Odale-Articles-Pingualuit
From OttawaRasc
IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
Pingualuit Impact Structure
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In Northern Quebec, Canada, there is a pristine simple crater that in 1999 was renamed the Pingualuit Meteorite Crater. It is visible as the small circular structure in the mid-right side of this Landsat image, and is larger than the smallest crater on the moon that is visible by telescope from earth. The crater is 3.44 km in diameter with a depth of 400 metres. The lake which occupies the crater is 267 metres deep and it is Quebec's deepest lake. The crater rim is over 100 metres above the surface of the enclosed lake with a pitch of 40 to 45 degrees down to the water. Uplift from the original impact extends outward to a distance equal to almost twice the diameter of the crater. |
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In 1943 it was the crew of a USAAF aircraft who first noticed and took pictures (similar to this high altitude image) of a circular structure imbedded in the bedrock of the Ungava Peninsula. Because of the remoteness of the structure, it was only in the 1950’s that geologic expeditions to the crater were initiated. The geologic data gleaned from the many expeditions to this structure, conclusively identified it as a meteorite crater. |
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The first expedition to the crater was initiated by a prospector, Frederick W. Chubb, who thought the circular structure was a kimberlite tube and thus a source of diamonds. Victor B. Meen, from the Royal Ontario Museum of Geology and Mineralogy in Toronto, accompanied Chubb on the expedition. During the short preliminary investigation no meteorites (or diamonds) were found among the boulders on the rim or on the surrounding plain. Nevertheless, Dr. Meen felt quite certain that the morphology of the formation indicated that it was caused by the impact of a huge meteorite and not from volcanic action. Dr. Meen estimated that since there were no Inuit legends about the structure, the impact of the meteorite must have occurred at least 3,000 years ago. He named it Chubb Crater after the sharp-eyed prospector. |
| Dr. Meen returned the following year in an expedition sponsored by the National Geographic Society. During that return trip he discovered a magnetic anomaly on the crater rim that he thought was due to the signature of the meteorite. This hypothesis has since been proven wrong. It is now known that the majority of the mass of a large meteorite that contacts the earth at cosmic velocity will vaporize upon impact. The resulting explosion will form a circular impact crater in the target bedrock. |
The 40Ar-39Ar dating method of the impactites determined the age of the impact to be 1.4 million years. This places the impact before the first major northern hemisphere continental glaciation in the middle Pleistocene [Note 1, 2].
| Bouguer Anomaly gravity studies reveal a profile with a well-defined negative anomaly that is symmetrical with the crater. As was found in the gravity investigations of the Brent, Holleford and Barringer meteorite craters, the negative anomaly field is most likely the expression of low-density fragmental material underlying the crater floor. These craters all produce negative gravity fields due to the low density fragmental rock underlying them and the expanded crustal rocks forming their rims [Note 3]. |
In the 1950’s I saw Dr. Meen on our old black and white TV when he appeared on a CBC television program describing his expedition to the then named Chubb crater. It fascinated me when he explained the concept that there are many impact craters still recognizable on our planet and I vowed that someday I would visit his crater specifically and possibly the others he mentioned! The Pingualuit Crater is of special importance because its discovery and identification as an impact crater gave rise to the identification of 26 authenticated impact structures on the Canadian Shield (Grieve, 1991).
My dream to visit this crater finally came true with first a trip in my airplane over the crater followed by a ground expedition.
On the aerial trip over the Pingualuit Crater, I was accompanied by Mr. Terry Peters, a personal friend and flying instructor. A flight to that remote area of the Quebec Arctic is not a trivial expedition and Terry's flying expertise was very welcome. The distance to the crater from Kuujjuaq (formerly Fort Chimo), our only reliable source of fuel in that area, demanded that I make exact calculations of fuel burn, fuel load and payload to ensure a safe flight between available airports. The weather also has to be factored into the planning to ensure a safe flight. Only by carrying extra fuel on board were we able to spend less than 20 minutes orbiting the Pingualuit Impact Crater and safely make it to one of the remote airports in the area (see below for Terry Peters' description of our flight over the crater).
| These images of the crater rim illustrate the glacial erosion that has removed the ejecta and some rim material. It is estimated that the original ground plane was as much as 15 meters above the present level. Even at our altitude the apparent size of the crater was deceiving. To give an idea of the scale, if I stood on the crest of the rim and threw a baseball as hard as I could toward the lake (and I used to make it to home plate from center field!), the ball would only make it two thirds of the way to the water! During my ground expedition to the crater a few years later, I actually did try and throw a rock into the water from the top of the crater rim. The rock maybe got a third of the way down to the water!! |
This image illustrates the Pingualuit Crater taken approximately 10 km away from the north-west, at an altitude of about 3000 feet above ground. Compare this image with my image of the Barringer Crater and note the difference in the relative size of the craters. I took the images for each crater from approximately the same altitude and distance.
