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


George W. Walker, U.S. Geological Survey, Menlo Park, California 94025
Bruce Nolf, Central Oregon Community College, Bend, Oregon

This road log gives point by point information on the geology from Bend to Harney Basin and into the Diamond Craters area at the southeast margin of Harney Basin. The route of the field trip and the locations of some important geographic landmarks are shown on figures 2A, 2B, and 2C; the road log is keyed to these figures by mileages between principal points of interest and by field trip stops.


Junction U.S. Hwys. 97 and 20 in Bend, Oregon, heading east on U.S Hwy. 20.
0.8 Jct. with Pilot Butte Road. Pilot Butte, a late Pliocene or early Pleistocene cinder cone, is lapped by a pumiceous air-fall dated at between 1 and 2 m.y.

For next few miles we travel on Pleistocene basalt cut by northwest-trending faults with displacements of up to 40 m; physiographic relief reduced or eliminated by sedimentation in downthrown blocks.
4.3 Jct. of road to Alfalfa. For several miles traverse young basalt partly covered by Mazama ash. Highway begins climb through older (7 m.y.) near vent basalt flows and flow breccias that characterize Horse Ridge. Northwest-trending faults, with up to 150 m vertical offset, cut these basalts.
20.6 STOP NO. 1. At road jct. Geologic features that can be seen from this point include (from oldest to youngest):

a. Pine Mountain to southeast, a kipuka of silicic to intermediate volcanic rocks dated at about 21 m.y.

b. To west and north, faulted late Miocene (~7 m.y.) basalt of Horse Ridge. Note fault scarp north of and parallel to the highway. North of this fault are several additional and parallel normal faults which have localized basaltic vents in the form of small lava cones and cinder cones.

c. To southwest the large volcanic edifice is Newberry Volcano. (Refer to MacLeod and others, this volume.)

d. To south, China Hat and East Butte represent silicic domes near west end of the age progression of silicic domal rocks along the Brothers fault zone (MacLeod and others, 1976). Age of China Hat is 0.78±0.20 m.y. and East Butte is 0.85±0.05 m.y.

e. Low terrain to south and southeast is underlain by young basalt flows and lacustrine and fluviatile sediments of probable Pleistocene age.

Note that normal faulting along this part of the Brothers fault zone is bracketed by the 7 m.y. basalt of Horse Ridge and the basin fill. Faulting of Newberry Volcano and of flows in Bend basin indicate some displacements in middle Pleistocene to Holocene time.

As we leave this stop, note thin-bedded lacustrine sediments in roadcut on north side of highway and a mile or so further on, and also on north side of highway, echelon normal faults that cut basalt of Horse Ridge.
24.4 Millican. Note scarp of another echelon fault north of highway.
30.3 Second flow from top of basalt sequence exposed on scarp north of highway has been dated at 6.5 m.y.
32.7 Younger, much less faulted flows interbedded with gravelly sediments adjacent to highway.
33.9 Jct. with State Hwy. 27.
41.1 STOP NO. 2. at Brothers rest area. Flat topped ridge to northwest is capped by basalt of late Miocene age, probably comparable in age to basalt of Horse Ridge. Irregular tree covered hills to the north and ridges forming northern skyline are composed of Eocene to middle Miocene volcanic and volcaniclastic rocks, including the Picture Gorge Basalt and the John Day and Clarno Formations. Foreground is underlain by Quaternary basalt and sediments. Source for some younger flows is low conical hill to the east-northeast with a tank on top. Rounded hill to south of highway is rhyolite of unknown age, probably older than the age progression.
48.1 Jct. with road south to Frederick (Fredrick) Butte. Fault scarp north of highway is in late Pliocene or Pleistocene basalt.
50.8 Jct. Camp Creek road. Turn northeast off highway on side trip. Go 3.6 miles to cattle guard.

STOP NO. 3. Rim to the northeast of cattle guard is 3.9 m.y. old ash-flow tuft which apparently came from vent area south of highway. Frederick Butte, to south southwest, is 3.9 m.y. old and composes one of several rhyolite domes on the east margin of a circular collapse area, probably the source of this ash-flow tuft.

Note that the ash-flow tuft and overlying basalt are cut by faults and here are down-dropped south of the fault. South of highway both basaltic and rhyolite vents can be seen as low hills. Hampton Butte, to east, is a volcanic pile that includes older rocks and intrusive rhyolite. The Clarno Formation crops out on west side and intrusive rhyolite forms conspicuous hill on south side of Hampton Butte. Return to U.S. Hwy 20 and continue east.
58.2 South of highway both north and south facing fault scarps are visible that cut Pliocene and Pleistocene basalts (younger than ash-flow tuft from Frederick Butte vent complex).

