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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

CLEARWATER WEST IMPACT CRATER

Clearwater West impact crater, Quebec

Clearwater West impact crater (on the left).

Age (ma): 290 ± 20
Diameter: 32 km
Location: Quebec, Canada. N 56° 13' W 74° 30'
Shock Metamorphism: PDF in quartz grains, shatter cones and spheroids derived by liquefaction of strongly shocked rock fragments.
K-Ar dating of the melt rocks. No meteorite component has been detected in the Clearwater West melt rocks (Reimold et al 1981). .

Image courtesy of NASA/LPI.

General Area: The Clearwater Lakes lie within the Canadian Shield, close to the treeline. Their circular form contrasts sharply with the linear and irregular lakes in this area of generally low relief. The area has been glaciated. The target rocks are crystalline.

Specific Features: Twin craters, formed simultaneously by the impact of an asteroidal pair. The eastern,smaller structure has a submerged central peak. The western, larger structure contains a prominent ring of islands, that are 6-10 km in diameter. This ring is capped by impact melt rocks. A fracture halo extending out 20-30 km from the structures is visible at low sun angles. The lakes are named after their exceedingly clear water.

The recognition of unique shock-produced “deformation lamellae” or planar deformation features (PDFs) in quartz in the 1960s was a critical development in the identification of ancient meteorite impact structures: a) first published description of “deformation lamellae” identified in breccias from the Clearwater West impact structure, as an abstract in the Journal of Geophysical Research (McIntyre 1962) (reproduced by permission of the American Geophysical Union); b) photomicrograph of quartz grain in breccia from Clearwater West, showing multiple sets of PDFs (McIntyre 1968). Plane-polarized light. The quartz grain is about 1.4 mm long (French 2004).

An article by Beals et al in the 1964 Journal of the RASC article (below) proposed that the Clearwater Lakes double structure may be the result of a cosmic impact. This report was published before any geological studies were performed at the site in order to find impact features.

Dence et al 1965

The red dot represents the approximate area of the Clearwater impact 290 million years ago in the Carboniferous Period.

The Clearwater Lake Crater pair is situated in crystalline bedrocks of the Canadian Shield. Data is consistent with the eastern and western structures being the result of simultaneous impacts. The size and separation of the two structures rule out the impact of disrupted single body by either atmospheric breakup or fragmentation within the Roche Limit, suggesting that the impacting bodies were a binary pair (Melosh et al, 1991). The Clearwater Lakes East and West* craters are a classic example of the impact of a contact binary asteroid creating this twin crater phenomenon, very rarely recognized on Earth (Grieve 2006).

*The larger Clearwater Lake West Crater (in the NASA/LPI image) shows a prominent ring of islands that has a diameter of about 10 kilometres (6 miles). The islands constitute a central uplifted area and are covered with units of breccias and impact melt. The shore line of the crater contains numerous small islands. The land rises from lake level to a poorly defined ind interrupted rim 5-10 km from the shore. Bathymetry indicates an annular trough between and the ring of islands, with depths of ~50m. The crater is classified generally as a complex meteorite crater with a peak-ring form (Grieve 2006).

The ring of islands reflects the eroded centrally uplifted rocks of the original crater and contains impact melt. Impact related lithologies are known only from the island ring and from drill core. Poorly formed shatter cones occur on the island ring but their orientations have not been studied in detail. The metagabbro of the central islands contains maskelynite and planar deformation features in quartz (Grieve 2006).

During an expedition to the structures in the winters of 1962-63 and 1963-64, drilling and gravity surveys were performed. The results were interpreted in favour of the structures being of impact origin (Dence, 1964; Dence et al, 1965):

The melt bearing units are approximately 100 meters thick;
Below the melt is a 10 meter thick breccia layer;
A 300 meter thick layer of fractured basement rocks complete the base of the crater, and;
Some of the larger fractures in the basement show displacement and contain pseudotachylite.

