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

Norman S. MacLeod, David R. Sherrod, U.S. Geological Survey, Menlo Park, California 94025
Lawrence A. Chitwood, U.S. Forest Service, Bend, Oregon 97701
and
Edwin M. McKee, U.S. Geological Survey, Menlo Park, California 94025

GEOLOGIC SUMMARY

Newberry volcano, centered about 20 miles southeast of Bend, Oregon, is among the largest Quaternary volcanoes in the conterminous United States. It covers an area in excess of 500 mi², and lavas from it extend northward many tens of miles beyond the volcano. The highest point on the volcano, Paulina Peak with an elevation of 7,984 ft. is about 4,000 ft higher than the terrain surrounding the volcano. The gently sloping flanks, embellished by more than 400 cinder cones, consist of basalt and basaltic andesite flows, andesitic to rhyolitic ash-flow and air-fall tuffs and other types of pyroclastic deposits, dacite to rhyolite domes and flows, and alluvial sediments produced during periods of erosion of the volcano. At Newberry's summit is a 4- to 5-mile-wide caldera that contains scenic Paulina and East Lakes. The caldera has been the site of numerous Holocene eruptions, mostly of rhyolitic composition, that occurred as recently as 1,400 years mgo.

Many geologists have studied various aspects of Newberry Volcano starting with I. C. Russell (1905) who visited it during a horseback reconnaissance of central and eastern Oregon in 1903. Howell Williams (1935, 1957) mapped the flanks of the volcano in reconnaissance and studied the caldera in more detail. His outstanding work forms the basis for subsequent investigations, most of which have focused on caldera rocks or young flank basalt flows. No comprehensive study has been made of the geology of Newberry's forest-covered and Mazama ash-covered flanks even though they form more than 95 percent of the area of the volcano. As part of a geothermal resource investigation of central and eastern Oregon, the first three authors have mapped the sixteen 7-1/2' quadrangles that cover the flanks of Newberry at a scale of 1:62,500, and reinterpreted and partly remapped the caldera. Highly generalized geologic sketch maps are shown in figures 1 and 2. The new mapping and K-Ar dating by the last author require substantial reinterpretation of the history of the volcano and of formation of the caldera and subsequent volcanic activity within it.

Newberry lies 40 miles east of the crest of the Cascade Range in a setting similar to Medicine Lake Volcano in California (Donnelly and others, this vol.). Both volcanoes have the same shape, are marked by summit calderas, contain abundant rhyolitic domes and flows, have widespread ash flows in addition to the more areally extensive basalt and basaltic-andesite flows and their related cinder cones, have similar petrochemistry, and have been the sites of eruptions of pumiceous tephra and obsidian flows during the last few thousand years.

Newberry lies at the west end of the High Lava Plains, a terrain formed of Miocene to Quaternary basalt flows and vents punctuated by rhyolitic domes and vent complexes (Walker and others, 1967; Greene and others, 1972; Walker and Nolf, this vol.). The rhyolitic rocks show a well-defined monotonic age progression starting at about 10 m.y. east of Harney basin and decreasing to less than 1 m.y. at Newberry's eastern border (Walker, 1974; MacLeod and others, 1975; McKee and others, 1976). Newberry rhyolites appear to be a continuation of these age-progressive rhyolitic rocks.

A complexly faulted terrain surrounds Newberry. The northeast-trending Walker Rim fault zone impinges on Newberry's southern flank but offsets only its older flows. A zone of faults that offsets older flows on the lower northern flank extends northwestward into the Cascade Range at Green Ridge. Although Newberry lavas obscure the relations of the Walker Rim and Green Ridge fault zones, they likely join beneath Newberry and represent but one curving fault system. The Brothers fault zone, a major west-northwest-trending zone of faults, extends across the extreme northeastern flank but does not apparently offset surficial Newberry flows. It probably extends at depth to join or abut against the Green Ridge-Walker Rim fault zones.

The north and south flanks of Newberry Volcano, which extend the greatest distances from the summit caldera, are almost exclusively veneered by basalt and basaltic andesite flows and associated vents.