Note ash-flow tuft rims north of highway.
61.6 Hampton Station. As we approach Glass Buttes, the faulted basalts on both sides of highway are probably Pliocene in age. Some of these basalts appear to lap against the rhyolite domes of Glass Buttes.
72.6 Jct. Buck Creek Road, to G. I. Ranch.
75.0 Jct. with BLM road to south. Side trip to outcrops of dated obsidian associated with Glass Buttes volcanic complex. Drive 2.7 miles to small reservoir.

STOP NO. 4. Outcrops of obsidian, dated at 4.9 m.y., on both sides of wash. Glass Buttes complex is elongate in northwest-southeast direction, approximately parallel to the Brothers fault zone. At the east end of this group of rhyolite domes are opalite-type mercury deposits and a thermal well. Return to highway.

Note large mafic vent north of highway that fed Plio-Pleistocene basalt flows. Proceed east on U.S. Hwy. 20.
81.4 Juniper-covered hills north of highway are another rhyolitic complex elongate northwest-southeast parallel to the fault zone.
84.8 Squaw Butte visible ahead. Squaw Butte sits in the middle of another rhyolitic complex, dated at 5.1 m.y., surrounded by basalt which in places laps onto the rhyolite.
96.2 Dry Mountain north of the highway is a large rounded volcanic pile of faulted middle Miocene hypersthene andesite lapped by 6-m.y.-old ash-flow tufts. A few miles north of Dry Mountain, pre-Cenozoic rocks are exposed, some as old as Devonian. Valley north of highway and south of Dry Mountain is filled with Pleistocene gravel and a few thin Pliocene and Pleistocene basalt flows.
102.4 Jct. U.S. Hwy 395 (Riley). Fault scarps cutting Pliocene or possibly late Miocene basalt flows to the north and northeast of highway.
103.7 Sharp peak on southern skyline is Iron Mountain, a rhyolite dome dated at 2-3 m.y. This appears to be the only exception in the well-defined age progression in silicic volcanism.
109.0 Enter area underlain by palagonitic (basaltic) bedded tufts of Pliocene and Pleistocene age.
109.7 Ditch on north side of highway provides best exposures of these tuffs.
111.6 STOP NO. 5: Highway rest area.
112.1 Palamino Buttes, south of highway, consists of 6.5-m.y.-old rhyolite; adjacent to Buttes on east is eroded basaltic cinder cone. Buttes lapped by basalt flows dated elsewhere at 2-3-m.y. Both the basalt and underlying rhyolite are cut by northwest-trending normal faults with decreased offset of the basalt.
116.1 South of highway for next several miles, rim of 2-3-m.y. basalt underlain by tuffaceous sedimentary rocks generally referred to as part of the Harney Formation. These sedimentary rocks rest on the Rattlesnake Ash-flow Tuft.
119.2 Exposure of Rattlesnake Ash-flow Tuft in roadcut.
120.5 Outcrop of rhyolite south of highway and additional outcrops to northeast and north of highway. These rhyolites are part of the Burns Butte volcanic complex which has been dated at 7.8 m.y. Within the complex are some mafic cinder cones.
123.0 Borrow pit on south side of highway exposes one of these mafic cinder cones. Clastic dikes cutting these cinder cones have recently been described by Peterson (1978).
126.1 City limits of Hines, Oregon.
128.9 Jct. of U.S. Hwy 20 (also U.S. Hwy 395) and State Hwy. 78 in the center of Burns, Oregon. Burns lies at the northwest margin of a nearly circular area that represents central Harney Basin, an area characterized mostly by flat-lying younger fill and some dune deposits. Central Harney Basin lies within a much larger structurally depressed area that coincides approximately with the physiographic basin outlined on fig. 2. The structural basin evolved in part concurrently with the eruption of late Miocene ash-flow tufts, which are traversed by the next stage of the field trip.

Turn north on U.S. Hwys. 20 and 395.
131.6 Jct. U.S. Hwy. 20 and 395, turn north on 395.
135.1 STOP NO. 6: Exposures, about 20 m thick, of Rattlesnake Ash-flow Tuft, which has been dated at about 6.4 m.y. on the basis of numerous analyses. A single flow and cooling unit of rhyolitic ash-flow tuft that rests on bedded sedimentary rocks. Consists of a well developed basal vitrophyre, an overlying glassy zone containing abundant spherulites, and a thick vapor-phase zone characterized by abundant lithophysae above that. The unit dips gently southward into Harney Basin. This ash-flow tuft originally covered nearly 50,000 km of east-central and southeast Oregon, reaching a maximum thickness near Iron Mountain (southwest part of Harney Basin) of more than 60 m.

