A new lithostratigraphic scheme for the Schinznach Formation (upper part of the Muschelkalk Group of northern Switzerland)

Pietsch, Johannes S.; Wetzel, Andreas; Jordan, Peter

doi:10.1007/s00015-016-0214-7

Published: 12 July 2016

A new lithostratigraphic scheme for the Schinznach Formation (upper part of the Muschelkalk Group of northern Switzerland)

Johannes S. Pietsch^1,2,
Andreas Wetzel¹ &
Peter Jordan^1,2

Swiss Journal of Geosciences volume 109, pages 285–307 (2016)Cite this article

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Abstract

The sediments of late Anisian and Ladinian age in northern Switzerland, which were formerly named Upper Muschelkalk and Lower Keuper, mostly consist of carbonates. They accumulated in a transitional area between central parts of the Central European Epicontinental Basin and its margin towards the Vindelician Swell. Oolitic intervals imply the former presence of shoals or ramps in this region. They characterise this transitional region (“Alemannische Fazies”) together with dolomites in the upper part of the sedimentary succession. The total thickness usually varies between 50 and 85 m. A general decrease in thickness towards southeast has been found. The newly named Schinznach Formation is defined as a mappable unit of the upper part of the Muschelkalk Group. It is precisely introduced for those sedimentary rocks formerly named Upper Muschelkalk and Lower Keuper in northern Switzerland between the Doubs River and the Lake Biel in the west and the Lake Constance in the east. Within this formation the informal lithostratigraphic subdivisions currently in use should be replaced by new terms in accordance with the rules of lithostratigraphic nomenclature. In the scheme presented here the Schinznach Formation comprises 5 members and 5 beds.

1 Introduction

1.1 Introductive remark

The terms Muschelkalk and Keuper are well established in international literature (e.g. Aigner 1985; Szulc 2000; Palermo et al. 2010) and are, therefore, used in translated form (Upper Muschelkalk), while regional units are notified in German. Informal units are set off by quotation marks.

1.2 History of Upper Muschelkalk stratigraphy in northern Switzerland

1.2.1 General overview

The term Muschelkalk was introduced into the geological literature by Füchsel (1761), primarily based on observations in central Germany. Later the superordinate term Trias was introduced by von Alberti (1834). Von Alberti (1826) described the “Steinsalz umschliessender Kalkstein” from southern Germany and divided these sediments into five subunits (Fig. 1). Later, Quenstedt (1843) established the term “Hauptmuschelkalk-Gebirge”, which was used henceforward for different parts of Upper Muschelkalk (Figs. 1, 2).

Roughly at the same time Merian (1821) described the sediments of the Swiss Jura mountains and established the term “Rauchgrauer Kalkstein” for the whole Muschelkalk. Despite the fact that he first grouped these sediments into the Jurassic, he later correlated the “Rauchgrauer Kalkstein” with the Muschelkalk of northern Germany (Merian 1831).

In 1867 Moesch established a stratigraphic scheme of the Upper Muschelkalk in the Aargau Jura distinguishing four members. He divided the lower part into “Thonkalkbänke” and “Encrinitenkalke”, introduced the name “Plattenkalk” for the middle part and called the upper part “Oberer Muschelkalkdolomit mit Hornstein”. A related scheme was elaborated by Schalch (1878), who established the term “Trigonodusdolomit” in northern Switzerland following Sandberger (1864), who introduced the term. Both Moesch and Schalch defined the “Lettenkohle” as an independent formation between the Muschelkalk and the overlying Keuper.

Several decades later, Disler (1914) published stratigraphic and tectonic investigations dealing with the “Rotliegendes” and the Triassic of the Rheinfelden area. He combined the “Trochitenkalk” and the “Nodosuskalk” into the “Hauptmuschelkalk” and he subsumed both units together with the “Trigonodusdolomit” to the Upper Muschelkalk. This division has been in general use since then.

Based on numerous measured sections, a detailed stratigraphic scheme of the Upper Muschelkalk in northern Switzerland was worked out by Merki (1961). He suggested the usage of the term “Hautpmuschelkalk” for “Trochitenkalk”, “Nodosuskalk” as well as for the “Trigonodusdolomit”. However, his suggestion was not accepted and, therefore, not in general usage (e.g. in the Geological Atlas of Switzerland). Thus, the Upper Muschelkalk was still divided into “Trigonodusdolomit” and “Hauptmuschelkalk” during the last decades, the latter comprising the subunits “Plattenkalk” and “Trochitenkalk”.

While in Germany there has been further endeavour on sequence stratigraphy of the Upper Muschelkalk and Lower Keuper (e.g. Aigner 1985; Aigner and Bachmann 1992; Urlichs and Mundlos 1990; Franz et al. 2013) and formal lithostratigraphic units have been established (e.g. Hagdorn and Simon 2005), there has not been much interest in the stratigraphy of Upper Muschelkalk in northern Switzerland during the last decades of the 20th century. Today the Upper Muschelkalk is prospected for geothermal energy production and for storage of gas or CO₂ (Alt-Epping and Diamond 2014). Therefore its reservoir properties are currently investigated (e.g. Aschwanden et al. 2014; Adams et al. 2014).

