Palynologisch konnten bei den vier untersuchten Proben keine Diff erenzen zur heutigen Vegetation festgestellt werden, insbesondere fehlten pliozäne oder altpleistocäne Pollen- und Sporenformen. Die 230Th /U-Alter, die Ergebnisse der palynologischen und der paläomagnetischen Untersuchungen weisen somit für die datierten Stalagmiten auf ein Alter zwischen 780 und 200 ka. Die 234U/238U- und die ESR-Methode lieferten keine schlüssigen Resultate.

Aus der bekannten Hebungsrate des Gebirges, dem korrosiven Abtrag der Karstoberfl äche sowie der Eintiefung des Muotatals wurde abgeleitet, dass die Verkarstung im Untersuchungsgebiet vor

©2010 Gebrüder Borntraeger Verlagsbuchhandlung, Stuttgart, Germany www.borntraeger-cramer.deDOI: 10.1127/0372-8854/2010/0054S2-0016 0372-8854/10/5402-0016 $ 5.50

Zeitschrift für Geomorphologie Vol. 54, Suppl. 2, 307–328 ArticleStuttgart, März 2010

308 Andres Wildberger et al.

4 bis 3 Ma (Mittleres bis Oberes Pliozän) begonnen hat. Die bislang datierten Karsthöhlensedimente sind nach diesem Modell also wesentlich jünger als der Anfang der Speleogenese.

Stichwörter. Tropfsteindatierung, Höhlengenese, Axen-Decke (Helvetikum), Silberen- Höhlensystem, Hölloch-Höhlensystem, Muotatal

1 Introduction

Caves and their sediments are well known as recorders of environmental change (e.g. Burns et al. 2001, Fairchild et al. 2006). However, normal karstic caves (caves formed by meteoric water) are also intimately linked to landscape evolution, especially valley deepening and subsequent lowering of the base level (Ford & Williams 2007). Caves may be useful in interpreting phases of valley deepening as well as the deepening rate (Stock et al. 2005, Häuselmann et al. 2007). Such information is particularly useful in areas where the surface record is absent due to erosion. Th is is the case in most mountain belts, where net erosion almost always prevails over accumulation (Kühni & Pfiffner 2001, Schlunegger & Hinderer 2003), and where repeated glacial advances have eff ectively obliterated older accumulations (Beck 1954, Schlüchter 2004).

Generally, the age of a cave system is only deducible in an indirect or relative manner (Audra et al. 2006):

• Th e age of the cave-containing rock gives a (mostly unrealistic) maximum value;

• Erosional removal of overlying formations (the beginning of speleogenesis under unconfi ned conditions) can possibly be detected in the sediments of the fore-land;

• Upper cave levels are normally older than lower ones;

• Cave levels and dated river terraces can be correlated;

• Speleothems can be dated.

With speleothems, we can date the period of speleogenesis during which the concerned part of the cave is no longer frequently fl ooded (transition from the epiphreatic to the vadose phase). Th erefore, dating of speleothems provides information about the age of the active phase of the younger or lower cave level. Dating speleothems is the most common method for dating speleogenesis (Häuselmann et al. 2008).

Th e Muota valley in Central Switzerland, with known caves of a total length of more than 250 km, is one of the major alpine karst regions of the country. Th e present paper makes a contribution to the understanding of the speleogenesis of this karstic region.

For the present study we sampled speleothems at high altitudes to overcome the prob-lems with open systems by fl ooding during glacial periods that were encountered earlier (Fig. 1; Wildberger et al. 1991). Th is paper presents results of stalagmite investigations from the Silberen cave system and the cave Himmelstor nearby.


2 Geological overview

Th e investigated karst systems are within the Axen nappe of Central Switzerland (Helvetic realm of the Alps). Th is nappe contains formations from Triassic to Paleogene periods. In the northern (frontal) part of the nappe, a stack of slices consisting mainly of Creta-ceous formations is found. Within this region a karst system has developed, giving rise to the Hölloch and Silberen cave. Th e main parts of these caves are within the Urgonian (Barremian – Aptian) and Seewen limestone (Cenomanian – Coniacian). In parts of the region, the Drusberg marls (Barremian) form a basal aquitard, while rocks of the Garschella layers (Aptian – Cenomanian) form an only locally karstifi ed layer between the Urgonian and the Seewen limestone. Th e individual slices are stacked one on top of the other in a fl at and ramp style. It should be noted that the main karstifi ed unit, the Urgonian limestone, about 200 m thick, can be doubled or tripled in thickness tectonically (Fig. 2).

Above the Axen nappe, there are erosional remains of higher nappes (klippe of the Roggenstöckli peak, reversed series of the Toralp nappe, Hantke 1961). Th e karstifi ed surface of the Axen nappe in the region of the Hölloch and Silberen cave systems and their associated watersheds are less than a few hundred meters below the overthrust plane of the overlying Drusberg nappe (Fig. 2).

Th e strata of the Drusberg nappe usually range from the basal Cretaceous to the Paleo gene. Th e lithology is comparable with corresponding formations of the Axen nappe. Urgonian limestone is also the main karst rock in the Drusberg nappe. Because there are thick marly Lower Cretaceous layers at the base of this nappe, the karst systems in the Axen and in the Drusberg nappes are completely separated from each other.

