Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-017-1450-4 ORIGINAL PAPER Structural complexity at and around the Triassic–Jurassic GSSP at Kuhjoch, Northern Calcareous Alps, Austria M. Palotai1,2 · J. Pálfy1,3 · Á. Sasvári4 Received: 16 February 2016 / Accepted: 24 January 2017 © The Author(s) 2017. This article is published with open access at Springerlink.com Abstract One of the key requirements for a Global Stra- Keywords GSSP · Jurassic · Triassic · Kuhjoch · totype Section and Point (GSSP) is the absence of tectonic Faulting · Folding disturbance. The GSSP for the Triassic–Jurassic system boundary was recently defined at Kuhjoch, Northern Cal- careous Alps, Austria. New field observations in the area Introduction of the Triassic–Jurassic boundary GSSP site demonstrate that the overturned, tight, and almost upright Karwendel Modern chronostratigraphy is underpinned by the Global syncline was formed at semibrittle deformation conditions, Stratotype Section and Point (GSSP) concept, whereby confirmed by axial planar foliation. Tight to isoclinal folds the lower boundary of each chronostratigraphic unit is at various scales were related to a tectonic transport to the defined at a certain level in the best available stratigraphic north. Brittle faulting occurred before and after folding as section for the investigated interval, as agreed by the confirmed by tilt tests (the rotation of structural data by the regulating bodies of the international stratigraphic com- average bedding). Foliation is ubiquitous in the incompe- munity (Remane 2003). The base of the Jurassic System tent units, including the Kendlbach Formation at the GSSP. was recently defined at Kuhjoch in the Northern Calcare- A reverse fault (inferred to be formed as a normal fault ous Alps of Austria (Hillebrandt et al. 2013). The Kuhjoch before folding) crosscuts the GSSP sections, results in the GSSP is claimed to be the best continuous, non-condensed partial tectonic omission of the Schattwald Beds, and thus section of sedimentary rocks of latest Triassic and earliest makes it impossible to measure a complete and continuous Jurassic age worldwide (Hillebrandt et al. 2013; Morton stratigraphic section across the whole Kendlbach Forma- 2012). Although many stratigraphical, paleontological and tion. Based on these observations, the Kuhjoch sections do geochemical aspects related to the GSSP have already been not fulfil the specific requirement for a GSSP regarding the covered (Hillebrandt et al. 2013), no detailed structural absence of tectonic disturbances near boundary level. geological study has been carried out to clarify the defor- mation history of the site and its broader area. Detailed, Electronic supplementary material The online version of this 1:10 000 scale mapping of the area was completed in the article (doi:10.1007/s00531-017-1450-4) contains supplementary 1970s and remained unpublished (Schütz 1974). material, which is available to authorised users. To provide a better understanding of the regional geo- logical context, deformation history and conditions as well * M. Palotai [email protected] as local structural geological phenomena of the GSSP, the authors investigated and interpreted an extended geological 1 Department of Geology, Eötvös Loránd University, Pázmány profile across the site. This exercise also enabled us to test Péter sétány 1/C, Budapest 1117, Hungary the repeatedly expressed assumption that the GSSP section 2 CASP, 181A Huntingdon Road, Cambridge CB3 0DH, UK is free of structural complexities that would limit its use as 3 MTA-MTM-ELTE Research Group for Paleontology, a stratigraphic standard (Hillebrandt et al. 2007, 2013). Pázmány Péter sétány 1/C, Budapest 1117, Hungary 4 Llandudno, Gwynedd LL30 1SA, UK Vol.:(0123456789)1 3 Int J Earth Sci (Geol Rundsch) Geological setting locality in the Eiberg basin suggest a maximum of 65 °C (Kuerschner et al. 2007). The Kuhjoch GSSP (Hillebrandt et al. 2013) is located on The local stratigraphy around the Kuhjoch section is the overturned southern limb of the Karwendel syncline summarised below on the basis of a detailed (1:10,000 (Eisbacher and Brandner 1996) (Fig. 1). The Karwendel scale) unpublished map of Schütz (1974) and the recently syncline forms a Paleogene fault propagation fold (Tan- compiled 1:50,000 scale map of Moser (2008), with further ner et al. 2003) in the Lechtal nappe, which in turn is part details of the Triassic–Jurassic boundary strata provided by of the upper Bavaric unit (or nappe system) in the western Hillebrandt et al. (2013), Richoz and Krystyn (2015) and Northern Calcareous Alps (Tollmann 1976b). Deformation Mette et al. (2016) (Fig. 2). The overall stratigraphic archi- is constrained to anchimetamorphic conditions as a maxi- tecture of