Age of microcontinents forming Laurentia

Reconstruction of the position of the continents at 600 Ma

Precambrian rocks are located north and west of the NCA. They belong to the North American craton, known as Laurentia, and form part of the Canadian Shield. North America is considered to be an old continent because the craton has been coherent since 1.7 Ga, compared to South America and Africa that were formed 0.7 Ga, and Eurasia, 0.3 Ga. Laurentia was formed by the aggregation of several microcontinents in the interval between Archean (~3.80 Ga) and Early Proterozoic (1.65 Ga) time. The Precambrian rocks of the NCA were formed during the youngest orogen indigenous to Laurentia. This orogen, called the Grenville orogen, occurred between 1.3 and 1.0 Ga and consisted of extensive reworking of older crust.

At the end of the Grenville orogen, the Laurentian Mountain Range extended over much of southern Ontario and Québec. Although the exact position of continents surrounding Laurentia at that time is uncertain, it is generally accepted that North America was at the centre of a late Precambrian Pangea. The position of continents at the end of the Precambrian Era, 600 Ma, at the breakup of the North American Pangea. At that time, Laurentia was located at the equator, oriented at 90° from its actual position, and Avalonia (Appalachian Mountains) and Florida were located far away from Laurentia, close to the African craton.

The Precambrian terrane ( Maps - GeoServ ) of the NCA is characterized by rolling hills that rise above the flat-lying Paleozoic rocks, as shown in the bedrock topography map ( Maps - GeoServ ), and is generally covered by only a thin layer of unconsolidated sediments as shown on the drift thickness map ( Maps - GeoServ ). The Precambrian rocks that are now exposed correspond to the roots of the Laurentian Mountain Range that was much higher has been eroded through weathering and glacier action during the billion years that followed their formation.

The geology of Precambrian rocks is extremely complex and can be mapped accurately only at scales of 1:25,000 or larger, due to the numerous faults, folds, and rapid successions of rock types; the original maps should be consulted to get information on the structure of the formations. The regional synthesis of bedrock geology ( Maps - GeoServ ) was compiled from maps published by various authors at different scales. Precambrian rocks underlie the entire region, forming a basement complex. They outcrop as the Gatineau Hills, north of the Ottawa River; as the Carp Ridge, a narrow band extending South eastbound in the West NCA; and as highlands to the Southwest where they form a geological structure known as the Frontenac Arch.

The bedrock geology map, groups the Precambrian rocks according to their origin, into intrusive (felsic, mafic and ultramafic), migmatic, metasedimentary and dyke formations. The intrusive rocks formed from slowly cooling magma , are characterized by large mineral crystals. Massive and foliated light coloured felsic intrusive rocks (granite, quartz, monzonite, syenite) are abundant and often form large batholiths (Wakefield Batholith) north of the Ottawa River. The dark coloured mafic and ultramafic intrusive rocks (diorite, gabbro, anorthosite) are much less abundant and outcrop only in a few areas. Contact zone rocks along the margins of intrusive rock bodies, which are partly igneous and partly metamorphic, are grouped as migmatic rocks (amphibolite, greenstone, gneisses). In these transition zones, the nature of the rocks is reflected by the nature of the invading and invaded rocks. Metasedimentary rocks, which are mostly located in the western part of the area, are differentiated by their origin: carbonate sedimentary rocks (limestone) were transformed to marble or lime silicate rocks, and non-carbonate rocks (shale and sandstone) were transformed to gneiss and quartzite. Dykes reported on this map are composed of pegmatite either associated with granite (pink pegmatite) or marble (white pegmatite). Several younger (675 to 450 Ma) east-west trending diabase dykes are also present in the area; although they are not shown on the map as they are not uniformly mapped on the source documents.