3 Samples and site description

Previously we had sampled stalagmites in the longest cave of the region, the Hölloch (length 194.5 km, depth 939 m; as of 2008, Wildberger et al. 1991). Th e Hölloch is a phreatic cave system with a few vadose infl uences (Fig. 1). For dating these speleothems we used the 14C and the 230Th /U methods. Th e oldest stalagmites were observed to be open systems with respect to uranium and carbon, thus no reliable absolute ages of the speleothems were obtained (e.g. top of stalagmite «older» than the bottom). However, it was suggested that these stalagmites possibly grew in MIS 5 and were fl ooded during highstands of the Pleisto cene valley glacier (Fig. 1). Th e loam-covered surface of these stalagmites provides evidence for this. Because of these results, it was decided to sample another set of stalagmites in higher caves of the area.

Th e Silberen cave system sampled in this study (length 37.7 km, depth 888 m; state 2008) is a phreatic cave with local vadose infl uences. Th is cave system is situated east and upstream of Hölloch cave. We assume it was above the water level during Upper Pleistocene (Fig. 1).

An additional sample was collected in a small cave named Himmelstor. Th is 100 m long cave is on the opposite side of the Muota valley. Th e known part of the cave is obstructed by detrital material (sand, gravel, transported speleothems). Th e Himmelstor cave does not belong to the Hölloch karst system, but it is drained to the same spring group, has a similar geology and therefore it may have a comparable speleogenesis.


Westkluft

Sintergang

Himmelstor

Seilfrässer

Fassadengang

Kristallkluft

Muo

taVa

lley

mai

nso

urce

702'

000

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000

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709'

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710'

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710'

000

203'000

203'000

204'000

204'000

205'000

205'000

Westkluft

Sintergang

Himmelstor

Kristallkluft

Seilfrässer

Fassadengang

702'

000

703'

000

704'

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705'

000

706'

000

707'

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708'

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709'

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ross

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tion

(fig.

2)

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1. M

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nd p

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cted

cro

ss se

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loch

and

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BödmerenDrusberg

NNW SSE

Charetalp

Wang-formation (Drusberg nappe only) Tertiary, Seewen limestone, Garschella-formation

Drusberg-formation

Quarternary

Urgonian limestone

Siliceous limestone

Öhrli limestone

Schilt-formation, Dogger

Palfris marl, Vitznau marl, Öhrli marl, Zementstein-formation

Quinten limestone

Lias

position of Silberen system and Hölloch cave

overthrust

fault

main overthrust(base of Drusberg nappe)

500

2'500

Drusberg nappe Axen nappe s.l.

m a.s.l.

500

2'500

m a.s.l.

after Hantke (1961)

Jura

ssic

Cre

tace

ous

0 0.5 1 2 km

Fig. 2. Geological cross-section (after Hantke 1961), position see Fig. 1.


Th e main criterion for the selection of the sampled stalagmites was their position above the last-glacial water table caused by the dam of the valley glacier. Additionally, the sample points were either hidden or plain speleothems without æsthetic qualities. Stalagmites form in vadose or upper epiphreatic conditions; their presence in phreatic passages indicates therefore that these passages were for the most part above the watertable during the growth of the stalagmites. Th e minimal age of fossilization of the passages can be dated this way.

Th ree samples from the Igluschacht cave (part of the Silberen cave system; Fig. 1) were taken. Th e sampling points (galleries «Fassadengang», «Seilfrässer» and «Westkluft») are within the Urgonian limestone in a phreatic gallery with a vadose overprint, oscillating around 2070 m asl. Based on the altitude, the gallery may be one of the oldest levels of the karst system. Horizontal phreatic galleries between 2000 and 2100 m asl (total length about 10 km in several caves) are widespread in the Silberen region. Carbonate dripstones do not grow under present climatic conditions: the upper limit of more than patchy vegetation is between 1800 and 2000 m asl. Th e sampled speleothems thus represent periods of warmer climate. Th is may be due to climate change and/or due to the alpine uplift of the sampling points.

A conelike stalagmite of the «Fassadengang» gallery, a phreatic gallery on a bedding plane, has a height of 19 cm and a maximum diameter of 8 cm (Figs. 3 and 4A). Th e carbonate crystals are arranged fan-like and perpendicular to the growth layers. Th e speleo them is porous, particularly along the cone axis. Th e pores are elongated parallel to the crystal axes and are in many cases longer than 2 cm. Th e diff erent layers of the stalagmite vary from white to slight beige. Th e 2nd youngest layer is colourless and transparent. Th e surface of the stalagmite is slightly corroded, probably due to dripping water, and has a white, beige and in some areas, a local rusty brown colour.

Another stalagmite of 35 cm in length and with a maximum diameter of 13 cm (Figs. 3 and 4B) was sampled in the «Seilfrässer», a phreatic gallery with a vadose overprint. Th e speleothem has growth layers along the axis as well as in radial direction. Th e long axes of the crystals are arranged perpendicular to the growth layers. Th e stalagmite is slightly porous in its central part. Th e narrow holes are parallel to the long axis of the stalagmite and have a length of more than 1 cm. Th e colour of the diff erent layers ranges from light-beige to white; the surface layer is beige and slightly corroded.

Th e «Westkluft» gallery has a phreatic origin and follows fractures and the rock strata. A candle-like stalagmite (height 40 cm, maximum diameter 4.5 cm) was sampled at the junction with the «Seilfrässer» gallery. Th e long axis of the coarsely crystallized speleothem is perpendicular to its growth layers. Th ese layers are more or less horizontal, only at the edge they bend off . Th e top end is smaller than the lower end, probably due to corrosion from dripping water. In the lowermost 24 cm a beige, slim stalagmite is incorporated. Th e younger part of the generally massive stalagmite is brownish, on the outside partly white and porous (Figs. 3 and 4C).

