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

PIKE CREEK VOLCANIC SERIES
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LITTLE ALVORD CREEK RHYOLITE

The exposures on the main scarp are not sufficient to permit the upper platy rhyolite flow to be traced northward from Pike Creek. Where last exposed it appears to be thinning rapidly towards the north. In Little Alvord Creek, however, another mile and a half to the north, a very thick flow of rhyolite occurs at the same horizon. The vent from which this lava was extruded is also exposed. It lies at least approximately in line with the two just described to the south, but the lava does not closely resemble the upper platy rhyolite in either its physical or its chemical characteristics (table III).

This lava, like the upper platy rhyolite, contains a few small phenocrysts of oligoclase and orthoclase, and also exhibits some felsitic flow structure, but not sufficient to cause it to split into plates. It lacks the irregular segregations of cryptocrystalline quartz that characterize the Pike Creek flow. Chemically, however, the distinction appears to be more definite. Although the silica content is almost equally high in both flows, there is marked difference in the oxides. In the Little Alvord Creek rhyolite, the potash is far lower, while the content of soda and lime is decidedly higher.

On the south side of Little Alvord Creek, at about a quarter of a mile from the scarp, there is a steep cliff about 400 feet high formed by this flow of rhyolite (fig. 34). In marked contrast, the north side of the valley is formed by the partially dissected dip slope of the tuffaceous sediments composing the southern limb of the laccolith. This slope, as previously mentioned, formed the retaining wall for the rhyolitic flow, but farther to the west it is cut by the vent from which the lava was extruded. The base of the flow is not exposed. Since it shows a basal margin of perlite that is approximately level, it presumably was extruded on a relatively flat surface, which should occur close to the level of the stream.

Fig. 34. A view of the southern wall of the valley of Little Alvord Creek showing the 400 foot flow of rhyolite overlain by the two flows of biotite-dacite.

This basal perlite is exposed for only about 100 yards and ceases with the development of vent characteristics, which consist in the presence of vertical lines of flowage in the rhyolite and in the intrusive relationship of the perlite on the north side of the valley. These exposures of the latter extend up the northern slope for several hundred feet and appear to define part of the rounded northern end of the rhyolitic vent.

To the east of this margin lie the inclined, well-stratified tuffs, which dip to the southwest at over 15° The inclination of these beds towards the vent suggests a genetic relationship, but the marked classification of the clastics demands an aqueous stratification. The slope may be explained by the deformation induced by the rhyolitic laccolith.

To the west, the perlite develops steeply inclined bands of spherulites. Some of these radiating masses attain a diameter of 18 inches. As usual, they mark the transition to the felsitic phase, which shows pronounced vertical flowage. This phase locally exhibits partitions covered with small nodes formed by conflicting spherulites.

Near the center of the vent, a breccia, formed of perlitic fragments in a light colored tuff, rises about 200 feet above the surface of the flow. This tuff is thought to be formed both by the local explosive activity and by the comminution of the perlite which injects it extensively. These injections rise steeply and then tend to curve into horizontal sheets which are usually but four or five feet in thickness, although some range up to 15 feet or more (fig. 35). As a rule these injections, which are considered to be near surface features of the rhyolitic vent, cannot be traced more than 30 or 40 feet. They appear to end rather bluntly.

Fig. 35. Perlitic injections curving to a horizontal position in the tuffaceous breccias above the rhyolite vent on the northern side of Little Alvord Creek. An 18 inch hammer is beneath the center of the lower injection. The basal exposure of the andesitic vent is visible at the top.

About 100 yards to the west at a lower elevation there is an exposure near the stream of massive black perlite showing irregular zones of coloration to brownish and reddish shades. This vitreous lava presumably formed the surface of the flow. As in the case of varicolored obsidians previously described by the writer,¹⁰ the variations in color are due to the oxidation of the glass adjacent to the lines of fracture in a flow breccia. The fragments were thus partially altered to a brownish or reddish shade. The hot gasses following these brecciated zones subsequently caused the refusion of the glass. With additional flowage the clean-cut outline of the breccia was locally destroyed.

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¹⁰R. E. Fuller, "The Mode of Origin of the Color of Certain Varicolored Obsidians," Jour. Geol., vol. XXXV, p. 570, 1927.

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UPPER TUFFS

The next stratigraphic unit of this volcanic series outcrops only in the exposure of the main sequence on the south side of the valley of Pike Creek (fig. 30). Here well-stratified tuffs about 40 feet in thickness are exposed overlying the irregular perlitic phase that caps the upper platy rhyolite. This bed is quite green towards the top, but shows some brownish shades in the lower part. Near the top there are two conglomeratic beds less then three feet in thickness. The fragments are of acidic lava and vary in shape from round to angular. Possibly owing to their original porosity these beds have suffered extreme silicification similar to that observed lower in the series. The alteration is probably due to the proximity of an adjacent vent, which will be described later.

