Terrestrial Impact Structures Research Articles

Bridging the Gap III: Impact cratering in nature, experiment, and modeling

Bridging the Gap III: Impact cratering in nature, experiment, and modeling

The structural inventory of a small complex impact crater: Jebel Waqf as Suwwan, Jordan

AbstractThe investigation of terrestrial impact structures is crucial to gain an in‐depth understanding of impact cratering processes in the solar system. Here, we use the impact structure Jebel Waqf as Suwwan, Jordan, as a representative for crater formation into a layered sedimentary target with contrasting rheology. The complex crater is moderately eroded (300–420 m) with an apparent diameter of 6.1 km and an original rim fault diameter of 7 km. Based on extensive field work, IKONOS imagery, and geophysical surveying we present a novel geological map of the entire crater structure that provides the basis for structural analysis. Parametric scaling indicates that the structural uplift (250–350 m) and the depth of the ring syncline (<200 m) are anomalously low. The very shallow relief of the crater along with a NE vergence of the asymmetric central uplift and the enhanced deformations in the up‐range and down‐range sectors of the annular moat and crater rim suggest that the impact was most likely a very oblique one (~20°). One of the major consequences of the presence of the rheologically anisotropic target was that extensive strata buckling occurred during impact cratering both on the decameter as well as on the hundred‐meter scale. The crater rim is defined by a circumferential normal fault dipping mostly toward the crater. Footwall strata beneath the rim fault are bent‐up in the down‐range sector but appear unaffected in the up‐range sector. The hanging wall displays various synthetic and antithetic rotations in the down‐range sector but always shows antithetic block rotation in the up‐range sector. At greater depth reverse faulting or folding is indicated at the rim indicating that the rim fault was already formed during the excavation stage.

Terrestrial impact structures as geoheritage: an assessment method of their scientific value and its application to Brazil.

Terrestrial impact structures are geological and geomorphological features with particular importance to understand the history and evolution of the planet. Impact structures are scattered around the world but in many countries these features are under threat, essentially due to anthropic factors. Impact structures with higher scientific value should be considered as geological heritage and, consequently, be subjected to geoconservation strategies. In order to select the most important impact structures to be properly conserved and managed, this paper proposes a quantitative assessment method of the scientific value of these structures. The eight Brazilian impact structures were used to test this method that has the potential to be applied to any geological context in any country. The structures known as Araguainha Dome-MT and Serra da Cangalha-TO reached a higher scientific value, which justifies the need to develop geoconservation strategies and a proper management.

A previously unrecognized high-temperature impactite from the Steen River impact structure, Alberta, Canada

Abstract Here, we report a previously unrecognized impactite from the Steen River impact structure in Alberta, Canada, which was intersected by continuous diamond drill core into the allochthonous proximal deposits of this buried 25-km-diameter complex crater. A suite of high-temperature minerals defines the matrix, formed by grain growth in a solid state by static recrystallization of an originally clastic matrix, deposited at temperatures ≥800 °C. This rock type is predominantly a result of the recrystallization of target material driven by the acceleration of hot gasses from volatilized sedimentary cover mixed with variably shocked crystalline basement. Approximately one-third of terrestrial impact structures occur in mixed target rocks; therefore, this type of impactite may be more common than previously realized. Contact metamorphism between entrained sedimentary target rocks and the juxtaposed hot matrix resulted in carbonate decomposition to form a rare spinel-group mineral, magnesioferrite. In crater environments, magnesioferrite has been found in the distal Chicxulub (Mexico) ejecta and may prove a novel indicator mineral for impact into carbonate-bearing target rocks.

