This paper discusses the philosophy and major aspects of the geology training of the Apollo 15 , 16 , and 17 astronauts. This training concentrated on monthly field trips that were intended to develop the crew's observational skills in recognizing basic geologic structures and rocks and translating observations into an interpretative framework for local geologic evolution. Individual field trips became increasingly mission-like as their training matured. The crews worked with predetermined traverses and progressively added diverse operational aspects, such as proper usage of sampling tools, photo-documentation of pertinent features and rocks, simulation of space-suit mobility, and use of a roving vehicle. These exercises also provided simulations and practice for all major science support functions that would reside in Mission Control during the actual mission. This combined training of surface explorers and ground support will be indispensable in rendering future planetary surface operations as efficient and scientifically rewarding as Apollo .

The diameter of an impact crater is one of the most basic and important parameters used in energy scaling and numerical modeling of the cratering process. However, within the impact and geological communities and literature, there is considerable confusion about crater sizes due to the occurrence of a variety of concentric features, any of which might be interpreted as defining a crater's diameter. The disparate types of data available for different craters make the use of consistent metrics difficult, especially when comparing terrestrial to extraterrestrial craters. Furthermore, assessment of the diameters of terrestrial craters can be greatly complicated due to post-impact modification by erosion and tectonic activity. We analyze the terminology used to describe crater geometry and size and attempt to clarify the confusion over what exactly the term “crater diameter” means, proposing a consistent terminology to help avert future ambiguities. We discuss several issues of crater-size in the context of four large terrestrial examples for which crater diameters have been disputed (Chicxulub, Sudbury, Vredefort, and Chesapeake Bay) with the aim of moving toward consistent application of terminology.

Prominent magnetic anomalies over large impact craters are attributed to remanent magnetization as thermal effects induce extremely high Koenigsberger values (remanent to induced magnetization ratio, Q). Magnetization of impact melt rocks, breccias, and the rocks underneath the crater floor is related to the thermal evolution of large impact craters, from a single heat pulse to long-lived hydrothermal processes and associated alteration and mineral deposits. The magnetic signature observed on large impact structures can be primarily the aggregate of three effects: (1) composition and properties of target rocks, (2) modification of magnetic carriers due to high pressure-temperature ( P-T ) conditions, and (3) natural remanent magnetization (NRM). Numerical modeling is used to predict the pressure and temperature distribution for varying size craters ranging from a 1.5-km-diameter simple crater to a 90-km-diameter complex crater. We first compare the results of two hydrocodes: the Simplified Arbitrary Lagrangian Eulerian code, version B (SALEB), to produce the final crater shape after a vertical impact; and the Solid, Vapor, Air (SOVA) code to model the initial stage of an oblique impact and to evaluate differences in the volumes of highly shocked materials within the crater created by a vertical impact versus an oblique one. After defining the accuracy of the numerical modeling and reliability of the resulting P-T conditions predicted, numerical modeling is applied for 1–90 km diameter impact structures. The P-T distributions of the target material at its final position obtained with the SALEB code are used for geophysical predictions. The calculated maximum P-T values and their initial and final spatial distributions can be linked with geological and physical and/or chemical processes that lead to different geophysical signatures. For a 5 km diameter crater, brecciation ( P > 1 GPa) affects all material in a 2.5 km hemisphere. Brecciation and fluid circulation trigger a hydrothermal system, which introduces chemical remanent magnetization (CRM) and modify magnetic carriers due to alteration. Pressures larger than 30 GPa induce a secondary component of remanent magnetization (shock remanent magnetization, SRM). These P-T conditions are generally restricted to a radius of <0.5 km and depth <0.8 km. Therefore, SRM and melting are confined to small regions, occasionally not even discernible by a regional airborne magnetic survey, and easily removed by erosive processes.

