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The Lac à l'Eau Claire (the official name, in French), also called the Clearwater Lakes in English, is a calque of Wiyâšâkamî in Northern East Cree (changed form of ''wâšâkamî'' or ''wâšekamî'' in more southerly Cree dialects) and Allait Qasigialingat by the Inuit, are a pair of annular lakes on the Canadian Shield in Quebec, Canada, near Hudson Bay. The lakes are actually a single body of water with a sprinkling of islands forming a "dotted line" between the eastern and western parts. The name is due to the clear water it holds. There are actually 25 lakes with that name in the province (26 if the ''Petit lac à l'Eau Claire'' — the Small Clearwater Lake — is included). These are the largest and northernmost, and the second largest natural lake in Quebec after Lake Mistassini.〔 In 1896, the explorer and geologist Albert Peter Low, a member of the Geological Survey of Canada, provided a probable explanation for the lakes' descriptive Cree name by highlighting the extraordinary clarity and depth of their icy waters.〔 ==Impact craters== The Clearwater Lakes occupy the near-circular depressions of two eroded impact craters (astroblemes).〔Robertson, P.B. & Grieve, R.A.F. 1975 Impact structures in Canada: Their recognition and characteristics. Journal of the Royal Astronomical Society of Canada, v. 69, pp. 1-21.〕 The eastern and western craters are and in diameter, respectively. Both craters were previously believed to have the same age, 290 ± 20 million years (Permian period),〔Reimold, W.U., Grieve, R.A.F. and Palme, H. 1981. Rb-Sr dating of the impact melt from East Clearwater, Quebec. Contrib. Mineral. Petrol. 76, 73–76.〕 promoting the long held idea that they formed simultaneously. According to this doublet impact crater theory initially proposed by Michael R. Dence and colleagues in 1965,〔Dence, M. R., Innes, M. J. S. and Beals, C. S., 1965. On the probable meteorite origin of the Clearwater Lakes, Quebec. Royal Astronom. Soc. Canada J. 59, 13–22.〕 the impactors may have been gravitationally bound as a binary asteroid, a suggestion also made by Thomas Wm. Hamilton in a 1978 letter to Sky & Telescope magazine in support of the then-controversial theory that asteroids may possess moons (such as, for example, asteroid 243 Ida with its satellite Dactyl〔Chapman, C.R., Veverka, J., Thomas, P.C., Klaasen, K., Belton, M.J.S., Harch, A., McEwen, A., Johnson, T.V., Helfenstein, P., Davies, M.E., Merline, W.J., Denk, T., 1995. Discovery and physical properties of Dactyl, a satellite of asteroid 243 Ida. Nature, 374, 783–784.〕). However, repeated 40Ar/39Ar dating of impact melt rocks from both impact craters suggests that Clearwater East has an age of approximately 460–470 million years, corresponding to the Ordovician time period, whereas Clearwater West was formed 286.2 ± 2.6 million years ago, in the early Permian.〔Bottomley, R.J., York, D., and Grieve, R.A.F. 1990. 40Argon-39Argon dating of impact craters. Proc. 20th Lunar Planet. Sci. Conf., LPI, Houston, pp. 421–431.〕〔Schmieder, M., Schwarz, W. H., Trieloff, M., Tohver, E., Buchner, E., Hopp, J. & Osinski, G. R. 2014. New 40Ar/39Ar dating of the Clearwater Lake impact structures (Québec, Canada) – Not the binary asteroid impact it seems? Geochim. Cosmochim. Acta (in press).〕 Both Clearwater impact structures also carry different geophysical (natural remanent magnetization) signatures〔Scott, R. G., Pilkington, M. and Tanczyk, E. I. 1997. Magnetic investigations of the West Hawk, Deep Bay, and Clearwater impact structures, Canada. Meteoritics Planet. Sci. 32, 293–308.〕 and different geochemical fingerprints of the impacting meteorite in the impact melt of each crater.〔Palme, H., Janssens, M.-J., Takahashi, H., Anders, E. and Hertogen, J. 1978. Meteoritic material at five large impact craters. Geochim. Cosmochim. Acta 42, 313–323.〕 Clearwater East and Clearwater West are both complex craters with distinct central peaks. These peaks are caused by the gravitational collapse of crater walls and subsequent rebound of the compressed crater floor. Lake water and sediments cover the central peak of Clearwater East, but bathymetric surveys of the lake floor and core drilling confirm the presence of a peak in its center.〔(A One-Two Punch ) by NASA Earth Observatory〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Clearwater Lakes」の詳細全文を読む スポンサード リンク
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