Comparisons of the 2005 Geologic Map of North America with the 1965 Map, Areas 1-4
2. Northwestern Canadian Shield
Extensive systematic mapping supported by geophysical surveys and a carefully coordinated program of radiometric dating by the Geological Survey of Canada has led to a vast increase in understanding of the assembly of the shield during the Archean and Proterozoic. On the 1965 map most of the rocks of the shield were lumped into 5 units: Precambrian granite and gneiss, lower Precambrian metamorphic rocks, middle Precambrian metamorphic rocks, Precambrian basic (mafic) rocks, and upper Precambrian sedimentary rocks. On the new map the shield rocks are subdivided into dozens of units that are distinguished by age, lithology and origin. In this part of the shield this increased knowledge has led to:
- Recognition of major provinces characterized by Archean and Proterozoic rocks with different tectonic histories. These include the Slave Province (A), the Rae Province (B), and the Hearne Province (C). The provinces are bounded by orogenic zones such as the Wopmay Orogen (D), the Thelon-Taltson Magmatic Zone (E), and the Snowbird Tectonic Zone (F). The provinces are believed to be disparate blocks of Archean continental crust amalgamated about 2 billion years ago along sutures marked by the intervening orogenic zones.
- The Athabasca Basin (G), the Thelon Basin (H), and the Baker Basin (I) are filled with unmetamorphosed sedimentary rocks, chiefly quartz-rich sandstone and conglomerate that unconformably overlie Archean and Proterozoic metamorphic and plutonic rocks. The shape of these basins is essentially the same as that portrayed on the 1965 map, but the beginning of deposition of these is now established as between 1.75 and 1.73 billion years ago, and deposition of these rocks probably continued until at least 1.6 billion years ago. Note also the addition of the Early Cretaceous Carswell impact structure in the western part of the Athabasca Basin.
- The widespread MacKenzie Dike Swarm dated at about 1.3 billion years consists of north-northwest trending mafic dikes that cut the Archean and Proterozoic rocks, including those in the Athabasca, Thelon, and Baker Basins, showing that the unmetamorphosed basin-fill was deposited before the dikes were intruded. The MacKenzie is one of more than three dozen dike swarms shown in various parts of the Canadian Shield on the new map. None were shown on the 1965 map.
Return to comparisons of map areas 1-4.
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