Structural geology of Central : results of seismic campaign in 2011 in cantons Nid- and

Autor(en): Ebert, Andreas / Genoni, Oliver / Häring, Markus

Objekttyp: Article

Zeitschrift: Swiss bulletin für angewandte Geologie = Swiss bulletin pour la géologie appliquée = Swiss bulletin per la geologia applicata = Swiss bulletin for applied geology

Band (Jahr): 18 (2013)

Heft 1

PDF erstellt am: 11.10.2021

Persistenter Link: http://doi.org/10.5169/seals-391139

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http://www.e-periodica.ch Swiss Bull, angew. Geol. Vol. 18/1.2013 S.51-59

Structural geology of Central Switzerland - results of seismic campaign in 2011 in cantons Nid- and Obwalden Andreas EbertJ, Oliver Genoni1, Markus Häringi

Key words: Helvetics, seismic, Central Switzerland, tectonics, Drusberg nappe

Abstract Zusammenfassung GVM (Gas Verbund Mittelland AG) commissioned in Als erster Schritt einer integralen Explorations- 2011 the acguisition of 113 km of 2D reflection seismic kampagne für geothermische Ressourcen und as a first step of an integrated exploration Kohlenwasserstoffe erteilte GVM (Gasverbund campaign for geothermal and hydrocarbon resources in Mittelland AG] 2011 den Auftrag zur Aufnahme von the cantons Nid- and Obwalden (Switzerland]. One 113 km Reflexionsseismik in den Kantonen Nid- of the aims of the seismic campaign was to define und Obwalden (Schweiz). Die Kampagne hatte the structure of the Helvetic nappes and its base, unter anderem zum Ziel, den Aufbau der Helvetischen the internal build-up of the underlying Tertiary Decken, der unterlagernden tertiären sediments and the autochthonous Mesozoic cover. Sedimente (Molasse, Flysch) und der autochthonen The new data allow defining the internal tectonics Einheiten (mesozoische Bedeckung und Grundgebirge) of the Drusberg nappe and its base. The deformation zu bestimmen. style of the Drusberg nappe is characterized by Die neuen Daten bestätigen weitgehend bestehende imbrication and ramp anticlines. The base of the strukturelle Modelle. Der interne Aufbau der Helvetic nappe forms a relatively flat, wide and Drusberg Decke ist gekennzeichnet durch Ver- gently structured synform south of a line Schwendi schuppung und Rampenantiklinalen. Die Basis der - - Stans. The base of the Helvetic nappes Helvetischen Decken ist wenig strukturiert und bildet reaches a max. depth of approximately 1.8-2.0 km eine weite und flache Synform südlich einer below the valley. Linie Schwendi - Alpnach - Stans. Die Basis It is not possible to distinguish between south erreicht an ihrer tiefsten Stelle Tiefen von ca. dipping Molasse and supposed North Helvetic Flysch. 1.8-2.0 km unter dem Sarner Aatal. The autochthonous Mesozoic cover is less In den darunter liegenden tertiären Sedimenten ist deformed. Few faults and folds may correlate with es nicht möglich, einzelne Einheiten zu unterscheiden. underlying Permo-Carboniferous troughs. The latter Die Grenze Molasse - Nord Helvetischer can be identified on the seismic sections by Flysch konnte nicht erkannt werden. In der autochthonen unconformities. mesozoischen Bedeckung können nur wenige Falten und Brüche ausgeschieden werden, welche häufig mit unterlagernden Permokarbon- Vorkommen korrelieren. Das Permokarbon kann auf einigen Linien an Hand von Diskordanzen gut identifiziert werden.

Geo Explorers Ltd, Wasserturmplatz 1, AhlO Liestal, [[email protected]]

