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

ALVORD CREEK BEDS

GENERAL CHARACTERISTICS

Light colored tuffaceous sediments outcrop locally at a number of places (fig. 24) in the lower thousand feet of Steens Mountain between Cottonwood and Toughey Creeks. In this distance, of over five miles, there are only a few good exposures and these for the most part are widely separated. The bedding as a rule is well defined and roughly horizontal, invariably testifying to the absence of compressional forces. The isolated exposures, however, could not be correlated, but owing to their close association they are classed as the Alvord Creek Beds.

Fig. 24. Aeroplane view of High Steens west of Alvord Ranch, which is in the foreground on the left. On the right the valley of Alvord Creek exhibits a broad cirque with the characteristic glaciated tributaries visible as snow fields just below the crest. The light colored exposures at the base of the mountain are formed by the Alvord Creek beds. These beds on the northern side of the valley may be seen to dip southward. The dark outcrops at the top of the south fork locality are caused by basaltic and andesitic intrusions.

In general, the sediments consist of stratified acidic tuffs, which are predominantly white in color. Owing to the subsequent volcanic activity, these sediments have suffered considerable alteration, which has affected both their color and consolidation. Greenish and brownish varieties are relatively common. The greenish coloration is definitely due to localized alteration, while the brownish shades, at least in one exposure, appear to have an exotic origin. In consequence, in the field, the color does not form a satisfactory criterion for correlation. In two localities, a content of fossil leaves indicates a similar age, but not necessarily the same horizon. Some exposures show small conglomeratic facies composed largely of subangular pebbles of acidic lavas, while others show beds of a very fine shale. These phases are too local in their distribution to permit the correlation of isolated exposures even if the same stratigraphic horizon were exposed. Aside from the local indication of water classification, the coarseness of the material probably depended chiefly on the proximity of the locality to a volcanic vent. In consequence, the size of fragments may have varied rapidly in distribution. It is possible that a correlation might be attained by detailed petrographic methods.

NORTH OF ALVORD CREEK

The greatest exposure (fig. 25) occurs on the main scarp a few hundred yards north of the valley of Alvord Creek. Here well-stratified beds dip southward at about 7° The lowest beds are of brownish tuffs that are exposed continuously for a height of approximately 500 feet. For the most part they are too badly altered to permit the survival of any relict structure. In some fragments, however, the alignment of a few minute feldspathic laths indicates an original trachytic texture and suggests a lava of intermediate composition. At about 200 feet above the base of the exposure, the tuff is relatively coarse and exhibits dark pumiceous fragments, which are usually not over 1.5 cm. in diameter. Stratification is locally apparent, but as a rule it is rather indistinct. An impregnation with zeolitic material is common. The lower part of this exposure shows a number of displacements. Although the lithologic homogeneity renders it difficult to be sure of the extent of movement, the distance in any instance did not appear to be more than a few feet. This lower brownish facies grades upwards into buff colored stratified tuffs about 50 feet in thickness. These in turn are overlain by about 200 feet of whitish tuffaceous sediments, which contain horizons of opalized shale rich in fossil leaves.

Fig. 25. The main exposures of the Alvord Creek beds north of Alvord Creek. A flow of basic andesite about 100 feet in thickness is interbedded in the upper part of the series. The fossil beds are a little over 100 feet below its base. The resistant horizons are due to local opalization.

Capping these beds is a flow of rather basic andesite close to 100 feet in thickness. This lava is overlain by less than 100 feet of white tuffs, which are poorly exposed beneath a second far greater andesitic flow of similar composition. This upper flow can be traced northward as a horizontal stratigraphic unit for over a mile to Cottonwood Creek. There it may be seen to be still underlain by well-stratified white tuffs about 100 feet in thickness. As in the section to the south, these sediments cap a lower andesitic flow, which is here cut by a vertical fault roughly parallel to the scarp. The eastern block, consisting of an acidic agglomerate several hundred feet in thickness, has been upthrown, thus hiding the base of the lower andesite.

Judging from these exposures and from the similarity between these two adjacent sections, the andesite that caps the upper tuffs is at least locally conformable. To the south, however, north of Alvord Creek, the tuffaceous sediments dip southward at about 7°. In this locality, the relationship of the capping andesite cannot be accurately determined; but it appears to lack this deformation.

SOUTH FORK OF ALVORD CREEK

About a half a mile farther to the south, on the northern side of the south fork of Alvord Creek (fig. 26), there are good exposures of stratified tuffs, which at least superficially resemble the section studied to the north. These tuffs are cut by small elliptical andesitic necks of dark glassy andesite, and by basaltic dikes, one of which is 75 feet in width. The induration from this dike has resulted in the formation of a cliff at the top of the exposure.

Fig. 26. Looking northward at the Alvord Creek beds north of the south fork of Alvord Creek. The cliff on the right is formed of tuffaceous sediments indurated by a 75 foot basaltic dike, which is visible in the center of the view. The fossil locality is about 50 feet below the base of this cliff near its southern end.

The beds, which are roughly horizontal, show a thickness of about 450 feet without exposing either the top or the base of the section. Again the lowermost exposures are of a brownish color (fig. 27), but this phase is eroded to a depth of only about 50 feet. Higher in the section the tuffs are very light colored. In this zone, about 30 feet below the base of the previously mentioned cliff, a partially opalized horizon of shale was found to be rich in fossil leaves. The uppermost phase of the section is rather agglomeratic. The top is close to 200 feet below the massive andesite, which overlies the two previously mentioned andesitic flows to the north of Alvord Creek.

