IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
CLEARWATER WEST IMPACT CRATER
Image courtesy of NASA/LPI.
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.
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.
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!
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.
1. Messenger at Mercury (Feb 2008) by Simon Hanmer.)
Possible Multiple Impact
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.