The seismic profile across Bedout is similar to one across the 40-km-diameter Mjølnir crater (S-16 below) in the Barents Sea (38), except the Mjølnir central uplift is much smaller (1.5-2 km high and 8 km wide). Mjølnir has a central uplift that extends well above the pre-impact surface (horizon UB, S-16), and is apparently the result of differential subsidence in the annular trough around the peak under the load of post-impact sediments (39). Permian strata at Bedout are overlain by ~3-5 km of sediment so it is possible that differential subsidence has altered the relief of the Bedout High since its initial formation.

S-16. High-resolution single channel seismic profiles and interpretations crossing the Mjolnir crater structure. The central uplift extends well above the pre-impact surface (horizon UB) and is attributed to differential subsidence in the annular trough around the peak due to loading of post-impact sediments. A similar process may have altered the elevation of the Bedout High. (Figure provided by Filippos Tsikalas; http://folk.uio.no/ftsikala/mjolnir/index.html). Click here for a larger view

Evidence for a P/Tr Impact in Gondwana
A large impact crater at Bedout is consistent with the global distribution of impact ejecta in the P/Tr boundary and helps explain apparent anomalies in the observed patterns. Large (>200-mm) impact ejecta fragments have, so far, only been found in the P/Tr boundary at sites relatively close to Bedout (Fig. 1). Meteorite fragments from the P/Tr boundary at Graphite Peak in Antarctica range in size from 50-400-mm (8, Fig. 1). We have found shocked quartz ranging from 150 up to 550 mm in size at Fraser Park, adjacent to the well-known site at Wybung Head in the Sydney Basin (4) (Fig. 12, S-1a,b) and up to 250 mm sized grains at Graphite Peak, Antarctica (Fig. 1, S-1).

Figure 12. Shocked quartz in the K/T (yellow circles) and P/Tr (red crosses) boundaries. Shown is the distribution of the maximum grain size of shocked quartz with distance from the proposed source crater. Solid line is a power regression through the K/T data (42). Click here for a larger view

The shocked quartz at Fraser Park and Graphite Peak comprises ~1% of the quartz fraction, compared to ~50% at many K/T boundary sites (40). Retallack et al. (4) suggested that such a small amount of shocked quartz in the P/Tr boundary may indicate a minor impact, but we interpret the low percentage as a product of dilution by re-working of the ejecta in a continental depositional environment. The P/Tr boundary layer in the Sydney Basin and in Graphite Peak is a 10-20 cm thick claystone breccia containing abundant rip-up clasts from the underlying soil (4), while the shocked quartz-rich distal K/T boundary deposits are composed mostly of ejecta and are <1 cm thick (41).

When the maximum grain size of shocked quartz from Fraser Park and Graphite Peak are plotted with respect to distance from Bedout, they match well with the maximum sizes for shocked quartz in the K/T boundary and their distance from Chicxulub (Fig. 12). Pope (42) demonstrated that the global shocked quartz distribution in the K/T boundary is best explained by dispersal by stratospheric winds and settling of the particles through the atmosphere. Such a dispersal mechanism is not efficient in latitudinal transport of debris and, therefore, an impact at Bedout would disperse shocked quartz mostly over the southern hemisphere. Thus, a large impact at Bedout is consistent with the size of the shocked quartz grains found in Australia and Antarctica and may also explain why such grains are not found further north.

Elsewhere, in China (Meishan) and Japan (Sasayama), Fe-Ni-Si metal nuggets, oxides, and spherules ~30-200-mm are found in the P/Tr boundary (5, 6, 8, 43). Similar sized spherules with refractory grains (Mg-Ni-Fe oxides and Si-Ca-Al oxides) from the K/T boundary are attributed to formation in the Chicxulub vapor plume (44), and a similar vapor plume origin has been proposed for the P/Tr spherules (5, 8). These high-energy vapor plume products are dispersed much more widely than the clastic debris (shocked quartz) (42), thus the presence of vapor plume condensates in China and Japan without shocked quartz is consistent with an impact at Bedout. The apparent absence of P/Tr impact ejecta from sites far to the north of Gondwana, in what is today North America, Europe, and most of Asia (formerly the Laurasia supercontinent), may also be a consequence of a far southern hemisphere impact at Bedout, but more work is needed to verify this hypothesis.