Odale-Articles-Brent

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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

BRENT IMPACT CRATER

The Brent meteorite crater is highlighted by the circle in this landsat image
The visual “bowl” shape of the Brent crater is accentuated in this low altitude image.
  • Age (ma): 396 ± 20
  • Diameter: 3.8 km
  • Location: Ontario, Canada. N 46° 05' W 78° 29'
  • Shock Metamorphism: Shatter cones and PDF in quartz and feldspar.
  • Dating Method: K-Ar studies on the coarsely crystalline melt rocks using post 1977 decay constants (J. Whitehead).
  • Impactor type: Ordinary chondrite; type L or LL - siderophile elements (PGE, Ni, Au) (Tangle, Hecht 2006).
  • Overflight of the Brent Impact Crater in my Cessna C177B - C-GOZM (GOZooM)

Possible link to the Late Devonian Extinction
The red dot represents the approximate area of the Brent impact 396 million years ago in the late Silurian or early Devonian Period.
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.

This high altitude image of the Brent crater shows a vague circular area with two bordering lakes.

Early in 1951, Mr. John A. Roberts was looking over some of the high altitude aerial photos, similar to the above image that his aviation company had taken for the Government of Canada. He noticed that Gilmour and Tecumseh Lakes form a semicircle in a circular feature straddling the boundary of Algonquin Park north of the village of Brent on Cedar Lake. After consultation with the Dominion Astronomer for Canada and the Geological Survey of Canada, investigations were initiated at the site. Over the next ten seasons topographical, geophysical and geological investigations (including diamond drilling of 12 holes into the crater) were performed. Greater than 5,000 metres of drill core were recovered. Brent aerial image courtesy of Earth Impact Database, UNB.

The Brent crater from my airplane, GOZooM, from about 3000' AGL.

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 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):

Odale-brent-04 brent cross section.gif
  • >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).


Aerial Exploration

A winter view of the Brent crater taken from the east-south-east at about 4000' AGL. Lake Nipissing is visible 75 km away on the horizon. Glaciers entered the crater from the right (north) in this image.
After impact, Ordovician seas deposited a thick layer of sediment over the crater. The crater was thus insulated from erosion until the last glaciers removed the sedimentary covering. The total amount of erosion undergone in the Brent crater area is estimated to be ~400 m (Grieve and Cintala, 1981). When the last glacier entered the crater over the north rim and reached the floor of the crater, it apparently spread out and gouged into the sedimentary rock along the northeastern and northwestern edges of the floor a little more deeply than elsewhere. The resultant slight depressions in the floor were filled with water when the final glacier retreated 11,000 years ago, creating the two lakes Tecumseh and Gilmour. The glacier also sculpted the area between the two lakes with a “ripple” superimposed on the landscape.
The Brent crater imaged from the south clearly shows the remnant of the northern rim. If this impact happened today, every tree in Algonquin Park would be flattened and covered with ejecta, Ottawa would experience a major earthquake and the most of the windows in the city’s buildings would be blown out!


Bouguer gravity anomaly over Gilmour and Tecumseh lakes region. The crater lies at the center (Modified from Millman et al, 1960.

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 (see West 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).

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.

Ground Exploration of the Brent Impact Crater – Part I

To follow our ground explorations of the Brent crater, please refer to this image where I have traced out the path of our explorations.
Barry, Chuck and Dale fresh and ready for a day of Brent crater explorations.
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.

The Brent crater taken from the observation tower looking north. The northern rim of the crater is clearly visible in the distance.

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.

A typical creek in the Brent crater. I explored these creeks trying (unsuccessfully) to find in situ impact breccia.

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.

Here Dale and Barry are on the southern crater rim bottom, about 1 metre above the crater floor. Note the vegetation in the background, not too thick and fairly easy to navigate through.

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.

Turning 180° from the previous photo we get a look at the foliage we have to trudge through on our Brent crater explorations.

Looking 180° from the previous photo onto the crater floor is yours truly (still at position #3). Note the vegetation in the background, the horizontal visibility in there is 4 or 5 metres at the most. That is where we are headed next for the “off trail” portion of our exploration! Even though the swamps were still semi-frozen with lots of remaining snow, we still got wet up to our knees!