Evidence for a spatially extensive hydrothermal system at the Ries impact structure, Germany

AbstractThe ~15 Ma, 26 km diameter Ries impact structure in south‐central Germany was one of the first terrestrial impact structures where evidence of impact‐associated hydrothermal alteration was recognized. Previous studies suggested that pervasive, high‐temperature hydrothermal activity was restricted to the area within the “inner ring” (i.e., the crater‐fill impactite units). Here we present mineralogical evidence for localized hydrothermal activity in the ejecta beyond the crater rim in two previously unstudied settings: a pervasively altered lens of suevite ejecta directly overlying the Bunte Breccia at the Aumühle quarry; and suevite ejecta at depth overlain by ~20 m of lacustrine sediments sampled by the Wörnitzostheim 1965 drill core. A comprehensive set of X‐ray diffraction analyses indicates five distinct alteration regimes (1) surficial ambient weathering characterized by smectite and a minor illitic component; (2) locally restricted hydrothermal activity characterized by an illitic component and minor smectite; (3) hydrothermal activity at depth characterized by smectite, a minor illitic component, and calcite; (4) hydrothermal activity at depth characterized by smectite, a minor illitic component, calcite, zeolites, and clinochlore; and (5) pervasive hydrothermal activity at depth characterized by smectite, a minor illitic component, and minor clinochlore. These data spatially extend the Ries postimpact hydrothermal system suggesting a much more extensive, complex, and dynamic system than previously thought. Constraining the mineralogical alteration regimes at the Ries impact structure may also further our understanding of impact‐associated phyllosilicate formation on Mars with implications for climate models and habitability.

The soda lake—mesosaline halite lake transition in the Ries impact crater basin (drilling Löpsingen 2012, Miocene, southern Germany)

Lacustrine sediments of impact craters form valuable climate archives, although chemical evolution and changes in the catchment area potentially superimpose, distort, or obliterate primary climate signals. The 15 Ma Nordlinger Ries in southern Germany, one of the most intensively studied terrestrial impact structures, harbors a well-preserved but controversially interpreted lacustrine sedimentary fill. While earlier studies proposed a climate-driven development from a playa to a mesosaline soda lake (Units A and B), which then decreased in salinity (Units C and D), new investigations suggest a chemical evolution from a playa and soda lake (Units A–C) to a mesosaline halite lake (Unit D), which then turned into a hypersaline halite lake, until an outlet formed. However, problems in the stratigraphic correlation of basin center and margin sediments impeded the recognition of the hypothetical soda to halite lake transition to date. A new drilling in the central crater now provides a solution for the problem. Unit C still comprises analcime-rich dolomite marl with reversely correlated δ13C and δ18O values, thereby reflecting a shallow, highly alkaline, saline meromictic lake (Na–Mg–CO3–SO4). In turn, Unit D is characterized by a change to cycles composed of lignite, diatomite, claystone, marl, and limestone. Gypsum pseudomorphs at the cycle tops indicate saline lake water (Na–Mg–Cl–SO4) with increased Ca2+ concentrations. Reworked, previously aragonitic, green algal tubes prove that early parts of Unit D sediments formed contemporaneously to basin margin green algal bioherms, contrary to previous assumptions. Therefore, the change from a highly alkaline soda lake to a mesosaline halite lake reflects increasing influx of waters from the Bunte Breccia into the lake, while suevite-derived weathering solutions decreased. Low-salinity conditions during Unit D are temporary phases during lake-level rise at the beginning of short-term cycles, whereas stable oxygen isotope ratios indicate meso- to hypersaline conditions at cycle tops. However, the long-term increase in salinity leading to continuous hypersaline conditions is only preserved in carbonates at the crater rim.

