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

PIKE CREEK VOLCANIC SERIES

As previously mentioned the dip slope of the southern limb of the laccolithic dome in part forms the northern wall of the valley of Little Alvord Creek. To the south of the creek for about three miles there is a great series of acidic flows and stratified tuffs, in all more than 1500 feet in thickness (fig. 29). Aside from the gentle westward dip of the fault block this series is flat lying. The flows composing its upper 1000 feet abut against the inclined strata forming the southern margin of the dome. The lowermost beds of this southern series, exposed in Pike Creek, about two miles to the south, consist of an acidic flow interbedded in stratified tuffs. Each of the three members thus formed is about 200 feet in thickness, although the base of the lower tuffs is not exposed. Locally these lower beds are conformable to the upper series, but if they could be traced northward they might be found either to be, like the upper members, nonconformable to the southern limb of the laccolith or to be the direct stratigraphic continuation of its domed beds with which a local viscous flow is interbedded.

Fig. 29. Aeroplane view of the southern end of High Steens from the east. The volcanic series of Pike Creek, which lies in the center, is prominently exposed. On the right the andesitic vent on the northern side of Little Alvord Creek is visible. Farther northward at the margin of the picture, the rhyolite vent above the laccolith may be seen just to the north of the most northerly exposure of the upper flow of biotite-dacite. At the base the playa formed by Alvord Desert is partially flooded.

Since the stratigraphic relationship of these acidic tuffs and lavas is best exposed in the valley of Pike Creek, that name is given to the series (fig. 30). To follow the chronology of events as well as the origin of its different members, it is necessary, however, to digress geographically both to the north to Little Alvord Creek and southward to Indian Creek.

Fig. 30. The volcanic series disclosed in an exposure about 1500 feet in height on the southern side of Pike Creek valley. At the base the lower platy rhyolite may be seen. Above it the dark exposures are formed by the middle tuffs. These are overlain by the upper platy rhyolite, which is shown by the light colored exposures in the center of the series. Above this are the upper tuffs and the lower biotite-dacite, which is not clearly defined. The upper flow of biotite-dacite forms the pinnacles at the summit.

In the valleys of both Pike Creek and Toughey Creek, the geology is greatly complicated by the previously mentioned step faults which roughly parallel the scarp as it extends towards Wildhorse Spur. These step faults are rarely more than 100 yards from the main scarp.

LOWER TUFFS

To the west of this zone, on the northern side of the deep valley of Pike Creek, the lowermost outcrops are formed by about 200 feet of stratified tuffs, the base of which is not exposed. These beds are chiefly of a pronounced greenish hue, due presumably to the alteration of the normal white acidic tuffs by the contact effect of the adjacent acidic vents.

Locally these beds are cut by five basaltic sills varying in thickness from a few inches to about 15 feet. The smallest of these, at its western extremity, diminishes to a thickness of only half an inch. The two larger ones, which are approximately of equal size, have both resulted in the induration of the adjacent sediments to a dark grey. This contact effect dies out within about five feet. The largest of the three smaller sills is less than four feet in thickness. The induration caused by these minor intrusions is almost negligible.

LOWER LAMINATED RHYOLITE

Directly above these tuffs in the valley of Pike Creek and extending westward for several hundred yards, there is some extremely laminated felsitic rhyolite (table III) of a reddish brown color. This acidic lava is about 200 feet in thickness. Overlying the tuffs, the laminations are largely horizontal and the mass is definitely a flow, but farther to the west it develops marked vent characteristics. These characteristics consist of highly inclined features such as lines of flowage, and bands of spherulites and lithophysae. At the surface, additional evidence is furnished by the presence of perlitic spines, surrounded by white tuffaceous material, which presumably is of contemporaneous origin. Unfortunately a crosscutting relationship of this rhyolite with the underlying highly altered tuffs is not exposed. Its only other exposure occurs immediately to the south in the valley of Toughey Creek, where it forms the base of the section.

TABLE III

In the valley of Pike Creek, the most striking feature of this lava is the extreme regularity of the parallel laminations. Although some of the banding is remarkably straight, much of it is slightly wavy (fig. 31) or even contorted. In the latter instance the extreme viscosity locally resulted in brecciation. When fresh, the rock shows an alternation of fine streaks of dark grey and reddish brown felsite. With kaolinization, the banding is more apparent, for the alteration is localized in certain streaks. With weathering following the lines of flowage, this rhyolite splits into paperlike laminations of a pale pinkish shade. Crystalline quartz is locally visible as minute segregations between the felsitic bands as well as in a network of transverse veinlets, which are chiefly apparent on the weathered plates.

