Facies character and evolution of a mixed carbonate–siliciclastic shelf: Upper Triassic–Lower Jurassic succession in the eastern Northern Calcareous Alps (Stumpfmauer, Austria)

Abstract

Western Tethys sedimentary successions constitute fundamental archives of Late Triassic–Early Jurassic environmental, carbonate production and tectonic changes. During the Late Triassic, the Northern Calcareous Alps (Austria) belonged to the Western Tethys passive margin, characterised by the deposition of the early-dolomitized peritidal Hauptdolomit (Norian) adjacent basinward to the Dachstein carbonate shelf and passing upward to the mixed carbonate–siliciclastic Kössen Formation (Upper Norian–Rhaetian). The Kössen Fm. was subdivided into the lower shallow-water Hochalm Member and the upper Eiberg Member, accumulated in an intraplatform basin coeval to shallow-water carbonates (Upper Rhaetian Limestone). The Eiberg Mb. and overlying Jurassic strata were extensively studied as a continuous marine record across the Triassic/Jurassic boundary. In contrast, shallow-water successions, time-equivalent to the Eiberg Mb. and Upper Rhaetian Limestone, located North of the Eiberg Basin, are poorly investigated. This study focusses on the approximately 350 m thick Norian to Lower Jurassic succession cropping out in the eastern Northern Calcareous Alps (Stumpfmauer). The 32 distinguished lithofacies compose seven, vertically superimposed, sedimentary units (A–G), belonging to the Hochalm Mb. of the Kössen Fm. (Unit A peritidal cyclothems, Unit B claystone/marlstone with fossiliferous beds, Unit C coral boundstone to floatstone), Upper Rhaetian Limestone (Unit D subtidal cyclothems with claystone), shallow-water carbonate strata transitional to Lower Jurassic (Unit E ooidal coated grain peloidal grainstone with basal transgressive lag, Unit F bivalve-rich, microbialite and oncoidal lithofacies, previously attributed to the Upper Rhaetian Limestone) and Hettangian–Sinemurian Kalksburg Fm. (Unit G cross-laminated coated grain peloidal grainstone with quartz and chert). The detailed lithofacies characterisation presented in this study contributes to the knowledge on the Northern Calcareous Alps stratigraphy and depositional environments across the Triassic/Jurassic boundary. The identified sedimentary units can be framed in the evolution of Western Tethys and share similarities with depositional systems from the Western Carpathians, Transdanubian Range, Southern Alps and Dinarides suggesting coherent sedimentary response and environmental, climate and tectonic controls in different palaeogeographic domains.