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

EARLY INTRUSIONS IN THE ALVORD CREEK BEDS

BASALTIC SILL

About a mile to the north of the mouth of Little Alvord Creek, a large basaltic sill is exposed where the Alvord Creek Beds are uparched by the rhyolitic laccolith. Viewed from the desert, its brownish outcrops cause the domed structure to be apparent. The basic intrusive, which is over 200 feet in thickness, is separated from the rhyolite by only about 100 feet of tuffaceous sediments. Above the laccolith, this basalt has suffered such marked alteration that from a megascopic examination it is almost indistinguishable from a basic tuff. In fact, to the north where the alteration is less extreme and the upper portion of the intrusion is relatively fresh, it required very careful observation to determine even the approximate contact with the underlying tuffs. This greenish brown contact zone has been partially silicified and impregnated with calcite.

The upper contact of this igneous mass is best exposed in the extremely altered zone above the acidic intrusion. Here the determination of its position and relationship also requires minute examination, for the stratified beds for the next 25 feet above are of approximately the same light brownish color. The upper part of the basalt is distinguishable from its high content of minute vesicles averaging about 1 mm. in diameter. These are filled with a dark powdery substance which appears to be formed of decomposed chlorophaeite. The actual contact is demarked by a thin layer of soft black material which is considered to be devitrified tachylite. This contact fluctuates slightly without causing any appreciable disturbance in the overlying beds.

A thin section of the less altered rock, from one of the most northerly exposures about 50 feet above the base, shows the sill to be formed of a feldspathic basalt. About 50 per cent of the rock is composed of roughly aligned labradorite laths which show a seriate development ranging from .1 mm. to 2 mm. in length.

The rock also contains a few grains of augite, which are usually about .5 mm. in diameter. The ground is formed of a brownish white semi-opaque substance, which shows locally a faint suggestion of spherulitic structure, due possibly to an incipient development of variolites in a glass. Throughout this ground there are irregular patches of yellowish brown chlorophaeite, which locally exhibits minute fibres of indeterminable birefringence. The formation of this basic magmatic residual depends largely on the retention of volatiles and is, therefore, quite commonly developed in basic intrusives. In this instance, it is not derived from the decomposition of the pyroxene, for the latter is unaltered. This colloidal residual, which in the past has been confused with palagonite,⁷ characteristically occurs both as thin impregnations and as rounded or amoebaform segregations.⁸

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⁷L.L. Fermor, "On the Composition and Nomenclature of Chlorophaeite and Palagonite, and on the Chlorophaeite Series," Rec. Geol. Surv. India, vol. LX, pp. 411-430, 1930. Martin A. Peacock, "The Distinction between Chlorophaeite and Palagonite," Geol. Mag., vol. LXVII, pp. 170-178, 1930.

⁸Robert Campbell and James W. Lunn, "Chlorophaeite in the Dolerites (tholeiites) of Dalmahoy and Kaimes Hills, Edinburgh," Min. Mag., vol. XX, pp. 435-440, 1925. "The Tholeiites and Dolerites of the Dalmahoy Syncline," Trans. Roy. Soc. Edinburgh, vol. LV, pp. 496-500, 1927. M. A. Peacock and R. E. Fuller, "Chlorophaeite, Sideromelane and Palagonite from the Columbia River Plateau," Am. Mineralogist, vol. 13, pp. 361-369, 1928. D. N. Wadia, "Palagonite-bearing Dolerite from Nagpur: Suggestion regarding the Nature and Origin of Palagonite," Rec. Geol. Surv. India, vol. LVII, pp. 338-343, 1925.

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RHYOLITIC LACCOLITH

The acidic intrusion, which is responsible for the local deformation of the Alvord Creek Beds and the alteration of the previously mentioned sill, is exposed at about a mile north of Little Alvord Creek. At the base of the scarp for several hundred yards there are scattered outcrops of light grey rhyolite, which exhibits marked cavernous weathering because of the erosion of locally kaolinized areas.

In the more resistant rock, phenocrysts of quartz and dull white feldspar are easily distinguished in a felsitic ground that is usually kaolinized to a whitish grey. The quartz, which composes about 20 per cent of the rock, attains a maximum diameter of about 4 mm., although the grains average less than half that size. In thin section, many of the individuals show a good crystal outline, which is locally modified by numerous sinuous embayments. The feldspar phenocrysts that constitute probably an equal percentage, are as an average decidedly larger. They were invariably found to be highly altered, but judging from their remnants they appear to consist wholly of orthoclase. The rock also contains a number of angular inclusions of a kaolinized iron stained material that is indeterminable but probably represents fragments of the intruded tuffs.

At the northern margin of the intrusive, the adjacent tuffs have been highly brecciated and altered to a dense pale greenish rock. At about 200 feet above the basal exposures, the contact of the intrusion with the white tuffs may still be observed curving upwards into the hill, as if forming a small segment of a dome. The actual contact is very sharp, but the adjacent rhyolite contains small fragments of indurated tuffs. Most of these fragments are angular and do not suggest resorption. The massive tuffs near the intrusion are highly shattered and locally injected by small apophyses a few inches wide and extending for merely a few feet.

About 50 feet higher on the slope, the rhyolite is again exposed, possibly in some measure on account of a small fault, but more probably it is caused chiefly by the slope of the contact being greater than that of the scarp. At these uppermost exposures, the intrusive is greatly kaolinized and, in part, streaked with bands formed of decomposed white spherulites. Near the contact, massive white quartz occurs in a few irregular veins, which are 2 or 3 inches in thickness.

