A buried crater beneath the Atlantic changed how scientists read Canada’s coastline

Deep beneath the waves off the coast of Nova Scotia lie what might be among the most significant examples of invisible impact structures ever discovered on the continental shelf. The crater has come to be called the Montagnais impact structure, but it has long remained something of a curiosity because of years of seismic surveys, drilling operations, and rock examination until its true significance became known – namely that the site represents the remnants of an asteroid collision from about 51 million years ago.Whereas other well-known craters owe their recognition to having been visible to geologists, what stands out about Montagnais is how the crater was actually discovered and pieced together. What makes this particular impact structure so unique is that it was not discovered in its visible state like other well-known examples.According to the Geological Survey of Canada, Montagnais became the first recognised impact structure formed by a meteorite striking the seafloor.A strange circular structure beneath the Scotian ShelfThe first hints were provided through studies conducted during offshore exploration for oil and natural gas in the Scotian Shelf region south of Nova Scotia. A large circular structure with a central raised area and shattered rock surrounding it was discovered using seismic reflection technology. It was at first not clear that the structure was caused by an asteroid impact.It was suggested that the formation might be due to volcanism or some other geological process since there was no crater exposed on the surface, but rather covered with marine sediments.A 1987 study published in Nature by Lubomir F. Jansa and Georgia Pe-Piper described the structure as an underwater extraterrestrial impact crater on the North Atlantic continental shelf. The paper noted the presence of breccia and shock deformation features that strongly supported an impact origin.Additional seismic surveys have revealed that the crater was approximately 45 kilometres wide and had features characteristic of a complex impact structure, including the presence of a central uplift. The US Geological Survey reports that Montagnais has some geological features in common with large craters formed by asteroid impacts on Earth, despite its underwater location.Drilling revealed the decisive evidenceOne of the best proofs was obtained when the exploratory drilling encountered the centre of the structure, where fractured basement rocks, breccia, and impact melts were encountered. In addition, the drilling had planar deformation features in quartz and feldspar. Such features are considered classic markers for the presence of shock waves that occur due to the impact of asteroids. Furthermore, there is evidence of suevite, a rock created during violent collisions.Additionally, the hole went through more than 500 metres of shattered and melted rock. The original volcanic deposits were finally reclassified as impact-generated deposits.Scientists stated that the combination of seismically-derived geometry, drill core evidence, and shock minerals was crucial. The shape by itself would not be able to prove an impact. The physical rock evidence helped turn Montagnais from an offshore mystery to a proven impact structure. Dating the collision to the early EoceneThe crater was later dated back to the Early Eocene era, at about 50.5-51 million years old. These dates were achieved through the use of radiometric techniques of dating, which included argon isotopic analysis in melt rocks retrieved from the drilling operation.This age was further supported by fossil-rich sediment layers surrounding the crater, formed after the impact.Montagnais provided an important insight into scientists’ assumptions regarding potential locations where such structures could persist. Montagnais shows that continental shelf regions and marine margins can serve as craters’ hiding places under thick layers of sediments for tens of millions of years. Later, studies found that the impact could cause significant slope instability and mass movements along some segments of the Scotian margin.The more general conclusion one can draw from studying the Montagnais is that a scientific approach should be applied to identifying such structures. In other words, the discovery was made due to numerous findings but not as a result of a single revelation.