MICROFACIES AND MACROFAUNAL ANALYSIS OF UPPER RHAETIAN EIBERG MEMBER SEQUENCES FROM THE NORTHERN CALCAREOUS ALPS, AUSTRIA: IMPLICATIONS FOR UNDERSTANDING THE END-TRIASSIC MASS EXTINCTION

The late Rhaetian time-interval is an important period for the study of paleoenvironmental and paleoecological changes leading up to the end-Triassic mass extinction. The Northern Calcareous Alps (NCA) in Austria preserve one of the most complete marine sedimentary records of upper Rhaetian strata in the world. We conducted a detailed microfacies and macrofaunal study of five key sections spanning the upper Rhaetian (upper Kössen Formation, Eiberg Member) in the NCA. Four of the studied sites (Kuhjoch, Eiberg, Juifen, and Hochalm) were deposited across the intraplatform Eiberg Basin with the fifth site (Restentalgraben) deposited on the terrigenous-influenced northern side of a small carbonate platform (Oberrhaet limestones), near the northwestern margin of the Eiberg Basin. Near the basin margin, upper Rhaetian strata are characterized by taxonomically diverse and rich benthic assemblages including scleractinian corals, bivalves, brachiopods, sponges, bryozoans, gastropods, echinoderms, ostracods and foraminifera. The Restentalgraben site records a progression from high-energy inner ramp facies to lower energy mid-ramp facies with tempestite deposits alternating between silty mudstones to bioclastic pack- to wacke- stones. Transition to the center of the basin is characterized by overall numerically and taxonomically impoverished benthic communities but with rich brachiopod and abundant ostracod assemblages. The basin facies are mainly characterized by alternating mudstones, pack- and wacke- stones representing a classic example of a detrital mud carbonate realm, previously described by Golebiowski and others. The uppermost part of the studied sections (except Hochalm) contains across the basin a distinctive thin black laminated mudstone bed, known as the T-bed, characterized by a low diversity eurytopic opportunistic bivalve assemblage, which marks the initial extinction phase in the NCA. A shallow marine benthic paleocommunity adjacent to the basin margin flourished while marine macrofauna occupying deeper parts of the basin were impoverished, compared to underlying strata, just before initiation of the end-Triassic mass extinction.