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Terranes of the North Cascades Terranes of the North Cascades: Bell Pass Mélange

Location of Bell Pass Terrane rocks
Location of Bell Pass Terrane rocks shown in green.

Summary: Bits and pieces of many things, including deep ocean deposits, pieces of ancient continental crust, and pieces of the mantle. Ages of constituents range from probably more than 570 (Precambrian) to about 200 (Triassic) million years.

On top of the Chilliwack River terrane rests the Bell Pass mélange. Mélange is French for mix, and geologic mélanges are very mixed rocks; they are hodge-podges on a grand scale. Much of the Bell Pass mélange is made up of sedimentary rocks, including sandstone, shale, and chert. Basalt is also abundant in some areas. The chert, shale, and basalt are not only mixed, but highly broken up. The continuous layers of bedding are gone; only blocks and pieces remain. The rocks look faulted, or, more expressively, smeared.

Deformed chert of Bell Pass melange.
Deformed chert of Bell Pass melange.

Blocks of hard basalt or chert stick out here and there as resistant knobs. But the mélange contains exotic rocks as well, things we do not expect to find mixed into unmetamorphosed sedimentary or volcanic rocks. Exotic blocks in the mélange are metamorphic rocks, such as gneiss and schist, which came from deep in the Earth’s crust, and ultramafic rocks dark colored, rich in iron and magnesium which came from deeper in the mantle. The sedimentary parts of the Bell Pass mélange are mostly oceanic in origin, but many of the exotic blocks are not.

Bell Pass Melange, south side of Suiattle Mountain.
Fragments of greenstone, sandstone, and other rocks in disrupted argillite. Bell Pass Melange below the Shuksan Thrust Fault, south side of Suiattle Mountain.

The blocks or "knockers" of gneiss (being hard, they resist the knocking of a geologist’s hammer) in the Bell Pass mélange were called the Yellow Aster Complex by Peter Misch, who named the formation for Yellow Aster Meadows, which is underlain by a large slab of gneiss. The gneiss was formed deep in the crust, at relatively high temperatures. Associated with it are igneous rocks. Both the gneiss and igneous rocks have been smashed and further metamorphosed at cooler temperatures. The origin of much Yellow Aster gneiss is obscure, but on Park Butte, gneiss rich in calcium minerals and associated with marble indicates that some Yellow Aster rocks were sedimentary rocks before high-temperature metamorphism. The association of metamorphosed sedimentary rocks and plutonic igneous rocks is suggestive of a continental setting, although it is surprising to find such rocks amidst the relatively unmetamorphosed oceanic rocks that comprise most of the Bell Pass mélange.

Foreground Yellow Aster meadows are held up by large slab of Yellow Aster granitic gneiss in the Bell Pass melange
Foreground Yellow Aster meadows are held up by large slab of Yellow Aster granitic gneiss in the Bell Pass melange. American Border Peak (left) and Yellow Aster Butte (right), both carved from volcanic rocks of the Chilliwack Group.

Even more startling in the mélange are blocks of ultramafic rock. They began existence in the mantle. Most ultramafic chunks in the Bell Pass mélange are small, a few feet to a few hundreds of feet across. The largest block of ultramafic rock in the Bell Pass melange is the Twin Sisters Mountain massif, made mostly of dunite, a rock consisting of the mineral olivine. Except for the remarkably unchanged Twin Sisters dunite, many of the ultramafic blocks are now partly or entirely made of serpentinite.

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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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