BRENT IMPACT CRATER
by: Charles O’Dale
- Type: Simple
- Age (ma): 396 ± 20a – DEVONIAN
- Diameter: 3.8 km
- Location: Ontario, Canada. N 46° 05′ W 78° 29′
- Shock Metamorphism: Shatter cones and PDF in quartz and feldspar.
- Impactor type: IA or IIIC non‐magmatic iron (Claeys 2010) – Ordinary chondrite; type L or LL – siderophile elements (PGE, Ni, Au) (Tangle, Hecht 2006).
a Dating Method: K-Ar studies on the coarsely crystalline melt rocks using post 1977 decay constants (J. Whitehead).
Characterization of the alteration present at the Brent impact structure, revealed at least the presence of a chloritization, Au‐depletion and K‐enrichment process in the melt‐fragment breccias.
Based on a multi‐signature approach by combining the moderately and highly siderophile elements, a precise meteorite classification into the IA non‐magmatic irons is possible. While the Ni/Cr, Co/Cr and Pd/Ir ratios point to a LL or L ordinary chondrite, the Ni/Co, other platinum group elements (PGE) and combined siderophile ratios do not. Based on a linear and magnified PGE pattern that is assumed to be representative for the impact meteorite, the IA or IIIC non‐magmatic irons are the only possibility. When all siderophile ratios are taken into account, the IA is by far the best fitting group.
EARTH SCIENCE PICTURE OF THE DAY
On the Google Earth images, note the small white square indicators. These are position reports from my SPOT personal locator beacon.
Gigapascal, 1 GPa = 1,000 MPa (Megapascal) = 109 Pascal, the SI unit of pressure. GPa is commonly used in the high-pressure range of shock deformation, 1 GPa = 10 kbar.
Geochemistry of the Brent impact structure, Ontario, Canada
Dr. Ph. Claeys, Dr. M. Elburg, Dr. S. Goderis, Dr. Leescommissie, Dr. P. Van den haute,Dr. F. Vanhaecke;
FACULTEIT WETENSCHAPPEN Vakgroep Geologie en Bodemkunde Academiejaar 2010–2011
The platinum group elements (PGE) comprise platinum (Pt), palladium (Pd), iridium (Ir), osmium (Os), rhodium (Rh) and ruthenium (Ru).
ABSTRACT: Approximately 470 million years ago one of the largest cosmic catastrophes occurred in our solar system since the accretion of the planets. A 200-km large asteroid was disrupted by a collision in the Main Asteroid Belt, which spawned fragments into Earth crossing orbits. This had tremendous consequences for the meteorite production and cratering rate during several millions of years following the event. The 7.5-km wide Lockne crater, central Sweden, is known to be a member of this family. We here provide evidence that Lockne and its nearby companion, the 0.7-km diameter, contemporaneous, Målingen crater, formed by the impact of a binary, presumably ‘rubble pile’ asteroid. This newly discovered crater doublet provides a unique reference for impacts by combined, and poorly consolidated projectiles, as well as for the development of binary asteroids.
At the time of this impact in the late Silurian or early Devonian, the most advanced creatures present on earth were marine crustaceans called trilobites, Europe and America are just about to collide, Ontario was approximately at the equator and plant life is just beginning to appear on land.The Brent meteorite crater is located within the northern boundary of Algonquin Park 75 km east of Lake Nipissing. It was named the “Brent crater” because of its proximity to the village of Brent, a divisional point on the Canadian National Railway’s transcontinental line. It is the largest known terrestrial crater with a simple, bowl-shaped form and perhaps the best known and possibly the most thoroughly studied fossil meteorite crater in the world.
A search for meteorite impact sites in Canada was initiated following the discovery and interpretation of the Pingualuit Meteorite Crater as an impact site (Meen 1950). On the strength of Meen’s discovery, Beals, the Dominion Astronomer for Canada, instituted a crater research program at the Dominion Observatory, which included a systematic search of aerial photographs (Grieve 1975). This led to the confirmation of the Holleford structure as an impact site. As the Observatory program became known, others reported unusual, circular topographic features in Canada such as Brent and Clearwater.
From these studies it was theorized that immediately after the meteorite impact the crater was 600 metres deep and its rim was over 100 metres high. But over the eons it was “modified” by Devonion period sedimentary deposits and an estimated 220 metres of vertical erosion (Grieve and Cintala, 1981). The most recent erosion was caused by four or more ice ages, the last of which ended over 11,000 years ago. The gradual addition of the sedimentary layers in the crater tended to compact the under-laid rubble layer causing the bottom of the shallow sea occupying the crater to sink. The sedimentary layer grew in the bottom of the deep basin (crater) and was protected from erosion of downstream running water and glaciers flowing over the crater. This image taken from the south-east illustrates the bowl shape remnant of the crater with the crater floor capped by a 250 metre thick layer of sedimentary rock. If it were not for this layer of sedimentary fill displacing the water, the Brent Crater would resemble the water filled Pingualuit Impact Crater.
General Area: South of the Ottawa River in the Canadian Shield in an area of rolling hills. The area has been glaciated. The target rocks are crystalline.Specific Features: Brent crater has been heavily eroded and is partially in-filled by post-crater sediments. There are two lakes within the crater, forming a semi-circular hoof-print shape. The remnants of the crater form a 3 km circular depression 60 m deep. The interior of the crater is noticeably smoother than the surrounding terrain and the structure clearly cuts across pre-existing folds and tectonic trends in the crystalline bedrock.