Lake Wanapitei, located within the Southern Province of Ontario, Canada, provides the setting for a unique study of an impact crater situated within a shield environment. Evidence for the 7.5-km-diameter Wanapitei impact includes a circular Bouguer gravity low centered over the central area of the lake and features of shock metamorphism in samples of glacial drift found on the southern shores. Geophysical studies of craters in hard-rock environments are often limited by the lack of markers used for exploration; this may be overcome with the use of the large igneous dike swarms that characterize Archean terrains. The 1.2 Ga Sudbury dike swarm predates the impact that is suggested to have generated Lake Wanapitei and provides the setting for a study to constrain the size and location of the impact crater. The swarm is clearly visible on aeromagnetic maps as high amplitude, linear features, suggesting they could be used as vertical markers indicative of structural changes having an effect on target rock susceptibilities. To fully establish the size of the crater, a total field magnetic map was produced to trace the Sudbury dikes through the proposed crater center. A gap in their signature, expressed as a 100 nT low, 2–3 km in width, constrains the size of the crater to <5 km. Numerical modeling suggests that a crater of this size will demagnetize target rocks, producing a low in the total magnetic field, up to a maximum diameter of 3 km. Dikes

Aeromagnetic surveys are a useful tool in the detection and analysis of terrestrial impact structures. Although gravity anomalies provide clearer and simpler signatures of impact craters, large regional-scale aeromagnetic surveys are more widely available. A combination of many factors, such as the nature of the target rocks, the impact-related magnetization, and effects of crater fill and post-impact cover results in a great variation of magnetic signatures in the terrestrial impact craters. In crystalline basement targets, the most common signature of a complex impact structure is a magnetic low with a central peak or ring uplift magnetic anomaly. Contributions to the magnetic signature include demagnetization, shock remagnetization, and thermal and chemical remanent magnetization effects. Impact craters in sedimentary targets usually are of small magnetic amplitude, depending on the lithology. The origin of the magnetic signatures in sedimentary targets is not well understood. Enhancement of magnetic signatures of terrestrial impact structures using filtering techniques is an important part of detection and analysis. Derivative and derivative-based (such as sunshading) techniques, along with separation filtering, are probably the most used methods. Here we present our new developments of algorithms for fractional order derivatives and circular shaded relief that have dramatically improved filter results. The fractional derivative order can be varied to optimize the separation of the impact magnetic signature. Given a chosen center location, the circular shaded relief algorithm treats all directions equally, thus preventing fade-out of features subparallel to the shading direction evident in conventional shaded relief. Unlike Hough transform based algorithms, the circular sunshading method is not sensitive to the radius of the circular feature being searched for, and no radius parameter is specified during the data processing We illustrate the new fractional derivative and circular shaded relief algorithms using selected Australian and Canadian impact crater data sets involving both crystalline basement and sedimentary targets.

Considering the Ries crater as an example for middle-sized complex impact structures on Earth, we use geophysical data to examine the structure underneath the crater and simulate the formation process by numerical modeling. In contrast to previous investigations, we show by reanalyzing seismic refraction profiles that some clues for structural uplift exist beneath the crater in a range of at least 1 km. We propose that the average P-wave velocity inside the crater is lower than outside the crater from the surface to ∼2.2 km depth, but that below this level, the velocity increases beneath the center, presumably due to uplifted basement rocks. In addition, we utilized magnetotelluric depth sounding to investigate the deep electrical structure beneath the crater. Two-dimensional inversion models of the data show anomalously high conductivity beneath the crater. Our best model features a zone of presumably brine-filled fractures in open pore space to a depth of ∼2 km. Furthermore, our numerical modeling results for the crater formation are consistent with surface and subsurface observations in the vicinity of the crater. In order to explain the structural differences between similarly sized craters and Ries, we investigate the sensitivity of crater shape and subsurface structure to varying target compositions. We show that for a reasonable range of constitutive material and acoustic fluidization parameters the model calculations produce a large variety of different crater shapes, even for the same amount of impact energy. In contrast to the conventional estimate of crater diameters, our results suggest that Ries crater is comparable in size with the Bosumtwi and Zhamanshin crater. Despite their apparent lack of similarity at first look, Ries and Bosumtwi are closely matched in terms of transient crater size (inner ring), aspect ratio, and structural elements, and we conclude that they both represent typical complex crater structures of the terrestrial impact record for their size range.

