Geologic Map of the Bend 30- × 60-Minute Quadrangle, 

Central Oregon 

By David R. Sherrod, Edward M. Taylor, Mark L. Ferns, William E. Scott, 

Richard M. Conrey, and Gary A. Smith 

Pamphlet to accompany

Geologic Investigations Series I–2683

2004

U.S. Department of the Interior

U.S. Geological Survey
1

Geologic Map of the Bend 30- × 60-Minute Quadrangle,

Central Oregon

By 

David R. Sherrod1

, Edward M. Taylor2, Mark L. Ferns3, William E. Scott4, 

Richard M. Conrey5, and Gary A. Smith6

DISCUSSION

INTRODUCTION

The Bend 30- × 60-minute quadrangle has been the 

locus of volcanism, faulting, and sedimentation for the past 

35 mil lion years. It encompasses parts of the Cascade Range 

and Blue Mountains geomorphic provinces, stretching from 

snowclad Quaternary stratovolcanoes on the west to bare 

rocky hills and sparsely forested juniper plains on the east. 

The Deschutes River and its large tributaries, the Metolius and 

Crooked Rivers, drain the area (fi g. 1, sheet 1). To po graph ic 

re lief ranges from 3,157 m (10,358 ft) at the top of South 

Sis ter to 590 m (1,940 ft) at the fl oor of the Deschutes and 

Crook ed Rivers where they exit the area at the north-central 

edge of the map area (cross sections A–A''' and B–B'''). The 

map encompasses a part of rapidly growing Deschutes Coun ty. 

The city of Bend, which has over 50,000 people liv ing in its 

urban growth boundary, lies at the south-central edge of the 

map. Redmond, Sisters, and a few small er vil lag es lie scattered along the major transportation routes of U.S. Highways 

97 and 20 (fi g. 1).

This geologic map depicts the geologic setting as a ba sis 

for structural and stratigraphic analysis of the Deschutes 

basin, a major hydrologic discharge area on the east fl ank 

of the Cascade Range. The map also provides a framework 

for studying potentially active faults of the Sisters fault 

zone, which trends northwest across the map area from 

Bend to beyond Sisters. A series of index maps shows the 

sources of mapping used for our geologic depiction and 

other sources consulted during the preparation of the map 

(fi g. 2, sheet 1).

ABOUT THE MAP UNITS

Conventional lithologic criteria were used to assign rocks 

and deposits to geologic map units. For sedimentary strata, 

grain size and shape, sorting, and bed thickness are the main 

features used in interpreting origin of deposits. For volcanic 

rocks, two major subdivisions distinguish lava flows or 

domes from pyroclastic-fl ow deposits or tephra-fall de pos its. 

Chemical composition is also used as much as pos si ble; compositional classifi cation relies on the rec om men da tions of the 

IUGS Subcommission on the Sys tem at ics of Igneous Rocks 

(Le Bas and Streckeisen, 1991) but is mod i fi ed to include a 

fi eld for rhyodacite and sim pli fi ed to use 72 percent SiO2

 as 

the cutoff for rhyolite, re gard less of alkali content. Thus, the 

compositional divisions are basalt (<52 percent SiO2), basaltic 

andesite (≥52 and <57 percent SiO2), andesite (≥57 and <63 

percent SiO2), dacite (≥63 and <68 percent SiO2), rhyodacite 

(≥68 and <72 per cent SiO 2), and rhyolite (at least 72 percent 

SiO2). Unanalyzed rocks were assigned compositions by 

visual comparison with analyzed rocks. Features such as 

phe noc ryst abundance or remanent magnetization provided 

ad di tion al criteria for dis tin guish ing volcanic units.

Formal stratigraphic member names have been es tab -

lished previously for some strata of the John Day For ma tion, 

which is found in the eastern part of the map area. The 

Deschutes Formation, in the central part of the map area, is 

also formally named, but separate strata within it are pres-

 ent ly named only informally. Few Cascade Range units 

have been formally named; exceptions are some of the 

major py ro clas tic-fl ow and tephra-fall deposits exposed in 

the Bend area. Therefore, informal stratigraphic names are 

ap plied to many units in the Cascade Range. For example, 

the broadly defi ned unit of Quaternary basaltic andesite 

(Qba) is sub di vid ed lo cal ly to distinguish the lava from major shield vol ca noes or other extensive lava fl ows that can be 

rec og nized separately.

Numerical ages are assigned to dated units or to units 

emplaced over a short period of time and for which the 

dep o si tion al episode is bracketed by events of known age. 

We use the standard conventions of reporting age in millions of years ago (mega-annum, abbreviated Ma), or, for 

ra dio car bon ages, in carbon-14 years before 1950 A.D. (14C 

yr B.P.). For stratigraphic units emplaced between 10,000 

yr B.P. and 200,000 yr B.P., age is reported in thousands of 

years ago (kilo-annum, abbreviated ka). All isotopic ages 

from the map area are listed in tables 1 and 2. Potassiumargon ages (table 2) have been recalculated using modern 

decay constants (Steiger and Jäger, 1977) and therefore 

1

U.S. Geological Survey, Ha waii National Park, HI 96718

2

Oregon State University, Corvallis, OR 97331

3

Oregon Department of Geology and Mineral Industries, Baker 

City, OR 97814 4

U.S. Geological Survey, Vancouver, WA 98661

5

Washington State University, Pullman, WA 99164

6

University of New Mexico, Albuquerque, NM 87131
2

may differ slightly from original source publications. A 

simplifi ed location map (fi g. 3, sheet 1) plots the samples 

reported in table 2.

In our discussion of carbon-14 ages, we distinguish between calibrated years and 14C years to avoid confusion. The 14C time scale diverges from conventional calendar or sidereal 

years because the relative abundance of 14C in the atmosphere 

has varied over time (for example, Faure, 1986). Actual age 

(in sidereal years) generally increases when ra dio car bon ages 

are calibrated for organic materials older than about 2,500 

14C years and decreases for the younger sam ples (Stuiver and 

Reimer, 1993).

Magnetic polarity reversals, commonly preserved by volcanic rocks and readily measured during fi eld work, pro vide 

another means to constrain the depositional period for some 

strata and, for Quaternary volcanic rocks, even to as sign limiting numerical ages. A simplifi ed stratigraphic col umn shows 

several stratigraphic units positioned on the pa le o mag net ic 

time scale (fi g. 4).

The Description of Map Units explains the basis for 

subdividing rocks into stratigraphic units shown on the geo-

 log ic map. It is preceded by the Correlation of Map Units, 

which shows the relative stratigraphic position of all units. 

The following explanatory text is devoted to describing in 

greater detail the age assigned to several stratigraphic units, 

thereby providing both a stratigraphic synopsis and a sum-

 ma ry of recent work.

JOHN DAY FORMATION

The John Day Formation in the map area comprises as 

much as 4,300 m in thickness of sandstone, shale, ash-fall 

and ash-fl ow tuff, and lava fl ows, including rhyolite domes. 

Regionally the formation is perhaps best known for fallout tuff 

and stream-reworked tuffaceous strata exposed at the Painted 

Hills unit of the John Day National Monument, 100 km east of 

the map area. Some of the fallout tuff may have been derived 

from ancestral volcanoes in the Cascade Range (Robinson and 

others, 1984), but ash-producing volcanoes were also present 

in the Deschutes basin at the time. The John Day Formation 

has low permeability owing to di age net ic and hydrothermal 

alteration of once-glassy material to clay and zeolite minerals, 

resulting in loss of porosity.

An extensive sequence of southeast-dipping John Day 

strata is exposed from Haystack Butte southeast nearly to 

the Crooked River, including the area of Smith Rock State 

Park. These beds form the upthrown block of the northeaststriking Cyrus Springs fault zone (Smith and others, 1998), 

and rocks as young as Prineville Basalt (unit Tp) are tilted 

southeasterly by the deformation. The Deschutes For ma tion, 

the next-youngest stratigraphic unit, is undeformed. Thus the 

age of deformation is bracketed between early Oligocene and 

late Miocene time. Stratigraphic units across the fault are 

poorly matched, suggesting more than 7 km of right-lateral 

slip (Smith and others, 1998).

The John Day Formation is divided into members A 

through I elsewhere in the central-Oregon region (Peck, 1964; 

Robinson, 1975). The base of most members is defi ned by 

extensive ash-fl ow tuffs. Of these tuffs, only the memberH basal tuff reaches into the map area, where it forms the 

young est John Day strata north of the Cyrus Springs fault 

zone. Additional temporal correlations are provided by newly 

ob tained isotopic ages of 28.82±0.23 Ma from rhyolite lava 

at Gray Butte (in unit Tjr) and 29.53±0.09 and 29.57±0.17 

Ma from the tuff of Haystack Reservoir (Tjth), which indicate these rocks correlate with member-G strata (Smith 

and oth ers, 1998). Member-G tuff north of the map area has 

newly obtained isotopic ages of 29.54±0.10 and 29.61±0.10 

Ma (both single-crystal sanidine ages by 40Ar/39Ar method; 

Smith and others, 1998). Previous age estimates were slight ly 

young er—a weighted mean age of 28.3±0.2 Ma on the basis of 

four sanidine K-Ar ages (Fiebelkorn and others, 1983; Robinson and others, 1990). A lapilli-fall deposit beneath rhyolitic 

lava at Gray Butte is correlated herein with a de pos it found 

near the base of member F northeast of the map area. This 

deposit, the tuff of Rodman Spring (Smith and others, 1998), 

has a 40Ar/39Ar sanidine age of 32.49±0.30 Ma. Correlations 

with members E and B are suggested by the composition of 

basalt and basaltic andesite lava fl ows in the lower part of the 

sequence north of Gray Butte (units Tjb and Tjba). A tentative 

correlation between poorly exposed, al tered welded tuff in the 

oldest beds of the map area (unit Tjl) and member-A welded 

tuff elsewhere in the region is the basis for extending the age 

of the John Day Formation with in the map area back as far 

as late Eocene time.

A K-Ar age of 30.8±0.5 Ma (whole rock; Fiebelkorn and 

others, 1983) was reported from basaltic andesite lava, but 

we interpret that rock as an intrusion of John Day age (Tjbi). 

The dated intrusion cuts rocks as young as the ba sal tic andesite 

unit (Tjba). Other ages ranging from about 19 to 17 Ma were 

reported by Obermiller (1987; table 2) but are too young in 

view of Oligocene paleontologic ages from interbedded fossil 

fl ora and the newly obtained isotopic ages ranging from about 

32 to 27 Ma. Isotopic whole-rock ages as young as 10 Ma were 

reported from rhyolite exposed at Gray Butte, but we consider 

these ages spurious because the dated material is extensively 

hydrated (Obermiller, 1987).

Powell Buttes, at the southeast edge of the map area, is 

made of rhyolite domes and associated strata also of John Day 

age. The Powell Buttes rocks are relatively undeformed and 

untilted, unlike the John Day strata in the Gray Butte area. A 

K-Ar age of 28.3±1.0 Ma was obtained from Powell Buttes 

east of the map area (table 2). Core from drill holes on the 

west fl ank of Powell Buttes penetrated moderately weathered 

basalt or basaltic andesite lava beginning at 240 m depth; a 

K-Ar age of 30.1±1.1 Ma (table 2) was obtained from a sample 

collected at 310- to 320-m depth (Brown and others, 1980a, p. 

5; Evans and Brown, 1981). This sample may be reasonably 

assigned to basaltic andesite of the John Day Formation (unit 

Tjba) on the basis of its age.
3

DESCHUTES FORMATION

Sedimentary strata, pyroclastic rocks, and fewer lava 

fl ows of the Deschutes Formation are found in picturesque 

ex po sures along can yons of the Deschutes River and its trib-

 u tar ies north of Bend. The Deschutes For ma tion sed i men ta ry 

and py ro clas tic rocks were de pos it ed chiefl y in a fl uvial ba sin 

on the east fl ank of the Cas cade Range. Closer to the range 

crest, lava fl ows be come in creas ing ly abundant near ma jor 

erup tive cen ters. Lava fl ows are also dominant in out crops 

and the subsurface to depths of 300 m near Bend. A few lava 

fl ows were erupted east and southeast of the basin (Smith and 

others, 1987a), and intrabasinal cinder cones and small shield 

volcanoes are lo cat ed in the area between Sis ters, Redmond, 

Figure 4. Correlation of selected dated samples with paleomagnetic time scale. Patterns show remanent magnetization: dark fill, normal

polarity; white fill, reversed polarity. Bars showing age and standard de vi a tion are similarly patterned. See table 2 for references to age data 

except those for Pelton basalt and Round Butte members of Smith (1986). Remanent magnetization determined by using portable fluxgate 

magnetometer. Time scale from Cande and Kent (1992).

Basalt at top of Deep Creek grade

0

1

2

3

4

5

Age

(Ma)

0.78

Brunhes Matuyama

2.60

3.553

Gauss Gilbert

Normal Polarity Reversed Polarity Reversed Polarity Normal Polarity

Paleomagnetic

time scale

(Chrons)

Dated samples

5.705 Basalt of Opal Springs

Basalt of Tetherow Butte

Basaltic andesite of Steamboat Rock

Round Butte member

 (not in southern Deschutes basin)

Basalt of Redmond

3.56±0.30

3.97±0.05

4.8±0.4

5.06±0.03

5.31±0.05

Basalt of Lower Desert 5.43±0.05

5.77±0.07 6

7

8

Pelton basalt member

 (not in southern Deschutes basin)

7.42±0.22

6.74±0.20

Andesite, Bull Spring area 4.7±0.1

Rhyodacite southwest of Steelhead Falls 

Most of Deschutes Formation

in Bend 30' x 60' quadrangle

Rhyolite of Cline Buttes

2.9±0.2

Black Butte volcano 1.43±0.33

Basalt of The Island

(older intracanyon lava) 1.19±0.08

Base of South Sister

Shevlin Park Tuff

Tumalo Tuff and Bend Pumice

Desert Spring Tuff

~0.09

~0.17

~0.3

~0.6

Age

(Ma)

DESCHUTES FORMATION

Andesite of McKinney Butte 3.3±0.2

Squaw Back Ridge volcano
4

and Bend. Iso to pic ages indicate that the Deschutes Formation formed between about 7.4 and 4.0 Ma (Armstrong and 

others, 1975; Smith, 1986; Smith and oth ers, 1987a), but the 

oldest parts are exposed only north and per haps east of the 

map area.

Lava fl ows and partially to moderately welded ash-fl ow 

tuff (ignimbrites) form stratigraphic markers through out the 

Deschutes Formation. Many have been named in for mal ly by 

Smith (1986), and his terminology is adopt ed herein. One ashfl ow tuff not previously named, the tuff of Fremont Can yon 

(Stensland, 1970, tuff 5), and a few lava fl ows are in tro duced 

as additional informally named mem bers of the Deschutes 

For ma tion.

As described by Smith (1986, 1987) and Smith and oth ers 

(1987a), sedimentation in the lower part of the Deschutes 

For ma tion was induced by pyroclastic vol ca n ism in the near by 

Cascade Range during late Miocene time. Sub stan tial volcaniclastic sediment was deposited in the wake of py ro clas tic 

eruptions that choked the drain age sys tem with vol ca nic 

de bris. Pyroclastic-fl ow de pos its are found com mon ly in the 

low er part of the Deschutes For ma tion, where they form the 

ig n im brite members of the unit. In contrast, the upper part of 

the Deschutes For ma tion lacks widespread pyroclastic-fl ow 

de pos its and is dom i nat ed by paleosols and minor ash-fall 

deposits, sug gest ing that sediment ceased to overwhelm the 

alluvial system and that pyroclastic fl ows no longer entered 

the basin. In the map area, this transition oc curred prior to 

the emplacement of lava fl ows of the Tetherow Butte mem-

 ber, one of which yielded a whole-rock age of 5.31±0.05 Ma 

(40Ar/39Ar; Smith, 1986) from a sample col lect ed north of 

the map area.

Most of the Deschutes Formation exposed in the map 

area was emplaced between 6.74 and 4.7 Ma. The older age 

is that of the rhyodacite southwest of Steelhead Falls (fi g. 4 

and table 2), and the younger age is from basaltic andesite (in 

unit Tda) that caps exposures of Deschutes Formation lava 

fl ows near Bull Spring.

PLIOCENE VOLCANIC AND SEDIMENTARY ROCKS 

YOUNGER THAN THE DESCHUTES FORMATION

Lava fl ows are the predominant pre-Quaternary strata 

younger than the Deschutes Formation in the map area. The 

fl ows mantle broad areas east of the Deschutes River and accumulated near vents to form small shield volcanoes west of 

the river. Sedimentary deposits probably once overlay much 

of the lava east of the Deschutes River but are pre served only 

adjacent to Powell Buttes and on the lower slopes of terrain 

underlain by John Day Formation strata northeast and northwest of Gray Butte.

Isotopic ages have been obtained from few Pliocene lava 

fl ows emplaced after Deschutes time. The basalt of Redmond 

(unit Tbr), a plains-forming lava fl ow that underlies the Redmond area, has an age of 3.56±0.30 Ma (40Ar/39Ar, whole rock; 

Smith, 1986). The basaltic andesite of Squaw Back Ridge 

(Tbas) forms a shield volcano in the north-cen tral part of the 

map area; its age is 2.9±0.2 Ma (K-Ar, whole rock; Armstrong 

and others, 1975). Andesite of McKinney Butte north of Sisters has an age of 3.3±0.2 Ma (K-Ar, whole rock; Armstrong 

and others, 1975).

The sedimentary deposits (unit QTs) are undated, al-

 though ash-fall beds are present locally within the unit and 

could provide ages. Conceivably some of the sedimentary 

deposits may be as young as Quaternary.

PLEISTOCENE PYROCLASTIC DEPOSITS FROM THE 

EAST FLANK OF THE HIGH CASCADES

Several pyroclastic eruptions of Pleistocene age are 

re cord ed by deposits exposed near Bend and in adjacent 

parts of the Cascade Range. From oldest to youngest, the 

major de pos its are the Desert Spring Tuff, the Bend Pum-

 ice and Tumalo Tuff (tephra-fall and ash-fl ow deposits of 

a single magmatic episode), and the Shevlin Park Tuff. 

Each is suf fi cient ly unique to form a marker bed useful for 

as sign ing rel a tive ages to over ly ing and underlying units. 

The most ex ten sive exposures oc cur along the Sis ters fault 

zone, thus aid ing our understanding of the fault history 

there. Our knowl edge of the age of these deposits is derived mainly by strati graph ic correlation with distal tephra 

in the Basin and Range prov ince of northern Cal i for nia 

and Nevada, the de tails of which are scattered through-

 out the geologic lit er a ture and con stant ly being refi ned. 

Other isolated pum ice-fall de pos its are found through out 

the map area, but their age and cor re la tion re main poorly 

understood.

Desert Spring Tuff

The Desert Spring Tuff is a rhyodacitic pyroclasticfl ow de pos it erupted about 0.6-0.7 Ma. This age is based 

on (1) a geochem i cal correlation of the Desert Spring Tuff 

with its dis tal fallout equiv a lent, the Rye Patch Dam ash 

bed in the Great Basin of west ern Nevada (Sarna-Wojcicki 

and others, 1989), and (2) the fact that the Rye Patch Dam 

ash bed un der lies the Lava Creek B ash bed with small 

strati graph ic separation in cores from Tulelake, Calif. 

The age of the Lava Creek B ash bed is 0.62 Ma (Izett and 

Wilcox, 1982).

An age of 0.63 Ma was suggested for the Desert Spring 

Tuff (Sarna-Wojcicki and others, 1989) by cor re lat ing it to 

sim i lar ash in a cored section near Tulelake, Calif., and thence 

by in ter po la tion between the ages of Lava Creek B bed stratigraphically close above it and the Brunhes-Matuyama Chron 

boundary below (Rieck and others, 1992). In the 1980s, 

the chron boundary was cus tom ar i ly taken to be 0.73 Ma, 

but subsequent work has de ter mined a more precise age 

of 0.78 Ma (Shackleton and others, 1990; Baksi and others, 1992). Pre sum ably the slightly older chron bound ary 

im plies a slight ly older age for the Desert Spring Tuff. The 
5

age range we sug gest (0.6–0.7 Ma) is as accurate as is war-

 rant ed cur rent ly, but more pre cise estimates could be made 

by as sum ing con stant sed i men ta tion rate in the Tule Lake 

basin dur ing the 0.17 mil lion years between known dated 

events (mag net ic chronozone bound ary and age of Lava 

Creek B ash bed).

Bend Pumice and Tumalo Tuff

A single eruptive sequence is recorded by rhyolitic fall-

 out tephra and overlying pyroclastic fl ow deposits, the Bend 

Pumice and Tumalo Tuff. The Bend Pumice has been geochem i cal ly correlated with the Loleta ash bed (Sarna-Wojcicki 

and others, 1987), whose age is probably about 0.4 to 0.3 Ma. 

Several K-Ar ages ranging from 0.19±0.08 to 0.44±0.01 Ma 

have been obtained from (1) plagioclase sep a rat ed from pumice 

in the Tumalo Tuff and (2) whole rock ob tained from obsidian 

clasts in epiclastic strata that im me di ate ly underlie the Tumalo 

Tuff (Sarna-Wojcicki and oth ers, 1989). A weighted mean of 

four ages from plagioclase in the Tumalo Tuff is 0.3±0.1 Ma 

and is the preferred age (Sarna-Wojcicki and others, 1989). 

In contrast, hornblende separated from dacitic pumice of the 

Tumalo Tuff has ages of 1.30±0.23 and 1.04±0.20 Ma (SarnaWojcicki and others, 1989). These old er ages are explained 

as a consequence of inherited ra dio gen ic argon or xenocrysts 

(nonmagmatic crys tals in cor po rat ed into the magma before or 

during eruption). Previously de ter mined plagioclase ages of 

3.98±1.9 Ma from the Tumalo Tuff and 2.50±2.0 Ma from 

the Bend Pumice had large analytical er rors (Fiebelkorn and 

others, 1983). An equally ambiguous glass age of 0.83±1.5 

Ma was obtained from the fresh core of a dacitic pumice bomb 

(Armstrong and others, 1975).