1.2.2 Muschelkalk-Keuper boundary

While refining the Triassic stratigraphy, the boundary between Muschelkalk and Keuper was placed at different levels by different authors (Figs. 1, 2). Von Alberti (1834) first added the “Lettenkohle” (today’s Asp Member) to the Keuper. Later he also included the “Unterer Dolomitischer Kalkstein” (today’s Stamberg Member) together with the “Lettenkohle” and the “Oberer Dolomit” as “Gruppe der Lettenkohle”, which was stated to belong to the Keuper (von Alberti 1864). Gressly (1853) placed the boundary between Keuper and Muschelkalk above the “Oberer Muschelkalkdolomit”. Moesch (1867) intercalated the “Lettenkohle” as an autonomous formation between the Muschelkalk and the Keuper. Schalch (1878) adopted this quartering of the Triassic into Buntsandstein, Muschelkalk, “Lettenkohlegruppe” and Keuper. Brombach (1903) again included the “Lettenkohle” into the Keuper as well as Mühlberg (1905, 1908, 1911), who changed his mind later on (Mühlberg 1915). Since Disler (1914) published his stratigraphic scheme of the Triassic, the “Lettenkohle” has been grouped into the Keuper in general use (e.g. for the Geological Atlas of Switzerland). Merki (1961) emphasised the many indicators of marine facies to occur in the “Lettenkeuper” and, therefore, he included the “Lettenkohle” in the Upper Muschelkalk. This allocation, however, did not become common usage in Switzerland.

1.2.3 Need for revision

In the context of the HARMOS project of the Swiss Geological Survey to harmonise the Swiss stratigraphic scheme (Morard et al. 2012, Strasky et al. 2016), the Upper Muschelkalk was redefined as Schinznach Formation (name approved by the Swiss Committee on Stratigraphy on 22nd November 2014). In its definition, the Schinznach Formation also includes the former Lower Keuper (“Lettenkohle”), formally renamed as Asp Member.

Before this revision, lithostratigraphy and allostratigraphy were mixed (e.g. usage of marker beds like the “Mergelhorizont”(now Dünnlenberg Bed) as boundary between “Trochitenkalk” and “Plattenkalk” by Merki 1961). Furthermore, many synonyms, such as “Plattenkalk”, “Nodosuskalk” and “Tonplattenregion” (see Figs. 1, 2), as well as different levels defined as boundaries between the subunits were used. For these reasons, a new stratigraphic scheme of the Schinznach Formation complying to the guidelines of the Swiss Committee on Stratigraphy (Remane et al. 2005) needed to be developed. The lithostratigraphic scheme of the Schinznach Formation presented here was approved by the Swiss Committee on Stratigraphy on 28th January 2016.

2 Materials and methods

The new lithostratigraphic subdivision of the Schinznach Formation is based on (1) already published sections that were measured in outcrops (their stratigraphic subdivision is here revised according to the new scheme) and (2) borehole data because the conditions of exposure are poor in general. Unfortunately, there are no sections outcropping that expose the whole sedimentary succession of the Schinznach Formation from base to top. Therefore, the Nagra borehole Weiach is proposed as boundary stratotype section of the Schinznach Formation.

Due to the regional character of this study, the coordinates of the sections are stated using the Swiss coordinate system CH1903+.

2.1 Biostratigraphy

A ceratite biostratigraphy is well established for central and southeastern parts of the Central European Epicontinental Basin (e.g. Wenger 1957; Kozur 1974; Urlichs and Mundlos 1990; Urlichs 1993a; Ockert and Rein 2000; Rein 2007a, b). However, due to rare occurrence of ceratites along the basin margins, it is difficult to correlate the margins with central parts based on ceratite zonation (Hagdorn et al. 1993).

In northern Switzerland ceratites were only found in the Dünnlenberg Bed and the overlying marls of the Liedertswil Member. The following ceratites probably were found in the Dünnlenberg Bed: Ceratites münsteri E. Phil. det.: Celliers 1907 (belongs to Ceratites spinosus Philippi according to Urlichs 2006), Ceratites compressus Sandb. det.: Celliers 1907 (=Ceratites compressus Philippi according to Urlichs 2006), Ceratites (Acanthoceratites) compressus compressus Philippi det.: Merki 1961 (=Ceratites compressus Philippi according to Urlichs 2006), Ceratites (Acanthoceratites) compressus subnudus Stolley det.: Paul 1971 (=Ceratites subnudus Stolley according to Urlichs 2006), Ceratites (Acanthoceratites) evolutus Philippi det.: Merki 1961 (=Ceratites evolutus Philippi according to Urlichs 2006), Ceratites (Acanthoceratites) evolutus evolutus Philippi det.: Merki 1961 (=Ceratites evolutus Philippi according to Urlichs 2006), Ceratites (Acanthoceratites) evolutus tenuis Riedel det.: Paul 1971 (=Ceratites evolutus Philippi according to Urlichs 2006) and Ceratites (Acanthoceratites) evolutus praecursor Riedel det.: Paul 1971 (=Ceratites obesus Wenger or Ceratites praecursor Riedel according to Urlichs 2006, because of the co-occurrence of ceratites evolutus and ceratites compressus probably the latter). The allocation of Ceratites münsteri (now Ceratites spinosus) is questionable. All other described ceratites can be assigned to the Compressus or Evolutus zones (Urlichs and Mundlos 1988; Urlichs and Mundlos 1990; Urlichs 1993). Therefore, the Dünnlenberg Bed seems to lie within the boundary interval between the Compressus and Evolutus zones.

While the Spiriferinabank (which is not developed as such in northern Switzerland) somewhat above the Dünnlenberg Bed (Paul 1971) corresponds to the boundary between the Compressus zone and the Evolutus zone (Kozur 1974; Ockert 1988; Urlichs 1993; Ockert and Rein 2000), the Dünnlenberg Bed obviously lies in the late Compressus zone and, therefore, has a late Anisian age (Illyrian; Brack et al. 1999; Kozur 1999; Brack et al. 2005).

