Palaeoenvironmental interpretation of a Triassic-Jurassic boundary section from Western Austria based on palaeoecological and geochemical data

Abstract

A section spanning the Triassic-Jurassic boundary is described from near the village of Lorüns in the Vorarlberg region of western Austria. At Lorüns, the uppermost Triassic is characterised by bedded carbonates of the Kössen Formation supporting a stenotopic fauna indicative of a shallow sub-tidal environment of normal marine salinity. The Triassic-Jurassic boundary may be represented as a sequence boundary developed on top of a 1.1 m thick red mudstone of the lower Schattwald Shale, which is interpreted to have been deposited in a marginal marine environment, possibly a mud flat. Above the boundary beds, the upper Schattwald Shale is characterised by thin-bedded marl and dark limestone beds with an earliest Hettangian macrofauna dominated by epifaunal filter-feeding bivalves, including ostreids, mytilids and oxytomids, which suggest a shallow, subtidal, salinity-controlled environment typical of an interplatform lagoon. Carbonate production rejuvenated in the later Early Hettangian with development of the Lorüns oolite, a shallow subtidal oolitic and oncolitic unit bearing echinoderms indicative of normal marine conditions. Low ThU ratios from the remainder of the section are a result of reduced thorium in carbonate-rich sediments and not from authigenic uranium in anoxic sediments. In the boundary beds evidence for marine anoxia (or dysoxia) is absent where ThU values, determined by gamma-ray spectrometry, are above 5. The negative excursion in δ13C and positive excursion in δ18O in the boundary beds may be due to secondary geochemical effects, due to organic diagenesis or the precipitation of caliche during paleosol development. Alternatively, the excursions may reflect a primary geochemical signal recording short-term decline in primary productivity. Comparison in δ18O and δ13C values between the Kössen Formation and Lorfüs oolite indicate no significant long-term geochemical changes are evident in the section and suggest that any environmental perturbations were restricted to the boundary beds and possible sequence boundary.