51 1 Introduction

In the cantons Nidwalden and Obwalden Carboniferous. Wells at Wellenberg relating (Central Switzerland), the surface geology is to the surveying of the impermeable Palfris well exposed and allows to construct the Formation by Nagra only reached the base complex nappe and fold structures of the of the Helvetic nappes. The 2,302 m deep Helvetic and Penninic nappes. As a result of geothermal well of Weggis only intersected this, numerous studies were performed in the USM and UMM of the Molasse the past. The first published cross sections sediments. across the Pilatus are given in Kaufmann In the course of the exploration for (1867). A selection of more recent and hydrocarbons in Switzerland during the sixties to important regional studies about the structural eighties several tens of kilometres of seismic geology of the Central Alpine nappes were acquired in Nid- and Obwalden. During are the PhD thesis of Menkveld (1995), the the seventies and early nineties LEAG (AG recent compilation of the Helvetic nappes of für Luzernisches Erdöl), respectively TGK Pfiffner (2011), and the extensive work of (Tiefengaskonsortium) shot 270 km of 2D Hantke (1961). reflection seismic (Fig. 1). While the LEAG However, all the presented information seismic is accessible, the TGK seismic is not about the deeper subsurface rely on available. When the LEAG seismic were extrapolations from the visible geology at the acquired, vibrators were less powerful and surface. Deep wells are missing. The closest were not able to provide the same energy as exploration well that penetrated the nowadays. The data quality is only sufficient Molasse and Mesozoic sediments is Entle- to sketch the autochthonous Mesozoic cover. buch-1 north of the Alpine front (Fig. 1). This It is not possible to define the internal 5,144 m deep well bottomed in the Upper structures and the base of the Helvetic

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77 nappes, the Flysch - Molasse contact, the IPS. Gasser Felstechnik AG made the drilling internal structures of the autochthonous and blasting where source points were not cover and the existence and extend of Per- accessible by vibrotrucks. mo-Carboniferous troughs below the autochthonous Mesozoic cover. Execution Permitting started end of June 2011. Acquisition took place during September 2011. 2 Seismic survey Along seven lines a total length of 112.55 km were acquired (Fig. 1). Additionally, two Motivation for a new survey pseudo lines were recorded (GVM 11-01/02 Currently, Switzerland depends 100% on and 11-04/07) by simultaneous recording imports in case of gas. Only one well, the along both parallel lines GVM 11-01 and 11- Entlebuch-1 well, produced a small gas 02, respectively GVM 11-04 and 11-07. while volume of around 74 Mio. m3 within the years shooting took place along one line. Despite 1984-1994. Comparing to neighboring countries the difficult topography and owing to the the exploration status in Switzerland is good and dense road network in Nid- and low. Most of the underground data from seismic Obwalden, even in the wooded mountain observations or drilling are old and in areas, it was possible to achieve fairly many locations of dissatisfying quality. straight lines. 97% of shot points were Several indications for potential gas sourced by vibrotrucks. accumulations in the subsurface make the area of The deployed vibrators of type HEMI Mertz the Central Alps attractive for an M 12 achieve a peak force of 47,700 lbf. Operation exploration. Such indications are e. g. the gas finding took place with four trucks wherever in the well Entlebuch-1 (Lahusen & Wyss possible. However due to limited space 1995), numerous gas seeps from to and/or environmental conditions vibrator Alpnach (e. g. Greber et al. 1995), gas blowouts count had occasionally to be reduced. For in the wells EWS Wilen (Wyss 2001) and blasting 2-2.5 kg of explosives (supergel 30) Wellenberg SB1 (e. g. Nagra 1997). were used in drill holes up to 15 m deep. The Based on these conditions, seismic surveys distance between the source points was were acquired in the past in Nid- and 50 m and 25 m between the receiver points. Obwalden. However, as described above, Hydrophones were used from Stansstad to the data quality is insufficient to describe Hergiswil across the «Vierwaldstättersee» the subsurface in detail. To close gaps and (line GVM 11-03). to improve the geological model, a new seismic survey was required. 3 Geological overview Organisation GVM (Gasverbund Mittelland AG) holds the The geology of the exploration area is exploration licenses for hydrocarbons in characterized by the Helvetic nappe stack Nid- and Obwalden. The initial exploration thrusted onto the Tertiary sediments (Sub- targets were to estimate the potential for alpine Molasse and North Heveltic Flysch). energy resources (gas and hot water) and Internal deformation (folding, imbrication, for gas storage. Geo Explorers Ltd was faulting) occurred in Oligocene and Miocene responsible for the project development, time. This period correlates with the Kiental seismic planning, management and the and Grindelwald deformation phase. The interpretation of the seismic data. DMT resulting Alpine nappe stack and the internal GmbH & Co. KG carried out the seismic data folds are clearly visible in the area, e. g. acquisition. Permitting was performed by the landmark mountain Pilatus.

