Bedrock in the southwestern corner of New Brunswick provides the setting and topographical control for the International Passamaquoddy Tidal Power Project, an engineering proposal for converting the region's tidal forces into hydroelectric energy. Although the Coldbrook Group (Precambrian) comprises the oldest rocks of the region, they arc of limited extent and Palaeozoic rocks predominate. New fossil (graptolite) evidence is presented for the Ordovician age of the Charlotte Group. The previously undivided Silurian rocks, extensively developed around the margin of Passamaquoddy Bay, are correlated with formations in adjacent areas of Maine. Intrusive Devonian granitic rocks trend northeast across the entire region. Postorogenic sedimentary rocks of the Perry Formation (Upper Devonian) directly w1derlie Passamaquoddy Bay. Major folds within the area appear to have been formed by a single compressive force, which produced regional strike-slip faults . Aeromagnetic maps of the region aid in tracing belts of volcanic rocks and in outlining structures and the distribution of intrusive rocks of various ages. A field excursion roadlog is provided that points out some key geological relationships within the area.
The Strait of Belle Isle area comprises three principal geological terranes and each is described separately. Precambrian terrane. The oldest rocks in the area are widespread leucocratic to melanocratic biotite-quartz-felspar gneiss including some quartz-rich gneiss, pelitic gneiss, amphibolite, and minor calcareous gneiss of Helikian or earlier age. The gneiss was intruded by plutons of the anorthositic suite, commencing with metagabbro throughout the area and continuing with mangerite and hornblende granite north of the Strait of Belle Isle. Small bodies of foliated granite were probably emplaced early in this sequence. Regional folding along northeast-trending axes accompanied by amphibolite to granulite facies metamorphism probably occurred after intrusion of the metagabbro and before emplacement of the hornblende granite. Later, more hydrous megacrystic to massive granitic plutons were emplaced diapirically, mostly within the gneiss of the northern Long Range. Phyllonite, up to 300 m thick, was formed by outward thrusting along the northeast margin of the megacrystic pluton at Canada Bay. Gneiss and megacrystic granite along the east margin of the area were folded along northeast-trending axes and subjected to greenschist fad es metamorphism during or soon after emplacement of the megacrystic plutons. Small bodies of fine grained, massive to foliated, mostly leucocratic rocks of probable cataclastic origin are present near the east coast of the Long Range. The protolith of these bodies is of the same age as the Grenville basement, but cataclasis is of late Grenville or later age or both. Normal faulting followed by reverse faulting is evident on Belle Isle and similar faults are present along the margin of the Long Range Grenville inlier. A zone of greenschist facies metamorphism of Ordovician age crosses the Precambrian rocks along the east margin of the area and may extend to the southern end of the Grenville inlier. Lower Paleozoic autochthonous strata. Hadrynian, Cambrian, and Ordovician autochthonous strata of the Strait of Belle Isle area are part of an easterly thickening wedge of strata extending to the southwest and underlying much of the northern Gulf of St. Lawrence. The strata were deposited on a shelf that gave way easterly to a shelf edge and then to a turbiditic fades which formed along the former continental slope and rise. The oldest strata comprise quartzite, conglomerate, siltstone, and minor basalt of the Bateau Formation on Belle Isle. Tholeiitic diabase dykes of the Long Range dyke swarm of latest Hadrynian age intrude the Bateau Formation and are feeders to tholeiitic basalt flows of the overlying Lighthouse Cove Formation. These flows are interlayered with basal arkosic sandstone, conglomerate and siltstone of the succeeding Bradore Formation. In all, these strata are as much as 700 m thick. A regolith is widely evident on the basement complex beneath the Bradore beds. The Cambrian to Middle Ordovician is represented by up to 1500 m of shallow water marine carbonates and some shale and quartz sandstone derived from the Precambrian rocks to the northwest as the proto- Atlantic ocean waters gradually transgressed to the northwest. Lower Ordovician carbonate of the St. George Group consists of about 600 m of calcitic dolomite and dolomitic limestone, which exhibits subtle lateral changes from one rock type to