Shevlin Park Tuff

Youngest of the Bend pyroclastic deposits is the Shevlin 

Park Tuff, an andesitic ash-fl ow tuff. Associated proximal or 

medial fallout deposits are unknown. The Shevlin Park Tuff 

is thought to be younger than about 0.17 Ma on the basis of 

several correlations.

(1) The unit has a distal fallout tephra thought to be the 

Summer Lake JJ ash bed on the basis of geochemical and 

paleomagnetic correlations (Gardner and others, 1992).

(2) The Summer Lake JJ ash bed is underlain by the 

Summer Lake KK ash bed, which is the distal fallout of 

an andesitic ash-fl ow tuff at Medicine Lake volcano, Calif. 

Strati graph ic separation between these two units is 30 to 40 

cm (Davis, 1985, fi g. 4).

(3) The Medicine Lake-derived andesitic ash-fl ow tuff 

has a newly determined 40Ar/39Ar age of 0.171±0.043 Ma 

(Herrero-Bervera and others, 1994); its age was previously 

known as 0.160±0.025 Ma on the basis of con ven tion al 

K-Ar ages from stratigraphically bracketing units at Med-

 i cine Lake volcano (J.M. Donnelly-Nolan and L.B. Gray, 

in Rieck and others, 1992).

Other Pleistocene pumice-fall deposits

Pumice lapilli and ash of fallout origin are exposed 

spo rad i cal ly in roadcuts, streambanks, and quarries throughout the area. Some of these deposits correlate with the Bend 

Pumice, itself the most notable of Pleistocene tephra-fall 

deposits in the area. Several deposits, however, lie above the 

Tumalo Tuff and are therefore younger than Bend Pumice. 

Compositionally the deposits range from rhyolite to andes ite, 

with andesitic fallout tephra being uncommon.

Aside from Bend Pumice, the only other Pleistocene 

tephra-fall deposit of known regional correlation is the pum-

 ice of Columbia Canal, which is as thick as 3 m in exposures 

along the Columbia Southern Canal, sec. 20, T. 17 S., R. 11 

E. This deposit has also been called the pumice of Columbia 

Canyon (Hill and Taylor, 1990), but we modify the title here 

to more closely approximate a named geographic feature. 

The relatively coarse size of pumiceous lapilli (to 2 cm) and 

obsidian clasts (to 4 cm) suggests the deposit was erupted 

from a nearby source, probably from a vent near Bearwallow 

Butte, about 10 km to the west.

The pumice of Columbia Canal is chemically similar 

to ash bed NN at Summer Lake (Sarna-Wojcicki and others, 1989). Ash bed NN is older than bed KK, whose age is 

0.171±0.043 Ma (Herrero-Bervera and others, 1994); stratigraphically, NN underlies KK by about 4.5 m (Davis, 1985). 

Given the large analytical uncertainty in the KK age and the 

large stratigraphic separation between beds NN and KK, it 

is diffi cult to place the age of NN more convincingly than 

between about 170,000 and 300,000 years.

Eruptive sources

The Desert Spring, Tumalo, and Shevlin Park Tuffs were 

erupted from volcanic centers west of Bend, on the basis of 

their distribution and diminished welding outward from the 

proposed source area. The two major fallout deposits, Bend 

Pumice and pumice of Columbia Canal, become coarser 

grained westward toward the same general area. This area has 

been called the Tumalo volcanic center and defi ned to include 

a 25-km-long, south-trending belt of rhyolite domes extending 

from Melvin Butte to Edison Butte (fi g. 1; Hill, 1988; Hill 

and Taylor, 1990). We prefer a more restricted defi nition that 

encompasses the volcanic highland from Melvin Butte south 

to Tumalo Creek valley, an area about 15 km across.

Neither the precise location nor the size and structure 

has been determined for the volcanoes that erupted the ma jor 

pyroclastic deposits. The largest buildup of middle and late 

Pleistocene rhyolite and rhyodacite domes (units Qr and 

Qrd), however, underlies the area surrounding Triangle Hill 

and includes domes compositionally similar to the Bend 

Pumice and Tumalo Tuff (Hill, 1992a). Silicic vent deposits 

(Qsv) are exposed locally adjacent to the domes and have 

been penetrated by drill hole CEBH–7, 1 km southwest of 

Tri an gle Hill (drill hole labeled on map sheet). The Triangle 
6

Hill area is also remarkable for its abundance of andesitic 

cinder cones (in unit Qc), similar in composition to the 

Shevlin Park Tuff. Elsewhere, most Quaternary cinder cones 

shown on the geo log ic map are basaltic andesite or basalt 

in composition, and nowhere do they form such a broadly 

co a lesced fi eld as in the Triangle Hill area. A 10-mGal negative gravity anomaly about 5 km across coincides with the 

clus ter of rhyolite domes and andesitic cinder cones in the 

Tri an gle Hill area (fi g. 5, sheet 1).

GLACIATION

Cascade Range glaciers have expanded and shrunk 

re peat ed ly in Quaternary time. Deposits of two major Pleis-

 tocene glaciations have been described from the map area, 

the older Jack Creek and younger Cabot Creek glaciations 

(Scott, 1977). The Cabot Creek glaciation is customarily 

di vid ed into Suttle Lake and Canyon Creek advances. Ar eal ly 

restricted Holocene glacier advances occurred at least twice 

during the past few thousand years, during what is com-

 mon ly referred to as Neoglaciation. The distinction among 

various deposits is based largely on soil profi le de vel op ment, 

thick ness of weathering rinds on clasts in soils formed in 

the de pos its, and the degree of preservation of moraines. 

For ex am ple, originally sharp-crested moraines become 

in creas ing ly rounded with time. The following summary of 

de pos its is drawn mainly from work by Scott (1977).

The Jack Creek glaciation is the oldest from which moraines are preserved. Mass wasting has modifi ed lateral and 

terminal moraines so they display broadly rounded crests. 

Till (unit Qgj) and outwash (Qoj) of the Jack Creek gla ci a tion 

bear soils about 2 m thick with textural B horizons (ev i dence 

of clay accumulation). Clasts in the B horizon have mean 

weathering-rind thicknesses of 0.5–0.7 mm. Deposits of Jack 

Creek age are restricted to an area east of Three Fin gered Jack. 

Evidently glaciers of the subsequent Suttle Lake advance were 

more extensive than those of Jack Creek age along most of 

the Cascade Range in the map area. The pres er va tion of till 

in the Jack Creek area was aided by the growth of Three Fingered Jack volcano, the presence of which forced glaciers of 

Suttle Lake age into the areas now occupied by Canyon and 

First Creeks, thus saving the Jack Creek mo raines from erosion and burial (Scott and others, 1996). Al though not dated 

directly, Jack Creek glaciation is be lieved correlative with 

the Hayden Creek glaciation in Wash ing ton, which is thought 

to be about 140,000 years in age (Colman and Pierce, 1981; 

Easterbrook, 1986).

Deposits of the Cabot Creek glaciation are commonly 

assigned to an older Suttle Lake advance (unit Qgs) and a 

younger Canyon Creek advance (unit Qgc). A mountain 

ice sheet covered the High Cascades during the Suttle Lake 

ad vance, which is the last major glacial advance in the area, 

and the resulting deposits form a widespread band at middle 

elevations on both fl anks of the High Cascades. Suttle Lake 

itself is impounded by prominent moraines left by the ice 

advance. Outwash of Suttle Lake age (Qos) fl oors much of 

the Metolius River valley and forms low terraces only a few 

meters above present streams. Deposits of Suttle Lake age bear 

soils about 1 m thick that have cambic B horizons. Weath er ing 

rinds on clasts in B horizons are 0.2 mm or thin ner. Although 

not directly dated, the Suttle Lake advance is correlative with 

the Evans Creek stade of the Fraser gla ci a tion of Washington on the basis of weathering similarities. The Evans Creek 

probably culminated in alpine areas about 20,000 years ago 

(Porter and others, 1983).

Deposits of the Canyon Creek advance (unit Qgc) of the 

Cabot Creek glaciation are restricted to cirques and val ley 

heads near the Cascade Range crest. Moraines are found on 

northerly and easterly cirques of Three Fingered Jack and 

Mount Washington and on the fl anks of the Three Sisters 

and Broken Top. The Canyon Creek advance is thought to be 

correlative with the Hyak advance of the Fraser gla ci a tion, 

which occurred in the Cascade Range of Washington between 

about 12,500 and 11,000 years ago (Porter and oth ers, 1983). 

Canyon Creek drift on Broken Top is overlain by Cayuse 

Crater scoria (see unit Qbcy), which is older than 9,500 years. 

No other data bear on the age of the Canyon Creek advance 

in the map area.

Two minor glacial advances, described here informally as 

early and late Neoglacial episodes, postdate the dep o si tion of 

ash from the Mazama eruption (6,845 14C yr B.P.; about 7,650 

years ago when calibrated to sidereal years). The deposits 

(unit Qgn) are too restricted to subdivide by age at the scale of 

this map. Moraines of early Neoglacial age are slightly more 

degraded than those of late Neoglacial age (Scott, 1990). The 

approximately 2,000-yr-old tephra from Rock Mesa and Devils 

Hill chain of vents lies upon mo raines of early Neoglacial age 

in the Three Sisters area but was buried or destroyed by glacial 

advances that deposited the younger moraines.

VOLCANOES OF THE THREE SISTERS AREA

The Three Sisters and Broken Top form a volcanic fi eld 

active since middle Pleistocene time. Taken together, the group 

accounts for 30–40 km3 of eruptive products. Oldest is Broken 

Top, a basaltic andesite volcano containing a few andesitic, 

dacitic, and rhyodacitic domes and lava fl ows (Tay lor, 1978, 

1990). A small-volume dacitic pyroclastic-fl ow deposit was 

emplaced high on the southwest fl ank and today is well-exposed 

in a cirque wall. Early-erupted lava on Bro ken Top’s east fl ank 

was emplaced before 213±9 ka, the age of interlayered rhyodacite of Tam MacArthur Rim (unit Qrdt), and some Broken 

Top lava is slightly younger (Hill, 1992a).

The Three Sisters themselves are progressively young er 

from north to south. North Sister, the oldest of the three, is a 

steep-sided basaltic andesite shield volcano. The North Sis ter 

is younger than 171 ka because its lava overlies the Shevlin 

Park Tuff (Taylor, 1990).

Middle and South Sisters are diverse in composition, 

in contrast to North Sister. The Middle Sister cone, entirely 
7

younger than North Sister, grew by eruptions of andesite, 

dacite, and basaltic andesite lava. Rhyolite is found only in 

a fl ow on the northwest side, which at its northwest terminus forms the Obsidian Cliffs. A highly porphyritic basaltic 

andes ite (SiO2

 approximately 52–53 percent; Taylor, 1987) 

was erupted late in cone growth and now mantles most of the 

volcano’s southwest sector. Dacitic lava fl ows are the young est 

known eruptive product from Middle Sister.

South Sister began to grow about the same time as Mid dle 

Sister, but its eruptions have continued into Ho locene time, 

making it the youngest of the Three Sisters. Its prod ucts range 

in composition from basaltic andesite to rhyolite, and rhyolite 

is more abundant than at any other Quaternary volcano in the 

map area. Dacitic lava from the volcano’s northeastern base 

has a K-Ar age of 93±11 ka (Hill, 1992a) and its summit 

is indented by a crater of presumed latest Pleistocene age 

(older than 10,000 yr B.P.). This age is de duced on the basis 

of substantial erosion that has gutted the summit cone and 

isolated lava fl ows on an eastern prom i nence that was once 

connected to the summit area (Wozniak, 1982). The main-cone 

eruptive sequence indicates no se quen tial pattern of composition or eruptive style. Explosive erup tions that produced 

thick near-vent tephra deposits oc curred on several occasions 

during Pleistocene and Ho locene time. The volcano’s summit 

comprises interlayered lava and sco ria of basaltic andesite, 

andesite, and dacite.

A latest Pleistocene or Holocene basaltic andesite lava 

fl ow erupted from Le Conte Crater, a cinder cone at the 

south west base of South Sister. Even younger Holocene 

rhyolite erupted from fi ssures on the southwest, southeast, 

and north east fl anks. The young rhyolitic eruptions (unit 

Qrrm), which formed the Rock Mesa and Devils Hill chain 

of vents, oc curred during two brief episodes between 2,300 

and 2,000 14C yr B.P. (Scott, 1987). In addition to thick stubby 

lava fl ows, the eruptions produced tephra showers and small 

py ro clas tic fl ows. The fallout accumulated to a thickness in 

excess of 10 m near the vents. Rapid snowmelt early in each 

eruption triggered a few small lahars (Scott, 1987).

South Sister poses a potential volcanic hazards threat 

if future eruptions resemble those of the recent past. Tephra 

fallout might accumulate to 1–2 cm thick in the Bend area, and 

small-volume lahars and pyroclastic fl ows would en dan ger 

anyone on the slopes or areas nearby.

Nearby Mount Bachelor, south of the map area, is a 

basaltic andesite shield better known for its downhill ski ing 

than its volcanic history. It forms the northernmost part of the 

Mount Bachelor volcanic chain, a string of shield vol ca noes 

active from about 18 to 7 ka and amassing 30-50 km3

 of 

lava and near-vent deposits (Scott, 1990; Scott and Gardner, 

1992; Gardner, 1994). Eruptions along the chain progressed 

generally northward, with youngest activity on the north 

fl ank of Mount Bachelor at a cinder vent known informally 

as Egan cone* (south edge of map area). Age of the Egan 

cone is known only imprecisely. The Egan cone and its lava 

fl ows are older than the Mazama ash bed (older than about 

7,650 calibrated yr B.P.). They were only slight ly weathered 

prior to Mazama time, so they may be only slight ly older than 

7,650 yr. On the other hand, pa le o mag net ic fi eld directions 

suggest that distal lava from Egan cone may be closer in age 

to lava from the summit cone of Mount Bach e lor, that is, about 

11,000–12,500 yr (Scott and Gardner, 1992).

HOLOCENE LAVA FLOWS NEAR SANTIAM AND 

MCKENZIE PASSES

Basalt and basaltic andesite lava fl ows were erupted 

from cinder cones and small shield volcanoes in the area from 

Santiam Pass to McKenzie Pass. Most are younger than the 

Mazama ash bed (younger than 7,650 yr), and their rug ged 

surfaces provide striking contrast to older glaciated lava in the 

region. Substantial ash issued from the vents and ac cu mu lat ed 

in downwind areas to as much as 3 m thick.

Radiocarbon ages from organic material trapped with in or 

closely underlying some eruptive products span the time from 

nearly 4,000 to 1,300 years ago and cluster into three groups. 

The oldest group (1), with only two ages, indicates eruptions 

4,000 to 3,000 years ago but is poorly con strained. Better 

established are (2) a group comprising several ages from the 

time between 2,900 and 2,500 years ago; and (3) the youngest 

group, with ages between about 2,000 and 1,300 years ago 

(table 1). The ages indicate no discernible geo graph ic pattern 

of age progression; eruptions occurred spo rad i cal ly in both 

the McKenzie Pass and western Santiam Pass areas through 

most of the approximately 2,700-year duration. Two small 

eruptive centers (in unit Qyc) are known from the eastern 

Santiam Pass area at Blue Lake and a chain of spatter cones 

on the lower northeast fl ank of Mount Wash ing ton.

In the following discussion, all ages are carbon-14 ages 

(reported as 14C years before present) and calculated using 

the preferred half-life of 5,568 yr. We have provided the 

com par i son between noncalibrated and calibrated ages (using the method of Stuiver and Reimer, 1993) for the Holocene 

lava fl ows near Santiam and McKenzie Passes (fi gs. 6A, B). 

None of the calibrations change our observations about the 

ap par ent ly random north-south position of vents with time 

(no age progression).

The oldest age from post-Mazama lava fl ows in the area 

of Santiam and McKenzie Passes is 3,850±215 14C yr B.P, 

the age assigned to the Fish Lake lava fl ow from Nash Cra ter 

(Taylor, 1968, 1990; Chatters, 1968). An age of 3,440±250 

14C yr B.P. was obtained from a charred conifer limb above 

fi ne ash from the Sand Mountain chain of vents and beneath 

coarse ash and lapilli from the Blue Lake vent (sampled near 

Suttle Lake). If correct, the age places a minimum age on 

part of the Sand Mountain volcanic fi eld and a maximum 

age for the Blue Lake volcano (can’t be older than 3,440 

14C yr B.P.). The age of Blue Lake volcano is discussed as 

part of the youngest episode.

*Informal geographic name
8

The middle group (2) has ages of eruptive activity 

dat ing back to about 3,000 14C yr B.P. An age of 2,800±150 14C yr B.P. is the maximum age for Twin Craters, a vent near 

the southern part of the Holocene lava fi eld near McKenzie 

Pass. Clear Lake, at the west-central edge of the fi eld, is 

thought to have been created by lava fl ows after 2,750±45 

14C yr B.P. (J.M. Licciardi, unpub. data, 1999). This age, newly 

de ter mined by accelerator mass spec tro met ric methods and 

pos sess ing small analytical un cer tain ty, was from a sample 

of wood from a drowned snag orig i nal ly col lect ed in 1964 by 

E.M. Taylor. A cor rob o rat ing age—2,705±200 14C yr B.P., also 

with large an a lyt i cal un cer tain ty—was obtained from wood 

collected from the out er layers of a different drowned tree 

in the lake (Benson, 1965). Another age determination from 

the same area—this sam ple from a charred log buried by the 

lava fl ow on the east side of the lake—produced an age of 

2,990±300 14C yr B.P. (E.M. Tay lor in Champion, 1980; also 

reported as 3,000 yr B.P. with out error in Taylor, 1968, 1981). 

All these ages overlap at the reported an a lyt i cal un cer tain ty 

(fi g. 6A), but the 2,750-yr age is most pre cise.

At the north end of the Sand Mountain volcanic fi eld, an 

age of 2,590±150 14C yr B.P. was obtained from char coal at 

the contact between soil developed on a lateral mo raine and 

scoriaceous fi ne ash from Nash Crater or the Sand Moun tain 

chain of vents (table 1). This age places a max i mum age on 

eruptive activity in the immediate area of Santiam Junc tion, 

including Little Nash Crater, the young est vent in that area. An 

age of 2,883±175 14C yr B.P. was determined from charcoal 

roots in a tree mold formed by lava from Little Belknap, in 

the southern part of the fi eld. Charred roots from another tree 

mold in the area were used to date lava from South Belknap 

cone*; its age is 2,635±50 14C yr B.P. (J.M. Licciardi, unpub. 

data, 1999). This South Belknap tree mold is the same site for 

charcoal collected in the 1960s by Tay lor, who obtained an age 

of 1,775±400 14C yr B.P. (E.M. Tay lor, in Champion, 1980). 

The older age (2,635) is pre ferred on the basis of high concentrations of cosmogenic 3

He that indicate a greater surface 

exposure age than the 1,775-yr age permits (J.M. Licciardi, 

written commun., 1999). The strati graph ic relations between 

Little Belknap and adjacent Belknap Crater are discussed more 

fully as part of the next group of ages.

Ages younger than 2,000 14C yr B.P. are also scattered 

across the lava fi eld and defi ne the youngest group (3). At 

the north end of the fi eld, the Lost Lake chain of cones was 

active about 1,950±150 14C yr B.P. (Taylor, 1968; Chatters, 

1968). Four In One Cone at the southerly limit of the lava 

fi eld has an age of about 1,980±160 14C yr B.P. The char coal 

from beneath the Four In One tephra overlies a thin, fi ne 

white silicic tephra thought to have been erupted from Rock 

Mesa or Devils Hill chain of vents between about 2,000 and 

2,300 years ago (W.E. Scott, unpub. data). An older age of 

2,550±165 14C yr B.P. from the Four In One tephra (Tay lor, 

1968; Chatters, 1968) is from the core of a tree already of 

substantial girth by the time of the Four In One eruption (old er 

age shown in gray on fi g. 6).

Stratigraphic relations of tephra and lava fl ows in di cate 

the following relative ages for four units in the southern part 

of the fi eld: Little Belknap (oldest), then Yapoah Crater, then 

Four In One Cone, then Collier Cone (youngest). Collier Cone 

has an age of 1,600±100 14C yr B.P. and the preferred age for 

Four In One Cone is 1,980±160 14C yr B.P.

Belknap Crater’s age is perhaps the most diffi cult to 

defi ne precisely, and the Belknap shield has probably grown 

through repeated eruptions. As mentioned previously, South 

Belknap cone* is considered to be about 2,635 14C yr B.P. 

Charcoal from beneath a fl ow that traveled down the west 

fl ank of Belknap Crater yielded two ages: 1,590±160 and 

1,400±100 14C yr B.P.; this lava emanated from Belknap’s 

northerly summit vent. These two ages overlap at the level of 

statistical certainty (fi g. 6A), allowing them to be in ter pret ed 

as roughly the same age or ages that differ by as much as 

eight centuries. An additional age criterion is provided by 

the relation between Belknap Crater and Little Belknap: ear ly 

Belknap lava fl ows were already emplaced before con struc tion 

of the Little Belknap volcano. The ages indicate that vents 

have been active near or at the site of Belknap Crater intermittently through an interval of at least 1,200 years.

Blue Lake crater and a chain of spatter cones between 

Blue Lake and Mount Washington may be the youngest vol-

 ca nic features in the Santiam and McKenzie Passes region. 

Neither vent fed lava fl ows. An age of 1,330±140 14C yr B.P. 

was obtained from charred forest litter collected be neath the 

spatter-cone cinders (table 1). The chain of spatter cones trends 

N. 18° E., on line with Blue Lake, 6 km distant. Te phra from 

Blue Lake crater is petrographically similar to that from the 

chain of spatter cones—moderately porphyritic with 10–15 

percent plagioclase phenocrysts as large as 3 mm and about 

1 percent olivine phenocrysts, 1 mm across. The align ment 

of the spatter cone chain of vents with Blue Lake cra ter and 

the petrographic similarity of their tephra lead us to sug gest 

that these vents were active during a single eruptive ep i sode 

about 1,330 14C years ago. An age of 3,440±250 14C yr B.P. 

had been previously assigned to Blue Lake cra ter (Taylor, 

1968, 1981), but the tree limb dated lay at the in ter face between tephra from Blue Lake crater and un der ly ing ash of 

Sand Mountain. We think it likely that the limb was part of a 

tree killed by the Sand Mountain eruptions and sub se quent ly 

buried by younger Blue Lake deposits—and not a casualty of 

the Blue Lake eruptions. Thus, the 3,440-yr age may provide 

only a maximum limiting age for the tephra from Blue Lake 

crater but a useful age for the ash of Sand Mountain. We 

failed to fi nd additional organic ma te ri al suit able for dating 

the tephra from Blue Lake crater.