The basalt flows form much of the surface in a broad region extending far north of the volcano (Peterson and Groh, 1976) as well as southward to the Fort Rock basin. Individual flows are a few feet to more than 100 ft thick and cover areas of less than 1 square mile to many tens of square miles. Flow margins are commonly well preserved even on older flows, but the flows are complexely interwoven and it is difficult and time consuming to trace individual flow boundaries. Most flows are of block or aa type; pahoehoe surfaces occur locally on a few lower flank flows. Lava tubes are common, and some extend uncollapsed for distances of 1 mile (Greeley, 1971); some lower flank flows may have been fed by tube systems. Casts of trees occur in many flows, particularly the younger ones.

The basalt and basaltic andesite flows can be readily divided into two groups on the basis of their age relative to Mazama ash (¹⁴C age 6,600-6,700 years) derived from the volcano at Crater Lake 70 miles distant. The youngest lava flows overlie Mazama ash, and their ¹⁴C ages range from 5,800 to 6,380 years (¹⁴C ages of this magnitude are generally about 800 years younger than actual ages). Carbon for isotopic dating was obtained from carbonized root systems at the bases of lava tree casts (Peterson and Groh, 1969) and from beneath cinder deposits that extend as plumes leeward of cinder vents related to the flows (Chitwood and others, 1977). The young flows may have erupted during a much shorter period of time than the age spread indicates, perhaps as little as a few weeks or several years. All other flows are covered by Mazama ash and are older than 6,700 ¹⁴C years; surface features on some flows suggest a relatively young age, perhaps 7,000 to 10,000 years, others are likely several tens or hundred of thousands of years old.

Figure l. Geologic sketch map of Newberry Volcano. Geology of the caldera is shown in fiqure 2. (click on image for an enlargement in a new window)

Seventy-three basaltic to andesitic flow rocks or bombs from associated vents analyzed by Higgins (1973) and Beyer (1973) contain 48 to 60 percent SiO₂. The mean silica content of these rocks is about 54 percent, similar to that of analyzed rocks from the adjacent Cascade Range. Of these analyzed Newberry rocks, only about 20 percent contain less than 52 percent SiO₂, about 60 percent lie between 52 and 56 percent, and the remaining 20 percent have 56 to 60 percent. Thus the dominant analyzed rock type is basaltic andesite similar to many rocks in the Cascade Range. The analyzed rocks are generally biased toward younger rocks, however, and many of the older voluminous flows on the lower flanks are basalt, mostly of high-alumina type (table 1, col. 1). The 6,000-year-old flows contain 52 to 57 percent SiO2 and are basaltic andesites (table 1, col. 2).

Table 1. Representative chemical analyses of Newberry rocks.
[n.r. = not reported]

More than 400 cinder cones and fissure vents have been identified on the flanks of Newberry—few other volcanoes in the world contain so many. They are concentrated in three broad zones that join on the upper part of the volcano. The eastern zone is a continuation of the High Lava Plains zone of basaltic vents and parallels the Brothers fault zone; except for cones high on the east flank, most cones in this zone appear relatively old. The northwestern zone of vents is collinear with the zone of faults on the lowermost flank that extends to Green Ridge in the Cascade Range, and the southwestern zone is collinear with the Walker Rim fault zone. Fissures and alined cinder cones generally parallel the belts in which they occur. The distribution of the vents, and particularly of alined vents and fissures, suggests that the northwest and southwest zones, and perhaps the faults that they parallel, are part of one broad arcuate zone that curves in the vicinity of Newberry's summit. Some alined cinder cones and fissure vents near the summit occur in arcuate zones parallel to the caldera rim and likely lie along ring fractures; some occur along faults whose caldera side is downdropped.

Most of the cinder cones are well preserved owing to their high porosity and consequent absorption rather than runoff of water. Larger cones are as much as 500 ft high, typical cones are 200 to 300 ft. Most are marked by summit craters and flows emerge from their bases. Cinders dispersed by prevailing winds during eruptions form aprons extending leeward from some cones such as Lava Butte (Chitwood and others, 1977) (stop 1, road log). Fissure vents consist of long ridges or trenchlike depressions formed by cinders, spatter and agglutinate flows. Small pit craters are developed along some fissure vents. Cinder cones and fissure vents on the lower flanks are generally devoid of fragments of rhyolitic rock, whereas many of those higher on the volcano contain rhyolitic inclusions (stop 3, road log).

