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Shock processing via impacts is a common occurrence during planet formation and evolution. Characterizing how rocks and minerals respond to extreme conditions is critical to understand and model realistic impact events.First, we review SiO2 at high pressure, including static compression and ab initio calculations of the melt curve and solid phase boundaries, and a particular focus on shock wave data. The first shock temperature measurements on α-quartz and fused silica displayed unusual drops in temperature with increasing pressure. The temperature excursions were interpreted as metastable superheating of the solid phase followed by a drop in temperature to the equilibrium melt curve. For several decades, this superheating interpretation has been accepted and inferred to occur in other materials. Upon compilation of available data, we propose a new interpretation of the melt curve and the SiO2 Hugoniot, focusing on the correlation of shock data with solid phase changes and recent static melt curve experiments. Finally, we suggest superheating does not occur and this feature can be explained by changes in the solid phases and physical properties of the liquid along the melt curve. We propose that the shock Hugoniots provides direct information about the melt curve in agreement with static measurements, including regions with negative slopes in temperature with increasing pressure.Next, we performed a series of gas gun experiments on quartz and fused silica, one of the major components of silicates and a common material on the surfaces of evolved bodies and major component of rocky bodies. We probed the thermodynamic region where these materials undergo melting with a focus on measuring shock and release temperatures. We found that the thermal histories of these samples provide insight on the phase transitions of these materials as well as support for proposed changes to the SiO2 melt curve and understanding of superheating or lack thereof.Finally, we performed a series of experiments on the pyroxenes enstatite (MgSiO3) and bronzite ((Mg,Fe)SiO3 and a pyrolitic (olivine + pyroxene) composition glass at the Sandia Z Machine. These experiments reach higher pressures than are available to gas gun platforms. Z experiments reach pressures similar to peak giant impact conditions and planetary interiors. We focused on defining the principal Hugoniots with temperatures. These materials are incredibly useful for understanding planet formation and impact modeling as pyroxene is the most common silicate in the mantle and pyrolite represents the composition of Earth’s mantle or a quenched magma ocean. Additionally, these materials are colored and contain iron, which have non-ideal optical properties for typical pyrometry diagnostics. With rigorous consideration of wavelength-dependent absorption, we determined temperatures and compared to similar experiments on the OMEGA EP laser platform that uses an alternative pyrometry diagnostic.

The Keurusselka structure is one of Finland's 12 meteorite impact structures-one of the world's oldest (1150 +/- 10 Ma) and the largest of its kind in Finland. Findings of numerous well-formed shatter cones in a wide area have helped define and prove Keurusselka's impact origin. Keurusselka is deeply eroded, making estimating its size challenging. Thus, various size ranges are based on the distribution of shatter cones. This study provides an overview of the earlier published studies in light of the unpublished data resulting from 2003 to 2019 field surveys. Two shatter cone outcrops 15 km from each other were sampled during a field survey in 2019. Thin section samples from these sites were studied with a polarizing microscope, and shock metamorphic features were identified and measured with the universal stage. We also compared topographic and bathymetric Lidar (light detection and ranging) data with the existing geophysical data and shatter cone occurrences. In situ outcrops-Metsomaki 9.6 km toward the W and Martinniemi 5.7 km toward SE from the crater's center-delimit the maximum radius of shatter cones found so far. Studies resulted in planar fractures parallel to (0001) in Metsomaki shatter cones. We determine the size of 37.5 km as the apparent diameter of the Keurusselka impact structure, whereas the 25 km in diameter semicircular feature represents faulted rim structures.

Shocked quartz and feldspar grains commonly exhibit planar microstructures, such as planar fractures, planar deformation features, and possibly microtwins, which are considered to have formed by shock metamorphism. Their orientation and frequency are typically reported to be randomly distributed across a sample. The goal of this study is to investigate whether such microstructures are completely random within a given sample, or whether their orientation might also retain information on the direction of the local shock wave propagation. For this work, we selected samples of shatter cones, which were cut normal to the striated surface and the striation direction, from three impact structures (Keurusselkä, Finland, and Charlevoix and Manicouagan, Canada). These samples show different stages of pre‐impact tectonic deformation. Additionally, we investigated several shocked granite samples, selected at different depths along the drill core recovered during the joint IODP‐ICDP Chicxulub Expedition 364 (Mexico). In this case, thin sections were cut along two orthogonal directions, one parallel and one normal to the drill core axis. All the results refer to optical microscopy and universal‐stage analyses performed on petrographic thin sections. Our results show that such shock‐related microstructures do have a preferred orientation, but also that relating their orientation with the possible shock wave propagation is quite challenging and potentially impossible. This is largely due to the lack of dedicated experiments to provide a key to interpret the observed preferred orientation and to the lack of information on postimpact orientation modifications, especially in the case of the drill core samples.

