Cretaceous Formations

During much of late Cretaceous time, the present site of the southern Cascades was largely if not entirely occupied by a shallow sea extending at least as far east as the John Day Basin. In the Roseburg region, a thick series of Cretaceous rocks rests on a surface of moderate relief cut in the plutonic and metamorphic bedrocks. These Cretaceous rocks approach within 40 miles of Crater Lake, where they dip beneath the Tertiary volcanic deposits of the Western Cascades (see map, plate 2).

Plate 2

At the beginning of Cretaceous time, the Klamath Mountains rose as a high island. Slowly this was reduced by erosion, until most of it had been worn below sea level before the close of the Cretaceous. A thin and broken strip of Chico (Upper Cretaceous) sediments now follows the eastern fringe of the Klamath Mountains from Jacksonville southward for 50 miles, to a point about 5 miles southeast of Yreka, on the edge of Shasta Valley. Small outliers also overlie the bedrocks to the west. Apparently the older Cretaceous formations, the Knoxville and Horsetown, which outcrop in the vicinity of Roseburg, are missing in this southern area.

The Chico formation rests on many kinds of schist and plutonic rocks, and its composition shows a corresponding variety. Most of it consists of fine-grained arkosic sandstone interbedded with shale and, especially near the base, with layers of coarse conglomerate. In many places, placer gold has been obtained from the lower part of the formation, as in the Jacksonville district and south of Hornbrook. Where the Chico was derived by erosion of metavolcanics and basic plutonics, it is characteristically a greenish graywacke; where it was derived from acid plutonics, it is generally a pale arkose rich in milky quartz. Elsewhere, it is markedly micaceous, indicating provenance among the metasedimentary formations of the "bedrock series." The maximum thickness of the Chico in the Ashland-Medford valley is approximately 600 feet.

All the Chico beds were laid down in shallow water. The predominance of sandstone, the prevalence of conglomeratic layers, the occurrence of much carbonaceous material in certain horizons and of abundant and large shallow-water fossils, and the manner in which the sediments vary with the nature of the neighboring bedrocks suggest near-shore deposition.

No Chico rocks are known between Yreka and the head of the Sacramento Valley, but there they reappear. On the west side of the valley, in the Coast Ranges, their thickness may be several thousand feet, but on the east side, on the flanks of the Sierra Nevada, they thin rapidly in the direction of Lassen Peak. Whether or not they pass beneath the Cascade Range in the Lassen region is a debatable question, though the occurrence of saline springs south of Lassen Peak suggests that the volcanic rocks may be underlain locally by marine deposits. biller was of the opinion that an arm of the Chico sea, the "Lassen Strait,'' ran between the Klamath Island and the Sierra Nevada and spread eastward as far as the Blue Mountains of northeastern Oregon. Patches of Cretaceous rocks in the Prineville and John Day regions lend some support to this view.

We may therefore suggest, though admitting lack of proof, that in Upper Cretaceous times the site of Crater Lake was part of a wide and shallow sea bordered by the Sierra Nevada and Blue Mountains and containing many low islands of bedrock, the largest of which lay where the Klamath and Siskiyou mountains now rise.

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