IMPACT-AFFECTED MARINE MICROFOSSILS FROM THE GULF OF CARPENTARIA, AUSTRALIA
We selected samples primarily from core GC04 (9.83°S, 135.35°E). We measured the magnetic susceptibility of the core, sieved samples into >150 μm, >63 μm, and >38 μm fractions, and examined the >150 μm samples under a binocular microscope. Samples of obvious microfossils were picked based on a melted appearance, and analyzed with EDAX as CaCO3. We found impact-related materials to 60 cm down, contrary to our calculations of a millimeters-thick ejecta layer. We examined the grains under an SEM for characteristic textures. A few samples were teardrop-shaped or elliptical, transparent, and vitreous in visible light, but analyzed as CaCO3. Splashes of native Fe and TiO2 were observed, some with a stacked appearance. Grains of rutile, melted apatite and ilmenite, zircon, SiO2, tin-rich BaSO4, and FeS were also found on various samples. Three microfossils exhibited quench textures. We identified one of these microfossils as G. ruber [10]; SEM imaging revealed melted on coccoliths, Fe splashes, and grooves. We also noted CaO in a separate sample, a breakdown product of CaCO3.
In addition to melted microfossils in GC04, we found SiO2 with triangular fractures, a candidate for shocked quartz. Microspherules were not abundant in the core, supporting our hypothesis of two low-angle impacts, which produce more distal ejecta [9]. It is our opinion that melted microfossils in the Gulf of Carpentaria are best explained as formed in impact-related processes.
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