We present evidence that in ~ 1650 BCE (~ 3600 years ago), a cosmic airburst destroyed Tall el-Hammam, a Middle-Bronze-Age city in the southern Jordan Valley northeast of the Dead Sea. The proposed airburst was larger than the 1908 explosion over Tunguska, Russia, where a ~ 50-m-wide bolide detonated with ~ 1000× more energy than the Hiroshima atomic bomb. A city-wide ~ 1.5-m-thick carbon-and-ash-rich destruction layer contains peak concentrations of shocked quartz (~ 5–10 GPa); melted pottery and mudbricks; diamond-like carbon; soot; Fe- and Si-rich spherules; CaCO3 spherules from melted plaster; and melted platinum, iridium, nickel, gold, silver, zircon, chromite, and quartz. Heating experiments indicate temperatures exceeded 2000 °C. Amid city-side devastation, the airburst demolished 12+ m of the 4-to-5-story palace complex and the massive 4-m-thick mudbrick rampart, while causing extreme disarticulation and skeletal fragmentation in nearby humans. An airburst-related influx of salt (~ 4 wt.%) produced hypersalinity, inhibited agriculture, and caused a ~ 300–600-year-long abandonment of ~ 120 regional settlements within a > 25-km radius. Tall el-Hammam may be the second oldest city/town destroyed by a cosmic airburst/impact, after Abu Hureyra, Syria, and possibly the earliest site with an oral tradition that was written down (Genesis). Tunguska-scale airbursts can devastate entire cities/regions and thus, pose a severe modern-day hazard.

Once an important accessory for petrographic microscope and a valuable tool in mineralogical and petrofabric work, the Universal Stage (U-stage) has been dubbed obsolete with the advent of electron microprobe and XRD, and subsequent phasing out of the production of the optical equipment. Modern-day fabric analysis also relies more on far more expensive texture goniometers, neutron activation analysis and electron back-scattered diffraction analysis (EBSD) equipment. However, routine measurements of crystallographic fabric elements of uniaxial mineral aggregates (quartz- and calcite c-axis), principal axes of important biaxial minerals like olivine, strength of mica (001) fabric, paleostress estimate from mechanical twins (e-twin) in calcite, and evaluation of meteorite impact texture in quartz as well as impact history of asteroids, can still be done by U-stage knowledgeably and in a cost effective manner, subject to adequate training by efficient instructors. In the cases of e-twin in individual calcite grains and planar deformation features in natural shocked quartz, U-stage measurement cannot still be replaced by EBSD. An abridged review of the science behind U-stage procedure with examples of actual application has been presented to rekindle the interest.

Studies on type-V fulgurites are very sparse in the literature. This work reports on the characterization of natural exogenic fulgurites found at the archaeological site of Cerro de la Ermita (Tiedra, Valladolid, Spain), which was firstly a Celtiberian and then a Roman locum sacrum. Data from X-ray powder diffraction, X-ray fluorescence spectroscopy, Fourier-transform infrared spectroscopy and Raman spectroscopy suggest that the fulgurites consist of naquite, piroxenes, iron oxides, shocked quartz and neo-formed cristobalite.

