Geological Survey Circular 838 Guides to Some Volcanic Terrances in Washington, Idaho, Oregon, and Northern California

STRATIGRAPHY

General Aspects

The John Day Formation crops out widely over north-central Oregon, occurring in three geographically separate areas (Figure 1). The type locality (eastern facies) lies east of the Blue Mountain uplift and includes the Picture Gorge district and the Mitchell area. Similar rocks cropping out south of the Ochoco Mountains constitute a southern facies, and a western facies is exposed in the area between the Blue Mountaina and the Cascade Range. Although rocks from these three areas are lithologically similar, consisting chiefly of tuffaceous claystone and air-fall tuff, certain systematic differences exist reflecting the presence of topographic barriers between the areas during depositon. Specifically, the western facies contains numerous intercalated ash-flow tuffs and lava flows which are rare or absent in the eastern and southern facies.

Considerable disagreement exists as to the nature and stratigraphic position of the boundary between the John Day and Clarno Formations. In areas east of the Blue Mountains the top of the Clarno Formation is generally marked by a well developed red saprolite. Local incorporation of this material into the lower part of the John Day Formation accounts in part for the red color of the lower member (Big Basin member of Fisher and Rensberger, 1972). Waters (1954) recognized a similar saprolite in the Horse Heaven area in the Blue Mountains (Figure 1) and suggested that it also marked the top of the Clarno Formation at that locality. However, Peck (1964) and Swanson and Robinson (1968) placed the contact between the two formations in the western facies at the base of a widespread ash-flow tuff (member A of the western facies) which unconformably overlies rocks of Clarno lithology and is several hundred feet stratigraphically above the Horse Heaven saprolite. At most localities the John Day Formation is unconformably overlain by basalts of the Columbia River Group. Where these are missing, the formation is unconformably overlain by younger units such as the Madras Formation, the Mascall Formation, or Quaternary basalt.

Eastern Facies

Merriam (190)) defined three members in the eastern facies of the John Day Formation based largely on color. However, most of the features used by Merriam to distinguish the members from one another vary laterally and define alteration facies rather than stratigraphic units. Hay (1962a; 1963) proposed a 3-fold subdivision based on the presence of a widespread ash-flow sheet (Picture Gorge Ignimbrite) near the middle of the Formation. Fisher and Rensberger (1972) divided the unit into four members, nearly comparable to those originally defined by Merriam (1901). From oldest to youngest, these are; a) the Big Basin Member, a basal sequence of red claystone, b) the Turtle Cove Member, characterized by green, buff, or pale red zeolitized tuff and tuffaceous claystone, c) the Kimberly Member characterized by light gray to buff, unzeolitized tuff and tuffaceous sediment and, d) the Haystack Valley Member composed of gray, largely fluviatile, volcaniclastic rocks. Several of the members, particularly the Kimberly and Big Basin members, are distinguished largely on the basis of diagenetic features and, hence, the contacts often transgress time-stratigraphic boundaries.

Big Basin Member - The Big Basin member is a thin, discontinous sequence of red and less abundant yellow tuffaceous claystone named for exposures in Big Basin. It is typically 15 to 40 meters thick but ranges from 0 to 75 meters. Locally, thin, discontinuous olivine basalt flows occur near its base. The claystones are poorly bedded, and crop out in low, rounded hills mantled by a thin "popcorn-textured" layer. The red color of most of the unit is due to incorporation of red saprolite from local exposures of the Clarno Formation and diagenetic alteration of air-fall tuff in well-drained upland areas. Yellow coloration formed in the tuffaceous claystone where drainage was poor and there was less opportunity for iron oxidation (Hay, 1962a; Fisher and Rensberger, (1972).

