Characterization and significance of shocked quartz from the Woodleigh impact structure, Western Australia

Abstract

We re-examined the buried Woodleigh structure in Western Australia, which has been inferred to be a multi-ringed, 120 km diameter impact crater, because the proposed size and possible synchronicity with one of the pre-Mesozoic mass extinction events has attracted controversy. We undertook a detailed study of the petrology and mineralogy of a number of samples of core from the Woodleigh-1 borehole that was drilled into the central uplift of the structure. Crystalline Proterozoic basement rocks comprising granites and gneisses in the Woodleigh-1 core contain minor brecciation in discrete veins and reveal clear evidence of shock metamorphism over the full extent of the core. Imaging of laboratory-etched quartz showed that a large number of grains contain shock deformation lamellae. Microstructural and crystallographic analysis of these lamellae by TEM showed that they are planar deformation features (PDFs) that have subsequently undergone annealing and water assisted recrystallization. The available geological, petrographic, and mineralogical evidence suggest that Woodleigh is an eroded impact crater that is nearer to 60 km than 120 km in diameter. Future drilling projects should better constrain the level of erosion, and may reveal any preserved impact lithologies.