A. B. Ryan
Mineral Development Division,
Newfoundland Department of Mines and Energy
(from Newfoundland Journal of Geological Education, v7, n2, 1983)
Mountains, Plates and Iapetus
The island of Newfoundland occurs at the northeastern terminus of the Appalachian Mountain system, a mountain chain which once paralleled the east coast of North America from Newfoundland to Alabama, and rivalled the present-day Rockies in their beauty. The forces of erosion, however, have taken their toll, and our Appalachians have been worn down to a gently rolling terrain, with only a few upland areas remaining to remind us of their former magnificence. Even though we have been robbed of our Rocky Mountain highs, the erosion of the Appalachians has revealed in Newfoundland a superb cross-section of the core of an ancient mountain belt, and made it the Mecca for earth scientists from around the world.
To understand why Newfoundland attracts such hordes of geoscientists we must consider plate tectonics, that concept which has revolutionized geology since the mid-1960's. It has been recognized that the earth's surface is in constant motion, that it is composed of giant crustal plates that break, separate, rub side-by-side against each other or collide, just the way the ice pans around our coastline do in Spring. Newfoundland marks the site where one of the earth's great continental plates split apart about 600 million years ago and then collided again some 200 million years later. When this ancient continental plate split, the two smaller plates began to drift away from each other, and in the space between them an ocean was formed. This ocean has been termed the Iapetus Ocean, and it reached at least 1000 km in width before the wayward drift of the continental pieces was halted, and the two began to come together again.
Warm Waters in the West
What was Western Newfoundland like during the time when the Iapetus Ocean lapped its shores? To understand this, we must study the 500-600 million year old sedimentary rocks that were deposited when the Iapetus Ocean formed. Such rocks are well preserved in the western part of Newfoundland as far east as a line running northeast through Grand Lake - a chunk of Newfoundland which geologists refer to as the Humber Zone (Figure 1).
The oldest rocks of the Humber Zone are the 1200-1800 million year old gneisses, granites and anorthosites which constitute the Long Range Mountains of the Great Northern Peninsula, and underlie Indian Head and Steel Mountain near Stephenville. These are remnants of the old Canadian Shield which split apart 600 million years ago, and they formed the basement on which the sediments of Iapetus were laid down. These sedimentary rocks, which were deposited between 600 million and 400 million years ago, are predominantly sandstones and carbonate rocks (limestones), and are well-exposed in roadside outcrops and along the coastline of the Port au Port Peninsula and the Great Northern Peninsula. These rocks represent ancient beach sands and shallow marine carbonates. The marine environment at this time probably resembled the present-day Bahama Banks - and there was no need to go south in winter! It teemed with shelly animals, whose fossil remains can be found in limestones all along the west coast.
East of this carbonate bank which covered the present-day Great Northern Peninsula, there was a continental slope, much like the tail of the Grand Banks, which gave way to the deep ocean floor. Along this slope were deposited shale, sandstone, deep-water limestone, volcanic rocks, and giant debris falls resulting from material breaking off the carbonate bank and rolling down the slope. Such rocks can be observed between White Bay and the Baie Verte highway.
Ophiolites and West Coast Splendour
When Iapetus began to close some 450 million years ago and the Appalachian Mountains were built up along the east coast of North America, the ocean floor became wrinkled and uplifted. Giant slabs (up to 40 x 10 km) of ocean floor (the ophiolite complexes) were squeezed out between the continental plates and obducted or emplaced across the old continental margin (Figure 2). Some of the best examples of these ophiolites in the world can be seen on the west coast. They form the scenic brown mountains of Western Newfoundland - Blow Me Down, North Arm Mountain, the Lewis Hills and Table Mountain - and the White Hills near St. Anthony. The discovery of these transported oceanic slabs led Dr. E.R.W. Neale, an eminent geologist and at one time vice-president (academic) of Memorial University of Newfoundland, to exclaim several years ago, "Surely this has to be the eighth wonder of the world!"
Deer Lake and Death Valley
The Iapetus Ocean had closed by 400 million years ago, and Western Newfoundland was an upland area in which vertical movements of crustal blocks along major fault zones led to the formation of two major basins in which accumulations of sediments occurred between 345 million and 280 million years ago. One of these basins extended from Deer Lake to Conche (the Deer Lake Basin) and the other extended from Stephenville to the Codroy Valley (the St. George's Bay Basin). These basins probably resembled the present-day Death Valley of California, surrounded by mountains and with great alluvial fans extending from the mountain scarps across a lowland with shallow lakes. Sedimentary rocks deposited in both basins are conglomerate, sandstone, siltstone and shale, commonly containing fossils of fish, plants and trees. Thin coal seams and oil shales occur in the Deer Lake Basin, and evaporites (gypsum, potash and salt) as well as coal and oil, occur in the St. George's Bay Basin.
Atlantic Ocean Forms
When Iapetus closed 400 million years ago, what was eventually to become North America and Europe were welded together as one land mass. Alas, this would not be for long - in geological terms. One hundred million years later the old continental plate split again, this time to form the Atlantic Ocean which we know today. Even now Europe is moving away from us at about 3 cm per year. For those who study Greek mythology, you'll know that Iapetus, after whom the old ocean was named, was the father of Atlas, after whom the present day ocean is named.
West Coast Mineral Wealth
The rocks of Western Newfoundland are important to the province's commerce. For instance, the carbonate rocks hosted the zinc mine at Daniel's Harbour, which closed in 1990, and are the source of limestone for the cement manufacturing plant at Corner Brook. The ophiolite complexes contain copper-bearing minerals which were once mined at York Harbour in the Bay of Islands, and they also contain chromite and asbestos prospects in the Lewis Hills. Gypsum is mined at Flat Bay, and the salt and potash deposits of the St. George's Bay basin may prove to be commercial in the future. Coal was mined near Howley and Robinsons during the early days of the Newfoundland Railway. Uranium exploration has been carried out in both Deer Lake and St. George's Bay Basins, but no economic deposits have yet been found. Onshore oil and gas seepages have been known on the west coast since the early 1800's when a resident of Parson's Pond found oil floating on the water there. About 50 wells have been drilled during the past century, and about one-half showed varying indications of hydrocarbons. In the early 1980s the Government of Newfoundland and Labrador designated 500,000 acres of the west coast available for oil and gas exploration. So, rocks of the ancient continental margin of Iapetus may one day generate activity rivalling the present-day offshore continental margin of the Atlantic.