[Truncated abstract. Please see the pdf version of the abstract for the complete text.] Thirty impact structures of confirmed or possible status are currently identified in Australia. Twenty-two of these structures are confirmed by the presence of meteorite fragments or shock metamorphic features that are diagnostic of meteorite impact. The remainder have an impact origin supported by strong secondary evidence. New impact structures are being discovered in Australia at a rate of about one every year, with geophysics a key tool in the identification of candidate structures for further investigation. It is estimated that between two and five times the current number of impact structures are yet to be discovered on the Australian continent. Past compilations of the geophysical signatures of impact structures, particularly of their potential field responses, have been focused on structures formed in mainly crystalline targets. From these studies the expected gravity response is an overall low due to fracturing of the target rocks, with a local gravity high common over the centre of large complex structures, due to the structural uplift of denser material. An overall demagnetisation of the target rocks by the high shock pressures generated by the impact is also expected, although central magnetic highs may also be produced by remanently magnetised melt or the uplift of magnetic rocks from depth. The geophysical signatures of fifteen Australian impact structures are discussed, including individual case studies on nine structures and a detailed study of the Yallalie structure. Only one of the structures discussed here was formed in crystalline rocks, with a further two in mixed sedimentary / crystalline targets. The other structures that were studied were formed in either Phanerozoic basins or mildly-deformed Proterozoic sedimentary rocks. The potential field responses of these structures show a greater variability than was expected, particularly between structures that were formed in different types of target rock. A positive gravity response is found over four structures formed in clastic sedimentary rocks deposited in a Phanerozoic basin. These anomalies are due to the emplacement of denser rock into the central uplift. A decrease in density due to brecciation is not apparent in this target rock type. Furthermore, it is suggested that by collapsing pore space and removing water, the density of wet sedimentary rocks may be locally increased by impact. Circular magnetic anomalies are found outside the central uplift of six impact structures formed in either Phanerozoic or weakly-metamorphosed Proterozoic sedimentary basins.Four possible sources for these anomalies are proposed; remanently magnetised melt or suevite surrounding the central uplift, creation of new magnetic minerals along internal faults within the crater by post-impact hydrothermal fluids, deformation of a flat-lying magnetic layer within the target stratigraphy, and magnetic (maghemite, heavy minerals) minerals concentrated within the post-impact crater fill. It is not possible to definitively identify an impact crater from geophysical evidence alone. Consequently, candidate structures selected from geophysical data, even those as strongly supported as Silverpit, should not be given equal status to structures that have been proven beyond doubt by diagnostic geological criteria. However, it is proposed that structures that possess several pieces of secondary evidence, such as circular shape, interpretation of characteristic geophysical features and crater morphometry, be reclassified as “provisional” impact structures and be given a status that is between “possible” and “probable”. A global compilation of the natural resources known to be associated with impact structures has been undertaken. Where possible, an economic value is calculated for the total definable resource for each structure. The prospectivity of impact structures for petroleum, mineral or water resources is reconfirmed by this work. Almost 20% of all known terrestrial impact structures are associated with some form of resource that is, or has been, exploited. The most numerous, and generally most valuable, of these resources are hydrocarbon accumulations stored in structural traps or brecciated rocks within, or around, the structure. The structural displacements resulting from crater formation can expose from beneath cover, or preserve from erosion, a pre-existing, or progenetic, mineral deposit. While the massive base-metal deposits of the Sudbury Mining Camp are perhaps the most famous of all impact-related economic resources, they require the preservation of the melt sheet formed by a very large (>150 km diameter) impact structure. The Sudbury mineralisation is probably unique on the Earth, but may be a valid target for metal exploration on other planets. Other types of natural resource include surface or ground water, deposits of chemical or organic-rich sedimentary material, hydrothermal ores and industrial diamonds.
| Identifer | oai:union.ndltd.org:ADTP/221092 |
| Date | January 2004 |
| Creators | Hawke, Philip James |
| Publisher | University of Western Australia. School of Earth and Geographical Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Philip James Hawke, http://www.itpo.uwa.edu.au/UWA-Computer-And-Software-Use-Regulations.html |
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