Spherule textures in the Neoarchaean Wittenoom impact layer, Western Australia: consistency in diversity

• Published in: Australian journal of earth sciences 2005-09, Vol.52 (4-5), p.773-783
• Main Authors: Scally, A., Simonson, B. M.
• Format: Article
• Language: English
• Subjects: impact ejecta; impact spherules; microkrystites; Precambrian; strewn field
• ISSN: 0812-0099; 1440-0952
• DOI: 10.1080/08120090500170484

The Wittenoom Formation (Hamersley Group, Western Australia) is a well-preserved Neoarchaean unit deposited in a deep shelf to upper slope setting. The upper part contains several laterally persistent marker beds, one of which is rich in well-preserved spherules of former silicate melt showing a diverse suite of internal textures. We quantified the relative abundances of these textures by point counting in spherule-rich samples selected from seven sites and found them to be surprisingly uniform for a lateral distance of > 350 km. They also appear to be uniform at one site where the layer is thicker and contains multiple zones rich in spherules. Given this homogeneity and by comparison to experimentally produced textures and K/T impact spherules, we infer that: (i) the homogeneously diverse nature of the ejecta is most consistent with an impact origin; (ii) the spherules were partially crystallised at the time they were deposited and therefore classifiable as microkrystites; (iii) the original impact melt was roughly basaltic in composition; (iv) the spherules were generated by a single impact then deposited in multiple pulses; (v) the K/T impact model is not directly applicable to the Wittenoom spherule layer; and (vi) the Wittenoom spherule layer was not formed by the same impact as the Carawine layer.