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
The High Cascades Between Mounts Shasta and Mazama
The lntracanyon and Associated Fissure Flows
Along the margin of the High Cascades are a series of pale-gray olivine basalt flows that poured down the principal canyons cut across the Western Cascades. Thayer21 has described such flows under the name Santiam basalts, along the North Santiam River, west of Mount Jefferson, and has shown that they poured down canyons cut in pre-Wisconsin time through the older lavas of the High Cascades. In that region, the intracanyon flows reach a thickness of 1600 feet.
Similar flows occupy the upper stretches of the gorges of the McKenzie River, the North Fork of the Middle Fork of the Willamette River, Salt Creek, the North Umpqua, Rogue River, and Butte Creek. The last three of these are shown on the map (plate 2), and all appear on Callaghan’s general map of the Oregon Cascades.22
All the intracanyon flows are traceable eastward to the bases of basaltic or basaltic andesite volcanoes in the High Cascades. Their sources, however, were not the central vents of those volcanoes. They seem, rather, to have been poured from fissures near the contact between the High and Western Cascades. Probably, as Callaghan suggests, they represent a long span of time, for although “most of them have been deeply trenched by glaciation and by streams,” others preserve surface forms suggestive of a recent origin. Possibly some of them date back to the Pliocene, and the youngest were erupted during the later history of Mount Mazama.
Nothing is more distinctive of these intracanyon flows than their uniform lithology. All are hypersthene-free, olivine-rich basalts. Some, it is true, are pale gray and holocrystalline, and others are notably vesicular, black, and rich in glass, but these are minor differences reflecting variations in gas content and rate of chilling. Noteworthy, also, is the great scarcity, and in most places the complete absence, of any interbedded pyroclastic materials. Clearly, thin flows such as these must have been extremely fluid to pour as far as they did down narrow canyons. Those in the gorge of the North Umpqua traveled more than 20 miles and accumulated to a thickness of more than 1000 feet.
Along the Rogue River, the intracanyon basalts can be traced downstream to a point just below McLeod. At Prospect, their total thickness approximates 450 feet. Eastward, they continue up the Rogue River almost to the northwest corner of Crater Lake National Park, and they outcrop in the tributary Castle Creek at the Natural Bridge, a short distance from the west boundary of the park. Obviously, the canyons of the Rogue and Castle Creek were already deeply incised when these flows were erupted, and Mount Mazama had been in existence for a long time.
Near McLeod, the Rogue River debouches from the narrow gorge carved through the intracanyon basalts and continues for about 15 miles in a broader valley cut across the lavas of the Western Cascades until it reaches the belt occupied by the sediments of the Umpqua formation, a short distance north of Medford. Here, the river passes close to two conspicuous mesas called the Table Rocks, where the tilted Umpqua beds are capped by a horizontal sheet of columnar olivine basalt, 150 feet thick. The freshness of the lava and the fact that it lies horizontally leave no doubt that this basalt belongs to the High Cascade and not the Western Cascade series; presumably it represents the products of a fissure eruption coeval with the intracanyon basalts higher up the Rogue River. A similar occurrence has been observed on Table Rock at the base of the Goose Nest volcano in Shasta Valley.