JOHN DAY FORMATION
Unconformably overlying the Clarno Formation is a sequence predominantly of tuff and welded tuff that is assigned to the John Day Formation of late Oligocene and early Miocene age. Exposures of the formation form a band roughly 8 miles wide that extends from Clarno southwestward across the Antelope-Ashwood area (fig. 1 and pl. 1).
The John Day Formation was named and described from exposures along the John Day River between Clarno and Picture Gorge (Marsh, 1875, p. 52; Merriam, 1901a) where it consists of 1,000 to 2,000 feet of richly fossiliferous, varicolored andesitic to rhyolitic tuff and tuffaceous claystone (Merriam, 1901b, p. 291-303; Calkins, 1902, p. 143-159). These were mostly deposited as ash falls (Coleman, 19491; and Hay 1962a; 1963), or as loess derived from a source area of fresh ash falls (Fisher and Wilcox, 1960). West of the John Day River, the formation contains increasingly abundant lapilli tuff, welded tuff, and rhyolitic domes and flows, as noted previously by Waters (1954), and less abundant flows of trachyandesite (Peck, 1961).
In the area between Willowdale and Ashwood (pl. 1), the formation is about 4,000 feet thick. Strongly to weakly welded rhyolitic ash flows make up about one-quarter of the formation and serve as distinctive mapping horizons; they are intercalated with poorly indurated tuff and lapilli tuffmostly massive ash-fall deposits but including much less abundant bedded water-laid tuff. In the lower part of the formation, local rhyolite flows are derived from a large complex of rhyolite domes. Trachyandesite flows of basaltic appearance occur near the base of the John Day. Although Hodge (1932a) originally assigned the lower and middle parts of these strata to his Clarno Formation, they are assigned to the John Day Formation in this report because they overlie the Clarno unconformably, contain a flora of late Oligocene age at their base (p. 18), and extend eastward into the type John Day Formation.
The John Day Formation has been divided into nine conformable members (columnar section, fig. 2) in the area between Ashwood and Willowdale (pl. 1). As the rocks of these members dip to the west and northwest with few exceptions, progressively younger beds are exposed from east to west. The members are described from oldest to youngest in the following pages.
Member A.The basal member unconformably overlies the Clarno Formation and consists of a 400-foot sequence of tuff and welded tuff. The sequence is well exposed on the county road 2 miles west of Ashwood. At the base is a strongly welded rhyolite ash-flow sheet, as much as 100 feet thick; it is composed chiefly of light-gray stony rhyolite containing sparse lithophysae but includes a 20-foot-thick vitric layer at the base. The rhyolite contains about 5 percent phenocrysts that average about 1 mm in diameter. Quartz and feldspar are equally abundant; biotite and hornblende are rare. The feldspar is chiefly optically monoclinic soda sanidine (()2V=42°-48°; nx=1.524, ny=1.528, nz=1.529±0.0012) but includes less abundant oligoclase, about An15-20 (ny=1.542±.002, ()2V=65°-70°) The ash-flow sheet is overlain by about 100 feet of poorly exposed tuff that has yielded fossil leaves of late Oligocene age (p. 18). A strongly welded rhyolite ash-flow sheet, as much as 200 feet thick, overlies the tuff and forms the upper part of member A. It is composed chiefly of light-gray stony rock containing moderately abundant lithophysae and about 2 percent phenocrysts of oligoclase (about An20). A vitric layer as much as 40 feet thick forms the base of the ash-flow sheet. The chemical analysis of a sample (DLP-58-42) of this layer is listed in table 2, column 1.