Figure 1. Index map showing approximate outcrop extent of the John Day Formation in north-central Oregon. Dashed lines enclose areas of known outcrop, dotted lines areas where rocks of probable John Day affiliation are present. (click on image for an enlargement in a new window)

Turtle Cove Member - The Turtle Cove member is the thickest and most widespread unit in the eastern facies of the Formation. Throughout most of the area it ranges from about 120 to 150 meters in thickness but it thins markedly northward against the Blue Mountain uplift where it is less than 15 meters thick (Fisher, 1967). It consists chiefly of varicolored, zeolitized, mostly fine-grained tuff and tuffaceous claystone with less important air-fall and ash-flow tuff. Outcrops in the lower part of the member display a typical ribbed or pinnacled 'badland' topography; those higher in the unit are smooth and rounded. Colors vary widely, reflecting varying degrees of diagenetic alteration. Most of the unit is light green with thin interbeds of light gray material; however, brown, yellow, and light red colors are common. The zeolitized tuffs and tuffaceous clay stones were originally composed largely of vitric shards and pumice fragments. Pumice fragments are generally less than 1 mm in length hut range up to 4 am. Sand-size crystals and rock fragments comprise 5-10 percent of most specimens. Crystals are chiefly andesine feldspar with lesser amounts of oligoclase, labradorite, and sanidine. Pyroxene, hornblende, biotite, magnetite, and ilmenite are usually present in trace amounts. Quartz is rare except in a few rhyolitic air-fall tuffs in the lower part of the formation. One such tuff, exposed in the Painted Hills area, contains abundant large soda-rich sanidine crystals mantled with myrmekite. This layer, generally less than 1 meter thick, is believed to be the air-fall equivalent of a distinctive crystal-rich ash-flow tuff (member G) in the western facies (Hay, 1961; Woodburne and Robinson, 1977).

The Picture Gorge Ignimbrite is a composite ash-flow-sheet, up to 75 meters thick, composed of two cooling units (Fisher, 1966a). It is typically coarse-grained, moderately to densely welded, and crystal poor. Crystals average less than 2 modal percent and consist of plagioclase, sanidine, quartz, and minor clinopyroxene.

Kimberly Member - The Kimberly member consists of gray to yellow gray, unzeolitized tuff and tuffaceous claystone similar to that of the underlying Turtle Cove member. The boundary between the two members is drawn almost entirely on a color change reflecting variations in diagenetic alteration and is chronologically variable.

The Kimberly member has a composite thickness of about 100 meters but the thickness varies considerably because of the variable lower boundary and because erosion in late John Day time stripped off much of the unit. Outcrops are usually smooth, even slopes often covered with light gray, powdery soil. Generally, the rocks form thick, poorly bedded, featureless sections without distinct marker units. Crystals comprise about 5 modal percent of these rocks and are chiefly andesine feldspar with minor amounts of clinopyroxene, magnetite, hornblende, biotite, and ilmenite. The absence of quartz and sanidine suggests an andesitic to dacitic composition. One distinctive rhyolite tuff characterized by abundant black obsidian shards occurs near the base of this member (Hay, 1963; Fisher and Rensberger, 1972).

Haystack Valley Member - The Haystack Valley member comprises a sequence of interbedded fluviatile and lacustrine tuffaceous sediments near the top of the formation. It has a maximum thickness of about 150 meters. The member consists largely of unzeolitized conglomerate, sandstone, and siltstone with some lacustrine tuff. Massive air-fall tuffs are commonly interbedded with the reworked and cross-bedded sediments. Individual beds of conglomerate and sandstone are often lenticular, filling steep-walled channels cut into the underlying Kimberly member. The composition of the reworked material suggests that this unit was derived solely from erosion of the lower John Day Formation (Fisher and Rensberger, 1972).

Southern Facies

Little detailed work has been done in the southern facies, which lies south of the Ochoco Mountains. Reconnaissance studies indicate that this section is most like the eastern facies, consisting chiefly of fine-grained tuff and tuffaceous claystone but having two prominent ash-flow sheets in the area south of Prineville.

The southern facies has not been formally divided into members but lithologic variations are similar to those of the eastern facies. A sequence of red claystone occurs discontinuously along the base of the formation and is similar to the Big Basin member of Fisher and Rensberger (1972). The overlying tuffs and tuffaceous claystones can be divided into zeolitized and unzeolitized sections that correspond approximately with the Turtle Cove and Kimberly Members.

The lower welded tuff south of Prineville Reservoir is a light gray to reddish-gray, stony rhyolite. It ranges from about 5 to 15 meters thick, generally thinning from north to south. The tuff is densely welded with a good eutaxtic structure, having flattened pumice fragments up to 10 cm long. Crystals comprise about 1-2% and are chiefly plagioclase with traces of magnetite and partly altered biotite. The matrix is a microcrystalline mixture of quartz and feldspar in which the original vitroclastic texture has been largely obliterated. Many of the larger pumice fragments have been zeolitized.