Geology and mineralogy of the Auki Crater, Tyrrhena Terra, Mars: A possible post impact-induced hydrothermal system

A variety of hydrothermal environments have been documented in terrestrial impact structures. Due to both past water interactions and meteoritic bombardment on the surface of Mars, several authors have predicted various scenarios that include the formation of hydrothermal systems. Geological and mineralogical evidence of past hydrothermal activity have only recently been found on Mars. Here, we present a geological and mineralogical study of the Auki Crater using the spectral and visible imagery data acquired by the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars), CTX (Context Camera) and HiRISE (High Resolution Imaging Science Experiment) instruments on board the NASA MRO mission.The Auki Crater is a complex crater that is ∼38km in diameter located in Tyrrhena Terra (96.8°E and 15.7°S) and shows a correlation between its mineralogy and morphology. The presence of minerals, such as smectite, silica, zeolite, serpentine, carbonate and chlorite, associated with morphological structures, such as mounds, polygonal terrains, fractures and veins, suggests that the Auki Crater may have hosted a post impact-induced hydrothermal system. Although the distribution of hydrated minerals in and around the central uplift and the stratigraphic relationships of some morphological units could also be explained by the excavation and exhumation of carbonate-rich bedrock units as a consequence of crater formation, we favor the hypothesis of impact-induced hydrothermal circulation within fractures and subsequent mineral deposition. The hydrothermal system could have been active for a relatively long period of time after the impact, thus producing a potential transient habitable environment.

Frictional melting processes and the generation of shock veins in terrestrial impact structures: Evidence from the Steen River impact structure, Alberta, Canada

Shock-produced melt within crystalline basement rocks of the Steen River impact structure (SRIS) are observed as thin (1–510μm wide), interlocking networks of dark veins which cut across and displace host rock minerals. Solid-state phase transformations, such as ferro-pargasite to an almandine–andradite–majorite garnet and amorphization of quartz and feldspar, are observed in zones adjacent to comparatively wider (50–500μm) sections of the shock veins. Shock pressure estimates based on the coupled substitution of Na+, Ti4+ and Si4+ for divalent cations, Al3+ and Cr3+ in garnet (14–19GPa) and the pressure required for plagioclase (Ab62–83) amorphization at elevated temperature (14–20GPa) are not appreciably different from those recorded by deformation effects observed in non-veined regions of the bulk rock (14–20GPa). This spatial distribution is the result of an elevated temperature gradient experienced by host rock minerals in contact with larger volumes of impact-generated melt and large deviatoric stresses experienced by minerals along vein margins.Micrometer-size equant crystals of almandine–pyrope–majorite garnet define the shock vein matrix, consistent with rapid quench (100–200ms) at 7.5–10GPa. Crystallization of the vein occurred during a 0.1–0.15s shock pressure pulse. Majoritic garnet, formed during shock compression by solid state transformation of pargasite along shock vein margins, is observed in TEM bright field images as nanometer-size gouge particles produced at strain rates in the supersonic field (106–108). These crystals are embedded in vesiculated glass, and this texture is interpreted as continued movement and heating along slip planes during pressure release. The deformation of high-pressure minerals formed during shock compression may be the first evidence of oscillatory slip in natural shock veins, which accounts for the production of friction melt via shear when little or no appreciable displacement is observed. Our observations of the mineralogy, chemistry and microtextures of shock veins within crystalline rocks of the SRIS allow us to propose a model for shock vein formation by shear-induced friction melting during shock compression.

