Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia
Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia
Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia
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Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia
Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia
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Both wells drilled through ~3 km of marine and fluvial sediments consisting of carbonates with occasional interbeded siltstones and mudstones (Tertiary to Cretaceous) and sandstones interbeded with claystones, siltstones and coal (Cretaceous to Triassic) before reaching a breccia (Late Permian; Fig. S-2, 15). Two of the 14 AGSO regional seismic lines cross over the Bedout High (Fig. 2). In addition, four wells penetrate Permian strata (two are shown in Fig. 2.) offshore that help to identify seismic reflectors that define the Bedout structure and stratigraphy. In both the Lagrange-1 and Bedout-1 cores and cuttings, fluviatile and marine Keraudren (Middle to Late Triassic) sediments are deposited directly on top of the breccia (Late Permian; Fig. 4, S-2, S-3.

Figure 4. Selected intact 5 cm (wide) core from Bedout-1 at 3035 to 3037 m (9960 to 9965 ft). These cores display variable sized, angular and subrounded glassy (impact-melted) fragments set in a matrix that is mostly chloritized and carbonate filled. The smaller glassy fragments examined in thin section throughout the core (Fig. S-8-S-14) displayed the same mineralogy and texture as the larger impact-melted glassy fragments seen in hand specimen. The lower photo is the Yucatan-6 impact melt breccia (from that has similar characteristics to Bedout-1, particularly in hand specimen (Yucatan-6 core photo modified from www.icdp-online.de/sites/chicxulub/ICDP-Chix/Figures).
Click here for larger view)
 
S-3. The Bedout-1 core (A) displays numerous large and sub-rounded and angular inclusions set in a dense glassy matrix. The term &lsquo;melt breccia&rsquo; is used to describe impact breccias that contain discrete fragments of rock and minerals together with bodies of melt in a glassy matrix of fine-grained material. The Bedout melt breccia is similar in appearance to impact breccias in the Chicxulub Yax-1 core (B) (Yax-1 figure provided by B. Dressler).
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Presently the Bedout High stands several kilometers above the surrounding basement (17). Deep crustal reflections and seismic refraction velocities also suggest that the Bedout High is under-pinned by elevated middle and lower crust. Both core and cuttings catalogued as basalts and referred to as a “volcanic breccia” were recovered from the top of the High. This regional “volcanism” associated with rifting of the continental margin of Australia is classified as the “Bedout Movement” of P/Tr age (18,19). Immediately after the formation of the Bedout High and termination of the Bedout Movement, there is a regional angular unconformity at the top of the High, consistent with uplift and erosion at the end-Permian (20). Coincident with the formation of the Bedout High is the rifting of the continental "Sibumasu sliver" off northeastern Gondwana (21). The resulting post-impact tectonism, uplift, faulting and erosion during the Triassic and Jurassic time periods regionally overprinted the Bedout structure and deformed the original complex crater morphology.

The Bedout Breccia
Both the Lagrange-1 and Bedout-1 exploration wells ended in what was inferred to be a “volcanic breccia”. Fifty-two meters, 30 m of cuttings and 22 m of core, were collected from the breccia unit in Bedout-1, and 391 m of cuttings were sampled from the breccia unit in the Lagrange-1 drill hole. The Bedout core displays a series of centimeter-sized green clasts, some with variable banding and others that are poorly sorted (chaotic dips of 30 to 50 degrees in hand specimen) (Fig. 4, S-3) over the entire core length (20). Most of the clasts throughout the core are dark green, massive and appear glassy in hand specimen but in thin section many are partially altered to fine grained chlorite or a mixture of fine grained plagioclase, carbonate, and Fe oxides. We identified unaltered glass and relic igneous mineral grains from the lower most section of the core (9986 ft) 3044 m (see Figs. 5, 6, 7a, 7b, 7c, 8, S-4, S-7a, S-7b, S-8, S-9, S-10, S-11, S-12, S-13, S-14) below.

Figure 5 above. (left inset) A typical core sample from the Bedout impact melt breccia at 3052 m (9986 ft.) displays a distribution of poorly sorted angular and sub-angular clasts in a dark glassy matrix (scale of picture is 6.5 mm - long dimension). Under higher magnification (right inset), the yellowish clast appears to be a partially melted carbonate clast with fossil ooids characteristic of a marine continental (carbonate reef ) margin environment. Click here for larger view)