The upper ash-flow sheet is generally about 10 meters thick-and consists of light gray to reddish-gray, densely welded tuff. The tuff is relatively fine-grained and aphyric. Flattened pumice fragments, generally less than 1 cm long, define a crude eutaxtic structure. These are replaced by a mixture of tridymite, alkali feldspar, and zeolite. Small andesitic rock fragments comprise about 1 modal percent.

Neither of these ash-flow sheets cann be correlated with welded tuffs in the eastern or western facies and they are believed to have originated from separate vents south of the Ochoco Mountains.

Western Facies

In the area west of the Blue Mountains and east of the Cascade Range a sequence of largely pyroclastic rocks, 600 to 1200 meters thick, lies between the Columbia River Basalt and the Clarno Formation. This seqeuence consists largely of tuff and tuffaceous sediment compositionally similar to that of the eastern and southern facies of the John Day Formation, but it also includes numerous basaltic to rhyolitic lava flows and silicic ash-flow tuffs. These rocks have been correlated with the type John Day Formation in the eastern facies on the basis of stratigraphic position and general lithologic similarity (Hodge, 1932; Waters, 1954; Peck, 1964; Swanson, 1969; Robinson, 1975; Woodburne and Robinson, 1977).

In the ares west of the Blue Mountains the John Day-Clarno contact is placed just below the basal ash-flow sheet of member A. This ash-flow sheet lies unconformably on rocks of Clarno lithology and is locally separated from them by a thin, red saprolite.

Peck (1964) subdivided this sequence into 9 conformable members (A-I) based largely on the presence of ash-flow sheets and lava flows (Figure 2). This subdivision, with minor additions, is valid over most of the western facies (Robinson, 1975). The only major exceptions are in the area between Fossil and Lonerock and in the Mutton Mountains. In the Fossil area course-grained lapilli tuffs and dacitic to basaltic lava flows occur near the base of the formation. The stratigraphic position of these units is uncertain but they may be correlative with the upper part of the western facies. In the Mutton Mountains the only ash-flow sheet recognized is that of member G. Other ash-flow tuffs and lava flows are absent and the section cannot be further subdivided.

Figure 2. Generalized composite stratigraphic columns for the western and eastern facies of the John Day Formation showing probable correlations. (click on image for an enlargement in a new window)

Member A - Member A is a widespread sequence of tuff, tuffaceous sediment, and welded tuff ranging in thickness from about 10 to 130 meters. At the base is a distinctive ash-flow sheet, up to 35 meters thick, that rests unconformably on the Clarno Formation. This is overlain by about 30 meters of poorly exposed lapilli tuff, tuff, and tuffaceous sediment which in turn is succeeded upward by a densely welded. fine-grained ash flow-tuff, usually less than 20 meters thick.

The basal ash-flow tuff is densely welded, relatively coarse-grained, and sparsely porphyritic. A thin, light gray vitrophyre occurs locally at its base but most of the unit is devitrified and sparsely lithophysal. Two cooling units, each with a basal vitrophyre, are exposed along the county road west of Ashwood; elsewhere only one cooling unit can be recognized. The tuff is characterized by a well developed eutaxitic structure with flattened pumice lapilli up to 8 cm long. Crystals average 5-8% and consist chiefly of quartz, sanidine, and plagioclase; some specimens also contain traces of green hornblende and opaque minerals. Rock fragments, mostly andesite and rhyolite, comprise 0.5 to 2%.

This is one of the most distinctive and widespread ash-flow tuffs in the western facies. It can be traced in nearly continuous outcrop from Grizzly on the south to the vicinity of Clarno on the north, a distance of nearly 80 km. It has been recognized in isolated outcrops along the crest of the Blue Mountain uplift (Swanson and Robinson, 1968; Swanson, 1969) and in a small outcrop at the base of the eastern facies along Rowe Creek (Woodburne and Robinson, 1977). Correlations between these separate exposures are based on grain size, phenocryst assemblage, mineral compositions, and stratigraphic position.

