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

Although the absence of the upper members of the local stratigraphic sequence suggests that Steens Mountain has undergone considerable erosion, the topography is still almost a direct expression of the structure. In general, it consists of a great structural mass dipping gently westward from a high eastern fault scarp, which is continuous for over 50 miles. The Southern and central portion of this scarp lies approximately on a north-south line, but to the north it swings eastward about 30° The mountain can be divided roughly into three parts, of which the most striking feature is an extremely simple high central block. On both the north and south, this is bounded by lower more complex units which will be referred to as Northern and Southern Steens.

NORTHERN STEENS

Northern Steens is bounded on the east by a continuous scarp over 25 miles in length with a predominant trend N. 30° E. (fig. 16). At its southern end it rises to a height of nearly 3,000 feet, but to the north it decreases to less than 1,000 feet. This decrease in elevation is accomplished largely by transverse faults trending roughly east-west. At its northern end the scarp is truncated by an east-west fault depression.

The southern half of Northern Steens is a relatively homogeneous tilted block dipping gently away from the eastern scarp for a distance of about 15 miles. The northern half, however, is far more complex. This region was well described by I. C. Russell¹ in his reconnaissance of 1882 in the following passage:

"A narrow belt of country to the eastward of the northern part of Stein Mountains is extremely rugged and difficult to traverse, owing to the abruptness of the upturned edges of the long, narrow blocks into which it has broken. The fault lines that have determined this topography are branches of the great fault along the eastern base of the main range, and trend approximately north and south."

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¹I. C. Russell, "A Geological Reconnaissance in Southern Oregon," U.S. Geol. Survey, Ann. Rept. 4, p. 439, 1882-1883.

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This tilted fault block structure has, as usual, been accompanied by the sedimentation of the depressed areas. Farther to the west this part of the mountain merges into a region of irregularly tilted small fault blocks.

HIGH STEENS

The central portion, known as High Steens, is structurally the simplest of the three units, although, due to erosion, its topography is locally more complex. For approximately 15 miles its eastern scarp towers about 5,500 feet above the desert (fig. 6), and reaches an elevation of at least 2,000 feet greater than that of Northern or Southern Steens.

Fig. 6. Aeroplane view of High Steens from the northeast. Mann Lake Ranch lies in the foreground. The scarp of Southern Steens and part of Pueblo Mountains may be seen on the extreme left.

The crest is formed by the eastern margin of a homoclinal block (fig. 7), which dips westward at about 3° for a distance of over 20 miles, until it reaches the richly alluviated lower valley of the Donner und Blitzen River. Although the block is truncated on the north by several definite fault scarps, this transition to Northern Steens is accompanied by a slight northerly dip. The scarp bounding High Steens (fig. 8) on the southern side is far more pronounced, although it gradually decreases in throw towards the west as the elevation of the northern block diminishes.

Fig. 7. Aeroplane view of High Steens from the northwest showing the gently tilted surface of the block. The scarp in the middle foreground is formed by one of the east-west faults that bounds the lower block of Northern Steens. Kieger Canyon is visible on the left.

Fig. 8. Aeroplane view of the scarp bounding the High Steens block on the south taken at a distance of several miles west of the summit. The more level block in the foreground is that portion of Southern Steens known as Smith Flat.