TABLE 2.Chemical analyses and norms of rocks from the John Day
Formation near Ashwood, north-central Oregon
Member B.A sequence of trachyandesite flows is well exposed on the county road 2-1/2 miles west of Ashwood. The sequence is as much as 1,500 feet thick east of Teller Butte but thins to the north and south and was not recognized in the Antelope Creek drainage in the northern part of the mapped area, nor in the Willow Creek drainage at the southern edge of the area (pl. 1). The flows are composed of very dark gray aphanitic rock containing sparse small vesicles (1/4 to 1 inch in diameter) and rare phenocrysts of olivine. The flows break into rounded fist-size pieces upon weathering. Thin lenses of ash-fall tuff locally lie between flows. In the trachyandesite, felted laths of plagioclase that average An45 are set in equally abundant pale-brown glass; clinopyroxene (ferriferous pigeonite) and magnetite are less abundant, and olivine is rare. A plug of trachyandesite in sec. 14, T. 11 S., R. 16 E., probably represents the vent from which the flows were extruded. A chemical analysis of the trachyandesite (sample DLP-58-41) is listed in table 2, column 2.
Member C.Rhyolite flows as much as 400 feet thick and a complex of rhyolite domes are exposed in the basins of Wilson Creek, Pony Creek, and part of Trout Creek. The flows are exposed along Wilson Creek, where they occupy about the same stratigraphic position as the trachyandesite flows of member B that crop out farther east; rhyolite may overlie the latter at a locality along the divide between Wilson and Trout Creeks, but the relationships there are not clear because of poor exposures. Rhyolite in the flows is indistinguishable from that in a large complex of domes that cut the Clarno Formation and the lower part of the John Day, forming a crescentic outcrop 7 to 10 miles in diameter north of Ashwood. Rudely columnar jointed rhyolite of the domes forms cliffs (fig. 3) that rise over 1,000 feet above Trout Creek. Rubbly rhyolite breccia interbedded with tuff at the south margin of the dome complex indicates an exogenous origin, and the lithologic similarity of dome rock and flows suggests that the flows were derived from the domes.
Both domes and flows consist of light-gray to pinkish-gray and red massive rhyolite and flow-banded rhyolite, the latter containing contorted layers of contrasting color ranging from 0.1 to 10 mm in thickness. The rhyolite typically contains about 2 percent phenocrysts as much as 2 mm long. These weather to give the rock a spotted appearance. The phenocrysts consist of sodic plagioclase and traces of altered pyroxene and opaque minerals, and lie in a devitrified groundmass of anhedral quartz and alkalic feldspar. A welded rhyolite ash flow is exposed beneath the rhyolite flow at one locality in Wilson Creek, and the chemical analysis of a sample (DLP-58-32) of this ash flow is listed in table 2, column 3.
Member D.A layer as much as 100 feet thick of poorly indurated light-gray to yellow tuff and less abundant lapilli tuff overlies both the rhyolite of member C and the trachyandesite flows of member B. In part of the area this layer is mapped separately as member D, but in places it has been grouped with member E.
Member E.A ledge-forming rhyolite ash-flow sheet, 100 to 400 feet thick, overlies members B, C and D, and is well exposed along the upper course of Pony Creek. The ash-flow sheet is strongly welded and is typically composed of platy layers alternating with thinner massive layers that contain abundant lens-shaped lithophysae. Except for a basal vitric layer, the sheet is composed of light-gray to pale-red stony rock in which shard structure has been largely destroyed by recrystallization and is rarely visible, even with a microscope. Only traces of phenocrysts are present, mostly calcic oligoclase but including less abundant quartz and rare altered pyroxene. The underlying tuff of member D has locally been mapped with member E. The chemical analysis of a sample (DLP-58-50) of the vitric base of the ash flow is listed in table 2, column 4.
Member F.This member consists of 300 to 900 feet of poorly indurated tuff and lapilli tuff and an underlying weakly welded ash flow that is well exposed at the Priday agate deposit south of Pony Butte. The tuff and lapilli tuff are colored in shades of green, yellow, red, and gray; they are mostly massive ash-fall deposits, but include some bedded water-laid tuff. The basal ash flow is 5 to 25 feet thick fairly well indurated, and is light gray speckled with abundant chips and blocks of black glass as much as 4 inches in diameter. In the basal 4 to 6 inches of the ash flow, fragments are progressively flattened downward and the rock is moderately to strongly welded. In many localities this basal welded portion is altered to clay and opal, and contains abundant chalcedony-filled spherulites (thunder-eggs) (see p. 23-25. The angular fragments of glass (n=1.50) in the ash flow are collapsed pumice lapilli in which tubular structures and perlitic cracks are conspicuous. The lapilli lie in a matrix of glass shards and ash that contain a few fragments of hyalopilitic lava and about 1 percent phenocrysts (averaging one-fourth mm in diameter) of calcic oligoclase (An25) and a trace of quartz and altered pyroxene.
