IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
CLEARWATER EAST IMPACT CRATER
40 Argon/39 Argon Dating of Impact Craters - this 1990 paper indicated that the Clearwater craters were formed in two separate events.
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
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 impact structures.
2014 CLEARWATER CRATER(S) UPDATE
Martin Schmieder, Winfried H. Schwarz, Mario Trieloff, Eric Tohver, Elmar Buchner, Jens Hopp and Gordon R. Osinski
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.
PRE 2014 CLEARWATER CRATER(S) DATA
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.
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).
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.
Just to the west of the crater’s common rim we started our exploration of Clearwater West.
|1. Sudbury Impact Structure happened almost two billion years before the nearby Wanapitei Impact Crater impact (right in the image), whereas the latest research indicates that the Clearwater impacts were not simultaneousOct. 2014. (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.))|
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
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.
Doyle, A., Surprise! Canadian Double Crater Formed by 2 Separate Impact Events. Astrobiology Magazine, March 13, 2015 07:00am ET
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. et al, 40 Argon/39 Argon Dating of Impact Craters Proceedings of the 20th Lunar and Planetary Science Conference 1990
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.