The internal structures of central uplifts of impact craters are among the most complex geologic features within Earth's crust. Upheaval Dome, Utah, is used as a reference and case study to display the internal geometry of a central uplift and to deduce mechanisms of uplift formation in impact craters within a sedimentary, siliciclastic target. Geological and structural data gained from our high-resolution mapping of the central part of the structure were combined with topographic data in an ArcGIS database. A three-dimensional visualization of the geometry of faults and strata within the central uplift is presented and interpreted with respect to their deformation history. Central uplift formation is induced by an inward and upward directed convergent flow of the crater floor during gravity-driven collapse of the transient crater cavity. Radial folds and a concentric stacking of imbricated thrust slices are prominent deformation features and result from a constrictive strain pattern. The arrangement of structural elements in the inner part of the Upheaval Dome roughly displays some bilateral symmetry, trending northwest. The dominance of northwest-dipping reverse faults indicates a material transport of top to the southeast, which may be caused by an oblique impact. Fault planes commonly dip steeply and are bent due to a passive distortion after activation. The macroscopic coherence of large target units and blocks and the anisotropy of the layered target cause remarkable deviations from an ideal convergent flow field. Stratified siliciclastic rocks are commonly deformed by localized brittle faulting, and massive sandstones are deformed by a distributed cataclastic flow. During crater collapse, pervasively crushed sandstones will flow locally as a granular medium, resulting in the formation of dikes. Acting as lubricants, they accommodate the complex mesoscale folding and faulting of the neighboring strata. A standard numerical model of impact cratering was designed for comparison with the observed structures and to estimate impact parameters like initial crater size, amount of erosion, and the time of impact. The best fit between model and field data is found when the White Rim Sandstone is buried ∼2000 m beneath the target surface. This most likely corresponds to an Upper Cretaceous age of Upheaval Dome during deposition of the Mancos shales. The initial diameter of the Upheaval Dome impact crater would have been ∼7–8 km.

This study reexamines seven reprocessed (increased vertical exaggeration) seismic reflection profiles that cross the eastern rim of the Chesapeake Bay impact crater. The eastern rim is expressed as an arcuate ridge that borders the crater in a fashion typical of the “raised” rim documented in many well preserved complex impact craters. The inner boundary of the eastern rim (rim wall) is formed by a series of crater-facing, steep scarps, 15–60 m high. In combination, these rim-wall scarps represent the footwalls of a system of crater-encircling normal faults, which are downthrown toward the crater. Outboard of the rim wall are several additional normal-fault blocks, whose bounding faults trend approximately parallel to the rim wall. The tops of the outboard fault blocks form two distinct, parallel, flat or gently sloping, terraces. The innermost terrace (Terrace 1) can be identified on each profile, but Terrace 2 is only sporadically present. The terraced fault blocks are composed mainly of nonmarine, poorly to moderately consolidated, siliciclastic sediments, belonging to the Lower Cretaceous Potomac Formation. Though the ridge-forming geometry of the eastern rim gives the appearance of a raised compressional feature, no compelling evidence of compressive forces is evident in the profiles studied. The structural mode, instead, is that of extension, with the clear dominance of normal faulting as the extensional mechanism.

Previous and recent geological, topographic, drilling, and geophysical data are summarized to acknowledge the 100 km original size of the relatively young and well-preserved multi-ring Popigai impact structure and to estimate the depth of its erosion. The relics of the bedrock rim are traced in the northern and western sectors of the crater, allowing the correct evaluation of the crater diameter. The depth of erosion varies in different crater sectors and concentric zones from 50 to 100 m to 300 m. The most significant erosion occurred on the crater rim. The crater rim has a complex inner structure and is composed of outwardly dragged and uplifted blocks of target rocks overlain by ejecta. During the 35 m.y. that have passed after the crater formation, ∼1000–1200 km 3 of unconsolidated allogenic breccias were eroded from the ejecta blanket on the crater rim and from the upper layers of breccias and suevites of the central depression.

