Odale articles Carswell

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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

CARSWELL IMPACT STRUCTURE

Carswell structure - Saskatchewan, Canada. Yellow = ATHABASCA FORMATION, Pink = PRECAMBRIAN BASEMENT COMPLEX, Grey = INTERIOR PLAINS.
Carswell structure - Google image. The superimposed circle illustrates the position of the 39 km diameter buried crater. Lake Athabasca is at the top of the image.
  • Age ma: 115a ± 10
  • Diameter: 39b km
  • Location: N 58° 27’ W 109° 30’
  • Dating Method: Ar-Ar methods, the step-wise heating data, gives 115 Ma for the age of the Carswell structure (Grieve 2006).
a RE: new data on Carswell) - Therefore, our research suggests that the Carswell event is older than the deposition of the Athabasca Basin and that the actual circular structure is quite probably the central peak, a local expression of a larger multi-ring impact structure hidden beneath the sediments (Duhamel et al 2005).
b (RE: new data on Carswell) - The Carswell impact structure is therefore older and larger than previously estimated. Using both Rondot’s and Grieve’s models for complex impact structure, the central uplift (38km) considered to be underneath the annular dolomitic unit would suggest a crater size in the basement of 118 to 125 km wide. This value is a minimal one and fits data from paleocurrents and isopach maps done by Ramaekers on the Athabasca Basin (Duhamel et al 2004).



The red dot represents the approximate area of the Carswell impact 115 million years ago in the Cretaceous Period.
The Carswell crater is the largest known impact crater in Saskatchewan and 4th-largest in Canada. A chain of small lakes trending from southwest to northeast has formed along the rim in this location. Shatter cone fragments from the crystalline core at Carswell confirms a cosmic impact event (Grieve 2006).

The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels and creating numerous shallow inland seas. These oceans and seas were populated with now extinct marine reptiles, ammonites and rudists, while dinosaurs continued to dominate on land. At the same time, new groups of mammals and birds, as well as flowering plants, appeared. The Cretaceous ended with a large mass extinction, the Cretaceous–Paleogene extinction event, in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. The end of the Cretaceous is defined by the K–Pg boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic Eras.

Carswell structure - Saskatchewan. (courtesy of University of New Brunswick).
Carswell structure - geomorphology (courtesy of University of New Brunswick).
The Carswell structure in the western Athabasca Basin, northern Saskatchewan (Canada), has previously been interpreted as an eroded impact structure with a minimum diameter of ~36 km, the outer margin of which is broadly defined by an outer ring of sediments composed of the only algal reefs observed in the Athabasca Group, the Carswell Formation. This ring surrounds an 18-km-wide uplifted basement core composed of gneiss units of Archean to Paleoproterozoic age that display shatter cones, planar deformation features (PDFs), pseudotachylyte veins, and impact melts and breccias (Cluff melt sheet, Cluff breccias) indicating that pressures and temperatures locally exceeded 60 GPa and 1500 °C. Detailed analysis of the basement–Athabasca Group contact from field and drill core samples indicates that shock features are not present in the Athabasca Group sediments in direct contact with the highly shocked basement gneisses. This pattern is inconsistent with a post–Athabasca Group age for the impact. Moreover, our study has revealed PDF-bearing quartz grains in basal units of the Athabasca Group 14 km south of the southern edge of the basement core (outside the estimated outer ring). The new proposed model suggests that the impact event is of pre-Athabasca (Proterozoic) age and that it produced a multiring structure that controlled the paleogeography of the Athabasca Group units in the western part of the basin. The model is well supported by basin analysis and gravity data. The Carswell Formation is the result of algal reefs building on peak-ring–related seamounts at the end of Athabasca Group deposition. The overturned bedding observed locally adjacent to the basement core is interpreted as the result of gravity-driven readjustment of the central uplift (Abstract - Genest et al 2010).


The superimposed circle illustrates the relative size of the Carswell structure imaged from 3000 feet AGL in GO ZooM.
Ground zero of the Carswell impact structure is imaged under the wing of GO ZooM The point of the bolide impact is illustrated by the small superimposed circle. The small airport that services the mine at Carswell is visible in the upper right.
The Carswell structure has been highly eroded. The principal morphological element is a ring of small hills -50 m high, with an inner diameter of 30 km and an outer diameter of -40 km. These represent upturned beds of erosionally resistant rocks. There is a central core of swampy ground with a diameter of 18 km. At the contact between the crystalline rocks of the core and upturned sediments, uranium deposits are mined.

Low relief area in the Canadian Shield. Area is sparsely forested and has been glaciated. The target rocks are metamorphosed sediments overlying crystalline rocks.




From: Innes, M. J. S., Recent advances in meteorite crater research at the Dominion Observatory, Ottawa, Canada. 1964

Carswell structure - magnetic intensity anomalies.
The gravity field of Carswell is remarkably similar to that produced by the Vredefort ring structure in South Africa (Maree 1944 & Innes 1964).
Carswell structure - This dip where Highway 955 crosses the Douglas River, marks the rim of the Carswell meteorite impact crater (courtesy of Natural Resources Canada).
Carswell structure - Bouguer gravity anomalies contoured at intervals of two milligals.
Just to the north of the Carswell structure we passed over the Athabasca Sand Dunes in GO ZooM. Lake Athabasca is visible in the upper left. Note the sand ground in the image above of the Carswell structure floor.
Carswell structure - Interior (pink) = GRANITE; Inner ring (yellow) = DISTURBED ATHABASCA FORMATION; Outer ring (green) = CARSWELL FORMATION (dolomite) - (Sawatzky).
The Athabasca basin, a historical shallow tropical sea basin filled with sediments from the Hudsonian mountains. The Athabasca basin was formed during the Statherian or Paleohelikian 1.7 to 1.6 billion years ago when coarse fluvial and marine clastic sediments were laid down containing gold, copper, lead, zinc, and uranium oxides. The highest grade uranium deposits in the world are found at the unconformity between these clastic layers and the Precambrian bedrock. The Athabasca Sand Hills protected by The Athabasca Sand Dunes Provincial Park are unique feature of the Canadian shield. The hills are located in northern Saskatchewan and border Lake Athabasca, which straddles the Alberta and Saskatchewan border.




References

Duhamel, I., Genest, S., Robert, F. and Tremblay A.: A RE-EVALUATION OF THE SIZE OF THE CARSWELL ASTROBLEME, SASKATCHEWAN, CANADA. 67th Annual Meteoritical Society Meeting (2004).

Duhamel, I., Genest, S., Robert, F. and Tremblay A.: CARSWELL IMPACT STRUCTURE, SASKATCHEWAN, CANADA: GEOLOGICAL, PETROGRAPHICAL AND GEOPHYSICAL RESULTS, AND IMPLICATIONS FOR THE AGE OF THE ASTROBLEME. 68th Annual Meteoritical Society Meeting (2005)

Genest, S., Robert, F., Duhamel, I.: The Carswell impact event, Saskatchewan, Canada: Evidence for a pre-Athabasca multiring basin? Geological Society of America, 2010.

Grieve R.A.F., Impact structures in Canada, Geological Association of Canada, 2006.

Innes, M. J. S., Recent advances in meteorite crater research at the Dominion Observatory, Ottawa, Canada. Meteoritics, v. 2, pp. 219-241. 1964.

Maree, B.D.: The Vredefort Structure as revealed by gravi-metric survey, Trans. Geol. Soc. S. Africa 1944.

Sawatzky, H.B.: Astroblemes in the Williston Basin. C.S.E.G. National Convention. 1974.

Earth Impact Database



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