The abstract is available here: https://uscholar.univie.ac.at/o:1095789

Terrestrial impact cratering events produce anomalous rock fragmentation and alteration, and form sedimentary basins that are out of equilibrium with the local geomorphological environment. Assessement of their initial deposits, crater erosion and infilling, lacustrine environments and biology can provide important insights into similar features on Mars. Here we report on fluviodeltaic conglomerates associated with the 26 km diameter Miocene Ries impact structure (target: sediments overlying crystalline basement) with respect to shock features, grain size distribution, roundness, and potential parent rocks (provenance). The poorly sorted conglomerates form coarsening-upwards cycles associated with a delta-mouth bar system, with subangular to subrounded pebbles derived from crystalline rock impact breccia deposits. Pebbles comprise a wide range of lithologies and states ranging from fresh and solid to intensively impact-fractured, friable crystalline rocks, including shatter cones. Shocked quartz grains, however, are restricted to the sand fraction. The latter observation points to suevite as an additional, but subordinate source of the conglomerates. Granulometric analysis indicates that, in contrast to being derived from weathering of bedrock, impact brecciation of the parent crystalline rocks generated the initial grain size distribution of the conglomerates (i.e., preprocessing). This concept of “impact preprocessing” helped to defne the provenance, the post-impact sedimentologic history and the sediment transport distances of the conglomerates. Post-impact fluvial transport leads to the outwash of fine fractions into the prodelta subenvironment, and an increase in roundness and mean diameter in the residual conglomerates (i.e. hydraulic sorting). On the basis of the characteristics of the ejecta deposits and the present-day locations and occurrences of crystalline rock impact breccias and blocks in the northwestern Ries crater segment analyzed, a transport distance of less than 3.5 km is derived for these impact-preprocessed conglomerates. Reconstruction of the initial distribution and composition of the primary ejecta layer indicates that the conglomerates composed solely of crystalline rock had a maximum transport distance less than 6.5 km, i.e., within the closed basin of the Ries impact structure. In contrast to traditional interpretations of conglomerates, requiring long transport distances from bedrock source regions to deposition to ensure sufficient rounding and sorting, the rounding and sorting associated with these short transport distances are explained by preprocessing of parent rocks by impact brecciation and enhanced susceptibility to abrasion. Consequently, transport distances of conglomeratic fragments associated with impact craters, such as Gale crater on Mars, are likely to have been only a few kilometers, and derived from nearby ejecta, most likely from the inner crater walls. This Ries crater-based preprocessing and transport model implies that the unusual Gale crater conglomeratic fragments are likely to represent the immediate target material excavated by the Gale cratering event, rather than fragments transported laterally from greater distances outside the crater.

Yallalie is a ~12 km diameter circular structure located ~200 km north of Perth, Australia. Previous studies have proposed that the buried structure is a complex impact crater based on geophysical data. Allochthonous breccia exposed near the structure has previously been interpreted as proximal impact ejecta; however, no diagnostic indicators of shock metamorphism have been found. Here we report multiple (27) shocked quartz grains containing planar fractures (PFs) and planar deformation features (PDFs) in the breccia. The PFs occur in up to five sets per grain, while the PDFs occur in up to four sets per grain. Universal stage measurements of all 27 shocked quartz grains confirms that the planar microstructures occur in known crystallographic orientations in quartz corresponding to shock compression from 5 to 20 GPa. Proximity to the buried structure (~4 km) and occurrence of shocked quartz indicates that the breccia represents either primary or reworked ejecta. Ejecta distribution simulated using iSALE hydrocode predicts the same distribution of shock levels at the site as those found in the breccia, which supports a primary ejecta interpretation, although local reworking cannot be excluded. The Yallalie impact event is stratigraphically constrained to have occurred in the interval from 89.8 to 83.6 Ma based on the occurrence of Coniacian clasts in the breccia and undisturbed overlying Santonian to Campanian sedimentary rocks. Yallalie is thus the first confirmed Upper Cretaceous impact structure in Australia.