Shuttle Radar Topography Mission (SRTM) data over the Chicxulub impact crater are imaged and compared to previously available topography data. While the two data sets contain different biases related to variations in terrain and vegetation cover, the correspondence of the two sets supports earlier interpretations that the complex structure of the buried crater is expressed in the topography of the northwestern Yucatán Peninsula, México.

Four new coreholes in the western annular trough of the buried, late Eocene Chesapeake Bay impact structure provide samples of shocked minerals, cataclastic rocks, possible impact melt, mixed sediments, and damaged microfossils. Parautochthonous Cretaceous sediments show an upward increase in collapse, sand fluidization, and mixed sediment injections. These impact-modified sediments are scoured and covered by the upper Eocene Exmore beds, which consist of highly mixed Cretaceous to Eocene sediment clasts and minor crystalline-rock clasts in a muddy quartz-glauconite sand matrix. The Exmore beds are interpreted as seawater-resurge debris flows. Shocked quartz is found as sparse grains and in rock fragments at all four sites in the Exmore, where these fallback remnants are mixed into the resurge deposit. Crystalline-rock clasts that exhibit shocked quartz or cataclastic fabrics include felsites, granitoids, and other plutonic rocks. Felsite from a monomict cataclasite boulder has a sensitive high-resolution ion microprobe U-Pb zircon age of 613 ± 4 Ma. Leucogranite from a polymict cataclasite boulder has a similar Neoproterozoic age based on muscovite 40 Ar/ 39 Ar data. Potassium-feldspar 40 Ar/ 39 Ar ages from this leucogranite show cooling through closure (∼150 °C) at ca. 261 Ma without discernible impact heating. Spherulitic felsite is under investigation as a possible impact melt. Types of crystalline clasts, and exotic sediment clasts and grains, in the Exmore vary according to location, which suggests different provenances across the structure. Fractured calcareous nannofossils and fused, bubbled, and curled dinoflagellate cysts coexist with shocked quartz in the Exmore, and this damage may record conditions of heat, pressure, and abrasion due to impact in a shallow-marine environment.

Discoveries of Chicxulub impact ejecta of the Albion Formation in road cuts and quarries in southern Quintana Roo, México and Belize, broaden our understanding of ejecta depositional processes in large impacts. There are numerous new exposures of ejecta near the Río Hondo in Quintana Roo México, located at distances of 330–350 km from the center of the Chicxulub crater. A single ejecta exposure was discovered near Armenia in central Belize, 470 km from Chicxulub. The Albion Formation is composed of two lithostratigraphic units: the spheroid bed and diamictite bed, originally identified at Albion Island, Belize. The new spheroid bed exposures range from 2 to 5 m thick and are composed of altered glass fragments, accretionary lapilli, and pebble-sized carbonate clasts in a fine-grained calcite matrix. The base of the spheroid bed is exposed at Ramonal South in México and at Albion Island and Armenia in Belize, and at all three locations, the spheroid bed was deposited on a weathered karst land surface that had emerged in the Late Cretaceous. The new diamictite bed exposures are composed of altered glass fragments and carbonate clasts up to 9.0 × 3.2 m in size. In all but one of the new exposures, the diamictite bed extends to the surface with observed thicknesses up to 8 m. At Agua Dulce in México, the weathered top of the diamictite bed is overlain by thin-bedded Tertiary carbonates. No diamictite bed is found in Armenia, where the spheroid bed is overlain with a limestone conglomerate containing altered glass shards and shocked quartz. These discoveries indicate that ejecta are emplaced in large terrestrial impacts by at least two distinct flows: (1) an initial flow involving a volatile-rich cloud of fine debris similar to a volcanic pyroclastic flow, which extends >4.7 crater radii (the spheroid bed), and (2) a later flow of coarse debris that may not extend much beyond 3.6 crater radii (the diamictite bed). The former deposit we attribute to material entrained in the impact vapor plume, and the latter to the turbulent collapse of the ejecta curtain.

