Yallalie: a buried structure of possible impact origin in the Perth Basin,Western Australia

An enigmatic buried structure, located in Mesozoic sedimentary rocks in the Perth Basin, Western Australia, was discovered in 1990 by Ampol Exploratio n. The basin-like Yallalie structure (centred on 30 degrees 26' 40.3" S, 11 5 degrees 46' 16.4"E) is circular in plan view and about 12 km in diameter. High-resolution, seismic-reflection profiles across the structure show a b asin-shaped area of chaotic reflections that extend to a depth of approxima tely 2 km below the surface. The structure has sharp boundaries with surrou nding faulted, but otherwise relatively undisturbed, rocks. In the centre o f the structure there is an uplifted area approximately 3-4 km across, simi lar to those described from complex meteorite impact structures. The seismi cally defined structure coincides with a circular topographic depression, a nd image processing of digital elevation data has allowed recognition of co ncentric and radial structures extending as far as 40 km from the centre of the depression. Gravity surveys show the structure to be associated with a positive gravity anomaly of about 30 gu. Aeromagnetic surveys have defined annular anomalies associated with the central uplifted section, and possib ly margins, of the structure. A search for siderophile element enrichments (by neutron activation analysis) in the rocks of the structure, which would indicate the presence of a meteorite component, proved negative. Quartz gr ains in cores that penetrate the structure show the development of prismati c cleavage fractures and irregular, slightly curved planes formed by brittl e fracture. An allochthonous breccia of Late Cretaceous rocks occurs a few kilometres west of the western margin of the structure. Quartz grains from a thin veneer of Tertiary sediments that drape the structure are essentiall y undeformed. However, multiple sets of closely spaced planar deformation f eatures in quartz, characteristic of highly shocked rocks, have yet to be o bserved in the rocks of the Yallalie structure and the allochthonous brecci a. The morphology of the Yallalie structure determined from topographic and geophysical data suggests strongly that it is of impact origin. Geological and geochemical evidence is equivocal, but is not inconsistent with this i nterpretation.