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

THE EASTERN SCARP OF STEENS MOUNTAIN

STRUCTURE

At the eastern scarp of Southern Steens the displacement has occurred in two main faults that converge towards the north. The southern continuation of the eastern one, which trends roughly north-south, defines Pueblo Mountain on the east. The resulting scarp varies greatly in height, depending largely on the differential resistance of the rocks exposed. The western fault trending in general N. 20° E. forms a more persistent topographic feature exposing a relatively uniform series of flows. Below this scarp an irregular broad shoulder, which has been extensively faulted, shows minor exposures of various volcanic rocks complicated locally by vent characteristics. No conclusive evidence was obtained on the stratigraphic relation of these lavas to the main series. It is possible that they represent a graben filling which was extruded during an early stage of the faulting.

The major faults defining this shoulder converge to form a single steep scarp which continues approximately due northward and merges into the western wall of Wildhorse Canyon. Here just as it reaches High Steens the southern fault dies. Northward, its place is taken by another scarp offset about four miles to the east (fig. 13). This northern scarp rises from the playa as a thin tilted block, known as Wildhorse Spur. These two scarps were probably once joined by one that survives on the south side of an isolated plateau north of the spur. This plateau forms a slightly depressed erosion remnant of Smith Flat.

Fig. 13. Aeroplane view of the summit of High Steens from the south. On the left the scarp of Southern Steens continues up Wildhorse Canyon, while the main northern scarp is offset about four miles to the east. The isolated plateau in the foreground is a remnant of the northeastern corner of Smith Flat, which has been slightly depressed by the Wildhorse Canyon fault.

West of it, Wildhorse Canyon continues northward for a total distance of about 10 miles. The position of the valley directly in line with the southern scarp must be due to a structural weakness caused by a minor continuation of the fault. Its presence parallel to the eastern scarp renders the southern crest of High Steens extremely serrate.

Owing to erosion and to minor deformation the eastern scarp of the spur is not very well defined, but it trends approximately N. 20° W. The northern continuation of this fault bounds the southern extremity of the High Steens block for about a mile, after which a slight re-entrant marks a change in the predominant direction of the scarp to approximately north-south. Between Pike Creek and Toughey Creek, which lie directly to the south of this point, there are a number of well-defined step faults (fig. 14) showing slickensided surfaces. These individually show a maximum displacement of several hundred feet. Only the most westerly one of these faults can be traced northward, owing to the above mentioned re-entrant. Bearing approximately the same orientation as the eastern margin of the spur, this fault cuts higher on the scarp to the north of Pike Creek. Its throw, however, gradually decreases and it disappears within about half a mile.

Fig. 14. In Toughey Creek a view of the main scarp shows one of the numerous step faults, which occur in this locality.

Similar step faults are relatively common throughout the region. The blocks liberated by this faulting as a rule vary in width from a few feet to several hundred yards. The displacement observed in the individual faults usually has been only a few hundred feet. In the lower scarps, where erosion has been less severe, these subsidiary faults form marked physiographic breaks, which in some instances may be traced for several miles (fig. 16). In the fault zone exposed near the southern end of High Steens however, the actual step fault blocks are either so narrow or so eroded that they have no obvious effect on the topography. These step faults cannot be confused with landslides, for the blocks lack the characteristic reverse rotation. The fault planes, where exposed, show no tendency to flatten. One in fact retains a uniform inclination for a vertical distance of almost 1,000 feet.

This locality was the only one where the actual surface of the faults forming the scarps was observed.³ These fault planes dip eastward at an inclination varying from about 50° to 65° They appear to average about 60° In a few instances inclined slickensided grooves indicate a slight lateral movement. In adjacent planes, however, the direction of the strike slip showed no regularity. Most of the polished surfaces testify to its absence.

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³R. E. Fuller and A. C. Waters, op. cit., fig. 11, p. 223.

