An Early Tertiary Meteorite Impact Structure at Eagle Butte, Alberta

John Visser and Doug Scott
EnCana Corporation, Calgary, AB

The Eagle Butte structure is located 35 km south of the city of Medicine Hat, Alberta. It has been interpreted as a buried meteorite impact crater, with a central uplift surrounded by a moat and fractured rim. The time of impact appears to be Early Tertiary.

The diameter of the crater, including the rim, was approximately 17 km. It is estimated that the floor of the moat was 180m below, and the top of the central uplift 200m above the regional surface. The rim on the northeast side of the crater was raised to a height of 120m. Presently, there is little or no surface expression of the Eagle Butte structure.

Missing sections are common in wells drilled within the moat. They are the result of material having been displaced into the centre of the crater, or out onto the rim. Multiple repeat sections are present in the rim area and within the central uplift. Chaotic bedding, observed in core from the crater centre, is indicative of severe slumping or brecciation near the site of impact.

Natural gas is trapped within the crater rim and possibly on the flanks of the central uplift. Impact related fracturing, as seen in core, significantly enhances productivity. Currently, gas production from the Second White Specks, Medicine Hat and Milk River formations exceeds 18 mmcf/d. Total recoverable reserves are estimated at 50 bcf.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005