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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A section from higher in the core at (9977 ft) 3041 m also contains several large and highly fractured plagioclase phenocrysts (Figs. S-8 and S-9). The basal 8 m of the core contains many small rock and mineral clasts in a predominately glassy matrix that has partially altered to chlorite. Brownish glass shows evidence of flow structure (Fig. S-10) and calcite is observed as veins and in cavities in several thin sections (Fig. S-11, S-13, S-14). The mineral clasts are mostly single and multiple aggregates of plagioclase and the lithic clasts are of glassy fragments. The complex mixtures and very different textures in the lower 8 m of the core are similar to the cores from inside the Chicxulub crater (23-26, Figs. 4, S-3, S-9, S-10, S-11, S-12, S-13, S-14).

The clast from 3044 m (9986 ft.) contains shocked minerals surrounded by a matrix that is almost entirely glass except where it has been altered to chlorite. The samples include shock-melted plagioclase that has been completely or partially converted to glass (Figs. 6 and 8), spherulitic glass (Figs. 7a and 7b) and pure silica glass (SiO2) (Fig. 7c). Plagioclase encloses diaplectic glass (maskelynite) with a composition identical to the surrounding plagioclase (An50) (Fig. 8, analyses 3 and 4 in Table S-1). We also identified magnesian iron titanium oxide [(Fe.84, Mg.14) TiO3] grains that are stoichiometrically ilmenite, heterogeneous silica glass, albite, sanidine and a partially melted carbonate (CaCO3) clast with fragmented ooids (27-28) (Figs. 5, 6, 7, 8, S-4 to S-7ab and Table S-1). Sanidine, identified optically and confirmed by microprobe analysis of a 10-mm grain (analysis 5 in Table S-1) has 43% albite in solid solution without any sign of segregation (microperthite).

The Lagrange-1 cuttings consist of various types of partly crystalline and partly glassy rock of mostly basaltic composition (Fig. S-7). Some of the fragments are identical to those found in the Bedout core (see Figs. S11-S15). One of these fragments, sample LG-3255 m (10,679 ft.), is shown in Fig. (S-7a,b). Fig. S-7a shows feldspar crystallites (laths) in “swallowtail” terminations, indicating rapid crystallization from the glassy matrix. The feldspar laths display heterogeneous compositions, and are mixtures of either pure albite (Table S-1 #1) or K-feldspar (Table S-1 #2) in their glassy matrix (Table S-1 #3) as seen in the back-scattered image (S-7b) of one of the grains.

We interpret these textures, chemistry, mineralogy, and mixture of different fragments as indicating that the basal 8 m of Bedout-1 is an impact melt breccia. The completely or partially melted and fractured plagioclase crystals and abundant glassy clasts are most diagnostic. The coexistence of titanium-rich silica glass in close proximity (within 1 mm) to titanium-poor, but slightly aluminous silica glass (analysis 23 and 24 in Table S-1, Fig. 7c) requires silicate liquid immiscibility that is not seen in terrestrial magmatic environments.

Partially melted and re-crystallized carbonate lithic fragments (Fig. 5) and spherulitic glasses (partially altered to chlorite, Fig. 7b and analysis 22 in Table S-1), with a different chemical composition from the glassy matrix, are again features attributable to an impact-generated melt breccia. Magnesian-ilmenite (analysis 20 in Table S-1) found as microlites in the matrix is also an uncommon mineral in volcanic rocks. The glassy rock clasts can be attributed to melting of target materials that contain Mg-rich sediments (e.g. dolomites) and common Fe-Ti oxides (e.g. magnetite, titanite ‘sphene’ and rutile), found in crustal environments. Overall, the composition of the minerals and glasses of the Bedout core are consistent with a heterolithic impact breccia or melt-rich suevite, formed by impact triggered heterogeneous melt formation and subsequent quenching and crystallization. Such compositions are unknown and unlikely to exist in terrestrial volcanic agglomerates, lava flows and intrusive pipes. Individually, some of these minerals may rarely occur in volcanic or plutonic rocks, but never in association with each other.

In particular, natural volcanic processes generate silicate melts up to but not exceeding about 78% silica. Taken as a whole, these features are most consistent with impact-generated melting. Volcanism associated with rifting (nor any other endogenous magmatic process) does not produce melts (glasses) like (#21) and (#22) in Table S-1. The overall textures of these heterolithic fragments, especially the Bedout glasses, are similar to the features of the Sudbury Onaping breccia and the melt breccias inside the Chicxulub crater (Fig. 4) (22-25).

Ar/Ar Dating of the Bedout Core
We undertook 40Ar/39Ar age measurements on feldspar concentrates from the Bedout-1 core and Lagrange-1 impact breccia by ‘step-heating’ and single crystal fusion experiments (29, also see SOM). 40Ar/39Ar dates on six individual plagioclases 3041 m (9977 ft.) from the Bedout-1 core indicate ages that are much younger than the overlying Triassic sediments. Petrographic and microprobe examination of the Bedout core 3044 m and 3041m (9986 ft. and 9977 ft.) revealed significant alteration in plagioclase grains and possibly extensive K addition (Figs. 6-8 and S-8, S-9) resulting in young 40Ar/39Ar ages. Individual feldspar grains display heterogeneous chemical compositions due to alteration or disequilibrium in the sample cuttings (Table S-1, Figs S-7a, S-7b). The glassy matrix from 3044 m (9986 ft.) had extremely low K (<0.1%) and proved unsuitable for 40Ar/39Ar dating. A plagioclase separate at 3255 m (10,679 ft.) from the Lagrange-1 cuttings, that displayed the least evidence of alteration or disequilibrium, has an 40Ar/39Ar age of 250.2 with a plateau portion between 8% and 90% gas release at 250.1 ± 4.5 Ma (1s), (S-17, S-18, S-19) consistent with the previous K-Ar measurement on a plagioclase separate from Lagrange-1 (253 ± 5 myrs; 30) (Fig. 9, SOM). Similar problems in dating plagioclase separates were observed in the Yucatan-6 melt rocks from the Chicxulub crater (24).

S-17. Fraction of Ar release with respect to K/Cl during step heating of the Lagrange-1 plagioclase separate. Click here for a larger view

S-18. Fraction of Ar release with respect to K/Ca during step heating of the Lagrange-1 plagioclase separate. Click here for a larger view

S-19. Ar step release (36Ar/40/Ar versus 39Ar/40Ar) for the Lagrange-1 plagioclase separate.
Click here for a larger view

Figure 9. Ar/Ar step-heating ages for the Lagrange-1 plagioclases from the top of the Bedout High 3255 m (10,679 ft.) indicate an age of 250.1 +/- 4.5 myrs. Click here for a 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