The Santa Fe structure in northern New Mexico is one of the few confirmed impact craters in the western USA. The history of the impact structure is obscure as it is tectonized and eroded to the extent that an intact crater is not preserved, and what remains is located in a complex geological setting. Shatter cones and shocked quartz were previously cited to confirm an impact origin; however, estimates for both impact age (350−1200 Ma) and crater diameter (6−13 km) remain poorly constrained. To further evaluate the extent of shock deformation, we investigated ∼6600 detrital zircon grains for shock features, using material collected from 15 drainages and other sites within an ∼5 km radius of known shatter cone outcrops. Six detrital shocked zircon grains were found at three locations, including two near shatter cones and one near brecciated granitoid. Follow-up studies of bedrock at two sites proximal to detrital shocked zircon occurrences led to the discovery of shocked zircon in situ in a shatter cone-bearing sample of biotite schist; shocked grains were not found in brecciated granitoid at the second site. Electron backscatter diffraction confirms the presence of {112} shock-twin lamellae in five shocked zircon grains, and secondary ion mass spectrometry U-Pb data for three detrital shocked grains yielded Pb-207/Pb-206 crystallization ages from 1715 ± 22 to 1472 ± 35 Ma. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb ages for detrital zircon grains at five of the investigated sites provide the first broad constraints on the local distribution of Paleo- to Mesoproterozoic bedrock in the area. The presence of shock-twinned zircon indicates that some exposed rocks at the Santa Fe structure may record impact pressures up to ∼20 GPa based on empirical studies, which is higher than previous reports of ∼10 GPa based on planar deformation features in shocked quartz. The 1472 ± 35 Ma date from a shock-twinned zircon yields the first direct radiometric maximum age constraint on the Santa Fe impact event, and expands the possible time period for impact to the Mesoproterozoic. Identification of shocked zircon in modern sediment led to the first discovery of shocked zircon in bedrock at this site, which is notable, as shocked zircon is otherwise not abundant in the studied rock samples. This study thus illustrates that detrital zircon surveys are an efficient way to search for diagnostic evidence of shock deformation at putative impact structures where shocked minerals may be present, but are not abundant in exposed bedrock.

Impact ejecta formation and emplacement is of great importance when it comes to understanding the process of impact cratering and consequences of impact events in general. Here we present a multidisciplinary investigation of a distal impact ejecta layer, the Blockhorizont, that occurs near Bernhardzell in eastern Switzerland. We provide unambiguous evidence that this layer is impact-related by confirming the presence of shocked quartz grains exhibiting multiple sets of planar deformation features. Average shock pressures recorded by the quartz grains are ~ 19 GPa for the investigated sample. U–Pb dating of zircon grains from bentonites in close stratigraphic context allows us to constrain the depositional age of the Blockhorizont to ~ 14.8 Ma. This age, in combination with geochemical and paleontological analysis of ejecta particles, is consistent with deposition of this material as distal impact ejecta from the Ries impact structure, located ~ 180 km away, in Germany. Our observations are important for constraining models of impact ejecta emplacement as ballistically and non-ballistically transported fragments, derived from vastly different depths in the pre-impact target, occur together within the ejecta layer. These observations make the Ries ejecta one of the most completely preserved ejecta deposit on Earth for an impact structure of that size., Scientific Reports, 11 (1), ISSN:2045-2322

The ~790 ka Australasian (micro)tektite strewn field is one of the most recent and best-known examples of impact ejecta emplacement as the result of a large-scale cratering event across a considerable part of Earth's surface (>10% in area). The Australasian strewn field is characterized by a tri-lobe pattern consisting of a large central distribution lobe, and two smaller side lobes extending to the west and east. Here, we report on the discovery of microtektite-like particles in sedimentary traps, containing abundant micrometeorite material, in the Sør Rondane Mountain (SRM) range of East Antarctica. The thirty-three glassy particles display a characteristic pale yellow color and are predominantly spherical in shape, except for a single dumbbell-shaped particle. The vitreous spherules range in size from 220 to 570 μm, with an average diameter of ~370 μm. This compares relatively well with the size distribution (75–778 μm) of Australasian microtektites previously recovered from the Transantarctic Mountains (TAM) and located ca. 2500–3000 km from the SRM. In addition, the chemical composition of the SRM particles exhibits limited variation and is nearly identical to the ‘normal-type’ (i.e., SCOPUS: ar.j, info:eu-repo/semantics/published

Funding to GRO from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant program and the Canadian Space Agency (CSA) Canadian Analogue Research Network and Field Investigation programs is gratefully acknowledged. Part of LF’s work was supported by the Department of Foreign Affairs and International Trade (DFAIT), Government of Canada. The origin of the Stac Fada Member has been debated for decades with several early hypotheses being proposed, but all invoking some connection to volcanic activity. In 2008, the discovery of shocked quartz led to the hypothesis that the Stac Fada Member represents part the continuous ejecta blanket of a meteorite impact crater, the location of which was, and remains, unknown. In this paper, we confirm the presence of shock-metamorphosed and -melted material in the Stac Fada Member; however, we also show that its properties are unlike any other confirmed and well documented proximal impact ejecta deposits on Earth. Instead, the properties of the Stac Fada Member are most similar to the Onaping Formation of the Sudbury impact structure (Canada) and impact melt-bearing breccias from the Chicxulub impact structure (Mexico). We thus propose that, like the Sudbury and Chicxulub deposits, Melt Fuel Coolant Interactions – akin to what occur during phreatomagmatic volcanic eruptions – played a fundamental role in the origin of the Stac Fada Member. We conclude that these rocks are not primary impact ejecta but instead were deposited beyond the extent of the continuous ejecta blanket as high-energy ground-hugging sediment gravity flows. Postprint

Microphotographs, Raman methodology, Raman spectra and SEM-EDS spectrum supporting the close association between shocked quartz, impact glass, and organic matter described in the text.

