Shevchuk, M. Diamond-like glass. The grain size of the material varies between silt and sharp-edged blocks up to the size of 1 m. In the majority, even the smaller fraction of limestone particles does not show any roundness. Frequently, limestone cobbles are covered with multiple sets of scratches and polish. For the cross-bedded diamictite exposed at the edge of a flat chain of hills a glacial deposit, e. The multiple, small-scale cross-bedding units as well as the transport over short distance point to a close-by, short-term process of formation.
It is interpreted as the result of a big Lake Chiemsee tsunami that was triggered in the Holocene Chiemgau impact event. The deposit also raises issues relevant to a Lake Chiemsee glacier. Here, we in particular point out that the peculiar findings in the Nalbach area are revealing remarkable similarities to impact features in the Holocene large Chiemgau impact strewn field in southeast Germany, and meanwhile the possibility that the Nalbach impact is a companion to the Chiemgau impact is seriously being discussed.
Click on the image to open the full text! Under the scanning electron microscope SEM : The odd world of the iron silicides from the Chiemgau impact meteorite crater strewn field click to enlarge.
Contribution to the mineralogy meeting of the Russian Academy of Sciences in Syktyvkar:. Shortly after the meeting between the 19th and 22nd May in Syktyvkar the Proceedings volume has been published:. We remind of the fact that in the beginning of the research on the Chiemgau impact the group of quite experienced local historians and amateur archeologists had detected the metallic iron silicides and, after having been aware of their relation to crateriform structures, had published the possible meteoritic origin of the matter.
Ultimately it is evident that these local historians who later got together with scientists from geosciences, astronomy, archeology and historical scholarship to establish the Chiemgau Impact Research Team CIRT , are proved right! As the case may be we suggest to save the file on the computer and to activate it with a pdf reading program. In addition to the report on the microtektites from the foothills of the Alps we present here the LPSC abstract paper click to open the full article :.
On reentry in the atmosphere and cooling, the glass bodies exhibiting characteristic shapes fall to Earth where they form part of the impact ejecta. By definition tektites that are smaller than 1 mm are called microtektites. Although the origin of tektites from impact events is generally accepted the exact mode of formation is not well understood. In the Chiemgau meteorite crater strewn field impact glasses are found widespread in various formations, and tektite-like bodies of a dense black glass with vesicles have attracted considerable attention Fig.
Dense black glass particles from the Chiemgau impact strewn field frequently exhibiting tektite-like shape and twisted form similar to irghizites from the Zhamanshin impact crater. These are NOT the Chiemgau microtektites! Outside the crater strewn field in the foothills of the Alps at some m altitude Fig. Location map for the Chiemgau impact meteorite crater strewn field and the sites of the microtektite finds.
Google maps. They show the very typical splash shapes like spheres, teardrops and irregular dumbbells Fig. They are fully transparent and have a mostly yellowish-brownish-grayish color. Bubble inclusions are frequent. Typically shaped microtektites from the soil in the foothills of the Alps near the Chiemgau impact crater strewn field. The SEM micrographs in Fig. SEM images of microtektite-like glass particles.
Note the tiny, micrometer-sized glass filaments lower right of the particle over it. Microtektites are known, e. Even volcanic spherules may show dumbbell and teardrop shapes, while industrially produced microscopic glass spheres e. This question has been answered already on the preceding pages where the fundamental, universal importance of impact cratering as a planetological, geological, and environmental process has been emphasized.
But it could be argued that this is not the first review of African impact structures, as this work was preceded by a review by Koeberl Despite extensive work and new discoveries on African impact structures and many other structures that have been proposed as such, only abstract form compilations of confirmed and suspected impact structures in Africa have been contributed since e. Throughout the two decades, not only many new impact structures have been discovered or proposed on the African continent, but a vast body of new work including many important contributions to this record and also of global significance has been published.
Impact cratering studies have evolved into a mainstream scientific discipline, with many investigative methods having been added or improved, and a global body of impact knowledge has been assembled that must be referred in any such review effort. This has drawn attention to the need for continuous promotion of impact cratering as a serious geological process that needs to be part of the university education of every student of geoscience — also in Africa. Which economic geology course can afford to avoid discussing the economic potential of impact structures such as Vredefort or Sudbury?
It is a fact that African countries are heavily reliant on revenue from ore resources, and basic knowledge about impact cratering and impact geology should be offered to every student of geoscience. This undertaking is also based on a strong incentive for us, namely to caution about the serious problem of uncritical promotion of alleged impact structures based on insufficient evidence, which unfortunately has become a widespread problem.
Finally, it is hoped that the African geocommunity will take note of the heritage value that many impact structures on this continent represent. A range of tools e. The recognition of the early s that the mass extinction at the Cretaceous-Paleogene boundary could be related to an impact catastrophe, and the debate about possible traces of primitive life in the Martian meteorite ALHA in the mids, must be credited with the subsequent elevation of impact science into geoscientific, planetological, and astrobiological mainstream research.
A detailed review of the methodology of impact cratering studies has recently been published by French and Koeberl Some of the tools of impact cratering studies. The large water body at the far right of this area is part of the Vaal Dam reservoir. Note the obvious course of the Vaal River through the Vredefort Dome, where its bed is largely determined by topographic and structural fault lines aspects. The prominent mountain land of the collar of the Vredefort Dome is also well visible.
Most recently, one application of this gun was to accelerate centimeter-sized projectiles of metal or iron meteorite onto large blocks of porous sandstone to investigate the response of such material, which is of course abundant in the upper crust of Earth, to hypervelocity impact see Kenkmann et al. Here, results of simulations based on geological and geophysical constraints from the Vredefort impact structure are shown. Top: isotherms after the impact event in the rock volume of the central uplift left part and surrounding ring syncline.
