『Abstract
The author, who investigated the Wolfe Creek, Australia, in 1962
and edited two Benchmark Sets of Readings on Meteorite Craters
and possible Astroblemes in 1977 and 1979, reviews the state of
knowledge at the present time. The text is concerned with terrestrial
impact structures, geological features, without any consideration
of extraterrestrial analogues. A handful of definitive publications
are drawn on to present the story of terrestrial impact in a single
article. The text covers historical aspects (briefly); the effect
of target variations; the paucity of human observation of such
large-scale events; distinction from volcanic (endogenous) structures;
modification by geological processes; the transience of the crater
initially formed on the target, and its subsequent modifications;
the global geographic distribution of the 174 structure now listed
(of which a number are dubious attributions); their distribution
in geological time (many ages being known only known to wide limits,
maximum or minimum values); their size distribution; calibrations
of impact frequencies; shock effects; processes on impacts; the
stages of formation; impact into shallow marine and deep sea targets;
impacts on ice (about which little is known); and finally the
input of impact into biotic extinctions. In this last lengthy
section, the summaries of the conclusions of scientists researching
impact on Earth and palaeontologists researching biotic impact
are set side by side. It is concluded that, if the recent foraminiferal
evidence obtained by Gerta Keller and associates is taken at its
face value, the case of impact as a sole agent in extinction is
non-existent: biotic extinction is clearly a complex process involving
a number of causes, in some cases it was staggered in time, and
different sets of organisms responded quite differently and surprisingly,
even in the same extinction event. Extraterrestrial impact may
have been one of the causes in some cases, but it may have been
regional rather than global in its effects. We may never know
how much input it had into the record of biotic extinction of
Earth? An enormous amount of new knowledge has arisen from detailed
studies of this new family of remarkable geological structures.
Keywords: impact structures; craters; shock metamorphism; geophysical
investigations; biotic extinctions』
Contents
1. Introduction
2. Historical
3. Impacts on the Earth and extraterrestrial bodies: target variations
on Earth
4. The paucity of human observation historically
5. Distinction from volcanic (endogenous) structures
6. Modification by geological processes
7. The terms ‘crater’ and impact structures: transient craters:
morphology
8. Global distribution
9. Age dating of the impacts
10. Size distribution
11. Impact frequencies
12. Processes on impact
13. Shock effects
13.1. Introduction
13.2. Subsolidus shock effects
13.3. Impact melting
13.4. Trace additions from the impactor
13.5. Volume of impact melt sheets
13.6. Pseudotachylite
13.7. Distribution of shock metamorphism
13.8. Proximal and distal ejecta
14. Formation of impact structures
15. Geophysical investigation of impact structures
15.1. Introduction
15.2. Gravity
15.3. Magnetism
15.4. Seismic reflectance and refraction
15.5. Electrical methods
16. Impacts into a marine environment
16.1. Introduction
16.2. The marine structures studied
17. Impacts onto ice
18. Impact structures and biological extinctions
18.1. The K/T extinction
18.2. Other extinctions
18.2.1. Late Eocene impactoclastic layers (〜35 Ma)
18.2.2. The Paleocene-Eocene boundary
18.2.3. Cretaceous 〜74 Ma: the Manson Iowa structure
18.2.4. The Cenomanian-Turonian boundary
18.2.5. The Jurassic-Cretaceous boundary
18.2.6. The early Toarcian
18.2.7. The Triassic-Jurassic boundary (200 Ma)
18.2.8. End-Permian (253 Ma)
18.2.9. Late Guadalupian (North American stage of Late Permian)
18.2.10. Devonian-Carboniferous (362.5 Ma)
18.2.11. Late Devonian (367 Ma)
18.2.12. Ordovician-Silurian boundary
18.2.13. The earlier events
18.3. Conclusion
19. Final discussion
Acknowledgments
References