Cow yoke

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Relief map: Tyrol
marker
Cow yoke​
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Tyrol

The Kuhjoch is 1760  m above sea level. A. high yoke in the Vorkarwendel , the relatively low north-eastern part of the Karwendel Mountains . As an international type locality for the lower limit ( Global Stratotype Section and Point , GSSP) of the Hettangium and thus the Triassic-Jura boundary, it is of outstanding geological importance. In addition, it is currently (as of 2017) the only GSSP in Austria.

location

The Kuhjoch is located in the southern, Austrian ( Tyrolean ) part of the Vorkarwendel on the 2.5 km long northern foothills of the Hölzelstaljoch * . It forms the notch between the main summit and the upstream one along the ridge line around 700 m north, around 1800  m above sea level. A. high secondary summit of the Hölzelstaljoch ("Kuhjochspitze"). There an artificial outcrop has been created on both sides of the ridge, immediately below the ridge line . The “golden nail”, which marks the base of the hettangium in the cut layer sequence, is located in the outcrop on the less steep eastern flank.

The easiest access is from the north via the Baumgartenbachtal. From the Hochstallalm-Niederleger ( 47 ° 29 ′ 40 ″  N , 11 ° 31 ′ 34 ″  E ) the yoke can be reached via the Hochstallalm-Hochleger ( 47 ° 29 ′ 11 ″  N , 11 ° 31 ′ 32 ″  E ) in 1, 5 to 2 hours can be reached on foot.

* Confusingly, in the German-speaking Alpine region, both a mountain and the notch between two mountains can have the component -joch in their name .

geology

General

The Karwendel Mountains are part of the Northern Limestone Alps , which are mainly made up of marine limestone from the Triassic . The Vorkarwendel is tectonically assigned to the Inntaldeck of the Bajuvarikum of the Oberostalpins . An important structural element of the Inn valley cover is the Karwendel trough , a large tectonic tectonic trough which, as is common in the northern limestone Alps, strikes east-west and at the core of which bite out early Cretaceous limestone and marl stones ( Schrambach formation ) . The Kuhjoch located in the southern limb of the trough which is formed nearly isoklinal in this area so that the layers there very steep occur , are even slightly tilts. The northern flank and the summit area of ​​the “Kuhjochspitze” are made up of the mighty and partly thick-banked Upper Jurassic limestones of the Oberalm formation . To the south, towards the main summit of the Hölzelstaljoch, the age of the geological units increases, but their thickness is significantly less. In the lying area of the Oberalm Formation, the Upper Jurassic Ruhpolding Formation ( Kieselkalke and Radiolarite ), the Middle or Lower Jurassic Scheibelberg Formation (red and gray limestones and Kieselkalke), the predominantly Lower Jurassic Adnet Group (Adnet- Formation and "Enzesberger Kalkstein" or Schnöll Formation; predominantly reddish limestones) as well as the Lower Jurassic-Upper Triassic Kendlbach Formation and the Upper Triassic Kössen Formation (both mainly consisting of marl and limestone).

GSSP

Position and marker

The base of the Hettangium lies on the Kuhjoch in the lower part of the 22 m thick Tiefengraben subformation there (formerly "Rhaetian Grenzmergel"), the lower of the two subformations of the Kendlbach Formation. The Tiefengraben subformation superimposed on the Eiberg subformation of the Kössen formation. Both subformations are interpreted as the result of a relatively deep marine (basinal) sedimentation. The Eiberg basin , which began to sink into the carbonate shelf of the Upper Eastern Alps in the late Upper Triassic, is considered to be the depository. While the Eiberg subformation consists of deep marine gray and black limestone and marlstones, the Tiefengraben subformation is significantly poorer in carbonates and instead comprises yellowish marl stones, followed by reddish claystones with silty , sometimes sandy layers in the upper part. The lithological change at the base of the Tiefengraben subformation, which is already heralded in the top 20 cm of the Eiberg subformation by a dark marl limestone to marlstone layer (the so-called T-layer ), comes about when the eustatic sea level drops at the end of the Triassic explained, which manifests itself in shallow water carbonates of approximately the same age from Upper Eastern Alpine through emersion surfaces (strata showing signs of subaeric exposure, especially weathering phenomena as a result of contact with fresh water).