Shieldlike vents occur at Spring and Green Butte on the southwest flank and Green Mountain to the northwest of Newberry. They are 1 to 3 miles across, have gentle slopes, and are more faulted and older than most surficial Newberry flows.

Many of the hills on Newberry's flanks are rhyolitic domes. In addition, pumice rings, obsidian flows, and small rhyolite or obsidian protrusions occur in many places. Most of the domes form rounded hills, such as McKay Butte on the west flank, that are 100 to 500 ft high and up to 4,000 ft across. The largest dome, which forms Paulina Peak (stop 6, road log), extends southwestward from the caldera wall for 3 miles. Its very elongate outcrop suggests that it was emplaced along a northeast-trending fissure or fault; an obsidian flow crops out farther down the slope on a direct extension of the axis of the Paulina Peak dome and may have been erupted from the same buried fissure or fault.

Several small rhyolitic outcrops may be the tops of rhyolite domes that are more extensive at depth. An example is along Paulina Creek on the west flank of Newberry where obsidian irregularly invades basaltic andesite flows over a small area. In addition, the common occurrence of rhyolite as fragments in cinder cones on the upper flanks attests to the relative abundance of rhyolite at depth on the upper part of the volcano.

K-Ar ages were determined on six rhyolite domes and flows. The ages range from 400,000 to 700,000 years, although many undated rhyolites are probably younger. Some small spinal protrusions, domes, and pumice rings on the upper southeast flank may be less than 10,000 years old. In contrast to the relative antiquity of many rhyolites on the flanks, those in the caldera are commonly younger than Mazama ash and as young as 1,400 years.

Ash flows, pumice falls, mudflows, and other pyroclastic deposits are common on the west and east flanks of Newberry and are likely present at depth on the north and south flanks below the veneer of basalt and basaltic andesite flows. The oldest known unit occurs on the lower northeast and east flank and consists of a widespread rhyolitic ash flow identified by G. W. Walker (pers. commun., 1973) during reconnaissance mapping of adjacent areas to the east. It has been referred to informally as the Teepee Draw ash flow for outcrops along Teepee Draw (stop 14, road log). The Teepee Draw ash flow crops out along ravine walls for at least 6 miles toward the caldera before being buried by younger rocks. Along some ravines it exceeds 70 ft in thickness, but it may be considerably thicker because its base is exposed only where kipukas of older basalt project through it; also the upper part is eroded. The ash flow crops out roughly over a 50° quadrant of the volcano, and, as it is apparently older than the surficial rocks on the other flanks, may be present at depth completely around the volcano. Original volume of the unit is difficult to estimate without information on distribution at depth on the other flanks, but it likely is much more than 10 cubic miles. The Teepee Draw ash flow probably relates to an early, perhaps the earliest, period of caldera collapse.

Farther up the northeast flank the Teepee Draw ash flow is buried to progressively greater depth by alluvial sediments derived by erosion of rocks higher on the slopes. Interbedded in or underlying this alluvium are basalt and andesite flows, and several other ash-flow tuff units (stops 12 and 13, road log). Some of these ash flows are widespread, others occur in only a few scattered localities (commonly plastered on the caldera side of cinder cones). Most of these post-Teepee Draw ash flows are characterized by dark-colored, probably dacitic, pumice.