Member G.A basal ledge-forming ash-flow sheet, 50-feet thick, and overlying poorly indurated tuff and lapilli tuff, 100 to 400 feet thick, constitute member G. Good exposures of the basal sheet can be found at the crest of the hogback that extends along the east side of the lower valley of Hay Creek. The sheet is composed of pink to purplish-red and very pale orange stony rock containing moderately abundant lithophysae and 10 to 20 percent phenocrysts. The phenocrysts, which consist of crystals and crystal fragments that average one-half mm in diameter, lie in a finely devitrified matrix derived from glass shards and ash; vitroclastic texture is evident microscopically but not in hand specimens. A chemical analysis of a sample of this ash flow sheet was made by Calkins (1902, p. 156) and is listed in table 2, column 5.
Soda sanidine cryptoperthite forms most of the phenocrysts, but less abundant quartz and rare oligoclase (()2V=55° to 60°) are also present. Myrmekitic intergrowths of quartz and sanidine partly rim many sanidine crystals (fig. 4) and occur as separate angular fragments. The sanidine crystals are stubby prisms elongate parallel to the axis. With a very few exceptions the crystals are optically monoclinic, lacking grid twinning and having parallel extinction of the trace of the 010 cleavage on 001 cleavage flakes. Zoning is apparent as a result of variable extinction in crystals cut nearly perpendicular to the acute bisectrix. The optic axial angles of nine sanidine grains, measured conoscopically, range from 39° to 48° and average 43°. nx=1.524-5, ny=1.529-30, nx=1.5301±0.001 in three grains from one sample.3 In X-ray diffraction charts that were obtained from sanidine separated from two samples, the 201 and 111 peaks are compound, indicating a cryptocrystalline nature of the sanidine. Diffraction charts of the same samples after homogenization by heating at 800° C for 6 hours closely resemble Donnay and Donnay's (1952, fig. 3, p. 123) pattern for Na0.61 K0.39AlSi3O8. The position of the 201 peak (=4.118 and 4.122, measured at one-fourth degree per min), in comparison with Bowen and Tuttle's curve (1950, fig. 2, p. 493), indicates an average composition of Or43AbAn57 (molecular percent). The absence of 111 and 130 peaks strongly suggests monoclinic symmetry.
The ash-flow sheet of member G may be correlative with a thin layer of crystal-rich air-fall tuff in the John Day Formation near Mitchell (east of the map area, see fig. 1), which has been described by Hay (1962b; 1963, p. 205). The air-fall tuff contains abundant crystals of soda sanidine, quartz, and myrmekite. A chemically analyzed sample of the sanidine has the molecular composition Or41Ab56An3, remarkably similar to that of the sanidine of the ash-flow sheet near Willowdale. The sanidine in the air-fall tuff is not cryptoperthitic, in contrast to crystals from the welded ash-flow sheet, but this difference no doubt reflects quicker cooling in the ash fall.
Member H.A ledge-forming rhyolite ash-flow sheet, 50 to 100 feet thick, and an overlying 300-foot thickness of poorly indurated pumice lapilli tuff and tuff compose member H. The ash-flow sheet is well exposed at the Pacific States Cut Stone Co. quarry, 1-1/2 miles south of Willowdale. This moderately welded sheet is varicolored in shades of red, orange, and gray and contains sparse lithophysae. Shards are visible in most hand specimens. The rocks contain only a trace of phenocrysts of sodic plagioclase and quartz, which lie in a matrix of moderately deformed shards and ash composed of partially devitrified glass. The chemical analysis of a sample (DLP-58-39A) from the vitric base of the ash-flow sheet is listed in table 2, column 6.
