Impact events invariably produce deposits containing jumbled, chaotic mixes of angular debris (impact breccias) in areas close to the impact crater(s).  Different names are given to impact breccias based on the nature of the matrix, the nature of the clasts, and the inferred origin of the breccia.





Odessa Breccia (10.5 cm across) - this deposit formed when the Odessa Iron Meteorite impacted the western Texas plains about 64,000 years ago.  The impact site is about 9-10 miles southwest of the town of Odessa, in Ector County, Texas, USA.  The main crater is somewhat subtle as seen from the ground.  Fragments of Odessa Meteorite have been collected from the surrounding fields for decades.  The impact breccia produced by this event is called the Odessa Breccia.  It's a distinctively creamy-pink colored unit containing an abundance of large to small angular fragments.






Glover Bluff Impact Breccia (field of view 11.9 cm across) - it's not well known that south-central Wisconsin was the site of a significant meteorite impact during the Early Ordovician.  The Glover Bluff Impact Crater in northern Marquette County, Wisconsin is not well-defined in terms of its size.  Size estimates vary from 8 to 22 km in diameter.

The age of the impact is fairly well constrained by the presence of an impact breccia (fallback breccia) in the Oneota Formation, which is below the Sauk-Tippecanoe Measequence boundary (Prairie du Chien Group, Lower Ordovician).  The Glover Bluff impact breccia is a multicolored jumble of angular fragments in a reddish matrix.







Lockne impact resurge breccia (cut surface; field of view 8.2 cm across) from Tandsbyn Gully, western edge of Lockne Impact Crater, west of Lake Lockne, JŠmtland, central Sweden.

The Lockne Impact Crater of central Sweden is a 7.5 km-diameter (but may be larger), epicontinental marine target crater, with an original water depth estimated at ~500 to 700 meters.  Deposits from marine impacts differ significantly from those generated by terrestrial impacts.  The rock shown above is an impact resurge breccia formed during the Lockne Impact (the Swedes call this rock "Loftarsten" or "Loftarstone"), and found stratigraphically atop the Lockne Breccia itself.

The impact resurge breccia deposit was generated by water, carrying with it abundant rock fragments, surging into crater area after the initial splash (an impact resurge turbidity flow).  The fragments in this rock are mostly granule- & sand-sized, and noticeably angular in shape.  Published research has found that most of these small fragments are limestone and impact melt rock.

Dating work indicates that the Lockne Impact occurred at about 455-458 million years ago, during the early Late Ordovician (stage 5, sensu Gradstein et al., 2004).

A recent publication announced that extraterrestrial chromite (FeCr2O4) has been found in the Lockne impact resurge breccia.  The chromite chemistry is consistent with an origin in an L-chondrite meteorite.  Chondrites are the most common meteorite type found impacting on Earth.  It's been observed that there's a relative abundance of preserved Ordovician-aged impact craters on Earth, and that many L-chondrites that fall to Earth nowadays have CRE ages corresponding to the Ordovician (for example, the Park Forest Meteorite).

So, the situation seems to be this: the L-chondrite parent body in the asteroid belt was significantly disrupted (presumably by a large impact) back in the Ordovician (~470 million years ago).  This resulted in an temporary increase in impact events on Earth (for example, the Lockne Impact).  L-chondrites that fall today are small surviving fragments from that ancient disruption event in the asteroid belt.  Neat story!



Some info. from:


Alwmark & Schmitz (2007) - Extraterrestrial chromite in the resurge deposits of the early Late Ordovician Lockne crater, central Sweden.  Earth and Planetary Science Letters 253: 291-303.


Gradstein et al. (2004) - A Geologic Time Scale 2004.  Cambridge.  Cambridge University Press.  589 pp.





Montoume Breccia (suevite) (field of view 8.1 cm across) - France's deeply eroded Rochechouart Impact Crater is a fascinating place, both for its rocks and the significance of its impact date.  Rochechouart is located in Haute-Vienne Department of west-central France.  Impact breccias have long been used in the area as building stone - spectacular examples can be seen in the Rochechouart Castle.  The rock shown here is one of many specific lithologies found in the Rochechouart Crater area.  It's a reddish suevite.  "Suevite" is a term used by impact geologists to refer to polymict impact breccias that include glassy and/or frothy melt clasts derived from impact melting.  This particular reddish suevite is informally called the "Montoume Breccia".  It contains angular clasts of granite, gneiss, schist, and glassy/frothy melt of varying sizes, all set in a clastic matrix.

