Astro Geology Articles Published in AstroNotes
by: Charles O'Dale
Searching for Impact Structures / Multiple Craters - AstroNotes May 2006
Throughout the solar system there are many examples of multiple crater impacts. One of the assumed causes of this class of impact crater is collisions with binary asteroids. In this article I will document confirmed and suspected multiple craters in Canada and I will add my personal hypothesis on a couple of enigmas about these structures.
An example of a binary asteroid is the Ida and Dactyl pair, imaged by the Galileo probe in 1993. A google search for binary asteroids reveals a long list of binary asteroids, with at least twenty six in the vicinity of the Earth’s orbit. What would be the sequence of events following the collision of a large binary asteroid with the earth?
The mass of air displaced by a meteorite of thousands or millions of tons will be insufficient to produce appreciable retardation during its passage through the atmosphere and it will reach the surface of the earth with little loss of velocity. Impact velocity could range anywhere from 12 kilometres/second to 70 kilometres/second. The high instantaneous pressure generated on impact is comparable to that at the centre of the earth and will be shared by the meteorite and by the volume of rock into which it impacts. The impact will have two principal effects;
· A pressure of this magnitude will give rise to a shock wave which will vaporize a certain proportion of the material involved, melt a somewhat larger proportion and shatter large volumes of rock extending to distances of the order of 50 times the diameter of the impacting meteorite, and;
· As the meteorite comes to rest, the mixture of highly compressed rock and meteorite fragments will receive an upward and outward acceleration mainly from the decompression of shocked material, comparable to a powerful explosive. 
The resultant crater will vary from simple to peak-ring (or multi-ringed as seen on the moon’s surface!). Descriptions of the various types of impact craters may be viewed here. The resulting raised rims and the relatively shallow depressions of the impact craters will be particularly vulnerable to the Earth’s geological processes of erosion and deposition. This makes certain structures very difficult to determine whether or not they are impact related. Fortunately, the work pioneered by the Canadian Dominion Observatory in the 1950’s and 1960’s, described by Dr. M. R. Dence and Dr. B. Robertson at our Ottawa RASC monthly meetings, allows us to unambiguously identify that certain structures on earth are in fact impact related. 
An excellent example of the result of a collision between the earth and a binary asteroid is at the Clearwater Lakes double impact craters. These structures are confirmed as impact related. The 32 km. diameter Clearwater Lake West (left) shows a prominent ring of islands with a diameter of about 10 km. The islands constitute a central uplifted area and are covered with units of breccias and impact melt. The central peak of the 21 km. diameter Clearwater Lake East (right) is submerged. This twin crater phenomenon is very rarely recognized on Earth.
But, coincidentally, there is another twin impact site relatively nearby that features two confirmed impact structures. They are in the Canadian Shield at Sudbury, Ontario, but these impact events are totally unrelated. The impact that resulted in the Sudbury Crater (left in the image) happened almost two billion years before the Wanapitei Crater impact (right). The Sudbury Structure is interpreted to represent the tectonized and deeply eroded remnant of a multi-ring or peak-ring impact with an estimated diameter of 250 km. Wanapitei is classified as a simple crater because of its estimated diameter of 3 km to 8 km with no evidence of a central uplift in the submerged crater. So, if you are worried about being bonked on the head by a large meteorite, move to Sudbury. That area has had its quota of large impacts in the recent eons!
The Manicouagan impact structure in central-northern Quebec is one of the largest impact craters still preserved on the surface of the Earth. The annular moat, prominent in space images, fills a ring where impact-brecciated rock was once eroded away by glaciation. The 100 km. diameter of the original crater is approximately three times the size of this circular lake. Erosion has removed about a kilometre of rock from the region. The inner plateau remaining in the center of the annular moat is made up of metamorphic and igneous rock types along with melt sheet and is not as susceptible to glacial erosion. Now, look closely at the upper left portion of the image of Manicouagan. Notice the two curved lakes branching out of the annular moat to the north-west. My hypothesis is that the area enclosed by these two curved lakes may be the remnant of a twin crater of Manicouagan. To my knowledge there has not been an impact related study of this area.
Last summer (2005) I had the opportunity to over-fly the Merewether structure on one of my aerial exploration trips to the northern part of Labrador. The Merewether structure is located north of the tree-line in Labrador approximately 93 kilometres south-west of the Saglek Fiord. It has been on the “list of suspected impact craters”  for at least 55 years and it remains an enigma. Irrefutable evidence to indicate an impact event here has not been discovered to this date (April 2006) and there are no known natural geological causes to explain the formation of the structures. This image illustrates the three almost perfectly circular bowl shaped craters that compose the Merewether structure. Their diameters are 198, >46, >15 metres respectively largest to smallest. I hypothesize that the Merewether structure may have been caused by an impactor that split into at least three parts upon its collision with the Earth’s atmosphere. The parts then impacted the earth at a position covered by glaciers. The impactor(s) then penetrated the ice to the underlying moraine material. The glacier removed any trace of the impactor(s) and eroded the craters.
