This handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures emphasizes terrestrial impact structures, field geology, and particularly the recognition and petrographic study of shock-metamorphic effects in terrestrial rocks. Individual chapters include: 1) Landscapes with Craters: Meteorite Impacts, Earth, and the Solar System; 2) Target Earth: Present, Past and Future; 3) Formation of Impact Craters; 4) Shock-Metamorphic Effects in Rocks and Minerals; 5) Shock-Metamorphosed Rocks (Impactities) in Impact Structures; 6) Impact Melts; 7) How to Find Impact Structures; and 8) What Next? Current Problems and Future Investigations.

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Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures

The convincing identification of terrestrial meteorite impact structures: What works, what doesn't, and why

https://people.earth.yale.edu/sites/default/files/files/Evans/52_12b-Zhang%2BNuna.pdf

Pre-Rodinia supercontinent Nuna shaping up: A global synthesis with new paleomagnetic results from North China

We have developed a Web-based program for quickly estimating the regional environmental consequences of a comet or asteroid impact on Earth (www.lpl.arizona.edu/ impacteffects). This paper details the observations, assumptions and equations upon which the program is based. It describes our approach to quantifying the principal impact processes that might affect the people, buildings, and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions. The program requires six inputs: impactor diameter, impactor density, impact velocity before atmospheric entry, impact angle, the distance from the impact at which the environmental effects are to be calculated, and the target type (sedimentary rock, crystalline rock, or a water layer above rock). The program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse, the thermal radiation emitted by the impact-generated vapor plume (fireball), and the intensity of seismic shaking. The program also approximates various dimensions of the impact crater and ejecta deposit, as well as estimating the severity of the air blast in both crater-forming and airburst impacts. We illustrate the utility of our program by examining the predicted environmental consequences across the United States of hypothetical impact scenarios occurring in Los Angeles. We find that the most wide-reaching environmental consequence is seismic shaking: both ejecta deposit thickness and air-blast pressure decay much more rapidly with distance than with seismic ground motion. Close to the impact site the most devastating effect is from thermal radiation; however, the curvature of the Earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon.

/pdf/earth-impact-effects-program-a-web-based-computer-program-5dcnlzwcnv.pdf

Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth

The breakup of the former Pangea supercontinent culminated in the modern drifting continents. Increased rifting caused the establishment of the Atlantic Ocean in the middle Jurassic and significant widening in Cretaceous. An explosion of calcareous nannoplankton and foraminifers in the warm seas created massive chalk deposits. A surge in submarine volcanic activity enhanced supergreenhouse conditions in the middle Cretaceous with high CO2 concentrations. Angiosperm plants bloomed on the dinosaur-dominated land during late Cretaceous. The Cretaceous dramatically ended with an asteroid impact, which resulted in a mass extinction.

The Cretaceous Period

The Chesapeake Bay Crater : Geology and Geophysics of a Late Eocene Submarine Impact Structure

Ballen quartz and cristobalite in impactite samples from five impact structures (Bosumtwi, Chicxulub, Mien, Ries, and Rochechouart) were investigated by optical microscopy, scanning electron microscopy (SEM), cathodoluminescence (CL), transmission electron microscopy (TEM), and Raman spectroscopy to better understand ballen formation. The occurrence of so-called “ballen quartz” has been reported from about one in five of the known terrestrial impact structures, mostly from clasts in impact melt rock and, more rarely, in suevite. “Ballen silica”, with either α-quartz or α-cristobalite structure, occurs as independent clasts or within diaplectic quartz glass or lechatelierite inclusions. Ballen are more or less spheroidal, in some cases elongate (ovoid) bodies that range in size from 8 to 214 μm, and either intersect or penetrate each other or abut each other. Based mostly on optical microscopic observations and Raman spectroscopy, we distinguish five types of ballen silica: α-cristobalite ballen with homogeneous extinction (type I); ballen α-quartz with homogeneous extinction (type II), with heterogeneous extinction (type III), and with intraballen recrystallisation (type IV); chert-like recristallized ballen α-quartz (type V). For the first time, coesite has been identified within ballen silica – in the form of tiny inclusions and exclusively within ballen of type I. The formation of ballen involves an impact-triggered solid-solid transition from α-quartz to diaplectic quartz glass, followed by the formation at high temperature of ballen of β-cristobalite and/or β-quartz, and finally back-transformation to α-cristobalite and/or α-quartz; or a solid-liquid transition from quartz to lechatelierite followed by nucleation and crystal growth at high temperature. The different types of ballen silica are interpreted as the result of back-transformation of β-cristobalite and/or β-quartz to α-cristobalite and/or to α-quartz with time. In nature, ballen silica has not been found anywhere else but associated with impact structures and, thus, these features could be added to the list of impact-diagnostic criteria.