The post-shock temperatures estimated by Gibson and Reimold from petrographic observations of the exposed strata are indicated for three locations — clearly the modelling achieved excellent agreement with the observed values. White lines and numbers indicate from which depth in kilometers the rocks along these lines have been uplifted.
Dashed lines suggest the estimated limits of erosion depth since formation of the Vredefort structure ca. Middle: The profile schematically shows the geology between the central part of the central uplift left and the outer Potchefstroom Synclinorium right , extending from mid-crustal granitoids of the core to the supracrustals of the collar of the Vredefort Dome, and then into the synclinorium comprising mainly Transvaal Supergroup metasediment. Bottom: Here the modelled variation of shock pressures is shown in black lettering, with the limits of known PDF and shatter cone SH occurrences schematically illustrated.
Impact studies are conducted on a range of scales, from remote sensing investigations of planetary surfaces, through kilometer- to meter-scale field investigations of impact structures and ejecta horizons, to hand specimen, to sub-millimeter, and even micrometer-to-nanometer-scale laboratory investigations.
Mineralogy is at the forefront of analysis of impact-induced deformation phenomena, from the deformation and transformation of target minerals to the formation and differentiation of impact melt rock. Geochemical techniques are vital for the tracing of elemental or isotopic relics of the projectile in impact-generated rocks — and understanding possible fractionation mechanisms that might hinder fingerprinting the meteorite type of an impactor. State-of-the-art geochronological methods allow, in some cases, to determine a precise age for an impact event and can provide information regarding correlation with an ejecta horizon or with environmental change.
Impact shock experiments with acceleration of chemically well-defined projectiles onto targets designed from minerals, rocks, or metals, by means of explosive-driven devices or compressed light-gas guns, allow to determine the behavior of materials under the extreme pressure and temperature conditions of impact hypervelocity shock physics , as well as the shock and temperature calibration of the shock metamorphic effects that are observed in naturally shocked materials from impact structures and ejecta components. Finally, a vital and highly prospective avenue of impact research has become the technique generally known as numerical modelling, which allows to focus on individual aspects parameter studies of cratering, from astronomical considerations such as the orbits of projectiles and obliquity of impact, via kilometer to sub-millimeter modelling of target stratigraphic succession, localized deformation features such as faults or folds, mineral assemblages, effect of pore space, inter alia and ejecta dissemination of ballistic ejecta and ejecta curtain materials, origin of ejecta within different levels of the target.
The understanding of physicochemical material behavior and the kinematics material flow, ejection and displacement of the phases involved in impact events has been dramatically enhanced by this methodology. The techniques of both these methodologies have been highly important for the initial recognition of impact structures and as mapping aids on the ground, as well as providing an impact crater density-based means for relative chronology of planetary surfaces, and geological analysis of craters.
In some cases it was possible to indeed verify some of the structures as impact-generated, but this is not possible on the basis of remote-sensed data alone — in every case in situ ground-truthing is required through geological assessment of a structure and laboratory-based verification of bona fide impact evidence e. Remote sensing data may also provide extremely useful — even essential — support for geological field work, especially in remote areas where other orientation means are not available.
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The more or less densely cratered, and thus variably old planetary surfaces of the Moon and terrestrial planets have been evaluated by crater counting techniques Hartmann et al. Especially the high-resolution imagery provided by spacecraft in the last decade e. The principle of crater-counting chronometry is simple: the older a surface, the more craters should be accumulated.
Obviously the technique is not that simple, with parameters such as gravity, atmospheric density, and position of a body in the solar system with respect to the source regions of bolides and, thus, impact flux, or the nature of target lithology that obviously reflects on the size of impact craters produced having to be considered. It is obviously advantageous to have some absolute ages for geological formations to provide benchmark values for the calibration of crater frequency statistical curves, and fortunately some such values are available for the lunar record.
The lunar crater production curves have then been adapted for the conditions gravity, atmospheric density on other planetary bodies such as Mars. Impact crater density in certain regions on Earth has also been used to calculate impact flux values for various periods in Earth evolution. Obviously, it is very difficult to generalize how many impact structures should be preserved in a given region of the Earth. The geologic history of that region determines how many structures could have been preserved from erosion, are perhaps covered by younger sediment, have been completely obliterated by sedimentation, or could be exhumed at any given time.
Ivanov , his Fig. Naturally, this average value ought to vary significantly from region to region, between oceanic and continental crust. Stable continental platforms may accumulate impact structures and preserve their erosional remnants for periods up to several billion years — the reason why the Scandinavian impact record is quite exceptional see Earth Impact Database website. Several structures were, thus, recognized in the course of economic exploration programs and then confirmed by drilling and subsequent hands-on laboratory analysis of drill core.
In Africa, the Kgagodi structure in Botswana was initially identified through gravity analysis as part of a hydrological exploration project Paya et al. Typically, impact structures that have not been eroded to, or beyond, the crater floor are characterized by negative gravity anomalies caused by impact-induced fragmentation and brecciation of the target rock and by high-porosity impact-breccia fills of the crater forms — in essence representing circular simple bowl-shape structures or annular complex crater forms with central uplift structure zones of reduced density.
In the case of complex structures, and where craters are deeply eroded, anomaly patterns may be more complex, as they would be largely determined by the subcrater basement geology that can be quite complex both lithologically and with regard to long-term geological evolution metamorphism, alteration, etc.