Vents for this and underlying late Miocene ash-flow tufts are buried beneath younger till in the lower central part of Harney Basin. Only indirect and meager evidence is available as to their location and character.
136.1 STOP NO. 7: Prater Creek Ash-flow Tuft exposed in road cut. Single flow and cooling unit that is 8.4 m.y. old, and is slightly more mafic than the Rattlesnake and Devine Canyon Ash-flow Tufts. This unit is characterized by low crystal content and abundant lithophysae. Maximum thickness is about 30 m; its extent is poorly known because it is largely buried by younger materials including the Rattlesnake Ash-flow Tuft. Unit is underlain by bedded tuffaceous sedimentary rocks; inter-ash-flow tuft units thin away from central Harney Basin.
136.8 Cross railroad tracks.
138.1 STOP NO. 8: Upper non-welded part of Devine Canyon Ash-Flow Tuft. Columned ash flow tuft across creek to the north-northwest is welded and compacted lower part of the same unit, samples of which have been dated from a number of widely separated localities at about 9.2 m.y.
142.2 STOP NO. 9: Outcrops on both sides of road of Devine Canyon Ash-flow Tuft. A single cooling, probably multiple-flow unit that covers more than 18,000 km centered on the east halt of Harney Basin and extending north, south, and east of the basin. Maximum thickness is about 35 to 40 m, although most of the unit is less than 10 m thick. Isopaching suggests a source caldera in the vicinity of Burns, perhaps with its west margin in the vicinity of Burns Butte. Contains as much as 30 percent crystals and crystal fragments, mostly sodic sanidine, some quartz, and minor, ubiquitous green iron-rich clinopyroxene. Thinner distal ends tend to be glassy, whereas thicker sections exhibit more devitrification and locally extensive vapor-phase alteration.

At this point turn around and return to Burns.

0.0 Jct. of U.S. Hwys. 20 and 395 with State Hwy. 78. Head east on 78.
1.7 Jct. State Hwys. 78 and 205. Turn south on 205.
12.2 STOP NO. 10: Viewpoint on spur gravel road, south rim of Wrights Point. Wrights Point consists of a 2.4-m.y. valley-fill basalt flow which has been eroded into positive relief. The point extends 6.5 km from here eastward; the vent is several kilometers to the west. Visible beneath the basalt flow are some of the best exposures of bedded tuffaceous sedimentary rocks of the Pliocene Harney Formation.

From this point looking to the south-southwest, the juniper-covered hill is Dog Mountain, original site of the type section ot the Harney Formation. Inasmuch as Dog Mountain is a palagonite tuft ring, atypical of the formation, road cuts on Wrights Point have been recently designated a reference section (Walker, in press). The tuft ring is late Pliocene. West of Dog Mountain are a number of additional young basaltic vents (tuft rings, cinder cones, lava cones, and dikes). East and southeast of Dog Mountain are many more young basaltic vents, including those at Malheur National Wildlife Refuge Headquarters, at Coyote Buttes, and Diamond Craters.

Steens Mountain (elev. 9,733 feet; 2,967 in), a west-tilted fault block capped by the middle Miocene Steens Basalt dominates the southeast skyline. On the east-southeast skyline is Duck Butte, a large rhyolitic dome complex about 10 m.y. old, considered by MacLeod and others (1976) as probably the oldest rhyolitic unit within the age progression.
22.9 Excellent view to the west of the rhyolitic dome at Iron Mountain.
23.8 Bridge at Narrows. Some geophysical data suggest that one or more source calderas for the late Miocene ash-flow tufts are centered about here (H. R. Blank, oral communication, 1974). Harney Lake, to west, is the current sump of the Harney Basin drainage system. Low hills to the east are young basaltic cinders, palagonite tuffs, and related flows.
25.4 Jct. with road to Malheur National Wildlife Refuge Headquarters and to Princeton, Oregon. Along this road are numerous young basaltic vents, mostly characterized by low rounded hills consisting of red cinders, common cored bombs, and agglutinated basalt.
27.8 STOP NO. 11: Roadcut in a crystal-poor pumicite that is thoroughly altered by zeolitization (erionite, clinoptilolite, etc) and may represent either an ash-flow tuft or a pumice slurry deposit that flowed into shallow water. Pumice fragments are neither collapsed nor welded.
30.9 Excavated basaltic cinder cone to east of highway. Rim on west side of road is welded pumiceous ash-flow tuft, probably Rattlesnake Ash-flow Tuft. Cap on Saddle Butte is a basalt flow interstratified with the Harney Basin depositional sequence.
33.9 Exposures in low roadcut of non-compacted, non-welded, partly altered crystal-rich Devine Canyon Ash-flow Tuft. Alteration suggests that this also may represent deposition of ash-flow in wet environment.
40.5 STOP NO. 12: On moderately well welded crystal-rich Devine Canyon Ash-flow Tuft (9.2 m.y.). This is overlain by pumiceous ash-flow tuft to the west and is underlain locally by as much as 150 m of bedded tuffaceous sediments which lap south and southeastward onto middle Miocene (~15 m.y.) Steens Basalt. The overlying pumiceous tuft also thins to the south.

