Overview

The ‘Explosion of Life’

Most rocks from the Cambrian Period in New Brunswick are found in the south along the Bay of Fundy from Grand Manan Island to Cradle Brook. They include the sedimentary rocks of the Saint John Group and igneous rocks from the early Cambrian.

The Saint John Group consists of sediments deposited in a shallow ocean environment 541 to 485 million years ago.  During Cambrian to Silurian time these rocks were rifted-off Protogondwana (a southern hemisphere continent) and attached as an exotic terrane (Avalon Terrane) to the margin of Laurentia (ancient North America). The Avalon Terrane, named after the Avalon Peninsula in southern Newfoundland, is sometimes called the Avalon ‘Composite’ Terrane. It is made up of smaller terranes, the fragment in New Brunswick being called the Caledonia Terrane. The Avalon Terrane is well known to geologists. When it broke up 200 million years ago during the formation of the Atlantic Ocean its parts were scattered across North America, Europe and Africa.

The Cambrian Period is one of the most interesting times in the evolution of life. The sparse diversity of Precambrian life was suddenly replaced by an expansion of life forms representing almost every major group of animals we see today. The ‘Cambrian Explosion’ is the name given to this event. 

One of the most common fossils of the Cambrian is the trilobite, a crab-like animal that inhabited the oceans. The first trilobites appeared in the fossil record about 525 million years ago along with brachiopods, echinoderms and other arthropods.

An older record of trace fossils, the burrows and trails left by animals, and ‘small shelly fossils’ signal the start of the Cambrian Period. Southern New Brunswick has some of the classic Cambrian fossil sites in North America where the first Cambrian trilobites in Canada were found in 1863 and where ‘small shelly fossils’ were discovered.

An alternate sequence of events proposes that around the Precambrian – Cambrian boundary 545 million years ago, the Avalon Terrane was separated from Protogondwana. Avalonia was located near the South Pole while Protogondwana was further north near the equator. Evidence for a wider separation at the Precambrian – Cambrian boundary includes sedimentary rocks suggesting warm oceans around Protogondwana, but cold waters near Avalonia; different kinds of fossils (including trilobites) found in Avalonia and Protogondwana; and rocks that suggest Avalonia was glaciated while Protogondwana was not.

Later in the Cambrian around 515 million years ago the rocks of Avalonia (now found in eastern Canada and the USA, and in western Europe) and Gondwana (now found in North Africa and Spain) have similar fossils. As the two continents came close to each other in the late Cambrian, animals in the shallow ocean mixed to become a common fauna.