Gosses Bluff - a latest Jurassic impact structure, central Australia. Part 2: seismic, magnetic, and gravity studies

Geophysical studies of Gosses Bluff facilitate the resolution of its three-dimensional structure, which includes a broadly hemispherical low-velocity zone with a radius of about 4 km centred at the present ground level. This zone correlates with a deformed ring dominated by block-fauliing, megabreccia, and annular breccia troughs. Seismic data show a drop in shallow seismic velocities from 4.6 km s-1 over bedrock to 2.2 km s-1 over breccia outcrop. The base of the structure is defined by continuous reflectors at a maximum depth of 3450 m beneath the surface. The subsurface geology was deduced by fitting the extrapolated stratigraphy into the sequence of migrated seismic horizons. The uppermost little-disturbed reflectors are correlated with the tops of the Pertatataka Formation and Bitter Springs Formation of the Late Proterozoic--early Palaeozoic Amadeus Basin sequence. Seismic data for the Bitter Springs Formation define a domal structure and local thickening under Gosses Bluff. The gravity field associated with Gosses Bluff outlines a remarkably symmetrical circular gravity low of 440 µm s-2 with a radius of about 10.8 km. An annular gravity low bounded by steep gradients is associated with the central uplift. The outer limit of the near-surface gravity structure is nearly vertical. The symmetrical gravity features can be modelled by shallow density variations, and no net mass excess or deficiency is defined several thousand metres centrally under the bluff. Likewise, no gravity expression of a deep-seated Gardiner-Tyler anticline, suggested by the seismic reflection data, is apparent. Several anticlines around the bluff are shown by gravity to have low-density cores. Total magnetic intensity (TMI) contours suggest a depth to magnetic basement of about 10 km under the bluff. A negative anomaly of 4 nT occurs between the central ring and an outcrop of flow breccia at Mount Pyroclast. Similar short-wavelength airborne magnetic anomalies of shallow sources over the eastern, southern, and western flanks of the bluff within 6 km of the centre correspond to flow breccia. The anomaly sources represent both remanent primary components of magnetisation, and remagnetisation formed during cooling, as confirmed by ground magnetic surveys in the Mount Pyroclast area. The pole position is consistent with a lurassic-Cretaceous-boundary age.