Odale-Articles-Wanapitei

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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

WANAPITEI IMPACT CRATER

The Wanapitei Impact Crater lies entirely within the central portion of the 9 km diameter Wanapitei Lake, visible at the top center.
A satellite 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.
  • Age (ma): 37.2 ± 1.2
  • 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).
  • Dating Method: K/Ar, 40Ar-39Ar (Bottomley et al 1974).
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.
The Lake Wanapitei impact structure (background) is adjacent to the flat Chelmsford Formation of the Sudbury Igneous Complex (foreground).
The Wanapitei Impact Crater (right) 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. 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).
Residual Bouguer gravity field over Wanapitei impact structure, indicating a symmetric gravity low of ~15 MGal (Dence and Popelar 1972).
Lake Wanapetei bathymetry (2002).
Geophysical evidence is in the form of a circular gravity low of ~15 mgal, documented in this Bouguer gravity anomaly map (left). 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.

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.

Lake Wanapetei magnetic survey (2002).
Lake Wanapetei, high altitude image. The impact structure is completely under the water of the lake.
Lake Wanapetei topographic (2003).
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

Lake Wanapetei from the north.
Lake Wanapetei from the west.
This image (left) of Wanapitei Lake from the north 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). 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 (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.

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 (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.

Lake Wanapetei suevite.
Lake Wanapetei suevite.
Soft, friable suevite (illustrated in the images left & right), 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

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.

Earth Impact Database


Additional Notes

Odale-clearwaterWest-01 clearwater high alt.jpg
In another coincidence relatively nearby in northern Quebec’s Canadian Shield, shown in this image, there is a double impact site, the Clearwater East and West impact craters. The Clearwater impacts are related and unlike Wanapitei and Sudbury, they simultaneously occurred 290 million years ago.



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