IMPACT BRECCIAS
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
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
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
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,
Jmtland, 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
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
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
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.
LAPPAJRVI IMPACT BRECCIA
Lappajrvi Impact Breccia (field of view 7.0 cm across) from the Lappajrvi
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 Lappajrvi 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 Lappajrvi 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
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.