IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
CLEARWATER WEST IMPACT CRATER
Image courtesy of NASA/LPI.
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
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 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.
PRE 2014 CLEARWATER CRATER(S) DATA
Indications of fluid immiscibility in glass from West Clearwater Lake impact crater, Quebec, Canada
M. R. Dence, W. von Engelhardt, A. G. Plant, L. S. Walter
Abstract - Glass from the West Clearwater Lake hypervelocity impact crater contains numerous spheroids, 10 to 500 μm across, which appear to have formed at high temperatures as fluids immiscible in the enclosing melt. The spheroids are distinguished from small, normal, largely void gas vesicles, which are also present, by being completely filled in all cases; by having fillings which vary in composition from spheroid to spheroid, even between spheroids in close association; and by indications that the present fillings are representative of the contents present before the matrix melt chilled. Most of the spheroids are classified petrographically into three types. Type I, the most numerous, includes all spheroids>100 μm and are filled with uncommon pale brown to yellow montmorillonites with an unusual structure intermediate between dioctahedral montmorillonite and saponite. Type II, brown and green, are filled with Fe-rich montmorillonites, while Type III are aluminia-rich montmorillonites crystallized into mica-like sheaves. Rare, small spheroids are filled with calcite or silica. In a few cases one spheroid encloses another of similar or different type. Electron microprobe analyses indicate that with few exceptions Types I and III spheroids belong to a Mg series of montmorillonites in which the main chemical variation is the substitution of Mg for Al. A second Fe-K series includes Type II and a few Type I spheroids and shows substitution of Fe by Al, relatively high K2O and, in the alumina-rich members, low SiO2. The close association of spheroids with deformed, embayed lechatelierite inclusions indicates that they formed while the latter were liquid, i.e. at temperatures above 1700°C, as rapidly moving impact melt engulfed highly shocked inclusions of quartz-bearing country rock. The preservation of spheroids in the West Clearwater Lake glass is attributed mainly to the position of the glass masses within the breccias lining the crater floor. It is considered that the glass in this location did not achieve free flight but, as part of a large mass, cooled relatively slowly through the high temperature regime in which the spheroids were generated, and then, when detached, chilled rapidly to preserve a record of this transient stage in their history.
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.
Doyle, A., Surprise! Canadian Double Crater Formed by 2 Separate Impact Events. Astrobiology Magazine, March 13, 2015 07:00am ET
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. 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.
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.