Ground zero at the Brent crater.

Why did I want to trek through that swamp and cedar grove? Why, to get to ground zero, the area of the original meteorite impact! I took this image in the winter from less than a thousand feet above the crater floor and thought “what a great view of the crater it must be from that point!” Tecumseh Lake is on the right (east) and Gilmour Lake is on the left (west). In the bottom (south) of the image is the edge of the swamp that we slogged through. The visibility was very restricted while we were in the swampy cedar grove and it would have been very easy to get disoriented in that mess. Note how the last glacier has sculpted ripples into the sedimentary fill between the lakes.

On the ground at ground zero in the Brent crater.

It was a good thing that the sun was out as had I forgotten my compass! Keeping the shadows in the correct relative place prevented us from being lost, well not much anyway! Like in this image, “I think we are here!?” Actually, we are at ground zero (position #4) in this image planning our return to the trail (and we did find it first try!). Here 390 million years ago an object 150 metres in diameter impacted with a velocity of at least 11 km/sec. If we were there at the time of impact, we would not have heard the approach or knew what hit us.

Vegetation at ground zero of the Brent crater.

This is another image at ground zero looking south west (position #4). Great view EH? We couldn’t see anything of the crater! But at least we could say that we stood exactly at the position of impact! Barry and Dale are in the background with a very strange tree formation in the foreground. We had stopped for a snack break. The majority of trees here on the sedimentary fill mound were deciduous while the trees in the swamp and on the rim tended to be coniferous.

Tecumseh Lake south: from here years later, Eric and I will expedite a canoe exploration of the crater AND I WILL FINALLY FIND IMPACT BRECCIA!

We made it back to the trail from ground zero without too much trouble and stopped here at the south edge of Tecumseh Lake (position #5). Tecumseh and Gilmour Lakes have the highest concentration of bicarbonate of any lake in Algonquin Park. Bicarbonate is derived from calcium carbonate which is limestone. Gilmour and Tecumseh, alone among Algonquin Park lakes, are lying on Devonian limestone bedrock. This limestone would not be here if it were not for the bowl of the crater where it collected and was protected. In this marsh at the edge of the lake pitcher plants grow. They trap bugs to enhance their diet which is deficient due to the nutrient poor soil.

Shattered bedrock within the Brent crater.

Following the trail and climbing the south rim we arrive at the only bedrock outcrop we were to find (position #6). This shattered bedrock has the characteristics of rock exposed to a nuclear blast or a meteorite impact. I think we can eliminate the possibility of a nuclear explosion happening here 396 million years ago! The force of the explosion is estimated to have been equivalent to the explosion of 250 megatons of TNT. I was fascinated to see the effect first hand, a wall of bedrock with this amount of damage!

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!

The highest point of the Brent crater rim.

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! If you look carefully in the center of the image you can almost see Tecumseh Lake. The haze on the horizon in the background is the west crater rim. From this point if you tried to walk downhill toward the crater for a better view the relative tree level goes up and blocks the view. From the sun angle you can tell it is getting late in the day, a full day of hiking!

Chuck and Dale on the highest point of the Brent crater rim.

Here Chuck and Dale are proudly standing at the highest point on the crater rim (position #8), a big two thumbs up! The rocks visible underfoot are all glacial till from who knows where! Oh well, at least we can claim that we had stood at the highest point on the crater rim as well as at ground zero (position #4). Reflecting back on the distance we walked to explore the crater has given us an appreciation of the energy that was required to create this crater in a matter of seconds!

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, Coral M., Hans Brouwer and I, a group of keen amateur crater geologists (rockhounds) from the Ottawa RASC 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.

This image illustrates the two crater tours our fearless explorers followed into the Brent Impact Crater. The points of interest we found (see text) are indicated by the capital letters.

That morning we met at the lookout station that overlooks the crater. Our exploration started at a dry creek bed in the south-east corner of the crater bowl and following it down to the crater floor. There was very little exposed rock in any of the creek beds as the crater wall was thickly covered by glacial till. Our first stop (position A) was at the “shattered rock” cliff that I had visited on my first expedition (illustrated in Part I as position #6).