Nevertheless the number of found ceratite specimens is low and it does not permit a precise and unequivocal age assignment. The taxonomic determination is not reproducible due to missing reference samples and, hence, it is still a matter of discussion (compare Wenger 1957 in Merki 1961 and Rein 2007b, in this study, the zonation suggested by Urlichs 1993 is used). Further biostratigraphic dating based on the conodont zonation (Kozur 1974) is therefore still necessary.

2.2 Isopach map

An isopach map of the Schinznach Formation (Fig. 4) was constructed using the data of 294 boreholes, of 38 outcrops exposing a minimum thickness of 30 m and of the geologic record of the Belchen tunnel (Fröhlicher and Kehrer 1968).

Wherever dipping- or well-path-deviation-data were available, they were considered to identify the true thickness. In the Folded Jura, dipping information was taken from the Geological Atlas of Switzerland.

In the Basel Tabular Jura many borehole data of the Schweizer Salinen AG were available. Locally they show a highly variable thickness of the Schinznach Formation. This variation is interpreted to result from drilling differently inclined strata that were tilted during the formation of the Upper Rhinegraben (e.g. Gürler et al. 1987). Therefore thickness outliers were not considered.

3 Schinznach Formation

Names previously in use and synonyms: “Oberer Muschelkalk” and “Lettenkohle” among others (see Figs. 1, 2).

Type locality: Bad Schinznach (Canton Aargau; coordinates: 2655.000/1256.725); Merki (1961); outcrop well exposed, but hardly accessible due to railway traffic; Fig. 6.

Type region: Eastern Folded Jura and northern Tabular Jura.

Boundary stratotype section: Nagra borehole Weiach (Canton Zurich; coordinates: 2676.745/1268.617); Matter et al. (1988a), Nagra (1989); Fig. 7.

Underlying strata: Zeglingen Formation (Jordan 2016); mainly composed of evaporites and in the upper part of laminated (stromatolitic) dolomites (“Anhydritdolomit”).

Lower boundary: Medium- to thick-bedded, sometimes dolomitic limestones and dolomites overlie thin bedded and laminated, stromatolitic dolomites of the Zeglingen Formation.

Overlying strata: Bänkerjoch Formation (Jordan et al. 2016); mainly composed of evaporites and mudrocks.

Upper boundary: Last occurrence of dm- to m-thick continuous dolomite or rauhwacke. (The onset of the Bänkerjoch Formation is marked by marls and/or sulphates).

Subdivision (from base to top): Leutschenberg Member, Kienberg Member, Liedertswil Member, Stamberg Member, Asp Member.

Occurrence: Northern Switzerland.

Thickness: Usually 50–85 m (Fig. 4). A general decrease in thickness towards southeast can be detected. Local maxima and minima seem to be associated to pre-existent tectonic structures.

Chronostratigraphic age: Middle Triassic (Anisian to Ladinian).

Description: The Schinznach Formation consists of often dolomitic greyish limestones and—especially within the upper part—beige dolomites, comprising (from base to top) macrofossil-poor limestones, bioclastic limestones, limestones with interbedded marls and locally macrofossil-rich dolomites. Oolites are intercalated in the lower and the upper third. At the top, above the dolomites, there are marls or clays and finally dolomites. The upper third of the Schinznach Formation locally contains anhydrite (e.g. Matter et al. 1988a).

4 Lithostratigraphic subunits of the Schinznach Formation

4.1 Leutschenberg Member

Names previously in use and synonyms: “Unterer Trochitenkalk” among others (see Figs. 1 and 2).

Type locality: Western hillside of the Leutschenberg near Zeglingen (Canton Aargau; coordinates: 2636.950/1251.250); Merki (1961); Fig. 8.

Underlying strata: Zeglingen Formation (Jordan 2016).

Overlying strata: Kienberg Member.

Subdivision: Fützen Bed.

Occurrence: Northern Switzerland.

Thickness: Between approximately 4 m and 10 m (Merki 1961).

Description and boundaries: The Leutschenberg Member represents the lowermost part of the Schinznach Formation. It mainly consists of bioturbated mud- and wackestones. Bedding planes and burrows are often dolomitised. The basal strata are locally dolomitic (Disler 1914; Merki 1961) and overlie stromatolitic dolomites of the Zeglingen Formation (Fig. 8).

At some localities strata rich in crinoid and shell detritus occur near the base of the Leutschenberg Member (Figs. 8, 10). They are between 5 cm (Fig. 10; see Merki 1961; Gsell 1968) and about 2 m thick (Merki 1961). Merki (1961) called these strata “Basale Trochitenbank”. They often represent the lowermost part of the Leutschenberg Member, but similar lithology may occur in middle part as well (Disler 1914; Merki 1961). Due to their multiple occurrence at different stratigraphic levels within the lower part of the Leutschenberg Member they cannot be defined as a single bed. The Basale Trochitenbank rather comprises different crinoid-detritus-rich strata. Where these strata occur at the base of the Leutschenberg Member, they often contain dolomite clasts originating from the underlying “Anhydritdolomit” representing the top of the Zeglingen Formation (Merki 1961).

The top of the Leutschenberg Member is defined by the first occurrence of successional float- to rudstones rich in crinoid and shell detritus above bioturbated mud- and wackestones (see below; Figs. 8, 10). Thus, the “Basale Trochitenbank” of Merki (1961) is defined to form part of the Leutschenberg Member.

4.1.1 Fützen Bed

Names previously in use and synonyms: “Liegend-Oolith” or “Basaloolith” among others (see Fig. 2).

Type locality: Wutachflüh (W Fützen, Germany; coordinates: 2680.650/1296.300); Paul (1971); Fig. 9. Traditionally, the subcommission on Permian–Triassic Stratigraphy (SKPT) of the German Commission on Stratigraphy (DSK) defines the Wutachflüh as type locality of the Liegendoolith Subformation, which together with the Marbach-Oolith corresponds to the Fützen Bed.

