Iridium anomaly from the Acraman impact ejecta horizon: impacts can produce sedimentary iridium peaks

Abstract

THE observation of anomalously high iridium concentrations at the Cretaceous/Tertiary (K/T) boundary1 triggered the still unresolved debate concerning the origin of sedimentary Ir anomalies. Several hypotheses have been presented, including meteorite impacts1,2 and intense volcanism3,4. Here we present data from the Acraman impact ejecta horizon, South Australia5, which conclusively links high sedimentary iridium concentrations to a major meteoroid-impact structure and its widely dispersed ejecta. An Ir anomaly (up to 2 parts per 109 (p.p.b.) Ir as well as Au, Pt, Pd, Ru and Cr anomalies) is present within the ejecta from the Acraman impact event preserved in late Precambrian (∼600 Myr BP) shales within the Adelaide Geosyncline. The highest Ir (and Cr) concentrations within the ejecta are associated with the coarsergrained clasts of acid volcanics (boulder to medium-sand size) suggesting that most of the Ir is carried by these fragments. The target rocks at the impact site are comparable acid volcanics that have very low Ir, Au, Pt, Pd, Ru and Cr concentrations, indicating a meteoritic origin for the anomalous Ir levels within the ejecta. To our knowledge, this is the first example in which an accepted, widely dispersed impact-ejecta blanket, itself uniquely linked to a major terrestrial impact structure5,6 has been shown to be anomalously high in cosmogenic siderophile elements. This finding strongly supports the hypothesis that Ir anomalies in sediments can be produced by terrestrial meteorite impact events.