2 3 GEOLOGY OF THE APPALACHIAN-CALEDONIAN OROGEN IN CANADA AND GREENLAND Geological Survey of Canada, Geology of Canada, No. 6 Edited by Harold Williams, 1995 Chapter 1 – Introduction (by Harold Williams) PREAMBLE AND DEFINITION The Canadian Appalachian region includes the provinces of insular Newfoundland, Nova Scotia, New Brunswick, Prince Edward Island, and the southern part of Quebec along the south side of the St. Lawrence River (Fig. 1.1). It has an area of approximately 500000 km2 and it is widest (600 km) at the Canada-United States International Boundary in New Brunswick and Nova Scotia. A larger unexposed area of Appalachian rocks and structures extends across the Gulf of St. Lawrence and seaward to the Atlantic continental edge. Because of its coastal setting and insular makeup, the region offers tremendous shoreline exposures along marine passages. The Appalachian region is a Paleozoic geological mountain belt or orogen. This means that its rocks have been affected by orogeny, the combined effects of folding, faulting, metamorphism, and plutonism. Paleozoic folds and faults of several generations trend northeastward. Regional metamorphic rocks occupy continuous belts in interior parts of the orogen, and granitic batholiths are common throughout its length (Maps 1 and 2). 4 5 The word "Appalachian" was first used in a geographic context for the morphological mountains in the southeast United States. It has displaced the word "Acadian" formerly applied to this region of eastern Canada. In the present context, the word "Appalachian" is used for the geological mountain belt without regard for its morphological expression. Like the Cordilleran and Innuitian orogens, the Appalachian Orogen occupies a position peripheral to the stable interior craton of North America (Fig. 1.2). Undeformed Paleozoic rocks of the craton overlie a crystalline Precambrian basement. The exposed basement forms the Canadian Shield. The cover rocks define the Interior Platform. The Canadian Appalachian region is bordered to the west by that part of the Canadian Shield known as the Grenville Structural Province, and by that part of the Interior Platform known as the St. Lawrence Platform. Rocks of the Appalachian Orogen are mainly of Paleozoic age, and they contrast with Paleozoic rocks of the St. Lawrence Platform. Apart from the obvious structural contrasts, the Paleozoic rocks of the orogen are thicker and contain discontinuous deep marine clastic and volcanic units that contrast with the sheet-like shallow water limestones and mature quartz sandstones of the platform. As well, there are major differences in contained Paleozoic faunas, metallogenic characteristics, and geophysical expression. 6 Along the west flank of the Appalachian Orogen, the Paleozoic rocks overlie a Grenville gneissic basement and are continuous and correlative with cover rocks of the St. Lawrence Platform, although they have slightly older units at the base of the section. This part of the orogen is known as the Appalachian miogeocline. East of the miogeocline, fault-bounded Zones of lower Paleozoic rocks exhibit sharp and rapid facies contrasts. Volcanic rocks are common and in places overlie an ophiolitic basement. In other places, mixed Paleozoic sedimentary and volcanic rocks overlie continental rocks that are unlike those of the Grenville Structural Province. These contrasts in lower Paleozoic rocks, coupled with contrasting basement relationships, allow the definition of a number of distinct geological zones or terranes that lie outboard of the miogeocline. The geological zonation in most common usage is that of the Humber (miogeocline), and successively outboard Dunnage, Gander, Avalon, and Meguma zones (Williams, 1976, 1978, 1979; Fig. 1.3 and Map 2). For more than a century, North American geologists viewed the Appalachian region as a fixed and permanent "geosyncline", which through deformation, metamorphism, and plutonism was transformed into a geological mountain belt. Since the advent of plate tectonics and the wide acceptance of continental drift, the development of orogens such as the Appalachians is viewed as the result of rifting, ocean opening, subduction, accretion of terranes during ocean closing, and eventual continental collision. Accordingly, the Appalachian miogeocline (Humber Zone) is viewed now as the Paleozoic passive margin of eastern North America. Outboard zones (Dunnage, Gander, Avalon, and Meguma) are suspect terranes, or composite suspect terranes, accreted to North America during the closing of a Paleozoic ocean. 