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GEOLOGICAL SOCIETY OF AUSTRALIA

WESTERN AUSTRALIAN DIVISION


 

West Australian Geologist (WAG)

Bi-monthly newsletter of the Western Australian Division of the Geological Society of Australia Inc.

 

Number 493: February  ̶  March 2012  (4.3 Mb PDF file)

 

Past Issues

 

Monthly Meetings

Time:  5.30 pm for 6.00 pm formal start (bar open upstairs before talk)

Venue:  Irish Club of WA, 61 Townshend Rd, Subiaco

 Download a map showing the location, or check out the venue with Google Maps.

 

FEBRUARY MEETING

Wednesday 1st February, 2012

 

Talk title: Impact structures in Western Australia

 

Speaker: Alex Bevan (Earth & Planetary Sciences, Western Australian Museum)

 

Abstract: Fourteen structures in Western Australia have been recognised to varying degrees of certainty as of impact origin; a further five enigmatic structures known only from geophysical anomalies have been suggested as possible impact structures but remain unproven. The structures range in size from a few tens of metres in diameter, to many tens of kilometres, and in age from a few thousand years to more than a billion years.

 

Three well-preserved bowl-shaped craters are associated with surviving fragments of the impactor. The recovered projectile fragments are mainly iron meteorites (Wolfe Creek, Veevers), but also include a mesosiderite (Dalgaranga). The Hickman crater lacks projectile fragments. Impact-generated thermomechanical damage in the surviving projectile fragments ranges from simple fractures, through shockhardening, to plastic and shear deformation, severe reheating, recrystallization and melting. Compression and viscous drag between projectile and target appears sufficient to melt and possibly vaporize significant volumes of projectiles, which could account for the paucity of surviving meteoritic materials. Mechanical fracture is an important mechanism for the disruption of the projectile, allowing survival, whereas shear-related ‘jetting’ may be responsible for vaporization. Suspected impactite glass, representing melted target rocks, has been recovered from Wolfe Creek Crater.

 

Ten large, deeply weathered, complex or transitional structures are either exposed at the surface (e.g. Yarrabubba, Spider Structure, Goat Paddock), or buried beneath post-impact sediments (e.g. Yallalie, Woodleigh). The Spider structure may be one of the few known examples of an oblique impact. The target rocks vary from sedimentary terrains (e.g., Yallalie, Spider Structure, Goat Paddock) to granite–greenstone terrain (Yarrabubba) and mixed targets (e.g., Shoemaker).

In the absence of projectile fragments, or of an unequivocal chemical signature of the impactor, diagnostic shock-metamorphic evidence of an impact origin includes multiple sets of Planar Deformation Features (PDFs) in quartz and feldspar (e.g., Woodleigh, Yarrabubba, Shoemaker), and shatter cones (e.g., Yarrabubba, Spider Structure, Glikson, Shoemaker). Other corroborating, although not diagnostic, evidence of impact includes breccias (e.g., Yallalie), pseudotachylites (e.g., Yarrabubba, Shoemaker), melt sheets (e.g., Yarrabubba) and geophysics (e.g., Yallalie). An unusually large central uplift at Yallalie indicates impact into volatile-rich target rocks, possibly sub-aqueously.

 

Post-impact hydrothermal alteration renders the generally open architecture of ancient impact structures susceptible to mineralization, possibly of economic importance.

 

About the speaker: After graduating from University College London in 1972, Dr Alex Bevan worked briefly at the Geological Society in London, and then for 13 years at the British Museum (Natural History) before emigrating to Australia to take up his current position. At the BMNH, Dr Bevan completed a PhD on the metallurgy of meteorites under the joint University of London/Government Laboratories scheme. The work involved periods of research at Manchester University, Lehigh University in Pennsylvania, and the Indian Museum in Calcutta. At the WA Museum he established a programme of meteorite recovery and study (WAMET) from the Nullarbor Region. The new material has expanded knowledge of early Solar System materials, and opened up many new lines of research, including the innovative use of ancient meteorite falls to interpret past climatic change. In addition, Dr Bevan has developed the State’s mineralogical collections, concentrating on documenting and interpreting some of WA’s world famous mineral deposits. Dr Bevan’s research focuses on the effects of catastrophic collisions between minor planets during the early history of the Solar System. His wider research interests extend to metallurgy and materials science, with special subjects including mineralogy, meteoritics (including impact studies), petrology, geochemistry, metallurgy.

 

 

MARCH MEETING

Wednesday 7th March, 2011

 

Talk title: The redox budget of subduction zones – and implications for the ore-deposit source zones

 

Speaker: Katy Evans

 

Abstract: Published in the February – March 2012 WAG.

 

 

Gibb Maitland Medal

 

Guidelines download: MS Word doc

 


 

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Last modified: 27 Jan 2012
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