Impactites of the Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— Contrary to the previous interpretation of a single allochthonous impactite lithology, combined field, optical, and analytical scanning electron microscopy (SEM) studies have revealed the presence of a series of impactites at the Haughton impact structure. In the crater interior, there is a consistent upward sequence from parautochthonous target rocks overlain by parautochthonous lithic (monomict) breccias, through allochthonous lithic (polymict) breccia, into pale grey allochthonous impact melt breccias. The groundmass of the pale grey impact melt breccias consists of microcrystalline calcite, silicate impact melt glass, and anhydrite. Analytical data and microtextures indicate that these phases represent a series of impact‐generated melts that were molten at the time of, and following, deposition. Impact melt glass clasts are present in approximately half of the samples studied. Consideration of the groundmass phases and impact glass clasts reveal that impactites of the crater interior contain shock‐melted sedimentary material from depths of >920 to <1880 m in the pre‐impact target sequence. Two principal impactites have been recognized in the near‐surface crater rim region of Haughton. Pale yellow‐brown allochthonous impact melt breccias and megablocks are overlain by pale grey allochthonous impact melt breccias. The former are derived from depths of >200 to <760 m and are interpreted as remnants of the continuous ejecta blanket. The pale grey impact melt breccias, although similar to the impact melt breccias of the crater interior, are more carbonate‐rich and do not appear to have incorporated clasts from the crystalline basement. Thus, the spatial distribution of the crater‐fill impactites at Haughton, the stratigraphic succession from target rocks to allochthonous impactites, the recognition of large volumes of impact melt breccias, and their probable original volume are all analogous to characteristics of coherent impact melt layers in comparatively sized structures formed in crystalline targets.     

Mid‐sized complex crater formation in mixed crystalline‐sedimentary targets: Insight from modeling and observation

Abstract— Large impact crater formation is an important geologic process that is not fully understood. The current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets. Real targets are rarely uniform; for example, the majority of Earth's surface is covered by sedimentary rocks and/or a water layer. The ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process. To advance understanding of the mechanics of crater collapse, and the effect of variations in target properties on crater formation, the first “Bridging the Gap” workshop recommended that geological observation and numerical modeling focussed on mid‐sized (15–30 km diameter) craters on Earth. These are large enough to be complex; small enough to be mapped, surveyed and modelled at high resolution; and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables. In this paper, we compare observations and numerical models of three 18–26 km diameter craters formed in different target lithology: Ries, Germany; Haughton, Canada; and El'gygytgyn, Russia. Based on the first‐order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid‐sized complex crater formation in a subaerial, mixed crystalline‐sedimentary target. We compared our results with interpreted geological profiles of Ries and Haughton, based on detailed new and published geological mapping and published geophysical surveys. Our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre‐impact sedimentary cover in each case. We conclude that the presence of an inner ring at Ries, and not at Haughton, is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post‐impact erosion. For constant impact energy, transient and final crater diameters increase with increasing sediment thickness.     

Geological overview and cratering model for the Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— The Haughton impact structure has been the focus of systematic, multi‐disciplinary field and laboratory research activities over the past several years. Regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to ?1880 m. New ⁴⁰Ar–³⁹Ar dates place the impact event at ?39 Ma, in the late Eocene. Haughton has an apparent crater diameter of ?23 km, with an estimated rim (final crater) diameter of ?16 km. The structure lacks a central topographic peak or peak ring, which is unusual for craters of this size. Geological mapping and sampling reveals that a series of different impactites are present at Haughton. The volumetrically dominant crater‐fill impact melt breccias contain a calcite‐anhydrite‐silicate glass groundmass, all of which have been shown to represent impact‐generated melt phases. These impactites are, therefore, stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets. The crater‐fill impactites provided a heat source that drove a post‐impact hydrothermal system. During this time, Haughton would have represented a transient, warm, wet microbial oasis. A subsequent episode of erosion, during which time substantial amounts of impactites were removed, was followed by the deposition of intra‐crater lacustrine sediments of the Haughton Formation during the Miocene. Present‐day intra‐crater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the High Arctic. Modern modification of the landscape is dominated by seasonal regional glacial and niveal melting, and local periglacial processes. The impact processing of target materials improved the opportunities for colonization and has provided several present‐day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain.     