Magnetic anomalies may be circular over a simple crater, or ring-shaped in cases of complex crater structures with central uplifts see below , but not necessarily so. Reviews of geophysical signatures of impact structures are found in Pilkington and Grieve, , Grieve and Pilkington, , and Pilkington and Hildebrand Besides potential field studies, seismic investigations have occasionally led to proposals of the presence of an impact structure, when a stratigraphic uplift is indicated in the inner parts of a crater-like feature.
For example, the km-diameter Montagnais crater on the continental shelf off the east coast of Nova Scotia Canada was recognized from seismic patterns acquired in the course of oil exploration Jansa et al. However, also geological investigations of large impact structures may be greatly enhanced by the application of geophysical methods, prominent examples being Chicxulub, Vredefort, and Sudbury see Grieve et al.
Where impact structures are largely or entirely buried, geophysical characterization is a prerequisite prior to selection of drilling sites. It should be emphasized once again that geophysical patterns are not conclusive in determining the impact origin of a structure. Nowhere has this been better illustrated than in the case of the alleged Bedout impact structure. Becker et al. Their evidence was an alleged resemblance of the gravity pattern in this region to the gravity signature over the Chicxulub structure.
They also claimed that they had found impact breccias with shock deformation features in a drill core extracted from Bedout, and that this material had the exact age of the Permian—Triassic boundary. This suggestion quickly drew enormous interest in the scientific community and the general public, but it was also severely scrutinized. Glikson rejected the alleged impact deformation evidence, Renne et al. Until now, the alleged shock metamorphic evidence in the form of diaplectic plagioclase glass maskelynite has not been confirmed. Ground penetrating radar has been employed on occasion in impact structures, but the limited penetration depth of this technique restricts its applicability to immediate subsurface studies, for example of the distribution of impact breccia in the environs of a crater e.
The high-porosity impact breccias of the crater fill also lend themselves to investigation by electrical methods, such as resistivity mapping Henkel, , and magnetotellurics has been used to investigate crater depths and maximum depth of subcrater deformation e. At this latter structure an intriguing annular pattern of radiometric element signatures gamma-ray spectrometry has also been mapped Vasconcelos et al.
Radiometric data have also been obtained over the Bosumtwi structure in Ghana, where an annular anomaly outside of the crater itself seems to be associated with widespread K alteration in the environs of the crater, specifically of the ejecta blanket Boamah and Koeberl, , Koeberl and Reimold, Overall, geophysical analysis is highly useful for the recognition of possible impact structures, in support of geological analysis of crater structures, and particularly where surface geological access is not provided. However, by itself, neither remote sensing nor geophysical data sets suffice to confirm the presence of an impact structure.
Geological ground-truthing, in conjunction with laboratory analysis of crater rocks, is required in every instance — with samples to be acquired either from field work or from drilling. Ground-truthing of an impact structure is essential. This entails — in both simple and complex impact structures — the search for definite macroscopic evidence of impact, such as shatter cones see below.
Also, lithologies that are unique with regard to regional geology — in particular breccia occurrences and melt rocks, could be essential for the further investigation of an impact structure, as they have the highest potential to exhibit diagnostic impact evidence in the form of shock deformation. As discussed below, impact melt rock and impact-produced pseudotachylitic breccias provide the best material for dating of an impact event.
Several field expressions of impact breccias and of a shatter cone are shown in Fig. Exposures of different impact breccias and a typical shatter cone. This view shows a polymict lithic breccia overlain by a thin layer of Bunte Breccia which is ballistically emplaced lithic breccia.
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The melt rock from this exposure was described in Hecht et al. Large, dismembered, blocks of different metasedimentary rock types form a polymict megabreccia, which is the result of in- and downward directed slumping at the edge of the impact structure. The clast content is monomict — only alkali granite clasts occur. Notably, geological analysis does not stop within the confines of a crater-like structure but entails comparison with the geology and deformation as found outside of the structure.
This includes searching for evidence that might indicate a zone of stratigraphic uplift in the inner part of the structure central uplift, see below , in comparison with regional stratigraphy. Scaling relationships linking the amount of stratigraphic uplift with the diameter of an impact structure e. The regional tectonic situation rock deformation, presence of fault or shear zones must be investigated, as local deformation enhancement may be the result of impact but could be due to tectonically induced changes to the crust as well.
It is well-known that strata at crater rims should be up- or even overturned — showing the characteristic inverse stratigraphy of impact crater rims. Asymmetric tectonic configurations as found in crater rims or in central uplift structures can be indicative of oblique impact e. In any case, it is necessary to keep an open mind during fieldwork: preconceived ideas that a given structure would have to be of impact origin may lead one onto the wrong track.
Crater-like structures can be produced by many other processes, such as sinkhole formation, volcanic processes maars, collapsed calderas, volcanic vents including kimberlite pipes , tectonic movements, landslides, karstification, or glacial processes. It is obviously necessary to consider the respective geological situation in its entirety — e. The critical reader may find these allegations in Ernstson et al. Crater structures in volcanic regions hold a particular challenge — considering that it is not impossible that impact cratering might affect volcanic terrains as well.
This problematic is highlighted by recent reports of an entire impact crater strewn field in the volcanic Bajada del Diablo area of Argentina, where many crater-like features have been related to impact but, to date, no conclusive pro-impact evidence — what-so-ever — has been recognized Acevedo et al. Obviously besides the less than obvious geophysical observation no tangible evidence to validate this allegation has ever been offered. Amazing how it has even been possible to assign an age to this spurious impact event… This case has highlighted a common flaw: misleading disregard for the fundamental scientific principle that requires obtaining proof for a new hypothesis before it is reported as fact!