Aerial Exploration

We approached these twin craters from the northeast under a 1500’ cloud layer, the weather patterns over each crater were completely different. Clearwater East had a couple of rain systems covering the south shore of the crater while Clearwater West was almost completely clear. Fuel constraints prevented a longer stay over the area to wait out the weather. After exploring as long as fuel would allow we departed to the west.

This map, courtesy of Lacs Guillaume, Delisle, et a L' Eau Claire Park Project, Quebec, documents the islands described in the images below (so we all can keep track of where my airplane was when I took the picutes).

In the following images note the vegetation. We are just south of the “tree line”, which to us means that if we had to make a forced landing, we could now find something to burn in order to make an emergency fire!

The common rim between the Clearwater East (visible in the background) and West craters, immediately east of Ile Kamiskutanikaw.

This image is taken immediately east of Ile Kamiskutanikaw over the common rim of the craters looking south. The Clearwater East crater is visible in the left background and the rim between the twin craters is visible in the right background. The rim common to Clearwater East and Clearwater West craters is still prominent. The northern rim of the Clearwater West crater, in the immediate foreground, had suffered severe erosion from the many glacial ice ages since the impact event.

Ile Kamiskutanikaw north in the Clearwater West impact crater.

We entered the Clearwater West crater from the north east. This image, looking south, is taken immediately north of Ile Kamiskutanikaw. Ile Kamiskutanikaw is the north eastern-most island section of the crater ring. From this position, in the western ring islands are visible to the right. The southern crater ring is prominent in the near background with the southern crater rim visible in the far background. From here we orbited the ring islands in a counter clockwise direction.

Ile Lepage north in the Clearwater West impact crater.

This image, looking south east, is taken immediately north of the mid point of Ile Lepage, the most northern island ring. Ile Kamiskutanikaw is visible just to the east in the mid-left background. The Clearwater East and West common crater rim is visible to the left and the southern ring islands are visible to the right. We continued flying west along the north coast of this island. The geology on these islands illustrates the impact melt.

Ile Lepage north-west in the Clearwater West impact crater.

Scanning right (west) from the previous illustration, still north of Ile Lepage, is the western-most point of the northern island. Looking southwest from here are Iles Wiskichanikw, the series of small islands that make up the western-most ring feature. We crossed the most northern island at its western-most point.

The western-most point of Ile Lapage in the Clearwater West impact crater.

Looking east from the western-most point of Ile Lapage the common crater rim between Clearwater East and West is just visible at the extreme left background. The circular form of the ring islands is obvious even from this low altitude. We continued south over Iles Wiskichanikw to Ile Atkinson, the southern-most island in the crater ring.

The western-most point of Ile Atkinson in the Clearwater West impact crater.

After flying over the small western islands we explored the southern-most island, Ile Atkinson. Looking northwest from the western-most point of Ile Atkinson, Iles Wiskichanikw are visible in the left foreground with the northwest crater rim in the far background. Ile Kamiskutanikaw is just visible in the right background. These images give an excellent perspective of the size of this crater.

Ile Atkinson south looking northeast in the Clearwater West impact crater.

We travelled east exploring along the south coast of Ile Atkinson. This image is taken looking northeast from the southern point of Ile Atkinson. The eastern point of Ile Atkinson is visible in the foreground, the northern crater rim of Clearwater West is just visible at the upper left, Ile Kamiskutanikaw is in the mid background and the common crater rim between Clearwater East and West is visible at the upper right of the image.

With the crystal clear water and pristine islands, I couldn’t help thinking that this is the perfect place for a cottage. The very short summers, cold winters and extreme isolation puts it in a bit of perspective though!

To fully explore these craters unfortunately would take more fuel than my airplane could carry. So, after exploring as long as fuel would allow we departed to the west for the village of Umiujaq. There we would top up our fuel from the containers we carried and continue on south and home.

Side Notes

1.

An example of a peak ring crater imaged during the robotic MESSENGER spacecraft's swing past Mercury. The Clearwater West Crater probably looked like this, pre-erosion. - Image courtesy MESSENGER and NASA.