Th e «Sintergang» gallery in the Schwyzerschacht cave (part of the Silberen cave system) is the fossil, phreatic part of the active, vadose canyon of the Twärenenschlucht. Together, they form a keyhole passage. Th e «Sintergang» gallery is situated at an altitude of approximately


Kristallkluft2 cross sections

sample

0 1 2 3 m

Westkluft

sample

Sintergang

drilling core 0

drilling cores 1-4

Seilfrässer

sample

Fassadengang

sample

Fig. 3. Passage cross-sections at the sampling points in the Silberen cave system with indication of the situation of the samples. Th e shape of the cross-sections is showing the mainly phreatic origin of the galleries.


10 cm

IgluschachtFassadengang

A

10 cm

IgluschachtSeilfrässer

B

Sintergangdrilling cores

10 cm

1 2 3

4

0

D Schwyzer Schacht

IgluschachtWestkluft

10 cm

C

Fig. 4. Pictures of the samples from the Silberen cave system (outside and cross-section). A – sample Fassadengang (Igluschacht); B – sample Seilfrässer (Igluschacht); C – sample Westkluft (Iglusch-acht); D – samples Sintergang (Schwyzer Schacht), at left cores 1–4 (lateral drilling), at right core 0 from the top of the stalagmite (see Fig. 3).


1670 m asl, near the base of the Urgonian limestone. Th ere are recent (active) and old (inactive) speleothems. Th e source area of the water lies in karst with vegetative cover and few trees.

A cone-shaped, inactive and superfi cially corroded stalagmite with a height of 3.5 m and a diameter of 2.2 m at the base (volume about 4.5 m3) was sampled (Figs. 3 and 4D). For this, a fuel-driven drilling machine was used. Th e fi rst core was taken between the lateral surface and the centre at the base of the stalagmite (drilling axis was inclined by 20°, diameter of the drilling core was 2.5 cm). After 1.25 m, the drillhole reached the base of the stalagmite indicated by a space between blocks on which the stalagmite is standing. A second core with a length of 0.3 m was drilled vertically downwards from the top of the cone shaped speleothem. All cores broke into fragments (Fig. 4D). In porous layers, the yield was less than 100%. Layered and massive parts can be distinguished by colour changes (white to brown). Discordant layers are explained by interruptions of the growth or corro-sion phases.

Th e «Kristallkluft» gallery in the Schwyzer Schacht cave is an inactive lateral gallery in the area of recent drainage, similar to the «Sintergang» gallery, but 200 m lower in altitude. In the «Kristallkluft» gallery a long ledge at the wall consists of fl owstone up to 50 cm thick. At the bottom side, gravels with diameters of up to 10 cm are found. Because of its detrital contamination, this speleothem was not dated.

Th e Himmelstor cave is also a phreatic gallery near the base of the Urgonian lime-stone. Due to the low altitude (1070 m asl), this cave was fl ooded during the last glacia-tion. Th e detritic sediments (mainly gravels and sand and some fragments of speleothems) were probably deposited in a vadose regime during a late glacial active phase. A stalagmite fragment with a length of 9 cm and a basal diameter of 5 cm was sampled from these sediments. Th e surface of this speleothem is yellowish to white; the core is white and dense. Th e long axes of the crystals are more or less perpendicularly oriented to the growth layers.

4 Methods

An overview of the applied methods is given in Table 1. During the fi rst phase of the project, the 230Th /U und 234U/238U activity ratios of the stalagmite samples were measured. However, the ages exceeded the dating range of both methods, or the samples behaved as open systems with respect to uranium according the characteristics described by Geyh & Müller (2005). If a sample is open with respect to U, it means that U will be leached when the sample is fl ooded. Several of the older samples were subsequently investigated by paleomagnetic methods, without reaching unequivocal ages even in combination with 234U/238U dating. Additionally, several samples were examined palynologically. Dating of some stalagmites by the ESR-technique was also attempted. Finally, the morphogenesis of the landscape was studied in order to reconstruct the speleogenesis.


4.1 Radiometric and TIMS 230U/Th dating

For 230U/Th dating by alpha spectrometry, 30 to 50 g of a stalagmite were dissolved in HNO3 and treated according to the procedure of Lauritzen (1996). Analyte separation was achieved by ion-exchanging resin, electroplating the extracted uranium and thorium on stainless-steel discs and activity was measured with an alpha spectrometer (University of Bergen, Norway). Th e accuracy is roughly 5 %.

Th e thermion mass-spectrometric determination of 230Th and 234U (TIMS) activity is the most precise measurement (accuracy around 0.5 %). Th e measurement of the 234U/230Th and 234U/238U activity ratios was made on less than 1.5 g carbonate samples only. Th e sam-ples were dissolved in a mixture of HNO3 and HCl and a 229Th and 233U/236U double spike was added. After several cleaning steps, uranium and thorium were precipitated by iron hydroxide (Fe(OH)3) and separated by ion exchange. Th e uranium and thorium solu-tions were evaporated on two rhenium fi laments. Th e isotope ratios were measured with a Finnigan MAT 262 mass spectrometer with RQP-fi lter (Leibniz Institute for Applied Geo-sciences, Hannover, BRD).

Table 1. Samples and applied methods (sample «Kristallkluft» is detritically contaminated and there-fore not dated).

