CLEARWATER EAST IMPACT CRATER
by: Charles O’Dale
- Type: Central peak
- Age (ma): ~460–470 Ma *(new data Schmieder 2014)
- Diameter: 26 km
- Location: Quebec, Canada. N 56° 05′ W 74° 20′
- Impactor type: Ordinary chondrite type LL – siderophile elements (PGE, Ni, Au) (Tangle, Hecht 2006).
- Shock Metamorphism: PDF in quartz, no shattercones.
Recent chromium isotope analyses suggests that ordinary chondrite-type material is present. The present study reviews and reinterprets the available platinum-group element (PGE) data in the light of new PGE data from meteorites and concludes that the PGE ratios in the impact melt are most consistent with ordinary (possibly type-L) chondrite source material, not carbonaceous chondrites. Therefore the structure was most probably formed by the impact of an asteroid composed of material similar to ordinary chondrites (Iain McDonald 2002).
Melt rock samples from Clearwater East are strongly enriched in Os, Ir, Ru, Rh and Pd relative to crustal concentrations. The average Os/Ir, Ru/Ir, Rh/Ir, Pd/Ir ratios of the melt samples are CI-chondritic (Schmidt 1997).
Rubidium-Strontium dating of the melt rocks. The impact melt of Clearwater East contained Iridium and Osmium implying a CI chondrite type meteorite impact (Reimold et al 1981).
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 oincidence. 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.
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 40 Ar/39 Ar 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 EastClearwater 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.
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.
Clearwater East impact structure: A re-interpretation of the projectile type using new platinum-group element data from meteorites Iain McDonald Abstract — Platinum-group element (PGE) concentrations and ratios obtained from samples of the Clearwater East impact melt have been used along with other siderophile element ratios to classify the impacting projectile as a carbonaceous chondrite. This is at odds with recent chromium isotope analyses that suggest ordinary chondrite-type material is present. The present study reviews and reinterprets the available PGE data in the light of new PGE data from meteorites and concludes that the PGE ratios in the impact melt are most consistent with ordinary (possibly type-L) chondrite source material, not carbonaceous chondrites. Therefore the structure was most probably formed by the impact of an asteroid composed of material similar to ordinary chondrites. (Meteoritics & Planetary Science Volume 37, Issue 3, pages 459–464, March 2002)
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 twin crater phenomenon, very rarely recognized on Earth (Grieve 2006)* but they were created millions of years apart! (2015). 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.
These images of Clearwater East were taken from over its northern rim looking south east and make a continuous east to west view of the crater from the north. The remnant of the eastern crater rim is visible in the left background in the image at left. The central peak of this crater is underwater at the centre of the lake. I did not want to get too close to those “dangerous to a small airplane” rain cells as I was many hundreds of km away from any civilization and possibly days from rescue.
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. Just to the west of the crater’s common rim we started our exploration of Clearwater West.
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)
I just wanted to share this with you.
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Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
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