Double-ring craters do occur as well on Earth's Moon, but they are much more common on Mercury, and smaller in size. This is a function of Mercury's greater gravity. The immediate vicinity of the impact crater is pockmarked by locally derived impact ejecta associated with radial secondary impact crater chains. The impact and pockmarking is clearly younger than the smooth volcanic plains to the left in this image. Because the major phase of impact cratering in the Solar System is associated with the Great Bombardment that ended at 3.8 billion years ago, the presence of smooth lavas that drown older craters - but which are also heavily impacted by younger ones - tells us that the lava plains are probably about 3.8 billion years old as well. This highlights yet another difference with Earth's Moon where the lavas that filled up the giant Impact Basins are 100's of millions of years younger than the end of the Great Bombardment. (from: Messenger at Mercury (Feb 2008) by Simon Hanmer.)

2.

RADARSAT radar image of the Sudbury (left) and Lake Wanapetei (right), double impact structures.

Coincidentally, there is another double impact site in the Canadian Shield at Sudbury. The impact that resulted in the Sudbury Impact Structure happened almost two billion years before the nearby Wanapitei Impact Crater impact (right in the image), whereas the Clearwater impacts were simultaneous. (Aerial Radar Courtesy of the Planetary and Space Science Centre at the (Aerial Radar Courtesy of the Planetary and Space Science Centre at the Image courtesy of Earth Impact Database, UNB, 2003.))

3.

Lunar craters, Ritter and Sabine. The Clearwater East/West craters are similar in relative size.

There are a pair of craters, Ritter and Sabine, visible on the moon in the south west corner of Mare Tranquillitatis at 2°N latitude 19°E (lunar coordinates for Ritter Crater). Observing these craters will give you an excellent perspective of the physical size of the Clearwater Craters as these twin craters on the moon are "almost" the exact dimension and orientation of the Clearwater Craters. In other words, an observer on the moon would see the twin Clearwater Impact Structures almost exactly as we see the Ritter/Sabine Craters on the moon from our planet (courtesy NASA)

4.

Richmond Gulf, Quebec.

Le Goulet gap at Richmond Gulf, Quebec

The geology of the Richmond Gulf area, just to the west of the Clearwater Craters, is fascinating!

The left image, taken just to the east of the Richmond Gulf, is typical of the geology of the area.

The right image is facing east through Le Goulet gap connecting Hudson Bay and the Richmond Gulf, in the background.

I just wanted to share this with you.

References

Beals, C. S., Ferguson, G. M., & Landau, A., [Scientists Report II.] A Search for Analogies Between Lunar and Terrestrial Topography on Photographs of the Canadian Shield, Part II, Journal of the Royal Astronomical Society of Canada, Vol. 50, p.257-258

Dence, M. R., A comparative structural and petrographic study of probable Canadian meteorite craters. Meteoritics, v. 2, pp. 249-270. 1964.

Dence, M. R., Innes, M.J.S. and Beals,C.S., On the probable meteorite origin of the Clearwater Lakes, Quebec. Journal of the Royal Astronomical Society of Canada, v. 59, pp. 13-22. 1965.

French, B. M.,The importance of being cratered:The new role of meteorite impact as a normal geological process. Meteoritics & Planetary Science 39, Nr 2, 169–197. 2004.

Grieve, R.A.F., Impact Structures in Canada. Geological Association of Canada, 2006.

Hanmer, S. Messenger at Mercury (Feb 2008). RASC Ottawa Centre.

Reimold, W. U., Grieve, R.A.F. and Palme,H., Rb-Sr dating of the impact melt from East Clearwater, Quebec. Contributions to Mineralogy and Petrology, v. 76, pp. 73-76. 1981.

Earth Impact Database

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Odale-Articles-ClearwaterWest

IMPACT CRATER EXPLORATIONS

CLEARWATER WEST IMPACT CRATER

Aerial Exploration

Side Notes

References

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