Blue Lake crater erupted a tephra plume of coarse 

blocks and lapilli that blanket the Suttle Lake trough. Isopach 

lines on the map show that the tephra plume was defl ected 

by gen tle winds blowing to the east-northeast. The plume 

is thick er than 2 m along the southwestern shore of Suttle 

Lake. Blue Lake itself is surrounded by a modest cone of 

cinder and agglutinate.
9

1000 1500 2000 2500 3000 3500 4000

1,600±100

1,400±100

Collier Cone

1,590±160

1,775±400

West-flank flow from Belknap

 (same sample, two analyses)

Flow from South Belknap cone* (age probably too young;

see other South Belknap age) 

1,980±160

Four In One Cone 2,550±165

1,950±150

Lost Lake chain of cones

Santiam Junction ash deposits, max. age

 (Little Nash Crater, max. age)

2,590±150

2,705±200

Clear Lake flow, max. age

 (from Sand Mountain chain of cones)

Twin Craters 2,800±150

Little Belknap

2,883±175

Flow from South Belknap cone* 2,635±50

3,440±250

3,850±215

Minimum age, part of Sand Mountain chain of cones

Maximum age, Blue Lake crater

Fish Lake lava flow

 (from Nash Crater)

1000 1500 2000 2500 3000 3500 4000

Belknap

ages

2,990±300

2,750±45

Rock Mesa-Devils Hill chain of vents (2,000-2,300 14C yr B.P.)

2,620±150

1,330±140

Spatter cone chain of vents*

14C yr B.P.

(age too old)

tree molds

max. age, beneath scoria

South Belknap and

Little Belknap ages

*Informal geographic name

Figure 6A. Carbon-14 ages from Santiam and McKenzie Passes: 6A, noncalibrated, in 14C yr B.P.; 6B, calibrated yr B.P. Carbon-14 

ages in the 2,000-2,300-yr range are little affected by calibration, so the stratigraphic horizon occupied by tephra from the Rock MesaDevils Hill chain of vents is shown in the same position on figures 6A and 6B.
10

Twin Craters 

tree molds

max. age, beneath scoria

1354 (1511) 1569

1260 (1296) 1355

Collier Cone

1309 (1505) 1690

1289 (1660) 2144 Flow from South Belknap cone*

1720 (1910) 2123

Four In One Cone 2354 (2727) 2785

1711 (1878) 2052

Lost Lake chain of cones

Santiam Junction ash deposits, max. age

 (Little Nash Crater, max. age)

2469 (2748) 2849

2519 (2778) 3060

Clear Lake flow, max. age

 (from Sand Mountain chain of cones)

2759 (2907) 3160

Flow from South Belknap cone*

2713 (2752) 2845

3386 (3689) 4062

3924 (4238) 4529

Minimum age, part of Sand Mountain chain of cones

Maximum age, Blue Lake crater

Fish Lake lava flow

 (from Nash Crater)

1000 1500 2000 2500 3000 3500 4000

Belknap

ages

South Belknap

and Little Belknap

ages

2772 (3186) 3475

Rock Mesa-Devils Hill chain of vents (2,000-2,300 calendar yr B.P.)

(age probably too young;

see other South Belknap age)

(multiple intercepts: 1897, 1908, 1924)

2758 (2848) 2950

(multiple intercepts: 3162,

 3192, 3203)

(multiple intercepts: 2882, 2914, 2925)

Little Belknap 2780 (2975) 3262

4500

Spatter cone chain of vents* 1069 (1274) 1341

1000 1500 2000 2500 3000 3500 4000 4500

West-flank flow from Belknap

 (same sample, two analyses)

(age too old)

2483 (2750) 2856

calendar yr B.P.

(multiple intercepts: 1639, 1642, 1699)

*Informal geographic name

Figure 6B. Carbon-14 ages from Santiam and McKenzie Passes: 6A, noncalibrated, in 14C yr B.P.; 6B, calibrated yr B.P. Carbon-14 ages 

in the 2,000-2,300-yr range are little affected by calibration, so the stratigraphic horizon occupied by tephra from the Rock Mesa-Devils 

Hill chain of vents is shown in the same position on figures 6A and 6B.
11

SISTERS FAULT ZONE AND TUMALO FAULT

The Sisters fault zone comprises nearly 50 mapped faults 

extending from Black Butte southeastward through Sisters and 

beyond Bend (Lawrence, 1976; Pezzopane and Weldon, 1993, 

their fi g. 2). Total length of the zone is 60 km; width ranges 

from 5 to 15 km. The Tumalo fault, longest single fault strand 

in the zone, can be traced nearly continuously for 47 km, reach-

 ing from 3 km south of the map area to 4 km north of Sisters. 

Shorter faults range in length from 0.5 to 20 km.

Sense of slip is unknown for any of the faults. Dip sep a -

ra tion is apparent, but some slip may be oblique. Dis place ment 

of a lobate fl ow of basaltic andesite that cross es the Tumalo 

fault suggests a small component of right-lateral separation 

(Mimura, 1992), but even the evidence for that example is 

arguable. Dip separation is as great as 60–70 m near Tumalo 

Dam on the basis of topographic escarpments underlain by 

Pliocene lava fl ows of the Deschutes For ma tion. Quaternary 

lava fl ows younger than 0.78 Ma in the same area have escarpments of only 6–10 m.

Lava fl ows in the suburbs southeast of Bend have es carp -

ments as high as 15 m. A K-Ar age of 2.7±0.3 Ma was reported 

from a lava fl ow in one of these escarpments (table 2, Sample 

No. 86–3), but we suspect the age is too old. That lava fl ow 

(in unit Qbn) was erupted from a vent on the north fl ank of 

Newberry volcano, possesses normal-polarity mag ne ti za tion, 

and is likely younger than 0.78 Ma. Pilot Butte lava in Bend 

is offset a minimum of 20 m (downthrown side buried). Pilot 

Butte and its lava is thought to be younger than 0.78 Ma on 

the basis of normal-polarity magnetization. A poorly exposed 

white rhyolitic tephra, of unknown cor re la tion but probably 

younger than the Bend Pumice, mantles the lower southwest 

fl ank of the butte. Little is known of Pilot Butte’s age, which 

is important for estimating the rate of faulting.

The Tumalo fault and a few other strands of the Sisters 

fault zone near Bend cut the Shevlin Park Tuff, which in di cates 

offset occurring more recently than about 170,000 years ago. A 

fault-inspection trench dug by M.A. Hemphill-Haley (Univ. of 

Oregon) north of Tumalo exposed fractured and slightly faulted 

alluvial deposits overlain by undeformed al lu vi um (fi eld evidence we viewed with Hemphill-Haley in October 1994). The 

undeformed alluvium has been in place since 25,000–50,000 yr 

B.P., an age crudely estimated by us on basis of soil weathering profi les. Thus, we tentatively accept about 25,000 yr as the 

youngest possible age of re cent faulting at that location. Corroborative fi eld in ves ti ga tions are lacking elsewhere along the 

zone but should be undertaken prior to siting critical facilities 

near or along faults in the Sisters fault zone. We remain skeptical that any of the faults have been active in the past 10,000 

yr, although it has been suggested that the Mazama ash bed 

(age 6,845±50 14C yr) is displaced near Bend (Fisk and others, 

1993). By all accounts, the Sisters fault zone or faults along it 

are con sid ered potentially active (U.S. Army Corps of Engineers, 1983a, 1983b; L.R. Squier Associates, 1984; Hawkins 

and others, 1988; Geomatrix Consultants, 1995).

Earliest age of offset is unknown. The Deschutes For-

 ma tion, oldest stratigraphic unit exposed along the fault zone 

west of Redmond, shows only rare examples of faulting dur ing 

deposition (Stensland, 1970; Smith, 1986; our map ping), but 

the Sisters fault zone is located mostly southwest of the main 

outcrop belt of the Deschutes Formation.

Early views held that the Sisters fault zone is the alongstrike extension of the Brothers fault zone (Lawrence, 1976; 

Peterson and others, 1976), which is gen er al ly viewed as 

containing discontinuous normal faults de vel oped along a 

right-lateral transverse fault zone that ex tends southeast of 

the map area across eastern Oregon. More re cent opinion 

accepts the Sisters and Brothers fault zones as distinct struc-

 tur al zones (Hawkins and others, 1988, 1989; MacLeod and 

Sherrod, 1988; Pezzopane and Weldon, 1993), although the 

gross structural styles may be similar. The Broth ers fault zone 

is buried by Pleistocene lava fl ows from Newberry volcano 

(unit Qbn) in the area where it ter mi nates, merges, or steps 

westward into the Sisters fault zone.

An interesting interpretation by Hawkins and others 

(1988) tries to reconcile recency of faulting and the trend of 

distinct fault zones where they nearly overlap. By that in ter -

pre ta tion, the Brothers fault zone terminates roughly where 

the Sisters fault zone begins (a 5- to 7-km gap separates the 

fault zones), and an even more westerly zone extends from the 

northwest rift system of Newberry volcano through the west 

side of Bend to Black Butte (Metolius fault zone of Hawkins 

and others, 1988). We see little point in sub di vid ing the Sisters 

fault zone into western and eastern parts, giv en the limited 

knowledge of fault history in the area.

GREEN RIDGE AND THE HIGH CASCADES GRABEN

The escarpment of Green Ridge (cross section A–A') 

marks the east side of a major north-trending graben (Tay lor, 

1981; Smith and Taylor, 1983). The graben is 30 km wide 

and 50 km long, reaching from south of Mount Jefferson to 

nearly the Three Sisters (fi g. 7). The western graben-bound ing 

faults form the Horse Creek fault zone. A strand of the Horse 

Creek fault system lies in the southwest corner of the map 

area (cross section B–B').

Dis place ment on the Green Ridge and Horse Creek fault 

zones took place in late Mi ocene and ear ly Pliocene time. Motion along the Green Ridge fault zone iso lat ed the Deschutes 

basin from now-bur ied vol ca nic cen ters in the High Cas cades 

be gin ning about 5.4 Ma (Smith and oth ers, 1987a). Rocks 

as young as about 5 Ma are ex posed at the top of the 650-m 

es carp ment of Green Ridge (Armstrong and oth ers, 1975), 

where as the downthrown block is man tled by Pliocene and 

Qua ter na ry sed i men ta ry de pos its. Dis place ment is at least 1 

km, on the basis of an age of 1.81 Ma from the base of drill 

hole SP 77–24 at Santiam Pass (cross sec tion A–A') (Hill and 

Priest, 1992).

The Horse Creek fault zone dis plac es 5- to 6-Ma strata 

as much as 670 m down along a fault north of the McKenzie 
12

River (Brown and oth ers, 1980b); cu mu la tive mapped offset is 

as much as 850 m south of the McKenzie River (Priest and oth-

 ers, 1988). An ad di tion al 400 m of off set may be in di cat ed by 

a steep unconformity per haps re sult ing from lava but tress ing 

a fault es carp ment in the vi cin i ty of Scott Creek (Priest and 

oth ers, 1988). Thus, de mon stra ble gra ben sub sid ence is on the 

order of about 1.3 km. Headward ero sion by the McKenzie 

River breached the es carp ment by late Pliocene time. About 

1.7 Ma, the basalt of Roney Creek fl owed from a source in 

the High Cas cades westward across the fault trace and along 

Figure 7. Map showing area of High Cas cades and bound ing faults on east and west sides. Gen er al ized from Sherrod 

and Smith (2000) and this map.

20

22

126

126

20

97

Detroit

McKenzie

Bridge

Broken

Top

North Sister

Mt. Washington

Three 

 Fingered

 Jack

SP 77-24

0 10 20 30 KILOMETERS

122° 121°45' 121°30' 121°15'

44°45'

44°30 '

44°15 '

44°

Terwilliger

Hot Spring Bend

Middle Sister

South Sister

Belknap

 Crater

Mt. Jefferson

EXPLANATION

Basalt and basaltic andesite (Holocene)

Basalt and basaltic andesite (Pleistocene

 and Pliocene)

Andesite to rhyolite (Holocene and

 Pleistocene)—Younger than 0.73 Ma

Andesite to rhyolite (Pleistocene and

 Pliocene)—Mostly older than 0.73 Ma

Shield volcano

Composite volcano

Contact

122°15'

HO

RSE C

REEK FAULT

ZONE

G

REEN RIDGE FAULT

ZONE

SISTERS

FAULT

ZONE

Older rocks west and east of High

 Cascades (Pliocene to Oligocene)

Fault with offset greater than 300 m—

 Dotted where concealed; ball and bar 

 on downthrown side

Fault with offset less than 300 m

SP 77-24

Geothermal well—Showing name

Sisters

W

E

S

T

E

R

N

C

A

S

C

A

D

E

S

D

E

S

CHUTES-UMATILLA

PLATEAU

H

I

G

H

C

A

S

C

A

D

E

S
13

the McKenzie River valley (Priest and others, 1988). The 

fault has been inactive since the em place ment of the basalt 

of Roney Creek.

Seismic refl ection ex per i ments along a line that tra versed 

the Cascade Range between lat 44°10' and 44°15' had high 

noise-to-signal ratios and failed to establish the mag ni tude 

of offset on the Horse Creek fault zone (Keach and oth ers, 

1989). The line ends near the town of Sisters, where offset on 

the Green Ridge fault zone may have decreased to less than 

100 m; no escarpment was imaged by the seismic refl ection 

pro fi l ing. The conclusion drawn from the seismic data was 

that the Green Ridge fault zone has diminishing structural 

relief southward toward the town of Sisters (Keach and others, 1989).

A magnetotelluric profi le that trends roughly east-west 

through the Middle Sister approached the Sisters fault zone 

near Three Creek Butte, 16 km south of the town of Sisters (fi g. 

7; line C–C' in Livelybrooks and others, 1989). This profi le 

showed a shallow conductive layer thickening abrupt ly from 

0.3 to 1.8 km as it passes westward near Three Creek Butte. 

The conductive layer is overlain by an 0.7-km-thick resistive 

layer of unvarying thickness. The in ter pre ta tion of fered was 

the existence of a major fault bur ied by Qua ter na ry deposits 

(Livelybrooks and others, 1989). The profi le’s geometry sug-

 gests that such a fault would project to the sur face about 4 km 

east of Three Creek Butte, in line with the on-strike pro jec tion 

of a fault in the Sisters fault zone shown on our geo log ic map 

(at about station No. 10 shown on fi g. 5).

Despite its magnitude of offset, the fault interpreted by 

Livelybrooks and others (1989) from the magnetotelluric 

data lacks expression on Bouguer gravity maps (Pitts and 

Couch, 1978; Couch and others, 1981). But gravity data rarely 

de mar cate well-known examples of some other prominent 

Cas cade Range graben-bounding faults, such as the Horse 

Creek fault. These failures probably result from insuffi cient 

den si ty contrasts across the fault zones.

To summarize the east-side graben-bounding faults lo-

 cat ed on the Bend geologic map, we are left with an image of 

faults that vary substantially in offset along their length. The 

Green Ridge fault zone merges south-southeastward into the 

Sisters fault zone, judging from the structural in ter pre ta tion 

provided by the magnetotelluric profi ling. Offset of 1–2 km 

characterizes some parts of the Green Ridge and Sisters fault 

zones. Many faults in the zone show only minor offset. The 

major faults lie toward the west limit of the fault zones; any 

faults farther west are buried by Quaternary lava fl ows and 

pyroclastic rocks of the Cascade Range and lack geo phys i cal 

expression.

ACKNOWLEDGMENTS

Access to many parts of the map area has been possible 

only through permission granted willingly by numerous 

prop er ty owners. We acknowledge the work by Donald E. 

Stensland (deceased), whose mapping in the late 1960s has 

proven exceptionally thorough and accurate. Much of his work 

is incorporated here with little or no modifi cation.

Our collaboration with colleagues Marshall Gannett 

(U.S. Geological Survey) and Ken Lite, Jr., Sarah Gates, and 

Karl Wozniak (Oregon Department of Water Resources) has 

sharpened our thinking about the hydrology of the Deschutes 

basin. We also have benefi ted from fi eld studies of the Sis ters 

fault zone by Mark Hemphill-Haley (University of Or e gon). 

Joe Licciardi (Oregon State University) shared his newly obtained radiocarbon ages and allowed us to in cor po rate them 

into our tables, fi gures, and discussion. Marvin Lanphere (U.S. 

Geological Survey) provided 40Ar/39Ar ages for two buttes in 

the Deschutes basin whose ages have puz zled geologists since 

the 1930s. As is true of most ge ol o gists working in central 

Oregon, we turned periodically to Larry Chitwood and Bob 

Jensen (both of the U.S. Forest Service, Deschutes National 

Forest) to share in their knowl edge of the area.

The manuscript was reviewed by Marshall Gannett, Bob 

Christiansen, Jan Zigler (all of U.S. Geological Survey), and 

Britt Hill (Southwest Research Institute), who graciously 

provided suggestions leading to an enriched publication.

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

m Man-modified land (Holocene)—Chiefly waste 

piles of diatomite and sand from strip 

mining along Deschutes River 9 km west 

of Terrebonne. Main period of mining was 

from 1936 to 1961 (Peterson and others, 

1976)

Qal Alluvium (Holocene and Pleistocene)—Un-

 con sol i dat ed deposits of sand and gravel 

along streams and in valley bottoms

Qe Eolian deposits (Holocene and Pleistocene)—

Wind-blown sand and silt. Fills de pres sions 

in lava flows in eastern part of map area; 

mapped only where forms extensive de pos its. 

In cludes reworked ash of Mazama ash bed 

in upper part. Thickness ranges from 3 to 

6 m in quarried exposures where base is 

exposed

Qls Landslide deposits (Holocene and Pleis tocene)—Slumped blocks of sedimentary 

rocks, tuff, and basalt along valley walls 

of Deschutes and Crooked Rivers and a 

few scattered chaotic deposits elsewhere 

in map area. Those adjacent to basalt 

of Katsuk and Talapus Buttes in High 

Cascades formed after late Pleistocene 

glaciers melted, removing lat er al support 

that buttressed the lava pla teaus adjacent 

to these buttes
14

Qt Talus and colluvium (Holocene and Pleis tocene)—Chiefly blocky deposits that blan ket 

steep slopes at higher elevation in map 

area. Probably deposited during or after 

last major glacial advance

Till (Holocene and Pleistocene)—Very poorly 

sorted, angular to sub-rounded pebbles, 

cobbles, and boulders in silty sand matrix. 

Chiefly forms ground and lateral moraines 

in map area. Includes minor alluvium where 

reworked by streams. Divided into:

Qgn Till of Neoglacial age (Holocene)—Forms steep, 

sharp-crested, barren or sparsely veg e tat ed 

moraines close to existing glaciers and perennial snowfi elds. In map area, found on 

North, Middle, and South Sisters, Bro ken 

Top, and Three Fingered Jack; in cludes 

deposits of early and late Neoglacial age 

as shown on more detailed maps (Taylor, 

1978; Wozniak, 1982; Scott and Gardner, 

1992). Younger than Mazama ash bed. Early 

Neoglacial deposits are locally overlain by 

tephra from Rock Mesa and Devils Hill 

chain of vents (Qrrm) and therefore are 

older than approximately 2,000 14C yr B.P., 

whereas late Neoglacial deposits are younger. 

Age determinations from elsewhere in the 

Cas cade Range suggest early Neoglaciation 

lasted from about 3,500 to 2,300 yr B.P. and 

late Neoglaciation (occasionally referred to 

as the “Little Ice Age”) occurred during the 

past several centuries (see discussion and 

ref er enc es in Scott and Gardner, 1992)

Qgc Till of Canyon Creek advance (of Cabot 

Creek glaciation of Scott, 1977) (Pleis tocene)—Forms moraines in cirques or just 

downslope from cirques; moraines lie chiefl y 

within 2 km downslope of terminal mo raines 

of Neoglacial age. Degree of weathering is 

similar to that on till of Suttle Lake ad vance, 

which sug gests that Canyon Creek ad vance 

occurred during waning phase of last ma jor 

gla ci a tion. Overlain by Mazama ash bed and 

locally derived scoria deposits; oldest dated 

of these has minimum limiting ra dio car bon 

age of 9,520 14C yr B.P. On basis of correlation to moraines of similar po si tion in 

Wash ing ton Cascade Range, more closely 

limiting min i mum age is 11,250 14C yr B.P. 

(Heine, 1996)

Qgs Till of Suttle Lake advance (of Cabot Creek glaciation of Scott, 1977) (Pleis tocene)—Forms 

belts of moraines that mark max i mum extent 

of glaciers during last major ice age, which 

on basis of regional correlations probably 

culminated about 20,000 yr B.P. Cambic B 

horizons typically 30–45 cm thick; weath-

 er ing rinds on basaltic clasts less than 0.2 

mm thick. Total depth of oxidation of soil 

profi les chiefl y less than 1 m

Qgj Till of Jack Creek glaciation of Scott (1977) 

(Pleistocene)—Forms extensive moraine belt 

east of Three Fingered Jack that predates 

formation of that volcano (Scott and oth ers, 

1996). Characterized by reddish-brown argillic 

B horizons; basaltic clasts bear weathering 

rinds that average about 0.5 mm thick. Total 

depth of soil oxidation about 1.5 to 2 m. 

Map pattern east of Three Fin gered Jack 

mimics that of moraines of Suttle Lake age 

near Suttle Lake, which suggests that, during 

Jack Creek glaciation, ice from broad upland 

area under what is now Three Fingered Jack 

formed a large east-fl owing outlet glacier. 

Subsequent growth of Three Fingered Jack 

restricted glaciers of Suttle Lake advance 

to adjacent valleys (of First and Canyon 

Creeks) at margins of former outlet valley. 

Age of Jack Creek gla ci a tion is probably 

either about 75 or 150 ka (Scott, 1977). 