Underlying strata: Zeglingen Formation or lowermost parts of the Leutschenberg Member.

Overlying strata: The Fützen Bed often represents the basal strata of the Leutschenberg Member.

Occurrence: Westernmost Tabular Jura and adjacent Dinkelberg (Merki 1961; Rieser 1964; Brüderlin 1969, 1971), easternmost Tabular Jura (Merki 1961; Peters et al. 1989; Nagra 1984), Klettgau area and Wutach region (Paul 1936, 1971; Brüderlin 1971; Nagra 1992b), and in the Folded Jura near Eptingen (unpublished borehole data Eptingen 85.R.305, Bohrkataster Basel-Landschaft) and Lostorf (Schmassmann 1977). Whether the oolites in the easternmost Tabular Jura and the Klettgau area form a continuous lithosome cannot be ascertained. Due to the drilling of oolites belonging to the Fützen Bed in the borehole Weiach (Matter et al. 1988a) it seems to be possible.

Thickness: Up to approximately 5 m (e.g. Paul 1971; Matter et al. 1988a; Peters et al. 1989).

Description: The Fützen Bed represents the lowest oolitic interval of the Schinznach Formation. It consists of a thick-bedded, calcareous oolite (Paul 1971). The basal strata may contain chert nodules (Paul 1971; Hofmann et al. 2000). Macrofossils are rare in the Fützen Bed (Merki 1961; Rieser 1964; Paul 1971). Most of the ooids are recrystallised (Merki 1961) or dolomitised (Merki 1961; Rieser 1964). For discussion of the lithostratigraphic definition of the oolitic intervals see below.

4.2 Kienberg Member

Names previously in use and synonyms: “Oberer Trochitenkalk” among others (see Figs. 1 and 2).

Type locality: Saalhof (SW Kienberg, Canton Solothurn; coordinates: 2640.710/1253.860) compare Gsell (1968), Merki (1961); abandoned quarry, lowermost part not outcropping anymore; Fig. 10.

Underlying strata: Leutschenberg Member.

Overlying strata: Liedertswil Member.

Subdivision: Saalhof Bed and Dünnlenberg Bed (multiple subordination).

Occurrence: Northern Switzerland.

Thickness: Between some 14 m and 22 m.

Description and boundaries: The Kienberg Member consists of strata rich in crinoid fragments and shell debris (“Trochitenbänke” or “Schillbänke”). They typically form coarsening-upward cycles. Some marl may be interbedded. Many of the strata rich in crinoid and shell detritus possess subbeds (Aigner 1985) that are micritic layers diagenetically attached along their erosive base. Like for the Leutschenberg Member, bedding planes and burrows are often dolomitised.

The base of the member is defined by the first continuous occurrence of float- or rudstones rich in crinoid and shell detritus above the bioturbated mud- and wackestones of the Leutschenberg Member. This boundary is often marked by a bed rich in Coenothyris vulgaris (newly defined as Saalhof Bed, see below). Of course, mud- to wackestones also occur in the Kienberg Member, in which they represent the lower part of the coarsening-upward cycles. It is a classifier that the crinoid- and shell-detritus-rich float- or rudstones exceed 5 m in thickness to form part of the Kienberg Member. Conversely, the “Basale Trochitenbank” of Merki (1961) and similar relatively thin strata rich in crinoid fragments in the lowermost part of the Schinznach Formation still belong to the Leutschenberg Member (see above).

Deviating from Merki (1961) who placed the top of the “Oberer Trochitenkalk” at the base of the Dünnlenberg Bed, the top of the Kienberg Member is now defined with the last occurrence of float- to rudstones rich in crinoid detritus with a minimal bed thickness of 10 cm. This means that at least ten percent of the stratum has to be built up by crinoid debris. Of course, the subbeds of the strata rich in crinoid detritus should not be taken into account in this threshold value. Some crinoid debris may occur in overlying strata, but mostly they are limited to the top of the strata, whereas the strata themselves are built up mainly by bivalve shells.

The stratigraphic level up to which these crinoid-rich strata are present varies (Merki 1961). In Germany the Spiriferinabank is seen as the last bed rich in crinoid detritus and is set as top of the Trochitenkalk Formation (Geyer and Gwinner 2011; Paul 1971; Hagdorn and Simon 2005). It lies little above the Dünnlenberg Bed (Paul 1971; compare Fig. 9). The last occurrence of Encrinus liliiformis, which is the ubiquitous crinoid in the Kienberg Member, is coupled with the Spiriferinabank (Hagdorn and Simon 1993; Hagdorn 1999) and, therefore, with the boundary between the Compressus zone and the Evolutus zone as well (see above). Because of the varying stratigraphic level reached by the strata rich in crinoid detritus, the Dünnlenberg Bed may belong to either the Kienberg Member or the Liedertswil Member (see below).

4.2.1 Saalhof Bed

Names previously in use and synonyms: “Coenothyrisbank” among others (e.g. “Terebratellage” (Brombach 1903); compare Fig. 2).

Type locality: Saalhof (SW Kienberg, Canton Solothurn; coordinates: 2640.710/1253.860); compare Merki (1961), Gsell (1968); abandoned quarry; Fig. 10.

Occurrence: Northern Switzerland, locally missing.

Thickness: 5 cm up to 70 cm (Merki 1961).

Description: The lowermost macrofossil-rich stratum of the Kienberg Member is often rich in Coenothyris vulgaris and is here newly named Saalhof Bed. These brachiopod shells are usually dolomitised. At the type locality it consists of two strata, of which the lower one contains brachiopod shells only in its middle part (Fig. 10).

