PALEOENVIRONMENTAL INTERPRETATION OF A TRIASSIC-JURASSIC BOUNDARY SECTION FROM WESTERN AUSTRIA BASED ON PALEOECOLOGICAL AND GEOCHEMICAL DATA

A section spanning the Triassic-Jurassic boundary is described from ne ar the village of Loruns in the Vorarlberg region of western Austria. At Loruns, the uppermost Triassic is characterised by bedded carbonate s of the Kossen Formation supporting a stenotopic fauna indicative of a shallow sub-tidal environment of normal marine salinity. The Triassi c-Jurassic boundary may be represented as a sequence boundary develope d 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 envir onment, possibly a mud Rat. Above the boundary beds, the upper Schattw ald 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 su ggest a shallow, subtidal, salinity-controlled environment typical of an interplatform lagoon. Carbonate production rejuvenated in the later Early Hettangian with development of the Loruns oolite, a shallow sub tidal oolitic and oncolitic unit bearing echinoderms indicative of nor mal marine conditions. Low Th/U ratios from the remainder of the secti on are a result of reduced thorium in carbonate-rich sediments and not from authigenic uranium in anoxic sediments. In the boundary beds evi dence for marine anoxia (or dysoxia) is absent where Th/U values, dete rmined by gamma-ray spectrometry, are above 5. The negative excursion in delta(13)C and positive excursion in delta(18)O in the boundary bed s may be due to secondary geochemical effects, due to organic diagenes is or the precipitation of caliche during paleosol development. Altern atively, the excursions may reflect a primary geochemical signal recor ding short-term decline in primary productivity. Comparison in delta(1 8)O and delta(13)C values between the Kossen Formation and Lorfus ooli te indicate no significant long-term geochemical changes are evident i n the section and suggest that any environmental perturbations were re stricted to the boundary beds and possible sequence boundary. (C) 1997 Elsevier Science B.V.