The Middle to Late Triassic Bänkerjoch and Klettgau Formations of Northern Switzerland
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Swiss J Geosci (2016) 109:257–284 DOI 10.1007/s00015-016-0218-3 The middle to late Triassic Ba¨nkerjoch and Klettgau formations of northern Switzerland 1,2 1,2 3 4 Peter Jordan • Johannes S. Pietsch • Hansruedi Bla¨si • Heinz Furrer • 2 4 1 5 Nicole Ku¨ndig • Nathan Looser • Andreas Wetzel • Gaudenz Deplazes Received: 13 November 2015 / Accepted: 24 May 2016 / Published online: 16 July 2016 Ó Swiss Geological Society 2016 Abstract In the context of the harmonisation of the Swiss Member includes coarse alluvial and fluvial sandstones of lithostratigraphic scheme, the Late Ladinian to Early Vindelician origin. It is restricted to the Lake Constance Norian Ba¨nkerjoch Formation and the Norian to Rhaetian area. For practical reasons, the Gruhalde Member (for- Klettgau Formation of northern Switzerland are formally merly «Obere Bunte Mergel» and «Knollenmergel») defined. The Ba¨nkerjoch Formation replaces the «Gipske- encompasses all playa sediments above the Gansingen uper». Delimitation generally follows the traditional Swiss Member and Berlingen Member. Evidence in good out- scheme with the onset and offset of sulphate facies as basal crops suggests that it consists of several sedimentary cycles and upper boundary. The Klettgau Formation includes six separated by long time spans of omission and erosion. The members (from base to top): The Ergolz Member Seebi Member (formerly «Stubensandstein») includes (formerly «Schilfsandstein» and «Untere Bunte Mergel») layers of coarse-grained sandstones of Vindelician origin consists of variegated silty dolomitic marl with dolocretes with calcitic, siliceous or clayey matrix. Locally, matrix and channels filled by fine-grained sand of Scandinavian fraction increases and becomes mostly dolomitic in the origin. The Gansingen Member (formerly «Gansinger upper part. The Belchen Member (formerly «Rha¨t») con- Dolomit s.l.») starts with partly porous dolomite with sists of partly fossiliferous sandstone and greenish dark Carnian bivalves and continues with an alternation of marl of estuarine to marine origin. Due to late Triassic and dolomite beds with thin dolomitic marl interlayers. At early Jurassic erosion, the Belchen Member is restricted to several places, the member has been eroded to great parts northwestern Switzerland and the Lake Constance area. during the Late Triassic. Towards east and southeast, dolomitic sulphates, locally altered to dolomite or lime- Keywords Northern Switzerland Á Ba¨nkerjoch formation Á stone, replace the marine carbonates. The Berlingen Klettgau formation Á Middle to late Triassic Á Lithostratigraphy Editorial handling: W. Winkler and A. Morard. 1 Introduction & Peter Jordan [email protected] In the context of the harmonisation of the Swiss strati- 1 Institut fu¨r Geologie und Pala¨ontologie, Universita¨t Basel, graphic scheme (HARMOS project, Morard 2012, Strasky Bernoullistrasse 32, 4056 Basel, Switzerland et al. 2016, this issue), the subdivision of the Triassic strata 2 Gruner Bo¨hringer AG, Mu¨hlegasse 10, 4104 Oberwil, of northern Switzerland into six formations has been Switzerland accepted by the Swiss Committee on Stratigraphy on 22nd 3 H.R. Bla¨si Geo-Consulting, Steinackerstrasse 2, November 2014 with the proviso to hand in the formal 3184 Wu¨nnewil, Switzerland definition later (Jordan 2016). In the present study, the 4 Pala¨ontologisches Institut und Museum, Universita¨tZu¨rich, Ba¨nkerjoch and Klettgau Formations of Middle to Late Karl-Schmid-Strasse 4, 8006 Zurich, Switzerland Triassic age are introduced and the subdivision of the latter 5 Nagra, Hardstrasse 73, Postfach 5430 Wettingen, Switzerland into six members is defined. 258 P. Jordan et al. Due to the poor outcrop conditions of the Triassic strata anhydrite, massive or in alternation with shale and dolo- in northern Switzerland, the concept of Remane et al. mite, starting with the first occurrence of sulphate sedi- (2005) asking for stratotypes has to be softened. With the ments and ending with the offset of sulphate facies (Fig. 2). agreement of the Swiss Committee on Stratigraphy the It differs slightly from the Grabfeld Formation (e.g., Etzold definition of formations and members is completed with and Schweizer 2005; Geyer et al. 2011), which replaced the borehole reference sections. The requirement for a bore- former «Gipskeuper» in Germany (see Sect. 2.3). hole-based reference locality is that the core material is properly stored and accessible for future research (Jordan 2.2 Type locality and reference sections 2016). Type localities have been chosen based on suit- ability and disposability of local names, present-day Today, the only accessible outcrops are abandoned gypsum accessibility, and existence of