The Obolon impact structure,Ukraine, and its ejecta deposits

Abstract— The Obolon impact structure, 18 km in diameter, is situated at the northeastern slope of the Ukrainian Shield near its margin with the Dnieper‐Donets Depression. The crater was formed in crystalline rocks of the Precambrian basement that are overlain by marine Carboniferous and continental Lower Triassic deposits. The post‐impact sediments comprise marine Middle Jurassic (Bajocian and Bathonian) and younger Mesozoic and Cenozoic deposits. Today the impact structure is buried beneath an about 300‐meter‐thick sedimentary rock sequence. Most information on the Obolon structure is derived from two boreholes in the western part of the crater. The lowest part of the section in the deepest borehole is composed by allogenic breccia of crystalline basement rocks overlain by clast‐rich impact melt rocks and suevites. Abundant shock metamorphic effects are planar deformation features (PDFs) in quartz and feldspars, kink bands in biotite, etc. Coesite and impact diamonds were found in clast‐rich impact melt rocks. Crater‐fill deposits are a series of sandstones and breccias with blocks of sedimentary rocks that are covered by a layer of crystalline rock breccia. Crystalline rock breccias, conglomeratic breccias, and sandstones with crystalline rock debris have been found in some boreholes around the Obolon impact structure to a distance of about 50 km from its center. Those deposits are always underlain by Lower Triassic continental red clay and overlain by Middle Jurassic marine clay. The K‐Ar age of impact melt glasses is 169 Ma, which corresponds to the Middle Jurassic (Bajocian) age. The composition of crater‐fill rocks within the crater and sediments outside the Obolon structure testify to its formation under submarine conditions.     

Shock metamorphism and impact melting in small impact craters on Earth: Evidence from Kamil crater,Egypt

Kamil is a 45 m diameter impact crater identified in 2008 in southern Egypt. It was generated by the hypervelocity impact of the Gebel Kamil iron meteorite on a sedimentary target, namely layered sandstones with subhorizontal bedding. We have carried out a petrographic study of samples from the crater wall and ejecta deposits collected during our first geophysical campaign (February 2010) in order to investigate shock effects recorded in these rocks. Ejecta samples reveal a wide range of shock features common in quartz‐rich target rocks. They have been divided into two categories, as a function of their abundance at thin section scale: (1) pervasive shock features (the most abundant), including fracturing, planar deformation features, and impact melt lapilli and bombs, and (2) localized shock features (the least abundant) including high‐pressure phases and localized impact melting in the form of intergranular melt, melt veins, and melt films in shatter cones. In particular, Kamil crater is the smallest impact crater where shatter cones, coesite, stishovite, diamond, and melt veins have been reported. Based on experimental calibrations reported in the literature, pervasive shock features suggest that the maximum shock pressure was between 30 and 60 GPa. Using the planar impact approximation, we calculate a vertical component of the impact velocity of at least 3.5 km s^?1. The wide range of shock features and their freshness make Kamil a natural laboratory for studying impact cratering and shock deformation processes in small impact structures.     

The terrestrial cratering record: I. Current status of observations

The location, size, and principal characteristics of the currently known proven and probable terrestrial impact structures are tabulated. Of the 78 known probable structures, only 3 are Precambrian and the majority are <300 my in age. A survey of the variation in preservation with size and age indicates that, unless protected by sedimentary cover, a structure <20 km in diameter has a recognizable life of <600 my. The depth-diameter relationships of terrestrial structures are similar to lunar craters; however, it is believed that terrestrial craters were always shallower than their lunar counterparts. Complex structures formed in sedimentary targets are shallower than those in crystalline targets, and the transition from simple to complex crater morphology occurs in sedimentary strata at approximately one-half the diameter of the morphology transition in crystalline rocks. This is a reflection of target strength. Although observations indicate that crater size, target strength, and surface gravity are variables in the formation of complex craters, they do not permit an unequivocal choice between collapse and rebound processes for the formation of complex structures. It may be that both processes act together in the modification of crater morphology during the later stages of excavation. The major emphasis of recent shock metamorphic studies has been toward the development of models of cratering processes. An important contribution has been the identification, through meteoritic contamination in the melt rocks, of the type of bolide at a number of probable impact structures. This has served to strengthen the link between the occurrence of shock metamorphic effects and their origin by hypervelocity meteorite impact.     

