IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

CLEARWATER EAST IMPACT CRATER

Clearwater East impact crater, Quebec Clearwater East impact crater (on the right). Image courtesy of NASA/LPI Age (ma): 290 ± 20 Diameter: 26 km Location: Quebec, Canada. N 56° 05' W 74° 20' Impactor type: Ordinary chondrite type LL - siderophile elements (PGE, Ni, Au) (Tangle, Hecht 2006). Shock Metamorphism: PDF in quartz, no shattercones. Recent chromium isotope analyses suggests that ordinary chondrite-type material is present. The present study reviews and reinterprets the available platinum-group element (PGE) data in the light of new PGE data from meteorites and concludes that the PGE ratios in the impact melt are most consistent with ordinary (possibly type-L) chondrite source material, not carbonaceous chondrites. Therefore the structure was most probably formed by the impact of an asteroid composed of material similar to ordinary chondrites (Iain McDonald 2002). Melt rock samples from Clearwater East are strongly enriched in Os, Ir, Ru, Rh and Pd relative to crustal concentrations. The average Os/Ir, Ru/Ir, Rh/Ir, Pd/Ir ratios of the melt samples are CI-chondritic (Schmidt 1997). Rubidium-Strontium dating of the melt rocks. The impact melt of Clearwater East contained Iridium and Osmium implying a CI chondrite type meteorite impact (Reimold et al 1981).

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.

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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.

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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 Lake 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. 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). Clearwater East is defined as a highly eroded roughly circular depression, filled with water that submerges the central peak of this complex meteorite crater. Melt rock samples from Clearwater East are strongly enriched in Os, Ir, Ru, Rh and Pd relative to crustal concentrations. This work confirms earlier findings and demonstrates similarly high enrichments of Ru and Rh. The average Os/Ir, Ru/Ir, Rh/Ir ratios of the melt samples from Clearwater east is CI-chondritic. Recent analyses of platinum group elemental abundances in the melt rocks suggest either a C1 or L-chondrite (Evans et al., 1993).

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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 constrains 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.

Clearwater East impact crater - north. Clearwater East impact crater - north east. These images of Clearwater East were taken from over its northern rim looking south east and make a continuous east to west view of the crater from the north. The remnant of the eastern crater rim is visible in the left background in the image at left. The central peak of this crater is underwater at the centre of the lake. I did not want to get too close to those “dangerous to a small airplane” rain cells as I was many hundreds of km away from any civilization and possibly days from rescue.

Clearwater East impact crater - west, looking at the common rim between the two craters (Clearwater east and west). The severity of the erosion on the rims of the craters is obvious in this image. But still, the common crater rim between Clearwater East (background) and Clearwater West is prominent. Just to the west of the crater’s common rim we started our exploration of Clearwater West.

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Side Notes

1. 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.))

2. Lunar craters, Ritter and Sabine (see text for explanation). 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)

3. 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.

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Possible Multiple Impact

MY HYPOTHESIS: The series of circular features in line (illustrated in the image) from north east of Clearwater Lakes East and West to south west of Kakiattukallak Lake may be related multiple impacts.

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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.

Evans, N. J., Gregoire, D.C., Grieve, R.A.F., Goodfellow, W.D. and Veizer,J., Use of platinum-group elements for impactor identification: Terrestrial impact craters and Cretaceous-Tertiary boundary. Geochemica et Cosmochimica Acta, v. 57, pp. 3737-3748. 1993.

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

Melosh, H.J., Stansberry, J. Doublet craters and the tial disruption of binary asteroids [abstract]: Meteoritics, v 26, p. 371-372, 1991.

McDonald, I. Clearwater East impact structure: A re-interpretation of the projectile type using new platinum-group element data from meteorites. Meteoritics & Planetary Science, March 2002

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.

Schmidt, Gerhard; Clues to the nature of the impacting bodies from platinum-group elements (rhenium and gold) in borehole samples from the Clearwater East crater..... Meteoritics and Planetary Science, 1997.

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

Wanless, R. K., Stevens, R.D., Lachance, G.R. and Rimsaite,J.Y.H., Age determinations and geologic studies, Part I. Isotopic ages, Report 5. Geological Survey of Canada Paper 64-17, 126 p. 1964.

Earth Impact Database

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