Evidence Of Shock Metamorphism Research Articles

Reconnaissance survey of the Duolun ring structure in Inner Mongolia: Not an impact structure

AbstractThe Duolun basin, which is located in Inner Mongolia, China, has been proposed to be an impact structure with an apparent rim diameter of about 70, or even 170 km. The designation as an impact structure was based on its nearly circular topography, consisting of an annular moat that surrounds an inner hummocky region, and the widespread occurrences of various igneous rocks, polymict breccias, and deformed crustal rocks. Critical shock metamorphic evidence is not available to support the impact hypothesis. We conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure. We collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations, especially those sharing similar appearances with impact breccias, suevites, impact melt rocks, and shatter cones. Field investigation, together with thin‐section examination, discovered that the suspected impact melt rocks are actually Early Cretaceous and Late Jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions, and the interpreted impact glass is typical volcanic glass. Petrographic analyses of all the samples reveal no indications for shock metamorphic overprint. All these lines of evidence suggest that the Duolun basin was not formed through impact cratering. The structural deformation and spatial distribution pattern of the igneous rocks suggest that the Duolun basin is most likely a Jurassic–Cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin, followed by resurgent doming of the central area.

Formation of shatter cones in MEMIN impact experiments

AbstractShatter cones are the only macroscopic feature considered as evidence for shock metamorphism. Their presence is diagnostic for the discovery and verification of impact structures. The occurrence of shatter cones is heterogeneous throughout the crater record and their geometry can diverge from the typical cone shape. The precise formation mechanism of shatter cones is still not resolved. In this study, we aim at better constraining the boundary conditions of shatter cone formation in impact experiments and test a novel approach to qualitatively and quantitatively describe shatter cone geometries by white light interferometry. We recovered several ejected fragments from MEMIN cratering experiments that show slightly curved, striated surfaces and conical geometries with apices of 36°–52°. These fragments fulfilling the morphological criteria of shatter cones were found in experiments with 20–80 cm sized target cubes of sandstone, quartzite and limestone, but not in highly porous tuff. Targets were impacted by aluminum, steel, and iron meteorite projectiles at velocities of 4.6–7.8 km s−1. The projectile sizes ranged from 2.5–12 mm in diameter and produced experimental peak pressures of up to 86 GPa. In experiments with lower impact velocities shatter cones could not be found. A thorough morphometric analysis of the experimentally generated shatter cones was made with 3D white light interferometry scans at micrometer accuracy. SEM analysis of the surfaces of recovered fragments showed vesicular melt films alternating with smoothly polished surfaces. We hypothesize that the vesicular melt films predominantly form at strain releasing steps and suggest that shatter cones are probably mixed mode fractures.

Mineralogy and defect microstructure of an olivine-dominated Itokawa dust particle: evidence for shock metamorphism, collisional fragmentation, and LL chondrite origin

We report here detailed analytical scanning and transmission electron microscopic investigations on an olivine-dominated dust particle (RB-QD04-0042) from the surface of asteroid 25143 Itokawa. The dust particle was returned to Earth by the Hayabusa spacecraft and was made available in the context of the first announcement of opportunity for Hayabusa sample investigation. Multiple thin slices were prepared from the precious particle by means of focused ion beam thinning, providing a unique three-dimensional access to its interior. The 40 × 50 μm sized olivine particle contains a spherical diopside inclusion and an intimate intergrowth of troilite and tetrataenite. The compositions of olivine (Fo69Fa31) and diopside (En48Wo42Fs10), as well as the high Ni content of the sulfide-metal alloy, indicate a LL ordinary chondrite origin in accord with previous classifications. Although no impact crater exists at the surface of RB-QD04-0042, transmission electron microscopy revealed the presence of various shock defects in constituent minerals. These defects are planar fractures and [001] screw dislocations in olivine, multiple {101} deformation twins in tetrataenite and basal (0001) stacking faults in troilite. These diagnostic shock indicators occur only in a small zone on one concave side of the dust particle characterized by a high fracture density. These observations can be explained by a collisional event that spalled off material from the particle's surface. Alternatively, the dust particle itself could be a spallation fragment of an impact into a larger regolith target. This suggests that Itokawa dust particles lacking visible microcraters on their surfaces might have still experienced shock metamorphism and were involved in collisional fragmentation that resulted in the formation of regolith.

Petrography, geochemistry, and Hf‐Nd isotope evolution of drill core samples and target rocks from the El'gygytgyn impact crater, NE Chukotka, Arctic Russia

AbstractEl'gygytgyn in northeast Chukotka (Russia) is a 3.6 Ma, 18‐km‐diameter impact structure. The impact crater was recently drilled in the framework of a project sponsored by the International Continental Scientific Drilling Program (ICDP). Target rocks at the El'gygytgyn area are dominated by the felsic members of the Late Cretaceous Okhotsk‐Chukotka Volcanic Belt (OCVB). Such a target lithology is unique among terrestrial impact craters, thereby providing the opportunity to study shock metamorphism in siliceous volcanic rocks. Here, we present a petrographic, geochemical, and isotopic study of the section of the drill core underneath the lacustrine sediments, extending from ∼316 m to 517 m below the lake bottom (blb). The drill core stratigraphy includes ∼80 m of suevite and a cross section through a volcanic suite, which consists of (1) a middle section (∼390–423 mblb) with dominant felsic tuffs and a few mafic members, and (2) a welded rhyolitic‐dacitic ignimbrite (∼423–517 mblb). The melt fragments embedded in the suevite are interpreted as being impact‐related by comparison with impact glasses from the crater and in opposition to the target rock, which does not include similar melts. A suevitic dyke crosscuts the lower section of the core at the depth 471.40 mblb. Evidence for shock metamorphism is concentrated in the upper 10 m of the drill core and almost limited to the suevitic breccia. The geochemical and isotope (Nd and Hf) composition of samples from the target and the drill core reveals relationships to the “Berlozhya magmatic assemblage” (BMA) arguing for similar source magmas. The primitive upper mantle (PUM)‐normalized trace element plot of rocks investigated here confirms a subduction‐related signature, as previously proposed for rocks from both OCVB and BMA.

Rock magnetic and paleomagnetic study of the Keurusselkä impact structure, central Finland

Abstract–There are 31 proven impact structures in Fennoscandia—one of the most densely crater‐populated areas of the Earth. The recently discovered Keurusselkä impact structure (62°08′ N, 24°37′ E) is located within the Central Finland Granitoid Complex, which formed 1890–1860 Ma ago during the Svecofennian orogeny. It is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism, e.g., planar deformation features in quartz. New petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented. The suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility. The presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization. Four different remanent magnetization directions were isolated: (1) a characteristic Svecofennian target rock component A with a mean direction ofD = 334.8°,I = 45.6°, α95 = 14.9° yielding a pole (Plat = 51.1°, Plon = 241.9°,A95 = 15.1°), (2) component B,D = 42.4°,I = 64.1°, α95 = 8.4° yielding a pole (Plat = 61.0°, Plon = 129.1°,A95 = 10.6°), (3) component C (D = 159.5°,I= 65.4°, α95 = 10.7°) yielding a pole (Plat = 21.0°, Plon = 39.3°,A95 = 15.6°), and (4) component E (D = 275.5°,I = 62.0°, α95 = 14.4°) yielding a pole (Plat = 39.7°, Plon = 314.3°,A95 = 19.7°). Components C and E are considered much younger, possibly Neoproterozoic overprints, compared with the components A and B. The pole of component B corresponds with the 1120 Ma pole of Salla diabase dyke and is in agreement with the40Ar/39Ar age of 1140 Ma from a pseudotachylitic breccia vein in a central part of the structure. Therefore, component B could be related to the impact, and thus represent the impact age.

Did the Middlesboro, Kentucky, bolide impact event influence coal rank?

Abstract The Middlesboro Basin, southeastern Kentucky, occurs on the Cumberland Overthrust Sheet and includes a ca. 5.5-km diameter impact structure. The Lower and Middle Pennsylvanian coal-bearing strata are faulted, with some evidence for shock metamorphism. The event post-dated the latest-Pennsylvanian–early-Permian thrusting and was likely prior to late-Mesozoic entrenchment of drainages. The impact of a 0.5-km meteor traveling at ca. 60,000 km/h would release about 1 EJ, the approximate equivalent of the instantaneous combustion of 30 Mt of coal. The coal rank, while increased slightly above the regional level, still is within the upper portion of the high volatile A bituminous rank range. This helps to constrain the depth of burial at the time of the impact. The coal would have had to have been at a depth of a few kilometers to have avoided a more substantial rank increase. In addition, it is possible that some of the coal rank increase might be attributable to movements along the cross-cutting Rocky Face fault, unrelated to the impact.

Aorounga and Gweni Fada impact structures, Chad: Remote sensing, petrography, and geochemistry of target rocks

Abstract— Shuttle Radar Topography Mission data was used to investigate the Aorounga and Gweni Fada impact structures in Chad as part of a new remote sensing study. We believe the results of various data treatments provide extensive new perspective on the macro‐structural and topographic divisions for these two impact structures. Our remote sensing studies indicate revised diameters of Aorounga and Gweni Fada of 16 and 22 km, respectively. We selected samples from these two structures for their petrographic, geochemical, and Rb‐Sr and Sm‐Nd isotopic characteristics. In samples from both structures, evidence for shock metamorphism was found in the form of single or multiple sets of planar deformation features in quartz, which confirms the impact origin for both the Aorounga and Gweni Fada structures. The crystallographic orientations of PDFs indicate maximum shock levels of 20–30 GPa for samples from the central parts of both structures. The PDF orientations are characteristic for the orientations observed elsewhere in shocked sandstones, with the higher angles in the orientation histograms being fairly abundant. Geochemically, the rocks are typical upper‐crustal sandstones.

Upper Eocene impact horizon in east-central Georgia

We report the discovery of shocked quartz grains in upper Eocene sediments from the Coastal Plain of east-central Georgia. The grains exhibit low refractive indices and contain one or more sets of planar deformation features formed during a hypervelocity impact. These grains were collected from a sand layer near the base of the Twiggs Clay Member of the Dry Branch Formation and most likely are ejecta from the Chesapeake Bay impact, which occurred between 35.7 and 36.0 Ma. In this layer, ∼1 in 250 quartz grains from the fine-sand size fraction and fewer medium-sand– sized grains show some evidence of shock metamorphism. This horizon could represent the source stratum for the Georgia tektites and potentially contains important evidence revealing the dynamics and environmental effects of the late Eocene cataclysm.

Silicate darkening in the Kobe CK chondrite: Evidence for shock metamorphism at high temperature

Abstract— The Kobe CK4 chondrite, like most metamorphosed CK chondrites, exhibits pronounced silicate darkening of matrix and chondrule mesostases. Our petrographic and scanning electron microscopic study reveals that the matrix of Kobe consists mostly of intermixtures of two types of fine‐grained olivine. One forms subhedral to anhedral normal crystals. The other fills interstices of the subhedral to anhedral olivine crystals, exhibiting a complex network of veinlets. The latter type of olivine contains high densities of small spherical vesicles (<0.05‐3 μm in diameter) and grains (<0.05‐5 μm) of magnetite and pentlandite as well as round to anhedral grains (1–10 μm) of plagioclase, low‐Ca pyroxene, diopside and chlorapatite. The vesicular olivine is particularly abundant in regions of matrix that exhibit a relatively high degree of darkening and commonly fills chondrule mesostases. The vesicular olivine is clearly the principal cause of the silicate darkening in Kobe. The internal texture of the vesicular olivine closely resembles those of local melts produced from the matrices of experimentally and naturally shocked carbonaceous chondrites. The occurrence and texture of the vesicular olivine suggest that it resulted from recrystallization of partially melted matrix olivine by shock.Kobe exhibits light shock effects in olivine that are consistent with shock stage S2 that is too low to explain the occurrence of olivine melting. We suggest that the vesicular olivine in Kobe was produced by shock metamorphism at a relatively mild shock pressure (<25 GPa) and a high temperature (>600 °C). Thus, it is probable that the shock effects in olivine, manifest as fracturing and deformation, were relatively minor, but heating was strong enough to cause partial melting of matrix olivine.

Evidence for shock metamorphism in sandstones from the Libyan Desert Glass strewn field

Abstract— Microscopic analysis of sandstones from the southern Libyan Desert Glass strewn field reveals a sequence of progressive deformation features, which range from unaffected rock to extreme brecciation. The quartz grains show a wide range of deformation features: crushing and fracturing, undulatory extinction, mosaicism, oriented cleavage, partial isotropization and multiple sets of planar deformation features. This preliminary report provides the first description of these shock indicators, which are typical for hypervelocity impact, in bedrock material that is assumed to be related to the Libyan Desert Glass.

Comment on “Origin of a late Eocene to pre‐Miocene buried crater and breccia lens at Fohn‐1, North Bonaparte Basin, Timor Sea: A probable extraterrestrial connection” by J. D. Gorter and A. Y. Glikson

Abstract— Gorter and Glikson (2000) proposed that the Fohn‐1 structure in the Timor Sea north of Australia probably is an impact structure. Examination of their evidence reveals that it is almost impossible to confirm any of their conclusions, as the authors do not provide sufficient detail of their experimental methods. Problems with the presentation of both seismic and geochemical data cast further doubt on their conclusions. Absence of evidence for shock metamorphism and non‐chondritic siderophile element patterns in the few samples analysed make it likely that Fohn‐1 is neither a probable, nor even a possible, impact structure.

Mutually constrained geophysical data for the evaluation of a proposed impact structure: Lake Hummeln, Sweden

Lake Hummeln covers a 1.2-km-wide near-circular depression in the Precambrian basement of the Baltic Shield. In the depression, more than 150-m-thick Cambrian and Ordovician marine sediments are underlain by non-volcanic breccia. The only explanation that does not meet contradictions is that the structure was caused by impact, but evidence of shock metamorphism is until now missing. This study focuses on the geophysical characterisation and includes magnetic and gravimetric modelling, constrained by information from resistivity measurements, geological mapping and a drill core. The Hummeln structure shows both a gravimetric and a magnetic low. The magnetic low is not broken by any short wavelength anomalies which would indicate the presence of remanent melt occurrences. The diameter of the magnetic low is about twice that of the proposed crater depression, and probably indicates fracturing. Both the magnetic and the gravimetric models show a structure that is consistent with the impact hypothesis. Resistivity measurements were carried out in the surroundings of the proposed crater. The fractured region indicated by the magnetometry does not produce a detectable resistivity anomaly. However, resistivity measurements proved useful in investigating the tectonic fracture zones crossing the area in the vicinity of the structure. Following the impact model, a reconstruction of the pre-erosional structure resulted in an approximately 50% wider original crater than seen today.

Discovery of microscopic evidence for shock metamorphism at the Serpent Mound structure, south-central Ohio: confirmation of an origin by impact

The origin of the Serpent Mound structure in south-central Ohio has been disputed for many years. Clearly, more evidence was needed to resolve the confusion concerning the origin of the Serpent Mound feature either by endogenic processes or by hypervelocity impact. A petrographic study of 21 samples taken from a core 903 m long drilled in the central uplift of the structure provides evidence of shock metamorphism in the form of multiple sets of planar deformation features in quartz grains, as well as the presence of clasts of altered impact-melt rock. Crystallographic orientations of the planar deformation features show maxima at the shock-characteristic planes of {10 1 ̄ 3} and {10 1 ̄ 2} and additional maxima at {10 1 ̄ 1} , {21 3 ̄ 1} , and {51 6 ̄ 1} . Geochemical analyses of impact breccias show minor enrichments in the abundances of the siderophile elements Cr, Co, Ni, and Ir, indicating the presence of a minor meteoritic component.

Petrographic and geochemical study of rocks at Shakiso, Ethiopia: no evidence for shock metamorphism

We have performed field work and petrographic and geochemical studies on rocks from the area around Shakiso, Ethiopia, where the existence of an impact crater, based on the asserted presence of shatter cones and breccias, was previously suggested. We did not find any evidence that would support this claim.

Iridium anomaly but no shocked quartz from Late Archean microkrystite layer: Oceanic impact ejecta?

Samples from a ca. 2.54 Ga layer rich in microkrystites (sand-size spherules of former silicate melt) in the Hamersley Group (Western Australia) are enriched in Ir up to 50 fold over associated carbonates and shales, but few of the other siderophile elements display significant anomalies or chondritic interelement ratios. However, similar concentrations and interelement ratios are observed in ejecta from the ca. 590 Ma Acraman impact structure and have been attributed to diagenetic redistribution. The microkrystite layer also contains sand-size quartz grains with internal textures typical of regionally metamorphosed rocks rather than evidence of shock metamorphism. We suggest that the microkrystites were created by an impact that took place in a deep ocean basin rather than on a continent, and that the associated quartz is epiclastic detritus brought in by unusually high energy waves and/or currents, not as ballistic ejecta.

THE PETROLEUM POTENTIAL OF AUSTRALIAN PHANEROZOIC IMPACT STRUCTURES

This paper examines proven, probable, possible and speculative impact structures in Australian Phanerozoic strata and their petroleum potential. There are two classes of crater: simple and complex. The former usually assumes a bowl shaped depression with a raised and overturned rim with a diameter rarely more than three kilometres, with complex structures generally occurring above diameters of two kilometres in sedimentary rocks and four kilometres in crystalline rocks. Complex craters are characterised by a central uplifted area and a classic 'sombrero' structure and can be very large and have diameters of over 800 km. Criteria for the identification of terrestrial impact structures include: (a) circular plan; (b) faulted rim structure; (c) flat floor with central uplift (may not always be present) or interior ring(s); (d) negative gravity anomaly; (e) magnetic low with subdued magnetic relief; (f) brecciated crater fill; (g) low seismic velocities in the crater fill; (h) shock metamorphism (coesite, multiple sets of planar shock lamellae in quartz grains, shatter cones); (i) meteoric material; (j) presence of melt rock; (k) distal ejecta; and occurrence of an annular trough between the central uplift and the outer faulted rim.Proven impact features, like Gosses Bluff and Mt Toondina, are well exposed, contain indisputable evidence of shock metamorphism, and have had extensive geophysical surveys conducted over them: these structures provide models to interpret completely buried structures. Subsurface impact structures have been detected in areas where there has been intensive seismic surveying in the search for hydrocarbons. The Tookoonooka, Talundilly and Mulkarra structures all occur in the Cooper-Eromanga Basin, an area of high intensity exploration. The best known wholly subsurface impact feature is the 66 km diameter Tookoonooka Structure in southwestern Queensland, which exhibits several of the accepted criteria for an impact origin, including shock metamorphism, and is classified as a probable complex type impact structure. The Talundilly Structure, a possible impact feature, lies 300 km to the northeast of the Tookoonooka Structure and is of the same general age. The two structures could reflect the impact of fragments of the same bolide. The Yallallie Structure lies in a moderately explored hydrocarbon province in the central Perth Basin. It has a classic 'sombrero' shape in section view with a central uplift and evidence of shock metamorphism. Yallallie is a probable complex impact structure. The Mulkarra Structure, located in northeastern South Australia, has been classified as a simple type of impact crater lacking a central uplift, but recent geophysical work indicates a probable complex impact origin. Other possible and speculative impact related features described here owe their recognition to good quality seismic surveying.'there are yet some geologists who, adhering to Lyellian dogma, devoutly refuse to accept that large objects fall out of the sky.' (Shoemaker, 1997).

香川県高松クレーター産ガラスの岩石学的研究

Takamatsu crater has been identified and named by Kono et al., (1994), based on the presence of a concentric gravity low in the southern part of Takamatsu Plain, northeast Shikoku, Japan. The crater is 1∼2 km deep and ca. 4 km across, and buried under the Miocene volcanic formation and alluvium deposit. Both impact and volcanic origins have been proposed for the crater, but remained unresolved. We searched for the evidence of impact origin of the Takamatsu crater, such as shock metamorphic features of minerals, variable and extraordinarily high contents of SiO2, FeO, and NiO, together with low Na2O content of fresh glass in the Miocene deposit. The glass fragments in the pyroclastic flow deposit in the southern part of the Takamatsu crater are characterized by xenocrysts of quartz and feldspar and lack of idiomorphic phenocrysts. We did not find evidence for shock metamorphism in the minerals. The glass shows heterogeneous appearence with variable red tints and high vesicularity under the microscope, somewhat similar to the suevite of Ries crater. The glass of the Takamatsu crater, however, shows uniform and rhyolitic chemical composition, which is common for the calc-alkalic volcanics of the Miocene Setouchi volcanic suite. Furthermore, chemical composition of the glass is almost identical to those of welded tuff blocks and bulk matrix of the pyroclastic flow deposit, indicating that the glass fragments are of volcanic origin. The glass and associated rhyolite are dated by K-Ar method at around 14 Ma, which coincides with the age of the Setouchi volcanic activity. Although deep drilling has not been carried out for determining the subsurface structure, present analytical results argued for cauldron origin of the Takamatsu crater.

Impact Origin of the Chesapeake Bay Structure and the Source of the North American Tektites

Seismic profiles, drill core samples, and gravity data suggest that a complex impact crater ∼35.5 million years old and 90 kilometers in diameter is buried beneath the lower Chesapeake Bay. The breccia that fills the structure contains evidence of shock metamorphism, including impact melt breccias and multiple sets of planar deformation features (shock lamellae) in quartz and feldspar. The age of the crater and the composition of some breccia clasts are consistent with the Chesapeake Bay impact structure being the source of the North American tektites.

Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico

Research Article| September 01, 1991 Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico Alan R. Hildebrand; Alan R. Hildebrand 1Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Glen T. Penfield; Glen T. Penfield 2Aerogravity Division, Carson Services Inc., 32A Blooming Glen Road, Perkasie, Pennsylvania 18944 Search for other works by this author on: GSW Google Scholar David A. Kring; David A. Kring 1Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Mark Pilkington; Mark Pilkington 3Geophysics Division, Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada Search for other works by this author on: GSW Google Scholar Antonio Camargo Z.; Antonio Camargo Z. 4Gerencia Exploración, Petróleos Méxicanos, Avenida Marina Nacional 329, México D.F. 11311, México Search for other works by this author on: GSW Google Scholar Stein B. Jacobsen; Stein B. Jacobsen 5Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar William V. Boynton William V. Boynton 1Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Geology (1991) 19 (9): 867–871. https://doi.org/10.1130/0091-7613(1991)019<0867:CCAPCT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alan R. Hildebrand, Glen T. Penfield, David A. Kring, Mark Pilkington, Antonio Camargo Z., Stein B. Jacobsen, William V. Boynton; Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico. Geology 1991;; 19 (9): 867–871. doi: https://doi.org/10.1130/0091-7613(1991)019<0867:CCAPCT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We suggest that a buried 180-km-diameter circular structure on the Yucatán Peninsula, Mexico, is an impact crater. Its size and shape are revealed by magnetic and gravity-field anomalies, as well as by oil wells drilled inside and near the structure. The stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with, and overlain by, breccias that contain evidence of shock metamorphism. The andesitic rocks have chemical and isotopic compositions similar to those of tektites found in Cretaceous/Tertiary (K/T) ejecta. A 90-m-thick K/T boundary breccia, also containing evidence of shock metamorphism, is present 50 km outside the crater's edge. This breccia probably represents the crater's ejecta blanket. The age of the crater is not precisely known, but a K/T boundary age is indicated. Because the crater is in a thick carbonate sequence, shock-produced CO2 from the impact may have caused a severe greenhouse warming. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Araguainha astrobleme/Central Brazil

The following facts have supported the origin of the Araguainha circular structure in Central Brazil by a meteoritic impact: (1) the almost circular contour (2) the impact-morphologic sequence including a central uplift, ring walls and a basin rim of escarpments (3) outcrops of suevites and mixed breccias (4) the evidence of shock metamorphism (5) the presence of shatter cones, and (6) negative anomalies of the total intensity of the magnetic field at the center of the ring structure.