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To the north of this point for over 10 miles, the scarp continues approximately on a north-south line. North of Alvord Creek, however, there are a number of low salients in the interfluvial areas. These form irregular benches at about 1,000 feet above the desert (fig. 6). Usually the exposures are not sufficient to permit the interpretation of their relationship to the main scarp. In the valley of Cottonwood Creek, however, a vertical fault, exposed at the western margin of the bench, indicates that the eastern side has been elevated. The extent of displacement was not determined but it must be several hundred feet. In Willow Creek, the next valley to the north, a similar fault shows a minor displacement. These faults are roughly parallel to the main scarp.

In proof of the tensional origin of the structure a few minor faults, exposed at about two miles west of the scarp, are of considerable importance. Although these are approximately parallel to the major fault, some of them dip westward. In the valley of Alvord Creek, the displacement of a dike proves the western block to be downthrown about 200 feet.⁴ To the north, in the valley of Mosquito Creek, two of the smaller faults converge downward, permitting the depression of a wedge-shaped block. The presence of interbedded grey tuffs near the base of the series renders this deformation apparent. This same bed of tuft adjacent to a basaltic vent on the northern margin of the lower cirque shows miniature normal faults dipping westward (fig. 15). The intersection of vertical faults with these normal ones has, in this instance, also permitted the depression of minute wedge-shaped blocks. The displacement is only a matter of inches, but it furnishes additional testimony of tensional forces.

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⁴R. E. Fuller and A. C. Waters, op. cit., fig. 12, p. 224.

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Fig. 15. View of the miniature faults exposed in the bedded tuffaceous sediments on the northern side of the lower cirque in Mosquito Creek. The inclined normal faults dip westward towards the main mass of the mountain. Some of the vertical fractures show a slight displacement, which permits the depression of miniature wedges.

Although individual exposures on High Steens are very precipitous, the scarp as a whole shows a slope close to 20°. On the other hand, the basaltic series at Bluejoint Rim, in the northern portion of Warner Valley, has preserved locally a slope of almost 70°, if one disregards the insignificant accumulation of talus at the base. There is no proof, however, that this surface represents the uneroded foot-wall of the fault. The extreme inclination probably is due in part, both to the vertical jointing of the rock, and to the erosion caused by the former presence of Bluejoint Lake. Possibly the sapping may have been increased by the presence of underlying tuffaceous beds now unexposed.

Elsewhere in the Great Basin the scarps show a relatively uniform slope, usually of about 30°, although the actual faults, as on Steens Mountain, have averaged close to 60°.⁵ In southeastern Oregon, however, the slope of the scarps varies greatly. This is probably due in part to the intensity of subsequent erosion at different elevations, but chiefly to the differential resistance of the various volcanic rocks, which are exposed.

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⁵W. M. Davis, "The Basin Range Problem," Nat. Acad. Sci. Proc., p. 389, 1925. J. Gilluly, "Basin Range Faulting along the Oquirrh Range, Utah," Geol. Soc. America Bull., vol. XXXIX, p. 1113, 1928. A. C. Lawson, "Recent Fault Scarps at Genoa, Nevada," Seismological Soc. America Bull., vol. II, pp. 193-200, 1912. I. J. Pack, "New Discoveries relating to the Wasatch Fault," American Jour. Sci., vol. XI, pp. 398-410, 1929.

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Although the scarps in the northern part of the Great Basin have a predominant north-south trend, many of them show marked local irregularities. Some show broad curves, which even define circular fault basins (fig. 16), while others show sharp changes that produce a marked zigzag course. These changes in direction generally are accompanied by subsidiary faults. In spite of these minor faults, the scarps as a rule are continuous. A good example of this type may be observed at the northern end of High Steens (fig. 17), where the scarp curves sharply eastward to about N. 30° E. This curve is attained by the formation of branching faults. The main fault may be observed to continue northward with diminishing displacement, while the eastern branch develops the main scarp, until another branch trending slightly farther to the east takes its place. These faults also permit the downthrow of the northern block. The major displacement, however, appears to have taken place in a fault that branches to the west from the main scarp at the southern end of the curve. This normal fault with a displacement of many hundred feet lies about N. 25° W.

