|
|
 |
|
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!"
Cross-section of the geology of Western
Newfoundland
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.
|
|
