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.

Voluminous flows of olivine basalt also poured down the canyon of Butte Creek from fissures at the western base of the Rustler Peak, Blue Rock, and Mount McLoughlin volcanoes. On lithological grounds it is impossible to say where the intracanyon flows begin and the lavas of the cones end, though the boundary is indicated approximately by a distinct topographic break at an elevation of about 3600 feet.

Finally, an area of olivine basalt, 60 square miles in extent, forms the eastern half of Shasta Valley, separating the older volcanics of the Western Cascades from the cones of the High Cascades. The source of this basalt must be in concealed fissures at the northwest base of Mount Shasta, and when the lava was erupted it must have been distinctly fluid, for it poured for a distance of almost 20 miles down a gentle gradient. Here and there large tubes traverse the lava, one of which, Pluto's Cave, is traceable for more than a mile and has a maximum bore of 70 by 80 feet. The preservation of ropy surfaces and many "Schollendome," and the general absence of weathering suggest that the flow is more likely Recent than Pleistocene in age.

To summarize: After the folding and faulting of the Western Cascade series and the intrusion of dioritic stocks into them at the close of the Miocene, quiet eruptions of olivine basalt and basic, olivine-bearing andesite built a broad north-south belt of shield volcanoes of Icelandic type. This process probably occupied most of Pliocene time. Throughout this long interval the lavas of the High Cascades underwent little or no differentiation. Toward the close of the Pliocene, or early in the Pleistocene period, a few vents in the High Cascade chain began to erupt hypersthene andesite. These continued to be active throughout the Pleistocene and some have ceased to erupt only within the past few thousand years. They now form the crowning peaks of the range, Mounts Shasta, Mazama, Hood, Adams, Rainier, and Baker. At some of these centers, particularly among those at the southern end of the Cascades, differentiation became more and more pronounced as activity proceeded, until eruptions of hypersthene andesite finally gave way to eruptions of dacite and basalt. On the other hand, at the northern end of the chain, on Mounts Rainier and Baker, hypersthene andesite remained the only type of lava even to the end.

While these andesitic cones grew, the eruption of olivine basalt from other centers did not cease. On the contrary, large basaltic shields were formed at the same time, and extensive floods of olivine basalt poured westward down canyons cut through the older lavas of the High Cascades and the underlying volcanics of the Western Cascades.

We are now in a position to appreciate more fully the details of the geology of Crater Lake National Park, the main theme of this report. Our concern in the pages that follow is with the growth and destruction of one of the andesitic volcanoes, Mount Mazama, and with the irregular basement left by the outpourings of older basaltic cones on which it grew.

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