A Brief Overview of the Geology of Southwestern Newfoundland

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The bedrock that underlies southwestern Newfoundland, from North Branch south to Port aux Basques and then east to Rose Blanche, is not dominated by either one of the 3 fundamental rock types (igneous, metamorphic and sedimentary). Rather the underlying geology draws its character from among them.

The oldest rocks in southwestern Newfoundland are the rocks that make up parts of the highest mountains in the area. These are ophilitic rocks, pieces of the ancient ocean floor, which were pushed up during mountain building processes, and which are roughly 500 million years old (Upper Cambrian). These pieces of ancient ocean floor rock were embedded within the mountains, which for the most part are made of tonalite and roughly 488 million years old (Lower Ordovician).

The rocks in the immediate Port aux Basques area are metamorphic rocks such as gneiss (pronounced "nice"), schist and amphibolite. These rocks are roughly 460 million years old. Minerals such as garnet, staurolite and kyanite are common in these rocks. Some nice kyanite crystals can be found in the rocks near the tourist chalet at Port aux Basques. The rocks between Port aux Basques and Rose Blanche are much like the rocks in the Port aux Basques area, however there are some areas which also have quite a bit of granite. In fact, some of the finest granite in Newfoundland is found at an abandoned quarry near Petites, which is near Rose Blanche. The granite from this quarry was used to build the main courthouse in St. John's, and pieces of the quarried granite can still be seen on the beach near Petites.

The rocks which underlie the Codroy Valley are the youngest in the area. They are Carboniferous in age, or roughly 325 million years old. For the most part, these rocks are sandstones, siltstones and shales. Small seams of coal can be found in some areas and there is even a small area where the concentration of uranium is high. Limestone is also quite common in some areas of the Codroy Valley, some of which is rich enough to be used for agricultural purposes.

There are 2 major geological faults in southwestern Newfoundland. The Cape Ray Fault is about 1 km wide and crosses the Trans-Canada Highway (TCH) between the entrance to John T Cheeseman Provincial Park and Barachois Brook. This fault extends for about 100kms (60 miles) and eventually crosses the Burgeo Highway.

The largest and most important fault in the area is the Long Range Fault (Cabot Fault) which runs from the bridge crossing the Trans-Canada Highway at McDougall's Brook to the northeast coast of Newfoundland at the Baie Verte Peninsula. This fault also runs through Nova Scotia and New Brunswick. The Long Range Fault once marked the boundary between North America and Europe before the two continents separated about 200 million years ago. Neither one of these faults is active.

Many of the oldest rocks in southwestern Newfoundland have elevated gold levels. Hence, they are good source rocks for gold deposits and gold is found in above average crustal abundances in many parts of the region. Galena deposits, large and small, are particularly rich in gold. One large galena deposit in the Cape Ray Fault has been estimated to contain 150,000 ounces of gold.

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Landslides (rockfalls) in Table Mountains of southwestern Newfoundland, approximately 20 km north of Channel - Port aux Basques (Wreckhouse area). These brightest coloured landslides are contemporaneous (occurred at the same time) with the magnitude 5.7 earthquake that occurred at Plaster Rock, New Brunswick on January 9, 1982 - they were widely noticed by local residents in the days following the Plaster Rock earthquake. The darker coloured (older) landslides are likely contemporaneous with earlier earthquakes including the magnitude 7.2 earthquake that occurred undersea off southern Newfoundland on November 18, 1929.

Brightest coloured landslides (rockfalls) occurred: January 9, 1982
Photos taken: September 12, 2011.

landslide  landslide2  landslide3

Below: a boulder in the Codroy Valley interpreted as a piece of Mischief Melange created by continental collision. Note the one dollar piece (loonie) near the top of the boulder for scale.

Mischief melange boulder in Codroy Valley

A link to a larger image of the above image is here

Below: paragneiss at Grand Bay West, Channel-Port aux Basques, Newfoundland

paragneiss

Links:

Geological Survey of Newfoundland and Labrador

Geoscience Newfoundland and Labrador

Canadian Journal of Earth Sciences

Microstructure, metamorphism, thermochronology and P-T-t-deformation history of the Port aux Basques gneisses, south-west Newfoundland, Canada


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