Ground Exploration of the Pingualuit Crater - August 2008
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Mr. Eric Kujala, a fellow RASC member, contacted me after he had read my original article (above) to inquire about my future exploration plans to the crater. At our first meeting we agreed to an informal “crater exploration” partnership with our primary desire to eventually expedite a ground exploration project to the Pingualuit Impact Crater. We investigated many avenues for access to the structure, even including walking the 90 km to the crater from Kangiqsujuaq (formerly Wakeham Bay). During the three years that we were investigating ways to get to Pingualuit, we explored many other impact craters. We did this on the ground using Eric’s canoe and, from the air using my airplane. All our planning efforts changed with the November 2007 opening of the Parc National des Pingualuit. An airstrip was constructed at the crater which meant that we could make it to the crater by simply chartering an airplane from Kuujjuaq. I could not use my airplane for the final leg of this trip due to the weight and fuel constraints caused by the extra bulk of our camping gear. Also, general aviation aircraft are not allowed to land in the park. So in August 2008, after a round-about trip flying in my airplane from Ottawa to Kuujjuaq, then air-chartering via Kangiqsujuaq, we finally arrived at our destination, the Pingualuit Impact Structure. We spent four days of exploration there living our dream. Our first day at the crater consisted of setting up our camp site and doing a reconnaissance of the local terrain. It was too late in the day for a productive trip up to the crater. I tucked my tent behind a fairly large rock for shelter from the gusting winds (they were steady at 20 knots+). From my camp site the rim of the crater was visible as a small hill in the distance, illustrated in the image. The rocky terrain we had to traverse to the crater is very obvious in the picture. The temperatures here in August ranged from just freezing at night to a pleasant 20° C during the day. The water in my canteen formed ice crystals during the nights. Early in the morning of the second day we proceeded up to the crater rim. The walking was extremely difficult with the ground in the area covered in layers with large rock fragments. The rim rose continuously during the 2.5 km walk from Lac Laflamme to the crater. We had to climb over two ridges before reaching the steep slope of the rim itself. The outside rim is covered with a jumbled heap of large fragments of granite blocks. This made it a challenge to safely climb up the 100m, 10° slope. These fragments cover the ground so completely that for a distance of nearly 5 km beyond the rim there is almost no other rock. After a climb of 100m up the 10° slope we finally made it to the top of the rim. In this picture you can see two very happy explorers savouring the moment, all our planning has finally come true!! The rim is so broad, that at its peak, we could not see the lake inside the crater nor the terrain immediately outside surrounding the rim. The lake within the crater only became visible when we climbed over the flat peak of the rim. From where we stood, it was over 3 km across to the opposite side of the rim. The rim is highest and widest at its north east position giving the crater a lopsided cup shape. On the rim it was perfectly silent, we could hear the waves breaking on the inner rim 150 m below us. The far rim is 3 km in the distance, a challenging hike! As illustrated in this image, the inner rim of the crater is a steep 30° descending talus slope. The boulders on the slope are very unstable making it unsafe for a descent to the lake at this point. The distance to the water is very deceptive. It looked so close it seemed that you could easily throw a rock into the water from where we stood on the rim. I tried and didn’t even get close to hitting the water! The hike around the crater took most of the day and I have to say it was not one of the easiest of hikes that I have experienced. There were frequent gullies that we had to traverse along the lip of the rim. This image gives you a good perspective as the people leading the hike are just visible on the rim in the far distance. Our Inuit guides were extremely helpful in showing us the various unique geological features of the crater. This included leading us to the only safe descent to the enclosed lake. The clarity of the lake is amazing to see firsthand. The water temperature was just above freezing. Close to the water we found wild blue berries. I personally made two trips to the crater during our four day visit. On the other two days I explored outside the rim documenting the effects of the impact on the local geology. The exposed bedrock of the Pingualuit Meteorite Crater’s target rock consists of a mélange of metamorphosed, Archean plutonic rocks cut by rare basic dykes (Shoemaker, 1962). This image shows one of the few in situ samples of bedrock that I had found around the crater. This bedrock example was completely shattered by the impact. The rim of the crater is visible as the small hill over 6km away on the horizon. About 5 km east of the crater I was fortunate to find a large example of highly shocked and melted impactite (illustrated in this image - the caribou antler placed on the impactite is for scale). Pingualuit impactite contains enrichments in Ir, Ni, Co and Cr suggesting that the impacting body was chondritic in composition with siderophile element enrichment (Grieve 1991). Pingualuit impactite originated from dike bodies in the crater rim, was eroded and carried 3-4 km north of the crater by water action. [Note 6] Being an amateur rock hound, I kept searching for any shattercones that would have been created by the impact, unfortunately without success. In addition I experienced a close encounter with a few caribou and found an old Inuit campsite. It was impossible to guess the age of this camp site, it could have been here undisturbed for tens or even many hundreds of years! I had a very spiritual feeling gazing at this old camp-site trying to picture the family that survived here. Having had Arctic survival training with the military, and now again experiencing this type of desolate terrain, I stated to our Inuit guides how respectful I am toward their ancestors in that they could successfully support and feed a family in this type of geography. I also have the utmost respect for past exploration teams who, day after day for months, walked to the crater rim from their base camp for their research. The two trips I made to the top of the crater rim totally exhausted me! Our exploration of the Pingualuit Impact Crater and local area was very rewarding, an experience Eric and I will treasure the rest of our lives [Note 7]. "Our Aerial Expedition" by Terry Peters - August 2001
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