the other. Fossils are not abundant and include graptolites, trilobites, and gastropods. Carbonate in the top of the group contains zinc occurrences. Following the Lower Ordovician deposition, the entire continental shelf was uplifted and exposed to subaerial erosion. After perhaps 5 million years, the shelf again subsided and 500 m of marine limestone of the Table Head Group was then deposited in a Middle Ordovician epicontinental sea upon a karst surface. Water on the shelf then deepened, and detritus for the first time came from the east when the Hare Bay Allochthon was being moved westward on to the shelf. The resulting black shale and minor greywacke of the Goose Tickle Formation, as much as 500 m thick, were ·deposited gradationally upon the carbonate. Hare Bay Allochthon. The Allochthon consists of a variety of sedimentary and volcanic and plutonic rocks including greywacke, polymictic conglomerate, quartz-pebble conglomerate, siltstone, shale, mafic pillow lava, peridotite, harzburgite, dunite, gabbro, and diorite. These rocks make up six contrasting rock assemblages which are separated by thrust faults. The lower structural slices consist of sedimentary rocks and the highest structural slice consists of an ophiolite suite. Melange zones commonly separate the thrust slices. One of these zones contains large exotic blocks. The commonest blocks are serpentinized peridotite, mafic volcanic rocks, amphibolite, foliated gabbro, greywacke, diorite, hornblendite and hornblende-biotite schist. These are contained in a matrix of black and green shale. Most rock types of the blocks may be matched directly to rocks in nearby structural slices. The rocks of the Hare Bay Allochthon originated in the east, in the region near what is now Notre Dame Bay, and record a part of the development and destruction of the ancient continental margin of eastern North America.
Ordovician strata outcrop in two belts, each with different facies, along the southwestern and southeastern sides of the Hudson Platform. Limestone and minor sandstone form a continuous belt, 840 miles long, extending from North Knife River, Manitoba to the Kenogami River, Ontario. The belt is from 10 to 40 miles wide and is truncated in the south by a major fault scarp. This remarkably uniform belt of Ordovician rocks represents the southwestern and western margins of the Hudson Bay and Moose River Basins respectively and includes the Bad Cache Rapids Group, the Churchill River Group and the Red Head Rapids Formation. The thickness of the strata varies, from southwest to northeast, from a few feet at the wedge-edge to over 600 feet in the subsurface. The average thickness of the composite Ordovician section in the northern part of the outcrop belt is about 300 feet. Arkosic sandstone, calcareous sandstone, and shale comprise a smaller Ordovician outcrop belt, 30 miles long, near the southeastern extremity of the Hudson Bay Lowlands in Quebec. Toward central Moose River Basin these strata grade into dolomite, 300 feet thick. In the Moose River Basin and along the southwestern edge of the Hudson Bay Basin, Archean crystalline rocks form the basement and the non-conformably overlying Ordovician strata are the oldest Paleozoic rocks.
Three ages of crystalline or highly deformed rocks provide a basement to Carboniferous sedimentary areas in the Canadian Appalachians. Regional distribution of the exposed part of the basement provides evidence of progressive stabilization of the entire area, viz., Ordovician and Silurian rocks are a cratonic cover to Precambrian rocks; slightly folded Silurian and Devonian sediments cover folded Ordovician rocks; and flat-lying Carboniferous rocks are a cratonic cover on earlier Palreozoic rocks. Progressive stabilization confined the late Palreozoic subsidence to a relatively narrow mobile belt and aided in producing a fragmentation of the basement along this belt of deep subsidence. Basement fragmentation produced 'basement blocks' (e.g., Caledonia Highlands, Cobequid Mountains, Antigonish Highlands) comprising inliers of Precambrian and Lower Palreozoic rocks witbin the area of greatest Carboniferous subsidence. Uplift and tilting of basement blocks greatly influenced the regional Carboniferous sedimentation (Bell, 1944). Isopach maps of six stratigraphic units of the Carboniferous cover provide information concerning the surface of the pre-Carboniferous basement, and tbe subsurface position of the preCarboniferous rocks is interpreted from available bore-bole, seismic, aeromagnetic, and gravity data.
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