In the 1990s, the Triassic-Jura border in the Karwendel was correlated with this lithology change, not least because it was accompanied by the disappearance of numerous Triassic macro and mesofossil taxa (especially the Ceratites and Conodonts ) from the fossil record . However, typical Triassic micro- and nanno fossils have been detected in the basal layers of the Tiefengraben subformation (see below). The Hettangian base is located on Kuhjoch on the current definition of 5.80 m above the base of the deep trench Subformation, marked by the first appearance (engl. First appearance date , FAD) of Neoammonoideen art Psiloceras spelae (represented by the subspecies P. s. tirolicum ) in the sequence of layers. The mostly small ammonites are in the rare preservation with the original aragonitic case material. The FAD of Psiloceras spelae closes an ammonite-free interval that follows the last appearance date (LAD) of the Ceratite species Choristoceras marshi in the T-layer. Further FADs for the lower hettangium of diagnostic ammonites on the Kuhjoch are that of Psiloceras ex gr. Tilmanni just 8 m and of Psiloceras cf. pacificum just 12 m above the base of the Tiefengraben subformation.

The suitability of the outcrops on the Kuhjoch as a GSSP is largely due to their paleogeographical position in the Eiberg Basin. The increased water depth enabled the sedimentation to persist at a relatively high rate even during the low eustatic sea level at the turn of the Triassic-Jurassic and, secondly, that ammonites, the most important macro guide fossils of the Mesozoic Era , could be embedded there. Furthermore, the border interval at the Kuhjoch shows other paleontological and geochemical markers that are suitable for locating the Triassic-Jura border in sequences in which no ammonites occur. The LAD of Choristoceras marshi is associated with a clearly negative δ 13 C org excursion ( initial carbon isotope excursion , CIE for short ), which occurs outside the Alps in St. Audrie's Bay ** on the north coast of Somerset (England ) has been proven relatively reliably. The FAD of the mussel genus Agerchlamys correlated to both the Kuhjoch (represented by A. textoria ) and Jura-limit interval outcrops Trias approximately δ in several other websites with this 13 C org -Anomalie and the disappearance of typical Triassic Makrofossiltaxa. *** Stratigraphically valuable microfossils are present on the Kuhjoch especially in the form of aragonite-shell planktonic foraminifera and palynomorphs. The FAD and LAD of the foraminifer Praegubkinella turgescens are 50 to 60 cm below and around 1.5 m above the FAD of Psiloceras spelae, respectively . The lowest, ammonite-free part of the Tiefengraben subformation, however, contains representatives of the very similar † but smaller Triassic genus Oberhauserella . With regard to palynomorphs, the FAD of Psiloceras spelae lies in an interval (2.55 m to 8.10 m above the base of the deep trench subformation), which palynostratigraphically corresponds to the Trachysporites - Heliosporites zone and, through the FAD, of the pollen species Cerebropollenites thiergartii and the disappearance of the species Vitreisporites bjuvensis , Ovalipollis pseudoalatus , and Rhaetipollis germanicus , which are still common in the basal part of the deep trench subformation ( Rhaetipollis - Porcellispora zone) . The palynostratigraphic finding has the advantage that it can also be used to correlate continental sequences.

