University of Washington Publications in Geology The Geomorphology and Volcanic Sequence of Steens Mountain in Southeastern Oregon

RHYOLITIC ROCKS OF COTTONWOOD CREEK AND WILLOW CREEK

Rhyolitic rocks, which appear to be unrelated to those previously described, are exposed at the foot of the mountain in the valleys of Willow Creek and Cottonwood Creek. In the latter, a rather homogeneous massive light grey rock outcrops for several hundred yards with a maximum thickness of about 300 feet.

This rock is high in angular fragments, but is so altered that the writer has been unable to determine whether it is a lava filled with inclusions or a remarkably homogeneous agglomerate partially silicified. The fact that no surface features were observed and that the mass, which is apparently continuous, shows a high quartz content in its upper part and a high orthoclase content towards the base of the exposure, suggests a tuffaceous origin. On the other hand in thin section it appears to show slight traces of flowage and no indications of palimpsest structure of volcanic ejectamenta. Its high content of quartz and orthoclase, however, definitely prove a rhyolitic composition.

This rhyolitic rock is separated from the northern continuation of the uppermost Alvord Creek Beds and the interbedded andesite by a vertical fault which parallels the scarp. No precise evidence on the direction of movement was observed. The fault zone, however, is small and does not suggest great displacement. In the upper exposures of the mountain there is no rhyolite, although the series may locally have been capped by acidic flows now eroded. In consequence, the downthrow of an hypothetical acidic cap would demand a displacement of at least 4000 feet, which would hardly be expected on a narrow, almost vertical fault zone. An interpretation of minor upthrow of the eastern block coincides with the exposures in Willow Creek valley about 2 miles to the north. There, rhyolite is definitely overlain by the later andesite and yet near the contact it has been upraised by a minor almost vertical fault.

Near the mouth of the Willow Creek valley there are scattered exposures of a light grey rhyolite, which for the most part is highly altered. These exposures, which continue for about a quarter of a mile, show a thickness of approximately 500 feet. They appear to be formed by the accumulation of lava above a vent which is close to the present fault line. Near the western margin of the exposure a poorly defined vertical fault has permitted the uplift of the eastern wall. The displacement cannot be much more than 100 feet, for the upper surface of the rhyolite sloping westward at about 20° is exposed beneath the overlying andesite. This fault presumably is the northern continuation of the one previously mentioned in Cottonwood Creek. The displacement is the reverse of that occurring in the customary step faults.

This rhyolite in Willow Creek shows a few scattered feldspathic phenocrysts, up to 4 mm. in length, in a cryptocrystalline ground, which exhibits very irregular flowage. The phenocrysts show no twinning and appear to be wholly of orthoclase, The flowage in part is defined by sinuous lenticular segregations of relatively coarsely crystalline quartz. Coinciding with this evidence, a chemical analysis (table V) proved the rock to be a rhyolite, but its composition did not closely correspond with any of those previously described. Probably, however, it is genetically connected with the rhyolitic agglomerate in Cottonwood Creek.

TABLE V