Eocene Formations

At the dose of the Cretaceous period, the distribution of land and sea was greatly changed. The sea retreated westward beyond the line of the Cascade Range, never to return, and its place was taken by a gently undulating plain. Weaver¹ suggests that uplift of the Cretaceous rocks in southwestern Oregon forced the sea even farther west than the present coast line, though to the north the recession was considerably less. During Middle Eocene time, slight downwarping of the plain permitted the sea to readvance over northwestern Oregon to a shore line that ran approximately from the vicinity of Roseburg through Portland to Seattle. Farther south, in the Klamath-Siskiyou region and in the Ashland-Medford valley, the Lower and Middle Eocene was a time of nondeposition, and it is likely that the mountains were rising isostatically in response to erosion.

Volcanic activity had already commenced in eastern Washington during the early Eocene, as is shown by the lavas and pyroclastic deposits of the Naches formation. Between Lower and Middle Eocene times volcanism increased in intensity and became more widespread. From Roseburg north to Vancouver Island, tremendous eruptions occurred. Most of the lavas were basic and many escaped from submarine fissures in the form of pillow basalts, as in the Roseburg region, 40 miles west of Crater Lake. Accompanying the flows were explosions of fragmental debris, so that the sediments accumulating in the Eocene seas were highly charged with tuff.

A shallow marine embayment continued to occupy most of northwestern Oregon in Upper Eocene times, its southern limit following a line westward from Roseburg to the present coast. Farther south, between Roseburg and the head of Evans Valley (see map, plate 2), Eocene sediments seem to be absent. This appears to have been a region of continuous erosion throughout the Eocene period. But still farther south, from Evans Valley, through the Ashland-Medford 'valley, as far as Shasta Valley in California, 60 miles distant, there is an unbroken belt of Upper Eocene sediments, referred to as the Umpqua formation.

Plate 2

However, unlike the Umpqua formation of the Roseburg area, which is entirely marine, the Umpqua here is largely, if not wholly, of fresh-water origin.

The fresh-water Umpqua formation rests disconformably and perhaps in part with slight angular unconformity on the Chico formation in this southern belt. According to Wells,² it attains a maximum thickness of about 8000 feet in the Medford quadrangle. In Evans Valley, it rests directly on the bedrocks and consists mainly of arkosic sands with pebbly layers, shales, and several thin seams of coal. Around Beagle, in Sams Valley, and under the lava cappings of Table Rock, the Umpqua marls and sands, largely derived from acid plutonic bedrocks, carry abundant fragments of silicified wood. In the Ashland-Medford valley, where the formation reaches its fullest development, the sediments are chiefly sandstones, shales, and conglomerates, the latter heavily charged with pebbles and boulders of quartzite, milky quartz, and chert. In the upper part of the formation, volcanic material becomes increasingly abundant. It is therefore impossible to draw any sharp boundary between the Umpqua and the overlying Western Cascade volcanic series. Close to the Rogue River, the transitional water-laid volcanic detritus reaches a thickness of 2000 feet.

Noteworthy in the fresh-water Umpqua beds of the Ashland-Medford valley are the coal seams which outcrop in their upper part. According to Diller, they improve in quality as they are followed northeastward in the direction of dip. The rich flora recovered from these coals implies the existence of swamps along the border of the rising Klamath Mountains.

In California, no exact line has yet been drawn between the Chico and Umpqua formations, but the latter consists chiefly of blue-black shales and sandstones, with occasional seams of coal, as at Agar on the edge of Shasta Valley. Here also the Umpqua becomes increasingly rich in tuffaceous material in its upper part.

In brief, a thick series of river-borne sediments accumulated during Upper Eocene times along the northeast and east slopes of the Klamath Mountains. Here and there, on a low, rolling surface, were swamps bordered by dense subtropical vegetation. Volcanic activity, which had started earlier farther north, extended to this region toward the close of the Eocene, and thick deposits of ash and agglomerate were laid down between layers of fluviatile material. Finally, before the onset of the main volcanic eruptions that built the Western Cascades, there was a gentle uplift of the Umpqua beds near the Oregon-California boundary, for whereas the Western Cascade series rests conformably on the Umpqua in the northern half of the Medford quadrangle, a marked angular discordance divides them to the south. Still farther south, along the edge of Shasta Valley, the Western Cascade lavas gradually overlap the Umpqua beds and the underlying Chico and finally come to rest on the bedrocks.

There is little doubt that the marine Umpqua beds of the Roseburg region and the fresh-water Umpqua beds of the Ashland-Medford valley pass eastward under Crater Lake and interfinger with the continental Eocene sediments and tuffs east of the Cascade Range. While the Umpqua formation was being deposited west of the Cascades, a broad plain existed to the east. At that time there was no high divide as now. On the eastern plains, volcanoes were erupting the lavas and tuffs of the Clarno formation in the John Day Basin, and streams were depositing torrential gravels. The conditions that must then have obtained have been well described by Chancy:³

In Eocene time the vegetation across the state (Oregon) was essentially uniform. The avocado (Persea), cinnamon (Cinnamomum), fig (Ficus), and persimmon (Diospyros) found in the Goshen flora near Eugene or in the Comstock flora to the south are identical with those which lived in the John Day Basin. Of more than forty species of trees, shrubs, and vines which make up the Clarno flora, there are only four or five which have not been found in older Tertiary rocks along the Pacific slope from Washington to California. The conclusion is inescapable that the mild and humid climate of the coastal borders extended inland to the John Day Basin of eastern Oregon, over the area now occupied by the Cascade Range. For if a mountain barrier had divided Oregon during the Eocene, the fossil floras would have been as different as they are across the state today. . . . Growing on the lower slopes of the Andes in Venezuela, on the savannas of Panama, and in the mountains of Costa Rica and Guatemala there are trees with leaves, fruits, and stems like those of the Clarno and other Eocene bras.

In the hills overlooking the plains, the subtropical vegetation was mixed with trees of temperate species, such as the redwood, alder, tan oak, and elm. Between the hills ran broad rivers that were dammed from time to time by flows of lava and outpourings of ash. Where the volcanoes were situated, it is impossible to. tell, but that they were more widely scattered than the later volcanoes of the Cascade Range cannot be doubted from the distribution of their products. The low coast line swung west and north by-Roseburg, and to the south the Klamath Mountains were slowly rising.

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