21.5 Western Canada during the Cenozoic
Two additional relatively small terranes collided with North America early in the Cenozoic. At around 55 Ma, metamorphosed sedimentary and volcanic rocks of the Pacific Rim Terrane were forced a few tens of kilometres underneath the west coast of Vancouver Island (Figure 21.26). These rocks are distributed along the west coast of the island and in the area around Victoria (Figure 21.27). At around 42 Ma, sea-floor pillow basalt and gabbro of the Crescent Terrane accreted to the southern margin of Vancouver Island and also to the adjacent part of Washington State. These terranes are shown as Outboard terranes in Figure 21.14.
The accretion of the Pacific Rim and Crescent Terranes had the effect of pushing Vancouver Island closer to the North American mainland, resulting in the uplift of the sediments deposited within the Nanaimo Basin to form islands in the Strait of Georgia (Figure 21.28) and mountains on Vancouver Island.
Following these events, the subduction of the Juan de Fuca Plate, which is a remnant of the former, much larger, Farallon Plate, was re-established at its current location farther to the west of Vancouver Island. This subduction, and that of the North America Plate beneath Alaska, has produced recently active volcanoes in Alaska, and all along the west coast from north of Vancouver Island to northern California (Figure 21.29). In southwestern B.C., there are several dormant volcanoes of Pleistocene age (including Garibaldi and Meager) that trend along a line that also passes through Mt. Baker in Washington State. About 40 km to the east is a trend of slightly older igneous complexes (Pliocene to Oligocene). The displacement between these belts could be explained by a westward shift in the position of the subduction zone over that time period.
The subduction and transform boundaries along this coast also generate relatively frequent earthquakes throughout this region, as illustrated in Exercise 11.1.
Sedimentation in the WCSB continued into the Cenozoic (Figure 21.22) with deposition of the Paskapoo Formation adjacent to the Rockies in Alberta (Figure 21.31), the Ravenscrag Formation in the Cypress Hills of southern Alberta and Saskatchewan, and the Turtle Hills Formation in southern Manitoba. All of these strata were deposited in terrestrial fluvial and deltaic environments, and all of them include coal deposits. Numerous mammalian (and other) fossils have been found in these rocks in Alberta and Saskatchewan. The mammals include primitive ungulates (ancestors to the deer and their relatives), a type of pangolin, a colugo (a gliding mammal that was possibly a primate ancestor), and some true primates in the suborder Plesiadapiformes, which became extinct and are not ancestors to any modern primates.
Exercises
Exercise 21.5 The Volume of the Paskapoo Formation
The area underlain by the Paleocene Paskapoo Formation is outlined in yellow on the map shown here. The Paskapoo ranges up to about