53 Stanserhorn, Buochserhorn, Schlierental anced cross sections. Only the wells at and Glaubenberg are built up of Penninic Wellenberg intersect the Helvetic nappe units (Schlierenflysch and Klippendecke) stack. Existing seismic data only allow a overlying Ultrahelvetic and Helvetic units. schematic outline of the autochthonous The Helvetic nappe stack is divided into two Mesozoic cover. Apart from that a lot of nappes, the Drusberg and Axen nappe. The open questions persist: structural higher nappe, the Drusberg nappe • The depth and shape of the base of the covers the area north a line Lungern - Ober- Helvetic nappes is unknown. rickenbach - Isenthal. It consists of Cretaceous • The extent of folding, imbrication or the carbonates and Tertiary sediments. coupling of both within the Drusberg The Cretaceous of the Drusberg nappe was nappe can only be assumed. detached from the Jurassic sequence of the • The interfingering of Molasse and North- Axen nappe within the Palfris shales. The Helvetic Flysch is unknown. front of the Drusberg nappe forms the cliffs • Existing seismic provides hardly any of Bürgenstock, Pilatus as far as Schratten- information about the internal deformation flue. The strongly folded and imbricated (e. g. traps, faults) of the autochthonous Axen nappe outcrops south of the Drusberg Mesozoic cover. The gas in nearby well nappe and north of a line - Joch- Entlebuch-1 and numerous gas seeps let pass - Surenenpass. suppose that Permocarboniferous sediments are enclosed within crystalline basement. Its presence has to be proven U Purpose of new seismic campaign on seismic profiles. • The location and extent of displacement of To date, the existing geological model of the Aar massif onto autochthonous is vague. Helvetics relies on surface data and bal¬

50

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l à

Fig. 2: Convoy of vibrotrucks recording in Nidwalden.

',/, New findings from seismic interpretation

Internal deformation Drusberg nappe Along the Sarner Aa valley a large sinistral Due to uncertainties in predicting the strike slip fault (e. g. Schindler 1980, al. 1996) is assumed. subsurface, several interpretations of the buildup Schindler et Arguments the of the Drusberg nappe exist (Figs. 4, 5; for this fault are non-fitting geology on Hantke 1961, Menkveld 1995; Pfiffner 2011). opposite valley slopes, earthquakes below Aa Based on surface geology only, it is difficult the Sarner valley (e. g. earthquake to define the internal deformation style of in 1964) with focal mechanisms consistent strike fault, a lot of the Drusberg nappe south of Pilatus. with the slip gas seeps and sinistral Depending of the authors, internal shortening striking along the valley numerous is either characterized by folding or faults in the valley slopes parallel to the axis. this assumed imbricating or a combination of both. Based valley Unfortunately, is visible seismic. However, on surface geology, folding is evident, e. g. at large fault not on that the fault does Bürgenstock and Mueterschwanderberg this does not mean not Reasons for this be (Hantke 1961). Balanced cross sections exist. invisibility can show that besides folding additional thrusting geological and technical: thick quaternary and is mandatory (Menkveld 1995). However, sediments may cause static problems seismic so far it was unclear whether folding or filter energy. imbrication is the dominant deformation mechanism and how the internal structural Base Helvetic nappes of the Helvetic in particular build-up looks like. The base nappes, is reflective The new seismic data (Fig. 6) indicate that the Drusberg nappe, highly all sections. It can also be distinguished imbrication is a predominant deformation on the units a style within the Drusberg nappe. Displacements from underlying by more transparent character. The Cretaceous of seem to be significant and range in an layering marls and limestones of the Drusberg order of up to few kilometers. Folding is nappe shows a good seismic reflectivity compared concomitant of ramp anticlines.

SE NNW flatus Figs 4 + 5 Melchtal Aa Sarner valley UH Axen nappe :)m • Drusberg nappe

Molasse North-Helvetic Flysch

Aar massif Mesozoic cover *OOOOOO \ x\ x\ v\ x\ \\ v autochthonous basement ^ O x O \N O \x \ ^\0^ SP: Subalpine Flysch 0\ UH: -10 km \0^ Ultrahelvetic units after Pfiffner2011 0\ ¦¦ \ ^A_

1 Fig. 3: Cross section from Pilatus to Melchtal (see trace on map in Fig. It shows the prevailing compilation of knowledge after Pfiffner 2011. State of knowledge predates the described seismic survey.

