BOW CITY IMPACT STRUCTURE

BOW CITY IMPACT STRUCTURE

  • Type: Central peak
  • Age Ma: <73 (Geological dating)a
  • Diameter: 8 km
  • Location: N 50° 27’ 00″ W 112° 21’ 36″

A maximum age of 73 Ma is estimated based on the age of the youngest sediments deformed.

The Bow City Structure situated south east of Calgary. Courtesy Breaking Science News at Sci-News.com Science Website
The Bow City 8 km diameter structure is indicated by the red circle. The lack of any obvious morphological expression at the surface suggests that the structure was eroded prior to and perhaps during glaciation. (courtesy Google)
The Bow City Impact structure from the north taken from approximately 4000 feet above ground level. “Ground zero” is image centre.

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)

Detailed map of the Bow City structure area showing well control. Surface location of wells shown as gray-filled circles. Surface location of cross section wells shown as solid black circles. Outline of the structure (based on subsurface structure maps) is shown as black, long-dashed line. The location of outcrop control data used in gridding shown as white-filled triangles. The approximate outer boundary of the central uplift (defined using the 710 m structure contour on the Belly River Group top) is shown as a black, short-dashed line. The inferred contact of the inlier of Dinosaur Park Formation rocks is shown in the center of the structure (approximated using the 820 m structure contour). Township grid shown as solid black line. The location of seismic lines shown as solid, dark gray lines. Map grid coordinates are in UTM Zone 12, NAD 83 datum. Twp = township; Rge = range; W4 = west of the Fourth Meridian. (GLOMBICK et al, 2014)
Topographic LiDAR map displaying the estimated outline of Bow City structure (highlighted by dashed line) and the structure measurements from outcrop. Donated legacy seismic lines are shown by the solid black lines. The blue lines represent the new high resolutions surveys conducted in 2013 by the University of Alberta. Coordinates are in UTM 12N, NAD 83. Modified from Glombick et al. (2014).
Image showing the remnants of a crater that UAlberta researchers theorize was left by a massive meteorite strike sometime in the last 70 million years. Colour variation shows metres above sea level.
3D models presenting the distinctive structural patterns. (a) Oblique view of the Belly River Group from northeast showing circular depression with central uplift. (b) Oblique view of the Belly River Group surface revealing the distinctive faults patterns in the outer zone. Colored blocks display the three listric faults patterns interpreted in seismic profiles. Vertical exaggeration is 1:7. (Xie, 2014)
Outcrop photographs from the Bow City crater: top – panoramic view; bottom – a close-up showing thrust faults, outlined in red; geologist kneeling on outcrop, black arrow, for scale. Image credit: Paul Glombick et al

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.

Seismic profile 86249 with interpreted reflectors. Common midpoint interval is 10 m and the bottom image represents the location of the line by red. The geometry and structure of the Bow City structure are revealed through the integration of outcrop, well, and seismic data. The general shape of the structure, as indicated from the structure maps, is a semicircular, bowl-shaped depression with a structural uplift in the center. The apparent diameter of the structure is approximately 8 km.(Glombick, 2014)

We took these images of the Bow City structure flying in from the west and circling the area counterclockwise.

 

References

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