A small ghost town in southeast Alberta has been harbouring an ancient secret — an asteroid crater nearly eight kilometres wide and almost a kilometre deep, which has remained buried and unknown for nearly 70 million years until it was discovered by a team from the University of Alberta and the Alberta Geological Survey.
The team, led by U of A geophysics graduate student Wei Xie, were first inspired to investigate the area near Bow City after data from boreholes revealed some striking differences from the rather mundane layered sediments of the ground around the area. Some layers showed signs of being overturned — so for example, layer A rests overtop of layer B in the areas around the crater site, but in the crater, layer B is overtop of layer A — while layers that existed in the areas around the crater were missing altogether from inside the crater. Both of these patterns have been seen at the sites of other impact craters, due to earth and rock being blasted upward and away from the point of impact.
This prompted them to examine seismic data gathered from the region by oil companies searching for oil and gas deposits, which supported the findings from the borehole data, and they found further support from geologic survey records going back as far as the 1940s that indicated unusual faults in local shale deposits that could have been caused by a massive impact.
"I was really surprised," Xie said about finding the crater. She presented the team's findings at the American Geophysical Union conference on Monday.
The seismic map of the crater shows the structure of it quite well, with its low-lying interior and characteristic central peak. The team also noted some potentially unique features of the crater, which indicate that some of the sediments were pushed directly outward from the impact, rather than being blown upwards (as would be expected).
Xie's team is now working on constructing a detailed map of the crater, by taking high-resolution seismic readings, and measurements of the local magnetic field and gravitational field. They hope to find evidence of the shock waves and melting that would have been caused by an impact, and may be drilling into the site sometime next year in search of these features.