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

STEENS MOUNTAIN ANDESITIC SERIES

BASIC ANDESITE

To the north of Alvord Creek, as previously mentioned, a flow of basic andesite, about 100 feet in thickness, is interbedded near the top of the Alvord Creek Beds. Above the uppermost tuffs, there is another flow of similar andesite (table VI), which is well exposed in both forks of Cottonwood Creek (fig. 47). This flow can be traced a short distance southward and then found again with similar relationship about another mile to the south on the northern side of Alvord Creek (fig. 46). Here it is exposed for several hundred yards. In the valley of Cottonwood Creek, the flow is over 200 feet thick, but to the south, it appears to be less than half that figure, although the base is not exposed.

TABLE VI

Chemically the two flows are almost identical. Petrographically the resemblance is equally strong. The rock, in both cases, is a dark grey aphanitic andesite that resembles megascopically a chilled basalt. In thin section, they both show a seriate development of andesine laths exhibiting marked flow alignment. In most specimens, the feldspar forms about 60 per cent of the rock. The size of the laths, in the lower flow especially, varies considerably. The largest observed is about 3 mm. in length, although the average in both flows is close to .2 mm. One specimen of the lower flow contains isolated grains of ophitic augite with individuals up to .8 mm. in length. The maximum dimension of this mafic as a rule is transverse to the alignment of the laths, which it encloses. In another specimen of the same flow, there are minute intersertal grains of a mafic that is presumably augite. In addition, a brownish glass and an orange colored deuteric residual form a common minor constituent. In the upper flow, the ground is a dark, rather opaque substance that is considered to be formed of partially decomposed glass. Small grains of magnetite are distributed throughout the rock.

The upper of these two flows is capped by a thin bed of coarse andesitic tuffs, These deposits, which locally show well-defined stratification, vary from a thick ness of about 20 feet in Cottonwood Creek to less than half that figure in the exposures on the northern side of Alvord Creek valley. In this locality, they exhibit many peculiarities that are difficult to explain conclusively. They rest on the vesicular flow breccia of the underlying andesite. At their base, they grade into similar tuffaceous material that is interstitial to these coarser blocks. Examination proves that these tuffs, at least in part, have been formed by the comminution of the margins of these extremely fragile vesicular andesitic blocks (fig. 38). In numerous instances, this interstitial tuffaceous material shows marked stratification roughly parallel to the irregular surface of the adjacent block.

Fig. 38. Interstitial tuffaceous material in the breccia capping the upper flow of basic andesite north of Alvord Creek.

In this same locality, several small lava domes project above the flow and rise a few feet higher than the erosional surface of the tuffs (fig. 39). The beds, although truncated, curve upwards at the steep contacts of the domes and suggest that they have been intruded by the viscous lava, on which they rest. On closer examination, however, the relation may be seen to be definitely due to deposition on an irregular surface. The steep contacts may be explained by the fact that the viscosity of the lava, which intruded its flow breccia, resulted in a slope greater than the angle of repose of the pyroclastics. The inclination may subsequently have been augmented by compacting, which would have increased in amount with the thickness of the bed. The perfection of the bedding locally demands aqueous agencies, but much of the stratification may be attributed to variations in the ejectamenta. The transition from the tuffaceous deposits to the interstitial material in the flow breccia may be due to the initiation of deposition while the flow was still advancing.

Fig. 39. A local irregularity in the contact of the tuffaceous beds overlying the upper flow of basic andesite. To the left, the surface of the flow also protrudes above the tuffs.