Odale-Articles-Brent
From OttawaRasc
IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
Brent Impact Structure
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The Brent meteorite crater, highlighted by the circle in the landsat image, is located within the northern boundary of Algonquin Park 75 km east of Lake Nipissing [Note 1]. 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 (Robertson & Grieve, RASC Journal, Feb 1975). This led to the confirmation of the Holleford crater as an impact site. As the Observatory program became known, others reported unusual, circular topographic features in Canada such as Brent and Clearwater (R.A.F. Grieve, RASC Observers Handbook, 2003).
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 [Note 2]. 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.
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
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).
This winter view of the crater from the east-south-east illustrates the eastern, northern and western rim remnant of the crater. Comparing this image with the landsat image illustrates the distance to Lake Nipissing from the crater, visible 75 km away on the horizon [Note 3]. When the last glacier entered the crater over the north rim (from the right in this image) 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.
This view from the south looking north clearly shows the remnant of the northern rim of the Brent crater. 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! 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.
A gravity anomaly at the Brent Crater produced by the sediments and fragmented rocks in the structure is illustrated in this Bouguer anomaly map (Millman et al. 1960). This gravity anomaly reinforces the meteoritic origin of this structure 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 2003 it is 12° 00’ W. The change is due to the drift of the magnetic north pole over the past 43 years [Note 4].
 The visual “bowl” shape of the crater is accentuated in this low altitude image. Even though almost 400 million years of erosion had occurred here (220 metres of vertical erosion), the bowl shape of the impact crater is still visible from this angle. At higher altitudes the three dimensional feature of the crater is difficult to resolve which probably explains the relatively recent discovery 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 Structure – 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 were standing at the observation tower (position #1 in the crater tour image) beginning our adventure at the Brent Crater south rim [Note 5]. [To follow our undertaking, please refer to this image where I have traced out the path of our explorations].
Here are our intrepid explorers, Barry, Chuck (author) and Dale at the observation platform on the south eastern rim of the Brent meteorite crater (#1). 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.
From the observation tower at Muskwa Lake on the southern rim (#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, visible at the right (north east) of 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 (#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 at one time 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 (#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. 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.
Looking 180° from the previous photo onto the crater floor is yours truly (still at #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!
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 there!” 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.
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 (#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.
This is another image at ground zero looking south west (#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.
We made it back to the trail from ground zero without too much trouble and stopped here at the south edge of Tecumseh Lake (#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.
Following the trail and climbing the south rim we arrive at the only bedrock outcrop we were to find (#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!
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 (#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!
 Here Chuck and Dale are proudly standing at the highest point on the crater rim (#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 (#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 structure 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 Structure – Part II
In April 2006, Coral MacDonell, 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.
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 (A) was at the “shattered rock” cliff that I had visited on my first expedition (illustrated above as #6).
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.
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 shattered rock deposit (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 structure. Modern day volcanoes are surrounded by rocks that are penetrated by veins of cooled lava. They are otherwise basically intact and little disturbed. 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.
There is a mini waterfall upstream where the creek meets the interface between the bedrock of the area and the easily eroded shattered rock of the crater. We followed the creek back up the crater wall for our lunch break.
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 (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.
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 structure gave us an appreciation for the magnitude of the event that occurred here 396 million years ago.
Ground Exploration of the Brent Impact Structure – Part III
Eric Kujala and I explored the lakes in the bottom of the Brent impact structure by canoe in the fall of 2007, to get a first hand appreciation of the size of 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 structure was covered by post impact sedimentary deposits. This had the effect of “preserving” the Brent impact structure. 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.
 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 structure 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. This layer was protected from glacial erosion by the crater’s bowl shape. |
 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. |
 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 structure. My previous ground exploration trips to the Brent structure were in search of the “elusive” impact breccia. Ironically, this time the breccia search was still on my mind but it was not the main reason for this trip. And what did we find, IMPACT BRECCIA!! Finally!! |
 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 structure, 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 structure. |
 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). Geochemical analyses show that the “melt” rocks are in fact melted target rock with ~1% contamination by chondritic material (Note 5). |
A side note; 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!!
Notes
- The Brent meteorite crater is a short 1.5 hour flight north of Ottawa. On the way the circular white shape of the mothballed 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.
- Aerial Radar Courtesy of the Planetary and Space Science Centre at the Image courtesy of Earth Impact Database, UNB, 2003.
- In the Landsat image observe Lake Nipissing to the west of the Brent meteorite crater. There are two circular features on Lake Nipissing whose geological features are unexplained. They are documented in the following two images taken from the north coast of the Lake Nipissing looking south.
- At the extreme east of Lake Nipissing is a circular feature that resembles a caldera.
- In the eastern area of Lake Nipissing are the Manitou Islands, a series of small islands placed in the form of a circle.
- A third feature, a semicircular indentation into the north coast of Lake Nipissing at Meadowside, was probably caused by wave action erosion of the soft coast material.
- “An optimisation study of gravity data from the Brent crater” by Dr. Rene e. Chavez, 1960.
- “Impact Structures in Canada”; Richard A.F. Grieve. Geological Association of Canada.