The Saalhof Bed does not occur in whole northern Switzerland. Whether it was eroded briefly after deposition as supposed by Merki (1961) or it was not deposited area-wide remains unclear.

4.2.2 Dünnlenberg Bed

Names previously in use and synonyms: “Mergelhorizont” or “Mergelhorizont III” (compare Fig. 2).

Type locality: Dünnlenberg (S Liedertswil, Canton Basel-Landschaft; coordinates: 2620.930/1248.530); compare Merki (1961), Herold (1992); Fig. 11.

Occurrence: Basel and Aargau Tabular Jura, Folded Jura west of Schinznach (Merki 1961), Wutach region (Paul 1936, Paul 1971); traceable across northern Switzerland using gamma-ray logs (Fig. 15).

Thickness: 10 to 40 cm.

Chronostratigraphic age: Late Anisian (Illyrian; compare discussion above).

Description: The Dünnlenberg Bed consists of grey or brownish marl, which can be dolomitic or calcareous. It lies within the upper part of the strata rich in crinoid and shell detritus, where macrofossil-poor mud- and wackestones and intervals only rich in shell detritus gradually and diachronously replace this facies. Therefore, it may belong to either the Kienberg Member or the Liedertswil Member. Although many outcrops show several marly horizons, only the lowermost horizon, which is thicker than about 5 cm is defined as Dünnlenberg Bed (Figs. 10, 11). In northern Switzerland ceratites were only found in the Dünnlenberg Bed and in the overlying marl of the uppermost part of the Kienberg Member (see biostratigraphic discussion).

In gamma-logs the Dünnlenberg Bed is characterised by a positive peak (Figs. 10, 11). While the Leutschenberg Member and the Kienberg Member are characterised by decreasing clay content (Figs. 10, 15) the Dünnlenberg Bed often represents the first significant peak. Locally mostly less pronounced peaks may occur further up (e.g. Schafisheim; Fig. 15), which may correspond to the marl- and glauconite-bearing strata overlying the Dünnlenberg Bed.

Discussion: The maximum flooding of the Upper Muschelkalk Sea was supposed to have occurred at different levels by different authors (Kozur 1974; Aigner 1985; Aigner and Bachmann 1992; Röhl 1990). Franz et al. (2013) prefer a comparatively early maximum flooding similar to Kozur (1974) because of the transgression of coastal plain-fluvial sediments of the Erfurt Formation (largely equivalent to the Asp Member) since the late Anisian. Franz et al. (2015) located the initial regression of the Upper Muschelkalk Sea in the interval of the Compressus to Evolutus zone. In southwestern Germany they ascribe the increasing abundance of clastics in the so-called “Tonhorizonte”, which occur above the Spiriferinabank, to this first regression. Due to the biostratigraphic assignment of the Dünnlenberg Bed to the interval from the Compressus to the Evolutus zone (compare discussion above) it seems plausible to link the Dünnlenberg Bed to this first regression as well.

4.3 Liedertswil Member

Names previously in use and synonyms: “Plattenkalk” among others (see Figs. 1, 2).

Type locality: Dünnlenberg (S Liedertswil, Canton Basel-Landschaft; coordinates: 2620.930/1248.530); compare Merki (1961), Herold (1992); Fig. 11.

Underlying strata: Kienberg Member.

Overlying strata: Stamberg Member.

Subdivision: Dünnlenberg Bed (multiple subordination, compare above), Eptingen Bed (multiple subordination).

Occurrence: Northern Switzerland.

Thickness: 1.5 m (Weiach borehole, Fig. 7) up to approximately 25 m; due to the diagenetic nature of the upper boundary of the Liedertswil Member it is possible that the member is missing at some localities (compare with description below).

Description and boundaries: The Liedertswil Member consists of limestones rich in shell detritus in the lower part and macrofossil-poor mud- and wackestones in the upper part. As discussed above, crinoid detritus may occur in some strata of the lower part. In most sections the Dünnlenberg Bed lies little above the base of the Liedertswil Member. Above the Dünnlenberg Bed other marl layers may occur. Locally ceratites can be found in these layers (see above; Braun 1920; Merki 1961). Strata above and below the marl layers may contain glauconite (Merki 1961; Herold 1992; Brüderlin 1969).

Especially the upper part of the Liedertswil Member is often dolomitised. In some regions the dolomitised Liedertswil Member contains chert nodules (e.g. Braun 1920; Merki 1961). Within the lower parts often bedding planes and burrows are dolomitised similar to underlying members.

The top of the Liedertswil Member is defined by the change from (dolomitic) limestone with single dolomitic strata to a both laterally continuous and pure dolomite. Due to the diagenetic nature of the upper boundary of the Liedertswil Member, the level of the boundary varies within the Schinznach Formation. It is therefore possible that the normally overlying Stamberg Member directly rests on the Kienberg Member, if dolomitisation is used as classification criterion and if dolomitisation affected the uppermost strata rich in crinoid detritus. In that case the Liedertswil Member is not developed as such.

The oolitic interval around the boundary interval between the Liedertswil Member and the Stamberg Member is defined as Eptingen Bed. Depending on dolomitisation-depth it lies either in one or in both of these members.

4.3.1 Eptingen Bed

Names previously in use and synonyms: “Eptinger Oolith” and “Giebenacher Oolith” among others (see Fig. 2).

Type locality: Stamberg (S Eptingen, Canton Basel-Landschaft; coordinates: 2628.625/1248.025); Merki (1961); not completely outcropping anymore; Fig. 12.