Geology,petrography, shock petrography,and geochemistry of impactites and target rocks from the Kärdla crater,Estonia

Abstract— The Kärdla crater is a 4 km‐wide impact structure of Late Ordovician age located on Hiiumaa Island, Estonia. The 455 Ma‐old buried crater was formed in shallow seawater in Precambrian crystalline target rocks that were covered with sedimentary rocks. Basement and breccia samples from 13 drill cores were studied mineralogically, petrographically, and geochemically. Geochemical analyses of major and trace elements were performed on 90 samples from allochthonous breccias, sub‐crater and surrounding basement rocks. The breccia units do not include any melt rocks or suevites. The remarkably poorly mixed sedimentary and crystalline rocks were deposited separately within the allochthonous breccia suites of the crater. The most intensely shockmetamorphosed allochthonous granitoid crystalline‐derived breccia layers contain planar deformation features (PDFs) in quartz, indicating shock pressures of 20–35 GPa. An apparent K‐enrichment and Ca‐Na‐depletion of feldspar‐ and hornblende‐bearing rocks in the allochthonous breccia units and sub‐crater basement is interpreted to be the result of early stage alteration in an impact‐induced hydrothermal system. The chemical composition of the breccias shows no definite sign of an extraterrestrial contamination. By modeling of the different breccia units with HMX‐mixing, the indigenous component was determined. From the abundances of the siderophile elements (Cr, Co, Ni, Ir, and Au) in the breccia samples, no unambiguous evidence for the incorporation of a meteoritic component above about 0.1 wt% chondrite‐equivalent was found.     

Impact‐induced impoverishment and transformation of a sandstone habitat for lithophytic microorganisms

Abstract— Sandstones are a common habitat for lithophytic microorganisms, including cryptoendoliths. We describe laboratory experiments on the colonization of impact metamorphosed sandstones from the Haughton impact structure, Canadian High Arctic. Colonization experiments with the coccoid cyanobacterium, Chroococcidiopsis sp. and the motile gram‐positive bacterium Bacillus subtilis, show that, in contrast to initially low porosity crystalline target rocks, which can become more porous as a result of impact bulking, by closing pore spaces the sedimentary cryptoendolithic habitat can be impoverished by impact. However, the heterogeneous distribution of collapsed pores, melt phases, and subsequent recrystallization, results in heterogeneous colonization patterns. Cavities and vesicles formed during melting can yield new habitats for both cryptoendoliths and chasmoendoliths, manifested in the natural cryptoendolithic colonization of shocked sandstones. By contrast, post‐impact thermal annealing and recrystallization of impact melt phases destroys the cavities and vesicles. In extreme cases, complete recrystallization of the rock fabric makes the material suitable only for epilithic, and potentially hypolithic, colonists. These experiments further our understanding of the influence of the target lithology on the effects of asteroid and comet impacts on habitats for lithophytic microorganisms.     

Melt in the impact breccias from the Eyreville drill cores,Chesapeake Bay impact structure,USA

The center of the 35.3 Ma Chesapeake Bay impact structure (85 km diameter) was drilled during 2005/2006 in an ICDP–USGS drilling project. The Eyreville drill cores include polymict impact breccias and associated rocks (1397–1551 m depth). Tens of melt particles from these impactites were studied by optical and electron microscopy, electron microprobe, and microRaman spectroscopy, and classified into six groups: m1—clear or brownish melt, m2—brownish melt altered to phyllosilicates, m3—colorless silica melt, m4—melt with pyroxene and plagioclase crystallites, m5—dark brown melt, and m6—melt with globular texture. These melt types have partly overlapping major element abundances, and large compositional variations due to the presence of schlieren, poorly mixed melt phases, partly digested clasts, and variable crystallization and alteration. The different melt types also vary in their abundance with depth in the drill core. Based on the chemical data, mixing calculations were performed to determine possible precursors of these melt particles. The calculations suggest that most melt types formed mainly from the thick sedimentary section of the target sequence (mainly the Potomac Formation), but an additional crystalline basement (schist/gneiss) precursor is likely for the most abundant melt types m2 and m5. Sedimentary rocks with compositions similar to those of the melt particles are present among the Eyreville core samples. Therefore, sedimentary target rocks were the main precursor of the Eyreville melt particles. However, the composition of the melt particles is not only the result of the precursor composition but also the result of changes during melting and solidification, as well as postimpact alteration, which must also be considered. The variability of the melt particle compositions reflects the variety of target rocks and indicates that there was no uniform melt source. Original heterogeneities, resulting from melting of different target rocks, may be preserved in impactites of some large impact structures that formed in volatile‐rich targets, because no large melt body exists, in which homogenization would have taken place.     