The combined petrological and rock magnetic study of the Cretaceous-Paleogene (K-P) boundary in northeastern México revealed compositionally and texturally complex Chicxulub ejecta deposits. The predominant silicic ejecta components are Fe-Mg–rich chlorite and Si-Al-K–rich glass spherules with carbonate inclusions and schlieren. Besides these silica phases, the most prominent ejecta component is carbonate. Carbonate occurs as lithic clasts, accretionary lapilli, melt globules (often with quench textures), and as microspar. The composition of the spherules provides evidence for a range of target rocks of mafic to intermediate composition, presumably situated in the northwestern sector of the Chicxulub impact structure. The abundance of carbonate ejecta suggests that this area received ejecta mainly from shallow, carbonate-rich lithologies. Rare µm-sized metallic and sulfidic Ni-Co–rich inclusions in the spherules indicate a possible contamination by meteoritic material. This complex composition underlines the similarities of ejecta in NE México to Chicxulub ejecta from K-P sections worldwide. Although the ejecta display a great variability, the magnetic susceptibility, remanence, and hysteresis properties of the ejecta deposits are fairly homogeneous, with dominantly paramagnetic susceptibilities and a weak ferromagnetic contribution from hematite and goethite. The absence of spinels and the ubiquitous presence of hematite and goethite points to high oxygen fugacity during the impact process. The microfacies and internal texture of the ejecta deposits show welding and fusing of components, as well as evidence for liquid immiscibility between silicic and carbonate melts. No evidence for binary mixing of ejecta phases was found. Therefore, Chicxulub ejecta in NE México probably derived from less energetic parts of the ejecta curtain. However, welding features of ejecta particles and enclosed marl clasts and/or benthic foraminifera from a siliciclastic environment suggest interaction of the—still hot—ejecta curtain with northern Mexican shelf sediments. In addition, an initial ground surge–like ejecta-dispersion mode seems possible.

Carbon, oxygen, and hydrogen isotope results from carbonate and silicate fractions of altered core samples from the Yaxcopoil-1 borehole drilled into the 65 Ma Chicxulub impact crater provide constraints on the physico-chemical parameters of the hydrothermal solutions, and their likely origin. Yaxcopoil-1 impactites were initially permeated with calcite and halite at ambient temperature. This was followed by thermal metamorphism (diopside after igneous augite) and widespread Na-K metasomatism (feldspar after igneous plagioclase), which were overprinted by abundant lower-temperature clay and calcite. Silicate fraction isotopic values have δ 18 O SMOW values between 10 and 23‰ indicating important isotopic exchange between impact melt (∼8‰) and Cretaceous limestone (∼26‰). Heavier δ 18 O values occur over depth intervals with intense feldspar alteration (813–833 m and 864–872 m). The δD SMOW values (−34 to −54‰) are chiefly influenced by smectite abundance and roughly mirror δ 18 O values. Carbonate fraction δ 18 O SMOW values (22–30‰) are controlled by calcite contents, and several exceed the limestone signature. Most δ 13 C PDB (−1 to +2‰) values also cluster around that of local limestone, but a number are significantly lighter (down to −7‰). Isotopic and fluid inclusion results indicate hydrothermal fluid temperatures between 270 and 100 °C, high salinities (∼20%), and minor kerogen contents. These data are compatible with mineralogical constraints, which further support an increase in oxidation state with decreasing temperature. Isotopic data point to a saline CO 2 -bearing fluid mixed with small amounts of reduced carbon, and decarbonation and infiltration processes. Combined results are most consistent with a basinal oilfield saline brine that was driven by impact-induced heat.