Note the several northwest-trending fault scarps in which units are stepped up to the south. This is a prominent northwest-trending zone which can be traced almost 100 km.
41.3 Exposures of densely welded Devine Canyon Ash-flow Tuft.
42.1 Turn left on paved road to Diamond, Oregon.
43.6 Rimrock on both sides of road is Devine Canyon Ash-flow Tuft. View to south-southwest of large northeast-trending, east-facing scarp along the fault zone that separates the west-tilted High Steens block from the Jackass Mountain block. This is one locality near the southern margin of the Brothers fault zone where a northeast-trending fault system curves into a northwest-trending system.
44.8 Excellent profile to the north of the Diamond Craters volcanic complex. This complex has been studied by Peterson and Groh (1964). From here can be seen a few vents and local gentle doming of basalt flows within the complex. The valley to the north and east is underlain by surficial deposits resting on Devine Canyon Ash-flow Tuft which is faulted and underlies the Diamond Craters complex.
48.9 Turn north on gravel road to Diamond Craters.
49.9 Left at "Y" on road to Diamond Craters. Shortly beyond this point we see basalt flows of the Diamond Craters complex that lap onto Devine Canyon Ash-flow Tuft.
52.1 Road jct. at borrow pit in cinder cone. The principal part of the complex is west and north of this point.

STOP NO. 13; END MILEAGE LOG. Travel from here is dependant on time available and interest of participants. Following are notes from selected viewpoints.

a. On hill top, approximately 1 km north of borrow pit jct., we are on the south margin of a 2.3-km-long northwest-trending graben along the crest of an elongate domal structure in the basalt. This graben was apparently the site of little or no eruptive activity. Summit collapse probably resulted from withdrawal of lava from a high-standing magma body beneath this up-arched structure. The complex includes five of these elongate domal structures all of which are oriented in a northwest-southeast direction.

Four of the five are characterized by various degrees ot summit fracture and collapse.

In the near foreground to the south is a prominent pit crater and related basalt flows. Many of these youngest vents are localized at the margin of domal structures (at point of maximum flexure) and represent points of lava egress at times of summit collapse.

The tendency to use degree of vegetative cover to determine relative ages of flows can be misleading. For example, in the foreground to the south, the flow with more sagebrush cover laps onto the less vegetated flow.

Based on evaluation of weathering features in this dry environment, we believe that this basaltic complex is late Pleistocene in age, rather than Holocene. Thus far no materials suitable for isotopic dating have been found, but an age of 17,000±2,000 years was obtained by Friedman and Peterson (1971) on hydration rinds of rhyolitic material enclosed in basalt bombs.

Jackass Mountain, to the west-southwest, is the high point on the Jackass Mountain fault block which is bounded by prominent scarps. Most of the rims to the south are faulted and eroded Devine Canyon Ash-flow Tuft. Some patches of Rattlesnake Ash-flow Tuft are present on the back (west) slope of Steens Mountain. The giant west-dipping Steen Mountain block is bounded on the east by a precipitous northeast-trending, 1,800-in-high fault scarp, and is lapped on the west by sedimentary rocks containing a Barstovian vertebrate fauna and by late Miocene ash-flow tufts and interbedded sedimentary rocks.

NOTE: For those interested, a loop road between Frenchglen and northern Catlow Valley traverses the dip slope and crest of the High Steens.

The lower country to the north of the Diamond Craters complex is locally underlain by accumulations of young basalt, mostly valley-filling flows. Some of these flows effectively closed off the pluvial drainage from Harney Basin.

On the skyline to the north, older rocks crop out north of the Brothers fault zone.

b. Approximately 2 km west of the graben is a large vent area referred to by Peterson and Groh (1964) as the central crater complex. This complex is localized by a partly tilled northwest-trending graben at the crest of another elongate domal structure. Probably graben development and eruptive activity were at least in part contemporaneous. More than 30 separate vents have been identified within this depression. Vent activity ranged from quiet emission of lava to highly explosive volcano-phreatic events. Blocks of Devine Canyon Ash-flow Tuft and associated units are found in the tephra; exotic blocks up to 50 m in maximum dimension are present. Other features within this central crater complex include exhumed conduits, driblet spires and spatter cones, and small-scale collapse structures.

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Last Updated: 28-Mar-2006