Cypress Hills region - Bedrock Geology and Physiography

Astroblemes around the Cypress Hills

Astroblemes are roughly circular impact structures, thought to be formed by meteor impact. If the impacts occurred in the recent past, the craters may have a distinct surface expression, as at the Holleford Meteorite Impact Crater in Ontario (see MacLennan 1988). However, when impacts occurred in the distant geologic past, astroblemes may be deeply buried and only detectable through geophysical methods and drilling.

The Eagle Butte astrobleme, at the west end of the Cypress Hills, appears on the latest edition of the Geology of Alberta map (1999). This map shows two astroblemes in Alberta; the second (Steen River) is in the far north of Alberta. However, the latest GSC Bedrock Geology map of the area (Okulitch et al. 1996) shows three astroblemes around the margin of the Cypress Hills: Eagle Butte, Maple Creek (which is actually south of Piapot), and Govenlock (south of the town of the same name and immediately north of the US border). In their note accompanying the map, their comments on these are as follows:

Three sub-circular structures within the Bearpaw and older formations north, west and south of the Cypress Hills are herein interpreted as astroblemes. The enigmatic Maple Creek structure is of mid-Maastrichtian age; the others are of the same age or younger. The Eagle Butte structure has been verified as an astrobleme, but the Govenlock structure remains to be studied. The three may have formed during impact of a cluster of meteorites that perhaps contributed to faunal declines preceding extinctions at the end of the Cretaceous.

For Eagle Butte, Westbroek (1997) says the following:

The Eagle Butte impact crater is located in southeastern Alberta. It is a complex crater about 18 km in diameter with structural uplift at its core of some 250-300 m (Sawatzky 1976). Its impact origin has been proved by the discovery of shatter cones at the surface (Lawton et al. 1993; Hodge 1994). An area of active hydrocarbon exploration, it has been extensively covered by seismic acquisition lines and the structure has been drilled in a number of localities. To date, a few gas accumulations have been found associated with the structure. (p. 21)

Robertson and Grieve (1975) list Eagle Butte as a "probable" impact crater, together with one at Elbow, Saskatchewan. However, this was written in 1975 and more work has been done since then. Ezeji-Okoye (1985) describes the Eagle Butte feature in more detail.

A recently-published compendium by Grieve (2006) summarizes the publicly-available data for the Eagle Butte structure. He comments on several occasions that subsurface and geophysical data are proprietary and not readily available. Grieve (2006: 72) notes deformation in the Lower and Upper Cretaceous formations, between the Alberta Shale and Bearpaw formations, with disturbance extending to at least 1.3 km depth. He suggests that the timing of the event is younger than Upper Cretaceous (because bedrock units of this age are disturbed) but notes that the age is not well-constrained. Hanova et al. (2005) provide visualizations of the north-east quadrant of the structure, based on well and seismic data. Their analysis shows multiple faults and curved fault planes. Although these do delineate a roughly circular feature, they note that the structure is complex and detected a "rose-petal structure, as opposed to simply only bowl-shaped faults".

The impact structure is not well-expressed at the surface, and is not obvious in the field. Presumably, glaciation and subsequent geomorphic processes would have removed any marked crater, if one had been present prior to glaciation. However, satellite imagery (see below) does show a roughly circular feature, marked especially in the stream network and field network.

You can view an image of Eagle Butte here.

The Whitecourt Impact Crater in northwest Alberta provides a useful comparative example. This small crater is estimated to have been been formed in the late Holocene and has a more obvious surface expression (Herd et al. 2008; Kofman et al. 2010).

You can view an image of the Whitecourt Crater here.

Finally, a useful article by Donofrio (1998) has some helpful maps showing the distribution of astroblemes in Canada, although it does not mention Eagle Butte or the newly-identified Whitecourt Impact Crater.

