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The Triassic/Jurassic boundary beds of the Karwendel Syncline (Austria) - initial report of a new GSSP candidate for the base of the Jurassic

von Hillebrandt A., Krystyn L., Kuerschner W.

Volumina Jurassica

2006

Vol. 4, no. 1

287-288

The northwestern part of the Northern Calcareous Alps (NCA) is famous for its fully marine and complete Triassic/Jurassic transition beds formed in an intraplatform through of the western Tethys called as Eiberg Basin (Krystyn et al. 2005). A classic boundary section (Kendlbachgraben) with a relatively rich Rhaetian invertebrate fauna is known since 150 years but with the handicap of a 15 m gap between the last Triassic (Choristoceras marshi) and the first Jurassic ammonite (Psiloceras calliphyllum). A wealth of macro- and microfaunistic information has since been added (Golebiowski 1990) favouring a TJB directly on top of the Koessen Formation (Eiberg Member), just above the disappearance of many Triassic fossil groups (ammonoids, conodonts, brachiopods). New palynological and isotopic research in the area (Kuerschner et al. in press) now points to a boundary located 6 m higher within the Tiefengraben Member (or Grenzmergel) of the lower Kendlbach Formation where along with a distinct δ ¹ ³C shift the appearance of Cerebropollenites thiergatii marks the onset of Jurassic palynomorphs (Fig. 1). Another drastic negative C isotope excursion occurs at the top of the Koessen Formation concomitant with the disappearance of Triassic macrofauna, and both isotopic events can be recognized in boundary sections in England, Greenland and Nevada. The Triassic/Jurassic boundary (TJB) beds of the Karwendel Syncline (northern Tyrol) are well exposed at many places, easily correlatable by palynology (Fig. 1) and show a thicker (about 20 m), lithologically different, marl-dominated Tiefengraben Member with reddish clays ("Schattwald beds" auct.) at the base. They are richer and more diverse in micro- and macrofauna, and they contain an ammonite horizon around 7 m above the Koessen top with a new psiloceratid ammonite unknown from Europe and the Tethys realm. Based on its less intended suture line, the involute conch (umbilical width 40%) and juvenile tubercles (Knötchenstadium) the species is close to the South American earliest psiloceratid P. tilmanni but differs in a subtriangular cross-section. The new "Liassic" ammonite layer corresponds closely to the turnover in the Forminifera and to the onset of "Jurassic" ostracods (e.g. Cytherelloidea pulchella) as well as palynomorphs, with C. thiergatii as a first order correlation tool to marginal marine and continental basins. Bivalves in the basal Grenzmergel may bridge the present macrofaunal gap down to the Koessen Formation top but have still to be evaluated; a nannoplankton analysis is already under way. The investigated sections provide an important new insight in the nature of the physical and biological changes occurring around the boundary with rapidly changing palynomorph associations that point to several short-termed climatic oscillations. In which way they have affected the environmental conditions across the TJB and have controlled the stratigraphic ranges of biomarkers will be an important issue of the ongoing studies. Irrespective of any extinction scenario we see the fossil and geochemical record of the Karwendel Syncline as a major improvement of our knowledge of this time interval and as justified reason to introduce there a section as GSSP candidate for the Triassic/Jurassic boundary.