This putative event has been alleged to have affected the climate at that time, and thus biodiversity in North America, including living conditions for the then foraging early Americans. In contrast, other workers have not been able to confirm the presence of any alleged impact evidence e. Various claims about Chiemgau and Antarctic impacts, and an alleged Younger Dryas impact catastrophe, have made it into the secondary literature already, despite the ongoing controversy about these speculative claims. The reader is cautioned to conscientiously evaluate the data for these and other inconclusive but high-profile cases.
Detailed sampling of country rocks and other lithologies is required to allow petrographic analysis, especially in search of shock metamorphic indicators. Should it be possible to utilize impact breccias of the suevite or impact melt rock types see below for chemical tracking of an impactor component, care should be taken to analyze representative samples of all possible target rocks. And as it is very desirable to constrain the age of impact events, the geological field work should also be conducted under consideration of searching for relative chronostratigraphic evidence and for lithologies that might allow absolute — or at least relative i.
General geological analysis of an impact structure may involve much more than the above mentioned, especially in the case of large, old, poly-metamorphosed terrains. The case in point is presented by the Vredefort impact structure, as reviewed in all facets by Gibson and Reimold Another detailed review of a comprehensive, multidisciplinary investigation of an impact structure in Africa is the geological record of the Bosumtwi structure in Ghana, by Koeberl and Reimold , also see below.
A thorough review of structural geological investigations of impact structures has been published by Kenkmann et al. Main aspects of analysis involved, inter alia , the presence and distribution of impactites, the macroscopic and microscopic analysis of mineral and rock deformation and its possible decrease down hole i. Much has been learned from these projects about impact cratering and post-impact overprint on such structures. Currently, a further consortium study on ICDP drill cores is underway, whereby drill cores retrieved from the Barberton Mountain Land of South Africa are investigated for information regarding early crustal and mantle processes, including tracking of the processes that led to the development of early life.
Another aim of this project has been to retrieve fresh material of Archean spherule layers see below, section 6. Basic methods for the study of distal impact ejecta are described in detail in Montanari and Koeberl Optical microscopic analysis on high-quality, polished thin sections of rock samples from possible impact structures is the mandatory, first step of laboratory analysis.
The prime objective is, of course, to identify the telltale, characteristic features of shock deformation shock metamorphism, see below; presence of unusual high-pressure polymorphs to confirm the existence of an impact structure. The level of deformation, if present, gives an indication of how deeply the structure may be eroded.
Where optical microscopy is insufficient to confirm the presence of shock metamorphic features, electron microscopic, and even transmission electron microscopic, analysis may be required. In addition, Raman spectroscopy may be useful to investigate the presence of high-pressure polymorphs of impact-diagnostic value. The combination of optical contrast and SEM-based cathodoluminescence analysis Hamers and Drury, , Hamers, is also a powerful technique.
Electron-backscatter diffraction EBSD analysis, coupled with electron microscopic techniques, has also been employed for detailed shock metamorphic analysis e. Evidence for melting is sought after, for the aforementioned importance of melt breccias for dating purposes and because melt rocks may provide a possibility to identify traces of an extraterrestrial projectile therein.
Any attempt to determine the nature of breccia formation and emplacement in an impact structure e. The same holds for the investigation of melt breccia petrology e. There has been quite some interest in the formation and emplacement of proximal and distal impact ejecta e. It should not be forgotten that a given type of information may provide crucial data for a further, different aspect of research, too. Detailed petrographic analysis of the different lithologies provided a means to suggest their likely places of origin in the developing and modifying crater structure. This, in turn, was critical evidence that allowed to construct a numerical model for the multi-stage development of this structure Kenkmann et al.
Detailed reviews of geochemical analysis of impact facies have been given by Koeberl, , Koeberl, , French and Koeberl, , and Koeberl et al. This involves both elemental and isotopic analysis aimed at determination of the origin of lithologies found in a crater, as well as the identification of an extraterrestrial i. The latter, if successful, serves as a definite criterion supporting the impact origin of a crater structure; and, of course, the nature of the projectile is important for understanding what has impacted Earth at various times throughout Earth evolution.
Siderophile element analysis comparison of elemental abundances in the impact breccia with the chemical compositions of the target rocks — the so-called indigenous components of elements that may be enriched in impact breccias due to the presence therein of extraterrestrial material and especially the analysis of the platinum-group elements have been utilized since the s.
In recent decades this has been successfully supplemented by Re—Os and Cr isotopic methods, whereby the Re—Os method has superior sensitivity and may determine meteoritic components as low as 0. For many years attempts have been made to date impact events i. Both terrestrial and extraterrestrial materials Apollo samples returned from the Moon and lunar meteorites, Martian meteorites have been investigated. In the last decades, since the onset of the debate about whether or not catastrophic impact events have caused, or contributed to, mass extinction events, a further incentive has been to match individual impact events with such paleo-biodiversity crises — or show that a multitude of impact events at specific times could have caused global — or at least regional — breakdown of the environment.
However, biostratigraphic and radiometric dating techniques have, in many cases, not provided very precise ages. As discussed in detail by Jourdan et al. And any attempt to correlate impact events with distal ejecta or mass extinction related horizons e. Continuing attempts of high-precision age dating are warranted — in conjunction with sensitive petrographic analysis of best-suitable impactites — which is mandatory for obtaining high-quality chronological results. One technique of choice is the 40 Ar— 39 Ar method, which may allow to separate inherited Ar from the target rocks from the signature of reset impact melt rock or another type of impact-generated melt rock known as pseudotachylitic breccia, and may also allow to identify post-impact overprint due to hydrothermal alteration or post-impact thermally-induced loss of radiogenic Ar.