Fast deformations in the quartz crystals during shock compression process have been investigated with two time-resolved methods of in-situ diffractions by x-ray free electron laser (XFEL) and large-scale molecular dynamic (MD) simulations in order to get insights in planar deformation feature (PDF) mechanism. PDFs in quartz provide strong evidence for impact and are used to estimate the impact conditions. The present experimental results on X-cut and Y-cut quartz single crystals at pressures below phase transition indicate ideal uniaxial compression and rotation of fractured grains to a direction to heat up locally due to dispersed particle velocity distribution in the dynamic movement. This could explain the observed disordering at given planes, as supported by the MD simulations.

International audience; Planar deformation features (PDFs) in quartz are a commonly used and well‐documented indicator of shock metamorphism in terrestrial rocks. The measurement of PDF orientations provides constraints on the shock pressure experienced by a rock sample. A total of 963 PDF sets were measured in 352 quartz grains in 11 granite samples from the basement of the Chicxulub impact structure’s peak ring (IODP‐ICDP Expedition 364 drill core), with the aim to quantify the shock pressure distribution and a possible decay of the recorded shock pressure with depth, in the attempt to better constrain shock wave propagation and attenuation within a peak ring. The investigated quartz grains are highly shocked (99.8% are shocked), with an average of 2.8 PDF sets per grain; this is significantly higher than in all previously investigated drill cores recovered from Chicxulub and also for most K‐Pg boundary samples (for which shocked quartz data are available). PDF orientations are roughly homogenous from a sample to another sample and mainly parallel to {10urn:x-wiley:10869379:media:maps13570:maps13570-math-00013} and {10urn:x-wiley:10869379:media:maps13570:maps13570-math-00024} orientations (these two orientations representing on average 68.6% of the total), then to {10urn:x-wiley:10869379:media:maps13570:maps13570-math-00032} orientation, known to form at higher shock pressure. Our shock pressure estimates are within a narrow range, between ~16 and 18 GPa, with a slight shock attenuation with increasing depth in the drill core. The relatively high shock pressure estimates, coupled with the rare occurrence of basal PDFs, i.e., parallel to the (0001) orientation, suggest that the granite basement in the peak ring could be one of the sources of the shocked quartz grains found in the most distal K‐Pg boundary sites.

Glaciofluvial sand draining the newly discovered Hiawatha impact crater under the Greenland Ice Sheet in North-West Greenland contains shocked quartz, glasses derived from direct mineral melts, carbon-bearing glasses, particles of charcoal and transformed ('glassy') charcoal, as well as low-reflectance carbonaceous grains with tiny carbon spherules and mineral fragments. Some of these grains are interpreted as ejecta and perhaps plume material containing sublimated and re-deposited carbon. The only plausible carbon source of this carbon is subfossil Arctic vegetation including small conifer and angiosperm trees older than 50 ka and likely around 2.3 Ma in age, supporting a very young age of the crater.