The west flank of the volcano contains two major tephra units. The older unit forms most of the lower two-thirds of the slope and overlies basalt flows and vent deposits that in many places are deeply eroded. This unit also occurs on the northeast flank where it occurs higher in the section than most of the ash flows. Although locally over 200 ft thick, it is rarely exposed. Most of the scree-covered roadcuts along the paved road on the west flank that leads to the caldera are in this unit (stop 2, road log). The unit consists of gray to black ash, lapilli, and small bombs and abundant accidental lithic fragments. The lapilli and bombs have characteristic cauliflowerlike surfaces and virtually all contain angular to rounded inclusions of rhyolite, dacite, and andesite; some inclusions are fused and frothed. Trenches dug in the deposit show that it is massive. In no place have we seen any indication of collapse of pumiceous lapilli or welding. All of the lapilli and bombs, as well as the ashy matrix, have the same normal natural remnant magnetization, as measured by a field fluxgate magnetometer, which suggests emplacement temperatures above the Curie point for the entire unit. Most of the unit was likely deposited as pyroclastic flows. In one area the unit is palagonitized and much more indurated and has characteristics more like that of a lahar. Lapilli and bombs identical to those in this unit are an ubiquitous and voluminous component of most alluvial deposits on the volcano. Furthermore, numerous gravel pits beyond the flanks of the volcano contain similar lapilli as a major component of the gravel. The original volume of the unit was probably several cubic miles. Its eruption could have been accompanied by caldera collapse.

The second major tephra unit on the west flank forms the smooth and gently dipping upper part of the flank extending for about 2 miles from the caldera rim. At localities farthest from the rim (stop 4, road log) the unit consists of many thin ash-flow units, commonly 3 to 20 ft thick. They are reddish to brownish in color and consist of andesitic scoria and pumice and accidental lithic fragments in a poorly sorted lithic- and crystal-rich ashy matrix. Bases of individual units are commonly welded. Toward the caldera rim, the ash flows progressively change character and in many places near the rim, as at Paulina Falls (stop 5, road log), thick units have the appearance of agglutinate flows. These deposits probably represent hot co-ignimbrite lag deposits.

Alluvial deposits occur over broad areas of the northeast, lower southeast, and upper south flanks where they form rounded slopes with virtually nonexistent exposure. Most of the deposits are gravel and sand, but the occurrence of boulders as float at some horizons along sides of ravines indicates that boulder beds are present also; pumice falls and ash flows are interbedded in the alluvial deposits in some areas. Because moat of the uppermost slopes of the volcano in areas where the alluvium is present are veneered by young pumice falls, we were not able to determine the origin of the deposits near the caldera. They may be fluvial but equally well may be of glacio-fluvial or glacial origin. Farther down the slope they are probably entirely fluvial, representing broad alluvial fans.

Much of the uppermost northeast, east, and southern flanks extending for about 1 to 2 miles outward from the caldera rim are formed of pumice and ash deposits derived from vents within the caldera. Most of the deposits are younger than Mazama ash, but scattered holes dug through them show that similar deposits underlie Mazama ash in a few places. Much of the east flank of the volcano is covered by an extensive pumice fall (atop 11, road log) derived from the vent for the Big Obsidian flow in the caldera (Sherrod and MacLeod, 1979). It extends as a plume oriented N. 80° E., is well over 10 ft thick near the caldera rim, and thins to about 10 in. at a distance of about 40 miles from the caldera.

Williams (1935, 1957) first recognized that the 4- to 5-mile-wide depression at the summit of the volcano is a caldera; Russell (1905) had originally suggested that it was a large glacial cirque. Owing to the absence of known ash flows on the flanks, Williams (1957) interpreted the caldera as resulting from "* * * drainage of the underlying reservoir either by subterranean migration of magma, or, more likely, by copious eruptions of basalt from flank fissures * * *," with summit collapse occurring along ring fractures. Higgins (1973), on the other hand, interpreted caldera formation as due to tectonic-volcanic collapse along fault zones that supposedly intersected at the summit. As noted by Peterson and Groh (1976), the main axis of the Brothers fault zone lies far north of the caldera, rather than crossing the summit, and faults within the zone do not appear to offset Newberry lavas. Also, most of the faults shown by Higgins on the upper part of Newberry that he uses as part of his structural interpretation could not be corroborated.

Ash flows and other tephra units are now known to be common and voluminous on the flanks. Thus the caldera seems much more likely to be the result of voluminous tephra eruptions from magma chambers below the former summit with concomitant collapse of the summit in a manner similar to that of most other calderas the size of Newberry's or larger. As there were several major tephra eruptions, it seems likely that collapse occurred several times, each collapse involving areas smaller than that of the present caldera. Accordingly, the present caldera is interpreted as several nested calderas of different age.