7 8 The boundary between the Appalachian Orogen and the St. Lawrence Platform is drawn at the structural front between the deformed rocks of the orogen and the undeformed rocks of the platform (Fig. 1.3). This boundary is coincident, or nearly so, with Logan's Line, a major structural junction between transported rocks from well within the orogen and structurally underlying rocks of the miogeocline. The exposed edge of the miogeocline occurs much farther east where it is marked by a narrow steep belt of ophiolite occurrences, the Baie Verte-Brompton Line (Williams and St-Julien, 1982). Stratigraphic and sedimentological analyses of the Canadian and United States Appalachians indicate that terranes east of the miogeocline were accreted during three main events (Williams and Hatcher, 1983). The accretionary events are approximately coeval with three major orogenic episodes that occurred during the Early-Middle Ordovician (Taconic), Silurian-Devonian (Acadian), and Carboniferous-Permian (Alleghanian). Opening of the present North Atlantic Ocean was initiated well east of the accreted Appalachian terranes. Thus, a variety of Appalachian terranes were stranded at the margin of the North American craton. The sinuous form of the miogeocline in Canada, expressed in the Quebec Reentrant, St. Lawrence Promontory, and Newfoundland Reentrant (Fig. 1.3), probably reflects an orthogonal ancient continental margin bounded by rifts and transform faults analogous to the modern Grand Banks (Thomas, 1977; Williams and Doolan, 1979). The closing of the Paleozoic Iapetus Ocean (Harland and Gayer, 1972) and opening ofthe North Atlantic explain why segments of the Paleozoic North American miogeocline are now found on the eastern side of the Atlantic as parts of the European plate, e.g. Hebredian foreland of the British Caledonides (Fig. 1.4). Other segments of the miogeocline, although not now part of the North American continent, remain within the American plate. The Greenland Caledonides represents such a segment and for that reason the geology of East Greenland is discussed in this volume (Chapter 12). PHYSIOGRAPHY AND GLACIATION The Canadian Appalachian region has a glaciated surface of highlands, uplands, lowlands, valleys, and fiords (Fig. 1.5). Its broken hummocky nature contrasts with the St. Lawrence Lowlands to the northwest and the smooth flat surface of the Atlantic Continental Shelf to the southeast. Highest elevations occur to the west and northwest where the rocks of the miogeocline form local highlands. From there, an undulating upland slopes gently southeastward to the coast, although it is dissected by valleys and interrupted by lowlands. 9 10 Lowlands of the Canadian Appalachian region occur in the vicinity of the Gulf of St. Lawrence and these are underlain by mainly subhorizontal Carboniferous rocks. Within the Gulf of St. Lawrence, there is no morphological distinction between the St. Lawrence and Appalachian lowland provinces. The absence of an elevated miogeocline makes this region unique for the length of the Appalachian mountain chain. The Atlantic Continental Shelf is underlain by Mesozoic and Cenozoic strata (Fig. 1.1). These thicken profoundly toward the shelf edge and they form Canada's newest petroleum frontier. Offshore landforms were developed during periods of Late Cretaceous-Tertiary subareal erosion, producing mesas, cuestas, interfluves, and stream valleys (King, 1972; Grant, 1989). Physiography of the Canadian Appalachian region probably relates to a long and continuous erosional cycle that had its beginning in the Jurassic Period, contemporaneous with continental breakup and the opening of the North Atlantic Ocean (King, 1972; Grant, 1989). The present setting of the Atlantic Continental Shelf, which is partly emerged as the Atlantic Coastal Plain farther south in the eastern United States, and the conspicuous drowned coastline of the Atlantic Provinces may reflect pre- Pleistocene tilting and tectonic subsidence (King, 1972; Grant, 1989). 11 Glacial erosion and deposition contributed to the landscape by surficial modification of former features. Pleistocene ice sheets advanced south and southeastward across the region, and the island of Newfoundland supported its own ice cap that flowed radially to the sea (Fig. 1.6). Fiord development is extensive around the Newfoundland coast but overdeepening is less evident elsewhere. Glacial retreat began about 20 000 years ago, and glacial rebound brought the surface of the Atlantic region to its present position (Grant, 1989; Occhietti, 1989). ACCESS AND CULTURE The Appalachian region is the oldest settled part of Canada. It is covered by a network of paved roads and secondary gravel roads in populated areas. Because forest industries are important, there are numerous logging roads and trails. Navigable rivers and large lakes provide access to other parts of the region. Float planes or helicopters are available for charter near most major centres. Some shoreline