the Karwendel syncline and more broadly the mum (less than 200 °C) as defined by illite crystallinity data Bavaric unit is complex and not free of controversies. Our (Kralik et al. 1987). The maximum temperature is inferred stratigraphic summary follows the views of Gawlick et al. to have not exceeded 50 °C based on local palynomorph (2009), which partly contradicts the widely used chart of and conodont (CAI, Conodont Alteration Index = 1) pres- Piller and Erhart (2004), albeit both are built on the strati- ervation (Hillebrandt et al. 2013; Kuerschner et al. 2007). graphic synthesis of the Northern Calcareous Alps by Regionally, similar CAI values of 1 in the Tirolic units of Tollmann (1976a). Paleogeographic reconstructions sug- the central Northern Calcareous Alps are thought to cor- gest that Triassic–Jurassic boundary strata were deposited respond to temperature ranges of 50–80 °C (Gawlick et al. in the Eiberg basin, a large, elongated intraplatform basin 1994), whereas palynomorph preservation from another developed in the latest Triassic on the passive margin of Fig. 1 Location of Kuhjoch in the Northern Calcareous Alps and geological map of the surroundings of Kuhjoch [modified after Schütz (1974) in Hillebrandt et al. (2013)] 1 3 Int J Earth Sci (Geol Rundsch) Fig. 2 Upper Triassic–Lower Ma Cretaceous stratigraphy of the 115 a b Adnet F. Karwendel syncline (a), and the a b g stratigraphic section of Kuhjoch 120 Aptian (b) [modified after Hillebrandt et al. (2013), Gawlick et al. 125 Member y Schrambach F. (2009), Schütz (1974) and Barremian Breitenber Krische et al. (2013)]. An arrow 130 Earl 25 m pair indicates the position of Cretaceous Hauterivian the Triassic–Jurassic boundary 135 according to Hillebrandt et al. Valanginian (2013) 140 Berriasian ?? 145 Oberalm F. Tithonian 150 20 m ? 155 Kimmeridgian Late ? 160 Oxfordian Ruhpolding F. Callovian 165 ? n e Bathonian iddl Bajocian 15 m 170 M hiatus ? Aalenian ?? 175 Jurassic efengraben Member Toarcian Ti 180 Scheibelberg F. 185 y Pliensbachian Kendlbach Formatio Earl 10 m 190 ?? T/J 195 Sinemurian Adnet F. 200 Hettangian Kendlbach F. 205 Rhaetian GSSP KössenF. 5 m 210 Beds c te Plattenkalk F. Schattwald 215 iassi La Tr . riassic T Norian 220 n F Hauptdolomit F. bed T- 225 Kösse Neotethys, which separated the Dachstein carbonate plat- limestones with characteristic platy bedding, and occa- form from the more northerly located rimmed platform sional dolomitic intervals and thin marl interbeds (Schütz attached to the European continental foreland (Krystyn 1974). et al. 2005). Stratigraphic relationships and facies patterns The uppermost Norian–Rhaetian Kössen Formation were disrupted during successive phases of Alpine defor- develops gradually from the underlying Plattenkalk by an mation (Linzer et al. 1995). increase of clay, pyrite and organic matter content, indicat- The Norian stage is represented by the Hauptdolomit ing deepening of the basin. The Kössen Formation com- and Plattenkalk formations. The Hauptdolomit comprises prises dark limestones and marls, whereas its uppermost thick-bedded dolostones deposited in a lagoon of an exten- 20-cm-thick bed (the so-called T-bed) is characterised by sive carbonate platform. The transition to the Plattenkalk dark colour, increased clay and organic content and platy is gradational, and the latter formation comprises grey weathering, suggesting anoxic conditions (Hillebrandt et al. 1 3 Int J Earth Sci (Geol Rundsch) 2007). The uppermost ca. 2 m of the Kössen Formation are Formation, whose oldest age is established elsewhere as the oldest strata exposed in the Kuhjoch GSSP trenches. mid-Berriasian (Krische et al. 2013). Its upper part is domi- The Kössen Formation is overlain with an abrupt litho- nated by marls, representing the youngest unit encountered logical change by the uppermost Rhaetian–lowermost Het- in our tectonically oriented field survey in the vicinity of tangian Tiefengraben Member of the Kendlbach Forma- the Kuhjoch section. tion. This succession starts with the Schattwald Beds, a succession of grey to brown clay-rich marls that are over- lain by yellow laminated marls and red, partly laminated Structural observations at the GSSP site silty clays, recording an increase of terrigenous influx. Hillebrandt et al. (2013) report a thickness of 2.8 m for The Global Stratotype Section consists of two subparal- this unit in the Kuhjoch East section and ca. 2 m at Kuh- lel artificial trenches on both sides of the Kuhjoch pass, joch West, where tectonic omission in the latter section is referred to as sections Kuhjoch West and East (Fig. 3). apparent from the lack of a palynomorph assemblage zone The proposal prepared for the official ballot by the Trias- and a sudden shift in the δ13C curve. Natural