We present a mechanism linking large impacts, such as Chicxulub, to significant continental sedimentary slope failures and gas hydrate releases, and hence to carbon isotope excursions. Extensive continental margin failures and seabed sediment liquefaction at the Cretaceous-Tertiary boundary up to thousands of kilometers from Chicxulub have been linked to this impact in previous studies: here we analyze the implied seismic shaking and explore its effects in terms of gas hydrate release from failed continental margin sediments. Paleoseismic analysis of published studies of liquefaction and slope failure at the Cretaceous-Tertiary boundary in North America and adjacent regions suggests that, due to low seismic attenuation in plate interior rocks, there was a sufficient seismic forcing to cause the observed widespread sediment liquefaction and failure along tens of thousands of kilometers of continental slope. The implied magnitude of the impact-related seismicity (equivalent to an earthquake with moment magnitude ≈11) is shown to be broadly consistent with the characteristics of the Chicxulub impact structure. An extended period of post-impact liquefaction and slope failure may account for the observed complexity of Cretaceous-Tertiary boundary sequences in Mexico and North America. We favor a seismic shaking model for the triggering of slope failure over previous models implicating impact generated tsunamis, because the shallow-water Chicxulub impact itself is now recognized as an inefficient tsunami source. We have calculated the potential storage of gas hydrates based on known environmental conditions during the Cretaceous. This suggests that slope failures caused by the Chicxulub impact could have released between 300 and 1300 GtC (best estimate ∼700 GtC) of methane from the destabilization of gas hydrates. This would produce a global carbon isotopic excursion of between −0.5‰ and −2‰ (best estimate ∼−1‰). This compares well with the observed carbon isotopic excursion of −1‰ in the planktonic foraminifera records across the K-T boundary. This large release of methane may also account for the recently reconstructed very high atmospheric pCO 2 levels after the Cretaceous-Tertiary boundary as our estimated gas hydrate releases could have increased atmospheric carbon dioxide by a maximum of 600–2300 ppm (best estimate ∼1200 ppm). This mechanism linking impacts to carbon isotope excursions may apply to other significant excursions, such as that at the Paleocene-Eocene Thermal Maximum. The difficulty in identifying impact craters means that many of the other abrupt carbon isotope excursions found in the geological record could be related to impacts and not to climatic changes.

The early Late Devonian (early Frasnian) Alamo Impact targeted an oceanic, off-platform site in southern Nevada, excavating a crater with a final diameter of 44–65 km. The original crater is now dismembered and buried beneath younger rocks. Consequently, its size and site must be deduced through multiple converging lines of geological and paleontological evidence. Previous and new evidence includes the catastrophically emplaced Alamo Breccia, tsunamites, shock-metamorphosed quartz grains, carbonate accretionary lapilli, an iridium anomaly, sub-Breccia clastic injection, deep-water Breccia channels, and ejecta material. We now demonstrate, on the basis of conodont microfossils in carbonate ejecta clasts within lapillistone blocks and widely distributed shocked-quartz and lithic-clast ejecta within the upper part of the Breccia, that the Alamo Impact excavated down at least into Upper Cambrian strata, at a depth of 1.7 km, and possibly into the underlying Proterozoic–Lower Cambrian Prospect Mountain Quartzite, ∼2.5 km beneath the Late Devonian seafloor. Distal tsunamites and probable ejecta are now documented as far north as Devils Gate, northern Nevada, and as far northeast as the Confusion Range, western Utah. A charcoal-bearing, early Frasnian estuarine deposit in the Bighorn Mountains, Wyoming, may provide the first evidence of an Alamo Impact fallout-generated forest fire. Our new data further document the widespread effects and size of the Alamo Impact, and constrain the likely present position of the tectonically transported crater to an area between the Timpahute and Hot Creek Ranges, southern Nevada.

