WANAPITEI IMPACT CRATER
by: Charles O’Dale
- Type: Flat floored?
- Age (ma): 37.2 ± 1.2a – EOCENE
- Diameter: 3 to 7.5 km, under study (Grieve 1994 & Eyles 2002)
- Location: Ontario, Canada N 46° 45′ W 80° 45′
- Impactor type: Ordinary chondrite; type L,LL – siderophile elements (PGE, Ni, Au) (Tangle, Hecht 2006).
- Shock Metamorphism: high-pressure polymorphs of silica, coesite and stishovite, diaplectic glasses of quartz and feldspar (Grieve and Ber, 1994). PDF in quartz and feldspar. Maskelynite, Impact melt & Coesite (Dence et al., 1974). Suevite (Grieve and Ber, 1994).
a Dating Method: K/Ar, 40Ar-39Ar – Two samples of impact-melt glass provide a K-Ar age of 37 ± 2 Ma. (Bottomley et al 1974).
b Maskelynite: A type of naturally occurring glass having the composition of plagioclase series feldspar, created by the vitrification of plagioclase by shock melting in meteorites and meteorite impacts. (Dence et al 1967)
Dence, M. R., Robertson, P. B., Wirthlin, R. L.
Glacial float from Lake Wanapitei, Ontario (latitude 46° 44.7′N, longitude 80° 44.6′W), which has an approximately circular central basin 8.6 km in diameter, includes breccias and glassy rocks containing abundant evidence of shock metamorphism. One glass-rich boulder, a strongly shocked feldspathic quartzite, contains coesite in clasts of diaplectic silica glass (refractive index 1.4605±0.0005) held in a matrix of mixed vesicular glasses including alkali feldspar glass. This association is indicative of shock pressure of 425–500 kbar, and is additional strong evidence supporting a hypervelocity impact origin for the basin.
Earth and Planetary Science Letters, Volume 22, Issue 2, p. 118-122
Using vertical dikes as a new approach to constraining the size of buried craters: An example from Lake Wanapitei, Canada
E. L’Heureux, H. Ugalde, B. Milkereit
Lake Wanapitei, located within the Southern Province of Ontario, Canada, provides the setting for a unique study of an impact crater situated within a shield environment. Evidence for the 7.5-km-diameter Wanapitei impact includes a circular Bouguer gravity low centered over the central area of the lake and features of shock metamorphism in samples of glacial drift found on the southern shores. Geophysical studies of craters in hard-rock environments are often limited by the lack of markers used for exploration; this may be overcome with the use of the large igneous dike swarms that characterize
Archean terrains. The 1.2 Ga Sudbury dike swarm predates the impact that is suggested to have generated Lake Wanapitei and provides the setting for a study to constrain the size and location of the impact crater. The swarm is clearly visible on aeromagnetic maps as high amplitude, linear features, suggesting they could be used as vertical markers indicative of structural changes having an effect on target rock susceptibilities. To fully establish the size of the crater, a total fi eld magnetic map was produced to trace the Sudbury dikes through the proposed crater center. A gap in their signature,
expressed as a 100 nT low, 2–3 km in width, constrains the size of the crater to <5 km. Numerical modeling suggests that a crater of this size will demagnetize target rocks, producing a low in the total magnetic fi eld, up to a maximum diameter of 3 km. Dikes within the central crater structure will be excavated, vaporized, and melted down to a depth of 1.3 km.
Geological Society of America, Special Paper 384 2005
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. 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.
General Area: Wanapitei is superimposed upon the eastern margin of the older, larger Sudbury structure. The area is generally timbered and has been glaciated. The target rocks are crystalline.
Specific Features: The crater is occupied by a lake with a semi-circular north-shore, and defines a circle 8.5 km in diameter. Elongate fingers of the lake to the south are the result of deepening by glaciation.
Wanapitei is superimposed on the Sudbury structure and clearly transects pre-existing structural trends. A circular fracture halo is developed to the north and west but is obscured to the south by glacial deposits.
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.
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).