Crater Field Research Articles

Ground truth for oblique impact processes: New insight from the Rio Cuarto, Argentina, crater field

New evidence for an impact origin of oblong rimmed depressions near Rio Cuarto, Cordoba Province, Argentina, includes shocked silicate phases (e.g., diaplectic glass), thermal decomposition of high-temperature mineral clasts (e.g., baddeleyite from zircon), rapid quenching, very low water contents (≤0.1 wt%), and generation of identical glasses in hypervelocity laboratory impact experiments. The results indicate that glasses with a wide range in major element concentrations can form from a single target type in a relatively small impact event. Impact glasses with the greatest volatile loss typically exhibit the greatest meteoritic contamination (as defined by Cr, Ni, and Ir abundances). The different impact glass types and the different degrees of impactor contamination are proposed to reflect proximity to the projectile-target interface during shallow penetration in an oblique impact, consistent with laboratory simulations and planetary analogues.

Recent grazing impacts on the Earth recorded in the Rio Cuarto crater field, Argentina

THE most probable angle of a meteoroid impact on a planet is 45°, and an impact at 15° from the horizontal or lower is as likely as at 75° or higher 1,2 . Yet little direct evidence for oblique impacts exists on the Earth, for two reasons. Unless the impact angle is very low, any asymmetry created during the initial transfer of energy from impactor to target is lost as the crater is formed 3 ; moreover, the shallow craters formed by oblique impact are more easily obscured by subsequent erosion. During routine flights two years ago, however, one of us (R.E.L.) noticed an anomalous alignment of oblong rimmed depressions (4 km x 1 km) on the otherwise featureless farmland of the Pampas in Argentina. We argue here, from sample analysis and by analogy with laboratory experiments, that these structures resulted from a low-angle impact and ricochet of a chondritic body originally 150–300 m in diameter.

Chemical classification of iron meteorites—IX. A new group (IIF), revision of IAB and IIICD, and data on 57 additional irons

Structural observations and concentrations of Ni, Ga, Ge and Ir allow the classification of 57 iron meteorites in addition to those described in the previous papers in this series; the number of classified independent iron meteorites is now 535. INAA for an additional six elements indicates that five previously studied irons having very high Ge Ga ratios are compositionally closely related and can be gathered together as group IIF. A previously unstudied iron, Dehesa, has the highest Ge Ga ratio known in an iron meteorite, a ratio 18 × higher than that in CI chondrites. Although such high Ge Ga ratios are found in the metal grains of oxidized unequilibrated chondrites, their preservation during core formation requires disequilibrium melting or significant compositional and temperature effects on metal/silicate distribution constants and/or activity coefficients. In terms of Ge Ga ratios and various other properties group IIF shows genetic links to the Eagle Station pallasites and CO CV chondrites. Klamath Falls is a new high-Ni, low-Ir member of group IIIF that extends the concentration ranges in this group and makes these comparable to the ranges in large igneous groups such as IIIAB. Groups IAB and IIICD have been revised to extend the lower Ni boundary of group IIICD down to 62 mg/g. The iron having by far the highest known Ni concentration (585 mg/g), Oktibbeha County, is a member of group IAB and extends the concentration ranges of all elements in this nonmagmatic group. Morasko, a IAB iron associated with a crater field in Poland, is paired with the Seeläsgen iron discovered 100 km away. All explosion craters from which meteorites have been recovered were produced by IAB and IIIAB irons.

Effects of atmospheric breakup on crater field formation

This paper investigates the physics of meteoroid breakup in the atmosphere and its implications for the observed features of strewn fields. There are several effects which cause dispersion of the meteoroid fragments: gravity, differential lift of the fragments, bow shock interaction just after breakup, centripetal separation by a rotating meteoroid, and possibly a dynamical transverse separation resulting from the crushing deceleration in the atmosphere. Of these, we show that gravity alone can produce the common pattern in which the largest crater occurs at the downrange end of the scatter ellipse. The average lift-to-drag ratio of the tumbling fragments must be less than about 10 −3, otherwise small fragments would produce small craters downrange of the main crater, and this is not generally observed. The cross-range dispersion is probably due to the combined effects of bow shock interaction, crushing deceleration, and possibly spinning of the meteoroid. A number of terrestrial strewn fields are discussed in the light of these ideas, which are formulated quantitatively for a range of meteoroid velocities, entry angles, and crushing strengths. It is found that when the crater size exceeds about 1 km, the separation between the fragments upon landing is a fraction of their own diameter, so that the crater formed by such a fragmented meteoroid is almost indistinguishable from that formed by a solid body of the same total mass and velocity.

RbSr study of impact glass and country rocks from the Henbury meteorite crater field

Rb-Sr data for the Henbury country rock (Winnall Beds) and the laterally equivalent Pertatataka Formation 95 miles ENE of Henbury, fall close to an isochron with age 730 m.y. ± 45 m.y. and an Sr 87 Sr 86 initial ratio of 0.724. The scatter about the isochron is interpreted as variation in initial Sr 87 Sr 86 reflecting small changes in Sr provenance. The impact glass data are particularly well-aligned over a 15% range in Rb Sr , and independently define an isochron of 675 ± 100 m.y. Certain of the sediments will fit the glass alignment to within experimental error. These data show that no selective loss of Rb relative to Sr occurred during melting. Comparison of Rb, Sr abundances of glasses and sediments suggests that, although similar, none of the analyzed subgreywackes are identical with the inferred parent material of the glasses, which would contain a greater clay fraction. Alternatively the composition of the glasses was biassed by selective melting of the clay matrix.