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Terranes of the North Cascades Terranes of the North Cascades: Chilliwack River Terrane

Location of Chilliwack Terrane rocks
Location of Chilliwack Terrane rocks shown in green.

Summary: Mildly metamorphosed volcanic and sedimentary rocks from a volcanic island arc or arcs. From 375 to 170 million years old (Devonian to Triassic). Highly folded and commonly upside down.

On top of the Nooksack terrane is the Chilliwack River terrane, named for the Chilliwack River in British Columbia . Volcanic rocks dominate, but sandstone, shale, and scattered limestone are also abundant. We know from many fossils that the Chilliwack River terrane ranges from roughly 375 to 170 million years in age (Devonian, Carboniferous, Permian, and Triassic). As the reader can see from the geologic time scale , the Chilliwack River terrane is older than the underlying Nooksack terrane. Such a topsy-turvy relationship is a sure sign of significant fault movement. Nevertheless, as with the Nooksack, the rocks of the Chilliwack River terrane were also formed as part of a volcanic arc (See Making Sense), probably one fringed with limestone reefs.

Before the Chilliwack River terrane was thrust over the Nooksack terrane, its beds were folded upside down.
Before the Chilliwack River terrane was thrust over the Nooksack terrane, its beds were folded upside down.

The Chilliwack River terrane is strongly folded and faulted and somewhat metamorphosed. Many of the shales have changed to phyllites and volcanic rocks have become greenstones. Limestone is metamorphosed to marble, but the recrystallization has not destroyed all the fossils. Careful study of sedimentary-bedding features, such as graded bedding formed in density flows, indicates that broad areas of Chilliwack rocks are entirely upside-down. No part of the Nooksack terrane is like that! Chilliwack rocks must have been folded and faulted before, or as they were shoved on top of, Nooksack rocks, which, for the most part, are only gently folded and almost always right side up.

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Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.


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