Map-unit label shown queried for deposits 

of uncertain age but thought to predate the 

Cabot Creek glaciation; these deposits located on west side of High Cascades south 

of Bunchgrass Ridge

Outwash (Holocene and Pleistocene)—Mod er -

ate ly rounded to well-rounded cobbles and 

peb bles in sandy matrix. Forms stratifi ed 

gravel deposits. Confi ned to valley fl oors 

at high er elevations but widens into broad 

outwash fans at middle elevations. Chiefl y 

Pleis tocene in age; most was deposited 

during Suttle Lake advance by meltwater 

streams from gla ciers that deposited till 

of that advance (unit Qgs). Outwash of 

Canyon Creek advance mapped as alluvium 

(Qal) be cause it rarely forms deposits with 

dis tinct geo mor phic form or located far from 

ac tive stream courses. Minor outwash re-

 sult ing from Neoglacial epochs (in past 

3,500 yr) is found on upper fl anks of the 

Middle and South Sisters and Bro ken Top. 

De pos its have been divided, on basis of 

weath er ing char ac ter is tics similar to till 

units, into:

Qon Outwash of Neoglacial age (Holocene)

Qos Outwash of Suttle Lake advance (Pleis tocene)

Qoj Outwash of Jack Creek glaciation (Pleis-

 tocene)

Qs Sand and gravel (Pleistocene)—Alluvium 
15

de pos it ed throughout middle and late 

Pleis tocene time. Locally, older and 

younger parts of se quence may be recognizable by geo mor phic form such as 

nested terraces. De pos its along Deschutes 

River are sand and gravel young er than 

Tumalo Tuff (Qtt) and both older and 

younger than basalt of Newberry volcano 

(Qbn). Near O’Neil, deposits form highstanding terraces adjacent to flood plain 

of Crooked River. Those terraces prob a bly 

deposited when lava flows from Newberry 

volcano (Qbn) dammed the Crooked 

Riv er, forming a higher base level for 

deposition of sand and gravel. Includes 

catastrophic flood deposits on plains 

southwest and north of Tumalo. These 

latter deposits, which con tain boulders as 

large as 3 m across, may have resulted 

from glacial outburst floods originating 

in Cascade Range. Deposits near Alfalfa 

are commonly about 8 m thick and have 

been locally quarried; clasts are chief ly 

basaltic lava but include about five per-

 cent rhyolitic lava. Those deposits may 

have formed by increased runoff during 

pluvial periods and outflow from a lake 

southeast of map area near Millican, with 

prov e nance for the rhyolite most likely 

being Pine Mountain but possibly any of 

several domes in the Dry River drainage 

as far east as Hampton Buttes

Qf Alluvial fan deposits (Pleistocene)—Poorly 

sort ed silt, sand, and subangular gravel. 

Found chiefly in broad fans surrounding 

Miocene and older rocks in eastern part 

of map area but includes some small fans 

scattered throughout area

Qsd Diatomite (Pleistocene)—Poorly indurated, 

earthy white diatomite with interbedded 

sand. As noted by Moore (1937), the 

deposit orig i nal ly lay under a cover of 

sand, appearing as brilliant white outcrops 

in the bluffs of the Deschutes River near 

Lower Bridge. Once as thick as 20 m; 

little of the original de pos it remains, 

however, and the site is now mostly occupied by irregularly heaped overburden 

and waste from strip mining. Dominant 

diatom species are Stephanodiscus niagarae (K.E. Lohman in Moore, 1937) and 

S. excentricus (Smith and others, 1987b), 

indicating a late Pliocene or Pleistocene 

age (Krebs and others, 1989). Volcanic 

ash bedded within the deposit has been 

ten ta tive ly correlated with the Loleta ash 

bed (distal-fallout equivalent of Bend 

Pumice) (Smith and others, 1987b; A. 

Sarna-Wojcicki, oral commun., 1995), 

which is thought to be about 0.3–0.4 

Ma in age. Alternative correlation with 

1.9-Ma ash bed found in drill core from a 

well near Tulelake, Calif. (T–749, 191 m 

depth; Rieck and others, 1992), is nearly 

as satisfactory on basis of sta tis ti cal comparison coefficients (A. Sarna-Wojcicki, 

oral commun., 1995). Age of main mass 

probably middle Pleistocene on ba sis of a 

geomorphic analysis; the deposit fills valley floor whose elevation is only slight ly 

higher than the surface later mantled by 

basalt of Newberry volcano (unit Qbn). 

Pre sum ably the pre-diatomite erosional 

stage is only slightly older than the basalt. 

Overlain by basalt of Newberry volcano 

(Qbn) in roadcuts east of Lower Bridge, 

but earlier lava flows from same unit may 

have dammed an ces tral river courses to 

create the lake in which diatomite was 

deposited (Smith, 1986). Small scattered 

areas shown east of Deschutes River are 

pond diatomite that fills to po graph i cal ly 

low areas on surface of basalt of Newberry 

volcano. These pond diatomite deposits 

are middle Pleistocene in age

QTs Sedimentary rocks and deposits (Pleistocene 

and Pliocene)—Poorly indurated sandstone and pumice-fall deposits. Exposed 

chiefly in canyons cut through overlying 

alluvial fan deposits (Qf) on southwest 

flank of Powell Buttes. At this location 

unit is litho log i cal ly similar to upper part 

of Deschutes For ma tion, but overlies basalt of Dry River (Tbdr) and therefore is 

younger; underlies east edge of basalt of 

Newberry volcano (Qbn). Ex po sure near 

Juniper Butte includes some moderately 

consolidated deposits whose lower part may 

be correlative with Deschutes Formation. 

Includes few small areas at west edge 

of map area corresponding to lacus trine 

and fluviatile sedimentary rocks de pos it ed 

in late Pliocene and Pleistocene time at 

west edge of High Cascades. In north west 

corner of map area these deposits include 

strata equivalent to Parkette Creek sed i -

men ta ry rocks of Black and others (1987). 

In southwest corner of map area, deposits 

are conglomerate poorly exposed in south 

can yon wall of Separation Creek
16

VOLCANIC ROCKS AND DEPOSITS OF THE

CAS CADE RANGE AND NEWBERRY VOLCANO

[Arranged generally by composition, although first 

twelve units are compositionally diverse and have 

been grouped separately owing to their overlapping 

geographic setting and brief period of eruptive activity. 

These twelve units and one additional unit discussed 

later, the rhyolite of Rock Mesa and Devils Hill chain 

of vents (Qrrm), are young er than the Mazama ash bed, 

a widespread tephra deposit erupt ed during climactic 

eruptions of Mount Mazama (Crater Lake National 

Park), 120 km south of map area]

Young volcanic rocks of Santiam and McKenzie Passes

Young lava fl ows (Holocene)—Youthful lava fl ows 

that form the Sand Mountain vol ca nic fi eld, 

Belknap Crater, and other young volcanic 

features in area of Santiam and McKenzie 

Passes. Includes ropy pahoehoe, clinkery aa, 

and blocky lava. Possesses sparsely veg e tat ed 

fl ow surfaces with well-preserved pres sure 

ridges, tumuli, and levees. Erupted from 

cinder cones (unit Qyc) and lava shields 

(vents indicated by red asterisk). Chiefl y 

basaltic andesite, but includes ba salt, mi nor 

andesite, and rare dacite. Emplaced be tween 

about 7,000 and 1,300 years ago. Ra dio car bon 

ages summarized in table 1 and shown in 

fi gure 5. Divided by com po si tion and erup-

 tive source into:

Qybc Basaltic andesite and andesite of Collier 

Cone—Lava ranges in composition from 

basaltic andesite to andesite, with rare 

dacite; SiO2 ranges from 56 to 65 percent. 

Phe noc ryst abundance ranges widely too, 

with andes ite typically more porphyritic 

than basaltic andesite or dacite. Comprises 

multiple emplacement units, each of which 

ranges in composition (Schick, 1994). Age 

1,600±100 14C yr B.P. on basis of charcoal 

beneath tephra from cone (Scott, 1990). 

Cone man tled on southwest side by till 

deposited during late Neoglaciation. Stipple 

shows a small cin der deposit on a lava 

flow from Collier Cone (3 km northwest 

of North Sister) and an oth er deposit on a 

lava flow 14 km north east of North Sister. 

This lat ter de pos it was con sid ered part of 

Shevlin Park Tuff by Tay lor and Ferns 

(1995) but is re in ter pret ed here in as a 

raft ed cin der de pos it

Qya Andesite of Four In One Cone—Erupted 

from chain of cones built above fissure 

trending N. 10° E. Northern four cones 

contiguous (hence the name), and two 

cones farther south (in unit Qyc) are 

surrounded and isolated by lava from 

Collier Cone (Qybc). Early-erupt ed 

lava (found in flow levees) are sparsely 

porphyritic basaltic andesite (SiO2 about 

56 percent), whereas later flows (found 

chief ly in gutters) are porphyritic andesite 

(SiO2 about 58-59 percent). Radiocarbon 

age of 1,980±160 14C yr B.P. obtained 

from charred needles and twigs in lower 

20 cm of te phra that mantles broad area 

east of fissure (Scott, 1990); this age most 

closely approximates age of eruption. An 

older age of 2,550±165 14C yr B.P. (Chatters, 1968; Taylor, 1968) was obtained 

from the cen ter of a large charred tree 

standing in fused cinders

Qyby Basaltic andesite of Yapoah Crater—Mod-

 er ate ly porphyritic lava flows. Com po si tion 

rang es from 54.8 to 56.9 percent SiO2. 

Emplaced before eruption of Collier Cone 

and Four In One Cone on basis of ashlava relations in upland area near vents. 

Young er than Little Belknap (Qyblk)

Qybk Basalt and basaltic andesite of Belknap 

Crater—Numerous lava flows erupted 

from unforested shield volcano surmounted by Belknap Crater. Typically 

contains small plagioclase and olivine 

phenocrysts about 1 mm across; sparse 

glomerocrysts reach 7 mm. Duration of 

eruptive activity likely spans a lengthy 

period. Charcoal encased in lava on 

west flank yielded ages of 1,400±100 

and 1,590±160 14C yr B.P. (Taylor, 1965; 

Chat ters, 1968; E.M. Taylor, in Cham pi on, 

1980). Eruptions at cone on south flank 

(South Belknap cone*) occurred about 

2,635±50 14C yr B.P. on basis of ra dio -

car bon age from charred roots in tree 

mold about 5 km southwest of McKenzie 

Pass (J.M. Licciardi, unpub. data, 1999). 

An earlier reported age from charcoal 

collected at this same site—1,775±400 

14C yr B.P. (E.M. Taylor, in Champion, 

1980; Taylor, 1990)—is thought too young 

because the con cen tra tion of cosmogenic 

3He in olivine phe noc rysts in the lava 

indicates a sur face-exposure age in excess 

of 2,000 yr B.P. (J.M. Licciardi, written 

commun., 1999). Early eruptive history 

obscure but partly pre dates vol ca no at 

Little Belknap
17

Qyt Young tephra—Thick blanket of ash and 

mi nor lapilli originating from cinder vents 

along Cascade Range crest. Composition 

ranges from basalt to andesite. Widely 

distributed across Santiam Pass and locally near McKenzie Pass but shown only 

where drift ed to substantial thickness and 

masking un der ly ing units. Erupted chiefly 

from Sand Mountain chain of vents and 

Belknap Cra ter. Less extensive areas also 

shown down wind from their source vents 

at Four In One Cone, Collier Cone, and 

Lost Lake chain of cones. Younger than 

Mazama ash bed; radiocarbon ages range 

from 3,440±250 to 1,600 14C yr B.P. for 

most deposits or lava from source vents

Qyblk Basaltic andesite of Little Belknap—Erupt ed 

from shield volcano located east of Belknap 

Crater. Radiocarbon age of 2,883±175 14C 

yr B.P. from roots within tree mold near 

Pacific Crest National Scenic Trail (Chat-

 ters, 1968; Taylor, 1968, 1990)

[Following four units form Sand Mountain volcanic fi eld]

Qybln Basaltic andesite of Little Nash Crater—

Contains abundant small plagioclase and 

sparse olivine phenocrysts. Composition 

about 56.8 percent SiO2

. Age known only 

to be young er than 2,590±150 14C yr B.P. 

on basis of charcoal from soil developed 

on till that underlies cinder deposits of 

vent (table 1, No. W–6018)

Qybll Basalt of Lost Lake chain of cones—Slight ly 

porphyritic, with 2–3 percent olivine phe-

 noc rysts 1–2-mm across. Radiocarbon age 

is 1,950±150 14C yr B.P.

Qybn Basaltic andesite of Nash Crater—Lava 

flows with sparse small olivine phenocrysts. 

Contains about 53.5 percent SiO2

Qybsm Basalt of Sand Mountain chain of cones—Lava 

flows. Composition ranges from ba salt to 

basaltic andesite (51.6–53.2 percent SiO2); 

the basalt contains sparse phe noc rysts of 

plagioclase and olivine, whereas ba sal tic 

andesite has only olivine phe noc rysts. In-

 cludes flows younger than about 2,750 14C 

yr B.P. (Clear Lake flows; J.M. Licciardi, 

unpub. data, 1999) and flows as old as 

3,850±215 14C yr B.P. (Fish Lake flow; 

Chatters, 1968; Taylor, 1968)

Qybt Basaltic andesite of Twin Craters—Lava 

flows with olivine and sparse plagioclase 

phe noc rysts. Contains about 53.6 percent 

SiO2

. Predates Belknap eruptions. Maximum age is 2,800±150 14C yr B.P. on 

basis of char coal in soil beneath Twin 

Craters scoria (ta ble 1)

Qybic Basalt of Inaccessible cone—Nearly aphyric 

lava flows form alignment of cinder cones, 

most southerly of which is Inaccessible 

cone* (Taylor, 1965). Predates some or 

all Sand Mountain flows and all Belknap 

erup tions; two cinder cones of alignment 

were al most completely buried by basalt 

and basaltic andesite of Belknap Crater 

(Qybk). Young er than Mazama ash

Basalt of the Cascade Range and Newberry volcano

Qb Basalt (Holocene? and Pleistocene)—Mod er ate ly 

porphyritic to aphyric, light- to dark-gray 

lava flows and flow breccia found through out 

Cascade Range and areas to east. Chiefly 

50–52 percent SiO2

 but includes rocks with 

as little as 48 percent SiO2, especially east 

of Cascade Range. Phenocrysts commonly 

olivine and plagioclase; clinopyroxene is 

uncommon. Age chiefly Pleistocene, but 

first three units may be Holocene. Divided 

lo cal ly into:

Qbsb Basalt of Sims Butte (Holocene or Pleis tocene)—Erupted between 7,600 and 20,000 

yr ago, because Mazama ash lies upon lava, 

and cinders from cone lie upon till of Suttle 

Lake age. The till has only about 8 cm 

of ox i da tion, suggesting that Sims Butte 

cin ders and ash were deposited soon after 

glacial retreat—closer to 20 ka than 7.6 ka 

(W.E. Scott, unpub. data, 1987); therefore 

age is most likely latest Pleistocene

Qbcy Basalt of Cayuse Crater (Holocene or 

Pleis tocene)—Moderately porphyritic 

lava flows erupted from Cayuse Crater 

and two small er cinder cones to northwest (Taylor, 1978). Olivine phenocrysts 

to 2 mm, 10–12 per cent (Taylor, 1978). 

Silica content ranges from 49.7 to 51.8 

percent on basis of 12 anal y ses. Older 

than Mazama ash bed. Age between about 

12,500 and 9,500 yr B.P., because scoria locally overlies till of Can yon Creek 

advance, which generally is regarded to 

be about 11,000–12,500 yr in age. A radiocarbon age of organic-rich sediment 

from beneath the scoria is 9,520±100 14C 

yr B.P. (Scott and Gardner, 1992), but 

this carbon sample contains dispersed 

organic debris, could not be pre treat ed, 

and could be contaminated by younger 
18

material. Therefore the 9,520-yr age is 

considered a minimum age

Qbec Basalt of Egan cone (Holocene or Pleis tocene)—Aphyric to slightly porphyritic lava 

flows erupted from informally named cin der 

cone at north base of Mount Bachelor, 

which lies just south of map area. Includes 

some basaltic andesite; proximal lava and 

scoria ap prox i mate ly 50.6–53 percent SiO2, 

whereas dis tal, presumably early-erupted 

lava has ap prox i mate ly 53–54.5 percent 

SiO2 (Scott and Gardner, 1992; Gardner, 

1994). Older than Mazama ash bed (older 

than about 7,650 calibrated yr B.P.). 

Age considered closer to Mazama time 

(perhaps only 1,000 yr older) on basis 

of minimal weathering prior to eruption 

of Mazama ash, although pa le o mag net ic 

field directions suggest that dis tal lava 

may be closer in age to summit cone of 

Mount Bachelor, about 11,000–12,500 yr 

(Scott and Gardner, 1992)

Qbkt Basalt of Katsuk and Talapus Buttes (Pleis tocene)—Porphyritic and glomeroporphyritic 

basalt erupted from 2.5-km-long align ment 

of vents west of Sparks Lake. Lava flows 

form steep-sided plateau with ice-contact 

features suggesting lava encountered and 

was buttressed by glacial ice. Slumping 

of lava has occurred in places along east 

and west plateau margin since deglaciation (Scott and Gardner, 1992)

Qbn Basalt of Newberry volcano (Pleis tocene)—Open-textured vesicular lava flows. 

Con tains about 49–50 percent SiO2 (McDannel, 1989; Linneman, 1990). Erupted 

from vents on north flank of Newberry 

volcano (south of map area) and flowed 

north across broad plain extending to 

Redmond. Beyond Redmond, the basalt 

flowed into Deschutes and Crook ed River 

canyons and, via the Deschutes can yon, 

reached Lake Billy Chinook, 5 km north of 

map area. Unit possesses normal-po lar i ty 

magnetization and is younger than 0.78 Ma. 

Overlies Tumalo Tuff along west margin 

near Bend; therefore younger than about 

0.4 Ma in that area. An age of 2.7±0.3 

Ma (K-Ar, whole rock) was reported 

from lava flows exposed in fault blocks 

in southeast part of city of Bend (Hawkins 

and others, 1988). This age is discordant 

with geologic set ting and probably is too 

old. Potassium-argon age of 5.96±2.08 Ma 

(Evans and Brown, 1981) is meaningless. 

Lava in eastern part of unit divided on 

basis of mineralogy into:

Qbn Basalt of Badlands—Surface extremely 

ir reg u lar, displaying numerous features 

char ac ter is tic of inflated pahoehoe lava 

flows such as pressure ridges, pressure 

plateaus, tumuli, and residual depressions 

(Chitwood, 1994). More extensive south 

of map area, in clud ing Badlands Roadless 

Area (Bergquist and others, 1990). Vent 

area is matter of de bate: unit forms a broad 

topographic rise that may be the discharge 

point for lava tubes drain ing from upslope 

on Newberry vol ca no (MacLeod and others, 

1995), or al ter na tive ly, the rise is a separate 

volcano (informal “Badlands volcano” of 

Hawkins and oth ers, 1988). Contains about 

50 percent SiO2

 (Linneman, 1990). Isotopic 

ages of 0.7±0.1 and 2.9±0.3 Ma (K-Ar, 

whole rock) were reported from this unit 

south of map area (Hawkins and others, 

1989) but are too old on basis of flow 

morphology. Interpreted to overlie sand 

and gravel (in unit Qs) near Alfalfa

QbnPorphyritic basalt—Contains olivine phe-

 noc rysts 1–3 mm across and 1–3 percent 

in abundance

Qbn Highly porphyritic basalt—Olivinerich lava flows. Contains 3–5 percent 

olivine phe noc rysts as large as 4 mm 

across. Forms lin ear ridges and pressure 

plateaus. Outcrop pat tern suggests unit 

likely inflated to form a sinuous train of 

tumuli. Subsequently, por phy rit ic basalt 

(Qbn) inundated same area and buried 

all but high-standing mass es of highly 

porphyritic basalt. Contains about 48.5 

percent SiO2 (Linneman, 1990)

Qbn Large-feldspar basalt—Contains scant 

pla gio clase phenocrysts as large as 7 mm

Qbn Basalt near Alfalfa—Fine-grained basalt, 

commonly with slight hematitic alteration 

of olivine and magnetite on weathered 

surfaces

Qbn Very fine grained basalt—Dark-gray to 

black aphanitic lava flows

Qbsc Basalt of cones at summit and southeast flank 

of Cache Mountain (Pleistocene)—Gla-

 ci at ed lava flows bearing sparse xenocrysts 

of quartz. Unusual for its lack of pla gio -

clase phenocrysts and abundance of olivine 

(1–2 mm, 5 percent) and clinopyroxene 

(1–2 mm, 1 percent). Contains about 50 

percent SiO2

 (Hughes and Taylor, 1986)

Qbcb Basalt of Condon Butte (Pleistocene)—Gla-
19

 ci at ed aphyric lava flows. Summit crater 

remains intact, suggesting relatively youth-

 ful age

Qbkm Basalt of Koosah Mountain (Pleis tocene)—Olivine- and plagioclase-bearing 

lava. Contains 51.4–52.6 percent SiO2. 

Most phenocrysts less than 2 mm long, 

but typ i cal ly contains a few outsized olivine crys tals as large as 7 mm across. 

Age may be younger than 100 ka on basis 

of preserved landforms

Qbbt Basalt of Burnt Top (Pleistocene)—Oli v ineor olivine- and plagioclase-bearing lava with 

sparse glomerocrysts. Contains 52 percent 

SiO2. Forms shield centered at Burnt Top. 

Includes a somewhat younger flow erupt ed 

from both summits of Burnt Top; that flow 

is more plagioclase rich than most lava in 

the shield. Of same general age as basalt 

of Koosah Mountain (Qbkm) on basis of 

sim i lar lack of erosion

Qbtb Basalt of Two Butte (Pleistocene)—Gla-

 ci at ed lava flows from cones south of 

Scott Moun tain. Contains abundant small 

olivine phe noc rysts

Qbc Basalt of Craig Lake (Pleistocene)—Mod er -

ate ly porphyritic lava flows. Contains 3–5 

per cent of olivine phenocrysts and scant 

clinopyroxene phenocrysts. Erupted from 

broad cinder vent west of Craig Lake in 

Mount Jefferson Wilderness Area

Qbwf Basalt of Wizard Falls (Pleistocene)—Ve-

 sic u lar, finely porphyritic lava with about 

2 per cent olivine phenocrysts smaller than 

1 mm. Contains 50 percent SiO2 (Conrey, 

1985). Erupted from cinder cone near base 

of Green Ridge. Lava displaced Metolius 

River and flowed 2 km north of quad ran gle 

to form Wizard Falls. Normal-polarity 

mag ne ti za tion

Qbg Basalt of Garrison Butte (Pleistocene)—

Fine-grained, open-textured lava flows. 