Directly shove the basal ash-flow tuff is a white, poorly bedded lapilli tuff containing abundant pumice. This is followed upward by red to yellowish-brown tuffaceous claystone and interlayered tuff locally containing numerous plant fossils. The upper ash-flow sheet is a light brown, fine-grained welded tuff with 1-2% plagioclase phenocrysts and 1-2% small rock fragments. A light gray vitrophyre occurs locally at the base but most of the unit is stony with very sparse lithophyase.

Member B - Member B is a series of dark gray trachyandesite flows that crop out chiefly in the Antelope-Ashwood area. This member is about 300 meters thick in the ares west of Ashwood but the sequence thins gradually to the north and south from this point. A thinner sequence, probably no more than 30 meters thick, crops out in the Antelope Valley, Locally several flows can be recognized, separated by 5-6 meters of brown tuff and tuffaceous sediment. The trachyandesite typically fractures into angular, fist-size fragments, forming rounded outcrops mantled by talus.

The trachyandesite is very fine-grained and nearly aphyric. Small phenocrysts of olivine, replaced by smectite, lie in a matrix of tiny plagioclase laths, granular pigeonite, opaques, and abundant interstitial glass. One flow in Eagle Valley, south of Antelope, is holocrystalline and medium grained with an intergranular to subophitic texture. Vesicles are sparse except in thin zones at the tops of flows.

Member C - A thick rhyolite flow and domal complex in the Ashwood-Antelope area comprises member C. The rhyolite flows, which are up to 125 meters thick along Wilson Creek, were probably erupted from the domal complex along Trout Creek, just north of Ashwood, The domal complex has crude, nearly vertical columnar jointing, variable flow banding, and a marginal rhyolite breccia interbedded with tuff along the southern edge.

Both the flows and the dome consist of light gray to purplish-gray, massive to flow banded, very sparsely porphyritic rhyolite. Phenocrysts of plagioclase and quartz comprise less than 2 modal percent and lie in a matrix of microcrystalline quartz and feldspar.

Member D - Member D is a thin, areally restricted sequence of tuff and lapilli tuff that lies above rhyolite flows of member C and below ash-flow tuffs of member E. The tuffs are light gray to yellowish-gray, poorly indurated, and moderately bedded. A maximum thickness of about 30 meters is exposed along Pony Creek between Ashwood and Willowdale. At most localities the unit is less than 5 meters thick.

Member E - A series of densely welded, highly lithophysal ash-flow tuffs, up to 120 meters thick, comprise member E. These are best exposed along Pony Creek between Ashwood and Willowdale where they form steep, columnar-jointed outcrops. A light gray basal vitrophyre is exposed locally but most of the unit consists of stony rhyolite. Outcrops of crystallized tuff are characterized by abundant large lithophysae arranged in crude layers. The tuffs are very fine-grained and generally lack any eutaxitic structure. Crystals of oligoclase and quartz comprise 1-2% and some specimens have trace amounts of altered pyroxens. Rock fragments are rare or absent. In many places the large lithophysal cavities have been filled with opal or chalcedony to form round or oval masses known locally as 'thunder eggs'. Peck (1964) included the weakly welded ash-flow tuff at the Priday agate deposit in the base of member F but Robinson (1975) considered it to be the top of member E.

Member F - Member F consists of 100 to 300 meters of varicolored tuff, lapilli tuff, and tuffaceous claystone with interlayered basalt flows near the base. The pyroclastic rocks are poorly indurated and poorly exposed, often being involved in small land slides or covered with talus from ash-flow sheets in overlying members.

At the base of the member is m coarse-grained, well bedded pumice lapilli tuff up to 3 meters thick, Overlying this is a 50- to 70-meter-thick section of brick-red, poorly bedded, tuffaceous claystone. The claystone is followed upward by predominately yellow, gray, and green altered tuffs and lapilli tuffs.

In Antelope Valley numerous alkali-olivine basalt flows are interbedded with tuffs and tuffaceous claystones in the lower part of the member (Robinson, 1969). Individual flows are typically 5-10 meters thick and extend along strike for up to 20 km. Outcrops are generally low, rounded hills mantled with small chips of weathered, yellow-brown basalt.