S-4. Top photomicrograph of Bedout 3044 m (9986 ft.) in plane light shows transparent plagioclase lath set in matrix of plagioclase microlites, opaques and altered glass. Lower photomicrograph shows same view in crossed nicols with maskelynite core (as isotropic glass) of plagioclase lath. Field of view is 330 µm. Click here for larger view)
Figure 6. Photomicrograph in plane polarized light of Bedout-1 at 3044 m (9986 ft.) showing a large plagioclase lath (yellow brown color indicative of alteration) that has been shock melted. Another lath at upper left has also been shock melted (slide width 550Ám). The shock melted plagioclase glass is in the process of alteration (green). The matrix is composed of opaque Fe-Ti oxides (black) and crystalline albite (clear). The lower picture is the same view under crossed nicols. All of the plagioclase laths are now extinct (black-opaque) at all orientations indicating conversion to maskelynite, shock melted glass that suggests an impact. Note that the two maskelynite laths are at slightly different orientations yet both are completely extinct.
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Figure 7 (A). Photomicrograph of shock melted fragments of spherulitic glass from Bedout 3044 m (9986 ft) set in a matrix of dark glass. Similar spherulitic glass has been identified in other terrestrial craters (Chicxulub, Sudbury). Width of slide is 1.0 mm. Click here for larger view) Figure 7 (B). Back Scattered Electron Image of another spherulitic glass fragment from Bedout at 3044 m (9986 ft). The large grain of nearly pure silica (Table S-1, analyses #22 and #24). Similar textures have been observed in BSE images of the Chicxulub suevite. Scale bar is 50 Ám. Click here for larger view)
Figure 7 (C) (left). Back Scattered Electron Image of high silica glass from Bedout 3044 m (9986 ft.) The large grain of nearly pure silica (analyses #22 and #24) in the center of the image is set in a matrix of plagioclase, altered glass and Fe-Ti oxides. Note the dark and light areas of the BSE image that correspond to different levels of impurities in the silica glass. Oridinary volcanic processes cannot produce glass of >85% silica. Scale bar is 50 Ám. Click here for larger view)

Figure 8. Shock melted plagioclase grains set in a matrix of albite, Fe-Ti Oxides and glass that is altering to chlorite 3044 m (9986 ft.). The rim of the large grain in the upper right is crystalline plagioclase (visible in X- nicols in figure inset) with a composition of An50 (analysis 3 in Table S-1). The core of this grain is isotropic glass with an identical composition of An50 plagioclase (analysis 4). Single plagioclase lath (left of center) contains andesine plag (An50) (right side of grain, point 6), diaplectic glass in the center and pure albite (point 7) on the left. (Click here for larger view)
 
S-7a,b. Lagrange-1 (LG-3255 m) cuttings sample (S-7a left photo) displays feldspar crystallites in 'swallowtail' terminations, indicative of rapid crystallization from the glassy matrix. The laths display heteorgeneous compositions (Table S-2) derived from the impact process and are chemically unique in comparison to ordinary volcanic feldspars. S-7b. (right photo) S-1 denotes the chemistry of albite, #2, feldspar and #3, the glass matrix (see Table S-2). (Click here for larger view)

S-9. (above) Altered plagioclase laths under crossed nicols from the Bedout-1 core at 3041 m (9977 ft.; B-9977). Note the shattered lamellae perpendicular to the length of the plagioclase crystals. Fractured/altered plagioclases in B-9977 have suffered argon loss and perhaps K addition resulting in anomalously young Ar/Ar ages. Long dimension of slide is 3 mm. (Click here for larger view)
S-8 (above). A single plagioclase lath with multiple sets of fractures from 3041 m ( B-9977 ft.). Lamellae structures are approximately perpendicular to the length of the crystal. Upper photo is in plane polarized light; lower photo is under crossed nicols. Long dimension of slide is 3 mm.
(Click here for larger view)


S-12 (above) Heterolithic glassy, microcrystalline fragments from 3036 m (9960 ft). Notice the different shapes and sizes of clasts with varying degrees of microcrystallinity of the plagioclase microlites. Brighter, white areas are carbonate. Upper photo is in plane polarized light; lower photo is taken under crossed nicols. Long dimension is 3mm.
(Click here for larger view)

S-10 (right) A glassy fragment from Bedout-1 at 3036 m (B-9960'). Upper photo is in plane light; lower photo is under crossed nicols. Several clasts contain mostly dark colored glass with microlites of plagioclase feldspar, rimmed by brownish glass with a flow pattern. This glass is partially chloritized. Notice the black fragment (likely coal) lower center. To the far upper/lower right are microcyrstalline feldspar (whitish) of distinctly different composition that contrast to the darker glassy fragments. Long dimension of slide is 3 mm.
(Click here for larger view)

S-11 (above) Multiple, glassy, microcrystalline clasts in 3036 m (9960 ft. B-9960). Variable glass compositions are indicated by different colors in plane polarized light (upper photo). Notice the brownish, glassy patches, partially chloritized within the larger, darker, glassy fragments. Alternatively, these glassy pathes could be be later pore/vug fillings/alteration phases now chloritized. The thin vein in the center is calcite. Long dimension is 3 mm.
(Click here for larger view)

S-13. Calcite and glassy vein between two dark, glassy feldspar microcrystalline fragments in 3037 m (9964 ft). Colors are enhanced in the lower photo under crossed nicols by insertion of a gypsum plate. Long dimension is 0.6 mm.
(Click here for larger view)
S-14. Heterolithic glassy, microcrystalline fragments with interstitial altered glass in 3037 m (9964 ft). Upper photo is in plane polarized light. Lower photo (crossed nicols) has interference colors enhanced by insertion of a gypsum plate. Long dimension of slide is 0.6 mm. (Click here for larger view)
 
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Contents . 1 . 2 . 3 . 4 . 5 . 6 . 7 . 8 . 9 . 10 . 11
Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia
Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia