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At least 73 records · Page 4Linked to original sources
USGS1999

Synchroneity of the K-T oceanic mass extinction and meteorite impact: Blake Nose, western North Atlantic

A 10-cm-thick layer of green spherules occurs precisely at the biostratigraphic boundary between the Cretaceous and Paleogene (K-T boundary) at Ocean Drilling Program Site 1049 (lat 30°08′N, long 76°06′W). The spherulitic layer contains abundant rock fragments (chalk, limestone, dolomite, chert, mica books, and schist) as well as shocked quartz, abundant large Cretaceous planktic foraminifera, and rounded clasts of clay as long as 4 mm interpreted as altered tektite glass probably derived from the Chicxulub impact structure. Most of the Cretaceous foraminifera present above the spherule layer are not survivors since small specimens are conspicuously rare compared to large individuals. Instead, the Cretaceous taxa in Paleocene sediments are thought to be reworked. The first Paleocene planktic foraminifera and calcareous nannofossil species are recorded immediately above the spherule bed, the upper part of which contains an iridium anomaly. Hence, deposition of the impact ejecta exactly coincided with the biostratigraphic K-T boundary and demonstrates that the impact event was synchronous with the evolutionary turnover in the oceans. These results are consistent with a reanalysis of the biostratigraphy of the K-T boundary stratotype, which argues that shallow-marine K-T boundary sections are not biostratigraphically more complete than deep-sea K-T boundary sites.

Geology↗
USGS1972

Meteorite magnetism

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Earthquake Information Bulletin (USGS)↗
USGS1967

Cohenite: Its occurrence and a proposed origin

Cohenite is found almost exclusively in meteorites containing from 6 to 8 wt.% Ni. On the basis of phase diagrams and kinetic data it is proposed that cohenite cannot form in meteorites having more than 8 wt.% Ni and that any cohenite which formed in meteorites having Ni content lower than 6 wt.% decomposed during cooling. A series of isothermal sections for the system Fe Ni C has been constructed between 750 and 600°C from published information on the three constitutent binary systems. The diagrams indicate that the presence of a few tenths of a per cent carbon in a Ni Fe alloy may reduce the temperature at which kamacite separates from taenite by more than 50°C. Hence C in iron meteorites may be partly responsible for the postulated supercooled nucleation of kamacite in meteorites proposed by recent authors. Cohenite found in meteorites probably formed over the temperature range 650-610°C. For compositions approximating those of metallic meteorites, the greater the C or Ni content of the alloy, the lower the temperature of formation of cohenite. The presence of cohenite in meteorites indicates neither high nor low pressures of formation. However, the absence of cohenite in meteorites containing the assemblage metal + graphite requires low pressures during cooling. Such meteorites therefore cooled in parent bodies of asteroidal size, or near the surface of large bodies.

Geochimica et Cosmochimica Acta↗
USGS1989

Rhenium-osmium isotope systematics of carbonaceous chondrites

Rhenium and osmium concentrations and Os isotopic compositions of eight carbonaceous chondrites, one LL3 ordinary chondrite, and two iron meteorites were determined by resonance ionization mass spectrometry. Iron meteorite 187 Re/ 186 Os and 187 Os/ 186 Os ratios plot on the previously determined iron meteorite isochron, but most chondrite data plot 1 to 2 percent above this meteorite isochron. This suggests either that irons have significantly younger Re-Os closure ages than chondrites or that chondrites were formed from precursor materials with different chemical histories from the precursors of irons. Some samples of Semarkona (LL3) and Murray (C2M) meteorites plot 4 to 6 percent above the iron meteorite isochron, well above the field delineated by other chondrites. Murray may have lost Re by aqueous leaching during its preterrestrial history. Semarkona could have experienced a similar loss of Re, but only slight aqueous alteration is evident in the meteorite. Therefore, the isotopic composition of Semarkona could reflect assembly of isotopically heterogeneous components subsequent to 4.55 billion years ago or Os isotopic heterogeneities in the primordial solar nebula.

Science↗
USGS1959

Impact mechanics at Meteor Crater, Arizona

Meteor Crator is a bowl-shaped depression encompassed by a rim composed chiefly of debris stacked in layers of different composition. Original bedrock stratigraphy is preserved, inverted, in the debris. The debris rests on older disturbed strata, which are turned up at moderate to steep angles in the wall of the crater and are locally overturned near the contact with the debris. These features of Meteor Crater correspond closely to those of a crater produced by nuclear explosion where depth of burial of the device was about 1/5 the diameter of the resultant crater. Studies of craters formed by detonation of nuclear devices show that structures of the crater rims are sensitive to the depth of explosion scaled to the yield of the device. The structure of Meteor Crater is such as would be produced by a very strong shock originating about at the level of the present crater floor, 400 feet below the original surface. At supersonic to hypersonic velocity an impacting meteorite penetrates the ground by a complex mechanism that includes compression of the target rocks and the meteorite by shock as well as hydrodynamic flow of the compressed material under high pressure and temperature. The depth of penetration of the meteorite, before it loses its integrity as a single body, is a function primarily of the velocity and shape of the meteorite and the densities and equations of state of the meteorite and target. The intensely compressed material then becomes dispersed in a large volume of breccia formed in the expanding shock wave. An impact velocity of about 15 km/sec is consonant with the geology of Meteor Crater in light of the experimental equation of state of iron and inferred compressibility of the target rocks. The kinetic energy of the meteorite is estimated by scaling to have been from 1.4 to 1.7 megatons TNT equivalent.

Open-File Report↗
USGS1961

Paper A: Terrestrial features of impact origin

Craters made by meteorites striking the earth are discussed, with attention given to geological evidence for discriminating them from craters formed by other structural processes. The major known meteorite craters are briefly described, as well as other features that are considered probable or possible meteorite craters on the basis of less conclusive evidence. Separate tabulations are given of (1) topographic features either known to be meteorite craters, or considered as possible meteorite craters through lack of conclusive evidence to the contrary, and (2) topographic features once considered to be possible meteorite impact craters, but which are now believed to be of other origin.

Conference Paper↗
USGS2005

Geology of five small Australian impact craters

Here we present detailed geological maps and cross-sections of Liverpool, Wolfe Creek, Boxhole, Veevers and Dalgaranga craters. Liverpool crater and Wolfe Creek Meteorite Crater are classic bowlshaped, Barringer-type craters, Liverpool was likely formed during the Neoproterozoic and was filled and covered with sediments soon thereafter. In the Cenozoic, this cover was exhumed exposing the crater's brecciated wall rocks. Wolfe Creek Meteorite Crater displays many striking features, including well-bedded ejecta units, crater-floor faults and sinkholes, a ringed aeromagnetic anomaly, rim-skirting dunes, and numerous iron-rich shale balls. Boxhole Meteorite Crater, Veevers Meteorite Crater and Dalgaranga crater are smaller, Odessa-type craters without fully developed, steep, overturned rims. Boxhole and Dalgaranga craters are developed in highly follated Precambrian basement rocks with a veneer of Holocene colluvium. The pre-existing structure at these two sites complicates structural analyses of the craters, and may have influenced target deformation during impact. Veevers Meteorite Crater is formed in Cenozoic laterites, and is one of the best-preserved impact craters on Earth. The craters discussed herein were formed in different target materials, ranging from crystalline rocks to loosely consolidated sediments, containing evidence that the impactors struck at an array of angles and velocities. This facilitates a comparative study of the influence of these factors on the structural and topographic form of small impact craters. ?? Geological Society of Australia.

Boxhole crater, Dalgaranga crater, Liverpool crate↗
USGS2016

Preservation of ancient impact ages on the R chondrite parent body: 40 Ar/ 39 Ar age of hornblende-bearing R chondrite LAP 04840

The hornblende- and biotite-bearing R chondrite LAP 04840 is a rare kind of meteorite possibly containing outer solar system water stored during metamorphism or postshock annealing deep within an asteroid. Because little is known regarding its age and origin, we determined 40 Ar/ 39 Ar ages on hornblende-rich separates of the meteorite, and obtained plateau ages of 4340(±40) to 4380(±30) Ma. These well-defined plateau ages, coupled with evidence for postshock annealing, indicate this meteorite records an ancient shock event and subsequent annealing. The age of 4340–4380 Ma (or 4.34–4.38 Ga) for this and other previously dated R chondrites is much older than most impact events recorded by ordinary chondrites and points to an ancient event or events that predated the late heavy bombardment that is recorded in so many meteorites and lunar samples.

Meteoritics and Planetary Science↗
USGS2008

Osmium isotope and highly siderophile element systematics of lunar impact melt breccias: Implications for the late accretion history of the Moon and Earth