The Canadian space agency planetary analogue materials suite

The Canadian Space Agency (CSA) recently commissioned the development of a suite of over fifty well-characterized planetary analogue materials. These materials are terrestrial rocks and minerals that are similar to those known or suspected to occur on the lunar or martian surfaces. These include: Mars analogue sedimentary, hydrothermal, igneous and low-temperature alteration rock suites; lunar analogue basaltic and anorthositic rock suites; and a generic impactite rock suite from a variety of terrestrial impact structures. Representative thin sections of the materials have been characterized by optical microscopy and electron probe microanalysis (EPMA). Reflectance spectra have been collected in the ultraviolet, visible, near-infrared and mid-infrared, covering 0.2–25μm. Thermal infrared emission spectra were collected from 5 to 50μm. Raman spectra with 532nm excitation, and laser-induced fluorescence spectra with 405nm excitation were also measured. Bulk chemical analysis was carried out using X-ray fluorescence, with Fe valence determined by wet chemistry. Chemical and mineralogical data were collected using a field-portable Terra XRD–XRF instrument similar to CheMin on the MSL Curiosity rover. Laser-induced breakdown spectroscopy (LIBS) data similar to those measured by ChemCam on MSL were collected for powdered samples, cut slab surfaces, and as depth profiles into weathered surfaces where present. Three-dimensional laser camera images of rock textures were collected for selected samples.The CSA intends to make available sample powders (<45μm and 45–1000μm grain sizes), thin sections, and bulk rock samples, and all analytical data collected in the initial characterisation study to the broader planetary science community.Aiming to complement existing planetary analogue rock and mineral libraries, the CSA suite represents a new resource for planetary scientists and engineers. We envision many potential applications for these materials in the definition, development and testing of new analytical instruments for use in planetary missions, as well as possible calibration and ground-truthing of remote sensing data sets. These materials may also be useful as reference materials for cross-calibration between different instruments and laboratories. Comparison of the analytical data for selected samples is useful for highlighting the relative strengths, weaknesses and synergies of different analytical techniques.

Meteorite traces on a shatter cone surface from the Agoudal impact site, Morocco

AbstractThe recently discovered Agoudal impact site in Morocco is a small, eroded impact structure with well-developed shatter cones. A scanning electron microscopic study of a shatter cone surface has revealed the presence of schreibersite – a phosphide very rare on Earth but common in iron meteorites – and Fe–Ni oxides. This is the first reported evidence for primary meteoritic matter adherent to shatter cones and suggests that the Agoudal crater was formed by the impact of an iron meteorite, probably the Agoudal IIAB iron. Shatter cones from other terrestrial impact structures might also hold valuable information about the nature of the impacting projectiles.

Rotational Pixel Swapping Method for Detection of Circular Features in Binary Images

We propose a new automatic method called the rotational pixel swapping (RPSW) method to detect circular features in binary images of remote sensing images. The method is based on a multiplication operation between the original image and the rotated images. We show that the RPSW selectively enhances rotational symmetric patterns and weakens nonrotational symmetric patterns, including noise components, without any noise reduction processes. The method can detect not only simple circles but also more complex circular features such as incomplete ring structures or several concentric rings. Furthermore, we demonstrate that the RPSW provides the stable detection of circular features such as terrestrial impact structures, which are irregular imperfect circular shapes, in binary images based on Earth-observation satellite images. The RPSW would provide a potential method of future surveys or statistical studies using huge data sets of multiband or hyperspectral images obtained by Earth-observation satellites.

Revisiting the Rochechouart impact structure, France

AbstractThe Rochechouart impact structure, south‐central France (45o50′N, 0o46′E), is a partly eroded, approximately 200 Myr, complex impact structure. The impactite suite at Rochechouart provides an excellent example of gradational boundaries and transitional lithologies that have been historically difficult to classify with standard impactite nomenclature. Here, we present the first detailed scanning electron microscopy‐based description of the Rochechouart impactites integrated with hand‐sample and petrographic observations with the goal of understanding the clast‐matrix relationships of transitional lithologies. Three main impact‐generated hydrothermal alteration assemblages are also recognized: (1) argillic‐like, (2) carbonate, and (3) oxide. Our results support the existence of a continuum between clast‐rich impact melt rocks and glass‐rich clastic breccias (suevites) that must be represented in universal classification schemes. This suite of impactites from the Rochechouart impact structure is used as a test case for a recently published classification scheme based on the nature of the groundmass setting a precedent for classification of impactites with limited to no geological context such as deeply eroded terrestrial impact structures and future sample return missions. The re‐evaluation of the melt‐bearing Rochechouart impactites questions the currently accepted size of the crater, suggesting a much larger original crater diameter.