Sample(short version in

capitals)

Geographicposition

Swissgridcoordinates

Altitude[m asl]

Dat

ing

230 T

h/U

(α-c

ount

ing)

Dat

ing

230 T

h/U

(TIM

S)

Pal

eom

agne

tism

234 U

/238

U

Pal

ynol

ogy

Ele

ctro

n-S

pin-

Res

onan

ceE

SR

Silberen-SystemIgluschachtFASSADENGANGSilberen-SystemIgluschachtSEILFRÄSSER

8°53'18" E46°59'08" N

8°53'30" E46°59'10" N

710'284/204'861

710'525/204'944

2043

2094

X

X

X

X

X

X X

X

X

Silberen-SystemIgluschachtWESTKLUFTHIMMELSTORentrance + 20mSilberen-SystemSchwyzer SchachtSINTERGANGSilberen-SystemSchwyzer SchachtKRISTALLKLUFT

8°53'25" E46°59'08" N

8°46'17" E46°57'49" N

710'453/204'866

701'996/202'264

8°52'16" E46°58'50" N

46°58'56" N8°52'03" E

708'995/204'290

708'600/204'305

2060

1070

X

X

1660

1476

X

(X) X

X

X


4.2 234U/ 238U dating

Th e base of 234U/238U dating is the higher solubility of 234U compared with 238U. Th e initial ratio of 234U/238U in groundwater and therefore in the speleothems is generally >1. Usually, this ratio can only be estimated. In closed systems, this ratio decreases with time towards 1 and allows a dating up to about 1 Ma, but the 234U/238U ages are less precise than the results of the 230Th /U method, even if the initial ratio is well known (Geyh 2005).

4.3 Paleomagnetism

Horizontally drilled cores from the stalagmites were demagnetized using alternating magnetic fi elds (fi eld strength up to 60 mT) in order to test the stability of the natural remnant magneti-zation (NRM) and to separate possible secondary and primary components of the NRM. All samples had a very weak magnetization (initial NRM values had about 1 to 2 · 10-8 emu/cm3 respectively 1 to 2 · 10-5 Am-1), but they were measurable with a modern supraconducting magnetometer (SQUID) in a shielded place. Th e interpretation of the results was based on the inclination, as the samples were taken without azimuthal orientation.

4.4 Palynology

For palynological analysis, the samples were treated as described by Bastin (1990) and Genty et al. (2001). Samples of 210 to 290 g in weight were crushed. Lycopodium spores were added (determination of pollen concentration after Stockmarr 1971), and the sam-ples were dissolved in 15% HCl. Quartz and argillaceous material were disintegrated by warm 40% HF within 35 to 50 minutes and subsequently washed repeatedly with warm HCl. Deviating from the technique applied by the cited authors, the residue was neither acetolyzed nor stained. Identifi cation and counting of the pollen and spores were performed on simple glycerine slide preparations.

4.5 Electron Spin Resonance

Electron spin resonance spectroscopy (ESR) allows the measurement of paramagnetic cen-tres within minerals (defects with unpaired electrons, EDC). Th e increasing intensity of these EDC with time and hereby accumulated radiation dose is the base of ESR dating. Speleothems give a complex ESR spectrum due to the occurrence of carbonate radicals, humic acids, and other substances. Th e g-value at 2.0007 is usually used for dating with an age range of 10 to 400 ka (max. 1000 ka) (Grün 1989).

Th e ESR-spectra of the pulverized samples were measured at room temperature with a Bruker ElexSys E500 cw spectrometer at 9.8 GHz (EPR research group at ETH Zurich). To check the radiation induced centres in the sample «Seilfrässer», pristine as well as x-ray exposed material was investigated.


5 Results

5.1 230Th /U Ages

Th e speleothems were rather rich in uranium (0.8–9.3 ppm). A detritus correction was not necessary because the 230Th /232Th activity ratio was >1000 (Table 2). Th e three high altitude samples (Igluschacht: «Fassadengang», «Seilfrässer», «Westkluft») gave minimum 230Th /U ages of >350 ka. Th erefore precise dates were not obtained.

Th e base and the top of the stalagmite from the Schwyzer Schacht («Sintergang») gave reli-able 230Th /U ages of 312 ± 18 ka and 212 ± 9 ka respectively. Th e uppermost part of the trans-ported concretion of the Himmelstor cave (about 228 ± 11 ka) yielded a comparable age.

Th e samples from higher altitudes (Igluschacht) appear to represent a closed system, as visible features (e.g. macroscopic pores from corrosion) which allow for uranium dissolu-tion were not observed (the holes parallel to the axes are explained by in situ recrystallization of the stalagmites). Nevertheless the 234U/238U activity ratio of the detrital stalagmite of the Himmelstor cave did not completely exclude leaching, which would explain an overesti-mated 230Th /U age.

5.2 Paleomagnetism

Th e NRM measurements of the two stalagmites from Igluschacht cave («Fassadengang», «Seilfrässer») yielded positive inclinations and therefore the same earth magnetic confi gura-tion as today. One of three samples from the stalagmite «Seilfrässer» showed a faint indica-tion of a reverse component during a short phase of alternating fi eld demagnetization. Th is weakly expressed component could be explained as a relic of a geomagnetic fi eld epoch with reversed fi eld polarity or as a recrystallization eff ect of interacting magnetic phases. Th e main characteristic NRM components, however, have clear normal inclination. Hence we assign the sample magnetization to the Brunhes chron (780–0 ka, Cande & Kent 1995). Magnetization within the Jaramillo- (970–900 ka), the Olduvai- (1870–1670 ka) or even older chrons cannot be excluded, however.