Occurrence: Folded Jura between Reigoldswil and Bänkerjoch (Merki 1961; Schmassmann 1977); Tabular Jura west of Rheinfelden and adjacent Dinkelberg (Merki 1961; Rieser 1964; Brüderlin 1969, 1971); Wutach region (Brüderlin 1969, 1971; Paul 1971); drilled in Weiach (Matter et al. 1988a), Benken (Nagra 2001) and Schlattingen (Albert et al. 2012).

Thickness: Up to 8 m (Merki 1961).

Description: The Eptingen Bed represents the middle oolitic interval of the Schinznach Formation. The ooids are mostly dolomitised (Merki 1961; Rieser 1964) and, therefore, lost their internal structure. If the dolomitisation also concerns the matrix, the ooid shapes often can be recognised under microscope only. In the region of Eptingen chert nodules occur in the oolitic interval. They enclose silicified ooids (Merki 1961). For discussion of the lithostratigraphic definition of the oolitic intervals see below.

4.4 Stamberg Member

Names previously in use and synonyms: “Trigonodusdolomit” among others (see Figs. 1 and 2).

Type locality: Stamberg (S Eptingen, Canton Basel-Landschaft; coordinates: 2628.625/1248.025); Merki (1961); not completely outcropping anymore; Fig. 12.

Underlying strata: Liedertswil Member or Kienberg Member when the dolomitisation reached down to the strata rich in crinoid detritus.

Overlying strata: Asp Member.

Subdivision: Eptingen Bed (multiple subordination) and Kaisten Bed.

Occurrence: Northern Switzerland.

Thickness: Locally highly variable due to the diagenetic character of the lower boundary. Common values between approximately 20 m and 30 m; higher values for example in the Schinznach region (about 35 m; Häring 1997; Arbeitsgruppe Geothermik 1988) and in Benken (37 m; Nagra 2001).

Description and boundaries: The lower boundary is defined by the change from (dolomitic) limestone with single dolomitic strata to a laterally continuous and several m-thick pure dolomite. Single calcareous beds may occur within the Stamberg Member (compare mineralogic composition of borehole Weiach; Nagra (1989); Fig. 15) but the dolomite content has to exceed 90 percent. Macroscopically the dolomite varies considerably. Especially the upper part of the Stamberg Member typically shows a higher weatherability, which may arise from the texture prior to dolomitisation. The lower part often cannot be separated sharply from the underlying limestones of the Liedertswil Member visually. Matter et al. (1988b) recognise an offset of 1.75 m between a boundary set by macroscopic observation and a boundary set by the effective dolomite content. Because the lower, less erodible part of the Stamberg Member resembles the limestones of the Liedertswil Member, former investigators divided the dolomite in typical “Trigonodusdolomit” and dolomitic “Plattenkalk” (e.g. Merki 1961; Matter et al. 1988a).

Due to the diagenetic nature of the lower boundary of the Stamberg Member, the level of the boundary varies within the Schinznach Formation. It is therefore possible that the Stamberg Member directly rests on the Kienberg Member, if dolomitisation reached down to the uppermost strata rich in crinoid detritus. The dolomites of the Stamberg Member nowadays have a mudstone appearance, but they mostly seem to have been mud- and wackestones prior to dolomitisation. Locally bivalve- and gastropod-rich strata occur within the dolomites (Herb 1957; Merki 1961). Especially the uppermost strata, which partly were defined to belong to the Lower Keuper formerly, are rich in macrofossils. Bonebeds occur in the upper part and contain, for example, fish and reptile teeth (Braun 1920).

The Stamberg Member locally contains chert nodules. In Liedertswil (Fig. 11) they occur in stromatolitic dolomites, as also known from southwestern Germany, where stromatolitic dolomites are present in the upper part of the Rottweil Formation (Alesi 1984). Locally the Stamberg Member seems to be sandy (Disler 1914; Vonderschmitt 1942) or contains anhydrite (e.g. Vonderschmitt 1942; Matter et al. 1988a). The top of the Stamberg Member is defined by the first decimetre-thick mud or argillaceous marl layer of the Asp Member.

4.4.1 Kaisten Bed

Names previously in use and synonyms: “Kaistener Schichten” among others (Fig. 2).

Type locality: Kaisten (Canton Aargau; coordinates: 2646.600/1265.675); Merki (1961); Fig. 13.

Occurrence: Wutach region (Paul 1971); Tabular Jura between the Aare (Nagra 1984; Matter et al. 1987; Brüderlin 1971) and Kaisten (Merki 1961).

Thickness: up to approximately 5 m (Kaisten, Fig. 13; Merki 1961).

Description: The Kaisten Bed represents the uppermost oolitic interval of the Schinznach Formation. It comprises the oolitic intervals described for the Wutach region (e.g. Paul 1971), the lower course of the Aare River (Nagra 1984; Matter et al. 1987; Brüderlin 1971), and the macrofossil-rich oolitic strata in the Kaisten area (“Kaistener Schichten”; Merki 1961). The ooids in this interval are completely dolomitised (Merki 1961; Rieser 1964) and often ooid shapes can only be recognised under the microscope.

While most occurrences of the uppermost oolitic interval were described either from boreholes (Riniken, Matter et al. 1987; Beznau, Nagra 1984) or from outcrops in adjacent Germany (Brüderlin 1971; Paul 1971), the type locality of the oolitic “Kaistener Schichten” proposed by Merki (1961) in the Aargau Tabular Jura was chosen to be the type region of the Kaisten Bed as well. Contrary to the “Kaistener Schichten” the Kaisten Bed only comprises the oolitic strata within the bivalve- and gastropod-rich strata described by Herb (1957). For discussion of the lithostratigraphic definition of the oolitic intervals see below.

4.5 Asp Member

Names previously in use and synonyms: “Lettenkohle” among others (Figs. 1 and 2).