The ages of the calderas and of the ash flows that we consider to be related to their formation are poorly known. Higgins (1973) interprets the caldera age as Holocene on the basis of the absence of obvious widespread glacial deposits. However, many rocks on the volcano, particularly on its upper flanks, are now known to be several hundred thousand years old, so absence of glacial features probably is not meaningful. Also, several of the larger volcanoes on the east side of of the Cascade crest, which are about the same height as Newberry, show no obvious glacial features, yet some of them are nearly a million or more years old.

The ring fractures along which collapse occurred are not exposed, yet their general location is indicated by arcuate zones (fig. 2), visible on aerial photographs. It is not possible to relate individual ash-flow units to collapse along specific ring fractures, Also it is possible that collapse occurred along parallel ring fractures at the same time. Two parallel walls on the southeast side of the caldera present an interesting problem in interpreting the origin of the caldera wall sequences (described later). If the inner wall is the younger, then the area between the inner and outer wall may be formed of old caldera-fill deposits. The inner wall is thickly mantled by young tephra deposits, and the only exposures are a relatively small area of rhyolite in the lower part of the slope. The remainder of the slope on this wall dips 30° to 40°, near the angle of repose, and has a very uniform smooth shape. If the young tephra deposits were locally underlain by indurated rocks, as are present in the other walls, it seems likely that the slope would be more irregular. Thus this caldera wall likely is dominantly or entirely formed of relatively unconsolidated fragmental rocks, probably caldera-fill deposits. The rhyolite exposed locally at the base of the wall may be a faulted dome that was originally within the caldera. The upper part of the northeast wall, above exposures of caldera wallrocks, may also be formed of caldera-fill deposits, the older caldera wall being farther north.

Figure 2. Geologic sketch map of Newberry caldera. (click on image for an enlargement in a new window)

The walls of the caldera are mostly covered by younger deposits (talus, pumice falls, etc.) and the wallrocks are only locally exposed. They are described by Williams (1935) and in more detail by Higgins (Higgins and Waters, 1968; Higgins, 1973). The most continuous exposures are along the north wall extending from the Red Slide, north of Paulina Lake, to the northeast obsidian flow, which drapes the wall northeast of East Lake. The base of the wall sequence is formed of platy rhyolite. Higher in the section are basaltic andesite flows, palagonite tuff, cinder and agglutinated spatter deposits, and at the top of the exposures are palagonite tuffs.

Exposures in the east wall are found only from about midway on East Lake northward to the Sheeps Rump, a cinder cone along the wall at the northeast corner of the caldera. The intermittent exposures consist of basaltic andesite or andesite flows, palagonite tuff, and pumice deposits. The last range from unwelded and uncollapsed pumice to densely welded deposits.

Other than exposures of rhyolite south of East Lake, the inner south caldera wall is unexposed and is covered by thick pumice fall deposits. The outer south wall has exposures only near the Big Obsidian flow and farther west below Paulina Peak. The best exposures, midway between Paulina Peak and the Big Obsidian flow, consist of a lower platy rhyolite, overlain by basaltic andesite flows, dikes, and cinder deposits with local interbedded pumice fall deposits, and with a cliff-forming obsidian flow at the top. At the west end of this exposure are outcrops of bedded pumiceous tuffs that have been fused near their contact with the overlying obsidian flow; part of the apparent base of the obsidian flow is fused tuff even on the east end of the outcrop. South of the Big Obsidian flow vent are exposures of andesitic or dacitic ash flows and basaltic andesite flows, and below Paulina Peak a ledge of basalt crops out in one small area.

Most of the caldera floor is formed of rhyolitic rocks including domes, flows, and pumiceous tephra deposits (ash flows, pumice falls, explosion breccias), but basaltic andesite and andesite flows and palagonite tuff rings occur in several places. Williams (1935) and Higgins (1973) have provided much useful data on the floor rocks, although we interpret the relative ages of many of the units differently. The floor rocks represent but the tip on the iceburg with respect to the total fill present in the caldera. A core hole, currently being drilled for the U.S. Geological Survey has so far penetrated 1,700 ft of caldera-fill deposits, mostly pumiceous tephra, with a few rhyolitic and dacitic flows, and lake sediments 1,000 ft below the surface.

