Step Faults and Their Role
Step faults (also known as grabens) appear when magma wells up under the Earth's crust, causing it to swell; the ground in the centre of the swelling eventually collapses in a series of faults shaped like a series of steps.
The first series of step faults formed in Témiscamingue appeared between 2.6 and 2.5 billion years ago. About 2.15 billion years ago, geological reactions to deposits of sand and gravel in the Huronian period caused magma to rise and led to the silver and cobalt mineralization of the Ontario Temiscamingue region, one of the reasons for today's mining activity in that area. In the Ordovician and Silurian periods, new seas covered the Témiscamingue territory, which eventually came to occupy the centre of the Pangean supercontinent. Later, upheavals prefiguring the break-up of Pangea created another series of depressions or grabens, including the depressions of Lake Témiscamingue and the Ottawa River. Témiscamingue's extremely rich and ancient geological life has given rise to the great variety of geological formations visible in the region today.
The step faults of Témiscamingue
The step faults of Témiscamingue have preserved traces of extremely ancient life forms. At the time when dinosaurs lived on Earth, Témiscamingue was above sea level and its surface, along with all fossil records of animal life, were subject to erosion. The glaciations of the Quaternary, as they modified the landscape, also pulverized geological layers and their evidence of Paleozoic life. Only the sections of rock that descended into the step faults escaped these forces of erosion and levelling. Step faults are visible around Lake Témiscamingue, where many significant sites reveal the geological and paleontological life of the region. For instance, Dawson Point and Mann Island carry sediments from the Silurian sea, while the Farr quarry is made up of fossil-bearing limestones from the Ordovician sea. As a whole, the Témiscamingue region is the most important Paleozoic enclave in the Canadian Shield.
- The history of the Témiscamingue step faults begins around 2.6 billion years ago. At the time, the region was located directly above a hot spot in the mantle.
- Fixed above the hot spot by convection currents, the Earth's crust liquefied. Its surface began to swell, then stretched and eventually cracked.
- The cracks became faults which collapsed in a series of steps from the centre out.
- This collapse created a site in which sedimentary rocks - gravel and sand - accumulated during the Huronian period.
- Much more recently, around 180 million years ago, the Pangean supercontinent was shaken by the first signs of its break-up. A new series of step faults were created in Témiscamingue; fossil-bearing rocks dropped 280 metres, and the depressions of Lake Témiscamingue and the Ottawa River were formed.