BOW CITY IMPACT STRUCTURE
by: Charles O’Dale
- Type: Central peak
- Age Ma: <73 (Geological dating)a
- Diameter: 8 km
- Location: N 50° 27’ 00″ W 112° 21’ 36″
a A maximum age of 73 Ma is estimated based on the age of the youngest sediments deformed.
Abstract Geological and geophysical evidence is presented for a newly discovered, probable remnant complex impact structure. The structure, located near Bow City, southern Alberta, has no obvious morphological expression at surface. The geometry of the structure in the shallow subsurface, mapped using downhole geophysical well logs, is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region. Detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region. Field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure. Evidence of deformation along the eastern margin of the central uplift includes thrust faulting, folding, and steeply dipping bedding. Normal faults were mapped along the northern margin of the annular region. Isopach maps reveal that structural thickening and thinning were accommodated primarily within the Belly River Group. Evidence from legacy 2-D seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure, including a series of listric normal faults in the annular region and complex faulting within the central uplift. The absence of any ejecta blanket, breccia, suevite, or melt sheet (based on available data) is consistent with the Bow City structure being the remnant of a deeply eroded, complex impact structure. Accordingly, the Bow City structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks. (GLOMBICK et al, 2014)
The impact site was first discovered in 2009 by geologist Paul Glombick, who at the time was working on a geological map of the area for the Alberta Geological Survey, focusing on the shallow subsurface, between zero and 500 metres in depth. Glombick relied on existing geophysical log data from the oil and gas industry when he discovered a bowl-shaped structure. After checking maps of the area dating back to the 1940s, he found evidence of faulting at the surface.
The Alberta Geological Survey contacted the U of A and Schmitt to explore further, peeking into the earth by analyzing seismic data donated by industry. Schmitt’s student, Todd Brown, later confirmed a crater-like structure.
“We know that the impact occurred within the last 70 million years, and in that time about 1.5 km of sediment has been eroded. That makes it really hard to pin down and actually date the impact,” said Dr Douglas Schmitt of the University of Alberta, a co-author of the paper published in the journal Meteoritics & Planetary Science. “Time and glaciers have buried and eroded much of the evidence, making it impossible at this point to say with full certainty the ring-like structure was caused by a meteorite impact, but that’s what seismic and geological evidence strongly suggests.”
Erosion has worn away all but the “roots” of the crater, leaving a semicircular depression eight kilometres across with a central peak. Schmitt says that when it formed, the crater likely reached a depth of 1.6 to 2.4 km—the kind of impact his graduate student Wei Xie calculated would have had devastating consequences for life in the area. (Glombick et al, 2014)
In conclusion, structural data supports the hypothesis that the Bow City structure was produced by a hypervelocity impact. Alternate hypotheses for formation of the structure have been considered and rejected on the basis of geological observations. Confirmation of an impact origin requires definitive evidence of shock metamorphism that at this time (2014) has not been found.
We took these images of the Bow City structure flying in from the west and circling the area counterclockwise.
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Paul Glombick, Douglas R. Schmitt, Wei Xie, Todd Bown, Ben Hathway and Christopher Banks; The Bow City structure, southern Alberta, Canada: The deep roots of a complex impact structure? METEORITICS & PLANETARY SCIENCE Volume 49, Issue 5, May 2014, Pages: 872–895
Xie W. Seismic Characterization of A Possible Buried Impact Structure near Bow City in Southern Alberta Department of Physics University of Alberta2014
Professor Douglas R. Schmitt
University of Alberta
The Bow City, Alberta, Structure is hidden from the surface and only discovered during regional stratigraphic mapping of near surface formations by the Alberta Geological Survey. A combination of legacy industry data and our own high-resolution profiling revealed both normal faulting due to block slumping at the crater edge and a complex central peak. Seismic wave speed tomographic analysis, too, show decreased P-wave speeds beneath the central peak.