GOW TRANSITIONAL IMPACT CRATER

Type: Transitional^a
Age ma: <250^b – TRIASSIC
Diameter: 5 km^c
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).

Saskatchewan, Canada – Gow Lake and Deep Bay craters,Carswell, Dumas, Elbow, Maple Creek and Viewfield impact structures. Yellow = ATHABASCA FORMATION, Pink = PRECAMBRIAN BASEMENT COMPLEX, Grey = INTERIOR PLAINS. Courtesy Breaking Science News at Sci-News.com Science Website.

Gow Crater , Saskatchewan – Note Calder Island (central peak) Google image.

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

Abstract
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.

A new geological map of Calder Island based on our field expedition. The darker tone for each lithology represents outcrops visited and mapped during fieldwork. The lighter tone signifies the inferred presence each lithology. Impact melt-bearing breccia outcrops are too small to appear on the map and so are indicated with a star symbol.

Optical photomicrographs in cross-polarized light of target rocks and lithic impact breccias. (a) Target rock from the crater rim region. Sample 4.12. (b) Parautochthonous target rock from Calder island displaying evidence for in situ brecciation. Sample 1.28. (c) Lithic impact breccia from Calder island. Sample 7.08. (d) Lithic impact breccia from Calder island. Sample 5.06. All these images display a mix of quartz, K-feldspar, and plagioclase, plus minor biotite visible in some images due to its brown pleochroism.

Optical photomicrographs and BSE images of clast-rich impact melt rocks (a–c) and red impact melt rocks (d–f). (a) Optical photomicrograph in plane-polarized light of the clast-rich impact melt rocks. All clasts in this view are either quartz or feldspar mineral clasts. Sample 1.17. (b) BSE image showing the fine-grained nature of the K-feldspar (Kfs)-dominated groundmass of the clast-rich impact melt rock. Several quartz mineral clasts are also present (Qz-cl). Sample 5.13. (c) Optical photomicrograph of PDFs in quartz in the clast-rich impact melt rock. Sample 2.01. (d) Optical photomicrograph in plane-polarized light of the red impact melt rocks. Note the clast-poor nature compared to (a). The brownish groundmass is dominated by K-feldspar (see (e)). The clasts visible in this image are quartz and biotite. Sample 2.10. (e) BSE image showing K-feldspar-dominated groundmass of the red impact melt rocks. Sample 2.19. (f) Clast of ballen silica in the red impact melt rock.

Optical photomicrographs and BSE images of impact melt-bearing breccia (a–c) and green impact melt rocks (d–f). (a) Optical photomicrograph of impact melt-bearing breccia with a large flow-textured vitric particle taking up the left half of the image. Sample 1.21. (b) BSE of impact melt-bearing breccia with prominent altered flow-textured vitric particles (Gl) in the upper half of the image. The clastic matrix is visible in the bottom half of the image (cf., (d) and (f)). Sample 7.03. (c) BSE image of a vitric particle within impact melt-bearing breccia. A quartz clast (Qtz-Cl) appears ductilely deformed and the groundmass is comprised of altered glass (Gl) plus microscopic plagioclase (Pl) crystallites. Sample 7.03. (d) BSE image of a typical green impact melt rock with a glassy groundmass. Sample 7.06. (e) BSE image of the same sample as (d) with a more crystalline groundmass of plagioclase (Pl) and minor interstitial glass. Sample 7.06. (f) Optical photomicrograph of a clast of ballen silica in the green impact melt rock.

Images from: Revisiting the Gow Lake impact structure, Saskatchewan, Canada,

2012 GOW FLYOVER

Gow Impact crater looking west from GOZooM, 2012. (Note Calder Island – central peak)

Gow Impact Crater – Saskatchewan (Courtesy NASA).

The Gow impact occurred at the time of the Permian–Triassic (P–Tr) extinction event.

The red dot represents the approximate area of the Gow impact 250 million years ago 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!!

Gow Crater north west shore from GOZooM, 2012.

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).

We took these images of the Gow Crater flying in from the east and leaving the area to the west, from GOZooM, 2012.

References

[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.