Hans and Chuck on the floor of the Brent crater.
Here on the crater floor (position A), Hans (left) and I take a rest. At this point we will leave the groomed trail and enter the bush for some hard slogging.

There is an “arc” of this shattered rock around the south-east bowl of the Brent Crater. From this first shattered rock exposure we descended to Tecumseh Lake that is situated on the floor of the crater. We then traveled north along the east shore of the lake to find the mouth of the second creek that I wanted to explore. We would follow this creek back up the crater rim in our search for the breccia. The slogging was pretty tough once we got off the groomed trail.

Mini grand-canyon on the floor of the Brent crater.
The steep south Brent crater rim is illustrated in the foreground of this image.
We encountered a very interesting structure bracketing the creek that we were following up the south-east crater rim. The creek had eroded a mini-canyon through the easily eroded shattered rock on the crater wall. (position B). The walls of shattered rock were over 10 metres high! Shattered rock deposits like these helped to confirm the impact origin of the Brent Crater. Modern day volcanoes are surrounded by rocks that are penetrated by veins of cooled lava. They are otherwise basically intact and little disturbed. There is a mini-waterfall just upstream from this canyon. A big meteorite striking the earth can deliver the energy equivalent of a hydrogen bomb, shocking and shattering the rocks like those visible here in the walls of the mini-canyon. We followed the creek back up the crater wall for our lunch break.

Directly behind where Coral is standing you can make out the steepness of the remnant of the crater wall at Brent. It has kept most of its form here despite 396 million years of erosion.

After lunch, our second tour into the crater started a bit further to the west, from the creek originating at Rand Lake. Again, there were no bedrock exposures along the creek as the glacial till was too thick. The creek did reveal talus deposits (position C) near the floor of the crater. The talus in the crater was formed when the crater wall was eroded creating built up piles of fallen rock fragments (talus). The motion from the water that filled the crater washed into the talus slope and eroded the sharp edges of the rock fragments and filled the spaces between the fragments with mud. Over time the mud solidified into gritty limestone.

Fossilized talus deposit on the floor of the Brent crater.
The Brent Talus deposit is close to the floor of the crater at the junction of two creeks.

From the talus deposits we ascended the crater wall following the creek to Maskwa Lake in our vain search for the impact breccia deposit. Even though we were unsuccessful in our search for the breccia, our trip through the impact crater gave us an appreciation for the magnitude of the event that occurred here 396 million years ago.


Ground Exploration of the Brent Impact Crater – Part III

In the fall of 2007, Eric Kujala and I explored the lakes within the crater by canoe to get a first hand appreciation of it's size. First we had to lug the canoe down into the crater! The portage down to the crater bottom from the road is not trivial; it is a physically demanding exercise. But, the trip through the lakes is well worth the effort.

Shortly after the initial impactor contact here 396 million years ago, the crater was covered and protected by a post impact sedimentary rock layer. This had the effect of “preserving” the crater's form. Usually a crater of this age on earth would have had substantial geological erosion and would not have conserved its “crater shape”. The remnant of the Presqu’ile impact structure is an example of the magnitude of erosion occurring without protection over a similar length of time.

Lake Tecumseh within the Brent crater.

Here the north Brent Crater rim is illustrated in the centre background of this image taken from within Tecumseh Lake. It is hard to imagine a 1 km thick layer of ice flowing over this rim heading south. About 10,000 years ago the protective sedimentary layer over the impact crater was finally eroded away by these glaciers. In the left (west) of this image is the glacial till deposit in the bottom centre of the crater. Under this till is a 200 Metre thick layer of sedimentary rock layer. This is a remnant of the sedimentary layer that "protected" the crater and was in turn protected from glacial erosion by the crater’s bowl shape.

North shore of Gilmour Lake looking west within the Brent crater.

The north shore of Gilmour Lake, illustrated here, is directly adjacent to the north rim of the crater. The Brent Crater rim is covered by a substantial layer of glacial till making it almost impossible to find any bedrock (and maybe in situ breccia). The bush in this area was described to me by a forest ranger as a “tree slum”. The dead brush between the trees makes this area almost impassable for exploration.

