The Western Cascade Volcanic Series

     Rocks of the Western Cascade Series

The volcanic rocks of the Western Cascades are made up principally of basic andesites, though they range from olivine basalts to rhyolites. They differ from the rocks of the High Cascades in the much higher proportion of pyroclastic debris, the presence of more interbedded sediment, and the much higher degree of alteration. This alteration is most pronounced in the neighborhood of intrusive bodies, but even far from intrusive contacts the lavas and tuffs are commonly chloritized. In many places, the flows carry carbonates, zeolites, quartz, pyrite, sericite, and epidote, produced presumably by fluids escaping from buried intrusions. In certain lavas, particularly near the base of the series, opal and chalcedony amygdules are abundant; in the interbedded tuffs, silicified wood and fossil leaves are widespread. These features make it easy to distinguish the volcanics of the Western from those of the High Cascades.

Much has already been written elsewhere concerning the volcanics of the Western Cascades, notably by Callaghan,⁹ Thayer,¹⁰ and Wells.¹¹ A brief description must therefore suffice.

At the southern end of the belt, in Shasta Valley, the series consists chiefly of dark, basic andesites and basalts. Some of the flows of andesite carry abundant hornblende, but by far the majority are characterized by augite and hypersthene, and many are heavily charged with amygdules of opal and agate. On the west side of the valley the rocks have been strangely eroded into a multitude of hillocks and ridges that seem from a distance like the products of a recent eruption. On the opposite side, higher in the series, the lavas are increasingly mixed with pyroclastic debris, some in the form of white rhyolite tuff, and some, as on Sheep Rock, in the form of coarse andesitic tuff breccia. Around Copco Lake and in the deep gorge of the Klamath River, andesitic lavas and interbedded rhyolite tuffs are clearly displayed, dipping gently eastward under the basalts of the High Cascades.

Farther north, in the Medford quadrangle, where the thickness of the series rapidly increases, the proportion of andesitic and rhyolitic tuff also increases. Layers of volcanic conglomerate and sand indicate that streams were vigorous during the accumulation of the lavas; lenses of shale, some crowded with leaves and others with diatoms, suggest that ponds were dammed by flows and piles of ash. Most of the lavas are black, somewhat glassy pyroxene andesites. With these are occasional flows of hornblende andesite and many of black olivine basalt, but flows of acid lava are quite exceptional. Layers of andesitic breccia and rhyolite tuff recur at many horizons, carrying plentiful leaves and fragments of silicified wood.

Still farther north, the proportion of gray andesitic lavas increases, though dark, glassy pyroxene andesites and olivine basalts continue to be the dominant flows along both the South and North Umpqua rivers. Hereabouts there are cliff-forming layers of rhyolite tuff up to 400 feet in thickness, and many bands of andesite tuff breccia with scattered logs of carbonized wood, probably products of volcanic mudflows (lahars).

It would serve no good purpose to describe the Western Cascade series in more detail. Splendid dip sections are revealed along the major canyons and highways; yet even where these are closely spaced, the sequence of lavas, pyroclastics, and sediments differs greatly.

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