Geological Survey Bulletin 1161-D Geologic Reconnaisance of the Antelope-Ashwood Area, North-Central Oregon

The lower and middle Tertiary rocks of the Antelope-Ashwood area are gently folded and broken along faults that in general trend northeastward.

Two broad poorly defined major anticlines cross the area; one extends diagonally northeastward from the exposed pre-Tertiary rocks near Hay Creek Ranch to the ridge crest west of Clarno, and the other, which is not shown on the geologic map, trends almost due east across the north edge of the area. In general the beds dip 10° or less on the flanks of these major folds, but they dip more steeply, averaging 15° to 25° in a belt extending north-northeastward along the east side of lower Hay and Trout Creeks. The beds dip as steeply as 75° on the west side of the area of exposed pre-Tertiary rocks near Hay Creek Ranch. Dips are reversed across several minor folds, most of which also trend northeastward.

The rocks on the western limb of the major northeastward-trending anticline are broken along a series of faults, most of which trend northeastward. The faults dip steeply westward, and the west side of most faults has been displaced upward relative to the east side—antithetic to the displacement by folding. A particularly well marked line of faults lies about 1 mile west of the Priday agate deposit (pl. 1). Near the deposit, beds of the John Day Formation west of the fault are displaced upward relative to those to the east (section A—A, p1. 1), with a maximum throw of about 1,000 feet. Farther north, Columbia River Basalt and beds of the John Day Formation that lie west of the fault are displaced downward relative to those to the east; on the canyon wall north of Trout Creek, most of member F and all of members G, H, and I are cut out along the fault, indicating a stratigraphic throw of about 1,500 feet.

Beds of the Dalles Formation and younger rocks do not appear to be folded or faulted in the mapped area. The latest recorded deformation in the Antelope-Ashwood area thus took place during the late Miocene.