Hydrocode modeling of the Sierra Madera impact structure

Abstract— We present the first hydrocode simulations of the formation of the Sierra Madera structure (west Texas, USA), which was caused by an impact into a thick sedimentary target sequence. We modeled Sierra Madera using the iSALE hydrocode, and here we present two best‐fit models: 1) a crater with a rim (final crater) diameter of ?12 km, in agreement with previous authors' interpretations of the original structure, and 2) a crater ?16 km in diameter with increased postimpact erosion. Both models fit some of the geologic observational data, but discrepancies with estimates of peak shock pressure, extent of deformation, and stratigraphic displacement remain. This study suggests that Sierra Madera may be a larger crater than previously reported and illustrates some of the challenges in simulating impact deformation of sedimentary lithologies. As many terrestrial craters possess some amount of sedimentary rocks in the target sequence, numerical models of impacts into sedimentary targets are essential to our understanding of target rock deformation and the mechanics of crater formation.     

Ries crater and suevite revisited—Observations and modeling Part II: Modeling

We present the results of numerical modeling of the formation of the Ries crater utilizing the two hydrocodes SOVA and iSALE. These standard models allow us to reproduce crater shape, size, and morphology, and composition and extension of the continuous ejecta blanket. Some of these results cannot, however, be readily reconciled with observations: the impact plume above the crater consists mainly of molten and vaporized sedimentary rocks, containing very little material in comparison with the ejecta curtain; at the end of the modification stage, the crater floor is covered by a thick layer of impact melt with a total volume of 6–11 km³; the thickness of true fallback material from the plume inside the crater does not exceed a couple of meters; ejecta from all stratigraphic units of the target are transported ballistically; no separation of sedimentary and crystalline rocks—as observed between suevites and Bunte Breccia at Ries—is noted. We also present numerical results quantifying the existing geological hypotheses of Ries ejecta emplacement from an impact plume, by melt flow, or by a pyroclastic density current. The results show that none of these mechanisms is consistent with physical constraints and/or observations. Finally, we suggest a new hypothesis of suevite formation and emplacement by postimpact interaction of hot impact melt with water or volatile‐rich sedimentary rocks.     

Heterogeneity of melts in impact deposits and implications for their origin (Ries suevite,Germany)

Impact melt‐bearing clastic deposits (suevites) are one of the most important records of the impact cratering process. A deeper understanding of their composition and formation is therefore essential. This study focuses on impact melt particles in suevite at Ries, Germany. Textures and chemical evidence indicate that the suevite contains three melt types that originate from different shock levels in the target. The most abundant melt type (“melt type 1”) represents well‐mixed whole‐rock melting of crystalline basement and includes incompletely mixed mafic melt schlieren (“melt type 1 mafic”). Polymineralic melt type 2 comprises mixes between monomineralic melt types 3 and melt type 1. Melt types 2 and 3 are located within melt type 1 as small patches or schlieren but also isolated within the suevite matrix. The main melt type 1 is heterogeneous with respect to trace elements, varying geographically around the crater: in the western sector, it has lower values in trace elements, e.g., Ba, Zr, Th, and Ce, than in the eastern sector. The west–east zoning likely reflects the heterogeneous nature of crystalline basement target rocks with lower trace element contents, e.g., Ba, Zr, Th, and Ce, in the west compared to the east. The chemical zoning pattern of suevite melt type 1 indicates that mixing during ejection and emplacement occurred only on a local (hundreds of meters) scale. The incomplete larger scale mixing indicated by the preservation of these local chemical signatures, and schlieren corroborate the assumption that mixing, ejection, and quenching were very rapid, short‐lived processes.     

