146 21.3 Colorado during the Paleozoic — Physical Geology – 2nd Edition

Figure 21.3.1 The Cambrian Burgess Shale at the Walcott Quarry, Yoho Park, B.C., with Wapta Mountain in the upper right.

While clastic and carbonate sediments were accumulating along the western edge of Laurentia, much of the interior of the continent was submerged under inland seas that were connected to ocean most of the time. This region is known as the Western Canada Sedimentary Basin (WCSB). The Paleozoic sediments that accumulated within this basin show up as the blue areas in Figures 21.1.3 and 21.2.1; however, their extent is much wider than that because Paleozoic sedimentary rocks also underly the Mesozoic rocks within most of the areas that are light green on those maps. By way of example, a schematic cross-section through the Paleozoic and Mesozoic rocks of southern Manitoba is given in Figure 21.3.2. The section extends from the Saskatchewan-Manitoba border on the left to just east of Winnipeg on the right, and shows the Paleozoic rocks overlain on the rocks of the Precambrian Superior Craton.

Fifteen different Paleozoic formations, ranging in age from Ordovician to Carboniferous, are shown in Figure 21.3.2. Of these, 11 are dominated by carbonate rocks (limestone or dolomite) that very likely formed in an ocean-connected marine environment. The non-carbonate formations are the lowermost one (resting on Precambrian rocks), which is sandstone of marine origin; the Devonian Prairie Evaporite Formation (in red)—the same formation from which potash is mined in Saskatchewan; and the upper two Devonian formations (in yellow), which are shale. When the Prairie Evaporite formed, the basin was isolated from the open ocean, and the rate of evaporation was greater than the rate of input from precipitation and river inflow. During that time, probably at least several million years, there were numerous changes in sea level or land level that allowed additional ocean water—and therefore additional salt—into the basin.

Figure 21.3.2 The Paleozoic sedimentary rocks of southern Manitoba along a section extending from the Saskatchewan border on the left to the Winnipeg area on the right. The section is 400 km wide and 1,800 m high, and the vertical exaggeration is about 100 times. The dip of the beds is also exaggerated by 100 times; their original and current attitudes are close to horizontal.

Figure 21.3.3 The distribution of continents in the early Carboniferous, showing the terranes that later became attached to the west coast of North America. The light blue areas are continental shelves, the white is ice of the Karoo Glaciation, and the red line shows subduction of oceanic crust beneath Laurentia. Panthalassic is the name for the huge ocean that preceded the Pacific Ocean.

There are Paleozoic rocks in the central and western parts of British Columbia and Yukon, but they formed far away and did not become part of North America until the Mesozoic. Subduction started along the western edge of Laurentia by the middle Paleozoic. That meant that oceanic crust was moving toward the continent, bringing small segments of exotic continental crust with it (Figure 21.3.3). These crustal blocks along western North America are called terranes, indicating that they are sections of the continent that have an exotic origin (Figure 21.3.4). Most of British Columbia is made up of terranes that include sedimentary rocks with fossils that imply an origin south of the equator, or volcanic rocks with magnetic orientations that indicate a southern-hemisphere origin.

Figure 21.3.4 The Carboniferous Mt. Mark Formation on Vancouver Island is part of the Wrangellia Terrane, which arrived on the edge of North America during the Cretaceous.

Media Attributions

• Figures 21.3.1, 21.3.2, 21.3.4, 21.3.5: © Steven Earle. CC BY.
• Figure 21.3.3: © Steven Earle. CC BY. Based on information from Christopher Scotese.

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