IMPACT CRATER/STRUCTURE GLOSSARY
by: Charles O’Dale
Of rocks whose primary constituents have not been formed in situ (formed elsewhere and clearly moved to their current location). Allochthonous impactites can be further subdivided into those within and around the final crater (proximal) and those some distance from the final crater (distal). The latter are always ejecta, including air – fall deposits.
Allochthonous – something unequivocally displaced (physically moved) from where it was to start with . Usually applies to things like nappes and thrust sheets that have been translated on major flat-lying faults during continental collisions and the like. However, could also apply at the scale of impact-generated movements too. Grieve may also be referring to thrust or nappe displacements in the bedrock that was impacted. but which pre-date the impact by a long time.
Material that is formed or introduced from somewhere other than the place it is presently found. In impact cratering this may refer to the fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event. In the case of the Beaverhead Impact structure, the crater remnants are found in a place other than where it and its constituents were formed.
[see – PARAUTOCHTHONOUS, AUTOCHTHONOUS]
Cosmic body in the asteroid belt between the orbits of Mars and Jupiter. The largest asteroid, Ceres, has a diameter of roughly 1,000 km. Asteroids from Earth-crossing orbits are potential impactors.
Any of the numerous small rocky bodies in orbit around the Sun. Most asteroids reside in the “main belt” between Mars and Jupiter, but some have orbits that cross the Earth’s orbit and could strike its surface.
[see – METEOR, METEORITE, METEOROID.]
“star wound”; crater formed by meteorite impact.
[see – CRATER CLASSIFICATION.]
[see – CRATER FORMATION]
Originating where found (formed in place).
[see – ALLOCHTHONOUS, PARAUTOCHTHONOUS.]
BARRINGER (aka CANYON DIABLO) CRATER
Called also meteor crater, Arizona crater, Coon Butte, Canyon Diablo crater, Crater Mound.
1,200 m-diameter and 175 m-depth impact crater within the Colorado Plateau in Arizona, USA. Archetype of a meteorite crater. At the end of the 19th century, Meteor crater marks the beginning of the bitter controversy about the cosmic or endogenetic origin of terrestrial craters and ring structures.
“Fire-ball”; meteor exploding in passing through the Earth’s atmosphere.
A clastic sedimentary rock composed of angular clasts in a consolidated matrix. Breccias can be produced in several geologic processes: tectonic breccia, volcanic breccia (eruption breccia, vent breccia), sedimentary breccia (e.g., rock fall breccia), collapse breccia (e.g., in karst areas). Depending on the origin of the clasts, monomictic (monogenetic, monolithologic) and polymictic (polygenetic, polylithologic) breccias may be distinguished.
The word is a loan from Italian indicating both loose gravel and stone made by cemented gravel. A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, tectonic breccia, igneous breccia, impact breccia and hydrothermal breccia.
Multicolored breccia; local name. Impact ejecta deposit of the Ries impact structure (Germany).
At the Ries Crater in the image, there are two varieties of impact metamorphism. A grey well-consolidated suevite overlying a coarser bunte breccia. The suevite contains numerous pieces of black deformed glass and altered basement rocks held in a fine matrix of altered glass. Bunte breccia is largely a turbulent mixture of deformed sedimentary rocks. The suevite is formed from the basement below 700 m, and the bunte breccia from the overlying sedimentary rocks. (Dence 2005; “Half a Century of Searching for Impact Craters in Canada” – RASC presentation.)
A dike in the common geological sense is a mostly tabular body of different materials (minerals, rocks, ores) cross-cutting the host rock. In impact structures, breccia dikes have played an important role in the understanding of the impact cratering process. It is generally suggested that for the most part the breccia dikes are formed in the excavation stage by injection of brecciated material into the walls and the floor of the expanding excavation cavity. Later formations of breccia dikes in the modification stage incorporating earlier formed ones may lead to generations of breccia dikes.
The rocks at an impact target site are melted, shattered, and mixed during the impact explosion. When the site finally settles and cools, a new composite rock, impact breccia in bodies tens to hundreds of meters in size, is the result.
Lithologies showing these unique diagnostic shock effects, formed at pressures ≥10 GPa, tend to be restricted to two locations:
- crater-fill materials (suevites, melt breccias, and fragmental impact breccias) deposited in the crater; and
- brecciated basement rocks, often containing shatter cones, near the center of the structure.
[see – MONOMICT, POLYMICT, SUEVITE.]
Refers to the composition of clasts. Monomict breccia will contain clasts of identical lithology and origin. If the dislocation metamorphism is impact-related, the produced cataclasite may be termed a monomict impact breccia.
[see – POLYMICT BRECCIA.]
Breccia composed of mixed clasts of different lithology and origin.
[see – MONOMICT BRECCIA]
A polymict impact breccia composed of fragments more or less shocked, and melt clasts in a clastic matrix. Glass can make up more than half of the volume of a suevite. The minerals in the rock fragments within suevites (also called suevitic breccias) commonly display shock-metamorphic effects. Suevite was named after a rock found at Ries crater in southern Germany.
Breccias are rocks made up of bits and pieces of other rocks (clasts) in a matrix of finer-grained rock fragments, glass, or crystallized melt.
Monomict breccia is a term applied to a breccia that is made up entirely one kind of rock. Monomict breccias are rare on the Moon because meteoroid impacts tend to mix different kinds of rocks.
Dimict breccias or dilithologic breccias are made up of only two lithologies. The term is usually applied to a common type of rock collected on the Apollo 16 mission that consists of anorthosite (light color) and mafic (dark, iron rich) crystallized impact melt in a mutually intrusive textural relationship. SaU 169 could be regarded as a dilithologic breccia.
Polymict breccia is a general term that encompasses all breccias that aren’t either monomict or dimict. Types of polymict breccias are glassy melt breccias, impact-melt breccias, granulitic breccias, regolith breccias, and fragmental breccias. Each of these breccia types has a different texture because the set of conditions that formed them differed.
An impact-melt breccia can be regarded as in igneous rock because it formed from the cooling of a melt. Regolith and fragmental breccias are the closest lunar equivalents to terrestrial sedimentary rocks. Granulitic breccias are metamorphic rocks in that they were some other type of breccia that was metamorphosed (recrystallized) by the heat of an impact.
Most brecciated lunar meteorites are regolith breccias.