Con tains about 50 percent SiO2. Erupted 

from cin der cones along chain that includes Gar ri son Butte. Some lava flowed 

northeast into Stevens Canyon, reaching 

nearly to confluence of Deschutes River. 

Normal-polarity magnetization

Qbh Basalt of Henkle Butte (Pleistocene)—

Oli v ine-bearing lava flows

Qbsp Basalt of Santiam Pass (Pleistocene)—Por-

 phy rit ic olivine basalt or basaltic andesite. 

Commonly contains glomerocrysts of 

pla gio clase and olivine, which makes it 

pet ro graph i cal ly similar to some lava from 

Three Fingered Jack volcano to the north. 

Cov ers broad area of Santiam Pass be tween 

U.S. Highway 20 and Big Lake; erupted 

from cinder cones along Cascade Range 

crest. Normal-polarity magnetization

Qbbl Basalt of Booth Lake (Pleistocene)—Clinopyroxene- and olivine-bearing lava flows. 

Erupted from vent on ridge south of Three 

Fingered Jack and flowed south and east 

from Cascade Range crest

Qblb Basalt of Little Brother (Pleistocene)—Lava 

flows with microphenocrysts of pla gio clase 

and clinopyroxene

Qbpp Basalt of Plainview (Pleistocene)—Wide-

 spread lava flows exposed on east side of 

Cas cade Range near Sisters and in Deep 

Canyon. Contains as much as 20 percent 

pla gio clase phenocrysts (Taylor and Ferns, 

1995). Vent buried by younger flows in 

upslope area of Cascade Range south of 

Sisters town. Normal-polarity magnetization; overlies Desert Spring Tuff and 

therefore is young er than about 0.6 Ma

Qbti Basalt of The Island (Pleistocene)—Open-

 tex tured, vesicular lava flows exposed 

along Crook ed River Gorge from Crooked 

River Ranch north to edge of map area. 

Extends northward downcanyon beyond 

quad ran gle for 17 km to Round Butte. 

Forms The Is land, a prom i nent mesa in 

vicinity of Cove Palisade State Park, 7 

km north of quad ran gle (Smith, 1986). 

Possesses reversed-polarity magnetization but otherwise is in dis tin guish able 

from normal-polarity basalt of Newberry 

vol ca no, which overlies it near Crooked 

River Ranch, or from basalt of Opal 

Springs, which underlies it along much 

of Crooked River Gorge in map area. 

Isotopic age is 1.19±0.08 Ma (40Ar/39Ar, 

whole rock; Smith, 1986) from sample 

near Crooked River Ranch. Re put ed ly 

erupted from Newberry vol ca no (for 

example, Peterson and others, 1976; 

Smith, 1986; Dill, 1992), but unit cannot be traced farther south than Crooked 

River Ranch. Thus, the upstream pathway 

for the re versed- polarity basalt must now 

be over lain en tire ly by normal-polarity 

basalt of Newberry volcano (unit Qbn). 

As a pos si bly con tra dic to ry observation, 

upper sur face of unit lies roughly at 730-m 

elevation in most locations (Dill, 1992; 

Ferns and oth ers, 1996b), which seems 

unlikely if the lava were flowing down-
20

stream along entire extent. An alternative 

explanation, therefore, is that the basalt 

of The Island was erupted from some 

yet-to-be-found fissure vent down stream 

from (north of) Crooked River Ranch and 

backed up along Crooked River

QTb Basalt (Pleistocene and Pliocene?)—Lava flows 

on west side of High Cascades exposed 

in canyon walls of White Branch, Sep a -

ra tion, Horse, and Eugene Creeks; and on 

east side near Overturf Butte in Bend city 

limits. Canyon-wall exposures (north and 

south of Oregon Highway 242) consist of 

open-tex tured lava flows, chiefly basalt, 

that ac cu mu lat ed at the west margin of the 

High Cascades during latest Pliocene(?) 

and Pleistocene time; these correspond 

to the basalt of Roney Creek as mapped 

farther west by Priest and oth ers (1988). 

Chiefly re versed-polarity mag ne ti za tion. 

Overturf Butte de pos its are lava flows 

and cinders un re lat ed to the west-side 

lava and possessing nor mal-polarity 

mag ne ti za tion

Tb Basalt (Pliocene)—Lava flows on west side 

of High Cascades exposed in canyon 

walls of Horse and Eugene Creeks. 

Chiefly open-textured basalt lava flows 

but includes minor ba sal tic andesite, 

thin beds of unwelded ash-flow tuff, 

sandstone, and conglomerate. Most lava 

possesses normal-polarity mag ne ti za tion, 

corresponding to eruption during Gauss 

Normal-Polarity Chron (2.60–3.55 Ma). 

Lowest part, however, may include lower 

Pliocene strata

Basaltic andesite of the Cascade Range

Qba Basaltic andesite (Holocene? and Pleis tocene)—Slightly porphyritic to aphyric, 

light- to dark-gray lava flows and flow 

breccia. Forms much of High Cascades 

throughout map area. Chiefly Pleistocene, 

but youngest unit may be Holocene in 

age, and second youngest unit may be 

partly Holocene. Possesses normal-polarity 

magnetization; age young er than 0.78 Ma. 

Oldest isotopic age is 0.63±0.09 Ma (whole 

rock, K-Ar) from scoria in as so ci at ed 

cinder cone (Qc, part) 1.5 km west of 

Triangle Hill (Hill, 1992a), but unit was 

emplaced as numerous lava flows from 

multiple vents that span a broad time 

pe ri od. Divided locally into:

Qbal Basaltic andesite of Le Conte Crater (Ho-

 locene or Pleistocene)—Porphyritic lava 

flows. Younger than last major glaciation 

but older than Mazama ash bed (emplaced 

some time between about 20,000 and 7,650 

years ago)

Qbam Basaltic andesite of Mount Bachelor 

(Ho locene? and Pleistocene)—Includes 

lava flows young er and some older than 

Can yon Creek ad vance (of Cabot Creek 

gla ci a tion). Old er than Mazama ash bed

 Basaltic andesite of South Sister (Pleis tocene)—Clinopyroxene- and olivine-bear ing 

lava flows. Contains 56–58 percent SiO2

(Wozniak, 1982). Divided on basis of in-

 ter ven ing andesite (Qass) into younger 

and older parts:

Qbass Basaltic andesite of summit cone—Sco ria 

and oxidized lava flows

QbassBasaltic andesite of upper flanks

Qbah Basaltic andesite of Hoodoo Butte (Pleis tocene)—Nearly aphyric lava flows with less 

than 1 percent olivine and clinopyroxene 

as microphenocrysts. Contains about 55 

per cent SiO2 (Black and others, 1987). 

Sum mit crater of cinder cone is relatively 

well preserved but lava has been glaciated, 

so unit is older than about 20,000 yr

Qbamx Basaltic andesite of Maxwell Butte (Pleis tocene)—Nearly aphyric basaltic andes ite. 

Contains 55–56 percent SiO2 (R.M. Conrey, 

unpub. data, 1990). Erupted from vent 

at Max well Butte. Forms thick, mediumgrained, light-gray glaciated lava flows on 

slopes sur round ing Maxwell Butte, whereas 

at lower el e va tions to west beyond glacial 

limits, forms relatively youthful-looking, 

dark-gray to black lava flows with rugged 

primary flow fea tures preserved locally

Qbasm Basaltic andesite of Scott Mountain (Pleis-

 tocene)—Lava flows at lower elevations 

not glaciated; may be about same age as 

ba sal tic andesite of Maxwell Butte

Qbatm Basaltic andesite of Tumalo Mountain 

(Pleis tocene)—Lava flows and cinder-cone 

de pos its, some of which are hyaloclastite. 

Forms shield volcano of Tumalo Moun tain 

(north of Mount Bachelor) and scoria cones 

trend ing south and northwest from summit 

(Taylor, 1978; Scott and Gardner, 1992). 

Phe noc rysts of plagioclase (as much as 2 

mm across, 25 percent of rock), clinopyroxene (as much as 1 mm across, 5 percent), 

oli v ine (as much as 0.5 mm across, 1 per-

 cent), and rare orthopyroxene; phenocrysts 
21

com mon ly found as glomerocrysts as large 

as 6 mm in di am e ter, 3–7 percent of rock 

(Taylor, 1978). Age between about 18,000 

and 150,000 years; glaciated during Suttle 

Lake advance (of Cabot Creek glaciation, 

18,000–22,000 yr ago), and unit overlies 

an older till thought to be about 150,000 

years old (Scott and Gardner, 1992)

 Basaltic andesite of Sixmile Butte lava field 

(Pleistocene)—Erupted from ten cinder 

cones in area between Black Butte and 

Black Crater. Emplaced prior to Suttle 

Lake glacial ad vance (pre-latest Pleistocene). Divided into:

Qbasx Basaltic andesite of Bluegrass Butte—

Aphyric

QbasxBasaltic andesite of Graham Butte—Small 

plagioclase and olivine grains, about 1 mm 

across and about 2–3 percent in combined 

abundance (more olivine than pla gio clase), 

commonly forming microglomerocrysts

Qbasx Basaltic andesite of Fivemile Butte—Scant 

plagioclase phenocrysts less than 1 mm 

long, about 1 percent of rock; olivine as 

large as 1 mm across, 1 percent

Qbasx Basaltic andesite of Fourmile Butte—

Aphyric to sparsely porphyritic

Qbasx Basaltic andesite of Sixmile Butte—Pla-

 gio clase phenocrysts seriate to 2 mm long, 

10 percent of rock; olivine phenocrysts 

to 1 mm across, 1 percent

Qbacs Basaltic andesite of Cold Spring (Pleis tocene)—Nearly aphyric lava, with less than 

1 per cent small phenocrysts of pla gio clase, 

clinopyroxene, and olivine. Contains 56–57 

percent SiO2 (Taylor, 1987). Issued from 

unnamed cinder cone 2.6 km southwest 

of Millican Crater. Flow surface is ex ten -

sive ly glaciated above 1,160-m elevation 

but cov ered with fresh scoria and blocks 

at low er el e va tion beyond reach of Pleistocene gla ciers

Qbamc Basaltic andesite of Millican Crater (Pleis tocene)—Porphyritic lava with glomerocrysts 

of plagioclase and olivine. Contains 53–54 

percent SiO2 (Taylor, 1987). Younger than 

basaltic andesite of Black Crater

Qbabc Basaltic andesite of Black Crater (Pleis-

 tocene)—Sparsely porphyritic lava with 

scant pla gio clase and even less abundant 

clinopyroxene. Contains 55–57 percent 

SiO2

 (Taylor, 1987)

Qbarh Basaltic andesite of Red Hill (Pleis tocene)—Olivine- and plagioclase-bearing 

lava erupt ed from small vent near south 

edge of map. Contains 55.2 percent SiO2. 

Char ac ter ized by thick, steep-sided lava 

flows, com mon ly with glassy or palagonitized margins. These features suggest 

eruption during intraglacial period. Overlies 

basalt of Koosah Moun tain (Qbkm). Age 

probably younger than 100 ka

Qbams Basaltic andesite of Middle Sister (Pleis tocene)—Moderately to highly por phy rit ic 

lava flows. Commonly contains abundant 

pla gio clase phenocrysts 2–4 mm across, 

as much as 50 percent in some flows, and 

olivine up to 3 mm across, 5 percent in 

most flows. Con tains 52–53 percent SiO2

(Taylor, 1987). Younger than Shevlin Park 

Tuff (younger than about 0.17 Ma); isotopic 

age of 0.4±0.2 Ma has large analytical 

error (Hill, 1992a)

Qbans Basaltic andesite of North Sister (Pleis tocene)—Vesicular lava flows with abun dant 

pla gio clase microphenocrysts and glomerocrysts of plagioclase and olivine. Contains 

54–55 percent SiO2 (Taylor, 1987). Oldest 

of the Three Sisters volcanoes. Vent deposits of central pyroclastic cone (in unit 

Qmv) in clude substantial palagonitic tuff on 

east flank, possibly the result of sub gla cial 

eruption during initial cone growth

Qbatf Basaltic andesite of Three Fingered Jack 

(Pleistocene)—Slightly to moderately por-

 phy rit ic lava flows with phenocrysts and 

microphenocrysts of plagioclase and oli v ine. 

Many lava flows contain glomerocrysts of 

plagioclase and olivine, chiefly those in the 

middle and upper flanks of the vol ca no. 

Outlying lava flows on lower east flank 

have primary flow surfaces unmodified by 

gla ci a tion, lie upslope of terminal moraines 

deposited during Jack Creek glaciation, and 

are overlain upslope by moraines of Suttle 

Lake age, indicating an age between Jack 

Creek and Suttle Lake glaciations (Scott 

and others, 1996)

Qbabp Basaltic andesite of Black Pine Spring 

(Pleis tocene)—Nearly aphyric lava, with 

less than 1 percent small phenocrysts of 

pla gio clase. Contains 54–55 percent SiO2

(Taylor, 1987). Erupted from cinder cone 

1 km southwest of Black Pine Spring 

campground. Dis placed near its terminus 

by segment of Sisters fault zone. Overlain 

by Shevlin Park Tuff. Nor mal-polarity magnetization and younger than 0.78 Ma

Qbat Basaltic andesite of Trout Creek Butte 

(Pleistocene)—Vesicular lava flows 
22

with sparse plagioclase phenocrysts and 

glomerocrysts of plagioclase and olivine. 

Contains 56–57 percent SiO2 (Taylor, 

1987). Normal-po lar i ty magnetization 

and young er than 0.78 Ma

Qbaw Basaltic andesite of Mount Washington 

(Pleistocene)—Plagioclase- and olivinebearing lava flows and breccia. Vent 

de pos its of cen tral pyroclastic cone (in 

unit Qmv) include substantial palagonitic 

tuff on north east flank, possibly the result 

of sub gla cial eruption during initial cone 

growth

Qbasp Basaltic andesite of Substitute Point (Pleis-

 tocene)—Olivine-bearing lava; contains 

53.6–56.2 percent SiO2. Erupted from 

north-northwest-trending sequence of 

three vents and their conduit-filling 

plugs (shown sep a rate ly as units Qmv

and Qi). Normal-po lar i ty magnetization 

and younger than 0.78 Ma

Qbapb Basaltic andesite of Pilot Butte (Pleis tocene)—Highly porphyritic lava; contains 

10–15 percent of blocky plagioclase phenocrysts as large as 5 mm across. Contains 

about 53.5 percent SiO2 and 20 percent 

Al2O3. Erupt ed from cinder cone that forms 

Pilot Butte in city of Bend. Thought to be 

younger than 0.78 Ma on basis of normal-polarity mag ne ti za tion. Predates many 

flows assigned to basalt of Newberry volcano (Qbn). Cin der cone is mantled by a 

rhyolitic tephra fall on its southwest side. 

This tephra is chem i cal ly similar to tephra 

exposed on south rim of Tumalo Creek at 

1,728-m elevation and likely younger than 

Bend Pumice (see appendix 1, locality 

Nos. 12 and 13). Stratigraphic relation 

of the Pilot Butte lava to Bend Pumice 

unknown; that tephra deposit should be 

thicker than 2 m in Pilot Butte area, but 

none is reported to overlie lava flows from 

Pilot Butte. However, Bend Pumice is com-

 mon ly eroded except where protected by 

over ly ing deposits and is absent on many 

fea tures that predate it in the Bend area

Qoba Older basaltic andesite (Pleistocene)—Older 

than 0.78 Ma; possesses reversed-polarity 

mag ne ti za tion or known to underlie reversely polarized strata. Includes slightly 

to mod er ate ly porphyritic lava flows of 

Black Butte, a mildly eroded, steep-sided 

lava cone whose pyroclastic core remains 

unexposed. Black Butte lava contains 5–10 

percent of pla gio clase phenocrysts 1–2 mm 

long and 3–5 percent of olivine phenocrysts 1 mm across. Potassium-argon age 

for Black Butte is 1.43±0.33 Ma (whole 

rock) (Hill and Priest, 1992; recalculated); 

earlier age de ter mi na tions of about 0.4 

Ma were known to be too young owing 

to reversed-polarity mag ne ti za tion of the 

lava (Armstrong and others, 1975). Black 

Butte sits astride a small graben-bounding 

fault, and its lava has been slight ly offset 

by a fault on the northwest flank. Map-unit 

symbol shown queried for older ba sal tic 

andesite at Cache Mountain, where lava 

possesses normal-polarity magnetization; 

sample from Cache Mountain has K-Ar 

age of 0.90±0.05 Ma (Armstrong and others, 1975). If age is correct, then Cache 

Moun tain strata were likely erupted during 

the Jaramillo Normal-Polarity subchron of 

the Matuyama Reversed-Polarity chron, 

0.98–1.04 Ma (time scale from Cande 

and Kent, 1992)

Andesite of the Cascade Range

Qa Andesite (Pleistocene)—Chiefly porphyritic 

lava flows, all with normal-polarity mag-

 ne ti za tion. Commonly contains phenocrysts 

of plagioclase, orthopyroxene, and clinopy rox ene. May contain olivine phe noc rysts. 

Di vid ed locally into:

Qass Andesite of South Sister—Porphyritic, platy 

to massive lava flows and breccia. Iso to pic 

age from early-erupted silicic andesite on 

north east flank is 93±11 ka (Hill, 1992a). 

Di vid ed at summit into:

Qass Andesite of summit

QassAndesite of east flank

Qah Andesite of Hogg Rock and Hayrick 

Butte—Prominent flat-topped buttes in 

area of Santiam Pass; their shape and 

locally glassy margins suggest they may 

have erupted into glacial ice. Slightly porphyritic, with pla gio clase, orthopyroxene, 

and trace amounts of olivine phenocrysts. 

Silica content about 59 percent (Davie, 1980; 

Hughes, 1983; Hill, 1992b). Potassiumargon age from Hogg Rock is 80±20 ka 

(Hill and Priest, 1992; age recalculated)

Qams Andesite of Middle Sister—Typically me-

 di um-gray, slightly porphyritic lava with 

black, glassy margins and base. Possesses 

per va sive platy fracturing throughout its 

extent. Fills broad canyon on west side 

of Middle Sister
23

Qoad Andesite and dacite of First Creek (Pleis-

 tocene)—Aphyric andesite and dacite 

lava and dacitic(?) pumiceous ash-flow 

tuff. Ex posed in First Creek valley on 

southeast flank of Three Fingered Jack. 

Possesses re versed-polarity magnetization; 

older than 0.78 Ma

Dacite, rhyodacite, and rhyolite of the Cascade Range

Qd Dacite (Pleistocene)—Generally porphyritic lava 

with 5–10 percent plagioclase phenocrysts 

and lesser amounts of clinopyroxene and 

orthopyroxene phenocrysts. Possesses 

nor mal-polarity magnetization. Divided 

lo cal ly into:

Qdss Dacite of South Sister

Qdms Dacite of Middle Sister

Qdlp Dacite of Lane Plateau—Contains about 65 

percent SiO2 (Hughes, 1983; Hill, 1992a)

Qdt Dacite of Todd Lake—Porphyritic lava flows 

west and north of Todd Lake. Includes 

interbedded ejecta and central plug (not 

mapped sep a rate ly), which together form 

the Todd Lake volcano of Taylor (1978) or 

Hill (1992a). Normal polarity mag ne ti za tion. 

Po tas si um-argon age is 0.460±0.030 Ma 

(whole rock; Hill, 1992a)

Qrd Rhyodacite (Pleistocene)—Lava flows and 

domes. Possesses normal-polarity magnetization. Divided locally into:

Qrdt Rhyodacite of Tam McArthur Rim—Glassy 

to holocrystalline, slightly porphyritic lava. 

Consists of two flows, near-vent pu mi ceous 

deposits, and east-striking dikes (Taylor, 

1978). Interbedded with basaltic andesite 

lava flows from Broken Top. Potassiumargon age is 213±9 ka (Hill, 1992a)

Qr Rhyolite (Holocene and Pleistocene)—Lava fl ows 

and domes. Found southeast of Condon 

Butte, on northwest and southeast sides of 

Mid dle Sister, south of South Sister, and 

interlayered in cone of South Sister. Also 

forms iso lat ed domes in area near Melvin 

and Three Creek Buttes and a dome complex near Triangle Hill east of Broken Top. 

Normal-polarity magnetization; younger than 

0.78 Ma. Iso to pic ages from unit (all K-Ar 

from whole rock) include 95±10 ka from 

rhyolitic obsidian at Obsidian Cliffs (Hill, 

1992a), 0.4±0.4 Ma from Melvin Butte 

dome (Armstrong and others, 1975), and 

0.2±0.9 Ma from Three Creek Butte dome 

(Armstrong and others, 1975); latter two 

ages are ren dered meaningless by the large 

standard deviation. An age of 1.63±1.1 Ma 

was obtained from dome at Bearwallow 

Butte (Fiebelkorn and others, 1983). This 

dome possesses normal-polarity magnetization and is probably younger than 0.78 

Ma, within the range of the reported age. 

Youngest rhyolite shown separately as:

Qrrm Rhyolite of Rock Mesa and Devils Hill 

chain of vents (Holocene)—Dense to pu-

 mi ceous, porphyritic rhyolite. Chiefl y thick 

lava flows and domes; shown stippled are 

fragmental deposits, including tephra-fall, py-

 ro clas tic-fl ow, pyroclastic-surge, and laharic 

de pos its. Fragmental deposits drape and are 

cut by cracked ground or gaping fractures. 

Erupted between 2,000 and 2,300 14C yr B.P. 

on basis of several radiocarbon ages from 

charcoal beneath associated tephra (Scott, 

1987). Dormant interval of at least 100 yr 

separates older Rock Mesa from younger 

Devils Hill chain of vents (Scott, 1987). 

