CONFIRMATION OF A NEW IMPACT CRATER IN WESTERN AUSTRALIA: SHOCKED QUARTZ AT THE YALLALIE STRUCTURE

Introduction:In order to confirm an impact structure, diagnostic evidence of shock deformation, i.e. shatter cones and/or shocked minerals, or traces of meteoritic anomalies must be documented [1]. Yallalie is a proposed impact structure, situated 200 km north of Perth, Western Australia, within the Mesozoic Perth Basin. Previous studies have documented a buried 12 km circular, geophysical anomaly [2,3]. The age of the structure has been suggested as late cretaceous, determined by overlying sedimentary units and clasts incorporated within proximal breccia [2]. Here we report the first confirmation of shocked quartz at the Yallalie structure.

Results and Discussion: Proximal, allocthonous Mungedar breccia, exposed ~4 km west of the Yallalie structure were collected in order to examine the rock for shock. Five quartz grains contain multiple planar fractures (PFs), while two grains contain planar deformation features (PDFs). The shocked grains were indexed by U-stage with PFs occurring along {11-22},(0001),{10-11} and {11-21} while PDFs occur along (0001). PFs in quartz has been proven to result from low intensity shock 5-10 GPa while PDFs require pressure between 10-30 GPa [1,4,5,6]. One quartz grain exhibits a ballen type II (α-quartz) microstructure. Ballen has been identified in previous studies to record retrogression of shock produced polymorphs [7]. Thus, the presence of both PFs and PDFs represent the first documented shocked mineral evidence confirming the hypothesis that Yallalie is an impact crater.

References: [1] French and Koeberl (2010) ESR. [2] Dentith et al (1999) Geol Mag, 619-632. [3] Hawke et al. 2006 Exp. Geophys. [4] Stöffler and Langenhorst (1994) Meteoritics, 29, 155–181. [5] French (2004) Meteoritics and Planetary Science 39, 169–197. [6] French et al (2004) GSA Bulletin, 116; p. 200–218. [7] Ferrière et al (2010) Large Meteorite Impacts and Planetary Evolution IV: GSA, 465, p. 609–618.