Dating terrestrial impact structures: U‐Pb depth profiles and (U‐Th)/He ages of zircon

AbstractWe investigate the presence of epitaxial overgrowth rims and “reset” zircon, complete loss of radiogenic lead (Pb*), from terrestrial impactites to constrain the occurrence of such phenomenon in impact environments and their possible use in dating impact events. We also explore (U‐Th)/He dating of zircon to evaluate this geochronometer in accurately identifying impact ages, particularly when no dateable melt sheet exists. Our results show that (U‐Th)/He ages of zircon from the brecciated, and presumably shocked target, can accurately date an impact event and provides another tool to determine impact ages when no melt sheet exists, an alternative to problematic interpretations of commonly used apparent 40Ar/39Ar plateau ages. No evidence of epitaxial overgrowth rims and/or reset zircon was observed, suggesting that zircons within shocked impactites have remarkably slow Pb diffusion and possibly explaining the relatively few reset grains reported in terrestrial impactites.

Chemical projectile–target interaction and liquid immiscibility in impact glass from the Wabar craters, Saudi Arabia

Impact glasses are usually strongly affected by secondary alteration and chemical weathering. Thus, in order to understand relevant formation processes, detailed petrographic studies on unweathered impact glasses are necessary as preserved heterogeneities in quenched impact glasses may serve as a tool to better understand their genesis. Here, we report on petrography and microchemistry of impact glasses from the Wabar impact craters (Saudi Arabia) that, with an age of ∼300years, are among the youngest terrestrial impact craters. The fact that parts of the IIIAB iron meteorite have survived impact and subsequent weathering is granting Wabar a special role among the presently 184 confirmed terrestrial impact structures. Electron microprobe analysis (EMPA) and transmission electron microscopy (TEM) obtained on the black impact melt/glass variety at Wabar suggest that meteoritic Fe was selectively mixed with high-silica target melt at high temperatures due to selective oxidation, resulting in high Fe/Ni ratios for the black melt (37 on average, individual values range from 13 to 449) and low Fe/Ni ratios for projectile droplets (“FeNi spheres” with a Fe/Ni ratio of 3 on average; Fe/Ni ratio for the meteorite is ∼12). The black melt shows emulsion textures that are the result of silicate liquid immiscibility. Liquid–liquid phase-separation resulted in the formation of a poorly polymerized, ultrabasic melt (Lfe) rich in divalent cations like Fe2+, Ca2+, or Mg2+, that is dispersed in a highly polymerized, high-silica melt (Lsi) matrix. The typical Wabar black melt emulsion displays a spheres-in-a-matrix texture of ∼10–20% Lfe homogeneously dispersed in the form of two sets of spheres and droplets (10–30nm and 0.1–0.4μm in diameter) in ∼80–90% Lsi matrix, plus occasionally disseminated FeNi spheres. Around large (>10μm) FeNi spheres, however, the typical emulsion texture changes to ∼21% Lsi dispersed in ∼79% Lfe. This change of texture is interpreted as evidence for the transfer of meteoritic Fe from the meteoritic FeNi spheres into the target melt due to selective oxidation of Fe over Ni and Co. Variations in the bulk composition of Wabar black melt largely depend on the volume ratios between immiscible ultrabasic Lfe, felsic Lsi, and remains of meteoritic FeNi spheres. Based on natural occurrences of phase-separated glasses (this work and literature) and quenching experiments (literature), there is growing evidence that liquid immiscibility is a major process in the formation of glassy impactites.