It is impossible to establish the horizontal extent of these sediments. Owing to the thickening of the underlying platy rhyolite to the south, they do not occur in Toughey Creek. The strata, however, may continue to the north, but unfortunately there is no exposure in which this may be determined.

BIOTITE-DACITE

Above these sediments are two great flows of biotite-dacite which are very similar both petrographically and chemically (table IV). They show feldspar and biotite in a dense ground that varies from light grey to brownish or reddish shades. As a rule the ground shows no flowage. The percentage of feldspar is somewhat variable, but it appears to be distinctly more plentiful in the upper flow, where it forms as much as 30 per cent of the rock. In the lower flow it probably does not attain half that figure.

TABLE IV

The size of the crystals is about the same in both flows, although the individuals in each are very variable. The larger masses are usually glomeroporphyritic feldspathic intergrowths 4 to 6 mm. in diameter. These as a rule contain glassy inclusions. Most of the feldspar, however, occurs in small angular fragments or in irregular grains, whose rounded outline indicates partial resorption. These smaller individuals average less than 1 mm. in length. Some of the feldspar is a sodic plagioclase but many of the fragments show no twinning or zoning and appear to be orthoclase.

Flakes of biotite are relatively common and range up to 2 mm. in width. Locally, in the vicinity of Pike Creek, the biotite in the lower flow is distinctly aligned in an aphanitic ground that exhibits irregular flowage by an alternation of pinkish and whitish bands. In this phase, the feldspar is completely altered to a cryptocrystalline aggregate that consists partly of quartz.

Both flows appear to have been erupted from the same vent after the intervention of an insignificant time interval. The lower flow is exposed in the valleys of Pike Creek and Little Alvord Creek with a relatively uniform thickness of about 200 feet. The upper flow shows a similar thickness in Little Alvord Creek valley, but increases gradually to the south, reaching a maximum between Pike Creek and Indian Creek, where presumably at the center of its vent, it forms exposures over 500 feet in height. In this region, the two flows appear to have merged into one.

In the main Pike Creek section (fig. 30), the lower biotite-dacite shows, above the upper tuffs, a basal perlitic phase about 50 feet in thickness. Adjacent to the sediments, the glass is altered to a pale greenish shade, resembling the color of the altered beds with which it is in contact. This lower perlite shows some highly inclined bands of spherulites. Above it, the massive lava shows almost vertical flow structure, which is very marked in the coarse perlitic spines at the upper surface. The lower flow of dacite is here close to 300 feet in thickness, although it decreases greatly within 100 yards to the west. These facts suggest that it is the locus of extrusion, but a crosscutting relationship is not visible. This locality is at the margin of the vent, from which the great overlying mass of the upper dacite was largely extruded, and its peculiar characteristics may be due to an earlier phase of the same volcanic center.

On the divide between Pike Creek and Little Alvord, the biotite-dacite outcrops persistently and is clearly defined by perlitic facies as two separate flows. In the valley of Little Alvord Creek, at about a mile from the scarp, these exposures end. A few hundred yards to the west, similar rock forms a large outcrop that intersects both horizons and suggests that they merge. The flowage in this exposure is irregular, but predominantly highly inclined. Two interpretations of its relationship appear possible; the exposure might be formed by a vent from which both flows were extruded, or the lower flow being highly viscous might have ended abruptly with a steep margin, over which the upper flow would have advanced and would thus have acquired irregularly inclined jointing. The margin of the thick flow of Little Alvord Creek Rhyolite may also have contributed to this rapid increase in gradient. Although the latter explanation is favored, the exposures are not of sufficient depth to offer conclusive proof.

Between Pike and Indian Creeks, the upper biotite-dacite has been stripped of its overlying rocks, and forms the surface of an irregular broad shoulder which is at least a mile in width. Here the rock is considerably altered and exhibits a minute hackly jointing, which causes it to erode in this interfluvial region like a coarsely granular rock. The shape of the wind eroded exposures, however, is largely controlled by highly inclined flow structure which gives rise to steep pinnacles and cliffs (fig. 36). Between the irregular prominences a gently rolling terrain supports a scattered growth of junipers (fig. 37). On the west, the precipitous exposures formed by the overlying basaltic flows rise abruptly over 2000 feet above this relatively level shoulder.

Fig. 36. Vertical jointing in the vent for the upper biotite-dacite north of Toughey Creek. The exposure on the left is approximately 75 feet in height.

Fig. 37. The broad divide between Toughey Creek and Pike Creek exposing the upper biotite-dacite. The basalt forms the precipitous exposures in the background.

The flow structure is very persistent in the eastern part of this broad area, but it is best exposed at the western end of Toughey Creek where it can be traced vertically for about 500 feet. In spite of the depth of erosion, the base of the mass is not exposed. Northward, however, at the margin of the Pike Creek valley, and southward at Indian Creek the lines of flowage are inclined as if emerging from a large vent centered near the head of Toughey Creek. The lava is, therefore, considered to have been extruded from an extensive vent lying beneath the broad divide between Pike and Toughey Creeks, and continuing southward to the northern part of Indian Creek.