[Geographic note: Devils Hill proper is an 

older rhyolite lava fl ow of unit Qr]

Trd Rhyodacite or dacite (Pliocene or Miocene)—Lava 

flow, probably a dome, exposed in floor 

of Eugene Creek on west side of Cascade 

Range, southwest corner of map area. Like ly 

cor rel a tive with a unit of andesite and dacite 

mapped west of map area by Priest and 

others (1988, their unit Tmpa)

Pyroclastic fl ow and fall deposits

[Rock Mesa-Devils Hill ash bed (Holocene). Distribution shown only 

by iso p achs. Surfi cial de pos it com pris ing two beds of white rhy olitic 

lapil li and ash found mostly south and east of the South Sister. De-

 pos its are as thick as 2 m near vent and extend 30–40 km down wind. 

Derived from erup tions at Rock Mesa (older) and Devils Hill chain of 

vents (younger) dur ing two brief episodes between 2,300 and 2,000 

14C yr B.P. (Scott, 1987); waning stag es of these erup tions produced 

rhyolitic lava fl ows (Qrrm).]

[Mazama ash of Powers and Wilcox (1964) (Holocene). Not shown 

on map. Surfi cial deposit (here in re ferred to as the Mazama ash bed) 

that forms important strati graph ic horizon. Derived from cli mac tic 

erup tion of Mount Mazama (Crater Lake National Park), 135 km 

south-southwest of Bend (fi g. 1); grain size and thick ness de crease 

north ward and east ward. Age about 6,845±50 14C yr B.P. (Bacon, 

1983), which cor re sponds to about 7,650 cal i brat ed yr B.P.]

Qp Pumice-fall deposits (Pleistocene)—Coarse- to 

medium-grained, light-gray to white pu-

 mi ceous lapilli, locally reworked. Chiefly 

rhyodacite and rhyolite in composition but 

includes minor dacite and rarely andesite 

(andesitic scoria black to brownish). 
24

Con sists of several unrelated deposits, 

in clud ing Bend Pumice (fallout tephra 

preceding emplacement of Tumalo Tuff), 

pumice of Columbia Canal (stratigraphically 

be tween Shevlin Park and Tumalo Tuffs; 

see text), and tephra from eruptions near 

Middle Sister, South Sister, and Broken 

Top. Forms map pa ble unit along Bottle 

Creek 4 km west of Bearwallow Butte in 

south-central part of map area (pumice of 

Bottle Creek); ex po sures there are at least 

20 m thick re sult ing from a single tephra 

shower, and base is not exposed. Most 

occurrences, however, range in thickness 

from 10 cm to 2–3 m and are marked on 

map by “x” owing to re strict ed exposure; 

several of these are discussed more fully 

in appendix 1. None of the deposits is 

thought to predate Bend Pumice, which 

is approximately 0.3–0.4 Ma in age

Qsp Shevlin Park Tuff (Pleistocene)—Dark-gray 

to black, fresh pyroclastic-flow deposits. 

Pum ice lapilli are mostly porphyritic andesite with phe noc rysts of plagioclase, 

hypersthene, augite, and opaque oxides 

(Mimura, 1992), but some lapilli are rhyodacite. Whole-pumice and glass analyses 

range chiefly from 57 to 62 per cent SiO2

(Hill and Taylor, 1989; Sarna-Wojcicki 

and others, 1989; Mimura, 1992), with 

a few analyses as high as 67.5 per cent 

SiO2 (our unpub. data). Deposit contains 

lithic fragments of basalt, andes ite, and 

rhyolite. Forms single cooling unit that 

originated as two pyroclastic flows. 

Lower pyroclastic-flow deposit is rich 

in fine ash; upper deposit is pumice 

rich. Dense welding in lower part and 

vapor-phase crystallization in upper part 

are ubiquitous in thicker parts of unit. 

Imbrication, which is well developed in 

lower part of each pyroclastic-flow deposit, 

in di cates that unit was erupted west of 

map area, perhaps from now-buried vent 

in high lands near Broken Top volcano, 

15 to 25 km west of Bend (Mimura and 

MacLeod, 1978; Mimura, 1984; Hill and 

Taylor, 1990). Distribution varies from 

surface mantling to valley filling. Maximum thickness 45 m. Normal-po lar i ty 

magnetization. Age slight ly younger than 

0.17 Ma on basis of strati graph ic position occupied by distal fallout tephra in 

an cient lakebeds of the northern Great 

Basin (see explanatory text)

Qcd Century Drive Tuff (Pleistocene)—Mod er ate ly 

porphyritic pyroclastic-flow deposits that 

range from nonwelded to moderately weld ed. 

Contains both rhyodacitic and andesitic 

pumice lapilli (Hill and Taylor, 1990; Hill, 

1992a), in contrast to chiefly andesitic 

Shevlin Park Tuff; but phenocryst mineralogy is otherwise similar. Also similar in 

age to Shevlin Park Tuff, which overlies it 

in exposures along Tumalo Creek, and the 

two units may be products of same eruptive episode. Three-km-long main deposit 

in Tumalo Creek is densely welded and 8 

m thick but cannot be shown separately 

from Shevlin Park Tuff at scale of map. 

Map-unit label shown queried for deposits 

of un cer tain correlation

Qrt Rhyodacite tuff (Pleistocene)—Densely weld ed, 

moderately porphyritic, stony welded tuff. 

Collapsed pumice contains about 67 per cent 

SiO2

; whole-rock analyses range from 66 

to 68 percent (Hill, 1992a; our unpub. 

data). Found only in Tumalo Creek valley 

and in south wall above Tumalo Lake. 

Western exposure once formed the floor 

of an an ces tral Tumalo Creek valley but 

sub se quent ly has been incised and now 

forms a ridge that projects into the valley. Probably similar in age to Shevlin 

Park and Century Drive Tuffs on basis of 

topographically similar canyon settings

Qtt Tumalo Tuff (Pleistocene)—As mapped, 

con sists chiefly of pink pyroclastic-flow 

deposits (Tumalo Tuff, aggregate thickness 24 m) but locally includes thick 

underlying pu mi ceous fallout deposit 

(Bend Pumice, maximum thickness 11 

m). Stippled where Bend Pumice is exposed over large areas owing to open-pit 

quarrying. Tephra fallout and py ro clas tic 

flow resulted from single eruptive ep i sode 

at vent west of Bend; pumiceous lapilli 

of the two deposits are mineralogically 

sim i lar, containing phenocrysts of plagioclase, orthopyroxene, and minor amphibole, 

titanomagnetite, apatite, and zircon (Hill 

and Taylor, 1990). Silica content ranges 

from 73.1 to 75.7 percent (Mimura, 1992; 

Hill, 1985, 1992a). Well-developed imbrication in lower part and locally in 

upper part of Tumalo Tuff indicates that 

pyroclastic debris flowed northeastward 

into Bend area (Mimura and MacLeod, 

1978; Mimura, 1984). Flows, thought to 

have erupted from uplands east of Bro-
25

ken Top volcano, have been chan neled by 

northeast-trending drainages (Hill, 1985; 

Hill and Taylor, 1990). Normal-polarity 

magnetization in ash-flow tuff; age about 

0.3–0.4 Ma on basis of several K-Ar ages 

(Sarna-Wojcicki and others, 1989)

Qds Desert Spring Tuff (Pleistocene)—Rhyodacitic 

ash-flow tuff. Ashy matrix is brownish 

or ange, purple, or dark gray. Dark-gray 

pu mi ceous lapilli commonly contain 

phe noc rysts of plagioclase, hypersthene, 

aug ite, and opaque minerals. Diagnostic 

char ac ter is tic is abun dant apatite needles 

enclosed with in most phe noc ryst phases. 

Contains 68–69 percent SiO2 (Mimura, 

1992). Commonly contains ba sal tic lithic 

frag ments. Thick ness ranges from 5 to 

11 m; lower part of unit is partially 

welded and columnar jointed. Imbrication in di cates that di rec tion of flow was 

from southwest to northeast (Mimura and 

MacLeod, 1978; Mimura, 1984). Nor malpolarity mag ne ti za tion. Age between 0.6 

and 0.7 Ma on basis of cor re la tion with 

distal tephra (see ex plan a to ry text)

VOLCANIC AND SEDIMENTARY ROCKS IN THE 

DESCHUTES BASIN

Tbal Basaltic andesite of Little Squaw Back (Pliocene)—Slightly porphyritic, light-gray lava 

fl ows that form shield volcano of Little Squaw 

Back. Contains phenocrysts of hy per sthene, 

plagioclase, and olivine. Age uncertain but 

not older than late Pliocene. The volcano 

lacks exposures suitable for directly test-

 ing its magnetization. A com put er analysis 

was used by R.J. Blakely (U.S. Geological 

Survey) to calculate the mag net ic anomalies 

that would be expected over the volcano 

for various magnetization di rec tions. A 

com par i son of these calculations with observed magnetic anomalies suggests that the 

vol ca no has dominantly reversed-po lar i ty 

magnetization. The analysis also indicates 

certain complexities in magnetization, ex-

 plain able by either a transitional polarity 

direction or some anomalous aspect of the 

volcano’s structure (R.J. Blakely, written 

commun., 1995)

Tbas Basaltic andesite of Squaw Back Ridge (Pliocene)—Slightly porphyritic lava flows that 

form shield volcano of Squaw Back Ridge. 

Oli v ine phenocrysts form 2–3 per cent of 

rock. In some samples, glomerocrysts of 

pla gio clase and olivine are present. Contains 54.4–56.0 percent SiO2 (Conrey, 

1985). Nor mal-polarity magnetization. 

Age is 2.9±0.2 Ma (Armstrong and others, 1975)

Tam Andesite of McKinney Butte (Pliocene)—

Aphyric high-iron andesite lava, also 

with un usu al ly high concentration of 

Na; average of four analyses indicates 

about 60 percent SiO2, 11 percent FeO, 

and 6 percent Na2O (E.M. Taylor, unpub. 

data). Brick-red weath er ing. Magnetic 

polarity uncertain; total of ten fluxgate 

magnetometer measurements at different 

localities produced mixed re sults, all with 

low magnetic intensity. Po tas si um-argon 

age is 3.3±0.2 Ma (whole rock; Armstrong 

and others, 1975)

Tbdr Basalt of Dry River (Pliocene)—Fine-grained 

oli v ine basalt, commonly open-textured. 

Forms rimrock of Crooked River east 

of O’Neil and broad plain farther south 

near ly to Alfalfa; also found west as far 

as Redmond. Erupt ed from vents southeast 

and north of Powell Buttes (outside of 

map area). Re versed-polarity magnetization; probably about same age as basalt 

of Redmond (Tbr). Map-unit symbol 

shown queried south of Al fal fa for basalt 

of uncertain polarity (field mea sure ments 

inconclusive)

Tbr Basalt of Redmond (Pliocene)—Slightly por-

 phy rit ic, open-textured basalt that caps 

pla teau be tween Redmond and Terrebonne. 

Once included as Deschutes-equivalent 

stratum (Robinson and Stensland, 1979, 

their “Redmond flow” of the Madras For-

 ma tion) but now considered a separate, 

overlying unit (Smith, 1986). Isotopic age 

is 3.56±0.30 Ma (40Ar/39Ar, whole rock) 

from sample near Terrebonne (Smith, 

1986). Normal-po lar i ty magnetization, 

but otherwise similar to basalt of Dry 

River (Tbdr), which occupies a sim i lar 

geomorphic setting. Outcrops exposing 

the stratigraphic relation between these 

two units have not been found

Deschutes Formation (Pliocene and Mi ocene)—Sequence of interbedded vol ca nic 

and sub aeri al sedimentary strata. Di vid ed 

into:

Tds Sedimentary rocks and deposits—Chiefly 

volcaniclastic sandstone, conglomerate, and 

breccia, which formed by fluvial pro cess es. 

Commonly includes minor primary vol ca nic 
26

deposits of fine vitric tuff, lapilli tuff, 

and volcanic breccia. Much of this tuff 

formed as fallout and pyroclastic flows; 

py ro clas tic-flow deposits (ash-flow tuff) 

mapped separately where well exposed 

(unit Tdt and its divisions)

Tdb Basalt—Medium- to dark-gray, fine- to 

me di um-grained, open-textured to compact olivine basalt, chiefly forming lava 

flows. Erupt ed from small to moderatesized cin der cones (unit Tdc). Includes 

basaltic andes ite and andesite not mapped 

sep a rate ly. Ba sal tic andesite may form as 

much as 30 percent of unit, but we have 

too few chem i cal analyses and flow-by-flow 

mapping to define the compositional proportions more thor ough ly. Basaltic andesite 

may pre dom i nate at Green Ridge, along 

north-central edge of map. Andesite is probably less than 5 per cent throughout area. 

Queried for lava of uncertain stratigraphic 

assignment south of Sisters. Colored line 

shows extent of de pos its where exposed 

within Deschutes For ma tion sed i men ta ry 

strata (unit Tds) along can yon walls of 

Deschutes River and its trib u tar ies. Locally 

divided into:

Tdbp Porphyritic basalt—Contains abundant 

pla gio clase phenocrysts. Comprises several oc cur renc es derived from multiple 

vents

Tdbl Basalt of Long Butte—Moderately to 

high ly porphyritic lava flows that form 

shield volcano of Long Butte. Contains 

blocky phenocrysts of plagioclase (3–4 

mm long) and less abundant olivine (1–2 

mm across)

Tdbt Basalt of Tetherow Butte (Tetherow Butte 

member of Smith, 1986)—Consists of two 

lava flows and scattered cinder deposits. 

Un usu al ly low Al2

O3 (13–14 percent) com-

 pared to other basalt in Cascade Range 

or Deschutes ba sin. Contains scattered 

glomerocrysts of plagioclase and ironrich augite (Smith, 1986). Lava flows 

have been named informally by Smith 

(1986): Agency Plains flow and overlying Crooked River flow (coincides with 

Crooked River flow as named by Robinson 

and Stensland, 1979). Contact between 

the two may be seen in cliffy exposures 

along Crooked River. Crooked River flow 

ter mi nates at about the north edge of map 

area (approximately lat 44°30' N.), but 

Agency Plains flow extends north of map 

area for nearly 40 km to South Junction 

(Smith, 1986). Normal-polarity magnetization. Isotopic age is 5.31±0.05 Ma from 

sample collected 5 km north of map area 

(40Ar/39Ar, whole rock; Smith, 1986), but 

unit likely is young er than 5.04 Ma if 

magnetization is to match the currently 

accepted paleomagnetic time scale (see 

fig. 4). Includes many small deposits of 

cinders and scoria that prob a bly were 

rafted by lava flowing away from vent 

area; these deposits shown stippled

Tdbld Basalt of Lower Desert (Lower Desert 

basalt member of Smith, 1986)—Opentex tured olivine basalt. Comprises Fly Lake 

and Canadian Bench flows as de fined by 

Smith (1986). Both possess normal-po lar i ty 

mag ne ti za tion, and the slightly older Canadian Bench flow, dated north of the 

map area, has isotopic ages of 5.0±0.5 

Ma (K-Ar, whole rock; Armstrong and 

others, 1975) and 5.43±0.05 Ma (40Ar/ 39Ar, whole rock; Smith, 1986). (Unit 

likely is younger than 5.04 Ma if the 

magnetization is to match the cur rent ly 

accepted paleomagnetic time scale; see 

fig. 4.) Contact between flows shown on 

map by dashed-line internal contact (from 

Smith, 1986), and age relation indicated 

by Y (younger) and O (older)

Tdbc Basalt of Cline Falls—Moderately por phy -

rit ic lava flow, with 15 percent phe noc rysts 

of plagioclase as large as 6 mm. Contains 

50.5 percent SiO2. Reversed-polarity mag-

 ne ti za tion

Tdbo Basalt of Opal Springs—Open-textured, 

vesicular lava flows. Normal-polarity mag-

 ne ti za tion. Isotopic age is 5.77±0.07 Ma 

(Smith, 1986)

Tdba Basaltic andesite—Aphyric to slightly por phy -

rit ic lava. Shown where suffi cient chem i cal 

analyses to specify composition. Lo cal ly 

divided into:

Tdbal Basaltic andesite of Laidlaw Butte—Sparse-

 ly porphyritic, containing mil li me ter-size 

phenocrysts of pla gio clase, augite, and rare 

olivine (McDannel, 1989). Normal-po lar i ty 

magnetization

Tdbas Basaltic andesite of Steamboat Rock—

Lithologically diverse unit of lava flows, 

lapilli tuff, and breccia; many of the 

py ro clas tic beds are hydromagmatic in 

origin (Smith, 1986). Dikes that fed unit 

are shown sep a rate ly as intrusions of unit 

Tbi. Near ly aphyric, fine-grained basaltic 
27

andesite with scant plagioclase phenocrysts 

less than or equal to 1 mm in length. 

Reversed-po lar i ty magnetization. Forms 

prominent shelves and isolated mesas 

owing to relatively re sis tant lava that has 

withstood regional den u da tion of surrounding Deschutes plain. Exhumed conduits 

filled with lapilli tuff form pin na cles on 

east canyon wall of Deschutes River 1.4 

km north of Steelhead Falls. Isotopic age 

is 5.06±0.03 (40Ar/39Ar, whole rock) from 

mesa south of Steamboat Rock (Smith, 

1986). Stipple indicates area of tuff and 

tuff brec cia that form two large cones

Tdbaof Basaltic andesite of Odin Falls—Augitebearing porphyritic lava fl ows. Contains about 

56 percent SiO2

. May have erupted from 

low lava mound in southwest corner of sec. 

35, T. 14 S., R. 12 E.; vent symbol there 

shown queried because near-vent cinders and 

sco ria are lacking and evidence is limited 

to to po graph ic expression. Reversed-polarity 

magnetization. Older than basaltic andes ite 

of Steamboat Rock (older than 5.06±0.03 

Ma), although the two units are nowhere 

in direct stratigraphic succession. Instead, 

thick fallout tephra beds beneath the Steamboat Rock unit can be traced southward in 

west canyon wall of Deschutes River to a 

po si tion above the Odin Falls unit

Tda Andesite—Lava flows. Shown near Bull 

Spring in south-central part of map area, 

near Thorn Spring at north-central edge, 

and along southeast wall of Deep Canyon 

in east-central part. Bull Spring exposure 

includes some interbedded dacite and 

capping basaltic andesite, and its magnetostratigraphy var ies from reversed polarity 

at base to nor mal polarity at top, with an 

intervening se quence of reversed-polarity 

olivine ba salt mapped separately (in unit 

Tdb). Capping basaltic andesite flow in 

Bull Spring area has K-Ar age of 4.7±0.1 

Ma (whole rock; Hill, 1992a). Elsewhere 

in map area, sparse andesite in Deschutes 

Formation is in clud ed in unit Tdb

Tdd Dacite—Lava of eroded dome north of 

Sis ters

Tdt Ash-flow tuff—Partially to moderately 

weld ed pyroclastic-flow deposits. Most 

contain scoria and pumiceous lapilli and 

bombs ranging in composition from andesite 

to rhyolite. Colored line shows extent of 

de pos its where exposed within Deschutes 

For ma tion sed i men ta ry strata (unit Tds) 

along can yon walls of Deschutes River 

and its trib u tar ies. Locally divided into:

Tdtd Tuff of Deep Canyon—Brownish- to ol ivegray ash-flow tuff. Moderately welded in 

basal part of some exposures, but gen er al ly 

only weakly sintered or nonwelded. Char-

 ac ter ized by abundant large black dacitic 

pumiceous lapilli (analyses in Smith, 1986; 

Hill, 1992a; R.M. Conrey, unpub. data, 

1992). Reversed-polarity magnetization. 

Equiv a lent to Deep Canyon ignimbrite 

member of Smith (1986). Colored line 

shows extent of de pos its where exposed 

within Deschutes For ma tion sed i men ta ry 

strata (unit Tds) along can yon walls of 

Deschutes River and its trib u tar ies

Tdtf Tuff of Fremont Canyon—Moderately to 

densely welded dark-brown to grayish-red 

ash-flow tuff. Commonly platy weath er ing. 

Contains phenocrysts of plagioclase and 

hypersthene. Reversed-polarity mag ne -

ti za tion. Colored line shows extent of 

deposits where ex posed within Deschutes 

Formation sed i men ta ry strata (unit Tds) 

along canyon walls of Deschutes River 

and its trib u tar ies

Tdtp Tuff of The Peninsula—Sintered to very 

slight ly welded ash-flow tuff. Contains 

pu mi ceous lapilli ranging from andesite to 

dacite to rhyolite in composition and from 

black to gray to white in color (Smith, 

1986). Re versed-polarity magnetization. Equiv a lent to Peninsula ignimbrite 

mem ber of Smith (1986). Colored line 

shows ex tent of de pos its where exposed 

within Deschutes For ma tion sed i men ta ry 

strata (unit Tds) along can yon walls of 

Deschutes River and its trib u tar ies

Tdtm Tuff of McKenzie Canyon—Slightly por-

 phy rit ic ash-flow tuff comprising three or 

four pyroclastic-flow deposits. Forms sin gle 

cooling unit. Lower parts generally lightcolored and poorly welded or nonwelded. 

Most distinctive and characteristic, how ev er, 

is compositionally distinct upper part, an 

extensive reddish-orange or brownish-or ange 

welded tuff that forms low outcrops and 

bold cliffs with crude columnar jointing. 

Upper part varies from white basal nonwelded zone about 1 m thick into moderately 

welded, high ly oxidized reddish-brown tuff. 

Weld ing and oxidation increase upward, 

sug gest ing that an upper nonwelded part 

may have been stripped by erosion during 

deposition of the Deschutes Formation. 
28

Pumice lapilli in lower parts are generally white, whereas in up per part they 

include white, black, and banded (black 

and white) lapilli. Banded dense pumice 

is diagnostic of the unit. Phe noc rysts are 

small (approximately 1 mm across) and 

consist of 2–5 percent feldspar and less 

than or about 1 percent ferromagnesian 

minerals (hypersthene, augite); olivine (rare) 

occurs in black pumiceous lapilli from upper part of unit (Stensland, 1970; Cannon, 

1985). Reversed-polarity magnetization. 

