Regional Geology of the Maritime Provinces

The geological evolution of the Maritimes started about one billion years ago, during the Precambrian, with the mountain building event know as the Grenville Orogeny. This former mountain range (or orogen) formed as continental fragments were welded together into the supercontinent Rodinia. In northernmost Cape Breton, the Blair River Block (Fig. 3) is believed to represent part of the Grenville Orogen. Rocks from there were metamorphosed about one billion years ago (1,000,000,000 or 1 Ga); they are the oldest rocks in the Maritimes.

The rest of the Maritimes is underlain by a series of "exotic" crustal fragments (terranes) that started to accrete (fuse) onto Laurentia (the ancestral core of North America) more than 400 million years ago. Between about 550 and 400 million years ago three terranes, drifting around what was then the southern hemisphere, came together to form the southern Maritimes (Fig. 4). These terranes were the Miramichi-Bras d'Or, the Avalon, and the Meguma (Fig. 3).

Figure 3: Maritimes mosaic: the collage of terranes and other crustal pieces that came together to form the Pre-Carboniferous geology of the Maritimes. From Atlantic Geoscience Society (2001); there are other, slightly different schemes and names in the literature, reflecting the complexity of the puzzle. But the general idea of a Maritimes mosaic coming together in the Cambrian to Devonian interval is agreed upon by all researchers

Figure 4: Global paleogeography of the Early Ordovician, 475 million years ago (left) and late Ordovician, 455 million years ago (right). From Atlantic Geoscience Society (2001)

The Miramichi-Bras d'Or Terrane, comprising Precambrian and Cambrian sedimentary and volcanic rocks originally lay close to the South Pole, immediately off the coast of Amazonia (a continent that represented most of present-day South America). After migrating north, the Miramichi-Bras d'Or Terrane collided with Laurentia near the equator, about 430 million years ago. This collision closed the Iapetus Ocean. Oceanic rocks that originated in Iapetus still underlie much of northern New Brunswick (Fig. 3).

Large parts of Nova Scotia and southern New Brunswick are underlain by the Avalon Terrane, which detached from Gondwana about 480 million years ago. The developing seaway between the Avalon Terrane and Gondwana became the Rheic Ocean. Like the Miramichi-Bras d'Or Terrane, the Avalon originated in the high latitudes of the southern hemisphere. The Avalon Terrane consists of late Precambrian rocks associated with volcanic settings. After drifting north for about 80 million years, about 400 million years ago this terrane docked alongside the southern edge of the Miramichi-Bras d'Or Terrane. The Avalon Terrane underlies most of northern Nova Scotia, including the Cobequids - and includes the famous Silurian fossil-bearing rocks at Arisaig, near Antigonish.

The last terrane to arrive was the Meguma, probably spawned off the continental margin of that part of Gondwana that is now northern Africa. The Meguma Terrane separated from Gondwana in high southern latitudes about 470 million years ago. Following a leisurely transit across the Rheic Ocean, Meguma obliquely docked with the Avalon terrane about 390 million years ago. (This part of the story is currently controversial, some geologists now considering that the Avalon and Meguma terranes had much longer and closer ties.) It thus provided the last of the pieces that built the Maritimes. It is also modern-day North America's most outboard exotic terrane. The contact of the Avalon and Meguma terranes is marked by the Cobequid-Chedabucto Fault System, which runs from Cape Chignecto in the northwest across much of mainland Nova Scotia to Chedabucto Bay in the east. It forms a major topographic feature along the north shore of the Minas Basin and was subsequently re-activated as the Atlantic Ocean opened - but more about that later.

The plate collisions and closing of the Iapetus and Rheic oceans marked a time of mountain building (orogeny) on the eastern coast of present-day North America. The modern Appalachians are all that remains of the ancient mountains that must have been very imposing - perhaps of Himalayan proportions.