12 Intrusions and Volcanic Necks in the Western Cascades

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

The Foundations of Mount Mazama

Intrusions and Volcanic Necks in the Western Cascades

The vents from which the lavas and pyroclastic rocks of the Western Cascades were erupted are indicated only by the deeply denuded fillings of the conduits, and even of these few have been recognized. The cones and craters that formerly existed were destroyed long ago.

Too little work has been done to enable us to say whether or not the early Tertiary volcanoes were arranged in any definite manner, though it appears that they were concentrated in a more or less north-south belt parallel to the later volcanoes of the High Cascades. In the Bohemia mining district, Buddington and Callaghan13 found many small plugs of diorite porphyry and dacite porphyry associated with a stock and several large dikes of granodiorite porphyry, around which the volcanic rocks were metamorphosed and mineralized. Other plugs are known from the North. Santiam, Quartzville, and Blue River mining districts. In the Calapooya Mountains, Wells and Waters14 discovered three necks cutting the Western Cascade lavas, one composed of fine-grained gabbro, a second of quartz-augite diorite, and a third of hypersthene basalt. About 20 miles west of Crater Lake, at the Buzzard (Alserena) mine, there is a large intrusion of rhyolite porphyry. Two small oval plugs of dacite porphyry cut the Western Cascade series near Agate Flat, north of Copco Lake. In Shasta Valley, a line of fine-grained rhyolite porphyry intrusions, some 8 miles long, runs in a northeast-southwest direction through Little Shasta (see map, plate 2). Adjoining these rhyolitic intrusions, close to Little Shasta,. are two oval masses of hornblende andesite porphyry, each about a mile across; a third mass of similar rock forms Gregory Mountain, on the outskirts of Montague. Finally, near the south end of Shasta Valley there are two small plugs of hypersthene-quartz diorite porphyry.

How many of these intrusions represent deeply eroded volcanic conduits, it is impossible to say, though some of them almost certainly mark the sites of former vents. An accumulation of lavas and pyroclastic rocks as vast as that of the Western Cascades must have been erupted from a great number of sources. The surprising thing is that so few of the feeders have thus far been found. Since only a few Tertiary intrusions are known to cut the bedrocks farther west, the principal source cannot lie in that direction. On the other hand, many vents may be buried beneath the lavas of the High Cascades to the east.

In addition to the necks and plugs already mentioned, there are countless intrusions, chiefly sills, along the western fringe of the Western Cascades. Scores bf these may be seen among the sediments of the Umpqua formation all the way from Shasta Valley to the Roseburg district, and many may be found among the overlying volcanics. These intrusions range in composition from dacite porphyry to norites, the commonest types being augite- and hypersthene-diorite porphyry. Their range in age is probably the same as that of the Western Cascade volcanic rocks, namely, Upper Eocene to Upper Miocene. Adequate descriptions of them may be found in the papers by Buddington and Callaghan and by Wells and Waters to which reference has already been made.

 

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