Intra‐crater sedimentary deposits at the Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— Detailed field mapping has revealed the presence of a series of intra‐crater sedimentary deposits within the interior of the Haughton impact structure, Devon Island, Canadian High Arctic. Coarse‐grained, well‐sorted, pale gray lithic sandstones (reworked impact melt breccias) unconformably overlie pristine impact melt breccias and attest to an episode of erosion, during which time significant quantities of impact melt breccias were removed. The reworked impact melt breccias are, in turn, unconformably overlain by paleolacustrine sediments of the Miocene Haughton Formation. Sediments of the Haughton Formation were clearly derived from pre‐impact lower Paleozoic target rocks of the Allen Bay Formation, which form the crater rim in the northern, western, and southern regions of the Haughton structure. Collectively, these field relationships indicate that the Haughton Formation was deposited up to several million years after the formation of the Haughton crater and that they do not, therefore, represent an immediate, post‐impact crater lake deposit. This is consistent with new isotopic dating of impactites from Haughton that indicate an Eocene age for the impact event (Sherlock et al. 2005). In addition, isolated deposits of post‐Miocene intra‐crater glacigenic and fluvioglacial sediments were found lying unconformably over remnants of the Haughton Formation, impact melt breccias, and other pre‐impact target rock formations. These deposits provide clear evidence for glaciation at the Haughton crater. The wealth and complexity of geological and climatological information preserved as intra‐crater deposits at Haughton suggests that craters on Mars with intra‐crater sedimentary records might present us with similar opportunities, but also possibly significant challenges.     

Hornblende alteration and fluid inclusions in Kärdla impact crater,Estonia: Evidence for impact‐induced hydrothermal activity

Abstract— The well‐preserved Kärdla impact crater, on Hiiumaa Island, Estonia, is a 4 km diameter structure formed in a shallow Ordovician sea ?455 Ma ago into a target composed of thin (?150 m) unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites, amphibolites and gneisses. The fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals. The most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift. Alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater. Chloritic mineral associations suggest formation temperatures of 100–300 °C, in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150–300 °C in allochthonous breccia. The rather low salinity of fluids in Kärdla crater (<13 wt% NaCl_eq) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the crater rim walls raised above sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement. The well‐developed hydrothermal system in Kärdla crater shows that the thermal history of the shock‐heated and uplifted rocks in the central crater area, rather than cooling of impact melt or suevite sheets, controlled the distribution and intensity of the impact‐induced hydrothermal processes.     

Structural evolution of the 40 km wide Araguainha impact structure,central Brazil

Abstract— The 40 km wide Araguainha structure in central Brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation. The dominant structural features preserved at Araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity. Slumping of the transient cavity walls resulted in inward‐verging inclined folds and a km‐scale anticline in the outer ring of the structure. The folding stage was followed by radial and concentric faulting, with downward displacement of kilometer‐scale blocks around the crater rim. The central uplift records evidence for km‐scale upward movement of crystalline basement rocks from the transient cavity floor, and lateral moment of sedimentary target rocks detached from the cavity walls. Much of the structural grain in the central uplift relates to structural stacking of km‐scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks. Outward‐plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets. Each individual sheet records an early stage of folding and thickening due to non‐coaxial strains, shortly before sheet imbrication. We attribute this folding and thickening phase to the kilometer‐scale inward movement of the target strata from the transient cavity walls to the central uplift. The outer parts of the central uplift record additional outward movement of the target rocks, possibly related to the collapse of the central uplift. An inner ring structure at 10–12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks.     

Carbonate-silicate liquid immiscibility upon impact melting: Ries Crater,Germany

Abstract— The 24 km diameter Ries impact crater in southern Germany is one of the most studied impact structures on Earth. The Ries impactor struck a Triassic to Upper Jurassic sedimentary sequence overlying Hercynian crystalline basement. At the time of impact (14.87 × 0.36 Ma; Storzer et al., 1995), the 350 m thick Malm limestone was present only to the south and east of the impact site. To the north and west, the Malm had been eroded away, exposing the underlying Dogger and Lias. The largest proportion of shocked target material is in the impact-melt-bearing breccia suevite. The suevite had been believed to be derived entirely from the crystalline basement. Calcite in the suevite has been interpreted as a postimpact hydrothermal deposit. From optical inspection of 540 thin sections of suevite from 32 sites, I find that calcite in the suevite shows textural evidence of liquid immiscibility with the silicate impact melt. Textural evidence of liquid immiscibility between silicate and carbonate melt in the Ries suevite includes carbonate globules within silicate glass, silicate globules embedded in carbonate, deformable and coalescing carbonate spheres within silicate glass, sharp menisci or cusps and budding between silicate and carbonate melt, fluidal textures and gas vesicles in carbonate schlieren, a quench crystallization sequence of the carbonate, spinifex textured quenched carbonate, separate carbonate spherules in the suevite mineral-fragment matrix, and inclusions of mineral fragments suspended in carbonate blebs. Given this evidence of liquid immiscibility, the carbonate in the suevite therefore has—like the silicate melt—a primary origin by impact-shock melting. Evidence of carbonate-silicate liquid immiscibility is abundant in the suevites from the southwest to east of the Ries crater. The rarer suevites to the west to northeast of the crater are nearly devoid of carbonate melts. This correspondence between the occurrence of outcropping limestones at the target surface and the formation of carbonate melt indicates that the Malm limestones are the source rocks of the carbonate impact melt. This correspondence shows that the suevites preserve a compositional memory of their source rocks. From the regional distribution of suevites with or without immiscible carbonate melts, it is inferred that the Ries impactor hit the steep Albtrauf escarpment at its toe, in an oblique impact from the north.     