** a former candidate for the Hettangium's GSSP, see below
*** for example on Ferguson Hill in Nevada (USA), also a former candidate for the GSSP of the Hettangium (see below ), but there is a much smaller interval, only around 1 m, between the initial CIE / Agerchlamys -FAD and the FAD of Psiloceras , but there are around 7 m of rock between the initial CIE / Agerchlamys -FAD and the LAD of Choristoceras ; in this regard, doubts are expressed at least with regard to the reliability of the δ 13 C org curve due to a weak contact metamorphic overprinting of the rocks on Ferguson Hill, and the supposed initial CIE is reinterpreted as a secondary peak, "CIE II", while an actually weaker negative one peak in the level of Choristoceras -LADs as an expression of the actual initial CIE applies
† Praegubkinella and Oberhauserella are both placed in the Oberhauserellidae family, and for some authors Praegubkinella is even a younger synonym of Oberhauserella

Historical

In the early 1980s, was of the Jurassic sub-committee ( International Subcommission on Jurassic Stratigraphy , ISJS) of the International Stratigraphic Commission ( International Commission on Stratigraphy , ICS), a working group (called for finding a GSSP for the Hettangian to life Triassic-Jurassic Boundary Working Group , TJBWG). This search turned out to be difficult, both with regard to the border marker and with regard to the location, and because none of the GSSP combinations found proved to be optimal, five outcrops and four different primary markers were formally put to a vote within the 75-member TJBWG in 2007 :

  • Ferguson Hill, an elevation in New York Canyon in Mineral County, Nevada (USA), with either the ammonite Psiloceras spelae or a distinctive δ 13 C org anomaly ("initial CIE") as the primary marker,
  • the cow yoke with Psiloceras spelae as the primary marker,
  • Kunga Island ( Queen Charlotte Islands ) in British Columbia (Canada) with a marked change in the radiolarian fauna as the primary marker,
  • St. Audrie's Bay in Somerset (England) with Psiloceras planorbis ‡ as primary marker and
  • Waterloo Bay in Northern Ireland with P. planorbis as the primary marker.

The Kuhjoch was one of the last two candidates to be presented informally within the TJBWG (2005). The first e-mail vote in February 2008 initially decided on the border marker, whereby P. spelae clearly prevailed over P. planorbis (13) with 36 out of 67 votes . In the second ballot at the beginning of March 2008, only the two localities for which P. spelae had been proposed as a marker were available, whereby the vote was clearly in favor of the Kuhjoch with 32 to 18 of 57 votes (7 official abstentions). Because the quorum of 60% required for a final election was not achieved, a confirmation vote took place at the end of March / beginning of April 2008, in which the choice of the cow yoke (options only “yes”, “no” or abstention) with 48 out of 61 Votes was confirmed. This decision was confirmed again in June 2008 by the 22 voting members of the ISJS (14 “yes”, 4 “no”, 3 abstentions, 1 vote not cast). In May 2009, then the ICS confirmed this choice and in April 2010, was finally Kuhjoch as GSSP for the Hettangian by the Executive Committee of the Federation of ICS, the International Union of Geological Sciences ( International Union of Geological Sciences , ratified IUGS). The inauguration took place in August 2011.

‡ The FAD of ammonites of the genus Psiloceras and in particular of the species P. planorbis has served as an informal key fossil ("working definition") for the basis of the Hettangian and Jura since the 1960s .

literature

  • A. v. Hillebrandt, L. Krystyn, WM Kürschner, NR Bonis, M. Ruhl, S. Richoz, MAN Schobben, M. Urlichs, PR Bown, K. Kment, CA McRoberts, M. Simms, A. Tomãsových: The Global Stratotype Sections and Point (GSSP) for the base of the Jurassic System at Kuhjoch (Karwendel Mountains, Northern Calcareous Alps, Tyrol, Austria). Episodes. Vol. 36, No. 3, 2013, pp. 162–198 ( PDF 7.9 MB)

