Geologic Ancestors to the Atlantic: The Geology of Newfoundland
By Harold Williams
Newfoundland, colloquially "The Rock," is a geological paradise
for its coastal rock exposures. These are the "smoking guns"
for the Appalachian model outlined in this account.
The axiom "the truth lies in the rocks" fully applies
to the Appalachian Orogen in Newfoundland.
The island of Newfoundland is the northeast extremity of a chain of deformed and elevated rocks called the Appalachian Orogen (Figure 1). The Appalachian Orogen evolved through a cycle of ocean opening, beginning 600 million years ago (600 Ma), then ocean closing ending with continental collision at 300 Ma. The fundamental geologic divisions of Newfoundland record the development of the margins and oceanic tract of this ocean, called Iapetus after the mythical Greek father of Atlantis. The Atlantic Ocean began to open at about 250 Ma and continues to open today at the slow rate of a few centimetres per year. Opening of the Atlantic dispersed correlative segments of the Appalachian Orogen to be found in east Greenland, Scandinavia, United Kingdom, western Europe, and northwest Africa (Figure 2).
Figure 1. Extent of the Appalachian Orogen in eastern North America and interpretation of its major zones.
The fundamental geological divisions of Newfoundland were well known before the theory of continental drift and opening-closing oceans was widely accepted in the late 1960s (Williams, 1964). Understanding these divisions led J. Tuzo Wilson (1966) to write his seminal paper “Did the Atlantic Close and then Reopen?” With wide acceptance of continental
drift and continental collisions as a model for the formation of geologic mountain belts like the Appalachians, Newfoundland became a testing ground for ideas and processes of mountain building. This is because its rocks represent a complete Appalachian cross section, exposed in wave-washed cliffs across the northeast coastline. Some Newfoundland rocks, such as those at Fortune Head on the Burin Peninsula and Green Point of western Newfoundland, are leading world examples of geologic time boundaries. Others, such as those of Gros Morne National Park, have received UNESCO World Heritage recognition, and still others, like the oldest multi-cell fossils at Mistaken Point on the Avalon Peninsula, are of provincial heritage status (Figure 3).
Port au Port: contorted sedimentary rocks
The Iapetus Ocean, although succeeded by
the modern Atlantic, was preceded by the Uranus Ocean, whose cycle gave rise to the collisional Grenville Structural Province of the Canadian Shield in southeast Labrador (Williams et al. 1999 and Figure 1). The phenomena of opening-closing oceans along the same general seam—first Uranus, then Iapetus, now the Atlantic—has been equated to the opening and closing of the accordion bellows, locally known as the Harry Hibbs Effect (Williams et al. 1999). Modern structural patterns
in the North Atlantic may therefore have been
determined by events that began before 1000 Ma.
Geologic Zones of Newfoundland
Figure 2. Restored North Atlantic region showing the axis of the North Atlantic Ocean spreading and dispersed correlative rocks (blue and yellow colours) of the Appalachian Orogen
The major geologic zones of the Newfoundland Appalachians are shown in Figure 3. The zones, from west to east, are Humber, Dunnage, Gander, and Avalon. These are the fundamental divisions based on the earliest rocks of the system, those older than about 450 Ma. The Humber Zone represents the ancient continental margin of eastern North America or the western margin of Iapetus. More easterly zones are accreted terranes or geologic entities, added to the North American margin during the closing of the Iapetus Ocean. According to this model, the Dunnage Zone represents vestiges of Iapetus, the Gander Zone represents the eastern margin of Iapetus, and the Avalon Zone originated somewhere east of Iapetus and is of African affinity. Most of the rocks that define these zones are traceable southwestward along the full length of the Appalachian Orogen (Figure 1). They are also extrapolated northeastward across the British Caledonides (Figure 2).
The rocks and structures of the Humber Zone (Figure 3) fit the model of an evolving continental margin and spreading Iapetus Ocean. This began with rifting of existing continental crust dated at 600 to 550 Ma. The rifting is evidenced by liquid injections that filled cracks in the older crust and fed volcanic eruptions. It also led to deposition of coarse fragmental sedimentary rocks. This was followed by the development of a passive continental shelf with mainly limestone deposition, like that of the present Bahamas, and contemporary continental slope/rise deposits. This lasted for about 100 million years. It ended with deposition of fragmental rocks of easterly derivation, which are the first intimation of offshore disturbance, and they are a harbinger to forthcoming catastrophic events. Destruction of the margin is marked by the transport of rocks from the compressed, uplifted continental slope and rise landward above the former continental shelf. These transported rocks are, in turn, structurally overlain by slabs of oceanic crust and mantle, such as the Tablelands of Gros Morne National Park. This analysis, based partly on the geology of Gros Morne National Park, greatly enhanced its recognition as a UNESCO World Heritage Site.