Equiv a lent to McKenzie Canyon ignimbrite 

member of Smith (1986). Colored line 

shows extent of de pos its where exposed 

within Deschutes For ma tion sedimentary 

strata (unit Tds) along can yon walls of 

Deschutes River and its trib u tar ies

Tdtl Tuff of Lower Bridge—Slightly por-

 phy rit ic ash-flow tuff comprising two 

py ro clas tic-flow deposits. Forms single 

cooling unit. Grayish-brown to grayishpurple weath er ing; grayish-pink where 

fresh. Nonwelded to weakly sintered; 

forms round ed slopes. Contains 10–15 

percent white pu mi ceous lapilli and 1–5 

percent volcanic lithic clasts as large as 2 

cm. Phenocrysts are small (approximately 

1 mm across) and consist of 10–15 percent 

plagioclase and 5–7 percent ferromagnesian minerals, chiefly augite with minor 

hypersthene and pargasitic am phib ole 

(Cannon, 1985). Biotite occurs in trace 

amounts (Stensland, 1970; Smith, 1986). 

Unit includes 0.5–1.7 m of muddy, poorly 

sort ed pumiceous lapilli and accretionary 

lapilli, probably the initial fallout deposits 

as so ci at ed with Lower Bridge eruptions. 

Reversed-polarity magnetization. Equivalent 

to Low er Bridge ignimbrite member of 

Smith (1986). Colored line shows extent of 

deposits where ex posed within Deschutes 

Formation sed i men ta ry strata (unit Tds) 

along canyon walls of Deschutes River 

and its tributaries

Tddf Debris-flow deposits—Poorly sorted volcaniclastic deposits. Includes small area 5 

km north of Cline Buttes, where de pos its 

are rich with clasts as much as 2 m across 

of tuff of Fremont Canyon (Tdtf). The 

abundant float of tuff led Stensland (1970) 

to map the area as bedrock exposure of 

densely welded tuff, but other rock types 

are found, too, and the tuff blocks are set 

in odd positions com pared to the orig i -

nal ly horizontal orientation expected in 

a pri ma ry deposit. Most wide spread deposits, how ev er, extend from Forked Horn 

Butte to areas north and northwest that 

comprise unsorted an gu lar debris (socalled Tetherow mudflow deposits of 

Stensland, 1970). Clasts char ac ter is tic 

of Deschutes Formation units are common in these de pos its. Also common 

are prominent blocks of brownish-gray 

rhyodacite. Several ex po sures have fea-

 tures characteristic of de bris-avalanche 

de pos its. As mapped, in cludes Forked 

Horn Butte, a poorly ex posed low hill. 

Roadcuts and trenchwork in new hous ing 

developments there expose only debris-flow 

deposits, many with dark-gray, por phy rit ic 

rhyodacite clasts as large as 2 m across. 

The butte was mapped pre vi ous ly as a 

Mi ocene, Oligocene, or perhaps Eocene 

silicic dome (Stearns, 1931; Williams, 

1957; Stensland, 1970; Robinson and 

Stensland, 1979; Smith, 1986)

Tdrcb Rhyolite of Cline Buttes (Miocene)—Sparse ly 

porphyritic, sugary (white) to stony (lightgray) rhyolite. Contains 1 to 2 percent 

of oligoclase phenocrysts (McDannel, 

1989). Most of unit is devitrified, but 

flow-band ed obsidian is exposed low on 

northeast flank of buttes in rock quarry. 

Normal-polarity magnetization. Isotopic 

age is 6.14±0.06 Ma (40Ar/39Ar; M.A. 

Lanphere, written commun., 1999)

Tdrsf Rhyodacite southwest of Steelhead Falls 

(Miocene)—Brownish-gray to black, flowbanded, moderately porphyritic lava. Forms 

sprawling mass thicker than 180 m (base not 

exposed). Normal-polarity magnetization. 

Isotopic age is 6.74±0.20 Ma (40Ar/39Ar; 

M.A. Lanphere, written commun., 1998). 

Rel a tive ly permeable and unaltered (Ferns 

and oth ers, 1996a), which is consistent 

with its late Miocene isotopic age because 

ear ly Mi ocene and older units in area tend 

to be altered and have low permeability

[Next two units are not part of the Deschutes For ma tion as 

presently defi ned. They are either of uncertain age relative to 

Deschutes Formation or relatively isolated from the Deschutes 

basin]

Tbab Basalt of Awbrey Butte (Pliocene or Mi ocene)—Lava flows of moderately to very 

por phy rit ic olivine basalt. Awbrey Butte is 

a small shield volcano of normal-polarity 
29

mag ne ti za tion. It is deeply weathered, overlain by re versed-polarity basaltic andesite 

as signed to the Deschutes Formation, and 

likely older than 4 Ma

Tbw Basalt of Willow Creek (Miocene)—Olivinebear ing lava flows. Contains about 51 

percent SiO2 (Thormahlen, 1984). Exposed 

in north east corner of map area but more 

ex ten sive to north and northeast. Erupted 

from vent 4 km northeast of map area 

and flowed west and then northerly along 

Willow Creek drain age mostly north of 

map area (Peck, 1964; Swanson, 1969; 

Robinson, 1975). Isotopic age from sample 

collected north of map area is 6.30±0.09 

Ma (40Ar/39Ar; Smith, 1986)

Unconformity to disconformity

Tp Prineville Basalt (Miocene)—Fine-grained 

aph a nit ic lava flows. Includes rocks 

with both nor mal- and reversed-polarity 

mag ne ti za tion (Hooper and others, 1993); 

the unit is thought to have erupted during a short time period about 15.8 Ma 

when Earth’s magnetic field was changing polarity from reversed to normal. Has 

been considered a chemical type within 

the Grande Ronde Basalt of the Columbia 

River Basalt Group (Swanson and others, 1979) or, more recently, a distinct 

member of the group (Tolan and others, 

1989). Other workers have considered it a 

separate, interfingering stratigraphic unit 

(Hooper and others, 1993)

John Day Formation (Miocene to Eocene)—

Chiefly rhyolitic ash-flow tuff, lava flows 

ranging from basalt to rhyolite, tuffaceous 

sed i men ta ry rocks, and vent deposits. Most 

renowned exposure in map area forms Smith 

Rock, a sequence of rhyolitic welded tuff 

and volcaniclastic beds on west flank of 

Gray Butte in northeast corner of map 

area (unit Tjts). Lowest part of unit in 

map area (Tjl) is undated but may be late 

Eocene in age. John Day Formation is as 

old as late Eocene to east of map area 

in Blue Moun tains (Bestland and others, 

1993). Divided into:

Tjt Tuff and tuffaceous sedimentary rocks 

(Mi ocene and Oligocene)—White to 

very pale orange silicified tuff and lapilli tuff. In cludes ash-flow tuff, fallout 

tuff, and tuffaceous sed i men ta ry deposits. 

Age is early Miocene and late Oligocene 

on basis of stratigraphic position beneath 

Prineville Basalt (Tp) and above rhyolite 

of Gray Butte (in unit Tjr)

Tjh Ignimbrite of member H (Oligocene)—Finegrained, welded and nonwelded rhyolitic 

tuff. Light green, white, or orange where 

fresh, and typically weathers orange or 

reddish brown. Mostly devitrified, with 

abundant lithophysae; dark-greenish-gray, 

perlitic vitrophyre exposed locally at base. 

Gen er al ly contains less than 5 percent 

pumice lapilli up to 1 cm across, less 

than 1 percent rock fragments, and less 

than 1 percent crystals of quartz, sanidine, and plagioclase. This welded tuff 

forms the base of member H as defined 

by Peck (1964) or Robinson (1975), but 

much of member H in map area has been 

stripped away by erosion. Isotopic age of 

27.62±0.63 Ma (40Ar/39Ar, sanidine) was 

obtained from sample collected at west 

end of Haystack Reservoir (Smith and 

others, 1998)

Tjts Tuff of Smith Rock (Oligocene)—Thickbed ded to very thickly bedded pumiceous 

lapil li tuff (pyroclastic-flow deposits), 

me di um-bed ded pumice-lithic lapillistone 

(prob a bly near-vent fallout deposits), 

and sandstone. Pu mi ceous lapilli generally aphyric. Bed ding be comes more 

pronounced and the rocks finer grained 

upsection in Smith Rock area as volcaniclastic sandstone increases in abundance

Tjr Rhyolite (Oligocene)—Chiefly lava flows 

and domes, most prominently Juniper 

Butte, Powell Buttes, and Gray Butte. 

Age of 28.3±1.0 (K-Ar, whole rock) was 

obtained from Powell Buttes dome (Evans 

and Brown, 1981). Age of 28.82±0.23 

Ma (40Ar/39Ar, anorthoclase) was obtained 

from rhyolite at Gray Butte (Smith and 

others, 1998)

Tjd Dacite (Oligocene)—Lava flows and domes. 

Part of Powell Buttes dome field

Tjtw Welded tuff (Oligocene)—Found near rhy o lite 

dome at Juniper Butte (Tjr) and thought 

to be derived from that vent on basis of 

sim i lar phenocryst mineralogy (Robinson 

and Stensland, 1979)

Tjth Tuff of Haystack Reservoir (Oligocene)—

Light-green, gray, and light-purple tuff and 

lapil li tuff. Includes dark-reddish-brown 

welded tuff near top of unit at Haystack 

Reservoir. Slightly porphyritic; commonly 

contains 1–5 percent total phenocrysts of 
30

plagioclase, quartz, and sanidine. Nonwelded 

part of unit consists of planebedded and 

crossbedded, accretionary lapilli-bearing, 

hydromagmatic pyroclastic surge deposits 

and locally interbedded, massive lapilli tuff 

of prob a ble pyroclastic-fl ow origin. Thick ness 

in creas es northward from 75 m to at least 

100 m near Haystack Reservoir. Probably 

erupted from a vent now plugged by rhy o lite 

at Juniper Butte (in unit Tjr), on basis of 

thickness variations, restriction of weld ed tuff 

to Haystack Reservoir area, fl ow di rec tions 

determined from surge crossbedding, and 

mineral content similar to the rhy o lite. Best 

exposures are along north and west sides of 

Haystack Reservoir. Sanidine 40Ar/39Ar ages 

are 29.53±0.09 Ma from hydromagmatic tuff 

and 29.57±0.17 Ma from capping welded 

tuff; both samples from west end of Hay stack 

Reservoir (Smith and others, 1998). These 

ages suggest correlation with mem ber G of 

John Day Formation. The cap ping welded 

tuff was mapped as welded tuff possibly 

derived from Juniper Butte (Robinson and 

Stensland, 1979, their unit Tjw)

Tjs Tuffaceous sedimentary rocks and tuff (Oligocene)—Tuffaceous sandstone, lam i nat ed 

tuff, and locally conglomerate. Coars ergrained strata composed pri ma ri ly of ba-

 sal tic and rhyolitic detritus. Locally includes 

lapillistone of fallout origin and minor ashfl ow tuff. Correlative with mem bers E(?), 

F, and G of John Day For ma tion as de fi ned 

elsewhere in region (see explanatory text). 

Outcrops near Gray Butte include a lapil lifall deposit, the tuff of Rodman Spring of 

Smith and oth ers (1998). These Gray Buttearea out crops also in clude overlying strata 

thought to correlate with lower part of 

member G. The tuff of Rod man Spring 

has been correlated to a de pos it near the 

base of member F, 25 km north east of 

map area; it has a 40Ar/39Ar sanidine age 

of 32.49±0.30 Ma from a sample col lect ed 

near Rodman Spring (Smith and others, 

1998). Outcrops in area from Hay stack 

Reservoir to Hay stack Butte contain the 

member-F tuff of Rod man Spring but 

also include underlying brown tuffaceous 

sand stone and conglomerate that probably 

cor re late, at least in part, to member E

Tjb Basalt and tuffaceous sedimentary rocks 

(Oligocene)—Lava flows are chiefly darkgray, medium-grained aphyric alkali ba salt 

with groundmass characterized by pla gio -

clase, olivine, and interstitial to subophitic 

ti tani f er ous clinopyroxene. Interbedded with 

tan and green tuffaceous sandstone and 

tan shale; shale is host to Trail Crossing 

flora (Ashwill, 1990). Sandstone contains 

abun dant, once-glassy basaltic ash and 

scoria now altered to iron-rich smectite 

and zeolite. Unit is 75 percent lava, 25 

percent sedimentary rocks. Lava petrographically and com po si tion al ly similar to 

alkali basalt found in members E, F, and 

G of John Day For ma tion, but flows near 

Gray Butte underlie a member-F fall out 

deposit and are there fore correlated with 

member E

Tjba Basaltic andesite and tuffaceous sed i men ta ry 

rocks (Oligocene)—In Gray Butte area 

com pris es chiefly black, fine-grained, 

aphyric lava flows containing microlites 

of pla gio clase and clinopyroxene in a 

variably al tered glassy mesostasis. Petrographically and compositionally similar 

to trachyandesite that defines member B, 

20 km northeast of map area near Ashwood (Peck, 1964; Robinson, 1969, 1975). 

Interbedded sedimentary rocks are brown 

sandstone, siltstone, and shale containing 

fragments of rhyolite, basalt, and basaltic 

andesite. Shale is locally car bon aceous and 

contains abundant leaf im pres sions, including the Canal and Nichols Spring flora 

(Ashwill, 1983) of Oligocene age. Petrified wood is rarely present. Unit is about 

50 percent lava, 50 percent sed i men ta ry 

strata. Beds of accretionary lapil li-bearing, 

hydromagmatic basaltic andesite tuff are 

present locally. Also in cludes a light-green, 

slightly welded, highly al tered rhyolitic 

welded tuff exposed near middle of unit. 

This tuff, which is 5–10 m thick, contains 1–3 percent phenocrysts of quartz, 

plagioclase, and altered anorthoclase. In 

Powell Buttes area, consists of small 

ex po sure of por phy rit ic, clinopyroxenebearing ba sal tic andesite lava

Tjl Lower John Day Formation (Oligocene 

and Eocene?)—Diverse sequence of 

strata com pris ing brown to green, wellcemented sandstone, siltstone, and shale; 

yellow-brown altered rhyolitic welded tuff; 

white ash-fall tuff; and porphyritic basaltic 

andesite lava flows. Siltstone and shale 

host the Kings Gap and Sumner Spring 

flora (Ashwill, 1983) of Oligocene age; 

sandstone in this part of the sequence 
31

consists almost entirely of rhyolitic and 

basaltic andesite detritus. Two welded tuffs 

form about 30–50 percent of lowest 100 m 

of stratigraphic section. Lower of the two 

is nonwelded to densely welded rhyolitic 

tuff, roughly 20 m thick, con tain ing 5–10 

percent phenocrysts of quartz and highly 

altered anorthoclase and pla gio clase. Also 

contains small metasedimentary rock fragments. Upper welded tuff, at least 15 m 

thick, is nonwelded to slightly welded, 

devitrified, and silicified; it locally con tains 

layers of lithophysal cavities. Phenocrysts 

of quartz and highly altered plagioclase and 

alkali feldspar form 1-2 percent of rock. 

The welded tuffs, which are tentatively 

cor re lat ed with welded tuff of member A 

else where in the region, are overlain by 

500 m of strata, of which medium-gray 

por phy rit ic ba sal tic andesite lava forms 

about 20 per cent. Lava contains approximately 15 per cent phe noc rysts of 

plagioclase, olivine, and scant clinopyroxene. Although once mapped as part of 

Eocene Clarno Formation (Robinson and 

Stensland, 1979), unit is now correlated 

with lower John Day Formation for these 

rea sons: (1) lack of andesite and dacite 

lava flows that characterize the near est 

exposures of Clarno Formation 20 km east 

(Thormahlen, 1984); (2) lack of andesitic 

detritus and abundance of rhyolitic and 

basaltic andes ite detritus in sandstone; 

(3) lack of a lat er it ic paleosol, which 

typifies the contact be tween John Day 

and Clarno Formations; and (4) tentative 

correlation of the rhyolitic weld ed tuffs 

to those found in latest Eocene mem ber 

A of the John Day Formation (Robinson 

and others, 1990)

VENT DEPOSITS AND INTRUSIONS

Qyc Younger cinder deposits (Holocene and 

Pleis tocene?)—Basaltic and basaltic 

andesite sco ria and minor lava flows 

forming rel a tive ly pristine cinder cones. 

Marks vents for ba sal tic andesite of Le 

Conte Crater (Qbal), basalt of Cayuse 

Crater (Qbcy), basalt of Egan cone* 

(Qbec), and numerous lava flows near 

McKenzie and Santiam Passes. Ha chured 

lines indicate crater rims. Age is Holocene 

except for the Sims Butte, Cayuse Crater, 

and Egan cone* deposits, which may be 

latest Pleistocene. Age of specific vents 

is dis cussed with descriptions for erupted 

lava (for example, see basalt of Belknap 

Crater, unit Qybk). Two vents, however, 

erupted no lava flows: Blue Lake vent 

(west of Suttle Lake) and an informally 

named Spatter cone chain of vents*, scoria 

and agglutinate (north east of Mount Washington). Both are younger than 3,440±250 

14C yr B.P. because their tephra overlies 

fine ash associated with eruptions from 

Sand Mountain chain (Tay lor, 1965, 1981; 

Chatters, 1968). An age of 1,330±140 14C 

yr B.P. was obtained from charcoal beneath tephra of the north ern most of the 

three fissure deposits of the Spatter cone 

chain of vents* (table 1). Southern and 

middle fissure deposits of Spatter cone 

chain of vents* are nearly connected by 

linear furrows mantled by nonmagmatic 

lithic tephra (Fractured ground in Map 

Explanation)

Qc Cinder deposits (Pleistocene)—Basaltic, ba-

 sal tic andesite, and minor andesite scoria 

and lava forming slightly to moderately 

weathered deposits in cinder cones 

throughout map area. Extent of erosion 

varies depending on el e va tion, age, and 

position of deposits rel a tive to Cascade 

Range crest, which forms a precipitation 

barrier or rain shadow. Most cones lack 

summit craters; hachured lines indicate 

crater rims on relatively young cones. 

Deposits mark vents for numerous units 

of basalt and basaltic andesite. Younger 

than 0.78 Ma on basis of normal-polarity 

mag ne ti za tion in related lava flows

Qmv Mafic vent deposits (Pleistocene)—Mod er ate ly 

bedded, near-vent ejecta forming cores of 

large, steep-sided shield volcanoes such 

as Mount Washington, North Sister, or 

Three Fingered Jack; commonly laced with 

dikes. Parts of some vents hyaloclastic, 

sug gest ing that rising magma interacted 

with snow and ice, shallow ground water, 

or surface water. Ranges in composition 

from basalt to basaltic andesite

Qsv Silicic vent deposits (Pleistocene)—Three oc-

 cur renc es of near-vent tephra-fall and surge 

deposits: (a) palagonitic tephra of rhyodacitic 

composition in upper North Fork of Tumalo 

Creek; (b) near-vent pumice cones(?) ex-

 posed in escarpment of Triangle Hill; (c) 

pu mi ceous rhyodacitic tephra beneath 

rhyodacite near Three Creek Lake
32

Qi Intrusions (Pleistocene)—Plugs and thick dikes 

chiefly of basaltic andesite but including 

basalt and andesite. Fills conduits that fed 

large mafic vents such as Three Fingered 

Jack or Mount Washington. Not shown are 

many thin dikes that lace the pyroclastic 

cones of several Cascade Range shield 

and com pos ite volcanoes such as Three 

Fingered Jack, Mount Washington, Broken 

Top, and the Three Sisters. Normal-polarity 

mag ne ti za tion; presumably entirely middle 

or late Pleistocene in age

Qoc Older cinder deposits (Pleistocene)—Similar 

to cinder deposits (unit Qc) but older than 

0.78 Ma on basis of reversed-polarity mag-

 ne ti za tion. Only one occurrence, 1.5 km 

north west of Suttle Lake. Other probable 

de pos its large ly stripped by glaciation, 

leav ing only rem nant conduit-filling plug 

(unit Qoi, old er in tru sions)

Qoi Older intrusions (Pleistocene)—Similar to in-

 tru sions (unit Qi) but older than 0.78 Ma 

on basis of reversed-polarity mag ne ti za tion. 

Con sists of two masses in southwest cor ner 

of map area and one mass in northwest 

corner near Round Lake

Tc Cinder deposits (Pliocene)—Cinder deposits 

mark ing vent for andesite of McKinney 

Butte (unit Tam), north of Sisters

Tbi Basaltic intrusions (Pliocene and Mi ocene?)—

Dikes and sills emplaced into the Deschutes 

For ma tion

Tdc Cinder deposits of Deschutes Formation (Pliocene and Miocene)—Basalt and ba sal tic 

andesite scoria and lava, forming cones 

and ir reg u lar accumulation of cinders. 

Marks vents for lava flows in Deschutes 

For ma tion. Major cinder cones for one 

Deschutes Formation lava sequence, the 

basalt of Tetherow Butte, are shown with 

labels Tdc and (Tdbt)

Tri Rhyolitic intrusions (Miocene or Oli gocene)—

Dikes and plugs of very fi ne grained rhy o lite 

emplaced into the John Day For ma tion near 

Smith Rock State Park. Dikes west of Gray 

Butte are porphyritic rhyolite

Tjbi Basalt and basaltic andesite intrusions of 

John Day age (Oligocene)—Dikes and 

sills north of Gray Butte. Petrographically and com po si tion al ly similar to, and 

probably of same age as, nearby John Day 

lava flows (in units Tjb and Tjba). An 

isotopic age of 30.8±0.5 Ma (K-Ar, wholerock) was reported from this unit 2 km 

northeast of Gray Butte (Fiebelkorn and 

others, 1983)

Tjmv Mafic vent deposits of John Day age (Oli-

 gocene)—One occurrence, which forms 

poorly ex posed, altered scoria and ag glu -

ti nate northwest of Gray Butte. May mark 

upper part of con duit that erupted scoria 

and spatter found in adjacent outcrops of 

ba sal tic andesite (unit Tjba)

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38

APPENDIX 1. DESCRIPTION OF OCCURRENCES FOR ISOLATED PUMICE DEPOSITS.

[Numbered by locality and keyed to simplified location map (fig. 8). Quadrangles are U.S. Geological Survey 7.5' topographiWhole-pumice chemical analyses from tephra and some comparative whole-rock analyses from rhyolite domes are listed sepa

Locality

No.