Thalassinoides isp. burrow networks filled with green to dark bluish-gray glauconite pellets occur in the Cretaceous to Paleogene Hornerstown Formation and in the uppermost 1–2 m of the underlying Upper Cretaceous Navesink and New Egypt Formations at many central New Jersey Cretaceous–Paleogene (K/Pg) boundary sections. Near the base of the Hornerstown Formation is a renowned bonebed, the Main Fossiliferous Layer (MFL), which contains isolated to articulated Cretaceous marine reptiles, turtles, birds, and crocodiles interpreted as a thanatocoenosis. We investigated the bulk chemical composition of the burrow fill and surrounding matrix in the largely unconsolidated sections of the K–Pg interval at two sites, Inversand and Walnridge (Meirs) Farm, New Jersey, using a Niton XL3t XRF analyzer to place them in proper stratigraphic context. Bulk matrix Zr and Ti concentrations decrease up section as a result of transgression, deepening, and increased distance from source areas. In contrast, an increase in K is attributed to greater amounts and maturity of the glauconite, due to decreasing sedimentation rates. Zr and Ti concentrations in burrow fills below the MFL are virtually the same as in overlying sediments above the MFL and much less than in the surrounding matrix. This suggests that burrows in the Cretaceous strata were passively filled with Paleogene sediments. K concentrations of burrow fills are generally intermediate between the high values encountered in the Paleogene matrix and the lower values of the Cretaceous matrix. Although this may have resulted from removal of glauconite during bioturbation and burrow filling, it is more likely that immature glauconite was sequestered in the burrow fills below the depth of glauconitization before it could attain full maturity. Therefore, burrows originating above the formational contact at or above the MFL penetrate as much as two meters into underlying Cretaceous sediments, and are passively filled with Paleogene sediment. Burrows originating above the MFL may translocate diagnostic Danian microfossils, shocked quartz, and Ir, effectively diffusing the K–Pg boundary. This research demonstrates that care must be taken in sampling pervasively bioturbated strata to avoid incorporating stratigraphically younger burrow-fill sediments.

Chesapeake Bay impact structure is one of the largest impact structures on the Earth. It is buried beneath the lower part of the Chesapeake Bay and probably is one of the best-preserved marine impact craters. The structure, discovered only in 1983 during oil exploration when tektites and shocked quartz have been found, is 35.3 Myr old and approximately 85 km in diameter. However, it has been suggested that the large diameter is partly a result of the specific target properties—rheologic contrast between unconsolidated sediments and crystalline basement on the continental shelf—and consequent resurge and sliding processes during the impact formation and estimate on the transient crater diameter was only about 28 km (Bartosova and Koeberl 2011).

The Popigai (100 km in diameter) and the Chesapeake Bay (40–85 km diameter) impact structures formed within ~10–20 k.y. in the late Eocene during a 2 m.y. period with enhanced flux of 3He-rich interplanetary dust to Earth. Ejecta from the Siberian Popigai impact structure have been found in late Eocene marine sediments at numerous deep-sea drilling sites around the globe and also in a few marine sections outcropped on land, like the Massignano section near Ancona in Italy. In the Massignano section, the Popigai layer is associated with an iridium anomaly, shocked quartz, and abundant clinopyroxene-bearing (cpx) spherules, altered to smectite and flattened to “pancake spherules.” The ejecta are also associated with a significant enrichment of H-chondritic chromite grains (>63 µm), likely representing unmelted fragments of the impactor. The Massignano section also contains abundant terrestrial chrome-spinel grains, making reconstructions of the micrometeorite flux very difficult. We therefore searched for an alternative section that would be more useful for these types of studies. Here, we report the discovery of such a section, and also the first discovery of the Popigai ejecta in another locality in Italy, the Monte Vaccaro section, 90 km west of Ancona. The Monte Vaccaro section biostratigraphy was established based on calcareous nannoplankton, which allowed the identification of a sequence of distinct bioevents showing a good correlation with the Massignano section. In both the Monte Vaccaro and Massignano sections, the Popigai ejecta layer occurs in calcareous nannofossil zone CNE 19. The ejecta layer in the Monte Vaccaro section contains shocked quartz, abundant pancake spherules, and an iridium anomaly of 700 ppt, which is three times higher than the peak Ir measured in the ejecta layer at Massignano. In a 105-kg-size sample from just above the ejecta layer at Monte Vaccaro, we also found an enrichment of H-chondritic chromite grains. Because of its condensed nature and low content of terrestrial spinel grains, the Monte Vaccaro section holds great potential for reconstructions of the micrometeorite flux to Earth during the late Eocene using spinels. (Less)

Here we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North-Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater and represents low-shock semi-autochthonous bombarded strata. In the overlying, more fine-grained GPB and TS, quartz grains with PDFs are relatively abundant (2–5% of the grain population), and with higher shock levels in the upper parts, suggesting that they have formed by reworking of more distal ejecta by resurge of water toward the crater in a marine setting. The absence of shocked quartz grains in the TC indicates that this unit represents later slumps associated with weathering and erosion of the protruding crater rim. Sparse shocked quartz grains (

Researchers present the first unambiguous discovery of a 31-km-wide impact crater buried beneath the Greenland Ice Sheet., We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.

