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On Earth, catastrophic impact of an asteroid or comet with truly global consequences has not happened during our written history, and the threat seems very small. Giant scars on our planet’s surface are relics of an impact history stretching back more than 2 billion years, and there is no assurance it cannot happen again. In Australia there are 36 structures ranging from tens of metres to tens of kilometres in diameter, and recognised to varying degrees of certainty as having been formed by giant meteorite impact. In clear and concise language this book begins with ancient beliefs and myths about craters and then explains how they are actually formed and provides details of their structure. Using the record in the rocks, the authors also assess the likelihood of future impacts and their possible effects.
"This volume contains a sizable suite of contributions dealing with regional impact records (Australia, Sweden), impact craters and impactites, early Archean impacts and geophysical characteristics of impact structures, shock metamorphic investigations, post-impact hydrothermalism, and structural geology and morphometry of impact structures - on Earth and Mars"--
This book presents a comprehensive overview of Australian impact structures and related mineralization, including a discussion of the significance of many of these structures for crustal evolution. The book focuses in particular on Archaean impact ejecta/fallout units in the Pilbara Craton of Western Australia, large exposed and buried impact structures, and on the geophysical evidence for possible to probable impact structures. Thanks to their long-term geological stability, Precambrian and younger terrains in the Australian continent contain 38 confirmed impact structures and 43 ring and dome structures, many of which constitute possible to probable asteroid impact structures. The impact structures have been the subject of more than half a century of studies and range from several tens of meter-large craters to buried structures larger than 100 km in diameter. Discoveries of impact fallout units in the Pilbara Craton have defined the Pilbara as one of the two best documented terrains where Archaean impact ejecta/fallout deposits are identified, the other terrain being the Kaapvaal Craton in southern Africa. A synthesis of evidence from both cratons indicates periods of large asteroid bombardments during ~3.47 – 2.48 billion years-ago, including peak bombardment about 3.25—3.22 billion years-ago. The latter period coincides with an abrupt transformation of an early Archaean granite-greenstone crust to mid to late Archaean semi-continental crustal regimes, underpinning the significance of heavy asteroid impact events for crustal evolution. Apart from proven impact structures, Australian terrains display a range of circular features, including morphological and drainage rings, circular lakes, volcanic craters, tectonic domes, oval granite bodies, mafic igneous plugs, salt diapirs, and magnetic, gravity and seismic anomalies, many of which are of a likely impact origin. Thermal and hydrothermal processes associated with impact cratering bear important consequences for the formation of mineral deposits, such as Ni at Sudbury, Pb-Zn at Siljan and Kentland. Impact structures may also provide sites for the accumulation of hydrocarbons, whereas in some instances fracturing associated with impact structures allows outward migration of oil and gas.
This book presents a comprehensive overview of Australian impact structures and related mineralization, including a discussion of the significance of many of these structures for crustal evolution. The book focuses in particular on Archaean impact ejecta/fallout units in the Pilbara Craton of Western Australia, large exposed and buried impact structures, and on the geophysical evidence for possible to probable impact structures. Thanks to their long-term geological stability, Precambrian and younger terrains in the Australian continent contain 38 confirmed impact structures and 43 ring and dome structures, many of which constitute possible to probable asteroid impact structures. The impact structures have been the subject of more than half a century of studies and range from several tens of meter-large craters to buried structures larger than 100 km in diameter. Discoveries of impact fallout units in the Pilbara Craton have defined the Pilbara as one of the two best documented terrains where Archaean impact ejecta/fallout deposits are identified, the other terrain being the Kaapvaal Craton in southern Africa. A synthesis of evidence from both cratons indicates periods of large asteroid bombardments during ̃3.47 - 2.48 billion years-ago, including peak bombardment about 3.25--3.22 billion years-ago. The latter period coincides with an abrupt transformation of an early Archaean granite-greenstone crust to mid to late Archaean semi-continental crustal regimes, underpinning the significance of heavy asteroid impact events for crustal evolution. Apart from proven impact structures, Australian terrains display a range of circular features, including morphological and drainage rings, circular lakes, volcanic craters, tectonic domes, oval granite bodies, mafic igneous plugs, salt diapirs, and magnetic, gravity and seismic anomalies, many of which are of a likely impact origin. Thermal and hydrothermal processes associated with impact cratering bear important consequences for the formation of mineral deposits, such as Ni at Sudbury, Pb-Zn at Siljan and Kentland. Impact structures may also provide sites for the accumulation of hydrocarbons, whereas in some instances fracturing associated with impact structures allows outward migration of oil and gas.
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. water, th.
When in 1981 Louis and Walter Alvarez, the father and son team, unearthed a tell-tale Iridium-rich sedimentary horizon at the 65 million years-old Cretaceous-Tertiary boundary at Gubbio, Italy, their find heralded a paradigm shift in the study of terrestrial evolution. Since the 1980s the discovery and study of asteroid impact ejecta in the oldest well-preserved terrains of Western Australia and South Africa, by Don Lowe, Gary Byerly, Bruce Simonson, Scott Hassler, the author and others, and the documentation of new exposed and buried impact structures in several continents, have led to a resurgence of the idea of the catastrophism theory of Cuvier, previously largely supplanted by the uniformitarian theory of Hutton and Lyell. Several mass extinction of species events are known to have occurred in temporal proximity to large asteroid impacts, global volcanic eruptions and continental splitting. Likely links are observed between asteroid clusters and the 580 Ma acritarch radiation, end-Devonian extinction, end-Triassic extinction and end-Jurassic extinction. New discoveries of ~3.5 – 3.2 Ga-old impact fallout units in South Africa have led Don Lowe and Gary Byerly to propose a protracted prolongation of the Late Heavy Bombardment (~3.95-3.85 Ga) in the Earth-Moon system. Given the difficulty in identifying asteroid impact ejecta units and buried impact structures, it is likely new discoveries of impact signatures are in store, which would further profoundly alter models of terrestrial evolution. .