Tookoonooka, a large buried early Cretaceous impact structure in the Eromanga Basin of southwestern Queensland, Australia.

The 66 km wide Tookoonooka impact structure (27 degrees 07'S, 142 degrees 50'E) was first recognised, from seismic profiles, as a circular structure consisting of a concentric arrangement of anticlines and synclines, which surround a complex central dome, approximately 22 km wide. A gravity low and a central magnetic high characterize the structure. Now buried by up to 900 m of Cretaceous and Tertiary clastic sediments, the Tookoonooka structure was formed approximately 128 Ma ago, during deposition of the paralic Cadna-owie Formation. Thin sections from a centrally located exploration well reveal an impact melt breccia, composed of local Ordovician quartz-mica schist bedrock. Detailed study of planar deformation features (PDFs) in quartz grains from this breccia show 64 lamellae sets in 25 grains. Most of the PDF measurements correspond to xi ¿mean 1122¿ and r/z ¿mean 1011¿/¿mean 0111¿ crystallographic indices, with five other orientations also measured. This distribution of PDFs corresponds to that found in quartz from impact structures in porous sedimentary rock targets, thus confirming an impact origin for Tookoonooka.