55 NNW SSE + 1000 -

Om z.- sber _nap -1000-L_lr^ Fig. h: Cross section from i A~ Pfiffner (2011) across the >' j i-i.~T; -' -. ^ — -^ Sarner Aatal, see detail of Fig. 3. Trace of cross section similar to those in •2000 - Figs. 5 and 6. after Pfilfner 201

to the underlying Tertiary sediments (see and Flysch) a south dipping layering is Figs. 6, 7). Since the Drusberg nappe recognisable (e. g. see Figs. 7, 8). However, outcrops in the Northern part of the investigation on the seismic sections it is not possible to area, its base can be easily assigned. trace reflections for long distances within The base of the Helvetic nappes dips steeply the Tertiary sediments. No clear reflection from the surface at Pilatus to SE (see Figs. 7, bands or patterns exist. The seismic 8) and forms a relatively flat and wide syn- contrast within the Tertiary layering is too form in the central part of the investigation weak. Additionally, the contact Molasse - area. This synform strikes from NE to SW North Helvetic Flysch cannot be and covers a flat base of approximately 15 distinguished. km in width south of a line Schwendi - Alp- Sometimes unconformities, onlap, pinch out nach - Stans. This synform is internally and swallow structures are identifiable within weakly structured. Only few faults and low- the Molasse unit. The size of these structures amplitude folds are present. The deepest ranges in an order of 1 to 3 km in parts of the base of the Helvetic nappes width. They can be correlated with channels reach a two-way-time depth of around and/or fans, smaller structures cannot be 0.9-0.95 sec. (datum 700 m 0 sec), resolved. corresponding to a depth of approximately 1.8-2.0 km below the Sarner Aa valley. Autochthonous Mesozoic cover The autochthonous Mesozoic sediment Molasse - Flysch transition sequence can easily be traced (Figs. 7, 8). Within the Tertiary sediment unit (Molasse Similar to the Drusberg nappe the alternat-

NNW SSE H000 Sarner Aa valley Fig. 5: Balanced cross section from Menkveld (1995) UH across the Sarner Aatal, 0 m Drusber see detail of Fig. 3. Trace of cross section simitar to tappe those in Figs, h and 6. UH: USM: -1000 Ultrahelvetics, USM Untere Süsswassermolasse, UMM: Untere Meeresmo- UMM lasse, NHF: North Helvtic -2000 NHF Flysch, hypothetical boundary between USM/UMM

alter Menkveld I and UMM/NFH. y, ing layering of limestones, marls, evaporates the Mesozoic cover correlate with underlaying and siliciclastic rocks show a good reflectivity. presumed Permo-Carboniferous troughs. The continuous reflection pattern of the Mesozoic unit allows a good correlation of Permo-Carboniferous sediments all seismic sections. As learned from several seismic interpretations Although affected by the Alpine thrusting - a in the Swiss Molasse Basin, a distinction stack of nappes is overlying the Mesozoic between Permo-Carboniferous cover - the Mesozoic unit is only weakly sediments and surrounding crystalline basement folded, imbricated or intersected by significant is ambiguous. Reflections below the faults. This implies a decoupling of Mesozoic unit can either be multiple reflections Alpine deformation in the overlaying Alpine of prominent Mesozoic reflectors or nappes from the autochthonous units. As real reflections within the Permo-Carboniferous mentioned above, the assumed Sarner Aa troughs. Distinct criteria for Permo-Carboniferous valley fault is likewise not visible in the reflections are unconformities, Mesozoic unit. oblique reflection patterns in comparison to Major inversion of Permo-Carboniferous the Mesozoic ones, discontinuous and troughs and a correlated folding and systematically alternating but complex reflection imbrications of overlaying Mesozoic cover, as it is bands. evident in the Entlebuch area (e. g. prospect Such criteria can be observed on the seismic Finsterwald), cannot be observed (Figs. 7, sections in figures 7 and 8. The discordant 8). However, the few existing thrusts within reflections below the Mesozoic unit can

NNW SSE seismic line: GVM 11-06 filtered and scaled poststack time £* i migration of CRS-stack

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interpreted reflexions „Base Helvetic nappes interpreted faults Q-!

Fig. 6: of of 11 Imbrication Drusberg nappe along section seismic profile GVM -06. The cross section is inclined to and structural strike of folding thrusting. Therefore dip of thrusts appears gentle and displacement is exaggerated. Interpretation on the upper right hand part of profile is based on surface geology.