An impact origin for hydrated silicates on Mars: A synthesis

Recent Mars‐orbiting spectrometers continue to detect surface materials containing hydrated silicates, particularly clays and amorphous phases (e.g., silica glasses), concentrated within the heavily cratered Noachian highlands crust. This paper provides a review, summary, and synthesis of observations from terrestrial impact structures with current Martian data. It is suggested that numerous and frequent impacts into the volatile‐rich silicate crust of Mars, through direct and indirect impact‐generated mechanisms, represent a plausible hypothesis that can explain the widespread distribution of hydrated silicates in the surface and subsurface of the heavily cratered Noachian highlands crust largely independent of climate. In addition to impact‐generated hydrothermal activity, devitrification, autometamorphism, and the voluminous production of impact “damaged” materials that are susceptible to alteration must be considered. When taken together, a drastically different early climate on Mars, in which water is stable at the surface for extended periods of time, cannot be ruled out; however, it is noted here that these additional impact mechanisms can operate and thereby extend the range of possible alteration settings to include climate conditions that may have been predominately colder and drier. Such a climate would not be dissimilar to the conditions of today, with the important exceptions of a higher geothermal gradient, and punctuated thermal disturbance to the cryosphere and hydrosphere from igneous activity and an exponentially higher impact flux.

Impact-generated hydrothermal systems on Earth and Mars

It has long been suggested that hydrothermal systems might have provided habitats for the origin and evolution of early life on Earth, and possibly other planets such as Mars. In this contribution we show that most impact events that result in the formation of complex impact craters (i.e., >2–4 and >5–10km diameter on Earth and Mars, respectively) are potentially capable of generating a hydrothermal system. Consideration of the impact cratering record on Earth suggests that the presence of an impact crater lake is critical for determining the longevity and size of the hydrothermal system. We show that there are six main locations within and around impact craters on Earth where impact-generated hydrothermal deposits can form: (1) crater-fill impact melt rocks and melt-bearing breccias; (2) interior of central uplifts; (3) outer margin of central uplifts; (4) impact ejecta deposits; (5) crater rim region; and (6) post-impact crater lake sediments. We suggest that these six locations are applicable to Mars as well. Evidence for impact-generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration depending on the setting and nature of the system. A variety of hydrothermal minerals have been documented in terrestrial impact structures and these can be grouped into three broad categories: (1) hydrothermally-altered target-rock assemblages; (2) primary hydrothermal minerals precipitated from solutions; and (3) secondary assemblages formed by the alteration of primary hydrothermal minerals. Target lithology and the origin of the hydrothermal fluids strongly influences the hydrothermal mineral assemblages formed in these post-impact hydrothermal systems. There is a growing body of evidence for impact-generated hydrothermal activity on Mars; although further detailed studies using high-resolution imagery and multispectral information are required. Such studies have only been done in detail for a handful of martian craters. The best example so far is from Toro Crater (Marzo, G.A., Davila, A.F., Tornabene, L.L., Dohm, J.M., Fairèn, A.G., Gross, C., Kneissl, T., Bishop, J.L., Roush, T.L., Mckay, C.P. [2010]. Icarus 208, 667–683). We also present new evidence for impact-generated hydrothermal deposits within an unnamed ∼32-km diameter crater ∼350km away from Toro and within the larger Holden Crater. Synthesizing observations of impact craters on Earth and Mars, we suggest that if there was life on Mars early in its history, then hydrothermal deposits associated with impact craters may provide the best, and most numerous, opportunities for finding preserved evidence for life on Mars. Moreover, hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long after hydrothermal activity has ceased.

Suspect cubic diamond “impact” proxy and a suspect lonsdaleite identification

The presence of nanometer-sized diamonds in purported Younger Dryas (YD) boundary-dated sediments, carbon spherules, and Greenland ice was cited as evidence of a YD impact event (1). Although cubic and hexagonal (lonsdaleite) diamond have been found in shocked metamorphosed meteorites and are associated with terrestrial impact structures, cubic diamonds are well known to occur in terrestrial deposits that have no associations with impact processes. For example, submicron and smaller cubic diamond crystals have been found recently in carbonaceous spherules isolated in upper soils from various German and Belgian sites (2). Lacking links to impact structures, these diamonds are evidently not products of impact …

The 40Ar/39Ar dating technique applied to planetary sciences and terrestrial impacts