Type locality: Asp (Canton Aargau; coordinates: 2646.375/1255.125); Furrer (1977); Fig. 14.

Boundary stratotype section: Nagra borehole Weiach (Canton Zurich; coordinates: 2676.745/1268.617); Matter et al. (1988a), Nagra (1989); Fig. 7.

Underlying strata: Stamberg Member.

Overlying strata: Bänkerjoch Formation (Jordan et al. 2016).

Occurrence: Northern Switzerland.

Thickness: Up to 15 m, but variable. In the Basel Tabular Jura and the Upper Rhine Graben from reported, but very likely too low value of 1 m (Schweizer Salinen AG, unpublished) to 15 m (Hauber 1991); in the Aargau Tabular Jura, in the Schaffhausen area and the Zürcher Weinland from approximately 4 m (see Nagra 1984, 1990, 2001; Albert et al. 2012) to 8 m (Arbeitsgruppe Geothermik 1988); in the region of Lake Constance from 7 m (Büchi et al. 1965) to 15 m (Lemcke and Wagner 1961).

Description and boundaries: The base of the Asp Member is defined by the first decimetre-thick mud or argillaceous marl layer on top of the dolomites of the Stamberg Member. Traditionally the base was set with a millimetre- to centimetre-thick condensation layer called “Grenzbonebed” (e.g. Merki 1961). In this case the boundary is located within the dolomite and is therefore not always recognisable, particularly in drillings.

Within the mud or argillaceous marl intervals, one (e.g. Asp, Fig. 14) or more (e.g. Eptingen, Fig. 12) thin dolomite beds can occur. They may be equivalent to the dolomite beds of the “Alberti-Schichten”, especially the Alberti-Bank, which has been distinguished in southwestern Germany (Essigmann 1979; Nitsch 2015), where the thickness increases (up to 33 m in the Kraichgau Trough; Etzold and Schweizer 2005). Formerly this argillaceous succession was called “Estherienschiefer” (Fig. 2) after Euestheria minuta (Zieten 1833), which locally occurs in some layers (e.g. Disler 1914; Merki 1961; Gsell 1968). In addition to conchostraca and other invertebrates, plant remains (Brändlin 1911; Braun 1920) and vertebrate fossil components were found. Among the latter are remains of fish and teeth (e.g. Schmidt et al. 1924; Furrer 1977), bones (e.g. Brändlin 1911; Disler 1914; Merki 1961) amongst others of Nothosauridae (Frank 1928; Peyer 1932). Locally sandy layers are interbedded (e.g. Brändlin 1911; Merki 1961; Diebold et al. 2006). The dolomites within the mudrock or marl can have a sandy appearance (Braun 1920; Diebold et al. 2006).

Dolomites follow above the argillaceous succession. Locally anhydrite nodules occur within the Asp Member (e.g. Matter et al. 1988a, b; Jordan et al. 2011). In outcrops the anhydrite-rich strata are often weathered to rauhwacke. Sandy or argillaceous layers often occur as well (Merki 1961; Essigmann 1979).

The top of the Asp Member is defined by the last occurrence of continuous dolomite or rauhwacke strata overlain by marls or sulphates that already belong to the Bänkerjoch Formation. Therefore, the alternating strata consisting of massive anhydrite and dolomite that locally occur in the lowest part of the Bänkerjoch Formation do not belong to the Asp Member.

The Asp Member shows a significant peak in gamma-logs (Figs. 7, 15), which can be used to localise the top of the Schinznach Formation. In the Nagra boreholes Schafisheim, Riniken, Weiach and Benken a smaller peak marks the top of the Asp Member. In Siblingen this peak lies few metres deeper.

Discussion: Traditionally the Asp Member was defined to form part of the Keuper by many authors (see above). In adjacent southwestern Germany the “Lettenkeuper” (today: Erfurt Formation) is classified as Lower Keuper (e.g. Menning 2000; Etzold and Schweizer 2005). Merki (1961) noticed the prevailing open marine character of the “Lettenkohle”. Therefore he allocated these sediments to the Upper Muschelkalk. This allocation was recently adopted by the Swiss Committee on Stratigraphy (decision of the 22nd November 2014).

As the dolomites constituting the top of the Asp Member resemble those of the Stamberg Member underneath, it is appropriate to include the Asp Member into the same formation, in particular if mappability is considered (Remane et al. 2005).

The Asp Member does not exactly correspond to the Erfurt Formation (Lettenkeuper) in southwestern Germany (Fig. 2). Traditionally the base of the Erfurt Formation is defined by a millimetre- to centimetre-thick condensed layer called “Grenzbonebed” (Essigmann 1979; Etzold and Schweizer 2005). The “Grenzbonebed” separates the lowest subunit of the Erfurt Formation (Basisschichten) from the Rottweil Formation underneath, which is largely equivalent to the Stamberg Member. A similar criterion was used in northern Switzerland to define the base of the “Lettenkohle” (e.g. Merki 1961). However, as the equivalent of the lowest subunit of the German Erfurt Formation is a dolomitic interval in northern Switzerland, it cannot be separated from the underlying dolomites of the Stamberg Member. This dolomitic interval is therefore also integrated to the Stamberg Member (Fig. 2).

Similarly the upper boundary of the Asp Member in northern Switzerland does not match the upper boundary of the Erfurt Formation (Lettenkeuper) in southwestern Germany. Due to the locally sulphate-rich facies of the Grüne Mergel and the significant lithologic differences between the upper dolomites of the Asp Member and the overlying sediments, the top of the Schinznach Formation is placed at the top of the dolomites of the Asp Member. These dolomites may be equivalent to the Linguladolomit of southwestern Germany (Essigmann 1979).