The western boundary of the Humber Zone is drawn where deformed rocks of the Appalachians pass into flat-lying rocks of the continental interior. The eastern boundary of the Humber Zone is a steep belt marked by discontinuous occurrences of Dunnage Zone oceanic crust and mantle rocks along the Baie Verte Line (Figure 3). The Advocate asbestos mine occurs in oceanic mantle rocks on the Baie Verte Peninsula. This and similar occurrences throughout the Quebec segment of the Appalachians formed one
of the world’s richest asbestos belts.
The Dunnage Zone is recognized by its abundant volcanic rocks, oceanic crust and mantle rocks, and chaotic mixtures of discrete resistant blocks surrounded by shales. Sedimentary rocks are all of deep marine deposition. Compared to the Humber Zone, its rock units are of variable thickness and are commonly discontinuous. The Dunnage Zone is widest and best preserved in northeast Newfoundland, because of matching morphological embayments in the margins of Iapetus. It is narrow or absent in southwest Newfoundland at the Cape Ray Fault, where matching Iapetan promontories took the brunt of collision. In the north, the Dunnage Zone is separated into two sub-zones by the Red Indian Line (Figure 3). The sub-zones show contrasts in kinds of rocks, sequence of units, and especially fauna, which have North American affinities in the west and eastern Iapetan affinities in the east. The boundary of the Dunnage Zone and the Gander Zone is the Gander River Ultrabasic Belt (Figure 3). Like the Baie Verte Line, it is marked by discontinuous occurrences of oceanic crust and mantle rocks.
The Gander Zone has a thick, monotonous sequence of quartz sandstones, siltstones and shales that grade eastward into deformed and altered rocks. Its analysis is far less sophisticated than that of the Humber Zone. Almost one half of its rocks are granitic intrusions, and one half of the remainder are deformed and altered beyond recognition. Since large granite intrusions are atypical of oceanic crust, it is concluded that sedimentary rocks of the Gander Zone lie above continental crust at the eastern margin of Iapetus.
The Avalon Zone is defined by its well-preserved sedimentary and volcanic rocks mainly older than 550 Ma. Overlying shales and sandstones contain a fauna completely different from fauna in equivalent rocks of the Humber Zone. The Avalon Zone extends 600 kilometres offshore to the Flemish Cap, making it the broadest zone of the entire Appalachians, more than twice the combined width of all other zones. The earliest deformations of its oldest rocks are unrelated to the Iapetus cycle. The western boundary of the Avalon Zone is the Dover-Hermitage Bay Fault (Figure 3). Boundaries of the Avalon Zone elsewhere are major faults that disrupt the continuity of rock units and account for its extreme variability in width from Newfoundland to the southern Appalachians (Figure 1).
Successor Belts and Basins
Figure 3. Generalized interpretive map of the Newfoundland Appalachians.
Rocks younger than about 430 Ma overlie those of the fundamental zones. Some are confined to earlier zones and others cross zone boundaries. They are sedimentary and volcanic rocks that show an upward change from marine to terrestrial rocks, with all rocks deformed together and cut by granite intrusions. These changes mark the final closing phases of Iapetus. The final stages of deformation are all more important in central Newfoundland and decrease westward across the Humber Zone and eastward across the Avalon Zone.
After Iapetus closure, the youngest Appalachian rocks are everywhere the same, mainly subaerial red and grey sedimentary rocks that include fluvial and lacustrine strata, coal measures, shallow marine limestone, and evaporites. Volcanic rocks are minor. These rocks are confined to western Newfoundland but they extend offshore and underlie much of the Gulf of St. Lawrence, the southern Grand Banks, and the shelf off northeast Newfoundland. Most of the rocks are undeformed, except along major fault lines. Some of the depositional areas began as extensional rifts, others evolved as wrench structural basins. There is no evidence for oceanic crust or continental margins in the record of these rocks.
Significance and Interpretation
Rocks of the fundamental zones and overlying deposits record the full history of the Iapetus cycle. Apart from kinds of rocks and structures, zones are expressed also by geophysics, paleontology, metallogeny, plutonism, metamorphism, isotopic signatures, and other features. Younger rocks show less contrast because they were deposited during the dying phases of the Iapetus cycle or after its closure.
Analyses of the Newfoundland Appalachians indicate that its elements were assembled during two major events. Interaction of the Humber and Dunnage zones was the first event. It is attributed to northwestward transport of oceanic crust and mantle and head-on collision between a Humber Zone continental margin and Dunnage Zone oceanic rocks. The Gander Zone and Dunnage Zone also interacted at this time with southeastward transport of oceanic crust and mantle. The second event is attributed to the addition of the Avalon Zone and its interaction with more western zones. The boundaries of earliest interaction are marked by chaotic mixtures of shales and oceanic crust/mantle complexes, implying head-on collisions. Later boundaries between eastern zones, such as the Dover-Hermitage Bay fault, are steep ductile shears or brittle faults, implying oblique movements.
Closure of Iapetus resulted in the assembly of a giant supercontinent, named Pangea. At that time Newfoundland sat at its hub. How different the world was then compared to our insular setting today! Rifting and breakup of Pangea initiated the opening of the Atlantic Ocean.