Shock microstructures in refractory accessory minerals such as zircon and monazite provide crucial evidence for deciphering impact-related deformation in a wide variety of planetary materials. Here we describe the first occurrence of shock deformation in xenotime, YPO 4 , from a shocked quartz–bearing shatter cone in granite at the Santa Fe impact structure (New Mexico, USA). Backscattered electron imaging shows that shocked xenotime grains near the surface of a shatter cone contain multiple orientations of closely spaced planar fractures. High-resolution electron backscatter diffraction mapping reveals that some of the planar microstructures in {112} contain deformation twin lamellae that range from 50 nm to 200 nm in width on the polished surface and occur in up to three crystallographic orientations. Other features attributed to impact, such as planar low-angle boundaries and planar deformation bands, record crystal-plastic deformation. Shatter cone formation and co-existing shocked quartz constrain minimum shock pressure experienced by the xenotime grains to 5–10 GPa. An upper limit of 20 GPa is tentatively assigned based on the absence of YPO 4 polymorphs and shock twins in co-existing zircon. We propose that {112} deformation twins in xenotime constitute a diagnostic record of shock metamorphism, similar to {112} twins in zircon; they have not previously been reported in nature and occur in a rock with conspicuous evidence of shock deformation. Documentation of deformation twins in xenotime, a widely applied U-Pb geochronometer, can be used to identify hypervelocity deformation in shocked rocks, detrital grains, and other materials, and may be particularly ideal for recording low-pressure (

Degree of bioturbation, Thalassinoides isp. morphology, and diameters were compared across the Cretaceous–Paleogene (K–Pg) boundary interval at three localities along the New Jersey coastal plain. Within this regionally extensive ichnoassemblage, mean burrow diameters decrease abruptly by 26–29% (n = 1767) at the base of the Main Fossiliferous Layer (MFL) or laterally equivalent horizons. The base of the MFL has been previously interpreted as the K–Pg boundary based on the last occurrence of Cretaceous marine reptiles, birds, and ammonites, as well as iridium anomalies and associated shocked quartz. Along with the mean, the maximum and minimum burrow diameters exhibit a negative shift, which indicates that the changes are the result of a directional reduction in diameter, rather than an artifact of decreased variance. As a proxy for the size of the tracemaker, a change in burrow diameter indicates a decrease in thalassinid crustacean body size. We interpret this shift as dwarfing within the endobenthic community as detrital food sources became scarce following the mass extinction. Despite the difference in size, there is no change in framework geometry. Ichnofabric indices generally increase up-section at each site across the K–Pg chronostratigraphic boundary, indicating a regional reduction in sedimentation rate, which is supported by a gradual increase in glauconite maturity. Overall, the ichnological evidence at these localities suggests that a prolonged period of negative feedback followed a short-term positive endobenthic response across the K–Pg boundary.

The occurrence of coesite in suevites from the Chesapeake Bay impact structure is confirmed within a variety of textural domains in situ by Raman spectroscopy for the first time and in mechanically separated grains by X-ray diffraction. Microtextures of coesite identified in situ investigated under transmitted light and by scanning electron microscope reveal coesite as micrometer-sized grains (1–3 μm) within amorphous silica of impact-melt clasts and as submicrometer-sized grains and polycrystalline aggregates within shocked quartz grains. Coesite-bearing quartz grains are present both idiomorphically with original grain margins intact and as highly strained grains that underwent shock-produced plastic deformation. Coesite commonly occurs in plastically deformed quartz grains within domains that appear brown (toasted) in transmitted light and rarely within quartz of spheroidal texture. The coesite likely developed by a mechanism of solid-state transformation from precursor quartz. Raman spectroscopy also showed a series of unidentified peaks associated with shocked quartz grains that likely represent unidentified silica phases, possibly including a moganite-like phase that has not previously been associated with coesite.