5.3 234U/ 238U-Data

Th e 234U/238U age of the high altitude samples was estimated by plausible assumptions of the initial activity ratio, and then compared with the results of the paleomagnetic inves-tigations (Table 3). Th e stalagmites «Fassadengang» and «Seilfrässer» were probably open systems with respect to uranium and therefore the 234U/238U ages are not reliable.

5.4 Palynology

Two samples with a fi nite 230Th /U age («Sintergang»: ages of 312 ± 18 and 212 ± 9 ka, A and B in Table 4) and two samples older than 350 ka were analysed («Westkluft» and «Seilfrässer, C and D in Table 4). Th e samples were only partly investigated, therefore only qualitative conclusions can be made:

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Tho

rium

[ppm

]

234 U

/ 238

U ±

1s

(α-S

pect

ro.)

± 2

σ (T

IMS

)

230 T

h / 2

32T

h23

0 Th

/ 234

U23

0 Th/

U a

ge

[ka]

± 2

s

Silb

eren

-Sys

tem

Iglu

scha

cht

FA

SS

AD

EN

GA

NG

Silb

eren

-Sys

tem

Iglu

scha

cht

SE

ILF

RÄ

SS

ER

Top

Top

2451

2452

0.78

1.63

– –

1.08

42 ±

0.0

1172

1.02

58 ±

0.0

0754

1374

2106

1.05

69 ±

0.0

1749

1.04

39 ±

0.0

1464

> 3

50or

open

sys

tem

> 3

50or

open

sys

tem

Silb

eren

-Sys

tem

Iglu

scha

cht

WE

ST

KLU

FT

HIM

ME

LST

OR

entr

ance

+ 2

0m

Silb

eren

-Sys

tem

Sch

wyz

er S

chac

htS

INT

ER

GA

NG

Top

Top

2468

2469

Bas

e +

few

cent

imet

ers

Top

- fe

wce

ntim

eter

s

TIM

S-H

v 30

3

TIM

S-H

v 34

1

0.65

4.19

– –

9.63

5.63

0.00

1

0.01

1

1.02

25 ±

0.0

1893

0.88

93 ±

0.0

0550

1921

1919

0.94

3 ±

0.0

02

0.95

2 ±

0.00

2

2454

0

1290

1.02

39 ±

0.0

2755

0.85

00 ±

0.0

1177

> 3

50or

open

sys

tem

228

± 1

2

0.92

49 ±

0.0

075

0.84

71 ±

0.0

101

312

± 1

8

212

± 9

Top

TIM

S-H

v 30

26.

030.

010.

956

±0.

002

1485

0.84

11 ±

0.0

084

207

± 7

Tabl

e 3.

Pot

enti

al a

ges o

f tw

o sp

eleo

them

sam

ples

der

ived

from

the

pres

ent a

ctiv

ity

rati

o of

234 U

/238 U

as a

func

tion

of t

he p

ossi

ble

prim

ary

acti

vity

ra

tio.

Hig

hlig

hted

are

the

pote

ntia

l age

s whi

ch c

oinc

ide

wit

h a

posi

tive

pal

eom

agne

tic

incl

inat

ion

(Bru

nhes

780

–0 k

a, Ja

ram

illo

970–

900

ka, a

nd

Old

uvai

187

0–16

70 k

a; C

and

e &

Ken

t 19

95).

Sam

ple

Pre

sent

act

ivity

rat

io

Fas

sade

ngan

g (T

op)

234

U/23

8U

± 1

1.08

40 ±

0.0

117

1.50

630

± 5

0

Orig

inal

act

ivity

rat

io

234

U/23

8U

2.00

880

± 5

0

3.00

1120

± 5

0

4.00

1265

± 5

0

5.00

1365

± 5

0

Sei

lfräs

ser

(Top

)1.

0258

± 0

.007

510

05 ±

45

1245

± 4

514

90 ±

45

Age

[ka

± 1

]1635

± 4

517

35 ±

45


• Th e pollen density was similar to that found by Bastin (1990) and Genty et al. (2001) in Holocene and Upper Pleistocene samples (0.8 to 2.5 pollen/g).

• Th e pollen grains were quite well preserved, although some grains were cracked, and torn-off bladders of conifer pollen were noted; the colour of the pollen was yellowish to yellow and could be well distinguished from the yellow-brown marker spores (acetolysed spores of Lycopodium).

• Th e observed pollen belong to various widespread pollen types of trees, shrubs, dif-ferent kinds of grass and various herbaceous plants. Spores of Polypodiaceae (ferns) and exotic types (e.g. older species) were not found. In fi ne-grained sediments of nearby Hölloch cave, Tertiary and Mesozoic palynomorphs were frequently found, though in low density (Groner 1985, 1990); these older pollen were obviously reworked. Th is possibility has to be excluded in carbonate concretions.

• Pollen assemblages in the fl owstone at 1660 m asl («Sintergang») and in those at about 2070 m asl («Seilfrässer», «Westkluft») are very similar. Nevertheless the low pollen yield (Table 4) does not allow reliable conclusions.

• Th e absence of Tertiary and Early Pleistocene pollen types in the stalagmite sam-ples is interpreted as an indication for an age within the Middle Pleistocene or younger (<780 ka).

5.5 Electron Spin Resonance ESR

Th e ESR signal at g=2.0007, which is suitable for dating, was not found in the samples. Only the pristine sample «Seilfrässer» had a low-intensity line at g=2.0036. As the ESR signal at g=2.0036 is light-sensitive, it disappears during time under light exposure. Th e intensity of the signal does not refl ect anymore the accumulated dose. Consequently, underestimated age is determined. Th is is also valid for the other samples without any signal.

