P-Q-R

IMPACT CRATER/STRUCTURE GLOSSARY

by: Charles O’Dale

The petrographic and geochemical study of actual rocks from the potential impact structure will bring final confirmation of the presence of an impact structure. In case of a structure that is not exposed on the surface, drill-core samples are essential. Good materials for the recognition of an impact origin are various types of breccia and melt rocks. These rocks often carry unambiguous evidence for the impact origin of a structure in the form of shocked mineral and lithic clasts or a contamination from the extraterrestrial projectile.

PDFs

[see – PLANAR DEFORMATION FEATURES.]

 

PARAUTOCHTHONOUS

Ground which has been disturbed by impact, thrust or nappe displacement, but where the displacement is small enough that the rocks are still in contact with their source (moved but appear to be in place).

The Sudbury impact structure sits on the Grenville-Superior craton collision. The south “rim” of the structure is a parautochthon distortion. The Grenville parautochthon (which sits NW of the Grenville Front), but which (though disturbed) is still clearly part of the Superior Province margin to the north.

[see – CRATON, AUTOCHTHONOUS, ALLOCHTHONOUS, PARAUTOCHTHONOUS.]

 

PEAK RING IMPACT CRATER/STRUCTURE

Peak ring craters develop within the rim of larger complex craters. The ring structure forms as the central peak collapses and creates a peak ring before all motion stops (Melosh 1989).

The 290 million year old Clearwater West Crater (illustrated to the LEFT) is  a surviving peak ring crater on this planet. The rim diameter is 36 km and the internal “peak ring” has a diameter of 10 km. An annular trough surrounds the ring.

[see – CRATER CLASSIFICATIONS]

[see-   CRATER FORMATION]

 

PLANETESIMALS

Bodies ranging in size from meters up to hundreds of kilometers in diameter that formed during the process that formed the planets by accretion. Most planetesimals accreted to form the planets. A rocky and/or icy body, a few to several tens of kilometers in size, that was produced in the solar nebula.

 

PLANAR DEFORMATION FEATURES

Upon bolide impact, the passage of the resultant shock wave through the rock changes the structure of some of the enclosed minerals.

Planar deformation features (PDFs) are not cracks in quartz, but are “… multiple sets of closed, extremely narrow, parallel planar regions …” that are typically less than 2-3 μm wide and spaced around 2-10 μm apart (French, 1998: 42).

Shock-characteristic planar deformation features (PDFs) in a quartz grain (in distal ejecta from the Manson impact crater, found in South Dakota). Width of the grain ca. 100 mm. Multiple intersecting sets of PDFs are clearly visible (Christian Koeberl).
Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals (usually quartz or feldspar), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain’s crystal structure.

 

PRESSURE-TEMPERATURE CONDITIONS for SHOCK METAMORPHISM

[see – SHOCK METAMORPHISM]

 

PSEUDOTACHYLITE (friction melt)

Pseudotachylite is formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. It may contain unshocked and shocked mineral and lithic clasts in a fine-grained aphanatic [aphanatic = very fine-grained], crystalline texture matrix. (A tachylite is a black volcanic glass formed by the chilling of basaltic magmas.)

Sudbury pseudotachylite dikes range from veins less than 1 mm thick to massive zones measuring up to 1 km thick and extending for approximately 45 km. Formations of SB are found up to 100 km north of the SIC . The pseudotachylite here is injected into the pink gneiss country rock (the toe of my boot is for scale).
This example is from the Vredfort Impact crater in Africa where pseudotachylite was first identified.

 

RADIOMETRIC DATING Reports of the National Center for Science Education – Brent Dalrymple,

 

RADIOMETRIC DATING

[see – DATING – RADIOMETRIC]

 

REIDITE (METAMORPHIC ZIRCON)

Reidite is a rare mineral,  a dense form (polymorph) of the fairly tough gemstone zircon, which is produced when the latter is subjected to very high pressures.  Reidite has been found only in four crater impacts: the Chesapeake Bay Crater in Virginia, Ries Crater in Germany, Xiuyan Crater in China, and Rock Elm Crater in Wisconsin in the United States (Wiki).

Zircon transforms into reidite when meteorites slam into the ground because shock waves from the impact cause a dramatic increase in temperature and pressure at the site. The high pressures cause the building blocks of the mineral to rearrange, becoming tightly repacked. The resulting mineral is similar in composition to zircon, but around 10% more dense. Reidite can also be formed under high-pressure or shock recovery laboratory experiments. In fact, reidite was only known from lab-made samples for around 30 years before it was first discovered in nature in 2001 (Reidite was finally identified in nature starting in 2001, at three impact sites: the Chesapeake Bay Crater in Virginia, Ries Crater in Germany and Xiuyan Crater in China.).