What particularly intriguing about the Rochechouart Crater is not its surficial appearance (it's so deeply eroded that the diameter can only be broadly estimated at 20-30 km), but its age.  The Rochechouart impact event has been dated to 203 million years, near the end of the Late Triassic.  Several other impact craters on Earth are also Late Triassic (for example, Quebec's very large Manicouagan Crater).  It's been suggested that the end-Triassic mass extinction may have been caused by a chain-of-impacts event similar to the 1994 impact of Comet Shoemaker-Levy 9 with Jupiter.  Recent crater date revisions have thrown doubt on the impact chain hypothesis.




Montoume Breccia (suevite) (field of view 4.5 cm across)





Gardnos Breccia (cut surface; field of view 11.8 cm across) - this attractive black-and-white rock is an impactite from southern Norway's Gardnos Impact Crater (also known as the Hallingdal Impact Crater).  Several breccia types have been documented from Gardnos, but this is the most widespread unit.  It consists of angular, variably-sized, whitish clasts of granitic gneiss (the dominant local Precambrian basement lithology) set in a black, relatively carbon-rich matrix.  This deposit has been inferred to be brecciated basement rocks beneath the original Gardnos Crater floor.  The crater itself is filled with impact melt rocks and post-impact siliciclastic sediments.

Age of Impact: undetermined, but constrained between 385 and 900 million years.

Locality: outcrop along the Dokkelvi River, northern Buskerud County, southern Norway.

Size of impact structure: ~5 km in diameter.



See the following for more info. on the Gardnos Impact Structure:


French, B.M., C. Koeberl, I. Gilmour, S.B. Shirey, J.A. Dons & J. Naterstad.  1997.  The Gardnos impact structure, Norway: petrology and geochemistry of target rocks and impactites.  Geochimica et Cosmochimica Acta 61: 873-904.


Gilmour, I., B.M. French, I.A. Franchi, J.I. Abbott, R.M. Hough, J. Newton & C. Koeberl.  2003.  Geochemistry of carbonaceous impactites from the Gardnos impact structure, Norway.  Geochimica et Cosmochimica Acta 67: 3889-3903.





Vredefort Granophyre (cut & polished surface; field of view 6.3 cm across) - the number one largest impact feature anywhere on Earth is South Africa's Vredefort Impact Structure (~300 km preserved diameter).  The cut & polished sample shown above is an impact melt rock from the Vredefort Granophyre.

Such rocks have been called impact melt breccias or melt matrix breccias or impact melt rocks or tagamites.  The dark-colored matrix of this rock lacks a clastic texture.  The black material is glassy-textured to finely-crystalline textured, and formed by cooling & solidification of impact-generated melt.

The large, light-colored clasts in the Vredefort Granophyre are typically composed of quartzite and granite that have been recrystallized to varying degrees, but other lithologies have also been reported.  Some of the clasts contain quartz having planar deformation features (PDFs), widely accepted to be diagnostic of an impact event.

The Vredefort Impact occured during the mid-Paleoproterozoic; it has been well dated to 2.023 billion years.  The original crater and impact breccia that filled the crater eroded away long ago.  The rocks now exposed in South Africa are materials estimated to have originally been ~7-10 km below the original surface.





LappajŠrvi Impact Breccia (field of view 7.0 cm across) from the LappajŠrvi Impact Structure in southwestern Finland (63¼ 09" N, 23¼ 40' E).  This sample contains some fragments of impact melt, so the term suevite can be applied to this rock.

The LappajŠrvi Impact Structure is located in east-central Vaasa Province, southwestern Finland.  It's represents the eroded remnants of a Late Cretaceous impact event.  The target area consists principally of Precambrian basement rocks of the Fennoscandian/Baltic Shield.  Impact rocks at LappajŠrvi include polymict impact breccias, suevites, and melt-matrix breccias.  Impact-generated microdiamonds have been found in the impact rocks of this structure as well.

Age: 73-77 million years (Campanian Stage, late Late Cretaceous).






Black Onaping Impact Breccia (field of view: 3.7 cm across) from the Sudbury Impact Structure in Ontario, Canada.  This attractive black impact breccia is part of a thick succession of impact breccias associated with the 1.85 billion year old Sudbury Impact Structure of southern Ontario, Canada.  The rock is carbon-rich, which accounts for its blackish color.  Several published studies have determined that the unusual carbon molecule C60 (buckminsterfullerene, or "buckyball") is present in these rocks.

The Sudbury Impact Structure is the 2nd-largest preserved impact crater on Earth.  The impact crater is gigantic, but is no longer circular.  Tectonic collision and regional metamorphism during the Precambrian have deformed the impact structure into an ellipsoidal shape.




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