Target rocks,impact glasses,and melt rocks from the Lonar impact crater,India: Petrography and geochemistry

Abstract— The Lonar crater, India, is the only well‐preserved simple crater on Earth in continental flood basalts; it is excavated in the Deccan trap basalts of Cretaceous‐Tertiary age. A representative set of target basalts, including the basalt flows excavated by the crater, and a variety of impact breccias and impact glasses, were analyzed for their major and trace element compositions. Impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes. Comparable geochemical patterns of immobile elements (e.g., REEs) for glass, melt rock and basalt indicates minimal fractionation between the target rocks and the impactites. We found only little indication of post‐impact hydrothermal alteration in terms of volatile trace element changes. No clear indication of an extraterrestrial component was found in any of our breccias and impact glasses, indicating either a low level of contamination, or a non‐chondritic or otherwise iridium‐poor impactor.     

Characterization and morphological reconstruction of the Terny impact structure,central Ukraine

The Terny impact structure, located in central Ukraine, displays a variety of diagnostic indicators of shock metamorphism, including shatter cones, planar deformation features in quartz, diaplectic glass, selective melting of minerals, and whole rock melting. The structure has been modified by erosion and subsequently buried by recent sediments. Although there are no natural outcrops of the deformed basement rocks within the area, mining exploration has provided surface and subsurface access to the structure, exposing impact melt rocks, shocked parautochthonous target rocks, and allochthonous impact breccias, including impact melt‐bearing breccias similar to suevites observed at the Ries structure. We have collected and studied samples from surface and subsurface exposures to a depth of approximately 750 m below the surface. This analysis indicates the Terny crater is centered on geographic coordinates 48.13° N, 33.52° E. The center location and the distribution of shock pressures constrain the transient crater diameter to be no less than approximately 8.4 km. Using widely accepted morphometric scaling relations, we estimate the pre‐erosional rim diameter of Terny crater to be approximately 16–19 km, making it close in original size to the well‐preserved El'gygytgyn crater in Siberia. Comparison with El'gygytgyn yields useful insights into the original morphology of the Terny crater and indicates that the amount of erosion Terny experienced prior to burial probably does not exceed 320 m.     

Steinheim suevite—A first report of melt‐bearing impactites from the Steinheim Basin (SW Germany)

Abstract– The 3.8 km Steinheim Basin in SW Germany is a complex impact crater with central uplift hosted by a sequence of Triassic to Jurassic sedimentary rocks. It exhibits a well‐preserved crater morphology, intensely brecciated limestone blocks that form the crater rim, as well as distinct shatter cones in limestones. In addition, an impact breccia mainly composed of Middle to Upper Jurassic limestones, marls, mudstones, and sandstones is known from drilling into the impact crater. No impact melt lithologies, however, have so far been reported from the Steinheim Basin. In samples of the breccia that were taken from the B‐26 drill core, we discovered small particles (up to millimeters in size) that are rich in SiO₂ (～50 wt%) and Al₂O₃ (～28 wt%), and contain particles of Fe‐Ni‐Co sulfides, as well as target rock clasts (shocked and unshocked quartz, feldspar, limestone) and droplet‐shaped particles of calcite. The particles exhibit distinct flow structures and relicts of schlieren and vesicles. From the geochemical composition and the textural properties, we interpret these particles as mixed silicate melt fragments widely recrystallized, altered, and/or transformed into hydrous phyllosilicates. Furthermore, we detected schlieren of lechatelierite and recrystallized carbonate melt. On the basis of impactite nomenclature, the melt‐bearing impact breccia in the Steinheim Basin can be denominated as Steinheim suevite. The geochemical character of the mixed melt particles points to Middle Jurassic sandstones (“Eisensandstein” Formation) that crop out at the center of the central uplift as the source for the melt fragments.