by: Charles O’Dale

a The age of the Eagle Butte structure is generally given as <65 million years. This is based on upper Cretaceous rocks that are shatter-coned and the Pleistocene glaciation that has eroded the structure (Grieve 2006).

The Eagle Butte structure is located in southeast Alberta, 35 km south of the City of Medicine Hat and immediately west of Cypress Hills Provincial Park (Visser 2005).
The Eagle Butte Impact Structure, circled in this aeronautical chart, is south of Medicine Hat, Alberta.
Eagle Butte from approximately 3000 feet AGL. The circle represents the approximate rim position of the structure.
The red dot represents the approximate area of the Eagle Butte impact.
The Eagle Butte impact occurred <65 million years ago in the Paleocene Period. At that time CO2 was at two times present levels and O2 at 27% of atmosphere (vs 20.946% today). The building of the central Rocky Mountains was underway and the Deccan Traps continued to erupt.
Around the time of the Eagle Butte impact, this little critter – the Common Ancestor of all Placental Mammals – was surviving in the Tertiary environment of 65 million years ago.

Sawatzky H.B. Buried impact craters in the Williston Basin and adjacent area Lunar and Planetary Institute 1977

The diameter of the Eagle Butte Structure is approximately as great as 17 km. Originally it consisted of a central uplift with a radius of 2.5 km, surrounded by a 2 km wide moat and a 4 km wide fractured rim. It is estimated that the floor of the moat was at least 180 m below, and the top of the central uplift more than 200 m above the regional surface. The rim on the northeast side of the crater was raised to a height of at least 120 m.

Presently, there is little or no surface expression of the Eagle Butte structure. Substantial erosion of the central uplift and crater rim took place from the Eocene to the Pleistocene. In contrast, the low lying area of the crater moat underwent little or no erosion. The entire uppermost Cretaceous section appears to be preserved within the moat. This would place the time of impact at the Cretaceous/Tertiary boundary (~65 Ma) or later (Visser 2005).

This superimposed circle within the Eagle Butte structure represents ground zero of the impact <65 million years ago.

The Eagle Butte structure was definied as an impact event with the discovery of shatter cones and planar fractures in quartz. The structure is covered with glacial overburden and is portrayed on geological maps as a circular to oval area, ~16 km in diameter with complex faulting. The subsurface character is that of a complex structure, with a structural trough and a central uplift (Grieve 2006).

2D seismic profile across Eagle Butte crater showing a central raised area, disturbed horizons with apparent displacements and multiple faults pattern. Image from Hanova [2005].
Pakowki Formation super imposed onto fault network. The Pakowki Formation is a stratigraphical unit of Campanian age in the Western Canadian Sedimentary Basin. The interpretation of 2D and 3D seismic data reveals a complexly faulted crater dominated by arcuate fault planes (Hildebrand et al, 2005).
Shatter cone from the Eagle Butte impact structure, from fellow RASC member Mark Z.

The Eagle Butte impact structure is located in southern Alberta with a ~15 km diameter of the circular outline. The unknown faulting pattern on the surface was first noticed by Haites and Van Hees [1962] and the impact origin was only confirmed recently by the discovery of shatter cones near the central uplift [Hanova et al., 2005]. Of all the impact structures found within the Western Canada Sedimentary Basin (WCSB), the Eagle Butte crater is one of the best studied with a combination of information from outcrop exposure, 2D and 3D seismic imaging, and well log mapping [Hanova et al., 2005; Sawatzky, 1976]. On the representative seismic image (Figure Hanova [2005]), significant impact features are displayed including a central raised core, a severely faulted annular syncline, listric normal faults2, and structural thinning and thickening. Based on the displacements of a target bed in the Cretaceous, the structure is estimated to have formed between the Cretaceous and the Lower Tertiary. 2 Listric faults: can be defined as bent normal faults with a concave upwards fault surface. Such faults usually develop in extensional regimes. (Xie W. 2014).



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.

Haites, T., and H. Van Hees (1962), The origin of some anomalies in the Plains of western Canada, Bulletin of Canadian Petroleum Geology, 10(9), 511-533.

Hanova, J., D. Lawton, J. Visser, A. Hildebrand, and L. Ferriere (2005), 3D Structural Interpretation of the Eagle Butte Impact Structure, Alberta, Canada, paper presented at 36th Annual Lunar and Planetary Science Conference.

Hildebrand, A.R., Hanova, D.C., Lawton, D.C., Visser, J., Ferriere, L. 3D STRUCTURAL INTERPRETATION OF THE EAGLE BUTTE IMPACT STRUCTURE, ALBERTA, CANADA.Department of Geology and Geophysics, University of Calgary. 2005

Sawatzky, H. (1976), Two Probable Late Cretaceous Astroblemes in Western Canada-Eagle Butte, Alberta and Dumas, Saskatchewan, Geophysics, 41(6), 1261-1271.

Visser, J and Scott, D. An Early Tertiary Meteorite Impact Structure at Eagle Butte, Alberta. EnCana Corporation – Calgary, Alberta. 2005

Xie W. Seismic Characterization of A Possible Buried Impact Structure near Bow City in Southern Alberta Department of Physics University of Alberta 2014

University of New Brunswick, 2012.