GOW TRANSITIONAL IMPACT CRATER
by: Charles O’Dale
- Type: Transitionala
- Age ma: <250b – TRIASSIC
- Diameter: 5 kmc
- Location: N 56° 27’ W 104° 29’
- Shock Metamorphism: quartz and feldspar PDF
a A transitional form between a simple bowl-shaped crater and a complex crater with a central uplift. G. R. Osinski et al – 15 May 2023
b Dating Method: 40Ar-39Ar, pre-1977 K-Ar, 40Ar-39Ar and Rb-Sr ages recalculated using the decay constants of Steiger and Jager (1977) Ages in millions of years (Ma) before present (UNB 2012).
c The Gow structure, with a diameter of ~5 km, is one of the smallest currently known complex impact structures in crystalline target rocks on earth (Grieve 2006).
May 2023 UPDATES, with permission of G. R. Osinski 2023.
Revisiting the Gow Lake impact structure, Saskatchewan, Canada, Meteoritics & Planetary Science First published: 15 May 2023
The ~5 km diameter Gow Lake impact structure formed in the Canadian Shield of northern Saskatchewan approximately 197 Myr ago. This structure has not been studied in detail since its discovery during a regional gravity survey in the early 1970s. We report here on field observations from a 2011 expedition that, when combined with subsequent laboratory studies, have revealed a wealth of new information about this poorly studied Canadian impact structure. Initially considered to be a prototypical central peak (i.e., a complex) impact structure, our observations demonstrate that Gow Lake is actually a transitional impact structure, making it one of only two identified on Earth. Despite its age, a well-preserved sequence of crater-fill impactites is preserved on Calder Island in the middle of Gow Lake. From the base upward, this stratigraphy is parautochthonous target rock, lithic impact breccia, clast-rich impact melt rock, red clast-poor impact melt rock, and green clast-poor impact melt rocks. Discontinuous lenses of impact melt-bearing breccia also occur near the top of the red impact melt rocks and in the uppermost green impact melt rocks. The vitric particles in these breccias display irregular and contorted outlines. This, together with their setting within crater-fill melt rocks, is indicative of an origin as flows within the transient cavity and not an airborne mode of origin. Following impact, a hydrothermal system was initiated, which resulted in alteration of the crater-fill impactites. Major alteration phases are nontronite clay, K-feldspar, and quartz.
The Gow impact occurred at the time of the Permian–Triassic (P–Tr) extinction event.
Gow Lake was named after James R. Gow, a war casualty. It is a roughly circular lake with a prominant central island located in the Precambrian Shield of northern Saskatchewan. The country rocks are predominantly quartz-feldspathic gneisses. Impact rock contains 40-45% identifiable clastic debris, which includes quartz and feldspar with planar deformation features (Osinski 2012).
Systematic mapping of the island revealed an almost complete stratigraphy from the brecciated and fractured “basement”, up through a series of lithic breccias, impact melt-bearing breccias, impact melt rocks and, towards the top, further impact melt-bearing breccias intercalated with the melt rocks. It is notable that the highest point on the island comprises a large, steeply sided outcrop of green impact melt rock. There is an almost complete sequence of impactites that represent the crater-fill of the Gow Lake impact structure. Such sequences of rocks are very rare at terrestrial impact structures and rarer still exposed at the surface. As such, Gow Lake offers important insights into the stratigraphy of impactites produced during impact into relatively homogenous crystalline target (Osinski 2012).
Gow Lake, in the Precambrian Shield of Saskatchewan, is circular, 4 km in diameter, and has a large central island.
These aerial images with the haze created by forest fires in BC, were taken from GO ZooM on our trip to the west coast in 2012. At the time we took these images, the Saskatchewan prairies 100 km to the south were experiencing severe thunderstorms and tornadoes!!
Granites and quartz-feldspathic gneisses are exposed around the perimeter of the lake, whereas the island is formed largely of brecciated equivalents. Most of the breccias are composed entirely of clastic material, but at one locality fine-grained felted matrices form a significant component of the breccias, and coronas of clear glass surround quartz grains. The latter breccias also contain microscopic features characteristic of shock metamorphism, among which multiple sets of planar deformation structures in quartz are particularly diagnostic. Similar shock metamorphic features have been widely reported from terrestrial meteorite craters; accordingly, Gow Lake is interpreted as a deeply eroded impact crater and the felted matrices as impact melts. A local negative gravity anomaly with an amplitude of 3 mGal centered on the lake is attributed mainly to highly fractured basement rocks underlying the lake, which model studies indicate may extend to a depth of 900 m. A provisional minimum age of 100 Ma is proposed for the crater. Thomas et al (1977).
[see – METEORITE]
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Grieve R.A.F., Impact structures in Canada, Geological Association of Canada, 2006.
G. R. Osinski, A. C. Singleton, A. Ozaruk, and J. R. Hansen, NEW INVESTIGATIONS OF THE GOW LAKE IMPACT STRUCTURE, SASKATCHEWAN, CANADA. University of Western Ontario, London, ON, Canada, 2012.
Gordon R. Osinski, Adam B. Coulter, Roberta L. Flemming, Alexandra Ozaruk, Annemarie E. Pickersgill, Alaura C. Singleton, Revisiting the Gow Lake impact structure, Saskatchewan, Canada, Meteoritics & Planetary Science First published: 15 May 2023
Thomas, M. D., Innes, M.J.S., Dence, M.R., Grieve, R.A.F. and Robertson,P.B., Gow Lake, Saskatchewan: Evidence for an origin by meteorite impact (abstract). Meteoritics, v. 12, pp. 370-371. 1977.
University of New Brunswick, Earth Impact Data Base, 2012.