Member I.The uppermost member of the John Day Formation consists of 600 to 750 feet of tuff, which are well exposed on the west side of the valley south of the confluence of Hay and Trout Creeks. The upper 450 to 600 feet is light-gray and yellowish-gray poorly indurated tuff and lapilli tuff, consisting chiefly of massive air-laid deposits but including less abundant finely bedded and crossbedded water-laid tuff. Vertebrate fossils occur in the tuff at several localities (p. 18). Underlying this is 25 feet of light-gray thinly bedded and cross-bedded water-laid tuff, which in turn overlies a poorly indurated ash-flow sheet, 125 feet thick. The sheet is nonwelded except for a weekly welded 25-foot layer at the base and consists of light-gray lapilli tuff containing abundant dark-gray angular lapilli of collapsed pumice as much as one-half inch in diameter in a "salt and pepper" matrix of glassy shards and ash (glass=1.50). The tuff contains less than 1 percent phenocrysts, mostly oligoclase, and 1 to 2 percent angular fragments of andesite and schist.
WELDED ASH FLOWS
The moderately to strongly welded ash-flow sheets are more resistant to erosion than the poorly indurated tuffs that enclose them; they crop out as ledges or cap hogbacks, and can be easily traced in the sparsely timbered area between Ashwood and Willowdale. Some of the ash-flow sheets may represent only a single ash flow, such as the sheet in the basal part of member F that contains thunder-eggs; others probably contain a number of ash flows and constitute one or more cooling units (as defined by Smith, 1960a, p. 157; 1960b, p. 812).
The welded ash-flow sheets are rudely columnar jointed (fig. 5) and generally are composed of alternating layers containing few and many lithophysae. The lithophysae-rich layers are massive, mostly 1 to 4 feet thick, and contain 10 to 30 percent lens-shaped lithophysae (fig. 6), which generally average 2 to 4 inches in greater diameter. The lithophysae-poor layers average 5 to 10 feet in thickness and are more resistant to erosion; they have a platy structure formed by abundant disc-shaped cavities less than a millimeter in thickness that lie in light-colored discontinuous bands. Both the disc-shaped cavities and lithophysae are lined with euhedral to anhedral crystals of quartz, tridymite, alkalic feldspar, iron-oxide minerals, and, rarely, cristobalite. Probably disc-shaped cavities formed in flattened pumice lapilli by vapor-phase crystallization, as described by Ross and Smith, (1961, p. 27). Possibly each pair of a massive and a platy layer represents a single ash flow. At a locality in the ash-flow sheet of member G (along the Wilson Creek Road in sec. 8, T. 10 S., R. 15 E.), the lithophysal cavities are irregular in shape and size and are concentrated in a pipe, which cuts across the layering of the sheet; the pipe was probably formed by fumerolic action during cooling of the sheet.
The middle and upper parts of each sheet consist of stony light-gray to reddish rock that is moderately to strongly welded and is finely devitrified, chiefly to chalcedonic quartz, alkalic feldspar, and tridymite. A marked upward increase in porosity was noted in some sheets, such as the basal one in member A and the ash-flow sheet in member H. In other sheets that may have been emplaced at higher temperatures, notably member E, no such increase was observed, and the top of the sheet is composed of dense stony rock. The vitric-crystal ash-flow sheet of member G, where exposed in sec. 28, T. 9 S., R. 15 E., is composed of two units, each of which becomes more porous upward. Each of these units is layered owing to uneven concentration of lithophysae, and may represent a number of ash flows that cooled as a unit.
At the base of each welded ash-flow sheet is a layer of nonwelded tuff, overlain by gray vitric welded tuff in which deformed shards are discernible with a hand lens. The welded tuff is isotropic and appears little altered in thin section, but X-ray diffractometer studies indicate that it is partially devitrified to cristobalite and alkalic feldspar, as shown in the charts of figure 7. The upper chart was prepared from the most devitrified of the analyzed samples of vitric welded tuff listed in table 2, and the lower chart was prepared from the least devitrifled. Chemical analyses (table 2) indicate that the welded tuff is hydrated, containing 4 to 6 percent water.