Individual evidence

  1. ↑ cf. Florian Böhm: Lithostratigraphy of the Adnet Group (Lower to Middle Jurassic, Salzburg, Austria). Pp. 231-268 in: Werner E. Piller (Ed.): Stratigraphia Austriaca. Series of publications by the Earth Science Commission, vol. 16. Austrian Academy of Sciences, Vienna 2003, ISBN 978-3-7001-3180-9 ( online ), p. 239
  2. ↑ see also Wolfgang Mette, Nicolas Thibault, Leopold Krystyn, Christoph Korte, Marie-Emilie Clémence, Micha Ruhl, Malgorzata Rizzi, Clemens V. Ullmann: Rhaetian (Late Triassic) biotic and carbon isotope events and intraplatform basin development in the Northern Calcareous Alps, Tyrol, Austria. Geo.Alp - A yearly journal devoted to Alpine geology. Vol. 13, 2016, pp. 233–256 ( PDF on the Geo.Alp website; 6 MB)
  3. ↑ a b Christopher A. McRoberts, Peter D. Ward, Stephen Hesselbo: A proposal for the base Hettangian Stage (= base Jurassic System) GSSP at New York Canyon (Nevada, USA) using carbon isotopes. International Subcommission on Jurassic Stratigraphy Newsletter. No. 34/1, 2007, pp. 43–49 ( PDF on the SUNY Cortland website; 850 kB)
  4. ↑ a b Spencer G. Lucas, David G. Taylor, Jean Guex, Lawrence H. Tanner, Karl Krainer: Updated proposal for Global Stratotype Section and Point for the base of the Jurassic System in the New York Canyon area, Nevada, USA. International Subcommission on Jurassic Stratigraphy Newsletter. No. 34/1, 2007, pp. 34–42 ( PDF on the UCL website; 9 MB)
  5. ↑ Hillebrandt et al .: The Global Stratotype Sections and Point (GSSP) for the base of the Jurassic System at Kuhjoch [...]. 2013 (see literature ), p. 192
  6. ↑ see e.g. B. О. А. Корчагин [OA Kortschagin]: Классификация мезозойских планктонных фораминифер (надсемейства planomalinacea, planomaceae , planomalinea, Rotalipotoracea, planomalacea, and Runcaceacotal, planomacea, and Runcaceacota, planomacea a, Rotalacotala, planomacea, and Runcaceae, planomacea, and Runcaceae. Труды геологического института РАН [Treatises of the Geological Institute of the Russian Academy of Sciences] Vol. 547. GEOS, Moscow 2003 ( available online at geokniga.org), p. 38 f.
  7. ^ Arnold Zeiss, Olaf Michelsen: International Subcommission on Jurassic Stratigraphy Newsletter. No. 8, 1982 ( PDF on the ISJS website; 220 kB)
  8. ^ Louise M. Longridge, Elizabeth S. Carter, James W. Haggart, Paul L. Smith: The Triassic-Jurassic transition at Kunga Island, Queen Charlotte Islands, British Columbia, Canada. International Subcommission on Jurassic Stratigraphy Newsletter. No. 34/1, 2007, pp. 21–33 ( PDF on the UCL website; 2 MB)
  9. ↑ Geoffrey Warrington, John CW Cope, Hugh C. Ivimey-Cook: The St Audrie's Bay - Doniford Bay section, Somerset, England: updated proposal for a candidate Global Stratotype Section and Point for the base of the Hettangian Stage, and of the Jurassic System. International Subcommission on Jurassic Stratigraphy Newsletter. No. 35/1, 2008, pp. 2–66 ( PDF on the ISJS website; 3 MB, entire issue)
  10. ^ Nicol Morton, Geoffrey Warrington, Gert Bloos: Selection and voting procedures for the base Hettangian. International Subcommission on Jurassic Stratigraphy Newsletter. No. 35/1, 2008, pp. 67–74 ( PDF on the ISJS website; 3 MB, entire issue)
  11. ↑ Spencer G. Lucas, Jean Guex, Lawrence H. Tanner, David Taylor, Wolfram M. Kuerschner, Viorel Atudorei, Annachiara Bartolini: Definition of the Triassic-Jurassic boundary. Albertiana. No. 32, 2005, pp. 12–16 ( PDF on the SUNY Cortland website; 4 MB, entire issue)

Coordinates: 47 ° 29 ′ 2 ″  N , 11 ° 31 ′ 50 ″  E