The 40Ar/39Ar technique is a powerful geochronological method derived from the K/Ar technique that can help to unravel the evolution of the solar system. The 40Ar/39Ar system can not only record the timing of volcanic and metamorphic processes on asteroids and planets, it finds domain of predilection in dating impact events throughout the solar system. The 40Ar/39Ar method is a robust analytical technique when the events to be dated are well understood and data are not over interpreted. The power of the 40Ar/39Ar technique resides in the ability to check the validity of age data internally by statistical means and multiple lines of evidence, and hence to evaluate when Ar age data are unreliable. Yet, too many ‘ages’ reported in the literature are still based on over-interpretation of perturbed age spectra. This review is by no means exhaustive and is centred on the most recent applications of the 40Ar/39Ar technique applied to planetary material, not the history of the planetary bodies themselves, or a historical review of the development of the argon dating technique. This paper presents selected case-study examples on terrestrial impact structures, the Moon and meteorites with ages recalculated using the latest decay constants. Currently, only 21 terrestrial impact structures are precisely and accurately dated, and the only proven age concordance is between the Chixculub impact and the Cretaceous/Paleogene mass extinction. 40Ar/39Ar dating of volcanic events on the Moon suggests that volcanism was concentrated between 3.8 and 3.1 Ga. The study of lunar volcanism would also benefit from dating of volcanic spherules for which only few data are available. Rigorous filtering of the 40Ar/39Ar age database of lunar melt breccias yielded concordant ages with high precision for two major basins of the Moon, but more precise age data would be needed to further test and validate the Late Heavy Bombardment (LHB) hypothesis. 40Ar/39Ar dating of lunar impact spherules shows an increase of ages <400 Ma suggesting a recent increase in the impact flux. The impact history of the LL parent body (bodies?) has yet to be well constrained but may mimic the LHB observed on the Moon, which would indicate that the LL parent body was quite large. Basaltic meteorites (HEDs) show an 40Ar/39Ar age range between 4.1 and 3.4 Ga, suggesting a diffuse LHB event; however, the spread of apparent ages may be a data-interpretation artefact, and the LHB parent body (bodies?) might have experienced a bombardment closer to the duration of the LHB age range than expected. Martian meteorites contain clues on Mars atmospheric and mantle argon compositions.

Geochemistry of Impactites

Geochemical analysis is an essential tool for the confirmation and study of impact structures and the characterization of the various rock types involved (target rocks, impact breccias, melt rocks, etc.). Concentrations and interelement ratios of the platinum-group elements, as well as the osmium and chromium isotope systems, allow quantification of extraterrestrial components and the identification of impactor types in impact deposits. In addition, chemolithostratigraphy can reveal the possible role of impacts in environmental change throughout the geologic record. This article deals predominantly with terrestrial impact structures.

V. Ben Meen and the Riddle of Chubb Crater

Aerial photographs of Chubb Crater, a striking 3.4 km-wide circular basin in the far north of Quebec, led the Ontario prospector Fred W. Chubb to think it might be an extinct volcano, and possibly the site of a diamond-bearing diatreme. V. Ben Meen, the Director of the Royal Ontario Museum of Geology and Mineralogy in Toronto, however, suspected it was an impact crater caused by a meteorite. Meen led two expeditions to the crater in 1950 and 1951. Despite early opposition and the initial absence of corroborative field evidence, he held on to a persistent belief in the crater's meteoritic origin. Later fieldwork ultimately provided strong evidence in support of this view. The discovery of the crater led to the development of a program at the Dominion Observatory in Ottawa to search for additional impact craters on the Canadian Shield, and the development of valuable criteria by which they could be authenticated. The craters discovered through the program fit well on the Baldwin curve relating crater depth to diameter, and lent strong support to the argument for the relationship between the meteoritic origin of lunar craters and terrestrial impact structures. Chubb Crater is of historical importance because it was the first meteorite crater to be recognized in Canada, and the first anywhere to be authenticated in the absence of associated meteorites.