It was often observed in fl owstone samples from other caves that they are not suitable for ESR dating (Grün 1989). Proper strategies for sampling and storage are required for ESR-dating; these precautions were not implemented in our study.

Table 4. Samples used for pollen analysis and pollen yield.

Sample(short version in

capitals)Position in stalagmite Colour

Lab-number

Pollenconcentration

[Pollen grains/g]

Analysedpollen

Silberen-SystemSchwyzer SchachtSINTERGANG

lateral drill core(Ø 2.5 cm), 0 ÷ 23 cmbelow surfacelateral drill core(Ø 2.5 cm), 74 ÷ 102 cmbelow surface

brown + beigeribboned

brown, beige+ light beige

ribboned

A

B

0.9

0.9

11

8

Silberen-SystemIgluschachtWESTKLUFTSilberen-SystemIgluschachtSEILFRÄSSER

12 ÷ 26 cm below top

0 ÷ 10 cm below top

brown

light beige +brown-beige

C

D

1.2

2.0

12

46


5.6 Combination of 230Th /U, 234U/ 238U, Paleomagnetic Records and Palynologic Analysis

Th e three high-altitude samples from the Silberen cave system («Fassadengang», «Seilfrässer» and «Westkluft») are (by 230Th /U and 234U/238U activity ratios) older than 350 ka (or they had behaved as open systems with respect to uranium). Th e two stalagmites «Fassadengang» and «Seilfrässer» have a positive paleomagnetic inclination. Along with the palynological indication of a Middle or Late Pleistocene age of the samples «Sintergang», «Westkluft» and «Seilfrässer», their data indicate that the stalagmites «Fassadengang» and «Seilfrässer» might have grown during the Brunhes epoch.

Th erefore, an age between <780 ka and >350 ka is assumed for the sample «Seilfrässer». In the case of the other two fl owstones of the high-altitude sites, paleomagnetism (sample «Fassadengang») and palynology («Westkluft»), respectively, data indicate a similar age.

6 Morphogenetic Considerations

Th e theoretical maximum age of karstifi cation in the Helvetic realm of central and eastern Switzerland – apart from paleokarst phenomena – is middle Miocene (17–11 Ma). At this time, the fi rst Helvetic gravel appears in the sediments of the alpine foreland (Upper Fresh-water Molasse; Speck 1953, Bürgisser 1980). Karst phenomena of this early phase have not been preserved due to erosion and corrosion of the land surface.

Karstifi cation and speleogenesis depend on lithogenesis, tectonic subsidence or uplift, erosion and corrosion, glaciation and climate, soil formation and vegetation. Cave growth as a consequence of corrosional and erosional capacity of water as well as precipitation of fl owstone is limited in an alpine context, above all, to interglacial or interstadial environ-mental conditions (Audra et al. 2006). During glacial phases, detritic loams rich in calcite were deposited in the caves. Varved/laminated sediments resulted from backfl oods due to damming by temperate valley glaciers (Maire 1990, Häuselmann 2002; corrosion of lime-stone under glacial conditions is discussed by Faulkner 2006). Th e simple model in Fig. 5 shows schematically some of the abovementioned infl uencing factors on the axes „time“ and „relative altitude“ in an alpine environment. Here, the extended period before the start of the speleogenesis is neglected.

Th e formation of cavities begins under phreatic conditions as soon as the unkarstifi able covering beds are suffi ciently thinned or at most fragmented, so that the water can enter the rock and fl ow away (point A1 in Fig. 5). By uplift and erosion/corrosion, or by deepening of the valley bottom, the developing cave passage reaches gradually an epiphreatic and later a vadose position (B1 and C1). Th e lowering of the base level makes the development of a new phreatic cave level possible (A2–B2). After reaching the vadose level, speleothems may grow in the upper passages during warmer climatic phases (C2). Th e proceeding erosion and corrosion lowers the land surface and brings the older galleries up to the surface, where they are present as cave ruins or so-called topless caves, which have subsequently deteriorated with time (Mihevc et al. 1998).

During glaciations, fl ooding of the lower parts of the formerly dry karst systems is expected due to damming by the valley glacier (D3 in Fig. 5). In contrast to interglacial


Fig.

5. S

ketc

h of

spel

eoge

nesis

in a

n al

pine

con

text

and

dist

ant f

rom

out

put p

oint

(ins

pire

d by

the

cond

ition

s in

the

Höl

loch

regi

on, C

entr

al

Switz

erla

nd).

Ass

um

pti

on

s: u

plif

t >

su

rfac

e er

osi

on

in k

arst

bac

kflo

od

ing

by

tem

per

ate

valle

y g

laci

er

A

beg

in o

f sp

eleo

gen

esis

B

end

of p

hre

atic

ph

ase

C

first

gen

erat

ion

of s

tala

gm

ites

D

seco

nd

gen

erat

ion

of s

tala

gm

ites

(plu

s fir

st g

ener

atio

n p

lus

det

riti

cal s

edim

ents

)

