WHITECOURT


Iron, IIIAB, octahedrite
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Found July 1, 2007
53° 59' 57" N., 115° 35' 51" W.

While searching with metal detectors in a forested region near Whitecourt, Alberta, Canada, two local residents, James R. "Sonny" Stevens and Rod Stevens uncovered four jagged, angular fragments of an iron meteorite. The fragments were located near the partially raised rim of a 36 m diameter, 5–10 m deep, circular impact crater of late Holocene age (<1,130 years old); the Whitecourt Meteorite Impact Crater is recorded in the Earth Impact Database, Planetary and Space Science Center, University of New Brunswick, Canada. The crater is now protected from meteorite searching under the Alberta Provincial Historic Resources Designation Act.

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Cross section through the crater along 110°.
Credit: KOFMAN, R. S., HERD, C. D. K. and FROESE, D. G. (2010), The Whitecourt meteorite impact crater, Alberta, Canada
Meteoritics & Planetary Science, 45: 1429-1445. doi: 10.1111/j.1945-5100.2010.01118.x

The Whitecourt crater was excavated by an impactor measuring ~0.7–1.6 m in diameter and weighing 1,285–16,728 kg. The target material was composed of sediments associated with the advance and retreat of the Laurentide Ice Sheet (Herd et al., 2008; Kofman et al., 2009). The impactor struck at an angle of 40–55° along a trajectory of 60–85°. Within the crater there is a sharp transition from poorly sorted, fine-grained glacial till (heterogeneous pebble diamict) overlying unconsolidated fine quartz sand at a depth of ~2.9 m, and it is considered that this boundary marks the base of the transient crater. Notably, amber-colored glass fragments are found above ~2.9 m (Kofman et al., 2008).

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Simon de Boer standing in Whitecourt crater.
Photo courtesy of Simon de Boer (2010)

Subsequent searches of the area have resulted in the recovery of ~2,500 meteorite fragments, some found up to a distance of 70 m away from the crater (Kofman et al., 2010). The meteorite fragments have a total combined weight of ~70 kg and range in size from <1 cm to 12.5 cm, corresponding to weights of 0.1 to 1,196 g, respectively (Herd et al., 2008). The meteorite fragments consist primarily of shrapnel-type morphologies, but at least one exception exists—a 6.51 kg mass exhibiting regmaglypts and fusion crust that was found 261 m east-northeast of the crater. In addition, it has been reported that an exceptional mass weighing ~27 kg, likely also retaining ablation features, was found in the Fall of 2010 and donated by the finders to the University of Alberta in Edmonton. Among other noteworthy fragments found outside of the protected zone is a 1,900 g fragment that exhibits possible ablation characteristics.

The crater is surrounded by an ejecta blanket covering 6,000 m² constituting 40% of the volume of the crater. Meteorite fragments recovered from under the ejecta blanket have experienced less oxidation than those recovered from within the crater. It is thought that the crater was excavated during a low-energy hypervelocity impact by a single projectile traveling towards the east-northeast which measured an estimated 1.1 m in diameter (Kofman et al., 2009).

The exact scenario by which the strewn field was formed, whether through atmospheric fragmentation, explosive disruption during impact, or a combination of the two, is still under debate. However, it is argued by Kofman et al. (2010) that the evidence is most consistent with a meteoroid traveling 4–10 km per second that experienced catastrophic disruption upon impact. Only a single 6.51 kg fragment (see above) was spalled off the incoming mass at a high enough altitude to develop regmaglypts and fusion crust.

Geochemical analysis and structural studies (i.e., etching) distinguish Whitecourt as a IIIAB medium octahedrite (Herd, University of Alberta). The iron meteorite shows some signs of shock and recrystallization. Planar microstructures are present in quartz grains located below the transient crater boundary. These provide evidence of a low degree of shock. This is consistent with the presence of Fe,Ni-oxide spherules and the rare samples of impactor partial melting. The specimen of Whitecourt shown above is a 10.2 g shrapnel-type fragment kindly donated to the Weir Collection by its finder, Simon de Boer, via Canadian Cultural Property Export Permit #103306. Pictured below is a Whitecourt slice that was expertly etched by Mirko Graul.

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Photo courtesy of Mirko Graul Meteorite

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