Phenocrysts constitute from a trace to 20 percent (by volume) of each ash-flow sheet. Soda sanidine, oligoclase, and quartz are most abundant; biotite, hornblende, pyroxene, zircon, and opaque minerals are rare.
Chemical analyses of samples from welded ash-flow sheets (table 2) show that they are rhyolitic in composition and contain an average of about 37 percent normative quartz and 57 percent normative feldspar (on a water-free basis) with the average composition Or46Ab50An6.
AGE AND CORRELATION
The John Day Formation in the Antelope-Ashwood area is late Oligocene and early Miocene in age. Fossil plants, collected from member A by Jack A. Wolfe at a locality in sec. 10, T. 10 S., R. 16 E. (pl. 1), are similar to the Bridge Greek and Willamette Junction floras and are of late Oligocene age (J. A. Wolfe, written communication, 1959). They comprise the following species:
Pinus wheeleri Cockerell
Fossil leaves collected from the lower part of the John Day Formation immediately east of the mapped area near Clarno are also assigned to the late Oligocene (J. A. Wolfe, oral communication 1961).
Fossil vertebrates have been collected from the upper member of the John Day Formation in the northern part of the Antelope-Ashwood area and also farther west in the valley of Trout Creek and near the confluence of Trout Creek and the Deschutes River (Hodge, 1932b). The fossils were identified by E. L. Packard, R. A. Stirton, and W. D. Matthew. The approximate position of three localities in the Antelope-Ashwood area (loc. 700, 702, and 709 of Hodge, 1932b, p. 697) are marked on plate 1. As reported by Hodge, the fossils are in part identical to those collected from the John Day at its type locality, fossils that are usually assigned to the early Miocene (Wood and others, 1941); some fossils from west of the map area, however, are of younger early Miocene age.
Volcanic rocks contemporary with those of the John Day Formation are widely exposed on the western slope of the Cascade Range in Oregon (Peck, 1960a, 1960b). They are mostly massive poorly welded ash-flow deposits of dacitic, andesitic, and rhyodacitic pumice lapilli tuff, which were extruded from a belt of vents near the eastern margin of the present Western Cascade Range. The tuffs typically contain about 10 percent phenocrysts of feldspar, chiefly andesine, and less abundant pyroxene and magnetite.
SOURCE OF THE VOLCANIC MATERIAL
Volcanic material in the John Day Formation in the Antelope-Ashwood area includes products of both local and distant volcanism. The rhyolite dome complex and its associated flows were emplaced through vents located a few miles north of Ashwood, and the trachy andesite flows from a vent (pl. 1) 8 miles south of Ashwood. Most of the ash-flow sheets thin northward and northeastward from Ashwood and become less strongly welded, indicating that the ash flows as well as some of the associated tuff and lapilli tuff were derived from vents near Ashwood or to the south or west.
The source of much of the tuff and lapilli tuff in the John Day, however, may well have been some distance from the Antelope-Ashwood area. Calkins (1902), Coleman (1949),4 Fisher and Wilcox (1960), and Hay (1962a; 1963) have determined that much of the tuff in the John Day Formation in the John Day basin is dacitic or andesitic in composition; phenocrysts are mostly andesine, and quartz is lacking in many tuffs. Sparse data obtained in the present study on ash-fall deposits in the upper part of the formation in the Antelope-Ashwood area agree with the earlier studies. These rocks thus differ in composition from locally derived rhyolitic volcanic rocks but are similar to contemporaneous pyroclastic rocks in the Cascade Range. Furthermore, lapilli tuffs are increasingly abundant from east to west toward the Cascades. A reasonable conclusion is that some of the tuff, lapilli tuff, and tuffaceous claystone in the John Day Formation, both in the Antelope-Ashwood area and along the John Day River, was formed from ash carried eastward in the air from vents in an ancestral Cascade Range, as suggested by Hodge (1932b, p. 701-702), Coleman (op. cit.), and Hay (1962a, 1963).
Last Updated: 28-Mar-2006