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

Rocks exposed near Antelope and Ashwood range in age from pre-Tertiary to Quaternary, but most of the area mapped (pl. 1) is underlain by lower and middle Tertiary volcanic rocks of rhyolitic andesitic, and basaltic composition. The stratigraphy of the mapped area is summarized in table 1.

PRE-TERTIARY ROCKS

Slate and less abundant graywacke, chert-granule conglomerate, and meta-andesite underlie the Eocene Clarno Formation southeast of Hay Creek Ranch (pl. 1). The rocks dip mostly 15° to 30° to the southeast and strike N. 30°-60° E. over an outcrop width of 2 miles, suggesting a thickness of more than 5,000 feet. No discordance between cleavage and bedding was observed.

The slate is dark gray, spangled on some cleavage surfaces with fine flakes of mica. Thin interbeds of fine- to medium-grained graywacke, locally cut by thin discontinuous quartz veins, make up less than 10 percent of the unit. Beds of chert-granule conglomerate are rare. An intercalated flow (?) of microporphyritic meta-andesite was noted at the forks of an intermittent stream in sec. 25, T. 11 S., R. 15 E.; the andesite has been altered to an aggregate of oligoclase, a carbonate mineral, clinozoisite, quartz, and sericite.

The age of the pre-Tertiary rocks is unknown. A few small poorly preserved clamlike fossils were collected from thinly interbedded slate and graywacke in sec. 30, T. 11 S., R. 16 E., but are not identifiable (D. L. Jones, oral communication, 1959).

TABLE 1.—Bedded rocks of the Antelope-Ashwood area, north-central Oregon
[Thickness: Nd, not determined]

CLARNO FORMATION

Volcanic rocks of the Eocene Clarno Formation unconformably overlie the pre-Tertiary rocks southeast of Hay Creek Ranch and crop out over most of the southeastern half of the mapped area. In a well-exposed section south of Ashwood along Trout Creek, the Clarno consists of lava flows and coarse volcanic breccia of porphyritic pyroxene andesite, less abundant tuff and tuffaceous mudstone, and sparse rhyolite. Most of the flows and breccia are altered, displaying a greenish cast on fresh surfaces. There is a thin layer of reddish-brown saprolite at the top of the Clarno, providing a distinctive marker horizon, and layers of saprolite are also locally interbedded in the upper part of the formation. In a few areas, such as sec. 3, T. 12 S., R. 15 E., relatively unaltered columnar-jointed flows of dark-gray porphyritic augite andesite occur at the top of the formation; these were included with the Clarno Formation in the mapping, but further study may show them to be younger. In sec. 21, T. 11 S., R. 16 E., a narrow intracanyon flow of this andesite underlies a welded rhyolite ash flow of the John Day Formation and overlies a lens of cobbly conglomerate; the conglomerate in turn overlies altered andesite of the Clarno Formation.

The Clarno Formation was not studied in any detail during the present investigation but has been described by Waters and others (1951, p. 111-115) from exposures in the nearby Horse Heaven mining district (pl. 1). There the Clarno is more than 5,800 feet thick and is divisible into four mappable units as follows (Waters and others, 1951, p. 112-114):

Unit 1.—A basal unit, 600 feet thick, of platy andesite flows interbedded with layers of varicolored clay.

Unit 2.—An overlying unit, 1,350 feet thick, of tuff, bedded tuffaceous clay, volcanic mudflow deposits, and a few thin lava flows, all of andesitic composition.

Unit 3.—About 1,750 feet of tuffaceous clay containing a few beds of coarse tuff and few andesite flows.

Unit 4.—White tuff, largely rhyolitic in composition, that is at least 2,100 feet thick.

The petrography of the andesite flows and breccia of the Clarno Formation has been described by Calkins (1902, p. 122-141) and by Waters and others (1951, p. 114-115). The flows are chiefly pilotaxitic and hyalopilitic augite andesite containing zoned phenocrysts of plagioclase slightly more sodic than An₅₀. Typically the andesites are altered, with the formation of iddingsite, zeolite, chlorite, carbonate, serpentine, clay minerals, and hematite.

The Clarno Formation has been dated as Eocene on the basis of fossil plants collected east of the Antelope-Ashwood area at localities on Cherry Creek, Current Creek, and near Clarno (Knowlton, 1902, p. 102-103; Arnold, 1952, p. 68-72; Scott, 1954). Vertebrate fossils also have been collected near Clarno in recent years but have not as yet been reported on.