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

COLUMBIA RIVER BASALT

Flows of Columbia River Basalt of middle Miocene age, at the southern margin of the Columbia Plateau, cap a prominent south-facing scarp in the northern part of the mapped area (pl. 1), and occur as scattered outliers farther south. The basalt thins southward from a maximum thickness of more than 800 feet as the result of erosion and the lapping out of successively higher flows. The basalt unconformably overlies the John Day Formation in the John Day basin (Merriam, 1901b, p. 299). Presumably it is also unconformable in the Antelope-Ashwood area, but widespread landslides at the contact obscure the relations, and the contact was not studied in sufficient detail to confirm an unconformity.

The basalt occurs as thick columnar-jointed flows of dense, fine-grained dark-gray rock. A thin section of a sample from the lower part of the basal flow in Cow Canyon consists of fine-grained hyaloophitic basalt containing poorly terminated laths of sodic labradorite (1/4 to 1/2 mm in length), granules and stubby prisms of augite, and scattered granules of magnetite in light-brown glass; the glass is crowded with tiny dendrites of magnetite and crystallites of plagioclase and pyroxene.

The Columbia River Basalt is dated as middle Miocene on the basis of lower Miocene vertebrate fossils from the underlying John Day Formation in the Antelope-Ashwood area and the John Day basin, and middle or upper Miocene vertebrate and plant fossils from the overlying Mascall Formation in the John Day basin (Downs, 1956).

DALLES FORMATION

Bedded tuffaceous strata of the Dalles Formation of Pliocene age unconformably overlie the John Day Formation and Columbia River Basalt west of Hay Creek, at the western edge of the mapped area. There the formation ranges from 0 to about 800 feet in thickness and becomes thicker to the west toward the center of a Pliocene basin near Madras. Equivalent strata near Madras were mapped as the Madras Formation by Hodge (1941).

The Dalles consists chiefly of light-colored, thin- to medium-bedded and crossbedded tuffaceous sandstone and siltstone. Coarse conglomerate containing cobbles and boulders of Columbia River Basalt locally lies near the base of the formation (for example, in the draw north of U.S. Highway 97 and east of Lyle Gap, in sec. 24, T. 14 S., R. 9 E.). Probably most of the formation in the mapped area is fluviatile.

The Dalles Formation is dated as Pliocene on the basis of fossil plants, vertebrates, and diatoms collected near Madras and The Dalles (Chaney, 1944).

BASALT OF PLIOCENE OR PLEISTOCENE AGE

Overlying the Dalles and older formations are flows of olivine basalt of Pliocene or Pleistocene age that were extruded from three small shield volcanoes in the southern part of the Antelope-Ashwood area. The most prominent series of flows forms a sinuous flat-topped ridge 12 miles long that extends northwestward from near Hay Creek Ranch; the ridge is cut at Lyle Gap by U.S. Highway 97. These flows, which were extruded from a vent 1-1/2 miles south of Teller Butte, first flowed due west 4-1/2 miles, and then turned northward along the former course of Hay Creek. Another series of flows were erupted from a vent in the southwestern corner of the mapped area, 3 miles northwest of Grizzly; they flowed down the broad valley of Willow Creek to the west and northwest, beyond the edge of the map area. The third series of flows cap a small, high plateau 11 miles east of Hay Creek Ranch. Each series of flows is about 100 feet or less in total thickness; the shield volcanoes from which they were extruded are roughly half a mile in diameter and rise 200 feet above the surrounding lava plain. A small cinder core, shown on plate 1, is located in T. 12 S., R. 15 E., 1 mile southeast of a basalt shield volcano.

The flows are mostly 10 to 30 feet thick, rudely columnar jointed, and have little associated breccia or intercalated tuff and ancient soil. The upper part of the flows are highly vesicular and have a ropy filamented surface coated with iron oxides. The vesicles are small (about one-half inch in diameter), round to elliptical, and have smooth surfaces that are free from coatings for the most part, although some are coated with clay, calcite, and cristobalite. Some of the lava flowed through tubes below the chilled surface of the lava; a filled tube about 5 feet in diameter is marked by a circular vesicular zone near the base of a 20-foot flow at Lyle Gap. The characteristics of the flows compare closely with those of highly fluid pahoehoe described by Wentworth and Macdonald (1953, p. 33-57).

The basalt is medium dark gray, fine grained, and has a porous appearance due to the abundance of minute voids. A network of laths of sodic labradorite about one-half mm long forms about 50 percent of the rock; prisms and granules of augite lie between and partly enclose the laths, making up another 30 percent; brown glass with crystallites totals 10 percent; blades of ilmenite and less abundant grains of magnetite total 5 percent, and subhedral grains of olivine make up the remaining 5 percent. An additional 10 to 20 percent of the rock consists of angular diktytaxitic voids that average three-fourths mm in diameter.

The precise age of the flows is not known. As they overlie the Pliocene Dalles Formation, they must be Pliocene or Quaternary in age. The preservation of the small shield volcanoes suggests a youthful age; since their formation, however, Hay Creek has cut a valley 800 feet deep and has severed the connection between the Hay Creek flows and their vent. On this basis the flows appear to be late Pliocene or early Pleistocene in age.

QUATERNARY LOESS, LANDSLIDE DEBRIS, AND ALLUVIUM

Massive deposits of pale-brown silt and fine sand, as much as 50 feet thick, cap a flat-topped ridge 7 miles north of Ashwood, and thinner unmapped deposits locally overlie Columbia River Basalt near the north edge of the Antelope-Ashwood area. The deposits resemble loess of the Pleistocene Palouse Formation, which is widely distributed in the Columbia Basin to the north, and they are presumed to be of the same age and origin.

Landslides are widespread in the Antelope-Ashwood area, but only a few of them are shown on the reconnaissance geologic map (pl. 1). Many slides have taken place where poorly indurated tuff or sedimentary rocks of the John Day and Dalles formations are capped by resistant layers of dense, competent rock, such as Columbia River Basalt, basalt of Pliocene or Pleistocene age, or welded ash-flow sheets of the John Day. Small slides have occurred also on the clayey saprolite at the base of the John Day. Slides are particularly widespread where the strata dip approximately parallel to the ground surface, as on the hillside west of Clarno.

Alluvial deposits of sand, silt, gravel, and pumice form the floors of some of the stream valleys; alluvium was mapped along the John Day River, Hay Creek, and Trout Creek, but smaller unmapped areas are present along other streams. Stream valleys are wide and alluvial deposits widespread where the streams are eroded in poorly indurated tuff and sedimentary rocks of the John Day and Dalles Formations; the streams have steep-walled canyons with narrow bottoms in more resistant rocks, such as the rhyolite dome complex and welded ash-flow sheets of the John Day.

INTRUSIVE ROCKS

Small plugs and dikes of biotite rhyolite intrude rocks of the Clarno Formation near Ashwood and southeast of Hay Creek Ranch (pl. 1). In the Horse Heaven mining district, Waters and others (1951) have mapped other rhyolite plugs and dikes, as well as plugs of hornblende andesite, augite andesite, and augite-hypersthene andesite. The rhyolite plugs probably are of late Oligocene or early Miocene age and may represent feeders of some of the ash flows in the John Day Formation.

A small elongate plug of trachyandesite intrudes tuff of member A of the John Day Formation 7 miles east of Hay Creek Ranch. Undoubtedly this plug fills the vent that fed at least some of the lithologically similar flows in the lower part of the John Day Formation; hence, the plug is of late Oligocene or early Miocene age.