KEGS Seminar - Dating, geology and geophysics of the Carswell impact structure, Athabasca Basin, Saskatchewan


Dr. Wouter Bleeker
Research Scientist, Geological Survey of Canada 

Tuesday, April 19th
4:30 p.m.  

Harrison Hall
Room 177
Geological Survey of Canada
601 Booth Street



The Carswell impact structure of northern Saskatchewan ranks among the largest preserved impact structures in North America. Its central uplift, ~19 km in diameter, pierces the ~2 km-thick Athabasca Basin and forms a pronounced circular structure on the geological map of Canada, perhaps only second in terms of “visibility” after the Triassic Manicouagan impact. The final crater diameter of Carswell is ~50 km, which would rank it as Canada’s fourth largest crater after Sudbury, Manicouagan, and Charlevoix.

Like for so many other impact structures, its recognition as a scar from extraterrestrial impact has been tortuous. Although remoteness and less than perfect outcrop exposure have certainly contributed to this, the main reason has been that the traditional geological science community of the 20th century has been slow to embrace impact geology as a key process in shaping Earth evolution. Discovery of the global K-T iridium layer, the Chicxulub impact crater, and isotope ratios showing that the Sudbury igneous complex is a crustal melt sheet finally changed this balance.

At structures such as Carswell, in the face of overwhelming evidence for impact, the explanation in terms endogenic crypto-explosion and doming has thus died a slow death. Intense brecciation, shatter cones, and an overall rebounded crater cross-sections were described by Innes and the Dominion Observatory impact group as early as 1964. And microscopic shock features were recognized shortly thereafter. Yet its age has remained controversial with estimates varying from Precambrian to Mesozoic. Pristine adularia crystals (a low-T K-feldspar polymorph) growing inside vesicles in melt rocks from the deeply eroded central uplift yield unambiguous 40Ar-39Ar ages of 481 Ma and suggest Carswell may be part of the early Ordovician impact spike. With the age finally resolved, important questions that remain are:

1) the overall depth of erosion;

2) whether meltsheet rocks may be preserved, perhaps as injections into the uplifted core; and

3) why have we not yet recognized the (global) fallout layers associated with Carswell and other Ordovician impact craters? 



Wouter Bleeker is a senior scientist with the Geological Survey of Canada in Ottawa. He obtained degrees in geology and ore petrology from the Free University of Amsterdam, the Netherlands. In collaboration with industry, his M.Sc. degree involved unravelling the structure and stratigraphy of an ore-bearing horizon in the Bergslagen District, central Sweden, where his work contributed to the discovery and economic mineral extraction of a massive sulphide deposit. He then spent two years in southern Africa as a lecturer at the University of Botswana. He obtained his Ph.D. from the University of New Brunswick with a dissertation on the structure and stratigraphy of the Thompson Nickel Belt and its nickel sulphide deposits. He then joined Falconbridge Exploration as their in-house structural specialist, before joining the GSC in 1994. With the GSC, and a through a number of global collaborations, he has worked on numerous Precambrian terrains and cratons around the world, notably the Slave craton. He has had a long-time fascination with impact structures.