NICHOLSON LAKE IMPACT STRUCTURE
by: Charles O’Dale
- Type: Complex
- Age (ma): 389 ± 6.7
- Diameter: 12.5 km
- Location: N 62° 40′ W 102° 41′ North West Territories, Canada.
- Shock Metamorphism: Monomict and polymict breccias (including suevites) are present, in addition to shocked gneiss and granitic basement rocks, some of which are shatter cone bearing (Spray et al 2017).
General Area: The Nicholson impact structure occupies a shallow depression in an area of subdued topography. Vegetation is limited, as the area is 150 km north of the tree-line. The target rocks are crystalline.
Specific Features: This crater is occupied by a roughly oval lake with a large promontory on the west and a central island. The lake contrasts with the generally linear lakes of the area. The structure is heavily eroded and glaciated. A central uplift region rises approximately 52 m above lake level. No rim is preserved and the regional elevations are within 10 m of the lake level.
Evidence for impact origin was found in 1965. Shatter cones were found in the gneisses along the western shore of the lake. Impact breccias were exposed on a promontory that forms a peninsula at the west side of the lake and in two central islands. Planar deformation features were identified in the breccias. (Dence et al 1968).
The Nicholson Lake impact structure is an ~12 km diameter, eroded, complex crater located within the Rae Domain of the western Churchill Province, Canada. This province has undergone a complex, polycyclic history spanning 3.4 to 1.8 Ga. At the regional scale, it comprises Neoarchean gneiss and greenstone belts of the Snow River Suite, intruded by biotite leucogranites of the Snow Island Suite and by Hudson granitoids. Middle Ordovician limestones and associated sedimentary rocks overlay the Precambrian basement at the time of impact. Though now largely removed from the region by erosion, the sedimentary rocks can be found in situ and as fragments in breccias within the crater impactites. Previous age constraints, based on Ar-Ar geochronology, indicate a formation age for Nicholson of <400 Ma. A semi-total Pb/U isochron yields a lower intercept age of 389 ± 6.7 Ma, which places it in the Middle Devonian (Givetian stage). This result is in keeping with its geological setting (i.e., having to be post-Middle Ordovician: <~470 Ma) and the previous Ar-Ar geochronology (<400 Ma) (Spray et al 2017).
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Dence, M.R., Innes, M., Robertson P., Recent geological and geophysical studies of Canadian craters. Dominion Obs. Ottawa.
Grieve R.A.F., Robertson P.B., IMPACT STRUCTURES IN CANADA, the Journal of the Royal Astronomical Society of Canada, February 1975
McGregor, C. R. M. McFarlane, and J. G. Spray, THE NICHOLSON LAKE IMPACT STRUCTURE, CANADA: SHOCK FEATURES AND AGE OF FORMATION. Planetary and Space Science Centre, University of New Brunswick 2017