Due to the complex systems of precursor rock remnants, impact-related new phases, and post-impact overprint s , many dating attempts have remained unsuccessful. To combine two or more techniques may improve the chance to be successful Jourdan et al. The recent combination of in situ ion microprobe dating and EBSD micro-structural analysis of the specimen dated Moser et al.
Laboratory acceleration of projectiles onto targets of metal, minerals, or rocks is a powerful technique for investigating shock deformation effects produced in these materials under controlled physicochemical conditions, for comparison with those deformations produced in natural impact events.
That shock deformation effects have been observed not only in samples from terrestrial impact structures but also in lunar rocks and in meteorites including those derived from the Moon and Mars demonstrates that they are characteristic of impact deformation. Shock experiments also allow the calibration of the onset of formation of specific shock effects with precise shock pressures, thus providing a means to investigate the attenuation of shock pressure in natural impact structures. Reviews of the techniques e.
A compilation of shock effects vs. Much of the shock experimentation of previous decades was done with single-crystal mineral targets, and there is extensive scope for continuing experimental shock deformation with rocks. Limited studies have been conducted with target materials pre-heated to temperatures typically observed in the upper and middle crust e. Upper crustal target rocks are often porous and wet sediments, and their shock behavior is so far not known very well.
This dramatic lowering of onset pressure for melt phase generation is achieved due to shock front interaction with the pore space, whereby shock pressures can be locally elevated by factors up to 6 times the nominal experimental shock pressure Kowitz et al. An application to naturally occurring shock metamorphism is the comparison of these experimental results with the glass-bearing arenites of the deeply eroded central uplift of the Oasis impact structure in Libya see below, section 6. Shock micro-deformation features. Cross-polarized light. The area shown displays light-colored, locally vesiculated feldspar glass and dark-brown oxidic remnants after a mafic precursor mineral.
A clast of diaplectic quartz glass contains aggregates of tiny coesite crystals.
Impact structures in Africa: A review
Two distinct systems of planar fractures are clearly recognizable. This kind of shock deformation in refractory minerals such as zircon or monazite has proven invaluable to identify bona fide shock evidence in very old impact structures. Simulation of impact cratering with numerical modelling techniques e.
Such modelling is extremely useful in setting baselines for the formation of deformation effects that are actually observed in nature. Both processes related to the target volume and those related to the formation and dissemination of impact ejecta ballistic ejecta and impact plume materials can be investigated by these techniques. The comparison of modelling results and field and laboratory findings is a powerful technique that has significantly enhanced our understanding of impact cratering, in general.
And, what is more — on all scales! It is possible to set the basic parameters of numerical modelling, namely the cell size of target and projectile, to very different values, which allows to model energy distribution and material response at very different scales — from planetary-scale impact events down to the effects on hand specimen sized targets or even further to pore space scales. Ivanov ; see Fig. An example with several progressive steps in a modeled cratering experiment is shown in Fig. The mechanics of impact cratering has been described in detail by, for example, Grieve, , Melosh, , Melosh, , and Melosh and Ivanov , and useful introductions to this topic are provided by French, , French and Koeberl, , and most recently, by Collins et al.
One generally distinguishes — during the short interval of cratering — i contact and compression phase, ii excavation phase, and iii collapse and modification phase after Melosh, , French, ; see Fig. Contact and compression phase: The process of impact cratering begins upon contact of the projectile with the target Fig.
The main result of this initial phase is the transfer of the kinetic energy of the projectile to the target via a shock wave. It propagates hemispherically through the target, as well as backward into the projectile. The pressures produced are much larger than the yield strength of either the target or the projectile, so that most of the projectile and part of the target at the sub-surface are vaporized. Schematic representation of the formation of a complex crater after Grieve, and Mohr-Westheide, The initial stage of excavation and compression relates to the formation of the transient crater TC.
The strength-degraded crater floor rebounds to form a central uplift CU , while collapse of the transient cavity wall initiates an inward-directed material flow that combines with the upward-directed flow inherent to uplift formation. The CU collapses in stage c to form the central peak CP and — in even larger structures — the peak ring PR of the final complex crater morphology. As discussed in detail by Grieve et al. The duration of the pressure pulse depends on the projectile diameter. Pressure release occurs when the shock wave reaches the back-end of the projectile and is reflected back as a rarefaction wave, which travels slightly faster through the target material than the initial shock wave.
Naturally, shock pulse durations in natural impact events are much longer than those generated in shock experiments with projectiles orders of magnitude smaller. Consequently, the kinetics of shock propagation are of vital importance and anybody comparing the effects of natural and experimental impact ought to bear this in mind. The projectile will be transformed, almost entirely, to vapor, with the remainder being incorporated into impact-generated melt. Solid particles from the projectiles involved with natural impacts have been recovered on occasion some tiny particles at Chicxulub, on the sea-floor from the Eltanin impact, and a decimeter-sized meteorite fragment in the Morokweng impact melt rock — see below , but this is exceedingly rare.
Much of the impact melt may pool in the interior of the crater — in large events into crystalline targets resulting in coherent melt bodies often of sheet geometry , and some is incorporated into ejecta suevite outside of the crater, impact glass, tektites in the form of small melt particles. Stoeffler MfN Berlin, Generally, impacts into crystalline targets generate proportionally more melt than impacts into porous sedimentary rock, where much of the impact energy is expanded in closing the pore space while producing thermal energy.
