Overview

The Search for Early Life

Precambrian time encompasses Earth history from about 4.6 billion to 542 million years ago, almost 90% of geologic time. The rocks of New Brunswick are a tale of the last billion years of Earth history. Precambrian age rocks are found in southern New Brunswick near Saint John. These rocks belong to the Green Head Group, a piece of an ancient supercontinent called Rodinia, which formed 1.1 billion years ago. Green Head Group rocks were once sediments, deposited on a shallow ocean shelf and on the deeper ocean slope.  They tell the story of early life and the breakup of the supercontinent. In some places, Green Head Group marble contains stromatolites, structures built from single-celled cyanobacteria and sediment.

As Rodinia split into pieces, two new oceans called Iapetus and Brazilide were created. On one side of the Iapetus Ocean, four continents called Laurentia, Baltica, Siberia, and Amazonia were formed. The single southern continent was called Protogondwana (composed of modern day Africa, Antarctica, Australia, India). Eventually the Brazilide Ocean began to close, reuniting Amazonia with Protogondwana to form a new continent called Gondwana. The Iapetus Ocean continued to get wider, pushing Laurentia, Baltica, and Siberia further away.

About 600 to 500 million years ago sand and mud eroding from the coast of Protogondwana was deposited on the ocean shelf around volcanic islands. Plate tectonic processes eventually fractured (Proto)Gondwana, tearing pieces away from the larger continent to form microcontinents called terranes. The volcanic islands and surrounding ocean floor became part of the Avalon Terrane. It ‘rifted-off’ as geologists say and drifted toward Laurentia (ancient North America). As the Avalon Terrane moved north, the Iapetus Ocean began to close in front bringing Avalon closer to Laurentia. A new ocean called the Rheic opened behind it, moving Avalon further and further away from Gondwana. At the same time the Brookville Terrane (part of a larger terrane called Ganderia) was also torn from the remnants of Rodinia. The Brookville and Avalon terranes were on a similar path, eventually to collide with Laurentia, creating New Brunswick’s Precambrian and early Cambrian geology.