Accretionary and melt impactoclasts from the Tookoonooka impact event, Australia

Abstract

The Tookoonooka impact structure is a subsurface structure of the Eromanga Basin in Australia. Impact ejecta have recently been discovered in the stratigraphy proximal to the structure. The ejecta includes accretionary and armored impacto-clasts. They are observed at multiple locations in drill core across central Australia, spanning 375,000 km2 within possible impact tsunami deposits. Typical characteristics of the accretionary impactoclasts include a distinctive brownish-gray color, flattened shapes, concentric zonation, and a variety of morphologies with and without obvious nuclei. Some complex accretionary impactoclasts include melt components. Apparent diameters of these impactoclasts in drill core are commonly less than 2 cm, but may be up to 9 cm. They occur in a variety of depositional contexts, including clast-supported breccia-conglomerate layers and “floating” within massive and planar-bedded sandstones. Microscopic and geochemical investigations reveal that they are pervasively altered. Many resemble the types of accretionary lapilli recognized from hydroclastic volcanic environments, which implies the presence of significant water at the time of impact. Tookoonooka is interpreted to have been a marine (likely paralic to shallow) impact event. It is proposed that hydroclastic types of accretionary impactoclasts at impact sites may be an indicator of wet or marine targets. Complex forms of accretionary impactoclasts may also lead to new understanding of impact vapor plume processes. The impactoclasts studied at Tookoonooka are consistent with an impact origin of the candidate ejecta. The consistent first occurrence of the impactoclasts at the base of the Wyandra Sandstone Member stratigraphically constrains the Tookoonooka impact age to 125 ± 1 Ma in the Lower Cretaceous.