Mazama ash is a useful datum with which to roughly subdivide the caldera rocks as older or younger than about 6,700 ¹⁴C years. All the mafic rocks are older than Mazama except for those along the East Lake fissure. The East Lake fissure has not yielded carbon for dating, but the summit basaltic andesite flows along the same fissure less than a mile north are about 6,090 ¹⁴C years old (S. W. Robinson, written commun., 1978). Included in the pre-Mazama basaltic rocks are the interlake basaltic andesite flow (east shore of Paulina Lake), Red Slide cinder vent and flows (north side of Paulina Lake), Sheeps Rump cinder cone and flow (northeast side of East Lake), and east-rim fissure and the associated flow from it that extends to East Lake. The palagonite tuff rings that occur southeast of Paulina Lake (Little Crater) and near the south shore of East Lake (stop 10, road log) are also pre-Mazama in age. Rhyolitic rocks of pre-Mazama age include two domes along the south shore of Paulina Lake, a large obsidian flow in the northeast corner of the caldera, an obsidian dome that crops out near the caldera wall south of East Lake with an obsidian flow that extends northward from the dome to East Lake, and a poorly exposed dome(?) south of the Central Pumice cone. In addition, rhyolitic pumice falls, lacustrine deposits (i.e., east shore of Paulina Lake), and landslide deposits (north of Paulina Peak) are known to locally underlie Mazama ash.

The post-Mazama rhyolitic deposits occur in the eastern half of the caldera (fig. 2). They include obsidian flows, pumice rings and cones, ash flows, pumice falls, and other pumiceous tephra deposits. Hydration-rind dating by Friedman (1977) indicates that the Central Pumice cone, between East and Paulina Lakes, and the Interlake and Game Hut obsidian flows that crop out north and south of it, are only slightly younger than Mazama ash (¹⁴C age, 6,600-6,700 years). The East Lake obsidian flows are apparently about 3,500 years old, and the Big Obsidian flow is about 1,400 years old.

Widespread pumiceous tephra deposits cover much of the eastern part of the caldera. They underlie the East Lake obsidian flows and probably are slightly older than the Central Pumice cone and the obsidian flows on its north and south sides. These tephra deposits may have been derived from several different vents, but holes dug through them indicate that they consist dominantly of 5 to 10 ft of massive to poorly bedded pumice with accidental fragments (palagonite tuff, basalt, rhyolite, etc.) overlain by several feet of well-bedded mud-armored pumice, accretionary lapilli, ash, and pumice (stop 9, road log).

The youngest period of volcanism within the caldera was associated with the vent for the Big Obsidian flow. It began with eruptions that produced a widespread pumice fall that covers the southern part of the caldera and the eastern flank of the volcano (Sherrod and MacLeod, 1979). ¹⁴C ages of l,720±200 (Higgins, 1969) and 1,550±120 (S. W. Robinson, written commun., 1978) years were obtained on carbon directly beneath the fall. On the basis of thickness measurements from 150 holes dug through the pumice fall, the axis of the fall extends N. 80° E., away from the vent for the Big Obsidian flow. At about 5-1/2 miles from the vent the fall is about 12 ft thick, and at 40 miles it decreases to about 10 in.

The pumice fall was followed by eruptions that produced an ash flow that extends over a broad area between the Big Obsidian flow and Paulina Lake. It is well exposed in roadcuts near the Big Obsidian flow (stop 7, road log). ¹⁴C ages of the ash flow are 1,270±60 and 1,390±200 years (Pierson and others, 1966; Meyer Reubin, in Friedman, 1977), with an older age (2,054±230 years) obtained many years mgo by Libby (1952). The final event was the eruption of the Big Obsidian flow and the domal protrusion that marks its vent. Slight collapse occurred over a one-half-mile-wide area around the vent before the flow was erupted. The flow extends northward from near the outer caldera wall to near the paved road in the caldera and, in its northern part, partly filled an older pumice ring (stop 8, road log).

