GeoNotes Cypress Hills and area
The MAPLE CREEK-WHITE VALLEY and EAGLE BUTTE ASTROBLEMES
Astrobleme location map at the northern periphery of the Cypress Hills. Image: Google Earth
An astrobleme is generally a circular scar on the earth’s surface from an ancient impact of a meteorite. Evidence of crushed and deformed bedrock is present. There are approximately 190 recognizable major astroblemes on earth (Fig. 1) with more to be discovered. Most astroblemes are visible and, if eroded, barely visible on the surface, and some are buried beneath the surface in sediments with very little or no surface expression. Those buried beneath the surface are discovered by energy companies drilling for oil and gas. Two examples of the latter are the Eagle Butte and Maple Creek-White Valley astroblemes.
Figure 1: Astroblemes sites around the world. The 250 km diameter Sudbury impact structure is the largest in North America. Millions of years are abbreviated ‘mya’ in this image. Image from Wikipedia.
The drill core samples revealed shatter cones, a distinctive feature of high energy impacts, and planar deformation features in quartz, another distinctive feature of impact structures. A Bouguer (pronounced Boo-ge-air) gravity anomaly showed a positive anomaly over the structure’s raised center. A north-south seismic line over the structure revealed a definitive complex impact crater with central uplifts, terraced rims, down-dropped troughs, and many normal faults (Fig. 2).
Figure 2: Top, a north-south seismic line over the Maple Creek-White Valley astrobleme reveals a seismic signature of a complex impact structure. Bottom, an interpretation of the above seismic data showing a central uplift with terraced rims and troughs, from Westbroak (1997).
The impact resulted in asymmetric deformation of country rocks suggesting the impact force was not equal in all directions, possibly due to the heterogeneity within the country rocks themselves. The astrobleme is 7 km in diameter with a structural disturbance depth of 1.3 km and a structural uplift of 600 m. The impact disrupted the youngest country rock, the lower Eastend Formation, giving an age of impact approximately 75 million years ago (abbreviated Ma) or Upper Cretaceous. According to some researchers, since then the structure has been glaciated three times and flooded more than once.
During the Upper Cretaceous, the North American geomorphological environment was very different than today. The shallow Bearpaw Sea had completely inundated western North America depositing the Upper Cretaceous Bearpaw Formation (77 Ma) which underlies the younger Eastend Formation (75-67 Ma).
The Maple Creek-White Valley impact structure has no economic significance at this time.
The Eagle Butte astrobleme, located at the northwestern end of the Cypress Hills, is approximately 31 km SSE from Dunmore, AB. It was discovered in 1975 and confirmed in 2005 when shatter cones were found near the central uplift. The crater is approximately 18 km in diameter with a structural uplift of 300 m at its core. The impact has structurally disrupted geological formations down to approximately 1.3 km or the Lower Cretaceous (113 Ma) Ellis and Rundle Formations. The astrobleme is less than 65 Ma (Late Upper Cretaceous). It is not exposed, and the structure has been substantially eroded by the Laurentide glacier.
Eagle Butte first came to the attention of geologists in an article published in 1962 when they noticed some structural peculiarities in that area. How they came to that conclusion is unclear, but it wasn’t until later when surface geology, seismic, and drill core sampling was conducted that a high-energy impact structure was noticed buried beneath the glacial till. The shatter cones and planer fractures in quartz confirmed a high energy impact structure. The seismic and drill core data revealed disturbed and displaced horizons, multiple fault patterns, fractured rim, troughs, and a central raised area indicative of a complex impact crater (Fig. 3).
Figure 3: Eagle Butte 2D north-south seismic line showing a central raised area, disturbed horizons, trough, fractured rim, and fault patterns. Arrows indicate movement along fault planes, from Hanova, et al (2005).The Eagle Butte area is of interest to energy companies because the lower Upper Cretaceous Second Whitespecks (93 Ma), Medicine Hat (86 Ma), and Milk River (84 Ma) Formations contain natural gas reserves. The Eagle Butte impact structure also contains natural gas trapped within the crater rim.
Sources:
Alberta Geological Survey (2020); Eagle Butte Meteorite Impact Structure, Southeastern Alberta: 3D Print Files (digital data, STL format); https://ags.aer.ca/publication/dig-2019-0030; accessed Nov 4, 2020
Alberta Geological Survey (2019); Alberta Table of Formations; Alberta Energy Sector, https://static.ags.aer.ca/files/document/INF/Table_of_Formations_2019.pdf, accessed Nov 3, 2020
Ernstson, Kord, Claudin, Fernando; Ernstson Claudin Impact Structures-Meteorite Craters; http://www.impact-structures.com/impact-rocks-impactites/the-shatter-cone-page/; University of Wurzburg, Germany/Museum of Geology Barcelona, Spain
Hanova, J, Lawton, D.C, et al; https://www.researchgate.net/publication/253771273_3D_Structural_Interpretation_of_the_Eagle_Butte_Impact_Structure_Alberta_Canada; Planetary Science XXXVI; 2005
O’Dale, Charles; Maple Creek (Aka-White Valley) Impact Structure; http://craterexplorer.ca/maple-creek-impact-structure/; Ottawa, ON
O’Dale, Charles; Eagle Butte Impact Structure; http://craterexplorer.ca/eagle-butte-impact-structure/; Ottawa, ON.
Sawatzky, H. B., Two Probable Late Cretaeous Astroblemes In Western Canada-Eagle Butte, AB and Dumas, SK; https://pubs.geoscienceworld.org/geophysics/article-abstract/41/6/1261/68030/two-probable-late-cretaceous-astroblemes-in; Geoophysics, Vol. 41, No. 6; Pgs 1261-1271; 1976
Westbroek, Hans-Henrik; Seismic Interpretation of Two Possible Meteorite Impact Craters: White Valley, Saskatchewan and Purple Springs, Alberta; https://dspace.ucalgary.ca/bitstream/handle/1880/26906/31378Westbroek.pdf?sequence=1&isAllowed=y; University of Calgary Faculty of Graduate Studies, Msc Thesis; Calgary, AB; 1997
Visser, John, Scott, Doug; An Early Tertiary Meteorite Impact Structure at Eagle Butte, Alberta (Abstract); http://www.searchanddiscovery.com/abstracts/html/2005/annual/abstracts/visser.htm; AAPG; Calgary, AB; 2005
List of Impact Craters On Earth; https://en.wikipedia.org/wiki/List_of_impact_craters_on_Earth; Wikipedia.org; Washington, DC; September 27, 2020
USGS; File:Geologic time scale; https://commons.wikimedia.org/wiki/File:Geologic_time_scale.jpg; Washington, DC; July 2010
Wikipedia, The Free Encyclopedia; Washington, DC; 2020
Upper Cretaceous (75 Ma). Red dot shows the location of the Maple Creek-White Valley astrobleme at time of impact. Note the inland Bearpaw Sea and the formation of the western cordilleran mountain ranges. Image from Charles O’ Dale.
Upper Cretaceous (65 Ma). Red dot shows the location of the Eagle Butte astrobleme at the time of impact. 
An example of a shatter cone structure. A shock metamorphic feature created by high energy impacts. Image from impact-structures.com.
An
example of a microscopic Planar deformation feature in shocked
quartz, from Wikipedia.
The Manicouagan Meteor Crater, Northern Quebec an example of an astrobleme. Image: Google Earth
A generalized geologic time scale. ‘Ma’ is an abbreviation millions of years of age. Image from USGS.
