2015 CLEARWATER CRATER(S) UPDATE
New Ar-Ar Dating of the East and West Clearwater Lake Impact Structures, Québec, Canada – Evidence
for Two Separate Impact Events
Martin Schmieder , Winfried H. Schwarz , Mario Trieloff , Eric Tohver , Elmar Buchner , Jens Hopp , Gordon Richard Osinski and Richard A F Grieve
Abstract: For 50 years, the two Clearwater Lake impact structures in Québec have been considered
as a typical crater doublet formed by the impact of a binary asteroid. New Ar/ Ar dating of melt rocks from
the ≥36 km West Clearwater Lake (WCL) impact structure yielded two Early Permian plateau ages with a
weighted mean age of 286.2 ± 2.2 (2.6) Ma (2σ; MSWD = 0.33; P = 0.57). Ar/ Ar results for two chloritized
melt rocks from the ~26 km East Clearwater Lake (ECL) impact structure produced age spectra suggestive of
extraneous argon. The age spectra corrected for the trapped argon component and inverse isochron plots
consistently yielded ages around ~460–470 Ma for ECL, reproducing the Ar/ Ar results by Bottomley et al.
(1990) and contradicting an earlier Rb–Sr age of 287 ± 26 Ma. The Ar-Ar dates obtained from four different
melt samples across the melt sheet favor an Ordovician age for the ECL impact and impact-induced
hydrothermal overprint. WCL and ECL, moreover, show different natural remanent magnetizations indicating
separate geologic histories. Whereas WCL has no resolvable geochemical impactor traces, the ECL melt rocks
carry a strong (possibly L-) chondritic impactor signature. The WCL impact affected a thin layer of Ordovician
target carbonates; such rocks are absent in the ECL impact breccia, which is overlain by >100 m of post-impact
sediments. Biostratigraphic dating of the fossil-poor post-impact deposits at ECL is currently underway. In the
light of the new Ar/ Ar dates and in combination with the paleomagnetic and geochemical findings, the close
spatial arrangement of WCL and ECL is probably pure coincidence. The two impact structures seem to
represent a ‘false doublet’ struck by impacts ~180 million years apart. ECL possibly represents one of several
2014 CLEARWATER CRATER(S) UPDATE
* New 40 Ar/39 Ar dating of the Clearwater Lake impact structures (Quebec, Canada) - Not the binary asteroid impact it seems?
Martin Schmieder, Winfried H. Schwarz, Mario Trieloff, Eric Tohver, Elmar Buchner, Jens Hopp and Gordon R. Osinski
Abstract – The two Clearwater Lake impact structures (Québec, Canada) are generally interpreted as a crater doublet formed by the impact of a binary asteroid. Here, arguments are presented that raise important questions about the proposed double impact scenario. New 40 Ar/39 Ar dating of two virtually fresh impact melt rock samples from the >36 km West Clearwater Lake impact structure yielded two statistically robust Early Permian plateau ages with a weighted mean of 286.2 ± 2.2 (2.6) Ma ( MSWD = 0.33; P = 0.57). In contrast, 40 Ar/39 Ar results for two chloritized melt rocks from the ~26 km East Clearwater Lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component. Although individually weakly robust, age spectra corrected for the trapped argon component and inverse isochron plots for the East Clearwater melt rocks consistently yielded apparent ages around ~460–470 Ma. No Permian signal was found in either of these melt aliquots. Our new 40Ar/39Ar results reproduce earlier 40 Ar/39 Ar plateau ages (~283 Ma and ~465 Ma, respectively) for the two impact structures by Bottomley et al. (1990) and are in conflict with a previous, statistically non-robust Rb-Sr age of 287  ± 26 Ma for East Clearwater. The combined cluster of apparent ages of ~460–470 Ma, derived from four different samples across the impact melt sheet, is very unlikely to represent a ‘false age effect’ due to the incorporation of extraneous argon into the melt; instead, it strongly favors a Middle Ordovician age for the East Clearwater impact and impact-generated hydrothermal chloritization. Moreover, the Clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories, an effect unexpected in the case of a Permian double impact. Whereas the West Clearwater impact affected Ordovician carbonates incorporated into the impact breccia, drill core reports from the 1960s concluded that clasts of Ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the East Clearwater Lake impact structure, which is overlain by >100 m of post-impact sandstones, shales and carbonates. No resolvable impactor contamination has so far been detected in the West Clearwater impact melt rocks, whereas East
