Odale-Articles-Mistastin

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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

MISTASTIN IMPACT CRATER

Mistastin impact crater, central Labrador.
Mistastin impact crater (centre right).
  • Age (ma): 36.4 ± 4
  • Diameter: 28 km
  • Location: Newfoundland/Labrador, Canada. N 55° 53' W 63° 18'
  • Shock Metamorphism: Shatter cones are poorly developed. PDF in quartz and feldspars (Taylor and Dence, 1969). Maskelynite*.
  • Dating Method: 40Ar-39Ar by J. Whitehead (pers. comm. 2001)

*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. Image courtesy of NASA.

General Area: Moderate relief, 200-300 m, close to the tree-line in the Canadian Shield. Area has been glaciated, with ice moving west to east. The target rocks are crystalline.
Specific Features: Structure is defined by a ring of low hills, 28 km in diameter, surrounding a depression filled by Lake Mistastin. A horseshoe-shaped island, riSing 130 m above the lake and -3 km in diameter represents a central uplift. Although this is a fairly young crater, much of the original topography has been removed by glacial erosion. A weak fracture halo surrounds the crater in the target rocks and is best expressed in the west.
RADARSAT radar image of the Mistastin impact crater.
The red dot represents the approximate area of the Mistastin impact 36.4 million years ago in the Paleogene Period.
The Mistastin Impact Crater, a heavily eroded complex meteorite crater is technically a “central peak basin structure”. The structure’s rim includes a 26 km diameter ring of low hills that have an elevation of up to 670 m above sea level. These hills surround a depression, the centre of which is filled by the 16 km diameter Lake Mistastin. The lake surface is 338 m above sea level, and about 150 m lower than the mean regional elevation. It contains a horseshoe shaped island ~3 km in diameter, which rises ~130 m above lake level. The island consists largely of shocked basement lithologies and is the eroded remanent of a central uplift (Grieve 2006).

The crater was originally thought to be volcanic in origin. In 1968 it was confirmed as a meteorite impact site with the discovery of shock metamorphism features, specifically PDF in quartz, and poorly developed shatter cones (Taylor and Dence, 1969).


Aerial Exploration

Mistastin impact crater - east (looking west).
The large depression made by the impact that created the Mistastin Meteorite Crater is illustrated in this image. When we cleared the western rim and entered the crater we were forced to fly below the crater rim level to stay out of the clouds.

The impact meteorite type may have been of iron composition (Wolfe et al., 1980). The target rock type is Precambrian, crystalline containing Proterozoic anorthosites, mangerite, granodiorite and quartz monzonite and exhibits a full range of shock features from brecciation to diaplectic glasses. If you ignored the lakes, the area almost looks like a moonscape.

(Courtesy of NASA/LPI) Eastward moving glaciers have drastically reduced the surface expression of this structure, removing most of the impact melt sheet and breccias and exposing the crater floor. Glacial erosion has also imparted an eastward elongation to the crater that is particularly evident in the shape of the lake that occupies the central area of the structure. Isolated patches of fill and sub floor target rocks are preserved (Taylor and Dense, 1969). At the time of the impact 36 million years ago, the continents were approximately in their present positions and a moderate biological extinction had occurred which is associated with a microtektite and an iridium impregnated geological layer. The earliest apes made their appearance 10 million years later.

Mistastin impact crater - west (looking east). Discovery Hill is visible in the centre background.
An outcrop of impact melt rocks known as "Discovery Hill", in the Mistastin impact crater.
The image left was taken from about 1000’ over the western rim of the crater. On the margins of the lake are vestiges of the 80 m thick impact melt sheet that contain evidence of meteoritic features in quartz, feldspar and diaplectic glasses (maskelynite). The impact melt sheet is visible in this image as the plateau just to the left of center at the edge of the lake. In the far background the eastern rim is visible between the clouds.

The sheet of impact melt (image right - Courtesy of GISP)overlies a thin layer of breccia on the crater floor and has an exposed thickness of 80 m at its base. The melt is fine grained, glassy with numerous country rock inclusions. At higher levels it is medium grained with micro-porphyritic and poikilitic textures similar to many Apollo 16 melt rocks. Reaction, assimilation and local partial melting of the inclusions occur and the partial melts show varying degrees of mixing with the impact melt#.

Discovery Hill at the Mistastin impact crater.
Discovery Hill at the Mistastin impact crater.

# The passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.

Horseshoe Island, the central peak of the Mistastin impact crater.
Horseshoe Island, the remnant of the central peak in the center of the lake, is 130 m above the lake level with a diameter of 3 km. In this image the northern rim of the Mistastin Crater is clearly visible in the background. It is part of the central uplift composed primarily of Precambrian crystalline adamellite with small amounts of anorthosites. These target rocks have experienced various grades of shock metamorphism before being incorporated into the melt sheet. Geologically, the closest comparison to Mistastin Crater is the Clearwater West Crater, which has a similar distribution of igneous rocks with shocked inclusions around central mounds of moderately shocked basement rocks. The arch shape of Horseshoe Island resembles the ring islands of the Clearwater West Meteorite Crater. Mistastin is the more deeply eroded and younger than Clearwater. The igneous rocks at Mistastin and Clearwater have many textural features in common. They are considered to be the product of shock melting of the country rocks with admixtures of fragments of less strongly shocked materials. The variations in texture and composition are probably the result of mixing in different proportions of shocked fragments and melts of the adamellite and anorthosites host rocks, together with variations in cooling history. (Taylor, Dence, 1968).
Mistastin impact crater, central Labrador.
Yours truly at the Mistastin impact crater.
We had to gain over 1000’ in altitude to safely clear the northern rim of the crater/basin. We continued our flying explorations further north.

Side Note

The Fraser River Valley in Labrador.
North of the Mistastin Impact Crater is some fascinating geology. In particular I wanted to share with you this image of the Fraser River Valley in Labrador, typical of the scenery we flew over in our exploration. This is one area of Canada I want to explore by foot!!

References

Grieve, R.A.F., Impact Structures in Canada. Geological Association of Canada, 2006.

Taylor, E. C., Dence, M. R., A probable meteorite origin for Mistastin Lake, Labrador. Canadian Journal of Earth Sciences, v. 6, pp. 39-45. 1969.

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



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