For a more detailed description of the structures described in this article please visit these articles.
 Dence (et al), ON THE PROBABLE METEORITE ORIGIN OF THE CLEARWATER LAKES, QUEBEC. The Journal of the Royal Astronomical Society, Vol. 59, No.1, 13-22.
 Dence, STRUCTURAL EVIDENCE FROM SHOCK METAMORPHISM IN SIMPLE AND COMPLEX IMPACT CRATERS: LINKING OBSERVATIONS TO THEORY. Meteoritics & Planetary Science, 30 Nr2, 267-286 (2004).
 Robertson & Grieve, IMPACT STRUCTURES IN CANADA: THEIR RECOGNITION AND CHARACTERISTICS. The Journal of the Royal Astronomical Society, Vol. 69, No.1, 1-21.
Exploring the Pingualuit Impact Crater - AstroNotes Oct 2008
By Charles O’Dale Ottawa Centre President
My crater exploration partner, Eric Kujala, and I had been planning a ground exploration to the Pingualuit Impact Crater for some years. The problem that prevented an earlier exploration was “how to get there”. All our planning efforts changed with the November 2007 opening of the Parc national des Pingualuit at the crater, see www.nunavik-tourism.com/parks.aspx. An airstrip was constructed at the crater which meant that we could make it to the crater by simply chartering an airplane from Kuujjuaq. In August 2008, Eric and I spent four days at the crater living our dream.
Our first day at the crater area 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. From the camp site the rim of the crater looked like a small hill in the distance. The temperatures here in August ranged from just freezing at night to a pleasant 20° C during the day. The water in my canteen froze during the nights.
Early in the morning of the next day we proceeded up to the crater rim. The walking was extremely difficult as the ground in the area is covered with large fragments of rock. The rim rose continuously in 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 composed of a jumbled heap of fragments of granite. These fragments cover the ground so completely that for a distance of nearly 5 km beyond the rim there is almost no other rock (see Figure 1).
After a climb of 100 m up a 10° slope we made it to the top of the rim. 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 (see Figure 2). Toward the centre of the crater we came to a steep 30° descending talus slope. The boulders on the slope are very unstable making it unsafe for a descent to the lake at that point (see Figure 3).
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 northeast position giving the crater a lopsided cup shape. It was perfectly silent; we could hear the waves breaking on the inner rim 150 m below us. 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!
Our 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 climb in and out of along the lip of the rim (see Figure 4). 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. We even found wild blue berries on the inner slope. Being an amateur rock hound, I kept searching unsuccessfully for any rock fragments on the crater rim that would have been created by the impact.
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 example of shattered bedrock that I found was most compelling. I could not even imagine the magnitude of the impact forces originating from more than 6 km away that caused this rock to shatter so thoroughly (see Figure 5). About 5 km east of the crater I was fortunate to find a large example of impactite. I also experienced a close encounter with a few caribou and found an old Inuit campsite. Unfortunately, I did not find any shattercones.
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 up here. I also have the utmost respect for past exploration teams, who day after day for months climbed 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! My exploration of the Pingualuit Impact Crater and local area was very rewarding, an experience I will treasure the rest of my life.
A technical description of the Pingualuit Impact Crater can be viewed at my web site.
Figure 1. The outer rim of the crater is covered with these granite blocks making it a challenge to safely climb up the 100 m, 10° slope. This rock field surrounded the crater to a distance of over 5 km.
Figure 2. The lake within the crater only became visible as we climbed over the flat peak of the rim. The far rim is 3 km in the distance, a challenging hike! Here Eric Kujalla and yours truly take a quick break before the hike around the crater.
Figure 3. The 30° inner slope of the rim is an unstable talus slope. I did not attempt to climb down to the water at this point.
Figure 4. There were numerous gullies that we had to transit in our trip around the crater. The image also gives you a good distance perspective as the people leading the hike are just visible on the rim in the distance.
Figure 5. This is one of the few in situ samples of bedrock that I had found around the crater. This example was completely shattered by the impact from over 6 km away. The rim of the crater is visible in the far background.
Geological Effect of Impactors on This Planet - AstroNotes Oct 2009
In this article I will describe the geological effects of impactors on this planet that I have researched and/or personally explored. My interest in this subject dates back to the 1950's when I first saw Dr. Meen from the Royal Ontario Museum describe his exploration of the Pingualuit (then Chubb) impact structure. My interest was rekindled in the 1990's with the exploration of one of the most fascinating craters to study on this planet, the Chicxulub Crater. One of the original researchers of this structure was Dr. A. Hildebrand who investigated the KT boundary outcrop on a side valley of the Brazos River in Texas. It was there that the first tsunami deposit from the impact was recognized and the data documented was used to find the crater location (Alvarez W. 1997). The Chicxulub Crater has a prominent ring of cenotes circling the impact structure. The cenotes outline the crater rim even though the crater itself is buried under ~ 1km of sediment.