References

Donofrio, R. R. 1998

North American Impact Structures Hold Giant Field Potential. Oil & Gas Journal (May 11, 1998):69-83.
Available in PDF form at http://www.edge.o u.edu/news/Donofrio.pdf .

Ezeji-Okoye, S. 1985

The Origin of the Eagle Butte Structure. PanCanadian Petroleum Limited, unpublished report 75 pages.

Grieve, R. A. F. 2006

Impact Structures in Canada. GEOtext No. 5. Geological Association of Canada, St John's, Newfoundland, Canada. x + 210 pages.
Provides a useful summary of twenty-eight impact structures in Canada, including maps, subsurface data, geophysical data, and detailed citations. The text includes discussion of the Eagle Butte (pp. 71-73) and Maple Creek (pp. 113-114) structures as well as several others that occur in the Prairie Provinces. The Whitecourt Meteorite Impact Crater, however, is too recent to be included. (02/Sep/2010).

Hanova, J., D. C. Lawton, J. Visser, A. R. Hildebrand, and L. Ferriere 2005

3D Structural Interpretation of the Eagle Butte Impact Structure, Alberta, Canada. . 36th Annual Lunar and Planetary Science Conference, March 14-18, 2005, in League City, Texas, Abstract no. 2355 2 pages.
On-line document available at htt p://www.lpi.usra.edu/meetings/lpsc2005/pdf/2355.pdf .

Herd, C. D. K., D. G. Froese, E. L. Walton, R. S. Kofman, E. P. K. Herd, and M. J. M. Duke 2008

Anatomy of a young impact event in central Alberta, Canada: Prospects for the missing Holocene impact record. Geology 36(12):955-958.
AEU SCI QE 1 G3455 DOI: 10.1130/G25236A

Hodge, P. 1994

Meteorite Craters and Impact Structures of the Earth. Cambridge University Press, Cambridge, England, UK. 124 pages.

Kofman, R. S., C. D. K. Herd, and D. G. Froese 2010

The Whitecourt meteorite impact crater, Alberta, Canada. Meteoritics and Planetary Science 45(9):1429-1445.
DOI: 10.1111/j.1945-5100.2010.01118.x

Lawton, D. C., R. R. Stewart, and R. Gault 1993

The Geophysical Expression of the Eagle Butte Impact Structure. . Paper presented at the 19th Annual Meeting of the Canadian Geophysical Union, Banff, Alberta.

MacLennan, M. J. 1988

The Holleford Meteorite Impact Crater. The Canadian Geographer 32(2):173-177. Canadian Landform Examples - 8, vol. 32.
This crater is located near Kingston, Ontario. This provides a useful description of a crater that is still clearly visible at the ground surface.

Okulitch A. V., D. A. Lopez, and T. Jerzykiewicz (compilers) 1996

Geology, Lethbridge, Alberta-Saskatchewan, Montana. Geological Survey of Canada, Map NM - 12 - G, scale 1:1,000,000. National Earth Science Series, Geological Atlas. Geological Survey of Canada, Ottawa, Ontario, Canada.
Consists of four map sheets.

Robertson, P. B., and R. A. F. Grieve 1975

Impact Structures in Canada: Their Recognition and Characteristics. Journal of the Royal Astronomical Society of Canada 69:1- 21.
This is available on-line (PDF format) through the Royal Astronomical Society of Canada website at http://www.rasc.ca/journal/ .

Sawatzky, H. B. 1976

Two Probable Late Cretaceous Astroblemes in Western Canada - Eagle Butte, Alberta and Dumas, Saskatchewan. Geophysics 41:1261-1271.

Westbroek, H. 1997

Seismic Interpretation of Two Possible Meteorite Impact Craters: White Valley, Saskatchewan and Purple Springs, Alberta. Unpublished M.Sc. dissertation. Department of Geology and Geophysics, Calgary, Alberta, Canada. 145 pages.
Available on-line at http://www.cr ewes.org/Theses/1997/Westbroek/ .