Quartz grains from the Ries impact structure containing shock-induced microstructures were investigated using Scanning Electron Microscopy in cathodoluminescence (SEM-CL), secondary electron (SEM-SE) and back-scattered electron (SEM-BSE) modes as well as Mott–Seitz analysis. The purpose of this study is to evaluate the mechanism by which CL detects Planar Deformation Features (PDFs) in quartz, which is one of the most important indicators of shock metamorphism in rock-forming minerals. PDFs are micron-scale features not easily identified using optical microscopy or scanning electron microscopy. The CL spectrum of PDFs in quartz that has suffered relatively high shock pressure shows no or a relatively weak emission band at around 385 nm, whereas an emission band with a maximum near 650 nm is observed independent of shock pressure. Thus, the ~385 nm intensity in shocked quartz demonstrates a tendency to decrease with increasing shock metamorphic stage, whereas the 650 nm band remains fairly constant. The re...

Advanced techniques such as scanning electron microscope hot-cathodoluminescence (SEM-CL) and electron microprobe analysis (EMPA) as well as petrography are applied to investigate the paragenetic sequence and provenance of intertidal–tidal flat sandstones of the Ora Formation which is exposed in northern Iraq. The Formation consists mostly of supermature quartzarenites with subordinate immature subarkoses and sublitharenites. The mean framework composition of the thinly bedded sandstones of Ora Formation is Q95.7F2.8R1.4 where 90% of the quartz grains are monocrystalline and texturally supermature. Meanwhile the mean composition of the thickly bedded sandstones is Q85.4F9.7R4.8 with 84% of the quartz grains are monocrystalline and texturally immature. The provenance of the Ora sandstones is dominantly craton interior and less recycled orogeny. The Ora Formation has undergone intensive and complex episodes of eogenesis, mesogenesis, and telogenesis. Compaction and quartz cementation is more dominant than other diagenetic processes evident from tight grain supported fabric which predated authigenic illite formation; this is also evident from close packing of detrital framework that resulted in reduction of primary porosity. Albitization has postdated eogenesis of K-feldspar cements. Cathodoluminescence study indicated four main distinctive fabrics in quartz grains, (1) healed fractures, (2) mottled textures, (3) low-intensity dark CL streaks and patches, and (4) shocked quartz. The results indicate the dominance of brown to dark blue CL for quartz of low-temperature metamorphic origin; bright blue colors for the felsic plutonic and high-temperature metamorphic quartz; with considerable amounts of detritus that has originated from felsic and mafic volcanic rocks and are characterized by red, violet colors luminescence.

Ejecta from the Connors Creek site in Michigan (500 km from the Sudbury Igneous Complex [SIC]), the Pine River site in western Ontario (650 km from the SIC), and the Coleraine site in Minnesota (980 km from the SIC) were petrographically and geochemically analyzed. Connors Creek was found to have approximately 2 m of ejecta, including shocked quartz, melt droplets, and accretionary lapilli; Pine River has similar deposits about 1 m in thickness, although with smaller lapilli; Coleraine contains only impact spherules in a 20 cm-thick layer (impact spherules being similar to microkrystites or microtektites). The ejecta transition from chaotic deposits of massively bedded impactoclastic material with locally derived detritus at Connors Creek to a deposit with apparently very little detrital material that is primarily composed of melt droplets at Pine River to a deposit that is almost entirely composed of melt spherules at Coleraine. The major and trace element compositions of the ejecta confirm the previously observed similarity of the ejecta deposits to the Onaping Formation in the SIC. Platinum-group element (PGE) concentrations from each of the sites were also measured, revealing significantly elevated PGE contents in the spherule samples compared with background values. PGE abundances in samples from the Pine River site can be reproduced by addition of approximately 0.2 wt% CI chondrite to the background composition of the underlying sediments in the core. PGE interelement ratios indicate that the Sudbury impact event was probably caused by a chondritic impactor.

The Malingen structure in Sweden has for a long time been suspected to be the result of an impact; however, no hard evidence, i.e., shock metamorphic features or traces of the impactor, has so far been presented. Here we show that quartz grains displaying planar deformation features (PDFs) oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure. The shocked material was recovered from basement breccias, below the sediment infill, and the distribution of the orientation of the shock-produced PDFs indicates that the studied material experienced low shock pressures. Based on our findings, we can exclude that the material is transported from the nearby Lockne impact structure, which means that the Malingen structure is a separate impact structure, the seventh confirmed impact structure in Sweden. Furthermore, sedimentological and biostratigraphic aspects of the deposits that fill the depression at Malingen are very similar to features at the Lockne impact structure. This implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on Earth (i.e., the Lockne-Malingen pair).