HORNUNG, T. (2006)

Palaeoclimate background and stratigraphic evidence of Late Norian / early Rhaetian polyphase synsedimentary tectonics in the Hallstatt Limestones of Berchtesgaden
(Rappoltstein, Southern Germany)


A short succession of thick-bedded Hallstatt Limestones (Juvavic nappe, central Northern Calcareous Alps) with two generations of synsedimentary fissures and five different stages of sedimentation (S1 to S5) is described from the northwestern edge of the Rappoltstein block (Berchtesgaden Mountains, Southern Germany). The normally bedded sequence consisting of grey and red tinted bioclastic wacke- and packstones, contains three fissures metre-sized in length and width and filled with an "ammonite rudstone" consisting mainly of Cochloceras sp. and Arcestes sp. The time-discrepancy between the sediment and fissure-fillings is evidenced by two different conodont assemblages and one ammonite assemblage: a nearly monospecific presence of Norigondolella steinbergensis with rare Epidondollela ex gr. bidentata in the thick-bedded bioclastic limestones points approximately to the E. bidentata conodont assemblage zone (A.Z.) / quinquepunctatus ammonite zone representing Sevatian 1; S1). Rare Misikella hernsteini and Epigondolella ex. gr. bidentata found in two of the three fissures indicate Sevatian II (hernsteini conodont A.Z.; S2). Fissure 1 is bordered by radial-fibrous cements indicating long-term submarine exposure (S3). A macrofossil assemblage of abundant Cochloceras sp. found in its sedimentary infill implies a Lower Rhaetian age (S4). Thin fractures filled with grey mudstones, crossing fissure 1, indicate a fifth age of sedimentation (S5) at the very end of the Hallstatt Limestone development. Stable isotope values obtained from whole rock samples (oxygen and carbon, V-PDB) and calcium-apatite of conodonts (oxygen, V-SMOW) provide supplementary information about palaeotemperature. Assuming that open marine Triassic seawater 18O values were close to -1 ‰ V-SMOW (for ice-free periods), calculations result in low temperatures of approximately 12-13°C: If the Hallstatt Basin was as shallow as suggested by earlier studies, the temperatures would be too low to be within the range of tolerance of co-existing reef-building corals growing on the adjacent carbonate platforms.


published in: - Austrian Journal of Earth Sciences 97/98: 106-119, Wien