Formation of the Lawn Hill Impact Structure

Abstract

The Ordovician-aged Lawn Hill Impact Structure is perhaps unique among documented craters, in that it is largely filled with slumped, allochthonous Cambrian carbonates partly fluidised by the impact shock. It is a complex geological feature, with a thin horizon of suevite beneath carbonate sheets. The bulk of the impact ejecta material was either deposited on top of the mobilised carbonates, and not preserved owing to erosion, or is present beneath the base of current drilling. The geometry of the crater in its final form has been modelled in three dimensions using gravity data and dipole–dipole-induced polarisation geophysics, with control from an extensive drill-hole library. Post-impact modification, including terrace formation and carbonate fill, has resulted in an asymmetric ring syncline, which extends to depths >600 m. In the data-rich area of the Century zinc–lead–silver deposit, on the southwest margin of the crater, both intact and remnants of the crater rim and impact ejecta blanket were preserved above the deposit. The Proterozoic ore deposit is located on a detached terrace, which slumped into the crater and was buried by allochthonous carbonates from the Georgina basin. The sequence of events can be shown to comprise: impact with crater formation and injection of carbonate dykes and inter-thrust wedges in the crater walls with concurrent suevite deposits inside and surge deposits outside the crater; this was rapidly followed by slumping of carbonate mega-blocks transported by a fluidised carbonate slurry into the crater, concomitant with collapse of Proterozoic terraces; finally, collapse of the central crater uplift generated imbricate slices of Cambrian carbonate among mainly Proterozoic rocks inside the ring syncline.

KEY POINTS

1. After impact, the Ordovician-aged Lawn Hill Impact Structure crater was filled with slumped terraces of Proterozoic sediments and Pb–Zn ore, fluidised Cambrian carbonate, plus mega-clasts of both of those, on top of a thin but persistent hematitic suevite.

2. Outside the crater, remnants of base surge and debris deposits represent the ejecta blanket.

3. The Century Zn–Pb deposit was preserved on a disrupted terrace; the crater rim was partially preserved above the deposit and the mineralised sequence truncated by the crater wall.

4. Geophysical and drilling data show a terraced, asymmetric ring syncline, up to at least 600 m thick in places.

Keywords:

• Lawn Hill Impact Structure
• meteorite crater
• meteorite impact
• ring syncline
• acoustic fluidisation
• Century mine

Acknowledgements

The authors acknowledge New Century Resources for their support and permission to publish. Claudia Bowman, Tom Blenkinsop and Andy Tomkins are thanked for their extremely helpful reviews.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The geophysical and drilling data supporting this work are the property of New Century Resources; such data may be available from them on request. The 2017 aeromagnetic survey was conducted at nominal 50 m line spacing on east–west lines, nominal 25 m height above ground, across the company’s Mining Leases. Gravity data are a composite of historic company data, at a spacing as close as 500 m, and publicly available Geological Survey of Queensland data.