The unloading of the projectile from high pressure constitutes the end of the contact and compression stage. The central uplift or peak ring structure is often more resistant to weathering than the surrounding crater fill of impact breccias and may, thus, be the only remnant of deeply eroded impact structures — e. Images of selected simple bowl-shape and complex terrestrial impact structures. The ones in the upper row and the left and right images in the center row are simple craters, the others are complex structures.
In order to explain the phenomenology of complex craters, theories of hydrodynamic response of the target material to hypervelocity impact have been invoked. Thereby, rheological properties of Bingham fluids are assumed for material beneath collapsing craters. The principle thought is that shocked rock volumes are set into vibration — equivalent to a material flow. Impact cratering workers have been debating how this fluidization process works at the atomic to grain scale in the affected rocks. For example, it has been discussed how it is possible for the Archean fabrics in the gneisses and migmatites of the core of the Vredefort Dome to be largely with the exception of zones of pseudotachylitic breccia development and fracturing at meso- to micro-scales preserved.
On the micro-scale, however, pervasive microfracturing is noted in the rocks of the Dome.
Cross sections through final simple-bowl-shape and complex impact structures are shown in Fig. D a — apparent diameter; d a — apparent depth; d t — true depth. Note that the distribution of melt-bearing and purely lithic breccias in the crater interior is only exemplary and schematic. D a — apparent crater diameter, after collapse of the transient cavity crater ; D cp — diameter of central peak structure.
Also indicated are estimated occurrences of diaplectic glasses, planar deformation features, and shatter cones. Crater filling breccias are composed of a mix of unshocked and shocked clasts, with melt particles. Impact-induced thermal overprint of rocks e. A series of cartoons Fig. Based on these time steps we are showing how such a moderately sized complex crater could have evolved. A series of cartoons representing time steps in the early development of a moderate size, complex crater and its twin — similar to the Ries Crater and Steinheim Basin situation in southern Germany.
A jet of tektites is ejected from the horizon just below the target surface. The vapor cloud begins to collapse while the ejecta curtain still travels outward. A central uplift is formed. The central uplift has collapsed and — in this scenario — has been replaced by a peak ring. Deposition of vapor plume material is still ongoing. The crater is filled with debris of all shock degrees.
Note that the processes concerning the final time step — development of the vapor plume and deposition of fallback into the crater and material mixing within the crater — are quite strongly debated at this time e. A most recent view Artemieva et al. Maybe already at this time, and through the crater collapse phase, there may be a secondary plume — the result of additional processes within the crater, perhaps indicating fuel—coolant interaction between an impact melt body and superheating water. This secondary plume may be much denser than the primary, and may only reach a height of a few kilometers, spreading outward.
According to N. The secondary plume propagates and collapses quite quickly, within minutes, but a major issue of discussion is when exactly the fuel—coolant inter-activity occurs, immediately after, or even during, crater collapse, or many years later. Also, possible sources of volatiles have remained debated. These models have been calculated for a typical Ries Crater size impact, and the authors have attempted to correlate modelling results and observations on the actual Ries impact breccia deposits.
Clearly, there are still uncertainties based on partial incongruence. Like many aspects of the highly dynamic impact process, especially formation and emplacement of polymict impact breccias see below remain to be fully understood. A detailed discussion of impact crater morphologies and the inherent nomenclature is given by Turtle et al. Impact crater morphology can be very different. It ranges from simple, bowl-shaped Fig. In contrast to volcanic crater structures, impact produces generally circular, shallow i.
Due to erosion and post-impact tectonic overprint, the primary morphology can, of course, be significantly changed. A case in point is the Vredefort impact structure that extends over the entire Witwatersrand basin. The NE—SW extension of the basin is clearly the result of post-impact tectonics, likely due to the collision of southern Africa and Antarctica in Kibaran Grenvillian times.
Also the Sudbury impact structure in Canada is extended from its original, more or less circular geometry, in NE—SW direction — again due to orogenic overprint during post-impact Grenvillian times. A distinctive feature of fresh impact craters is — in contrast to volcanic crater structures — that they have overturned after some erosion, still strongly upturned rim stratigraphy. The rims of volcanic craters are generally characterized by flat stratification or, at best, weak upturning of the uppermost strata.
Selection of complex impact structures on various bodies in the solar system. D The km-diameter Gosses Bluff impact structure, Australia, showing mostly the 6-km-diameter deeply eroded central uplift as a ring-structure in its center Landsat image. Small i. Their morphologies are frequently modelled as hyperbolae, although they, too, suffer to a degree from wall collapse so that the final crater shape, in comparison to the early transient crater form, is characterized by a somewhat wider and flatter geometry.
In contrast to BP that is entirely formed in sedimentary strata, the 3. For complex craters, the nature of the target seems to influence how wide and high a central uplift structure may become. The It has been discussed that they could have formed due to the downward and outward directed compression vectors, or alternatively, that they are the result of inward-motion of material in the course of central uplifting Wagner et al. The low, inward-directed dip of the BP ring fault would conform to the latter hypothesis Koeberl et al. Very large craters may exhibit concentric ring structures that allow classifying them as multi-ring impact basins Spudis, As discussed above, morphology and geophysical anomalies may provide hints at the possible presence of an impact structure, but they do not suffice as proof.
Contrary to this, evidence of shock metamorphism is diagnostic, and chemical evidence that demonstrates the presence of traces of an extraterrestrial projectile is similarly conclusive. Physical expressions of shock wave compression and immediately subsequent decompression are irreversible deformation effects, e. These deformation and transformation effects are collectively known as shock or impact metamorphism. In Fig.
