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

SOUTHERN STEENS AND ITS RELATION TO PUEBLO MOUNTAIN

Southern Steens is a true horst, bounded on the western side by another well defined fault (fig. 9) which continues southward to the northern end of Pueblo Mountain (fig. 10). At the northern end, this scarp has an orientation about N. 30° W., but traced southward, it swings toward the west in a fairly even curve, which increases more sharply near the southern extremity until it reaches approximately N. 70° E. Where the scarp first appears above the alluvium north of Catlow Valley it is very low, but as its even arc to the south cuts higher on the gently tilted fault block it increases in magnitude to well over 1,000 feet.

Fig. 9. Aeroplane view showing the curving western scarp of Steens rising over 1000 feet above Catlow Valley. The scarp on the right divides Southern Steens. The higher block is Smith Flat.

Fig. 10. Aeroplane view from the north of the curving western scarp at the northern end of the Pueblo Mountain block. This scarp is the southern continuation of the one defining Southern Steens. Lone Mountain is visible in the middle distance. R. E. Fuller and A. C. Waters, op. cit., fig. 6, p. 218.

Southern Steens is divided by a fault trending roughly N. 60° W. into a northern and southern part. The northern division, which is about 10 miles across, is a very level and homogeneous block known as Smith Flat. At its eastern scarp this block dips westward at 2° or 3° but within a few miles to the west, the slope gradually decreases. Towards the western fault there is a reverse dip of about 1°, forming a very shallow, poorly defined sag on the north-south axis. On the north, Smith Flat is bounded by the southern scarp of High Steens. Owing to the slightly greater dip of the latter, the fault gradually increases in throw as it is traced eastward (fig. 8).

The downthrow of the fault dividing Southern Steens is to the south. Several miles south of this scarp the southern division is cut by many subsidiary faults, which are accompanied by the tilting of the small blocks, raising their summits considerably higher than the even slope of Smith's Flat. These blocks progressively decrease in elevation to the west. At the main eastern scarp a rugged crest is formed by a trachyte flow which dips S. 60° W. at about 12°. This inclination closely corresponds to that of the lavas forming the Pueblo Mountain series, which gradually increase in dip to approximately 20° as they continue southward.

Viewed from the north, the stratigraphic sequence and structure of the two mountains appear continuous (fig. 11). Between the southern portion of Steens and the northern continuation of Pueblo Mountain there is, however, a down faulted area known as Broad Valley, forming the only break in the Pueblo-Steens Range, which is nearly 100 miles in length. This depression is due to the presence of a number of minor tilted blocks which lie at low elevation and have a predominant strike of approximately N. 65° W., and a dip to the southwest of about 10° to 15°.

Fig. 11. Aeroplane view from the north of the tilted blocks forming the southernmost limit of Southern Steens. Broad Valley lies in the middle distance to the north of the extensive re-entrant. Still farther south on the extreme left the older metamorphics form the dome-like eastern crest of Pueblo Mountain, while the southern continuation of the great volcanic series is exposed on the right. R. E. Fuller and A. C. Waters, op. cit., fig. 7, p. 219.

To the south of Southern Steens at the eastern end of Broad Valley, an extensive re-entrant in the main scarp is bounded on the west by a high escarpment which exposes the previously mentioned tilted series of lavas. Continuing southward with a gradual increase in elevation, this tilted block forms the western ridge of Pueblo Mountain. The northern part of this ridge is undoubtedly defined on the east by a fault, but the throw must decrease with the elevation of the eastern ridge. To the west the dip of these tilted lavas decreases rapidly until the block is approximately flat adjacent to the western scarp.

The re-entrant which this ridge bounds is filled at least at its northern end with soft sediments whose conglomeratic facies contain the predominant types of lava characteristic of Steens and Pueblo Mountains. With north-south faulting or a continuation of it, the block on which these sediments rest has been tilted to the west at an angle of about 5° raising the eastern end 400 feet or more. These soft, easily eroded deposits still retain a distinct scarp which connects the main eastern fault of Steens with that bounding the metamorphics forming the eastern ridge of Pueblo Mountain. This evidence of recent faulting supports the conclusions which W. M. Davis attained from studies farther to the north.² Similar testimony is also clearly defined at several other localities in this region (fig. 12).

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²W. M. Davis, "Mountain Ranges of the Great Basin," Mus. Comp. Zool. Bull. (Harvard University), vol. XLII, p. 1, 1903-1905.

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Fig. 12. View of the scarp southwest of Upper Alvord Basin showing in the foreground the displacement caused by a recent fault. The partially dissected miniature scarp, which is about six feet in height, can be traced for at least a mile.