Terranes of the North Cascades: Methow Terrane

Location of Methow Terrane rocks shown in green.

Summary: Sandstone and shale from ocean sediments that filled the ancient Methow Ocean about 200 to 100 million years ago (Jurassic and Cretaceous). Sandstone and conglomerate deposited by rivers and streams on top of Methow Ocean sedimentary rocks, and volcanic rocks from a short-lived volcanic arc about 100 million years ago (Cretaceous).

For over 100 million years after the Hozomeen terrane formed, it remained an ocean floor, and on which sand and mud collected on top of it. In Canada, rocks record the growth of a volcanic arc on this old Hozomeen ocean floor about 180 million years ago (Jurassic), but rocks in the U.S. part of the North Cascades only record the continued filling of the Methow Ocean with mud, sand, a few density flows of pebbles and cobbles, and perhaps a few layers of volcanic ash blown down from the volcanic arc in Canada. By about 110 million years ago (Cretaceous), the Methow Ocean had pretty well filled up. The youngest deposits of this period are shallow-marine sandstones. The Hozomeen terrane was buried.

Setting for the accumulation of sediments in the Methow Ocean (after a drawing by David G. Howell).

Tectonic plate movements began (or continued), pushing up the ocean floor west of what is now the Methow area. Some of the Hozomeen terrane was faulted up and shoved eastward, overriding the deposits of the Methow Ocean. About 100 million years ago, the ocean deepened, probably because its western edge was depressed by the weight of the thrust sheet of Hozomeen terrane, much as the flexing of a diving board is bent by the weight of a diver standing on its end. But as the upthrust Hozomeen terrane rose above the ocean, erosion tore it down. The now-deep ocean to the east filled with sand, mud and pebbles flushed off the uplifted and now eroding thrust sheet. This debris is particularly rich in pebbles and sand-sized grains of chert, which contrast with the quartz, feldspar, and mica grains of sand eroded from uplifted granitic batholiths on the east side of the now rapidly filling ocean. All these sediments became the "Rocks of the Methow Ocean", which are best seen near Harts Pass and, farther north, in the Pasayten Wilderness. Much of the sediment was deposited by density flows (see Sand in the Sea: How Does It Move?). Fossilized sea-shells help establish the history of the Methow Ocean by indicating the age of the strata and the depth of water in which these organisms lived.

Conglomerate of the Methow Ocean near Woody Pass. Cobbles of granitic rocks (white with black speckles) may have come from mountains eroded in Mexico.

By about 95 million years ago (still in the Cretaceous), the Methow ocean had all but filled up. Rivers and streams from adjoining highlands poured out mud, sand, and gravels onto the drying ocean deposits, capping the ocean basin with non-marine materials. In addition, volcanic rocks were deposited on the flanks of a new volcanic arc rising along the east side of the filled ocean. The composition and even the appearance was probably much like the Cascade Volcanic Arc today. A few small granitic plutons the roots of the volcanoes intrude the Pasayten Group and Rocks of the Methow Ocean. We call all the younger sedimentary and volcanic rocks the Pasayten Group. Some of these younger materials were probably deposited on one or more deltas at the edge of the ancient ocean, a scene not unlike that of the lower Skagit or Fraser Rivers today. The Pasayten Group crops out along Highway 20 north of Winthrop and in the cores of big downfolds in the Pasayten Wilderness.

Something extra: Breaking and Folding the Rocks of the Methow Domain