IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
(Herd et al 2008)
The <1,100 yr old Whitecourt meteorite impact crater, located south of Whitecourt, Alberta, Canada, is a well-preserved bowl-shaped structure having a depth and diameter of approximately 6 and 36 m, respectively. There are fewer than a dozen known terrestrial sites of similar size and age. Unlike most of these sites, however, the Whitecourt crater contains nearly all of the features associated with small impact craters including meteorites, ejecta blanket, observable transient crater boundary, raised rim, and associated shock indicators. This study indicates that the crater formed from the impact of an approximately 1 m diameter type IIIAB iron meteoroid traveling east-northeast at less than approximately 10 km s−1, striking the surface at an angle between 40° and 55° to horizontal. It appears that the main mass survived atmospheric transit relatively intact, with fragmentation and partial melting during impact. Most meteoritic material has a jagged, shrapnel-like morphology and is distributed downrange of the crater (Kofman et al - Meteoritics & Planetary Science 2010). Target sediments consist of Quaternary glacial deposits.
Local hunters had used this unusual "hole in the ground" south of the town of Whitecourt as a meeting point for many years. Deer could often be found drinking rainwater that collected in the bottom of the crater. It was in July 2007 that local residents contacted Dr. Chris Herd, the curator of the Alberta Meteorite Collection and a meteorite researcher at the University of Alberta, after recovering several metallic fragments next to this large asymmetric hole in the ground. Analysis of one of the fragments with a scanning electron microscope (SEM) verified that it was a meteorite. This, and follow-up field work, confirmed that the well known structure was indeed an impact crater.
The crater contains nearly all the features similar to other Barringer type (simple) bowl-shaped craters. This includes meteorites, an ejecta blanket, an observable transient crater boundary, a raised rim, and a number of associated shock indicators. The shock effects are limited to planar microstructures (PMs) observed in quartz grains and Fe-Ni oxide spherules. Evidence of target sediment melting has not been found. A raised rim, which typically circumnavigates simple craters, only extends between the bearings ~020° and ~110°; the opposing side of the crater shows little evidence of uplift. Surface contours within the crater are relatively circular and evenly spaced indicating that there has been no preferential crater wall steepening. There is a slight shift along the south wall that appears to be in response to creep (Kofman et al, 2010).
An option of the 2012 RASC General Assembly (GA) in Edmonton was a post-conference field trip to the Whitecourt Crater guided by Dr. Herd. The crater is just south of the town of Whitecourt, a two hour bus ride away from the GA site in Edmonton. It was a "no brainer" that Gillian and I were going to be on that expedition.
The Impacting Meteorite
The Whitecourt impacting meteorite is identified as a medium octahedrite (Om) IIIAB that was derived 4.5 billion years ago from the core of a >50 km diameter asteroid. From the analysis of the widmanstatten patterns (Goldstein 1964) within the Whitecourt meteorites, it was determined that the core of this asteroid cooled at a rate of 50°C per million years.
The Whitecourt Crater morphology, lack of a large buried meteoroid mass, meteorite morphology, meteorite dust, and Fe-Ni oxide spherules suggest the main mass of the meteoroid was completely disrupted on impact and that this was a hypervelocity event of >4 km/s (Kofman et al, 2010).
Most meteoroids retain their hypervelocities and completely vaporize in an explosion when they impact the earth. In some cases the earth’s atmosphere slows the meteoroids to terminal velocity so they are relatively intact meteorites upon impact. These meteorites have rounded edges with regmaglypts. Regmaglypts are thumbprint-like depressions caused by the uneven flow of air during passage through the atmosphere and the consequent surface melting and ablation.
The smaller size of the Whitecourt hypervelocity impactor (approximately 1 metre diameter upon impact) is unusual as there are surviving meteorites elsewhere several times larger than this that did not cause an explosive (E = mv²) type crater. These larger meteoroids for some reason completely lost their hypervelocities in the atmosphere and impacted at terminal velocity without causing the mv² explosion. In contrast, this relatively small Whitecourt meteorite retained enough energy to explode on contact vaporizing much of the original meteoroid and forming the crater and associated shrapnel (from a conversation with Dr. C. Herd).
The Whitecourt impact event produced octahedrite meteorites having two different post impact shapes.
Some of the meteorites (image above) have sharp shrapnel-type edges. The hypothesis is that most of the meteorite was vaporized upon hypervelocity impact, but on contact, some parts were ejected in the explosion as shrapnel before the main body was vaporized.
Other meteorites found at the site have the rounded edges with regmaglypts. These meteorites broke away from the main body while still in the atmosphere and impacted around the crater at terminal velocity.
In order to complete our exploration of the Whitecourt Crater, Gillian and I flew our bird, C-GOZM, to the precise coordinates of the crater stored in the GPS. Little did we know how invisible the site is from low altitude. We could not see the crater at all while flying over the coordinates (while concentrating on not hitting anything!). All we could do was aim the wing at the GPS location, take the images and hope for the best.
The image at left shows the crater under the wing. See it? ..... Neither do I !
Give up? ... Open the icon at right for the crater location.
The perspective of the images is from the SE, the same as the LiDAR illustration at the top of this report.
Goldstein J.L., Ogilvie, R.E., The Growth of the Widmanstatten Pattern in Metallic Meteorites. Smithsonian Astrophysical Observatory, Cambridge Mass, 30 OCT 1964.
Kofman R.S., Herd C.D.K., Froese D.G., The Whitecourt meteorite impact crater, Alberta, Canada. Meteoritics & Planetary Science 1-17 (2010)
Kofman R.S., Herd C.D.K., Froese D.G., The Results of the Investigation of the Whitecourt Crater (Alberta, Canada), GeoCanada 2010 – Working with the Earth
Herd, C. D., Froese, D. G., Walton, E. L., Kofman, R. S., Herd, E. P., & Duke, M. J., 2008, Anatomy of a Young Impact Event in Central Alberta: Prospects for the 'Missing' Holocene Impact Record: Geology, 36, 955-958.
Herd, C. D., Froese, D. G., Walton, E. L., Kofman, R. S., Herd, E. P., & Duke, M. J., 2008, The Discovery of a Late Holocene Impact Crater Near Whitecourt Alberta: 71st Meeting of the Meteoritical Society, 2008.