Fig. 31. Typical exposure of the lower flow of platy rhyolite in the Pike Creek series.

In thin section the rock is cryptocrystalline, but the different laminations vary in coarseness. The quartz occurs in the center of thin, rather opaque streaks, which are of finer texture and of a darker shade of reddish brown. Many of these segregations of quartz are lenticular in shape. One of the largest observed, which is about .3 mm. in width, contains individual quartz grains over 1 mm. in length. Others consist of idiomorphic crystals separated largely by opaque iron stained material, but with their crystal faces projecting into the felsitic matrix. The major axes of these crystals parallel the lines of flowage.

The formation of the quartz apparently preceded the complete solidification of the lava. The network of veinlets also appears to be endogenetic, for the overlying stratified beds lack the silicification exhibited by the perlite and its associated tuffaceous material. Both here and higher in the series, the silicification of perlite has been rendered apparent by differential erosion. In extreme cases a whitish, honeycombed structure is formed, on account of the resistance of a network of siliceous veinlets, which roughly follow the mesh formed by the curving cracks. The glass, which has been at least partially devitrified by the solutions, is easily altered and subsequently eroded.

MIDDLE TUFFS AND THEIR STRATIGRAPHIC EQUIVALENT TO THE SOUTH

Capping this lower rhyolite are well-bedded tuffaceous sediments close to 300 feet in thickness. These beds, which dip predominantly to the northwest at about 15° are exposed on the south side of Pike Creek at about a half mile from its mouth and also to the south in Toughey Creek. They were not observed in Indian Creek. Instead, in this locality a badly altered acidic lava outcrops at approximately the same stratigraphic horizon.

The beds consist chiefly of whitish material that appears to be wholly of tuffaceous origin, but they also contain a number of horizons, usually two to three feet in thickness, which are formed principally of angular to subangular fragments of acidic lava. Although the fragments are predominantly less than an inch in diameter, they were found locally to be even a foot across. The angular fragments consist chiefly of platy lava resembling the rhyolitic flow immediately below.

Although these beds show no marked alteration at their base, which is in contact with a reddish, soil-like layer capping the underlying perlite, they have been highly silicified at a number of horizons. Perhaps because of their initial porosity, this action usually has been confined principally to the horizons containing the coarse fragmental material. Immediately beneath the capping flow of rhyolite, however, the silicification is more general and so extreme that the rock resembles a glassy acidic lava, but it grades into more normal tuff when traced downward about five feet. This alteration is invariably accompanied by the greenish coloration of the rock.

The localization of the alteration at scattered horizons, which probably were originally porous, suggests that the beds have been subjected to the horizontal access of altering solutions. These would have advanced along well-defined stratification and would have been concentrated in the more porous beds, especially when in contact with a relatively impervious layer. These solutions are easily explained by the presence of the vent for the overlying flow only a few hundred yards to the west.

In the valley of Indian Creek, local exposures apparently corresponding with this horizon are formed by a lava which is too badly altered to permit petrographic determination. In a felsitic ground with a marked spherulitic tendency, it shows a high content of thick feldspathic crystals about 5 mm. in length. This rock is cut by vertical lines of brecciation, which are highly silicified and locally impregnated with a low concentration of cinnabar. These zones of brecciation were not observed higher in the section. At what appears to be the approximate base of the flow, they are truncated by a thick diabasic sill which forms the lowest member of the local section. This basic rock is extremely fresh and shows no possible relation to the ore. The latter is therefore thought to have been deposited by the exhalations which accompanied earlier volcanic activity and to have been subsequently truncated by the sill.

UPPER LAMINATED RHYOLITE

The middle tuffs are capped by another flow of laminated rhyolite. Although the banding of this lava lacks the extreme regularity that characterizes the lower flow, it causes the rock to split into pinkish plates which strongly resemble those of the lower rhyolite even to the presence of minute siliceous veinlets. The felsitic flow structure is usually apparent from an alternation of greyish or reddish colors varying from a dark to a light shade. The lighter bands are usually completely kaolinized, while the dark constituent is found in thin section to be composed of cryptocrystalline quartz. This quartz as a rule occurs either in very irregular anastomosing bands that are roughly parallel, or in small segregations that may show slight alignment. Some specimens contain a few feldspathic phenocrysts that consist of orthoclase and highly sodic plagioclase.

