IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
WANAPITEI IMPACT CRATER
Update: Evidence for a second L chondrite impact in the Late Eocene: Preliminary results from the Wanapitei crater, Canada. R. Tagle1 (et al) 2006
The Wanapitei impact melt rocks contains about 1% of an extraterrestrial component and, based on Ni/Ir, Ni/Cr and Co/Cr ratios an L or LL chondrite projectile is advocated. Two samples of impact-melt glass provide a K-Ar age of 37 ± 2 Ma. Wanapitei crater is formed in the late Eocene, along with the two largest structures in the Cenozoic, the 100-km Popigai (35.7 ± 0.2 Ma) in northern Siberia and the 85-km Chesapeake Bay (35.5 ± 0.6 Ma) offshore Virginia. Two craters, Popigai and Wanapitei were formed by the same type of projectile, an L chondrite, supporting the hypothesis that a major disruption of the L parent body triggered an asteroid shower in the Late Eocene.
|Sudbury Impact Structure to the west of Wanapitei Lake. In the image (right), taken from over the Sudbury Impact Structure, Wanapitei Lake is visible in the background. The close proximity of these two impact structures is strictly coincidence. The Sudbury impact happened over 1.5 billion years before the one at Wanapitei. In the foreground is the flat Chelmsford Formation, the center of the Sudbury Igneous Complex (SIC). In the mid background is the rugged north eastern rim of the SIC (Robertson et al 1975). A gravity survey in 1969 drew attention to the Wanapitei Crater and it was suspected as a possible meteorite crater in 1972 with the discovery of boulders of breccia, with abundant shock metamprphic effects (Grieve 2006). The presence of coesite, which can be formed at pressures of 425-500 kilobars and temperatures near 1000°C, has confirmed the meteoritic origin of the Lake Wanapitei Impact Crater in Ontario (Dence et al. 1974) as well as other sites (Cohen et al. 1961). It is classified as a simple meteorite crater because of its estimated diameter of 3 km (E. L’Heureux et al, 2003) to ~7-8 km and because there is no evidence of a central uplift in the submerged crater (Dence and Popelar, 1972). New geological studies (2003) thus far indicate that if the observed circular structure is due to a meteorite impact, it is at most 3 to 4 km diameter (indicated by the circle in the landsat image). The new diameter of 3 km has not been widely accepted as yet (Eyles 2002).|
West Hawk and Brent) that have been identified as impact events by similar gravity anomalies. The data was corrected using water depth measurements from the Ontario Department of Lands and Forests.
Bathymetry taken in 2002 (right)has provided new and more precise depth estimates. The bathymetric structure of the lake may also be suggestive of larger regional deformation, such as a fault system running North-South through the area (Eyles 2002).
A shallow magnetic survey was run in August 2002 (below). It indicated a circular low approximately 2.5 km wide and placed over the greatest depths of the lake (>100m). It is therefore not known whether the magnetic low can be absolutely attributed to an impact structure or whether it is due to the large water depths. This magnetic low however is slightly more south than the location of the gravity low. The estimated maximum and minimum sizes for the impact crater are indicated.
|Earth Impact Database, 2003.|
The outline of the lake has been enlarged and modified by erosion, illustrated in this image (right) of the structure taken from the west. Approximately 300m of the original surface in this area has been removed by erosion. Breccia was scoured from the crater floor by glaciation and deposited on the southern shoreline (Dence and Popelar, 1972).
I found this area geologically fascinating while exploring it from the air. With two impact features to observe, at 8000’ and higher, the eastern portion of the SIC seems almost distorted by the nearby Wanapitei feature.
Petrographic evidence comes from rock samples demonstrating shock metamorphic effects (quartzite fragments and the presence of glass) that have been found in glacial drift on the southern shores of the lake (image above). These include boulders of suevite and glassy breccia as well as samples of coesite. Analysis of this glacial drift revealed ratios of Ir, Os, Pd, Ni, Cr and Co (Wolf et al., 1980). More recent work using platinum-group elements (PGE) (Evans et al., 1993) confirms an LL-chondrite meteorite as the most likely type of impacting body. This area is probably one of the most geologically studied areas on this planet!
Dressler observed deformation lamellae in a few quartz grains at three locations in the south western region of the lake. There are only a few samples that indicate these shock metamorphic features, none of which were found in their natural or original position or place (Eyles 2002). Shatter cones have been found on certain islands in the southern part of the lake as well as on shore, but cannot be unequivocally attributed to the Wanapitei Impact due to the close proximity of the Sudbury Impact Structure.
Bottomley, R. J., York, D. and Grieve,R.A.F., Possible source craters for the North American tektites--A geochronological investigation (abstract). EOS, v. 60, p. 309. 1979.
Dence, M. R., Popelar, J., Evidence for an impact origin for Lake Wanapitei, Ontario: Geological Association of Canada Special Paper No.10 p. 117-124, 1972.
Dence, M. R., Robertson, P.B. and Wirthlin,R.L., Coesite from the Lake Wanapitei crater, Ontario. Earth and Planetary Science Letters, v. 22, pp. 118-122. 1974.
Evans, N. J., Gregoire, D.C., Grieve, R.A.F., Goodfellow, W.D. and Veizer,J., Use of platinum-group elements for impactor identification: Terrestrial impact craters and Cretaceous-Tertiary boundary. Geochemica et Cosmochimica Acta, v. 57, pp. 3737-3748. 1993.
Eyles, N, E. L’Heureux, H. Ugalde, B. Milkereit, J. Boyce and W. Morris; MAGNETIC, GRAVITY AND SEISMIC CONSTRAINTS ON THE NATURE OF THE WANAPITEI LAKE IMPACT CRATER. Proceedings for the 3rd International Conference on Large Meteorite Impacts, Germany August 2003. A study to determine the crater’s size and location within the lake, through the identification of impact characteristics such as disruptions in local geology and geophysical trends.
Grieve, R.A.F., IMPACT STRUCTURES IN CANADA. GEOtext 5, 2006
Grieve, R.A.F.,Robertson P.B., IMPACT STRUCTURES IN CANADA: THEIR RECOGNITION AND CHARACTERISTICS Journal of the Royal Astronomical Society of Canada, V69, 1-21, Feb 1975
Grieve, R. A. F., Ber, T., Shocked lithologies at the Wanapitei impact structure, Ontario, Canada. Meteoritics, v. 29, 621-631. 1994.
Grieve, R. A. F., Ber, T., Characterization of an impact even from non in situ samples: The Wanapitei sample. Second International Workshop. Impact Cratering and Evolution of Planet Earth. The Identification and Characterization of Impacts. 1994.
Robertson, P. B., Grieve, R. A. F., Impact structures in Canada: Their recognition and characteristics. Journal of the Royal Astronomical Society of Canada, v. 69, pp. 1-21. 1975.
TAGLE, R. and HECHT, L., Geochemical identification of projectiles in impact rocks. Meteoritics & Planetary Science Volume 41, 26 JAN 2010.
Wolf, R., Woodrow, A.B. and Grieve,R.A.F., Meteoritic material at four Canadian impact craters. Geochimica et Cosmochimica Acta, v. 44, pp. 1015-1022. 1980.
Additional NotesEast and West impact craters. The Clearwater impacts are related and unlike Wanapitei and Sudbury, they simultaneously occurred 290 million years ago.