FORT RAE, NORTH WEST TERRITORIES

Type: Volcanic Crater
Diameter: ~2.8 Km
Age: before or during the palaeozoic – 541 to 252 million years ago (mya)
Location: North West Territories, Canada N 62° 39.4’ W 115° 48.1’
Great Slave Lake, NWT “Circular Structure”

Fort Rae structure – south west (looking north east) from GOZooM.

During our 2012 exploration of Canada above the Arctic Circle we were constantly on the lookout for any impact structures/craters and circular geologic structures. Fort Rae was noted in an article from a 1964 Journal of the Royal Canadian Astronomical Society and we wanted to ensured that we collected images of the structure before flying further south. It is suspected that the circular tongue of land in Great Slave Lake is a volcanic eruption at the west edge of the Slave Craton.

Fort Rae structure – west (looking east) from GOZooM.

(quote from JRASC) “This object was drawn to our attention by Mr. E.J. Meek, head of the Air Photo Library. It is located on a tongue of land jutting out from the north-eastern shore of North Arm, Great Slave Lake. It is oblong or oval in shape with a long axis of 1.75 miles and a short axis of 1.3 miles. A report received from Dr. C.S. Lord of the Geological Survey indicates that this point of land is at least partly covered by Palaeozoic sediments. A study of the crater with the stereoscope has indicated that the slope of the inner and outer walls are similar and in this respect it does not resemble known meteorite craters. It is further noted that the average elevation of the floor of the crater is at or slightly above the level of Great Slave Lake whereas, over most of its circumference, the outer periphery of the crater dips below the level of the lake. In this respect also this object differs from meteorite or lunar craters and is probably due to a different origin. It is possible that this crater is due to volcanic action” (Beals et al, 1964).

GROUND EXPLORATION

Fort Rae structure from the north. This image was taken by fellow RASC member Stephen Bedingfield while he was documenting the transit of Venus at this location.

Fellow crater explorer, Geoff Foster, paddled to the middle of the Fort Rae structure and climbed to the top of the feature. (Courtesy, Geoff Foster)

There was a lot of calcium precipitate coming through the cracks and forming neat little nodules on all the rocks. (Courtesy, Geoff Foster)

Palaeozoic sediments (Courtesy, Geoff Foster)

Fort Rae was first established on a prominent peninsula on the north shore of the north arm of Great Slave Lake in 1852 as a wintering provision post for the Hudson’s Bay Company. It was named for Scotsman explorer John Rae, who was among the explorers looking for remains of Sir John Franklin’s expedition in the Arctic. It became an important trading post for the Dogrib Dene.

THE SLAVE CRATON

The Slave craton is a Canadian geological formation located in the Northwest Territories and Nunavut. This craton is approximately 300,000 square kilometres (120,000 sq mi) in size and forms part of the Canadian Shield. It is dominated by ca. 2.73-2.63 Ga greenstones and turbidite sequences and ca. 2.72-2.58 Ga plutonic rock, with large parts of the craton underlain by older gneiss and granitoid units. The cratonic block extends from the Great Slave Lake at about 61 °N to Coronation Gulf on the Arctic Ocean at 69 °N. It covers longitudinally the area between about 105 °W to 117 °W. The Slave includes the Acasta Gneiss which is one of the oldest dated rock units on Earth at 4.03 Ga.

The crust of the Slave craton is thought to have amalgamated during a 2.69 Ga collision between a proto-Slave western basement complex, known as the Central Slave Basement Complex, and an eastern putative island arc terrane (Hackett River) along a N-S suture. Along the Acasta River, this basement complex yields protolith ages up to ca. 4.03 Ga.

Ages represented by the craton are the Cambrian, Cretaceous, Eocene, Jurassic, Permian and Siluro-Ordovician, based on the craton’s known kimberlites which presently number in the hundreds (Bleeker et al).

An impact melt origin for Earth’s oldest known evolved rocks
Abstract
Earth’s oldest evolved (felsic) rocks, the 4.02-billion-year-old Idiwhaa gneisses of the Acasta Gneiss Complex, northwest Canada, have compositions that are distinct from the felsic rocks that typify Earth’s ancient continental nuclei, implying that they formed through a different process. Using phase equilibria and trace element modelling, we show that the Idiwhaa gneisses were produced by partial melting of iron-rich hydrated basaltic rocks (amphibolites) at very low pressures, equating to the uppermost ~3 km of a Hadean crust that was dominantly mafic in composition. The heat required for partial melting at such shallow levels is most easily explained through meteorite impacts. Hydrodynamic impact modelling shows not only that this scenario is physically plausible, but also that the region of shallow partial melting appropriate to formation of the Idiwhaa gneisses would have been widespread. Given the predicted high flux of meteorites in the late Hadean, impact melting may have been the predominant mechanism that generated Hadean felsic rocks (Johnson et al 2018).

“Our modelling shows that the Acasta River rocks derived from the melting of pre-existing iron-rich basaltic rock, which formed the uppermost layers of crust on the primitive Earth,” says Johnson.

“The uppermost three kilometres of mafic crust would have been melted in producing the rocks we see today.

“It would have needed something special to produce the 900 degree Celsius temperatures needed to generate these early felsic rocks at such low pressures, and that probably means a drastic event, most likely the intense heating caused by meteorite bombardment.”

It’s entirely likely that following the barrage this type of felsic rock would have been very common. However, after billions of years and drastic changes due to plate tectonics, almost none remain. Today, only this small area in northern Canada survives as a window into the violent early years of a young Earth (NEWS GEOSCIENCE 15 AUGUST 2018).

Acasta gneiss, NWT Canada–This ancient stone, nearly 4 billion years old.

REFERENCE

Geology. Diamond Head (a Fort Rae analog) is part of the system of cones, vents, and their associated eruption flows that are collectively known to geologists as the Honolulu Volcanic Series, eruptions from the Koʻolau Volcano that took place long after the volcano formed and had gone dormant.

Banks Peninsula New Zealand (a Fort Rae analog) was an active volcanic complex.

Beals, C. S., Ferguson, G. M., & Landau, A., Scientists Report II. A Search for Analogies Between Lunar and Terrestrial Topography on Photographs of the Canadian Shield, Part II,Journal of the Royal Astronomical Society of Canada, Vol. 50, p.257-258

Bleeker W., Ketchum J., Davis B., Sircombe K, Stern R., Waldron ., The Slave Craton From On Top: The Crustal View.

Tim E. Johnson, Nicholas J. Gardiner, Katarina Miljković, Christopher J. Spencer, Christopher L. Kirkland, Phil A. Bland & Hugh Smithies., An impact melt origin for Earth’s oldest known evolved rocks Nature Geoscience (2018)