An enigmatic buried structure, located in Mesozoic sedimentary rocks in the Perth Basin,
Western Australia, was discovered in 1990 by Ampol Exploration. The basin-like Yallalie structure
(centred on 30° 26′ 40.3″ S, 115° 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 basin-shaped area of chaotic reflections
that extend to a depth of approximately 2 km below the surface. The structure has sharp
boundaries with surrounding faulted, but otherwise relatively undisturbed, rocks. In the centre of the
structure there is an uplifted area approximately 3–4 km across, similar to those described from complex
meteorite impact structures. The seismically defined structure coincides with a circular topographic
depression, and image processing of digital elevation data has allowed recognition of
concentric 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
possibly 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 grains in cores that penetrate the structure show the development of prismatic
cleavage fractures and irregular, slightly curved planes formed by brittle 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 essentially undeformed.
However, multiple sets of closely spaced planar deformation features in quartz, characteristic
of highly shocked rocks, have yet to be observed in the rocks of the Yallalie structure and the
allochthonous breccia. 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 interpretation.