The basalts are usually holocrystalline, medium- to coarse-grained, and aphyric. They contain 10-15 percent olivine, mostly altered to smectite, abundant titaniferous augite, plagioclase, ilmenite, and minor interstitial alkali feldspar. Local pegmatitic segregations consist chiefly of titanaugite, ilmenite, and plagioclase. These highly titaniferous and alkalic basalts are chemically distinct from lava flows in the Clarno Formation, the Columbia River Basalt Group, and younger basalts in north-central Oregon (Robinson, 1969)

Member G - This member consist of a basal ash-flow sheet, about 15 meters thick, overlain by 30 to 120 meters of poorly bedded, poorly indurated tuff and lapilli tuff. The basal ash-flow sheet is typically reddish-brown, densely welded, and completely crystalized, forming resistant ridge-like outcrops. It is characterized by abundant large phenocrysts of soda-rich sanidine mantled by myrmekitic intergrowths of quartz and feldspar. It crops out chiefly in the Antelope-Ashwood-Willowdale area but a similar tuff occurs in the Mutton Mountains about 20 km west of Willowdale and an air-fall equivalent occurs in the eastern facies.

The ash-flow sheet shows systematic variations in thickness, degree of welding, and crystallization from south to north. In the vicinity of Hay Creek Ranch, east of Madras, the tuff ranges from about 15 to 30 meters thick and is densely welded, completely devitrified, and moderately lithophysal. Twenty kilometers to the north along Oregon Highway 206 between Willowdale and Antelope, the tuff is 5-6 meters thick, moderately welded, and completely glassy. Farther to the northeast, between Antelope and Clarno, an air-fall tuff containing similar large crystals of soda-rich sanidine mantled with myrmekite occurs in the same stratigraphic position. In this area the air-fall tuff is usually about 1 meter thick but, where extensively reworked it may attain a thickness up to 10 meters. This air-fall tuff has been correlated with a similar tuff in the Painted Hills area east of the Blue Mountains (Hay, 1963; Woodburne and Robinson, 1977) and is believed to provide a tie between the eastern and western facies.

The tuffs and lapilli tuffs above the basal ash-flow sheet are light gray to light brownish-gray, poorly bedded, and moderately to weakly indurated. Outcrops are poorly exposed, often being covered with talus from overlying ash-flow sheets. At the base, there is usually a 2- to 10-meter-thick sequence of lithic lapilli tuff, overlain by 20 to 30 meters of light brown, fine-grained tuff with about 5% white pumice lapilli, which give the rock a characteristic spotted apppearance. Locally, this is overlain by 5-6 meters of poorly bedded, light gray, fine-grained ash. At the top of the section is a sequence of poorly bedded, poorly indurated, light brown to light gray tuff, up to 50 meters thick.

Most of the tuffs and lapilli tuffs are crystal poor, containing 2-5% plagiocase feldspar and traces of altered ferromagnesian minerals. Quartz and sanidine are rare to absent. Most specimens are diagenetically altered, containing abundant zeolite, chiefly clinoptilolite.

Member H - Member H consists of a 5- to 15-meter-thick basal ash-flow sheet overlain by about 30 to 40 meters of weakly indurated tuff and lapilli tuff. The basal ash-flow sheet is very widespread cropping out almost continuously from the vicinity of Juniper Butte to the area northeast of Clarno (Figure 1). Generally, the tuff becomes thinner and less densely welded to the north and northeast. It is typically yellowish- to reddish-brown, very fine-grained, and very sparsely phyric. Crystals of sodic plagioclase and quartz are present in trace amounts. At many localities there is a light gray, glassy layer at the base but most of the unit is devitrified. Outcrops east and south of Madras contain numerous lithophysal cavities but such features are sparse elsewhere.

The overlying tuffs and lapilli tuffs are poorly exposed, forming low, talus-mantled outcrops. They consist chiefly of light brown to buff, very poorly bedded, fine-grained tuff. Most rocks contain 5-10% of small white pumice lapilli producing a distinct speckled appearance. Outcrops also contain thin layers of light gray, slightly more resistant tuff up to 0.5 meters thick. Crystals of plagioclase comprise 1-2% of most tuffs and small lithic fragments are sometimes present.

Member I - A basal ash-flow sheet, 20 to 50 meters thick, and an overlying 200- to 300-meter-thick sequence of tuff crops out extensively in the western facies from the area south of Madras to the vicinity of Fossil. Thicknesses vary considerably due to erosion at the top of the formation.