Fig. 16. Aeroplane view from the south of the circular fault basin known as Upper Alvord. The playa is about two miles in diameter. In the distance, to the left of the center, the scarp of Northern Steens shows the offset with the formation of the step fault (fig. 20). To the east, the minor blocks parallel to the main fault are clearly defined.

Fig. 17. Aeroplane view of the northern end of High Steens from the east. The curvature in the scarp, attained by the branching faults, occurs at the junction with the lower block known as Northern Steens.

To the north of this curve, the eastern scarp of Northern Steens continues for nearly 30 miles, gradually diminishing in elevation from approximately 3,000 feet to a few hundred. The most marked irregularity occurs about 15 miles to the north of High Steens at Stone House Creek. Here the scarp is offset over a quarter of a mile to the east. The eastern scarp emerges from the valley alluvium as a tilted block plunging southward at about 7° (fig. 18). Traced northward it flattens within the distance of a mile to form a well-defined step fault block (fig. 16) several hundred yards in width, with a surface about 300 feet below the local crest of the mountain. Gradually increasing in elevation, this thin subsidiary block continues for several miles until truncated by a transverse scarp. Northward the surface of this step fault block widens from less than a quarter of a mile to probably close to a mile, due to the fact that the orientation of the high scarp is approximately N. 20° E., while that of the eastern scarp is about N. 30° E.

Fig. 18. View of the scarp of Northern Steens about ten miles north of the Mann Lake Ranch. The southern end of the inclined step fault block is clearly defined. This block becomes horizontal after attaining an elevation of about 1000 feet.

This step fault block dips westward at about 3° which corresponds to the inclination of the main mountain mass. At the southern end of this block, there is a longitudinal depression on its surface adjacent to the northern continuation of the main scarp. This depression is less than 100 feet in depth, and is bounded on the east by a sharp escarpment which slightly suggests another fault. It is more probable, however, that the basin is due to erosion originating in a manner similar to Wildhorse Canyon. The depression has been blocked to the south by a landslide so that it now forms a small lake basin filled with sediments (fig. 19).

Fig. 19. Looking southward on the top of the above mentioned step fault block. The formation of the playa lake is attributed to the damming of an erosional valley by a landslide.

To the north the eastern scarp of Northern Steens retains a remarkably straight orientation, at about N. 30° E., and an approximately even elevation of the crest. At intervals, however, it is cut by branching faults trending northward. The pronounced modification of Northern Steens by these faults has previously been mentioned.

On the scarp of Northern Steens, the effect of drag is still locally visible. This effect is apparent from the rapid change in the inclination of the beds on approaching the scarp. From a slight dip away from the fault plane they curve until almost parallel to the present slope. As a rule this indication of drag survives only on the lower scarps where erosion has been less severe. It is also clearly defined on the western margin of Southern Steens and east of Alvord Desert. At the base of the scarp of High Steens there are some isolated exposures showing highly inclined structure which is also attributed to drag.

Opposite to the southern end of Northern Steens a number of isolated fault blocks rise above the playa fiats (fig. 20). The dip of these blocks is very varied. To the east of them, lies the circular fault basin referred to as Upper Alvord although actually its floor is slightly lower than the playa to the south. To the north, however, the structure is dominantly parallel to that of Northern Steens, and two blocks dipping to the northwest define elongate alluviated valleys (fig. 16). These blocks, which are also cut by transverse faults, lie in the center of a graben bounded on the east by a high scarp paralleling that of Northern Steens.

Fig. 20. Isolated fault blocks at the northern end of Alvord Desert viewed from the south. A small recent fault cuts their eastern scarp. Note the encroachment of vegetation at the margin of the playa.