Characterisation of ballen quartz and cristobalite in impact breccias: new observations and constraints on ballen formation

During a recent drilling project sponsored by the International Continental Scientific Drilling Progam (ICDP), two boreholes (LB-07A and LB-08A) were drilled into the crater fill of the Bosumtwi impact structure and the underlying basement, into the deep crater moat and the outer flank of the central uplift, respectively. The Bosumtwi impact structure in Ghana (West Africa), which is 10.5 km in diameter and 1.07 Myr old, is largely filled by Lake Bosumtwi. Here we present the lithostratigraphy of drill core LB-08A (recovered between 235.6 and 451.33 m depth below lake level) as well as the first mineralogical and petrographic observations of samples from this core. This drill core consists of approximately 25 m of polymict, clast-supported lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment that displays a large variation in lithology and grain size. The lithologies present in the central uplift are metasediments composed dominantly of fine-grained to gritty meta-graywacke, phyllite, and slate, as well as suevite and polymict lithic impact breccia. The suevites, principally present between 235.6 and 240.5 m and between 257.6 and 262.2 m, display a fine-grained fragmental matrix (about 39 to 45 vol%) and a variety of lithic and mineral clasts that include meta-graywacke, phyllite, slate, quartzite, carbon-rich organic shale, and calcite, as well as melt particles, fractured quartz, unshocked quartz, unshocked feldspar, quartz with planar deformation features (PDFs), diaplectic quartz glass, mica, epidote, sphene, and opaque minerals). The crater-fill suevite contains calcite clasts but no granite clasts, in contrast to suevite from outside the northern crater rim. The presence of melt particles in suevite samples from the uppermost 25 meters of the core and in suevite dikelets in the basement is an indicator of shock pressures exceeding 45 GPa. Quartz grains present in suevite and polymict lithic impact breccia abundantly display 1 to (rarely) 4 sets of PDFs per grain. The shock pressures recorded by the PDFs in quartz grains in the polymict impact breccia range from 10 to ~30 GPa. We also observed a decrease of the abundance of shocked quartz grains in the brecciated basement with increasing depth. Meta-graywacke samples from the basement are heterogeneously shocked, with shock pressures locally ranging up to 2530 GPa. Suevites from this borehole show a lower proportion of melt particles and diaplectic quartz glass than suevites from outside the northern crater rim (fallback impact breccia), as well as a lack of ballen quartz, which is present in the external breccias. Similar variations of melt-particle abundance and shockmetamorphic grade between impact-breccia deposits within the crater and fallout impact breccia outside the crater have been observed at the Ries impact structure, Germany.

http://www.univie.ac.at/geochemistry/koeberl/publikation_list/304-Bosumtwi-core-LB08-lithology-Ferriere-MAPS2007.pdf

Drill core LB‐08A, Bosumtwi impact structure, Ghana: Petrographic and shock metamorphic studies of material from the central uplift

Shock wave attenuation rate and formation of central uplifts are not precisely constrained for moderately sized complex impact structures. The distribution of shock metamorphism in drilled basement rocks from the 10.5-kilometer-diameter Bosumtwi crater, and results of numerical modeling of inelastic rock deformation and modification processes during uplift, constrained with petrographic data, allowed reconstruction of the pre-impact position of the drilled rocks and revealed a shock attenuation by ∼5 gigapascals in the uppermost 200 meters of the central uplift. The proportion of shocked quartz grains and the average number of planar deformation feature sets per grain provide a sensitive indication of minor changes in shock pressure. The results further imply that for moderately sized craters the rise of the central uplift is dominated by brittle failure.

Shock metamorphism of Bosumtwi impact crater rocks, shock attenuation, and uplift formation.

Paleomagnetic and rock magnetic study of the Vredefort impact structure and the Johannesburg Dome, Kaapvaal Craton, South Africa—Implications for the apparent polar wander path of the Kaapvaal Craton during the Mesoproterozoic

Wolf Uwe Reimold

Papers

The Chesapeake Bay Crater : Geology and Geophysics of a Late Eocene Submarine Impact Structure

Characterisation of ballen quartz and cristobalite in impact breccias: new observations and constraints on ballen formation

Drill core LB‐08A, Bosumtwi impact structure, Ghana: Petrographic and shock metamorphic studies of material from the central uplift

Shock metamorphism of Bosumtwi impact crater rocks, shock attenuation, and uplift formation.

Paleomagnetic and rock magnetic study of the Vredefort impact structure and the Johannesburg Dome, Kaapvaal Craton, South Africa—Implications for the apparent polar wander path of the Kaapvaal Craton during the Mesoproterozoic