CLEARWATER EAST IMPACT CRATER
by: Charles O’Dale
- Type: Central peak
- Age (ma): ~460–470 Ma *(new data Schmieder 2014) – ORDOVICIAN
- 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.
The magnetization of rocks.
The magnetic field of the Earth can be “captured” by certain types of rocks, and this magnetic signature can be used to study the Earth’s magnetic field throughout history. The magnetic poles of the Earth are not fixed, and pole reversals have occurred many times in the past.
The rocks from the West Lake show that they were formed during a “superchron,” which is an unusually extended period of time where no reversals occurred. This superchron, known as the Permo-Carboniferous Reversed Superchron, lasted from 316 to 265 million years ago, which agrees with the age found by the argon dating.
The rocks from the East Lake tell a different story. They have a number of different magnetic polarizations, which indicate viscous remnant magnetization. This is magnetization that is acquired slowly over a long period of time. The more complex magnetic history points to the rocks being much older than the West Lake, as they have had more time to be altered.
Martin Schmieder, Winfried H. Schwarz, Mario Trieloff, Eric Tohver, Elmar Buchner, Jens Hopp and Gordon R. Osinski