Ottawa-Bonnechere Graben - Geology

Geology

Since the Late Proterozoic to Early Paleozoic, erosion has removed the volcanic peaks, exposing a number of relic volcanic pipes, such as Callander Bay and the Manitou Islands in Lake Nipissing. These features are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. Batholiths and dikes were also exposed by erosion, such as the Timber Lake, Mulock, West Arm, Powassan and Bonfield batholiths. The expressions of a thick pile of dominantly mafic, bimodal volcanics and the Tibbit Hill volcanics in the Humber Zone of the Quebec Appalachians are believed to be related to the formation of the Ottawa-Bonnechere Graben. The precise age of these volcanics is unknown but they are either early Cambrian and late Precambrian. This volcanism was probably coeval with the emplacement of the Grenville dike swarm.

Minor but significant igneous activity occurred during the Mesozoic era, including kimberlite emplacement during the Jurassic period, and the development of alkalic intrusions along the Ottawa-Bonnechere Graben and elsewhere in Ontario. This second episode of alkalic volcanism occurred along the eastern part of the graben in the early Cretaceous. The products of this event are the Monteregian Hills in Montérégie, Quebec. These are thought to have formed as a result of the North American Plate sliding westward over a long-lived center of upwelling magma called the New England hotspot, and is the eroded remnants of intrusive stocks. These intrusive stocks have been variously interpreted as the feeder intrusions of long extinct volcanoes, which would have been active about 125 million years ago, or as intrusives that never breached the surface in volcanic activity. Of all these features, Mont Saint-Hilaire is the best known as a source of rare specimens.

Along the northern side of the Ottawa-Bonnechere Graben lies a dramatic escarpment that forms the southern edge of the Gatineau Hills. This escarpment, called the Eardley Escarpment, makes this part of the graben an attractive location for rock climbers and hikers, offering a beautiful view of the relatively flat fields below, which extend to the Ottawa River.

On or near a branch of the Ottawa-Bonnechere Graben lies the Brent impact crater. It is 3.8 km (2 mi) in diameter and the age is estimated about 400 million years (Early Devonian). The impact crater, which was first recognized in 1951 from aerial photographs, formed in Precambrian gneisses. Geophysical and diamond drilling investigations show that the crater has a present depth of about 425 m (1,394 ft) but is partly filled with sedimentary rocks with a thickness of about 274 m (899 ft). The rocks beneath the crater floor are thoroughly fragmented over a depth of about 610 m (2,001 ft). Like the similar Pingualuit crater, the Brent crater is attributed to the high speed impact of a giant meteorite. It is calculated that the impact released energy equaling 250 megatons of TNT and occurred when this area was probably covered by a shallow sea.