A down-slope upper triassic reef mound: Aflenz limestone, Hochschwab Mountains, Northern Calcareous Alps

Abstract

A small reef mound occurring within the well-bedded, chert-bearing Norian Aflenz Limestones of the Aflenzer Burgeralm (south eastern Hochschwab Mountains, Styria, Austria) is described with respect to microfacies and paleontological criteria. The mound was formed in a down-slope position near the transition between basinal sediments (Aflenz Limestone, exhibiting a shallowing-wards sequence) and marginal platform carbonates (Dachstein Limestone). The transitional zone is characterized by a change from open-marine pelagic carbonates with allochthonous intercalations at the base to protected platform environments as well as shallow-marine platform slope deposits near the top. The biota of the reef mound (corals, calcisponges, hydrozoans, calcareous algae, foraminifera, microporoblematica) correspond with those of Dachsteinkalk reefs. Three communities can be distinguished according to dominant corals and calcisponges (Thecosmilia-Paradeningeria community,Astraeomorpha community,Paravesicocaulis community). The microfacies of the massive, partly dolomitized and stylolitized reef limestone is characterized by a bioclastic floatstone at the base of the mound, overlain by sponge bafflestone, coral-bafflestone, spongiostromate bindstone, coral-sponge framestone, and sponge-hydrozoan bafflestone, without regular distributional patterns. The bedded limestones below the reef carbonates consist of bioclastic lime mudstones with radiolarians; the overlying bedded limestones are composed of peloidal and bioclastic grainstones and packstones as well as rudstones indicating a termination of the reef growth by a rapid allochthonous sedimentation perhaps caused by slumping or storm effects.