WANAPITEI IMPACT CRATER

Type: Flat floored?
Age (ma): 37.2 ± 1.2^a – 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).

^aDating 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)

The Wanapitei Impact Crater lies entirely within the central portion of the 9 km diameter Wanapitei Lake, visible at the top center of this landsat image. The Landsat image illustrates the Sudbury Impact Structure to the west of Wanapitei Lake.

RADARSAT radar image of the dual impact structures, Sudbury (left) and Lake Wanapitei. The close proximity of these two impact structures is strictly coincidence. The Sudbury impact happened over 1.5 billion years before Wanapitei.

Dimensions and depths of Wanapitei Lake (from Dence and Popelar, 1972).

This image of Wanapitei Lake from GOZooM illustrates the “semi circular” shape of the north coast of the feature with a highly indented southern margin. Some evidence of circumferential lineaments exterior, and possibly related, to the Wanapitei Impact Crater is documented in the foreground (Grieve and Ber, 1994).

Coesite from the Lake Wanapitei crater, Ontario

Dence, M. R., Robertson, P. B., Wirthlin, R. L.

Abstract

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

ABSTRACT
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.

Lake Wanapitei regional setting. Northwest-trending lines represent Sudbury dikes and the 7.5-km-diameter circle indicates the impact crater proposed by Dence and Popelar (1972). **(A)** Geology. Stars indicate locations of planar deformation features in quartz; dots indicate samples of suevite and shock metamorphosed rocks (extracted from Dressler et al., 1997). SIC—Sudbury Igneous Complex

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.

The red dot represents the approximate area of the Wanapitei impact 37 million years ago in the Paleogene Period.`

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.

In this image taken from GOZooM 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). The Lake Wanapitei impact structure (background) is adjacent to the flat Chelmsford Formation of the Sudbury Igneous Complex (foreground).

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).

Geophysical evidence is in the form of a circular gravity low of ~15 mgal, documented in this Bouguer gravity anomaly map. The low is observed over the north-central, island free area of the lake. This gravity anomaly reinforces the meteoritic origin of this structure similar to other structures (see 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.

Bathymetric Map: Lake Wanapitei is 13,256.80 hectares in size with a shoreline perimeter of 160.40 kilometers. The maximum depth of the lake is 142 meters (466 feet).

Bathymetry taken in 2002 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. 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.

Lake Wanapetei, high altitude image. The impact structure is completely under the water of the lake. Smoke from the infamous “Sudbury Stack” is seen bisecting the lake.

Topographic evidence includes the shape and drainage pattern of the lake as well as an apparent concentric pattern of streams and smaller lakes within 5 km of Wanapitei, illustrated in this airborne C-band radar image (CIRIS). Although not thoroughly mapped, Dressler observed a similar circular pattern in joints and fractures of the region.The high altitude images are courtesy of Earth Impact Database, 2003.

Aerial Exploration

Target rocks are Precambrian crystalline and metasedimentary of the Southern structural province Archean gneisses, Huronian Gowganda (metasedimentary rocks) and Mississagi formations. They are intruded by Nipissing and younger diabase dikes.

The outline of the lake has been enlarged and modified by erosion, illustrated in this image 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).

Lake Wanapetei from the south.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. 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.

Soft, friable suevite, found on the south shore of Lake Wanapitei Impact Crater, is not found in outcrops. It was apparently scooped up from the lakebed by glacial activity and deposited in places along the southern shore of the lake.

Suevite is an impact fallbacks breccia, formed when a meteorite strikes the earth and blasts “target rock” high into the atmosphere. Some target rock falls back into the newly formed crater, and is compacted to form suevite. Suevite typically contains fragments of shock-metamorphosed rocks and glass set in a matrix of fine-grained minerals, rock, and glass fragments.

References

[see – METEORITE]

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.

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

Dence, M.R., Bunch T.E. Cohen A.J. NATURAL TERRESTRIAL MASKELYNITE, THE AMERICAN MINERALOGIST 1967

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.

Earth Impact Database

Side Note

RADARSAT radar image of the Sudbury (left) and Lake Wanapetei (right), double impact structures. Their close proximity is strictly coincidence. The Wanapitei impact happened over 1.8 billion years after Sudbury.

The Clearwater West (left) and East impact craters in northern Quebec’s Canadian Shield. Their close proximity is strictly coincidence. The Clearwater impacts, shown in this image, are related only by their geological position, recent dating puts the Clearwater East at ~460–470 Ma and Clearwater West impact at 286.2 ± 2.2 (2.6) Ma.

(Aerial Radar Courtesy of the Planetary and Space Science Centre at the (Aerial Radar Courtesy of the Planetary and Space Science Centre at the Image courtesy of Earth Impact Database, UNB, 2003.))