I will document two other structures, the Chesapeake and St. Martin Meteorite Craters, which are similarly buried. They are totally unrelated to each other with respect to time, size and place, but like Chicxulub, have many similar structural and stratigraphic features that have affected the geology around them. I will document these geological anomalies and conclude with a hypothesis based on the empirical data described here.
The Chesapeake Impact Crater
About 35 million years ago a 3-5 kilometres in diameter impactor hit the western Atlantic Ocean on a shallow shelf, creating the Chesapeake Bay impact crater. At this time the sea level was much higher and the coastline was in the vicinity of Richmond, VA. The crater is approximately 200 km southeast of Washington, D.C. and is now buried 300-500 metres beneath the southern part of Chesapeake Bay. Analysis of seismic profiling has determined that the crater is 85km in diameter and 1.3km deep. It is a complex peak-ring crater with an inner and outer rim, a relatively flat-floored annular trough, and an inner basin that penetrates the basement. The inner basin includes a central uplift surrounded by a series of concentric valleys and ridges.
A 1.3km thick rubble bed of impact breccia fills the crater and forms a thin ejecta blanket around it. Compaction of this breccia produced a subsidence differential, causing the land surface over the breccia to remain lower than the land surface over sediments outside the crater. Another consequence of the impact is that all ground-water aquifers were truncated and excavated by the impact. In place of those aquifers is a reservoir of briny water that is 1.5 times saltier than normal seawater.
Most rivers in the area, like the Rappahannock, flow southeastward to the Atlantic. In contrast, the York and James rivers make sharp turns to the northeast where the outer rim of the crater traverses the lower York-James Peninsula. The abrupt diversions of the lower courses of the James and York Rivers (indicated by the small circles in the map above) coincide with the Chesapeake crater rim. The cause of these diversions is the differential subsidence of the outlaying country rock compared to the breccia within the Chesapeake Bay impact crater forcing a structural sag over the subsiding breccia. The river diversions are at the "rim" of this sag.
Chesapeake Crater References
- D.S. Powars and T.S. Bruce, USGS, Feb. 2000; THE EFFECTS OF THE CHESAPEAKE BAY IMPACT CRATER ON THE GEOLOGICAL FRAMEWORK AND CORRELATION OF HYDROGEOLOGIC UNITS OF THE LOWER YORK-JAMES PENINSULA, VIRGINIA
- Poag C. Wylie 1999, Chesapeake Invader
The St. Martin Meteorite Impact Crater
About 219 million years ago a ~2 kilometres in diameter impactor hit north of Lake St. Martin between Lake Winnipeg to the east and Lake Manitoba to the west. At the time of impact the area was covered by ancient seas and dinosaurs ruled the Earth. The country bedrock was Ordovician to Devonian sandstones overlying Archean-aged granite of the Superior Province of the Canadian Shield. The crater is now buried by over 100 m of Jurassic red beds from these ancient seas. In recent times glacial drift has added an additional covering layer. This "cover" has made St. Martin one of the best preserved craters on this planet. Drilling and geophysical data has revealed a peripheral depression of 21.5 km in diameter and an outer limit of structural disturbance with a diameter of 38.5 km. The structure is classified as a complex crater with a central uplifted area. The uplifted area includes a surrounding annular trough. Outcrops of Precambrian granite at the crater's outer limit may indicate an inner ring of a larger central peak basin crater.
Drilling has revealed carbonate breccia, granitic breccia, suevitic breccias and impact melt rocks under the impact structure. A study (2007) detailed in the journal Geology suggests meteor impacts with the Earth can produce effects of a more subtle and insidious kind than just catastrophic extinction. The scientists said a good example was found at the Canadian town of Gypsumville, Manitoba, located within the St. Martin impact crater. Domestic wells in the town have elevated salinity, sulfate and fluoride concentrations. The groundwater with elevated fluoride is shown to occur exclusively within the impact structure, and the study is thought to be the first to document enhanced groundwater fluoride concentrations associated with impact structures.
The circle, superimposed on this aeronautical chart of the impact area, indicates the maximum geological extension of the St. Martin impact crater. Note on the chart that on the North East point of the crater the Dauphin River has an almost 180° diversion. This extreme change in direction of the river coincides with the North East extension of the crater rim. This abrupt diversion phenomenon is also documented in the Chesapeake impact structure.
The Dauphin River then follows the crater rim to the East and flows into Lake Winnipeg (Image right). My hypothesis is that the cause of this extreme diversion (Image left) of the Dauphin River at the St. Martin crater rim is a result of the subsidence differential of the outlaying bedrock compared to the breccia fill within the impact structure. To my knowledge, there is no other report describing the cause of this river's diversion at this specific location.
St. Martin Crater References
- Grieve, R.A.F. 2006, Impact Structures in Canada (Geological Association of Canada).
- Geology (Lower Hutt, New Zealand (UPI) Jan 23, 2007).