Considering the long time over which eruptions took place on Newberry, the volcano should be considered dormant but capable of future eruptions even though about 1,300-1,400 years have transpired since the last eruptions. Newberry is ideally suited for those who wish to see diverse volcanic features. Its rocks range widely in composition, and examples from it could be used to illustrate a nearly complete atlas of the types of products of volcanism.

LIST OF REFERENCES

Beyer, R. L., 1973, Magma differentiation at Newberry crater in central Oregon: Eugene, University of Oregon, Ph. D. thesis, 84 p.

Chitwood, L. A., Jenson, R. A., and Groh, E. A., 1977, The age of Lava Butte: The Ore Bin, v. 39, no. 10, p. 157-164.

Friedman, Irving, 1977, Hydration dating of volcanism at Newberry Crater, Oregon: U.S. Geological Survey Journal of Research, v. 5, no. 3, p. 337-342.

Greeley, Ronald, 1971, Geology of selected lava tubes in the Bend area, Oregon: Oregon Department of Geology and Mineral Industries Bulletin 71, 47 p.

Greene, R. C., Walker, G. W., and Corcoran, R. E., 1972, Geologic map of the Burns quadrangle, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-680, scale 1:250,000.

Higgins, M. W., 1969, Airfall ash and pumice lapilli deposits from Central Pumice cone, Newberry caldera, Oregon, in Geological Survey Research 1969: U.S. Geological Survey Professional Paper 650-D, p. D26-D 32.

______ 1973, Petrology of Newberry volcano, central Oregon: Geological Society of America Bulletin, v. 84, p. 455-488.

Higgins, M. W., and Waters, A. G., 1968, Newberry caldera field trip, in Andesite Conference Guidebook; Oregon Department of Geology and Mineral Industries Bulletin, p. 59-77.

Laidley, R. A., and McKay, D. S., 1971, Geochemical examination of obsidians from Newberry caldera, Oregon: Contributions to Mineralogy and Petrology, v. 30, p. 336-342.

Libby, W. F., 1952, Chicago radiocarbon dates III: Science, v. 161, p. 673-681.

MacLeod, M. S., Walker, G. W., and McKee, E. H., 1975, Geothermal significance of eastward increase in age of upper Cenozoic rhyolitic domes in southeast Oregon: Second United Nations Symposium on the Development and Use of Geothermal Resources, Proceedings, v. 1, p. 465-474.

McKee, E. H., MacLeod, M. S., and Walker, G. W., 1976, Potassium-argon ages of Late Cenozoic silicic volcanic rocks, southeast Oregon: Isochron/West, no. 15, p. 37-41.

Peterson, M. V., and Groh, E. A., 1969, The ages of some Holocene volcanic eruptions in the Newberry volcano area, Oregon: The Ore Bin, v. 31, p. 73-87.

______ 1976, Geology and mineral resources of Deschutes County, Oregon: Oregon Department of Geology and Mineral Industries Bulletin 89, 66 p.

Pierson, F. J., Jr., Davis, E. M., and Tamers, M. A., 1966, University of Texas radiocarbon dates IV: Radiocarbon, v. 8, p. 453-466.

Russell, I. C., 1905, Preliminary report on the geology and water resources of central Oregon: U.S. Geological Survey Bulletin 252, 138 p.

Sherrod, D. R., and MacLeod, M. S., 1979, The last eruptions at Newberry volcano, central Oregon [abs.]: Geological Society of America, Abstracts with Programs, v. 11, no. 3, p. 127.

Walker, G. W., 1974, Some implications of Late Cenozoic volcanism to geothermal potential in the High Lava Plains of south-central Oregon: The Ore Bin, v. 36, no. 7, p. 109-119.

Walker, G. W., Peterson, M. V., and Greene, R. C., 1967, Reconnaissance geologic map of the east half of the Crescent quadrangle, Lake, Deschutes, and Crook Counties, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-493, scale 1:250,000.

Williams, Howell, 1935, Newberry volcano of central Oregon: Geological Society of America Bulletin, v. 46, p. 253-304.

______ 1957, A geologic map of the Bend quadrangle, Oregon, and a reconnaissance geologic map of the central portion of the Nigh Cascade Mountains: Oregon Department of Geology and Mineral Industries Map, scales 1:125,000 and 1:250,000.