Sequences of Upper Proterozoic, dominantly clastic, sedimentary rocks, generally assigned to the Windermere Supergroup, are commonly more than 2000 m thick and are exposed almost continuously throughout the length of the eastern Cordillera (Fig. 6.1, 6.2). In the Purcell and Mackenzie mountains the supergroup unconformably overlies strata of the Purcell and Mackenzie Mountains supergroups, respectively. In and near the Omineca Belt the rocks unconformably overlie basement of granitic gneiss ranging in age from 728 Ma to more than 2 Ga (see Chapter 4). Elsewhere, the lower contact of the supergroup is not exposed.
In many places the supergroup is overlain unconformably by clastic rocks, commonly sandstone, of Early Cambrian age, but in more western areas, whereLower Cambrian rocks may be fine grained, an unconformable relationship is difficult to demonstrate.
Characteristically, in the central and southern Cordillera, the lower part of the Windermere Supergroup comprises thick, monotonous sequences of gritty, feldspathic sandstone, siltstone and shale, variably metamorphosed from low greenschist to upper amphibolite facies. The upper part of the Windermere is more calcareous and much more variable in lithology. One or more carbonate formations are of regional significance although correlationsbetween regions are dubious. Conspicuous maroon, purple and green shale occurs in the uppermost formation of the group in the Selwyn, northern Rocky, Cassiar, Omineca and Cariboo mountains. In the Selwyn Mountains the varicoloured shales overlie thick successions of gritty sandstone that are younger than most grits assigned to the Windermere Supergroup elsewhere. Diamictite, at least locally reflecting glacial
Figures & Tables
Geology of the Cordilleran Orogen in Canada
Seven stratigraphic chapters cover time slices from Precambrian to Neogene; also included are chapters covering tectonic framework, paleomagnetism, physiographic evolution, Quaternary glaciation, volcanic and plutonic regimes, metamorphism, structural styles, metallogeny, energy and ground resources, and natural hazards. A tectonic synthesis chapter and a summary of outstanding problems round out the volume. Accompanying plates include physiographic, tectonic assemblage, terrane and metamorphic maps, correlation charts, structural cross sections, and special maps showing distribution of Proterozoic and Miocene plutonic suites and the metallogeny of terranes.