The basal ash-flow sheet is thickest and most densely welded in the area between Madras and Grizzly. Here it is consists of 50 to 60 meters of densely welded tuff with a thick, black vitrophyre at the base. It thins and becomes less densely welded to the north, being largely unwelded near Willowdale. The tuff is coarse-grained, containing large flattened or irregular pumice fragments and pieces of black obsidian. Small oligoclase crystals comprise about 1% and rock fragments, chiefly andesite and rare shist, make up 1-2%.

The overlying tuffs are mostly light gray to yellowish-gray, poorly bedded, and massive, but a few layers are thinly bedded and cross-bedded. Outcrops are steep, even slopes often mantled with talus from the overlying Columbia River Basalt. The tuffs contain 1-2% crystals, chiefly andesine, sparse lithic fragments, and sometimes small pieces of black, vesicular glass. Tuffs in the lower part of the member are usually zeolitized but those in the upper 50 to 300 meters are still glassy but hydrated. These tuffs often contain well preserved vertebrate fossils (Woodburne and Robinson, 1977; Dingus, 1979).

Exposures of the John Day Formation between Fossil and Lonerock cannot be correlated directly with the facies but most of these rocks consist of fine-grained tuff similar to that in the upper parts of members H and I. At the base of the sequence is a cream-colored to gray, relatively coarse-grained, poorly sorted, very poorly bedded, pumice lapilli tuff. The tuff has a maximum thickness of about 250 meters directly north of Fossil and thins to the northeast and southwest. No welding is observed in this unit but the poor sorting and lack of bedding suggest an ash-flow origin.

In the vicinity of Fossil several lava flows are interbedded with the basal tuff, West and north of Fossil is a reddish-brown rhyodacite (Robinson, 1969), 5 to 10 meters thick, with crude platy jointing. Farther east at least two flows of alkali-olivine-basalt occur at the same stratigraphic level.

This lower sequence is locally overlain by a thin, discontinuous welded tuff, possibly correlative with the basal ash-flow sheet of member H. The tuff is 3 to 5 meters thick and moderately to densely welded with a weakly developed eutaxitic structure. A thin layer of light gray glass occurs at the base of the tuff, passing upward into a weakly welded, reddish-gray, stony rock. Sparse crystals are chiefly oligoclase.

Above the welded tuff is a 200-to 300-meter-thick sequence of fine-grained, moderately indurated, poorly bedded tuff similar to that in the upper part of members H and I. The uppermost tuffs contain small black fragments of fresh vesicular glass; those near the base are diagenetically altered and zeolitized.

Correlation Of Eastern And Western Facies

The western facies of the John Day Formation differs from the eastern facies in having generally coarser-grained air-fall tuffs and tuffaceous claystones and more abundant lava flows and ash-flow sheets. These differences are believed to reflect a western source for most of the pyroclastic material and the presence of a topographic barrier along the Blue Mountain axis through most of John Day time.

Two specific correlations between the eastern and western facies are proposed (Figure 2). A small outcrop of ash-flow tuff at the base of the eastern facies along Rowe Creek (Hay, 1963; Robinson, 1975) is correlated with the basal ash-flow sheet of member A in the western facies (Swanson and Robinson, 1968; Woodburne and Robinson, 1977). This correlation is based largely on phenocryst mineralogy, including the presence of a distinctive barium-rich sanidine (Woodburne and Robinson, 1977). If this correlation is correct it implies that uplift along this part of the Blue Mountains took place after about 36 m.y.

A distinctive rhyolitic air-fall tuff in the eastern facies (Hay, 1963) is correlated with the basal ash-flow tuff of member G and its air-fall equivalent in the western facies. These tuffs are mineralogically similar, having abundant phenocrysts, 1-4 mm in diameter, of distinctive soda-rich sanidine mantled with myrmekite. This correlation suggests that the bulk of the western facies is equivalent to about the lower 75 meters of the eastern facies.

If these correlations are correct, the Picture Gorge Ignimbrite has approximately the same stratigraphic position as the basal ash-flow sheet of member H of the western facies but these appear to be different units based on texture and mineralogy (Fisher, 1966). The general absence of ash-flow sheets in the eastern facies and thickness variations within the formation (Fisher, 1967) imply that the Blue Mountains formed a topographic barrier through out middle and late John Day time.