East shore of Gilmour Lake looking south within the Brent crater. The south rim of the crater is visible in the distance.

Looking south-west along Gilmour Lake we can see the south crater rim in the distance. This image gives you a true appreciation of the size of this impact crater.

My previous ground exploration trips to the crater resulted in unsuccessful searches for the “elusive” impact breccia. Serendipitously, on this trip we did we find, IMPACT BRECCIA!! Finally!!

Brent crater impact breccia.

We were very fortunate to spot these breccia examples. They WERE NOT found as in situ deposits but were most probably placed here by the glaciers (glacial erratics). So, scientifically, without material analysis we cannot absolutely claim that this is breccia from the Brent Crater impact, BUT, the circumstantial evidence is almost conclusive. The other explanation is that these deposits were from another impact site further to the north and just “happened” to be dropped off here within the Brent Crater.

Brent crater impact breccia.

The impact melt is visible here as the “greyish” material between and cementing the country rock fragments. K-Ar dating of the recrystallized melt-bearing breccia gave ages of 310-365 Ma (Shafiquallah et al., 1968), since updated by K-Ar studies on the coarsely crystalline melt rocks using post 1977 decay constants. Pre-1977 K-Ar, Ar-Ar and Rb-Sr ages recalculated using the decay constants of Steiger and Jager (1977) Ages in millions of years (Ma) before present. Geochemical analyses show that the “melt” rocks are in fact melted target rock with ~1% contamination by chondritic material.


Side notes;

1. beavers are very quiet at night! Many of the little critters brushed by my tent while I was “trying” to sleep. They smell of wet dog!!

2.
Algonquin radio observatory.
The Brent Impact Crater is a short 1.5 hour flight north of Ottawa. On the way the circular white shape of the Algonquin Radio Observatory at Lake Traverse is very obvious. I’m dating myself, I had a tour through the complex when it was operational in the early 1970s.

References

Chavez, R.E., An optimisation study of gravity data from the Brent Crater. First Break, Feb. 1986.

Grieve, R. A. F., Cintala, M. J., A method for estimating the initial impact conditions of terrestrial cratering events exemplified by its application to Brent Crater, Ontario. Proceedings Lunar and Planetary Science Conference 12th, pp. 1607-1621. 1981.

Grieve, R. A. F., Dence, M. R., Principle characteristics of the impactites at Brent Crater, Ontario, Canada (abstract). Lunar and Planetary Science IX, pp. 416-418. 1978.

Grieve, R. A. F., The melt rocks at Brent Crater, Ontario, Canada. Proceedings Lunar and Planetary Science Conference 9th, pp. 2579-2608. 1978.

Grieve, R. A. F., The petro-chemistry of the melt rocks at Brent Crater and their implications for the conditions of impact (abstract). Meteoritics, v. 13, pp. 484-486. 1978.

Grieve, R.A.F.,Robertson P.B., IMPACT STRUCTURES IN CANADA: THEIR RECOGNITION AND CHARACTERISTICS Journal of the Royal Astronomical Society of Canada, V69, 1-21, Feb 1975

Meen, V.B., CHUBB CRATER - A METEOR(sic) CRATER, Journal of the Royal Astronomical Society of Canada, V44, 169-180, 1950.

Millman, P. A., Liberty, B.A., Clark, J.F., Willmore, P. and Innes,M.J.S., The Brent Crater. Ottawa Dominion Observatory Publication, v. 24, 43 p. 1960.

Palme, H., Grieve, R.A.F. and Wolf,R., Identification of the projectile at Brent Crater, and further considerations of projectile types at terrestrial craters. Geochimica et Cosmochimica Acta, v. 45, pp. 2417-2424. 1981.

Shafiqullah, M., Tupper, W.M. and Cole,T.J.S., K-Ar ages on rocks from the crater at Brent, Ontario. Earth and Planetary Science Letters, v. 5, pp. 148-152. 1968.

TAGLE, R. and HECHT, L., Geochemical identification of projectiles in impact rocks. Meteoritics & Planetary Science Volume 41, 26 JAN 2010.

Earth Impact Database


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