To characterize the compositions of materials accreted to the Earth-Moon system between about 4.5 and 3.8 Ga, we have determined Os isotopic compositions and some highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances in 48 subsamples of six lunar breccias. These are: Apollo 17 poikilitic melt breccias 72395 and 76215; Apollo 17 aphanitic melt breccias 73215 and 73255; Apollo 14 polymict breccia 14321; and lunar meteorite NWA482, a crystallized impact melt. Plots of Ir versus other HSE define excellent linear correlations, indicating that all data sets likely represent dominantly two-component mixtures of a low-HSE target, presumably endogenous component, and a high-HSE, presumably exogenous component. Linear regressions of these trends yield intercepts that are statistically indistinguishable from zero for all HSE, except for Ru and Pd in two samples. The slopes of the linear regressions are insensitive to target rock contributions of Ru and Pd of the magnitude observed; thus, the trendline slopes approximate the elemental ratios present in the impactor components contributed to these rocks. The 187Os/188Os and regression-derived elemental ratios for the Apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites. The HSE in the Apollo 17 aphanitic melt breccias, however, might partially or entirely reflect the HSE characteristics of HSE-rich granulitic breccia clasts that were incorporated in the impact melt at the time of its creation. In this case, the HSE characteristics of these rocks may reflect those of an impactor that predated the impact event that led to the creation of the melt breccias. The impactor components in the Apollo 17 poikilitic melt breccias and in the Apollo 14 breccia have higher 187Os/188Os, Pt/Ir, and Ru/Ir and lower Os/Ir than most chondrites. These compositions suggest that the impactors they represent were chemically distinct from known chondrite types, and possibly represent a type of primitive material not currently delivered to Earth as meteorites. ?? 2008 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗
USGS1992

Impact origin of the Avak Structure, Arctic Alaska, and genesis of the Barrow gas fields

Geophysical and subsurface geologic data suggest that the Avak structure, which underlies the Arctic Coastal Plain 12 km southeast of Barrow, Alaska, is a hypervelocity meteorite or comet impact structure. The structure is a roughly circular area of uplifted, chaotically deformed Upper Triassic to Lower Cretaceous sedimentary rocks 8 km in diameter that is bounded by a ring of anastomosing, inwardly dipping, listric normal faults 12 km in diameter. A zone of gently outward-dipping sedimentary country rocks forms a discontinuous ring of "rim anticlines" within the peripheral ring of normal faults. Beyond these anticlines, the sedimentary rocks are almost flat-lying. Basement consists of strongly deformed Ordovician and Silurian argillite. Density and acoustic impedance con rasts between the argillite and the overlying strata produce gravity and seismic-reflection signatures that define a ring of anticlines around the disturbed zone and a structural high surrounded by an annular structural low at its center. In the adjacent Barrow gas fields, the tops of the informally named Neocomian "pebble shale" unit and the gas-producing Lower Jurassic Barrow sand (local usage) lie at average subsea depths of 488 m and 670 m, respectively. In the Avak 1 well, drilled on the central high, the pebble shale and the Barrow sand lie near the surface, documenting more than 500 m of relative uplift at the high. The cores in this well have steep dips (30-90 degrees), mixed breccia with Franklinian argillite clasts 10 and 90 m above basement, quartz grains with shock mosaicism and multiple sets of shock lamellae, oriented concussion fractures in sand-size quartz grains, and shatter cones resembling those found in the peripheral zones of well-documented impact structures. In addition, above-background levels o fractured quartz grains in Barrow sand were found as far as 19 km beyond the rim of the Avak structure. Data concerning the age of the Avak structure are not definitive. If submarine landslide deposits in the upper part of the Aptian and Albian Torok Formation, in the subsurface 200 km to the east, were triggered by the Avak event, then the Avak meteorite struck a submerged marine shelf about 100 + or - 5 Ma. However, the impact features found at Avak (shatter cones, concussion fractures, shock lamellae and shock mosaicism in quartz grains, and widespread cataclasis) characterize the distal zones of meteorite impact structures. Fused rocks, plastic deformation, and shock-metamorphic minerals found in more proximal zones of impact structures are apparently missing. These observations, and the lack of Avak ejecta in cuttings and cores from the Torok Formation and Nanushuk Group (Albian to middle Cenomanian) in surrounding test wells, indicate that the impact event postdated these beds. In this case, the Avak meteorite struck a Late Cretaceous or Tertiary marine shelf or coastal plain between the Cenomanian (ca. 95 Ma), and deposition of the basal beds of the overlying late Pliocene and Quaternary Gubik Formation (ca. 3 Ma).

Alaska↗
USGS1970

Atmospheric collection of debris from the Revelstoke and Allende fireballs

In two separate events, Revelstoke and Allende, the air through which a fireball had been observed to pass was sampled for meteoritic debris. Particulate matter was collected on fibrous filters, which were mounted on aircraft and flown downwind from the site of the meteorite fall at altitudes of 10,000–12,000 m. From Revelstoke, a highly distinctive assemblage of particles was collected. Included were large numbers of magnetite spherules, transparent siliceous spherules, and several types of irregular nickel-bearing particles. The Allende collections yielded only slightly more magnetite and siliceous spherules than background and no nickel-bearing particles. Revelstoke and Allende are believed to be representative of two different types of events. In the Revelstoke type, large amounts of meteoritic debris are left in the atmosphere and little reaches the ground in large coherent fragments; in the Allende type, little material remains in the atmosphere but large fragments reach the ground in the fall area.

Geochimica et Cosmochimica Acta↗