Just below this uppermost exposure of the intrusive, a well slickensided surface in the tuffs marks a vertical fault trending roughly N. 70° E. From an examination of this surface, the side towards the center of the laccolith was found to form the downthrow. The displacement was undetermined, but apparently small. Possibly only by chance its direction almost corresponds with that of some highly silicified lines of brecciation that cut the overlying series. These lines are approximately N. 60° E. Owing to differential erosion, these zones of silicification, on cutting the badly decomposed basalt, outcrop as resistant dikelike masses, one to two feet thick. The orientation of these lines of fracture furnishes an additional suggestion that the major axis of the laccolith continues to the southwest, thereby causing the deformation observed in the valley of Little Alvord Creek.

ELONGATE ACIDIC VENT

Above the sill, at the center of the laccolith, stratified tuffs exhibit a thickness of about 250 feet. Their exposure ceases with the outcrop of a light grey rhyolite which superficially suggests a flow, for it can be traced horizontally to the north for possibly 100 yards before it is covered by a landslide. To the south, however, the rhyolite grades into a perlitic phase which has intruded the tuffs and locally has upturned the uppermost beds.

About 50 feet above this intrusive contact more perlite is exposed, apparently as the northern margin of a well-defined vent that forms an elongate exposure continuing southward for over a quarter of a mile. At its northern end this volcanic neck is marked by a series of exposures trending approximately east and west. Traced downwards they swing southward as if defining a curving margin.

To the south of the perlitic margin and immediately parallel to it there is a wall-like exposure about 50 feet wide, which weathers out as a series of thin partitions about one to four inches in width. These partitions are covered with rounded excrescences, which are the surface expression of a solid mosaic of conflicting spherulites, ranging approximately from 1 to 10 cm. in diameter. As a whole, the surface usually attains an irregular reniform structure. Some of the rounded nodes exhibit a crater-like depression in the center, due apparently to the collapse of a hollow spherulite. The easily eroded material between these thin partitions usually is of a soft pale greenish substance, which, where less altered, is found to be formed by the decomposition of perlitic glass.

To the south of this marginal phase there are continuous exposures of a lava that appears identical to the grey rhyolite except for a brick red color and highly inclined flow structure. This phase ends abruptly at a distance of over a quarter of a mile to the south in a steep cliff about 300 feet high (fig. 28). Approximately a hundred yards farther, across a soil covered slope, there is another precipitous exposure at slightly higher elevation. This is formed by the southernmost limit of the great flow of biotite-dacite that caps the series to the south. Although the surface of this flow is comparatively level, the base, showing a fairly fresh perlitic phase, increases rapidly in elevation as if it were abutting on the marginal accumulation of an earlier vent. In consequence, it is considered by the writer to be of later origin.

Fig. 28. The lower thousand feet of the main scarp about a half mile north of Little Alvord Creek. The rhyolite vent forms the large horizontal exposure in the center of the picture. To the left of it is the upper flow of biotite-dacite. The principal outcrops directly beneath the northern part of the vent are attributed to landslides. In the center at the southern margin, inclined tuffaceous beds are visible.

The elongate exposure to the north appears to consist of the elliptical neck of a vent with no obvious extrusive material. The western margin is not exposed, but the outcrops and flowage suggest a trend slightly west of north. The lava shows pronounced near-surface characteristics, and, therefore, cannot have suffered extensive erosion, although its glassy features have been removed. At the time of activity it probably was surrounded by tuffs.

On the shoulder of the scarp below the steep cliff formed by the southern end of this vent there are scattered exposures of a similar lava of the same near-surface type. Immediately above this exposure, there is a re-entrant in the upper escarpment. The position of the lower outcrops is, therefore, attributed to landslide, although the primary irregularity in the steeply inclined lines of flowage renders it difficult to be sure of the reverse rotation of the block.

This volcanic vent and its satellitic intrusion to the north show no indication of deformation and are considered to be of later origin than the laccolith. In spite of their close association, these elongate volcanic necks show no petrographic evidence of the genetic relationship to the underlying intrusion. On the other hand, both the rock forming the rhyolite vent and the upper flow of biotite dacite, with which it is almost in contact on the south, appear to be somewhat similar petrographically, although they are both very variable in their phases. Judging from chemical analyses, however, at least the satellitic intrusion to the north is definitely rhyolitic and distinct from the biotite-dacite. The rhyolite is higher both in silica and potash and decidedly lower in its content of soda, lime, iron, and magnesia (table II).

TABLE II

Petrographically the rhyolite and the dacite both contain oligoclase and orthoclase. In each, the feldspar shows a glomerophorphyritic tendency and usually exhibits many glassy inclusions. In the rhyolite, the phenocrysts as a rule appear to be far more complex, for a number of the larger crystals, which range from 2 to 3 mm. in diameter, contain aggregates of smaller grains. Biotite, although it usually constitutes less than one per cent, is common to both of them, but the flakes in the rhyolite are seldom visible megascopically. They are generally less than 1 mm. in length, while those in the dacite are several times that size. In addition, the felsitic groundmass in the rhyolite is less altered and locally shows well-developed spherulites. In both types the contents of feldspathic phenocrysts varies from about 10 to 20 per cent of the rock.