On the Nuussuaq peninsula, Western Greenland sedimentary deposits of glass spherules also contain high Ir, Co, Ni, and Cu anomalies. The iron-rich silicate glass spherules (to ∼3 wt% NiO, ∼35 wt% FeO) are highly circular in cross section. They show surface dissolution, smectite replacement and calcite infilling of vesicles, though many glasses are optically unaltered. They are strikingly heterogeneous, with schlieren outlining counter-flowing convection cells. Their pronounced Fe-Ni correlation is unlike volcanic suites, but is explained by mixing between basaltic melt and an enriched iron-nickel source. Distinctive nickel-spinel (∼7–10 wt% NiO) contains very nickel-rich cores. Occasional glass spherules show compositional gradients toward resorbed silicates, (plagioclase, clinopyroxene); isotropic plagioclase has anomalous texture comparable to impact-melted lunar breccias. Their anomalously high copper and sulfur (to ∼1%) have lead to an explanation as products of fire-fountaining of exotic or picritic Disko lavas; they would be perhaps the only non-impact occurrence of Ni spinel. Since their discovery, better criteria for recognition of spherules ejected from large impacts have been established, and greater variations in meteorite chemistry as potential projectiles have been described. New mineralogical and petrographic textural data for the Nuussuaq spherules suggest they should be reinterpreted as impact ejecta; the highly oxidized Ni-spinel is a very characteristic signature of meteorite impact ejecta. Delicate preservation features rule out substantial sedimentary reworking, and spherule bed thicknesses imply a large source crater. Stratigraphically, the spherule beds are poorly constrained, but nannofossils and magnetostratigraphy place them close to onset of the West Greenland flood lavas (ca. 61–62 Ma). They share many characteristics with massive native iron localities in dykes and lavas up to >100 km away on Disko (Qeqertarsuaq) Island, but their precise relationship remains to be established.

The distribution and origin of impact diamonds in the ejecta blanket of the Ries crater, Germany, was investigated. Impact diamonds are present in the fallout suevite, whereas the cataclastic crystalline breccias, lithic impact breccia (Bunte Breccia) and clast-rich impact melt rock do not contain diamonds. No regional concentrations of impact diamonds in the fallout suevite could be detected. The average concentration of diamonds is ∼0.1–0.2 ppm. The carriers of impact diamonds are specific suevite components, such as graphite-bearing crystalline rock fragments of shock stage III, and most likely small fragments thereof in melt fragments and suevite matrix. Impact diamonds occur as pseudohexagonal, transparent, and birefringent plates, which reach sizes up to 300 µm. Their color is commonly greenish, but can also be black, gray, yellow, or colorless. Most of the impact diamonds have a fibrous or spongy internal structure and extensional microfractures, which lead to a characteristic porosity. This is the result of a volume decrease due to the phase transformation of graphite to diamond. The Raman characteristics of these impact diamonds are discussed in detail. The strong morphologic similarity of impact diamonds to the precursor graphite from the crystalline target rocks indicate a solid state martensitic phase transformation which occurs at shock pressures of 45–55 GPa.

Shocked quartz from the Charlevoix impact structure has been investigated by optical and scanning electron microscopy, combined with electron backscatter diffraction techniques. The apparent shock pressure recorded by specific sets of planar deformation features (PDFs) in quartz shows a systematic variation with distance (0–10 km) from the center of the structure from ∼5–20 GPa. The occurrence of basal PDFs at distances of ∼2–10 km from the center of the structure indicates a high deviatoric stress component of the shock wave–associated stress tensor. Grain size effects and a greater mineralogical heterogeneity are proposed to be the main cause for slightly lower shock pressures recorded by PDFs in finer-grained granitic gneisses in the southeastern part of the structure, compared to coarse-grained charnockitic gneisses to the northwest at similar distances from the center of the structure. The influence of the crystallographic orientation of quartz on the orientation distribution of planar microstructures appears to superimpose an influence of the orientation of the impact-related stress field. Based on the appearance of Dauphiné twins that are associated with PDFs and the occurrence of PDFs with orientations that correspond to positive and negative rhombohedra, quartz is suspected to have locally been in the β-modification state. Dauphiné twinning is proposed to be mainly due to a reversion to α-quartz during cooling. These findings imply that the uplifted, preheated target rocks have locally been shock-heated to the α-β transition temperature.