Lithic impact breccia and suevite
One of the most common signs and evidence of a comet or asteroid impact event are the breccia rocks at and near the impact site. Breccia (from the Latin word for broken) is a conglomeration where fragments of rock from one or more different sources are embedded within a finer grained matrix.
One of the most common signs and evidence of a comet or asteroid impact event are the breccia rocks at and near the impact site. Breccia (from the Latin word for broken) is a conglomeration where fragments of rock from one or more different sources are embedded within a finer grained matrix.
Le signe le plus fréquent d'un impact d'astéroïde est la présence des brèches qui se trouvent près du site de l'impact. Les brèches sont une des trois sortes de roches qui forment la famille des conglomérations (et elles sont constituées de morceaux anguleux de roches diverses réunis par un ciment).
Technically defined as lithic breccia (lithic as in ‘rock’) the fragments they contain are known as ‘clasts’. Lithic breccia can be either monomictic or polymictic, with the difference being the monomictic breccia have clasts of one type / source of origin, whereas the polymictic have clasts from multiple sources.
A Suevite is a type of impact breccia which also contain impact melts, often seen as small, dark, glassy or crystalline spheroids. Often the impact melts here are partial melts which did not become completely liquified. An impact breccia melt is where the matrix is of melted rock. The terminology of rocks associated with asteroid/comet impact has been the subject of some discussion over the past decade. The interested reader is referred to refer [1] for a more detailed definition and discussion.
Quartz and feldspar within impact breccias can also show planar deformation features (PDF), which are best seen using a microscope. Due to the very high pressures required for the formation of PDF, far higher than seen in tectonic or volcanic activity, PDF are considered as conclusive evidence of an impact event.
Rochechouart impact
During the Rochechouart impact event at the end of the Triassic period, 201million years ago, extensive amounts of impact melt, lithic breccia and suevite were produced. Three distinct types of impact breccia are found in the region; Rochechouart lithic breccia; Montoume suevite; and Chassenon suevite. These rock types feature in the walls of many of the older buildings of the region. Our short book describes these rocks in detail and a selection of photographs of each, together with impact melt and shatter cone images are available to view here on our web-pages.
As part of our spring field trip we have again visited the tiny hamlet of Montoume and the images below are from the abandon Montoume quarry.
A Suevite is a type of impact breccia which also contain impact melts, often seen as small, dark, glassy or crystalline spheroids. Often the impact melts here are partial melts which did not become completely liquified. An impact breccia melt is where the matrix is of melted rock. The terminology of rocks associated with asteroid/comet impact has been the subject of some discussion over the past decade. The interested reader is referred to refer [1] for a more detailed definition and discussion.
Quartz and feldspar within impact breccias can also show planar deformation features (PDF), which are best seen using a microscope. Due to the very high pressures required for the formation of PDF, far higher than seen in tectonic or volcanic activity, PDF are considered as conclusive evidence of an impact event.
Rochechouart impact
During the Rochechouart impact event at the end of the Triassic period, 201million years ago, extensive amounts of impact melt, lithic breccia and suevite were produced. Three distinct types of impact breccia are found in the region; Rochechouart lithic breccia; Montoume suevite; and Chassenon suevite. These rock types feature in the walls of many of the older buildings of the region. Our short book describes these rocks in detail and a selection of photographs of each, together with impact melt and shatter cone images are available to view here on our web-pages.
As part of our spring field trip we have again visited the tiny hamlet of Montoume and the images below are from the abandon Montoume quarry.
Montoume quarry, and a large polymictic suevite boulder.
References
[1] IUGS Impactite terminology
http://www.bgs.ac.uk/SCMR/docs/papers/paper_11.pdf
[2] Limousin breccias, suevites, impact melt and Shatter cone images
[1] IUGS Impactite terminology
http://www.bgs.ac.uk/SCMR/docs/papers/paper_11.pdf
[2] Limousin breccias, suevites, impact melt and Shatter cone images