Paleogeographic reconstruction of North America (Laurentia) during the Ordovician. Setting of the Canadian Shield and adjacent areas during the Early/Middle Ordovician. Major shallow marine ingressions on the Canadian Shield; Northern Europe (Baltica) approaches Laurentia narrowing the Iapetus Ocean. The ~460-470 Ma East Clearwater impact (E) probably occurred under near-coastal to shallow-marine conditions. Osinski et al 2014
Clearwater carries a distinct ordinary (possibly L-) chondritic impactor signature in its melt rocks. East Clearwater Lake might thus represent one among a long list of Ordovician impact structures in North America and northern Europe that were presumably generated in response to the L-chondrite asteroid breakup event ~470 Ma ago. Paleogeographic reconstructions show that the Ordovician East Clearwater impact probably occurred in a near-coastal to shallow marine setting, while the Permian West Clearwater impact hit continental Pangaea. Along with the new 40 Ar/39 Ar data, the paleomagnetic, sedimentologic, and paleogeographic findings suggest that the close spatial arrangement of the two Clearwater lakes is probably pure coincidence. The two impact structures seem to represent a ‘false doublet’ struck by impacts separated by ~180 million years in time. The new results for the Clearwater Lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on Earth and on other planetary bodies in the inner Solar System.
Jens Ormö, Erik Sturkell, Carl Alwmark & Jay Melosh
ABSTRACT: Approximately 470 million years ago one of the largest cosmic catastrophes occurred in our solar system since the accretion of the planets. A 200-km large asteroid was disrupted by a collision in the Main Asteroid Belt, which spawned fragments into Earth crossing orbits. This had tremendous consequences for the meteorite production and cratering rate during several millions of years following the event. The 7.5-km wide Lockne crater, central Sweden, is known to be a member of this family. We here provide evidence that Lockne and its nearby companion, the 0.7-km diameter, contemporaneous, Målingen crater, formed by the impact of a binary, presumably ‘rubble pile’ asteroid. This newly discovered crater doublet provides a unique reference for impacts by combined, and poorly consolidated projectiles, as well as for the development of binary asteroids.
Paleogeography of Baltica and neighboring cratons at the time of the increased cosmic bombardment following the ~470 Ma asteroid breakup event illustrating the resulting known craters (red dots). Clearwater East is dated to this event (~460–470 Ma).
Light blue color represents areas of shallow epicontinental seas, and dark blue areas of deep ocean. This distribution may, however, have varied somewhat due to periodical transgressions and regressions of the sea. The timeline documents the related meteorite falls (black dot and line).
|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 red dot represents the approximate area of the Clearwater impact 290 million years ago in the Carboniferous Period.
*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).
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.
|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.
Clearwater East impact crater - west, looking at the common rim between the two craters (Clearwater east and west).
Just to the west of the crater’s common rim we started our exploration of Clearwater West.
2. |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)
Lunar craters, Ritter and Sabine (see text for explanation).
3.|The geology of the Richmond Gulf area, just to the west of the Clearwater Craters, is fascinating!
Le Goulet gap at Richmond Gulf, Quebec
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.
Possible Multiple Impact
MY HYPOTHESIS: The series of circular features in line (illustrated in the image) from north east of Clearwater Lakes East
to south west of Kakiattukallak Lake
may be related multiple impacts.
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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