4.6 Lithostratigraphic rank of oolitic intervals and multiple subordination

Within the Schinznach Formation, oolitic intervals often occur as single strata with thicknesses of only a few decimetres, though they may build up oolitic strata of a few metres as well. Therefore, they represent units that occupy an intermediate position between a member and a bed with respect to lithostratigraphic terminology. Furthermore, it is not clear because of lacking outcrops, if the oolitic bodies within an interval are laterally genetically linked with each other. Therefore, they are seen as open shoals, which formed in similar stratigraphic levels. Since these levels seem to correspond to environmental conditions that favoured ooid formation, they may have some stratigraphic significance.

The lowermost oolitic interval is completely embedded within the Leutschenberg Member. The uppermost interval is always completely dolomitised and, therefore, obviously belongs to the Stamberg Member. Consequently, a definition on a hierarchical rank lower than member is desirable. Therefore, in the present paper each oolitic interval was classified in the rank of a bed. The oolites are thus divided into a lower, middle and an upper interval, which now are defined as Fützen Bed, Eptingen Bed and Kaisten Bed, respectively.

Depending on the more or less pronounced dolomitisation of the oolites of the Eptingen Bed, this unit either belongs to the Liedertswil Member (only partly or not affected by dolomitisation) or to the Stamberg Member (complete dolomitisation). Such multiple subordination, though somewhat problematic for database management, is allowed by the Swiss guidelines for stratigraphic nomenclature (Fig. 1 of Remane et al. 2005). Multiple subordination is also proposed for the marly Dünnlenberg Bed, though for somewhat different genetic reasons.

5 Concluding remarks

The Schinznach Formation comprises sediments previously classified as Upper Muschelkalk and Lower Keuper in northern Switzerland. These sediments are mainly composed of often dolomitic limestone and dolomite and usually have a total thickness of 50 - 85 m. Though the thickness would allow for distinguishing more formations, the lithologic similarity of the individual subunits is in favour of the definition of only one formation constituting a mappable unit. Similarly the Asp Member, which was formerly classified as Lower Keuper, is now defined to form part to the Schinznach Formation because of lithologic similarity with the rest of the Schinznach Formation and pronounced differences to the Bänkerjoch Formation.

The main characteristics of these sediments are represented by five members, two marker beds and three oolitic intervals, which are also defined as beds. This scheme widely corresponds to subdivisions previously suggested (e.g. Merki 1961). It transfers previously used subdivisions in a mere lithostratigraphic system, whereas the former subdivisions mixed lithostratigraphy and allostratigraphy.

The Schinznach Formation outcrops in northern Switzerland between the Doubs River and the Lake Biel in the west and the Lake Constance in the east. Towards south a transition to a facies related to the margin of the Central European Epicontinental Basin is observed (borehole Entlebuch; Vollmayr and Wendt 1987). Therefore the lithostratigraphic scheme presented here is not directly applicable to these transitional sediments of Middle Triassic age. Further investigations (detailed logged core drillings) of the marginal facies are needed to either ascribe it to the Schinznach Formation with the definition of new members or to establish a new formation. The same holds true for a potential extension of the Schinznach Formation towards the west.

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Acknowledgments

This study was financially supported by the Bundesamt für Landestopografie swisstopo – Bereich Landesgeologie. W. Neubert and G. Fuchs (Schweizer Salinen AG) gave access to unpublished borehole data. Dr. G. Deplazes and Dr. M. Ruff gave access to further information according Nagra boreholes. The manuscript benefited much from the thoughtful reviews of Dr. T. Bolliger (Aathal), Dr. H. Hagdorn (Ingelfingen) and Dr. A. Morard (Bern).

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Institut für Geologie und Paläontologie, Universität Basel, Bernoullistrasse 32, 4056, Basel, Switzerland
Johannes S. Pietsch, Andreas Wetzel & Peter Jordan
Gruner Böhringer AG, Mühlegasse 10, 4104, Oberwil, Switzerland
Johannes S. Pietsch & Peter Jordan

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Johannes S. Pietsch
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Andreas Wetzel
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Peter Jordan
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Correspondence to Johannes S. Pietsch.

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Editorial handling: W. Winkler and A. Morard.

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Pietsch, J.S., Wetzel, A. & Jordan, P. A new lithostratigraphic scheme for the Schinznach Formation (upper part of the Muschelkalk Group of northern Switzerland). Swiss J Geosci 109, 285–307 (2016). https://doi.org/10.1007/s00015-016-0214-7

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Received: 19 November 2015
Accepted: 10 May 2016
Published: 12 July 2016
Issue Date: September 2016
DOI: https://doi.org/10.1007/s00015-016-0214-7

A new lithostratigraphic scheme for the Schinznach Formation (upper part of the Muschelkalk Group of northern Switzerland)

Abstract

1 Introduction

1.1 Introductive remark

1.2 History of Upper Muschelkalk stratigraphy in northern Switzerland

1.2.1 General overview

1.2.2 Muschelkalk-Keuper boundary

1.2.3 Need for revision

2 Materials and methods

2.1 Biostratigraphy

2.2 Isopach map

3 Schinznach Formation

4 Lithostratigraphic subunits of the Schinznach Formation

4.1 Leutschenberg Member

4.1.1 Fützen Bed

4.2 Kienberg Member

4.2.1 Saalhof Bed

4.2.2 Dünnlenberg Bed

4.3 Liedertswil Member

4.3.1 Eptingen Bed

4.4 Stamberg Member

4.4.1 Kaisten Bed

4.5 Asp Member

4.6 Lithostratigraphic rank of oolitic intervals and multiple subordination

5 Concluding remarks

References

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