It is important to remember that impact structures are mostly formed in upper crust, with only very large structures reaching into the middle or even lower crust. See text for detail. Estimated shock temperatures are also marked at boundaries between shock zones.
Progressive refers to the continuous increase of shock and temperature conditions in direction towards the point of impact i. Close to the point of impact, at generation of the shock front, temperature conditions are comparable to those on the surface of the Sun. Shock pressure may reach many hundreds, perhaps thousands of GPa. The bulk of affected target material will be vaporized instantaneously, as is much of the projectile.
Finally, formation of diaplectic glass syn. Clearly the bulk of the impact affected rock volume is not shocked above the HEL of the mineral constituents.
Sedimentary Geology of Mars
For this reason, extensive investigations are carried out on experimentally and naturally weakly shocked i. The impact-diagnostic value of these phenomena is still unclear. For example, it is not impossible that single sets of PFs be formed under tectonic conditions, but multiple sets of PFs of different crystallographic orientation formed in a quartz host grain would be atypical for tectonic deformation and may well be shown to signal an effect of impact. In addition, so-called feather features FFs have been noted in quartz in rocks from many impact structures but have only been rarely produced experimentally Poelchau and Kenkmann, , Kowitz et al.
No information of FFs found in tectonically deformed rock has been reported yet, which makes this a very promising shock-characteristic deformation phenomenon, but it has also not yet been fully investigated whether they may form under normal tectonic conditions. This is impossible to reconcile with the Poelchau and Kenkmann hypothesis that FFs are formed under shear stress. A further important observation has been reported in a number of papers on impact structures formed in sedimentary targets. For example, French et al.
Kowitz et al. Shock melting was noted preferably in pockets originally filled with phyllosilicate minerals. Further investigation of these glasses is in progress, but already now it can be surmised whether such glass formations may be diagnostic for low-shock overprint of porous sedimentary rocks.
A further type of melt produced in the low-shock experiments with porous sandstone concerns silica glass melt along shear fractures in the shock experimental sample assemblies, and are thought to reporesent the result of friction melting Kowitz et al. PDFs — planar deformation features formed in a range of important rock-forming minerals, including quartz, feldspars, olivine — are the most widely applied recognition criterion for shock metamorphism.
Detailed scanning electron microscopy e. When PDFs are thermally overprinted post-impact annealing , the original glassy phase becomes annealed and remains only recognizable due to the straight trails of fluid inclusions exsolved from the primary phase, which still mirror the original locations of PDFs. Other impact-diagnostic thermal alteration features Fig.
Checkerboard feldspar Fig. An equivalent to checkerboard feldspar, occurring in quartz, was described by Buchanan and Reimold Based on the different shock textures produced at different shock pressure levels, shock metamorphic schemes have been set up for different minerals e. Table 1 after Reimold, shows such generalized schemes for quartz and feldspar, and in Fig. Shock-induced thermal alteration. Plane polarized light. Note the typical H-shape plagioclase crystals in the matrix that indicate crystallization under relatively fast cooling.
The plagioclase clast has been partially melted by the superheated impact melt, with melt channels developed predominantly along crystallographic orientations. The continuous outer rim of the clast indicates reaction with the surrounding melt. Note the crystallographically controlled narrow zones of feldspathic melt dark grey between rectangular remnants of the original crystal.
The right part of the image shows garben texture of finest-grained crystals grown from the melt matrix of the Granophyre. Shock metamorphic effects in quartz and feldspar, in relation to increasing shock pressure. Note: At shock pressures in excess of 50 GPa bulk rock melting becomes important. The limits of the shock pressure regimes quoted are strongly dependent on the pre-shock temperature of the target rock. The information tabled is based on the findings of Huffman and Reimold and Grieve et al. A comprehensive scheme of shock pressures vs.
The mineral quartz is the preferred study object for the search of shock metamorphic evidence. Above all, quartz displays a range of shock metamorphic effects that are shock barometers well calibrated by shock experiments: PFs, mosaicism, PDFs, diaplectic quartz glass, high-pressure polymorphs, and finally quartz fusion. PDF analysis is, furthermore, facilitated by the uniaxial character of this mineral that allows straightforward determination of crystallographic orientations by universal-stage or spindle-stage analysis.
At elevated shock conditions, certain minerals may be transformed to otherwise rare or non-existent high-pressure polymorphs. Coesite and stishovite have been found in many impact structures, with coesite observed under crustal conditions as well in kimberlite and in rocks associated with subduction zones but stishovite being only known from impact structures. Reidite, the high-pressure polymorph with scheelite-structure after zircon has also been observed widely in impact structures.
At even higher impact conditions, zircon will dissociate to silica plus baddeleyite ZrO 2. Where target rock contains graphite, chances are that this mineral may be converted to shock diamond or lonsdaleite. A host of high-pressure polymorphs is also known from shocked meteorites, including high-pressure phases of feldspar, pyroxene, olivine, and other minerals e. It must be expressly emphasized here that shock deformation is a very heterogeneous phenomenon shock heterogeneity due to the complex behavior of compression waves in a heterogeneous, multi-particle and — possibly — texturally complex rock.
Waves may be reflected, scattered, or refracted, resulting in shock attenuation or amplification. Unshocked and highly shocked grains may occur in shocked target rock right next to each other, with assemblages of grains of all shock stages in microscopic volumes being possible.
This holds for shocked and unshocked particles in impact breccias next section and shocked rock, as well as clasts in impact melt rock, which normally exhibit a wide range of shock metamorphic conditions, from unshocked to partially or wholly melted. Different minerals display different affinity for multiple shock effect generation. In contrast, pyroxene does not allow correlating deformation effects with different levels of shock pressure.