published records. All type quarries: Riepel south of the Ba¨nkerjoch Pass and Sta¨gli- localities of the formations and members have recently matt south of the Staffelegg Pass (both located in the been recorded, some of them after specific excavations. municipality of Ku¨ttigen AG), as well as a quarry in However, some of the proposed type localities are rather Oberehrendingen AG at the northern flank of the La¨gern nominal than formal, as the base and the top have never range. All three quarries are localised in the Folded Jura, been accessible or the development of the strata is where the sulphate rocks acted as viscous horizons during incomplete or untypical. Further explanations are given in thrusting and folding. Thus, all three outcrops show tec- the respective type locality descriptions. tonic overprint. The least tectonic overprint occurs in the To define the two formations and the members of the Riepel quarry, where also the present-day outcrop condi- Klettgau Formation several concepts have been considered. tions and accessibility are quite good. The stratigraphic The recently reviewed stratigraphic scheme of southern succession of the Riepel quarry (coord. 2645.300/ Germany (Etzold and Schweizer 2005; Geyer et al. 2011) 1253.850) has been repeatedly recorded (Schindler 1962; could not be directly adopted for several reasons: Most of Rick 1990; Kleiner 2009; Meier 2011) under differing the German formations are defined in the centre of the outcrop conditions and from various scientific points of Germanic Basin and wedge out towards the South. Con- view. It is the only outcrop where the top and base of the sequently, the formations reach only a few metres of formation have been exposed. Further, it can easily be thickness in the High Rhine region and in the adjacent correlated to the chosen borehole reference section at Swiss area. Remane et al. (2005) define formations as Weiach (Matter et al. 1988) (see below). The Riepel sec- mappable units. Referring to the current concept of the tion includes the upper four stratigraphic units I to IV of the Geological Atlas at 1:25,000 scale of the Swiss Geological current informal stratigraphic scheme developed for Nagra Survey, a mappable unit has to have a thickness of at least wells (Matter et al. 1988), including the markerbed 2 (see 30 m. A second approach to combine German formations below). The lowermost unit V is apparently missing, as and to adopt them as members failed by significant dif- well as the markerbed 1 expected in the uppermost unit I. ferences in delimitating the stratigraphic succession in For those reasons, the Riepel quarry was chosen as the type present-day German and traditional Swiss schemes. Addi- locality (Fig. 3), whereas the name of the formation derives tionally many recent outcrop and borehole records and from the nearby Ba¨nkerjoch road pass. studies refer to the traditional stratigraphic nomenclature. The corresponding sequence (drilling depth In this context, the shifting of formation or member 739.35–814.08 m) of the Weiach well (coord. 2676.745/ delimitations can create confusions or misinterpretations. 1268.618, Matter et al. 1988), was chosen as the reference However, the opportunity has been used to review and section. It shows a complete succession, including all five formally define the formations and members for the informal subdivisions of Matter et al. (1988) and Dronkert stratigraphic interval in question and to discuss their lateral et al. (1990) and both marker horizons of Nagra (1992b) variations and relation to the stratigraphic units of the (see below). Furthermore it represents the typical facies of adjacent German territory. northeastern Switzerland and relates well to the outcrops in southern Germany. 2Ba¨nkerjoch formation 2.3 Delimitation 2.1 Main characteristics The delimitation of the Ba¨nkerjoch Formation follows the traditional Swiss definition of the former «Gipskeuper». It The Ba¨nkerjoch Formation corresponds to what was used differs from the current German scheme where the Grabfeld to be the «Gipskeuper» in traditional (informal) Swiss Formation starts with the Grundgips Member. According to stratigraphy (Fig. 1). It is a succession of gypsum and that scheme, the underlying Grenzschichten Submember, Gr and Schweizer ( summarizing the work of Etzold partly indicated in theGerman column nomenclature is also Lateral equivalence with the Formations ( Ba subdivision scheme of the Switzerland with the new and Late Triassic ofnomenclature northern for the Middle historical stratigraphic Fig. 1 The middle to late Triassic Ba ¨ . Muschelkalk Group nkerjoch and Klettgau Correlation of the right column 2005 ). MK ). ¨ nkerjoch and Klettgau formations of northern Switzerland 259 Mühlberg (1908) Schmassmann Bitterli-Brunner & Hofmann et Diebold et Bitterli-Dreher Etzold & Équipe du projet