Fig. 27. The lowermost beds to the north of the south fork of Alvord Creek. The undisturbed stratification furnishes strong testimony against compressional faulting.

NORTH OF LITTLE ALVORD CREEK

It is also impossible to determine the stratigraphic relationship of the previously mentioned tuffs to those exposed on the scarp southward as far as Little Alvord Creek. In this locality, similar tuffs have been intruded by a basic sill approximately 200 feet in thickness and subsequently uparched by an intrusive mass of rhyolite, which outcrops at the base of the scarp for several hundred yards. Although its base is not exposed, this rhyolite is presumably in the form of a laccolith.

Judging from the inclination both of the contact and of the overlying beds, the exposure is formed by the eastern margin of the intrusive. On the northern limb near the contact, the beds have a strike of N. 30° W. and dip approximately 11° N.E. Adjacent to the southern contact the structure shows a southerly dip of about equal magnitude, but the exposures do not permit an accurate reading. The possible height of this dome is hidden by an elongate acidic vent, which cuts the arch parallel to the scarp at about 1,000 feet above the desert.

For about half a mile to the south, to the valley of Little Alvord Creek, the scarp exhibits a few small outcrops of tuffs, which are either approximately horizontal or dip slightly to the south. Adjacent to the scarp, however, on the northern wall of this valley, the tuffaceous series has a strike of N. 80° E. and dips approximately 15° S. About a quarter of a mile farther to the west, the strike is approximately N. 45° W. and the beds have a slightly greater dip to the southwest. Although no conclusive evidence is available, it is possible that this deformation is due to a southern continuation of the laccolith.

These inclined beds at Little Alvord Creek form the retaining wall for a thick rhyolite flow extruded from a vent which cuts them. Although tuffaceous material associated with this vent is indistinguishable from some facies of the series, the fine stratification of the latter locally demands aqueous deposition and precludes the possibility of a genetic relationship. Judging from the horizontal exposure formed by the chilled basal phase of this flow, it was extruded to the south on a relatively level surface. The absence of lower outcrops, however, makes it impossible to determine whether this surface was formed by non-conformable deposits resting on the tuffaceous series or by the direct continuation of these beds flattening out away from the margin of the laccolith.

In the valley of Pike Creek, nearly 2 miles to the south, there are exposed beneath the stratigraphic continuation of this flow over 400 feet of stratified tuffs and agglomerates with a lower 200 foot flow of rhyolite interbedded in them. The base of these beds is not exposed. Although they may well correlate with the uppermost Alvord Creek Beds, the relationship at the best is only hypothetical.

FOSSIL CONTENT

North of Alvord Creek the fossil leaves were found in partially opalized thin bedded shales. This horizon, which is about four feet in thickness, occurs approximately 100 feet below the interbedded flow of basic andesite. Owing to its hackly fracture, the highly opalized rock, which is confined to a layer about six inches thick, is not very productive. Fortunately the weathered phase of the upper and lower margins, which were possibly less silicified, split readily.

The other locality was found near the southern margin of the main exposure north of the south fork of Alvord Creek. It is about 50 feet below a wall formed by the induration of the great basaltic dike, which cuts about N. 10° E. Here the fossils were again confined to an opalized thin bedded shale. The specimens were more satisfactory owing to the fact that the horizon is exposed in a shallow prospect hole.

After a preliminary examination, Dr. Ralph W. Chaney considered these fossils to demand a direct correlation of the beds with the Mascall Formation,² which is exposed in the John Day Valley above the Columbia River Basalt. In submitting the following list of the fossils (table 1) he stated,³ "There are a number of specimens represented by incomplete material or at present unknown to me which will increase the list somewhat. But it seems unlikely that the resemblance to the Mascall will be greatly affected by such additions. In a list of this length the percentage figures have no great importance, but I have put them in for what they are worth. Rosa hilliae is a typical Florissant species, and is commonly considered to be a Miocene species, although I have found it to be in the upper Oligocene as well."

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²J. C. Merriam, "A Contribution to the Geology of The John Day Valley," Univ. Calif. Publ., Bull. Dept. Geol. Sci., vol. 2, no. 9, 1901. R. W. Chancy, "The Mascall Flora; its Distribution and Climatic Relation," Carnegie Inst. Washington Publ., no. 349, 1925.

³R. W. Chaney, personal communication.

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TABLE I

About five miles east of the southern end of Alvord Desert there are some well-stratified tuffaceous deposits rich in fossil leaves and diatomite. These beds, which are known as the Trout Creek Formation,⁴ have been considered to be of Eocene age and to occur at the base of the volcanic series.⁵ Vertebrate remains, however, recently collected by Stock "indicated quite clearly that the beds were not earlier than the Middle Miocene."⁶ Although this evidence permits the Trout Creek Formation to be tentatively correlated with the Alvord Creek Beds, its stratigraphic relation to the adjacent flows suggests a far later origin. No definite statement can be made, however, for the flows observed in association with it have no resemblance to any members of the Steens Mountain series exposed on the opposite side of the desert.

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⁴W. D. Smith, op. cit., p. 206.

⁵Gerald A. Waring, "Geology and Water Resources of the Harney Basin Region," U.S. Geol. Sur., Water Supply Paper 231, p. 20, 1909.

⁶Chester Stock, personal communication.

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