Intragranular fracturing, enhanced cleavage, and some twinning are the only shock effects known for this mineral, and their impact-diagnostic value is, at best, limited. Zircon and monazite are also very useful: due to their strong refractivity, they are the minerals of choice when searching for shock deformation in old, Proterozoic and Archean, rocks that may have been subjected to polymetamorphism and extensive alteration. In addition, these two minerals are highly useful for U—Th—Pb-based geochronology.
In accordance with the different shock behaviors of minerals, different rock types also display different shock behavior. Where they are available, it is desirable to sample impact breccias next section for investigation of possible evidence for shock metamorphism. Where, however, only monomict clastic breccia or unbrecciated rock can be sampled, it may still be possible to detect shocked minerals — albeit in comparatively lower abundance.
It must also be carefully observed that there can be other mineral deformation styles that are not impact-diagnostic, such as non-planar features including: fractures, fluid inclusion trails, deformation bands of tectonic origin. Great care must be taken not to confuse these with bona fide shock microdeformation. The literature is full of erroneous reports of shock deformation, which unfortunately is continuously promulgated due to still persisting widespread lack of proper tuition about impact deformation and the requisite methodology for their investigation.
A recent example for this is the report of a possible impact structure at Maniitsoq Greenland by Garde et al. Despite repetitive publication of their alleged PDFs from this site, the impact-relevance of these alleged shock features did not improve. Reimold et al. Kowitz and W.
A Record of Large Impacts in Sedimentary Deposits
They noted several — in comparison with PDFs — broad and dense fluid inclusion trails. When tilting the thin section on the U-stage by ca. Thus, some features that with normal optical microscopic observation will be — and have been in the past! On the contrary, there are cases of previously published supposed PDFs that must be readdressed by proper methodology.
Reliable mega- to macroscopic shock deformation occurs only in the form of shatter cones Fig. Such apical areas may measure just a millimeter or two in width, or may involve centimeter- to decimeter-wide planar areas. Striations may also converge towards a fracture plane that acted as generation plane. Shatter cones are known from a large number of impact structures and as they have not been observed in any other settings, are considered diagnostic of impact structures.
However, care must be taken in their recognition, as some other fracture phenomena e. Comparison of shatter cones and other striated fracture phenomena. The late impact researcher Jared Morrow for scale. Note the well displayed relationships between joints and cone structures in a and b. Lugli et al. Note the striated cone features in the upper part of the specimen. A number of hypotheses have been promoted for the origin of shatter cones, including shock wave scattering or refraction on heterogeneities such as pebbles, pores, or fractures; interference of multiple joint sets of distinct curviplanar geometries.
These ideas have been discussed most recently by Sagy et al. Planar deformation features have been found in many shatter cone-bearing rocks, and Nicolaysen and Reimold reported the presence of microscopic glass development on fractures inside of a shatter cone specimen — thus, suggesting that a shear component was involved in the formation of such striated fractures, resulting in local friction melting.
Nicolaysen and Reimold also drew attention to the apparent relationship between shatter cones and more linear arrangements of striations on fracture surfaces actually already recorded by Manton, They mapped out up to a dozen different sets of multipli-repeated curviplanar fractures in specific rock volumes in the Vredefort Dome. Their detailed fieldwork resulted in the observation that orientations of shatter cone striations seemingly are related to MSJS trace intersections on stereoplots.
To date, the actual relationship between shatter cones and MSJS has, however, not yet been clarified. Small shatter cones have also been produced experimentally. More recently, Zaag et al. The largest manifestations of shatter cones are known from the Slate Islands impact structure in Canada Sharpton et al. While it is desirable to have shock metamorphic indicators that are easily observed, preferably by widely accessible optical microscopy, it has been frequently necessary to revert to not as readily available, sophisticated but time-consuming transmission electron microscopy TEM to resolve features not explainable by optical microscopy alone or that have been controversial as shock features or tectonic deformation, or where the origin was obscured due to metamorphic overprint.
Recent advances of scanning electron microscopy SEM and associated techniques such as color or color composite cathodo-luminescence CL study or electron backscatter diffraction EBSD have lead to an enlarged arsenal for shock metamorphic investigations. Another advantage of SEM applications is that the study of polished thin sections with such an instrument allows immediate correlation of optical and electron optical microscopic observations on the same specimen. Pioneering work in this regard on quartz has been carried out by Hamers ; also Hamers and Drury, They found that PDF are readily distinguished from tectonic deformation lamellae by both limited wavelength grayscale and composite color SEM-CL analysis.
CL signals of tectonic lamellae range from blue to red. Several causes for the red luminescence of PDFs are discussed by the authors. This red luminescence was interpreted as due to damage caused by the electron beam preferentially along dislocations, fluid inclusions, and twin boundaries. Electron-backscatter diffraction EBSD analysis, coupled with electron microscopic techniques, has also been employed for detailed shock metamorphic analysis of zircon by, e.
SEM techniques now have the potential to bridge the gap between optical and transmission electron microscopy. Within and around impact structures, a series of lithologies formed as a direct consequence of the impact event occur prior to erosion. These authors distinguished proximal — occurring within and just around a crater — and distal impact facies. The latter comprises ejecta that can be distributed — according to the magnitude of the impact event — regionally around a crater structure, or on continental and even global scales.
This includes the tektite and microtektite occurrences of many hundreds to thousands of kilometers extent. Four tektite strewn fields are currently known on Earth Fig. Several sites in Cambodia have so far been investigated in vain.