Structural Inheritance and Iapetus-Atlantic Comparisons
On the scale of the North Atlantic region (Figure 2), there is an obvious spatial relationship between the location of present Atlantic continental margins and the distribution of Appalachian/ Caledonian rocks related to the Iapetus cycle. Offsets in the present Atlantic margin and present oceanic fracture zones coincide with Appalachian zone boundaries. Thus, the modern oceanic Charlie Gibbs fracture zone is aligned with the offshore projection of the Dover Fault, indicating that a major offset in the modern Atlantic Ocean crust coincides with the Gander-Avalon collisional zone boundary.
South of Newfoundland, the Appalachians are split longitudinally, parallel to the Atlantic spreading centre, whereas to the north, the bifurcation of the mid-Atlantic ridge in the Labrador Sea cuts across the Appalachian Orogen and older structural provinces of the Canadian Shield, oblivious to earlier structures. This circumstance exposed the superb cross-section of the Appalachian Orogen in Newfoundland (Figure 2).
The North Atlantic Ocean and its margins provide an actualistic model for the Iapetus Ocean, whose destruction led to the Appalachian Orogen. Just as Atlantic rifting involved a broad area of several hundred kilometres, so, too, did Iapetan rifting extend well inland toward the continental interior. The transition from rifting to continental drifting at the Atlantic margin, defined by seismic reflection, deep drilling, and the age of adjacent oceanic crust, has an Iapetan counterpart in the Appalachian Humber Zone. The widths of the North Atlantic continental shelf\ slope\rise and the thicknesses of sediments are comparable to restored widths of the Humber Zone and thicknesses of its sedimentary rocks. The form of the North Atlantic margin at the Tail of the Bank mimics an Iapetan promontory in the Gulf of St. Lawrence and provides an explanation for the sinuosity of the Humber Zone along the Appalachian Orogen (Figure 1). The crust and mantle beneath the North Atlantic is analogous to Iapetan volcanic rocks and its oceanic crust and mantle rocks of the Dunnage Zone, and Atlantic micro-continents and oceanic volcanic islands and seamounts are typical of some Appalachian unexplained terranes.
In the scenario of real estate exchanges that resulted from opening and closing of Iapetus, then opening of the North Atlantic, segments of the ancient North American Humber Zone are now found on the eastern side of the Atlantic, such as in Scotland and Northern Ireland (Figure 2). South of the Grand Banks, the North Atlantic opened well outboard of the Iapetus collisional zone, leaving a variety of terranes stranded at the margin of North America. The bifurcation of the mid Atlantic spreading centre into the Labrador Sea separated Greenland from North America (Figure 2).
Newfoundland, colloquially “The Rock,” is a geological paradise for its coastal rock exposures. These are the “smoking guns” for the Appalachian model outlined in this account. The axiom “the truth lies in the rocks” fully applies to the Appalachian Orogen in Newfoundland.
Most of the Newfoundland bays and harbours are much younger features, or fiords cut by glacial ice that flowed outward from a central ice cap within the last 20,000 years. Many of these cross the bedrock grain and present continuous rock sections, such as The Narrows of St. John’s harbour, the many ponds and inlets of western Newfoundland, and the long, deep harbours of the south coast. This scouring by glacial ice removed most of the topsoil leaving exposed bedrock, bogs, and boulder fields.
Geologists have struggled with explaining geologic mountain belts, like the Appalachians, since the earliest Newfoundland studies of J.B. Jukes and Alexander Murray well over 100 years ago. Current realistic models of drifting continents and mobile oceans are far superior to former models of fixed continents and permanent oceans. The database continues to grow at accelerated rates and our present concerns are on topics virtually unknown to geologists a generation or two ago. But, as D.M. Baird, first professor of geology at Memorial University, has said, “How strange it is that the more we seem to find out …. the horizon is still there, always inviting us to go closer. Where will the horizon be teasing us to approach in 25 or 50 or 100 years? Will we be then as far away from where we stand now as our present position is from the world of J.B. Jukes and Alexander Murray?”
The interested reader is advised to visit the Johnson Geo Centre at Signal Hill for further
Thanks are tended to Larry Nolan of the Newfoundland Department of Mines for supplying the digitized geological map of Newfoundland used in Figure 3 and to Charles Conway of Memorial University for the modifications used in this article.
Williams, Harold, 1964; The Appalachians in northeastern Newfoundland – a two-sided symmetrical system;
American Journal of Science, vol. 262, p. 1137-1158.
Williams, Harold, Dehler, Sonya A., Grant, Alan C., and Oakey, Gordon, N. , 1999, Tectonics of Atlantic Canada, Geoscience Canada, vol. 26, no. 2, p. 51-70.
Wilson, J. Tuzo, 1966; Did the Atlantic close and then re-open?; Nature, vol. 211, no. 5050, p. 676-681.