Geophysical Evidence
Confirmation that the Bedout High consists of an impact breccia and melt sheet led us to re-interpret some seismic lines provided by the Australian Geological Survey (AGSO; 18) including line S120-01 (Fig. 10 below), originally interpreted by Gorter (13,14).

Figure 10. Re-interpreted 1994 AGSO multichannel seismic line s120-01. This interpretation shows the central uplift of the inferred Bedout impact structure deforming end-Permian (dark blue line) and older sequences (Pre-Permian, orange dashed line and Top Precambrian basement, red dashed line), overprinted by younger faults associated with late-Triassic to mid-Jurassic rifting. The “pre-Permian strata”, is inferred only from seismic character (15), yet appear to show uplift with the basement. These reflectors are carried from wells in the adjacent onshore Canning basin. Click here for a larger view

Our revised chronostratigraphy for line S120-01 includes the Lagrange-1 and Bedout-1 stratigraphic sections, correlation with adjacent onshore seismics and wells (31) and the Ar-Ar and K-Ar dating of the melt breccia. The top of the Permian (blue line) is conformable with the Bedout High whereas Triassic sediments (light green) unconformably onlap onto the structure (Fig. 10, S-15 below).

S-15 1994 AGSO multichannel seismic line S120-04 showing both wells, a more pronounced annular depression, central uplift and end-Permian (blue line) and older sequences (orange, Pre-Permian; red, Precambrian). The reflectors for the older sediments are not well resolved as the lines are carried toward the central uplift and in the deeper Precambrian basement and appear dashed on the interpretation. Click here for a larger view

The revised seismic section shows a broad uplifted core of basement (red line) 40 to 60 km in diameter elevated a minimum of 6 to 9 km. The “pre-Permian” strata (Fig. 10) inferred only from seismic character correlations (15), are not well imaged in the seismic data and yet appear to show uplift with the basement core. Alternatively, as the deeper material has yet to be sampled and dated, these sequences could all be end-Permian crater-fill impact debris. We also detect a slight uplift of Permian and earlier strata at a radius of ~100 km from the center of the Bedout High, but it is not clear that this is a concentric feature. A 2D-velocity model derived from OBS wide-angle reflection and refraction data collected along the S120-01 line (32) reveal a central uplift beneath the Bedout high, with some 6 to 7 km of vertical structural relief on mid-crustal iso-velocities. Although less well resolved, the data also suggest possible variations in Moho depth beneath Bedout (33). It is difficult to assess though whether this Moho topography is, like Chicxulub, the result of the dynamic effects of the crater (and Bedout High) forming process extending down to the base of the crust (33-35), or is the result of later rifting of the continental margin.

The Bedout structure was emergent in the Early to Middle Triassic and probably deeply eroded. Onshore in the Canning Basin much of the Permian and Early Triassic section is missing: over 0.5-1 km of section overall, and as much as 2 km on topographic highs (36). We do not know the depth of erosion at Bedout, but it is probable that the unconformity at the top of the Permian represents missing section. The Lagrange-1 well extends for several hundred meters through the impact melt breccia, but it is uncertain how much more of the High is actual impact melt breccia. The isostatic-residual gravity model for the Bedout structure and the Bouguer gravity over Chicxulub both show a semi-circular gravity low surrounding the expression for the central peak (Fig. 11 below).

Figure 11. (left) Blow-up of isostatic residual gravity model over Bedout as compared with Bouguer gravity over Chicxulub (modified after 12) at approximately the same scale. Diameters of the central uplift (~40km) and transient crater (~100km, dashed circle) inferred from the gravity model for Bedout are similar in size to these features inferred for the Chicxulub impact structure. Gravity signature at Bedout is significantly reduced and more subdued than Chicxulub owing to its greater depth of burial (Bedout gravity model by Andrew Lockwood, GSWA, Perth Australia). Click here for a larger view

Unfortunately, the resolution of the offshore gravity data is not of sufficient quality to obtain a vertical derivative image generally used to highlight the more subtle gradients, and assist in assessing the geomorphometric parameters, including size, of the Bedout structure. The outer edge of the gravity low has a diameter of ~100 km and is similar in size to the more well resolved Chicxulub gravity low (Fig. 11).

Comparisons of the Bedout Structure with other Impact Structures
As first noted by Gorter (13,14), the geophysical expression of the Bedout High is similar to the central uplift in other large impact craters. Fracturing and brecciation, caused by the impact of large meteorites with the crust, produce a characteristic negative gravity anomaly surrounding a gravity-high, a feature that led to the initial discovery of Chicxulub (e.g., 12). Such an anomaly exists at Bedout (Fig. 11), but it is somewhat obscured by other complex crustal features derived from younger tectonic overprinting (e.g. Triassic and Jurassic). The gravity high in the center of large terrestrial craters is due to the central uplift elevating denser basement rocks. At Bedout the gravity high is clearly associated with a structural high. The central uplift at Chicxulub is poorly imaged seismically, consists mostly of ~6-7 km uplift of mid-crustal iso-velocities and is ~40-60 km in diameter (33). These dimensions compare well with the Bedout High, suggesting Bedout may be about the same size as Chicxulub (~200 km diameter). The slight uplift noted at a radius of ~100 km at Bedout, may be a subtle expression of the outer rim, but this is speculative. If the Bedout High is a central uplift similar to the one at Chicxulub, then the erosion at Bedout could be extensive, as the top of the Chicxulub central uplift lies about 3.5 km below the crater floor (37).