E ca

ve e

ntr

ance

faci

es

stal

agm

ite

stal

agm

ite

wit

h lo

amy

cove

r (d

ue

to g

laci

al in

un

dat

ion

)

blo

ck (c

ave

entr

ance

faci

es)

no

n-k

arst

ic o

verb

urd

en(n

on

-kar

stic

form

atio

ns,

up

per

nap

pes

)

gla

cier

gla

cier

wat

er ta

ble

A1

A2

B3

B4

B5

C4

D4

D3

E2

C3

C2

E1

C1

B2

B1

A3

cave

leve

l 1ca

ve le

vel 2

cave

leve

l 3

cave

leve

l 4ca

ve le

vel 5

pre

sen

t10

x ka

alti

tud

e

tim

e ax

is (

wit

ho

ut

scal

e)

mo

der

n

bas

e le

vel

surface erosionuplift

evolutio

n of

a cave le

vel

isochron

Fig5

.ai 1

6.12

.08

Wi/B

N/K

D


fl oods with decreasing watertable in direction of the spring, the waterlevel during glacial backfl ooding is almost horizontal due to the practical absence of fl ux. Such episodes with temperate glaciers have been demonstrated for the Würm glaciation (Wildberger 1992), when the water level in the Hölloch cave was up to 500 m above the low water level of today (200 m above the present fl ood water level, Wildberger et al. 1991). Th is phenomenon occurred repeatedly during former glaciations.

During such fl ood events, recrystallizations were possible, giving rise to open systems with respect to uranium, rendering absolute dating more diffi cult. Recrystallization of cal-cite is a slow process which is caused by intercrystalline pore water rather than fl ooding. Recrystallization may provoke transport of trace substances (uranium, magnetite) but only over short distances, thus allowing for large samples to remain datable in spite of open system conditions. Th is was evidenced in St. Beatus cave, where ages of correct stratigraphic order were obtained from corroded, eroded and recrystallized speleothems that were cored by a core-driller (Häuselmann et al. 2008).

Th e recent uplift relative to the foreland amounts to 500 mm/ka (Kahle et al. 1997). Dating by the fi ssion track method of biotite and apatite in Central Switzerland provided evidence of a more or less uniform uplift rate during the last 10 Ma (Michalski & Soom 1990, Müller et al. 1997). Th e linear incision rate of the main valleys by fl uviatile or gla-cial erosion is comparable with the uplift (Bernet et al. 2001). Lateral valleys and the karst systems are following the incision of the main valley. Th e gradient and the generally concave longitudinal profi le of the alpine rivers thus remain more or less constant during uplift.

Th e eff ect of a rising valley fl oor by deposition of river sediments is a temporary phe-nomenon during the evolution of the mountain relief. We assume a postglacial uplift of the water level of about 20 m in the recent active cave level: geophysical investigations have demonstrated a rocky threshold downstream of the spring covered by 20 m of gravels (Wildberger & Müller 2007).

Under the present climatic conditions, the surface lowering by corrosion amounts to about 15 mm/ka (karst without soil) and up to 80 mm/ka (forest karst), respectively (Bögli 1980, Häuselmann 2008). Erosion and transport of solid material is negligible in a holokarstic environment. In non-karstifi ed sediments of the Helvetic series, particularly under (peri)glacial conditions, the lowering of the surface would reach at least the same values or more: 200 mm/ka in the neighbouring catchment of the river Linth (Lambert 1980); in other alpine watersheds of Switzerland the surface erosion rate amounts to 100 and 400 mm/ka and rises up to 1000 mm/ka in small catchments (Jäckli 1958, Klemenz 1993). During cold periods, the surface lowering in karstic areas is generally low due to protection by ice and snow; some glacial erosion is seen in places with «roches mouton-nées» and «Schichttreppenkarst». Finally the lack of vegetation reduces the rate of corrosion (Bögli 1964, 1970, Klemenz 1993). On average, the tectonic uplift rate of 500 mm/ka in the Muota valley is more important than the denudation, and as a consequence, the diff er-ence in altitude between the valley bottom and the karstic catchment increases with time (Fig. 5).


7 Discussion

Assuming an equivalence of uplift rate (500 m/Ma) and linear incision rate of the valley bottom as well as assuming that both values remain more or less constant during longer time spans, it can be concluded that the age of phreatic galleries in the Silberen cave system, today at an altitude of 2000 to 2100 m asl and therefore 1400 m above the valley fl oor, is about 3 Ma. Accordingly, phreatic galleries 500 m deeper at 1650 m asl («Sintergang» in the Silberen system) have an age of about 2 Ma. Modelling of speleogenesis (e.g. Dreybrodt et al. 2005) has shown that the rate of cave level formation can follow the abovementioned incision of the river.

During long periods of speleogenesis, a natural denudation rate of the superfi cial area of 80 mm/ka in karst covered by vegetation can be assumed; this rate gives a total denudation of about 250 m since the Upper Pliocene. Th e non-karstic layers (especially the Palfris marls of the Lower Cretaceous at the base of the Drusberg nappe) were situated ca. 250 m above the present surface (Fig. 2). Th erefore the estimated rate of denudation is compatible with a start of the karstifi cation in the Bödmeren-Silberen region about 4 to 3 Ma ago (Middle to Upper Pliocene). Th e oldest products of speleogenesis (caves, speleothems a.s.f.) could have disappeared by erosion or corrosion.

Th e maximum ages of speleogenesis seems to contradict the 230Th /U ages of the dated stalagmites:

• 230 ka, «Himmelstor» (broken stalagmite);• 210 and 310 ka as probable separate growth phases in the «Sintergang» (Silberen

system);• Th e stalagmites in the uppermost sampling places («Fassagengang», «Seilfrässer»,

«Westkluft», galleries in the Silberen system) might have behaved as open systems with respect to uranium. According to the palynological results the age <780 ka seems likely; this is also compatible with the results of the paleomagnetic records (if the remanent magnetisation has not been infl uenced by recrystallisation).