The topographical, geophysical and geological investigations carried out at the crater have documented the contents in the bowl shaped depression as (from the top down):
- >250 metres of sedimentary fill (deposited after the impact in the Devonian period) – limestone, dolostone, sandstone, siltstone, shale and gypsum;
- ~600 metres of brecciated zone;
- ~20 metres of melt zone;
- ~50 metres of fractured crystalline basement over the bedrock, and;
- Bedrock, 1065 metres under the surface of the center of the crater floor, consisting of Precambrian crystalline igneous-metamorphic basement complex mainly of gneiss of granodioritic composition of the Grenville structural province (Grieve, 1978).
The meteorite type is inferred as an L or LL chondrite from analysis of the impact melt samples for siderophile trace elements and for a Ni-Cr correlation (Palme et al., 1981).
Old K–Ar Mineral Ages from the Grenville Province, Ontario
M. R. Dence, J. B. Hartung, J. F. Sutter
ABSTRACT – Hornblende-rich concentrates from quartz–feldspar gneisses of the Grenville Province near Brent Ontario, have yielded K–Ar apparent ages of 1570 to 1480 ± 80 m.y., while coexisting biotite- and feldspar-rich separates give ‘normal’ Grenville K–Ar ages near 900 ± 40 m.y. Comparison with the nearest Rb–Sr isochron dates suggests that the indicated hornblende K–Ar age represents a minimum age for time of crystallization of the gneisses in the Brent area and that the younger ages for minerals with lower blocking temperatures indicate a later thermal event in the metamorphic history of the Grenville Province.
A gravity anomaly at the Brent Crater produced by the sediments and fragmented rocks in the crater reinforces the meteoritic origin of this crater similar to other structures (seeWest Hawk and Wanapitei) that have been identified as impact events by similar gravity anomalies. It is interesting to note that in this gravity map that was published in 1960 the magnetic north had an indicated west declination (variation) of 10° 05’ W. Today in 2012 it is 12° 00’ W. The change is due to the drift of the magnetic north pole over the past 52 years (Chavez 1986).
Ground Exploration of the Brent Impact Crater – Part I
Not being satisfied with aerial explorations of the crater, I just had to visit the feature on foot for a full appreciation of what happened there. At one of my Ottawa RASC presentations I mentioned my plans and offered a day of adventure for anyone who wished to accompany me. Barry Mathews and Dale Morland expressed an interest and before you knew it we ( L-R Barry, Chuck (author) and Dale) were standing at the observation tower (position #1 in the crater tour image) beginning our adventure at the Brent Crater south rim. Tecumseh Lake is visible over Barry’s right shoulder. We planned the tour for mid-spring 2003, to avoid the bugs. Also, some of the swamps would still be semi-frozen, allowing us to explore areas that would normally be isolated in the summer because of the bogs. The exploration trip I had planned had a few “off trail” segments.
In image at left taken from the observation tower (position #1) the Brent Crater is visible to the north and northwest. The far rim, about 4 kilometres away, rises about 150 metres above Tecumseh Lake which is visible at the right (north east) in the image. Gilmour Lake is hidden behind the glacier sculpted sedimentary fill visible as the small wide hill in the mid background. Later that day we were going to be standing on top of that hill. As we later found out, this view from the tower is the best view of the crater we would see from the ground.
From the observation tower we followed the trail down the south east rim to the crater floor and saw plenty of wild life tracks in the snow. Some of the tracks were pretty big! That’s OK though, I think I could out run the other two guys!? Near the bottom of the rim a little creek has carved out a gully in the soft gritty limestone rock material that is not found anywhere else in the Park (position #2). This rock was formed when erosion of the crater rim built up a pile of fallen rock fragments called talus (the fossils of the Burgess Shale are also encased in talus). The sharp edges of these rocks were slowly rounded off by wave action of the sea water that partially filled the crater during the Devonian period. Mud filled the spaces between the fragments and eventually solidified into gritty limestone. The original talus fragments are now imbedded in the limestone. The ferns that grow here are “bulblet bladder fern,” a species common in the limestone areas of southern Ontario, but not found anywhere else in Algonquin Park.We followed the trail down the south rim to the bottom of the crater (position #3). The hiking was not difficult on the trail. Fortunately for us, the original 45° angle of the crater rim had long since been eroded to a semi-gentle slope.
|Over 400 million years of erosion had erased 220 metres of bedrock, the bowl shape of the impact crater is still visible. At higher altitudes the three dimensional feature of the crater is difficult to resolve which probably explains the relatively recent recognition of this crater. Maps published as late as 1946 do not accurately depict the two lakes in the crater.|
At the end of the trail is a mail box with a log book inside. We signed and dated the book and found that we were the first explorers of the crater for 2003!
Normally the ground tour of the Brent Crater would be complete at this point. But I noticed on the topographical maps that the highest point of the crater rim is on the north east portion of the rim and is accessible by road. Well, we just have to go and see the great view of the crater from up there and it would be a pleasant drive! Unfortunately snow had blocked the road and we were forced to “foot” it (position #7). Well, after about a 3 kilometre walk (through snow and mud) we made it to the highest point on the crater rim and you can see from this image the great view we had!
After a long slog back to the van, we headed home to Ottawa. What a great day!
Ground Exploration of the Brent Impact Crater – Part II
In April 2006 a group of keen amateur crater geologists (rockhounds) from the Ottawa RASC re-explored the Brent Crater. The purpose of the expedition was to find a deposit of impact breccia that I understood was on a creek bed somewhere in the south-east arc of the crater. Various papers on the Brent Crater that I had studied indicated this. My planning centered on the creeks in the south-east rim area and how we could systematically explore them. Again I chose the early spring for the expedition in order to avoid the “bugs”. Our search for the breccia deposit was in vain but we did encounter a spectacular structure related to the impact along with other geological impact features.