57 only be attributed to Permo-Carboniferous reflectors. The gas seeps suggest the presence of coal bearing Permo-Carboniferous.

6 Conclusions

The latest seismic data provide significant new findings and confirm structural models. The data confirm imbrication and ramp folding within the Drusberg nappe. Moreover, they show the presence of Permo-Carboniferous sediments. They are likely to contain the source rock providing the gas of the local gas seeps. Attractive leads for gas were identified below the Helvetic nappes and in Acknowledgements the autochthonous Mesozoic cover. We thank GVM for the permission to publish parts Prospects for further exploration are given. of their data.

ff% ^^p«^«%çsa^?V=-:„

Top Cretaceous Dursberg nappe ¦ ¦ ¦ 1 - —¦- at. JSa^,, v». .ja» ...-?- m

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."Top autochthonous Mesozoic cover

3ase Mesozoic cover =?y I presumably Permo- N Carboniferous sediments

Fig. 7: Section of seismic line 11-02 (NMO stack). The reflections of Cretaceous sediments of the Drusberg nappe and the Mesozoic cover of the autochthonous are visible. Reflections below the Mesozoic cover can presumably be attributed to Permo-Carboniferous sediments. Trace of presented section is given as a black line on the map.

'.8 •-. affletas : aV'--."V **¦=&_¦&._. :e -. --'¦¦-

51 |~ -Cretaceous Of- Drusberq naDDi ~>

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autochthonous ' Mesozoic sadiments / ,.'¦.:;:¦

1j presumablitPermo-Carboiilferous sediments

I presumablyrire'siimahlv autochthonous-crystallineaiitnnhthrìnnuQ-rn basement -' - ^j; "-

Fig. 8: Sections of seismic line 11-02 (interpreted migrated CRS stack), same detail as in Fig. 7. Simil ar to Fig. 7, the reflections of Cretaceous sediments of the Drusberg nappe and the Mesozoic cover o f the autochthonous can be traced. Reflections below the Mesozoic cover can be attributed to a Permo- Car- boniferous trough. Trace of presented section is given as a black line on the map.

References Pfiffner, 0. A.. 2011 : Structural Map of the Helvetic Zone of the Swiss Alps including Vorarlberg Greber, E., Leu, W. & Wyss. R. 1995: Erdgasmdika- (Austria) and Haute Savoie (France). Geological tionen in der Schweiz. Grundlagen zur Special Map 1:100000, Swiss Geological Survey, Charakterisierung des Gasgefahrenpotentials im Untergrund. 128, Federal Office of Topography swisstopo, Bull. Schweizer Ingenieur und Architekt., Wabern. 24, 8. Juni 1995,567-572. Schindler. C. 1980: Geologisch-geotechnische Hantke, R. 1961: Tektonik der helvetischen Voruntersuchung der N8 zwischen Samen und Kalkalpen zwischen Obwalden und dem St. Galler dem Brünig. Strasse und Verkehr, 66/5, 157-163 Rheintal. Vierteljahrsschrift der Naturforschenden Schindler, C, Beer, C, Mayer-Rosa, D., Rüttener, C.' Gesellschaft Zürich, 106, p. 211. E., Wagner, J.-J., Jaquet, J.-M., Frischknecht, Lahusen, H. L. & Wyss, R. 1995: Erdöl- und Erdgas- 1996: Integrierte Auswertung von seismischen exploration in der Schweiz: Ein Rückblick. und bodenspezifischen Parametern: Bulletin der Vereinigung schweizerischer Gefährdungskarten im Kanton Obwalden. Geologische Petroleum-Geologen und -Ingenieure, 62/141, 43-72. Berichte Landeshydrologie und-geologie 19 p Kaufmann, F. J. 1867: Der Pilatus. - Beitr. Geol. 61. Karte Schweiz 5. Wyss, R. 2001 : Der Gasausbruch aus einer Menkveld. J. W. 1995: Der geologische Bau des Erdsondenbohrung in Wilen (0W|. Bulletin für Helvetikums der Innerschweiz. PhD thesis. Angewandte Geologie, 6/1, 25-40. Universität Bern, p. 165. Nagra 1997: Geosynthese Wellenberg 1996. Ergebnisse der Untersuchungsphasen I und 11. Nagra Technischer Bericht NTB 96-01, p. 511.

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