Th e great stalagmite in the «Sintergang» (Schwyzer Schacht, part of the Silberen system) was formed at 312 ± 18 ka (few centimetres above the base) and 207 ± 7 ka (top of the cone). Base and top might have grown mainly during the interglacial warm phases (isotope stages 9 and 7). It has to be assumed that the growth had been interrupted or considerably slowed down during the intermediate cold phase (MIS 8), although there is no hard evi-dence for that. Th e arithmetic maximum growth velocity (height divided by the age diff er-ence between base and top) is 3.3 cm/ka and therefore a minimum for the growth velocity during both climatic warm phases.

Th e stalagmite of the Himmelstor cave has a 230Th /U age of 228 ± 12 ka corresponding to a growth period in MIS 7. Th e speleothem was incorporated in a gravely-sandy cave fi lling. It was broken and transported after its formation. Because there is only little damage, the transport distance is assumed to be less than some hundred meters. Th e detritic sediment might have been imported by late glacial or stadial meltwater at the end or in the middle


of the last glacial epoch (about 10 or 40 ka) or at the end of the penultimate ice-age (about 140 ka), respectively.

Speleogenesis began long before the growth of speleothems. Although a multitude of complementary dating methods was used, the uncertainty of the age range of the formation of the dripstones remains rather large:

• Th e growth of the stalagmite in the «Sintergang» at an altitude of 1670 m asl began at 310 ka and therefore the phreatic galleries «Fassadengang», «Seilfrässer» and «Westkluft» at about 2070 m asl have remained dry since (if not drown during a later glacial fl ood).

• Th e maximum age of the three above-mentioned samples is 780 ka according to the paleomagnetic and palynological results.

It seems obvious that caves forming under surfaces with an altitude of more than 1800 m asl, without vegetation and active speleothem growth today, have a relatively old (Upper Pleistocene-Pliocene) age. A long-lasting uplift with the present rate of 500 m/Ma could suffi ciently explain climatic conditions suitable for speleothem growth at higher altitudes. Th e oldest speleothems are found at more than 2000 m asl, the youngest inactive ones at 850 m asl in the uppermost, recent epiphreatic zone of the Hölloch cave. In comparison, active speleothems are growing between about 700 m asl (epiphreatic zone) and 1800 m asl (ca. upper limit of consistent vegetation).

Based on the assumption that the high-altitude speleothems are younger than 780 ka and began to grow within the upper part of the epiphreatic zone (about 100 m above the phreatic zone as today, where fl ooding is irregular and therefore corrosion of the speleothems not visible), the valley has incised about 1.3 km during 780 ka (sampled stalagmite on 2070 m asl, modern upper limit of the epiphreatic zone at 800 m asl, lower limit of active dripstone growth at 700 m asl, base level at 640 m asl). Th is corresponds to an uplift rate which is three times larger than that of today. Th is seems implausible. Th erefore the galleries at 2070 m asl are assumed to have fossilized (i.e., became inactive) long before the sampled stalagmites had grown. A similar apparent discrepancy of the supposed ages of a cave and its speleothems was also found in other cave systems (e.g. Stock et al. 2005).

Our hypothesis concerning the age of the cave (2–3 Ma) apparently contradicts the ages from comparable alpine cave systems (Audra et al. 2006), where the oldest parts are postulated to be more than 4 Ma old. Häuselmann et al. (2007) dated detritic sedi-ments with the exposure methods of cosmogenic nuclides at Siebenhengste (Switzerland): 4.4 ± 0.6 Ma. Audra (1994): Eisriesenwelt (A), Lower Miocene and Chartreuse (F), Upper Miocene for the beginning of speleogenesis. Th e comparatively low age of the investigated caves in the Muota valley may be the consequence of a more recent uncapping of the karstifi able limestones (Maire 1990). Th e nevertheless intense karstifi cation between the mountain top and the valley fl oor could depend – apart from geologic factors – on the high precipitation in this area (today about 2900 mm/a at 1600 m asl).


Acknowledgements

Th e authors thank especially Franz Auf der Maur, Gregor Bättig, Peter Beeler and Toni Pulfer for their assistance in sampling, Beat Geyer for sample preparation, D. Oezen, S. Mogwitz and Stein-Erik Lauritzen for dating and Kathy Dubach, Beat Niederberger and Felix Ziegler for the illustrations.

We thank also the reviewers Philippe Audra, Jo De Waele and Marc Luetscher for their con-tribution for a better understanding of the paper. Art Palmer kindly corrected the English.

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Addresses of authors:Dr. Andres Wildberger, Hölloch Cave Research Association AGH, Im Tiergarten 49, CH-8055 Zürich, Switzerland, E-mail:wildfi [email protected]. Dr. Mebus A. Geyh, University Marburg, D-35 032 Marburg, Germany, E-mail: [email protected]. Urs Groner, Engelstrasse 5, CH-8004 Zürich, SwitzerlandDr. Philipp Häuselmamm, Swiss Institute of Speleology and Karst Studies SISKA, P.O.B. 818, CH-2301 La Chaux-de-Fonds, Switzerland, E-mail: [email protected]. Dr. Friedrich Heller, Institute of Geophysics, ETH Zurich, CH-8093 Zürich, Switzerland, E-mail: [email protected]. Michael Ploetze, Institute for Geotechnical Engineering, ETH Zurich, CH-8093 Zürich, Switzerland, E-mail: [email protected]

dating speleothems from the silberen cave system and surrounding areas: speleogenesis in the muota...

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