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
||Junction U.S. Hwys. 97 and 20 in Bend, Oregon,
heading east on U.S Hwy. 20.|
||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
||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.|
||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
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.
||Millican. Note scarp of another echelon fault north
||Second flow from top of basalt sequence exposed on
scarp north of highway has been dated at 6.5 m.y.|
||Younger, much less faulted flows interbedded with
gravelly sediments adjacent to highway.|
||Jct. with State Hwy. 27.|
||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.|
||Jct. with road south to Frederick (Fredrick) Butte.
Fault scarp north of highway is in late Pliocene or Pleistocene basalt.|
||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
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.
||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.
||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
||Jct. Buck Creek Road, to G. I. Ranch.|
||Jct. with BLM road to south. Side trip to outcrops of
dated obsidian associated with Glass Buttes volcanic complex. Drive 2.7 miles to
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.
||Juniper-covered hills north of highway are another
rhyolitic complex elongate northwest-southeast parallel to the fault
||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
||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.|
||Jct. U.S. Hwy 395 (Riley). Fault scarps cutting
Pliocene or possibly late Miocene basalt
flows to the north and northeast of highway.|
||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
||Enter area underlain by palagonitic (basaltic) bedded
tufts of Pliocene and Pleistocene age.|
||Ditch on north side of highway provides best exposures of these
||STOP NO. 5: Highway rest area.|
||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
||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.|
||Exposure of Rattlesnake Ash-flow Tuft in roadcut.|
||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.|
||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).|
||City limits of Hines, Oregon.|
||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.
||Jct. U.S. Hwy. 20 and 395, turn north on 395.|
||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
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.
||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
||Cross railroad tracks.|
||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.|
||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
At this point turn around and return to Burns.
LOG FROM BURNS TO DIAMOND CRATERS
||Jct. of U.S. Hwys. 20 and 395 with State Hwy. 78.
Head east on 78.|
||Jct. State Hwys. 78 and 205. Turn south on 205.|
||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
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.
||Excellent view to the west of the rhyolitic dome at
||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
||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
||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
||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.|
||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.|
||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.
||Exposures of densely welded Devine Canyon Ash-flow Tuft.|
||Turn left on paved road to Diamond, Oregon.|
||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
||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.|
||Turn north on gravel road to Diamond Craters.|
||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.|
||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
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
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.