Abstract
One of the key requirements for a Global Stratotype Section and Point (GSSP) is the absence of tectonic disturbance. The GSSP for the Triassic–Jurassic system boundary was recently defined at Kuhjoch, Northern Calcareous Alps, Austria. New field observations in the area of the Triassic–Jurassic boundary GSSP site demonstrate that the overturned, tight, and almost upright Karwendel syncline was formed at semibrittle deformation conditions, confirmed by axial planar foliation. Tight to isoclinal folds at various scales were related to a tectonic transport to the north. Brittle faulting occurred before and after folding as confirmed by tilt tests (the rotation of structural data by the average bedding). Foliation is ubiquitous in the incompetent units, including the Kendlbach Formation at the GSSP. A reverse fault (inferred to be formed as a normal fault before folding) crosscuts the GSSP sections, results in the partial tectonic omission of the Schattwald Beds, and thus makes it impossible to measure a complete and continuous stratigraphic section across the whole Kendlbach Formation. Based on these observations, the Kuhjoch sections do not fulfil the specific requirement for a GSSP regarding the absence of tectonic disturbances near boundary level.
Highlights
Modern chronostratigraphy is underpinned by the Global Stratotype Section and Point (GSSP) concept, whereby the lower boundary of each chronostratigraphic unit is defined at a certain level in the best available stratigraphic section for the investigated interval, as agreed by the regulating bodies of the international stratigraphic community (Remane 2003)
To provide a better understanding of the regional geological context, deformation history and conditions as well as local structural geological phenomena of the GSSP, the authors investigated and interpreted an extended geological profile across the site. This exercise enabled us to test the repeatedly expressed assumption that the GSSP section is free of structural complexities that would limit its use as a stratigraphic standard (Hillebrandt et al 2007, 2013)
The summary paper of Hillebrandt et al (2013) presented a synthesis of local stratigraphical and geological knowledge related to the Kuhjoch GSSP derived from a large number of earlier studies (Bonis et al 2009, 2010; Hillebrandt and Krystyn 2009; Hillebrandt et al 2007; Morton 2012; Ruhl et al 2009, 2011); in the following discussion it is used as a key reference
Summary
Modern chronostratigraphy is underpinned by the Global Stratotype Section and Point (GSSP) concept, whereby the lower boundary of each chronostratigraphic unit is defined at a certain level in the best available stratigraphic section for the investigated interval, as agreed by the regulating bodies of the international stratigraphic community (Remane 2003). Following the general consideration of the relative dip angle of bedding and foliation in overturned folds (Fig. 4c) and accepting the overturned fold geometry confirmed by data of Schütz (1974) and our regional findings (“Structural observations in the Karwendel syncline”), the steeper plane set was tentatively established to be the bedding, the shallow dipping features were accepted as foliation (Fig. 4a, b) In both the eastern and the western trenches, the contact between the Kössen limestone and the Tiefengraben Member is tectonic: a steep, roughly east–west striking fault separates these formations In the central part of the section, a well-developed set of strike-slip faults (Fig. 8d) was observed in the Oberalm limestone (Fig. 9) Fault zones in both the Lower Jurassic formations and the Kössen limestone were observed in the close proximity of the GSSP. Brittle faulting occurred both before, after and most likely during the Paleogene folding phase (Tanner et al 2003) which produced the overturned Karwendel syncline
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