In the main section, on the south side of Pike Creek, the thickness of this flow is approximately 250 feet, including both the upper and lower perlitic margins. The upper perlite, which shows near surface features, is over 30 feet in thickness. Its contact with the underlying platy phase is not exposed, but towards its base it develops horizontal bands of spherulites which suggest a transition. The lower perlitic selvage is about 15 feet in thickness.

Traced less than 200 yards to the west, this rhyolite loses its horizontal banding and merges into an irregular mass of lava which crosscuts the underlying tuff. Here unfortunately the actual contact is not exposed. Owing to the gradient of the stream, the width of the vent cannot be determined, but to the west the characteristic vertical jointing is well defined for at least 100 yards. In this lava, 100 feet or more above the stream, there are many irregular breccias formed of light colored aphanitic rock, which was presumably derived from the tuffaceous sediments at the time of extrusion. These fragments, which have locally suffered a brownish discoloration, prove in thin section to be so completely silicified and kaolinized that their origin cannot be determined.

This rhyolite thickens greatly to the south, and on the north side of Toughey Creek, at a distance of only about a quarter of a mile, it is close to 500 feet in thickness, even disregarding the step faults, which cause a repetition of the flow at the face of the scarp.⁹ The increased thickness is due largely to the position of these exposures directly over the southern continuation of the vent or immediately adjacent to it. Although subject to considerable irregularity, the platy jointing of this lava in Toughey Creek is in general steeply inclined. On erosion it forms steep cliffs (fig. 32) and pinnacles that rise from a fine platy talus.

---

⁹Although there are innumerable minor faults on the scarp, the major displacement previously mentioned by the author is incorrect, owing to the fact that the presence of two rather similar platy flows was not realized. R. E. Fuller and A. C. Waters, op. cit., fig. 10, p. 222.

---

Fig. 32. Vertical jointing in the upper platy rhyolite on the northern wall of Toughey Creek. The cliff on the left is about 100 feet in height.

On the northern wall of the valley, west of the step faults exposed on the scarp, the middle tuffaceous beds outcrop beneath the rhyolite. To the west, however, the rhyolite curves downward in an even arc, crosscutting this strata.

Defining this basal contact, a dark mass of vitrophyre about 30 feet in thickness outcrops for almost 100 yards (fig. 33). Again the gradient of the stream prevents the determination of the width of the vent, but its occurrence to the north in the valley of Pike Creek suggests that both valleys intersect an elongate volcanic neck which has an approximate north and south axis.

Fig. 33. View of Toughey Creek showing the middle tuffaceous beds in the foreground.

Both the vitrophyre and the overlying felsite, near its lower contact, develop spherulites which are concentrated in zones approximately parallel to the flowage. At the margin of the vent, directly above the steeply inclined phase of the vitrophyre, some spherulites surrounded by altered glass attain a diameter of about 3 feet. These show a slightly reniform surface of pale greenish shade, while the inner part, which is free from alteration, is reddish brown. The center of some of the masses consists of a single spherulite. The outer zone, however, in thin section shows a succession of conflicting segments of spherulites which radiate roughly from the center of the mass. Except for a microscopic band of quartz delineating these segments, the component minerals of the spherulites were not determined. Their birefringence, however, is low, and the fibrous constituent shows a positive elongation. The extinction is roughly parallel, although mottled. The rock is rather opaque, partly because of the presence of minute crystallites.

The classification of this lava as a rhyolite depends chiefly on a chemical analysis of one of these large spherulites. With a high potash content, its composition coincided very closely with that of the lower platy flow (table III). At least a large part of the plagioclase in the norm may be attributed to the presence of embayed phenocrysts of sodic oligoclase.

This lava may be traced to the south into the valley of Indian Creek. In the southern part of the valley, as it swings northward, the platy rhyolite appears to be divided into two flows by an irregular horizontal perlitic zone, but farther to the north, the glassy lava cuts through both phases, possibly because of some type of auto-intrusion. In the same locality, however, vertical flowage and highly inclined spherulitic zones suggest a locus of extrusion, which may possibly be caused by another intersection of the fissure-like vent exposed to the north. All the varied facies appeared to be definitely syngenetic, but the exposures did not permit a conclusive interpretation of the relationship.

On the right wall of the valley the dark sheet of vitrophyrc defines the curving contact of the upper platy rhyolite, which forms the precipitous exposures both above and to the west. The cliff on the right in the distance is formed by the upper flow of biotite-dacite.