Quadrangle Chemical analysis

(Sample No., table 3)

Description

North Sister Unanalyzed On west ridge of Little Brother; single, thick silicic tephra fall deposits. Age and correla

have resulted from precursory eruptions leading to emplacement of rhyolite of Obsidian 

Trout Creek

Butte

830920-5 Tephra deposit mantling cinder cone at Pole Creek cinder pit, 3.3 km south of Trout Cre

Three Creek

Butte

RC95-178 Tephra-fall deposit beneath Shevlin Park Tuff, roadcut. The roadcut has sloughed and r

expose the deposit. Thickness greater than 1.5 m; base not reached by shoveling. ProbColumbia Canal on basis of stratigraphic position and chemical similarity to lapilli from section (Locality No. 10).

Three Creek

Butte

830920-6 Scattered pumiceous lapilli along Forest Road 16 where it surmounts a thick basaltic an

3823' benchmark, 5.5 km north-northeast of Melvin Butte.

Three Creek

Butte

3S086 Pumice south-southwest of Three Creek Butte; tephra-fall deposits exposed along loggi

thick; base not reached by auguring. Multiple emplacement units indicated by marked bgrain size, and varying proportion of lithic clasts. Pumiceous lapilli to 2 cm. Deposit pglaciation and lies at edge of lateral moraine. The lapilli chemically resemble rhyolite aunit Qr), 18 km to the west-northwest (B.E. Hill, written commun.); those analyses are and 3S139 (table 3).

Three Creek

Butte

Unanalyzed Small gravel pit; tephra-fall deposit overlies gravel. Gravel has carbonate stains and we

as 0.5 mm and therefore predates the Suttle Lake advance of Cabot Creek glaciation (EvFraser glaciation). No thickness noted.

1

234567 Tumalo Dam Unanalyzed Tephra-fall deposit lying beneath Shevlin Park Tuff and above Tumalo Tuff. Stratigrap

may be pumice of Columbia Canal (Locality No. 10, below).

crossing (table 3) are chemically similar to Locality No. 8, pumice that mantles cinder c

12 Tumalo Falls 830803-6 (from white

lapillli)

Pumice of Pilot Butte, collected 19 km west-southwest of Pilot Butte at East Tumalo qudeposit on flank of cinder cone (quarry). Thickness at least 2 m; base not exposed. Conpumiceous lapilli to 7 cm, with about 5 percent of dark lapilli. White lapilli are slightly phenocrysts of plagioclase and clinopyroxene, 1-2 mm, ≤1 percent. Dark lapilli (andesaphyric. Chemically similar to Locality No. 13.

13 Bend 3S007 Pumice of Pilot Butte, from tephra-fall deposit mantling southwest flank of Pilot Butte 

Chemically similar to Locality No. 12.

8 Tumalo Falls 3S018 Pumice of Bottle Creek, from tephra-fall deposit on flank of cinder cone north of Bearw

Thickness roughly 50 cm. Lapilli to 6 cm in upper part; ash increases in lower part, witoverall. Chemically similar to Locality No. 11, along Bottle Creek.

9 Shevlin Park 3S001 Tephra-fall deposit that mantles Deschutes-age strata near Bull Spring. Coarse lapilli ex

Thickness unknown. Chemically similar to Bend Pumice but hydrated (Hill, 1992a).

10 Shevlin Park 3S079 Pumice of Columbia Canal, at Columbia Canal waterfall exposure. Was named pumice 

by Hill and Taylor (1990), but title modified here to more closely match a named geogrUnderlies Shevlin Park Tuff; overlies Tumalo Tuff. Chemically similar to analyses from Glass shards have composition similar to Summer Lake ash bed NN (Sarna-Wojcicki anbed NN lies 4.5 m beneath ash bed JJ (Shevlin Park Tuff distal fallout) at the Summer L(fig. 4 in Davis, 1985).

11 Tumalo Falls 3S024 and 830920-1

(same location)

Pumice of Bottle Creek, along Bottle Creek. Two exposures, one lying 0.9 km northweextending 250 m downcreek from road in south bank. Thickness substantial; at least 20 road and at least 5 m thick east of road, but base not exposed in either locality. Samples 
39

122° 121°45' 121°30' 121°15' 

44°15' 

44°

Figure 8. Location of isolated pumice deposits younger than Bend Pumice or of uncertain correlation. Numbers correspond 

to localities described in appendix 1. Occurrences also shown on geologic map (labeled Qp). Chemical analyses listed in 

table 3 by Location No. Base from U.S. Geological Survey, Bend quadrangle, 1971; contour interval 1,000 ft.

0 5 10 KILOMETERS

1

12

13

3

5

6 7

8

9 10

11

2

4

20

20

97

126

126

Bend

Sisters

Tumalo

Mtn

South

Sister

Pilot Butte

Melvin Butte

Trout Creek

Butte

Mt

Washington

Black

Butte

Broken

 Top

North

Sister

Middle Sister
40

Table 1. Radiocarbon ages for Holocene volcanic rocks at Santiam and McKenzie Passes, Oregon[All ages are calculated using half-life of 5,568 years and reported as 14C year B.P. Description of material dated may vary somewha

descriptions, several of which were incorrect. Column labeled “Reported variants” is included for thoroughness, in the event that agelsewhere in the geologic literature create the impression of additional radiocarbon results not reported here]

Lab No. Field No. Age Description of material dated Map unit References GAK-1922 TFJ-269 1,330±140 Charred forest litter from base of Spatter cone chain of vents* Qyc (part) E.M. Taylor,

unpub. data

WSU-370+ TFJ-60A 1,400±100 Charcoal excavated from radial system of large roots in buried soil at

base of a tree mold in west Belknap lava flow

Qybk (part) E.M. Taylor, in

Champion, 1980 “1,49

“abou

WSU-292 TFJ-60 1,590±160 Second analysis on same sample of charcoal as WSU-370 (above) Qybk (part) Taylor, 1965;

Chatters, 1968

see a

W-5705 850909-2 1,600±100 Charcoal from beneath tephra plume of Collier Cone Qybc Scott, 1990WSU-450 TS-381 1,775±400 Charred roots in tree mold 1.3 km west of benchmark 5036, McKenzie

Pass Highway, collected in 1964. Probably too young; see Lab No.AA30523, below

Qybk (part) E.M. Taylor, in

Champion, 1980;Taylor, 1990

“1,80

WSU-371 TFJ-204 1,950±150 Charred wood beneath coarse cinders from cone at southwest base of

Potato Hill; collected along Jack Pine road south of U.S. Highway 20

Qybll Chatters, 1968;

Taylor, 1968

“2,00

W-6017 870725-1 1,980±160 Charred needles and twigs in lower 20 cm of tephra from Four In One

Cone, ~300 m east of vents. The dated andesitic tephra overlies a thinfine white tephra, which is thought to be from Rock Mesa or DevilsHill chain of vents (Qrrm)

Qya Scott, 1990

WSU-365 TS-374 2,550±165 Charcoal from core of 0.4-m-diameter upright stump enclosed in fused

cinders 200 m east of Four In One Cone. Age too old; preferred age is1,980±160 yr B.P. (see W-6017, above)

Qya Chatters, 1968;

Taylor, 1968

“2,60

W-6018 870804-1 2,590±150 Charcoal on contact where soil developed on till is overlain by 1.8 m

of scoriaceous ash from Nash Crater and north part of Sand Mountainchain of vents. That ash is overlain by 2 m of scoria from Little NashCrater. Interpreted as maximum age for volcanic activity near SantiamJunction

Maximumage forQybln

W.E. Scott, unpub.data

GAK-1921 TFJ-230 2,620±150 Charcoal from small limbs buried by coarse lapilli tephra about 650 m

west of the central point of Twin Crater's south crater

Qybt E.M. Taylor,

unpub. data

“2,60
41

AA30523 SB1998 2,635±50 Charred roots in tree mold 1.3 km west of benchmark 5036, McKenzie

Pass Highway; collected in 1998. Dates flows from vents close toSouth Belknap

Qybk (part) J.M. Licciardi,

unpub. data, 1999

not reported§ not

reported§

2,705±200

(outer wood);3,200±220(inner wood)

Inner and outer layers from 0.3-m-diameter submerged stump in ClearLake; younger age is maximum age for lava flows from south end ofSand Mountain chain of vents

Qybsm Benson, 1965;

Taylor, 1965

“appr(Tayl

(Tayl(Tayl

AA30522 CL1964 2,750±45 Wood from center of 0.3-m-diameter rooted snag submerged in Clear

Lake, a new analysis using sample material collected in 1964 by E.M.Taylor. Dates part of eruptive activity at Sand Mountain chain of vents

Qybsm J.M. Licciardi,

unpub. data, 1999

W-6021 870728-4 2,800±150 Charcoal fragments from upper 1 cm of soil in till and Mazama ash

that is buried by 3 m of scoria from Twin Craters; places maximum age for Twin Craters. (See age of 2620±150 from tree molds in GAK-1921, above)

Qybt W.E. Scott, unpub.

data

WSU-364 TFJ-207 2,883±175 Charred roots in tree mold along margin of Little Belknap lava, near

Skyline trail. Age appears too old, as Little Belknap Crater largelypostdates Belknap Crater

Qyblk Chatters, 1968;

Taylor, 1968,1990

“2,90

WSU-449 TFJ-213 2,990±300 Charred roots under lava, east shore Clear Lake. Dates part of eruptive

activity at Sand Mountain chain of vents

Qybsm E.M. Taylor, in

Champion, 1980;Taylor, 1990

“3,00

WSU-291 S-16 3,440±250 Long, delicate charred limb at interface between tephra of Blue Lake

(above) and ash of Sand Mountain (beneath), from large cut in loggingroad on south side of Suttle Lake. Interpreted here as tree killed by ashof Sand Mountain and later blanketed by tephra of Blue Lake

Qyt Taylor, 1965,

1990; Chatters,1968

Blue of 3,5(Tayl

WSU-372 TFJ-205 3,850±215 Charred root bark mixed with soil and rootlets near Old Santiam

Wagon Road. Dates Fish Lake lava flow of Taylor (1968) orHackleman Creek flow of Champion (1980, p. 177)

Qybsm Chatters, 1968;

Taylor, 1968,1990

“3,80

* Informal geographic name+ Laboratory number incorrectly reported as WSU-270 by Champion (1980)§ Neither laboratory nor field numbers were reported by Benson (1965). Samples were analyzed by Isotopes, Inc.

Table 1. Radiocarbon ages for Holocene volcanic rocks at Santiam and McKenzie Passes, Oregon—Continued

Lab No. Field No. Age Description of material dated Map unit References 
42

Table 2. Potassium-argon and 40Ar/ 39Ar isotopic ages from map area. See figure 3 for sample location map

[Grouped by stratigraphic unit and generally arranged from youngest to oldest. All ages are K-Ar except those shown by *, which a

Ar ages are recalculated where necessary to conform with currently accepted constants for radioactive decay and abundance of 40K 

1977). Symbols indicating “quality” show usefulness of age in stratigraphic interpretations: +, age thought meaningful; o, age probaccuracy may be far poorer than indicated by the reported precison; -, age meaningless (owing to large analytical error) or incorrect knowledge obtained by all ages and regional stratigraphic relationships). Lithologically, material dated is lava except where noted]

Map

No.

Strat.

unit

Age

(Ma)

Quality Sample

No. 

Geologic unit or

geographic location

Quadrangle

(1:24,000 scale)

Lat. N

Long. W

Rock type Mate

date

Volcanic rocks of High Cascades or lava flows erupted from north flank of Newberry volcano

1 Qah 0.08±0.02a + TFJ-2 Hogg Rock Three Fingered

Jack

44°25.53'121°52.11'

Andesite Whole 

2 Qass 0.093±0.011 + 3S162 South Sister, early-erupted lava

on northeast flank

Trout Creek Butte 44°08.29'

121°43.20'

Dacite Whole 

34

QbamsQr

0.4±0.2 - 3S159 Middle Sister North Sister 44°07.89'

121°45.95'

Basaltic andesite Whole 

0.095±0.010 + 3S139 Obsidian Cliffs North Sister 44°10.64'

121°48.98'

Rhyolite Whole 

5 Qsp 2.6±2.2 - BD-2 Shevlin Park Tuff Bend 44°03.71'

121°20.18'

Andesiticpumice

Plagio

6 Qrdt7 Qc

0.213±0.009 + 3S046 Tam McArthur Rim Broken Top 44°05.57'

121°38.54'

Rhyodacite Whole 

0.340±0.020 o 3S010 Triangle Hill Tumalo Falls 44°05.93'

121°32.41'

Andesite bomb Whole 

8 Qba 0.63±0.09 o 3S015 Lava 1.5 km west of Triangle Hill Tumalo Falls 44°06.22'

121°33.36'

Basaltic andesite Plagio

9 Qtt 0.83±1.5a - B-4 Tumalo Tuff Tumalo Dam 44°08.10'

121°22.65'

Dacitic pumice Whole 

10 Qtt 3.98±1.9 - M5-41 Tumalo Tuff Bend 44°03.80'

121°21.61'

Ash-flow tuff Plagio

11 Qtt 2.50±2.0 - M5-40 Bend Pumice (fallout tephra) Bend 44°03.70'

121°21.51'

Ash flow Plagio

12 Qbn 2.7±0.3 - 86-3 Lava from North flank of

Newberry volcano

Bend Airport 44°00.49'

121°14.39'

Basalt Whole 

13 Qbn 5.96±2.08 - Pb-2;

UT-210

Quaternary lava west of PowellButte

O'Neil 44°15.73'

121°04.82'

Basalt Whole 

14 Qba 0.12±0.3a o TS-137 Husband volcano South Sister 44°07.29'

121°51.27'

Basaltic andesite Whole 

15 Qba 0.44±0.12 + TFJ-321 Potato Hill road Santiam Junction 44°25.57'w

121°55.49'

Basaltic andesite Whole 

16 Qdt 0.460±0.030 + 3S067 Todd Lake volcano Broken Top 44°01.35'

121°43.28'

Dacite Whole 

17 Qr 0.4±0.4 - BT-31 Melvin Butte dome Three Creek Butte 44°10.61'

121°36.16'

Dacite Whole 

18 Qr 0.2±0.9 - BT-72 Three Creek Butte dome Three Creek Butte 44°09.06'

121°34.54'

Dacite Whole 

19 Qr 1.63±1.1 - M6-45 Bearwallow Butte dome Tumalo Falls 44°04.70'

121°33.20'

Rhyodacite Plagio

20 Qoba 0.56±0.14 - TFJ-427 U.S. 20 roadcut N. of Blue Lake,

reversed-polarity TRM

Three FingeredJack

44°25.17'121°46.32'

Basaltic andesite Whole 
43

21 Qoba 0.72±0.19 + 81/108 Highway 20, 0.5 km north of

Blue Lake

Three FingeredJack

44°25.16'121°46.22'

Basaltic andesite Whole 

22 Qoba 0.90±0.05 + TFJ-363 Cache Mountain, older part Three Fingered

Jack

44°23.25'121°46.03'

Basaltic andesite Whole 

23 Qoba 0.46±0.30 - S-80 Black Butte volcano, reversedpolarity TRM Black Butte 44°24.41' 121°37.53' Basaltic andesite Whole 

24 Qoba 0.48±0.2a - S-23 Black Butte volcano, reversedpolarity TRM Black Butte 44°24.41' 121°37.53' Basaltic andesite Whole 

25 Qoba 1.43±0.33 + 81/110 Black Butte, 1 km northeast of

summit

Little Squaw Back 44°24.41'

121°37.53'

Basaltic andesite Whole 

26 Qbti *1.19±0.08 + D4 Intracanyon lava, Crooked River,

reversed-polarity TRM

Opal City 44°25.96'

121°14.48'

Basalt Whole 

27 Qoba 1.6±0.3 + TFJ-431 State Hwy 126, 0.6 mi SE of

Carmen Reservoir

Clear Lake 44°19.75'

121°59.61'

Basaltic andesite Whole 

28 Qoba 1.1±0.2 + TFJ-256 State Hwy 126, 0.6 mi SE of

Carmen Reservoir

Clear Lake 44°19.75'

121°59.61'

Basaltic andesite Whole 

Drill core from High Cascades: Quaternary rocks

29 Qoba 1.00±0.03 o SP 77-24,

502 m

Drill hole, east side of SantiamPass (502 m depth)

Three FingeredJack

44°25.46'121°50.40'

Basalt Whole 

30 Qoba 0.91±0.06 o SP 77-24,

698 m

Drill hole, east side of SantiamPass (698 m depth)

Three FingeredJack

44°25.46'121°50.40'

Basaltic andesite Whole 

31 Qoba 1.81±0.05 o SP 77-24,

928 m

Drill hole, east side of SantiamPass (928 m depth)

Three FingeredJack

44°25.46'121°50.40'

Basaltic andesite Whole 

Drill core from High Cascades: probably Quaternary but units not determined

32 not known 0.15±0.05 - 88-2-1819 Devils Lake drill hole 83-2, core at 1819-ft depth South Sister 44°02.01' 121°45.84' Basalt Plagioc

33 not known 0.073±0.034 - 88-4-810 Trout Creek drill hole 82-4, core at 810-ft depth Trout Creek Butte 44°13.06' 121°39.71' Basalt Plagioc

Pliocene basalt and basaltic andesite

34 Tbas 2.9±0.2 + B-22 Summit, Squaw Back Ridge shield

volcano

Squaw Back Ridge 44°28.70'

121°28.59'

Basaltic andesite Whole 

35 Tbdr 8.83±1.36 - Pb-1;

UT-216

Pliocene lava west of PowellButte

O'Neil 44°15.02'

121°03.20'

Basalt Plagioc

36 Tam 3.3±0.2 + S-84 Slope above Camp Polk site

north of Sisters

Sisters 44°19.18'

121°31.07'

Andesite Whole 

37 Tbr *3.56±0.30 + D3a Plateau-forming flow near

Redmond

Opal City 44°23.20'

121°13.02'

Basalt Whole 

Deschutes Formation

38 Tda 4.7±0.1 + 3S002 Deschutes Formation lava, near

Bull Spring

Shevlin Park 44°06.63'

121°29.22'

Basaltic andesite Whole 

39 Tdb 4.9±0.4 + HB-7 Lava flow at top of Deep Canyon

grade (Oreg. Hwy 126)

Henkle Butte 44°17.38'

121°24.80'

Basaltic andesite Whole 

40 Tdbas *5.06±0.03 + D1a Basaltic andesite of Steamboat 

Rock, Deschutes Formation

Cline Falls 44°21.45'

121°16.03'

Basaltic andesite Whole 

41 Tdb 5.1±0.2 + S-74 Green Ridge summit, south end Little Squaw Back 44°25.61'

121°36.00'

Basaltic andesite Whole 

42 Tdbo *5.77±0.07 + D5 Basalt of Opal Springs,

Deschutes Formation

Opal City 44°26.11'

121°14.50'

Basalt Whole 

43 Tdrcb *6.14±0.06 + S94-B78 Rhyolite of Cline Buttes,

Deschutes Formation

Cline Falls 44°15.89'

121°17.50'

Rhyolite Plagioc

Table 2. Potassium-argon and 40Ar/ 39Ar isotopic ages from map area. See figure 3 for sample location map—Continued

Map

No.

Strat.

unit

Age

(Ma)

Quality Sample

No. 

Geologic unit or 

geographic location

Quadrangle

(1:24,000 scale)

Lat. N

Long. W

Rock type Mate

dat
44

44 Tdrsf *6.74±0.20 + S94-B100 Rhyodacite southwest of

Steelhead Falls, Deschutes Fm.

Steelhead Falls 44°23.21'

121°22.44'

Rhyodacite Plagioc

John Day Formation

45 Tjr 10.05±0.45 - SR-G-1 Gray Butte rhyolite, Smith Rock

area

Gray Butte 44°25.1'?

121°05.8'?

Vitrophyre -- 

46 Tjr 17.76±0.44 - SR-RH-4 Gray Butte rhyolite Gray Butte 44°24.0'?

121°07.2'?

Rhyolite -- 

47 Tjr *28.82±0.23 + GSO95-41 Gray Butte rhyolite, basal

vitrophyre

Gray Butte 44°24.30'

121°06.79'

Rhyolite Anorth

48 Tjr† 28.3±1.0 + PB-5;

AH-34

Powell Buttes dome Powell Buttes† 44°11.86'†

120°58.03'†

Rhyolite Anorth

49 Tjh *27.62±0.63 + GSO95-134 West end of Haystack Reservoir,

quarry

Opal City 44°29.87'

121°09.37'

Welded tuff Sanidin

50 Tjts 15.39±0.29 - SR-T-5 Tuff of Smith Rock, Smith Rock

area

Gray Butte 44°23.5'?

121°05.7?

Rhyolitic tuff -- 

51 Tjth *29.57±0.17 + GSO95-132 West end of Haystack Reservoir,

quarry

Opal City 44°29.76'

121°09.36'

Welded tuff Sanidin

52 Tjth *29.53±0.09 + GSO95-133 West end of Haystack Reservoir,

quarry

Opal City 44°29.85'

121°09.28'

Hydromagmatictuff

Sanidin

53 Tjbi¥ 30.8±0.5 + 10-6-78-2 Northeast of Gray Butte Gray Butte 44°26.10'

121°05.40'

Basalt(intrusion)¥

Whole 

54 Tjs *32.49±0.30 + GSO95-130 Tuff of Rodman Spring, at

Rodman Spring

Gray Butte 44°27.83'

121°06.91'

Fallout lapillituff

Sanidin

55 Tjb 17.54±0.98 - SR-B-1 West of Gray Butte Gray Butte 44°25.3'?

121°07.2'?

Basaltic andesite -- 

56 Tjb 18.85±0.51 - 24 Canal flora area Opal City 44°24.0'?

121°09.4'?

Basalt -- 

Drill core sample in or beneath John Day Formation strata in Bend quadrangle

57 Tjb?§ 1.53±0.77 - DOGMIBas81-1 Powell Buttes, subsurface(?) Powell Butte 44°11.63' 121°03.04' -- Whole 

58 Tjb?§§ 30.1±1.1 + DOGMIBas81-2 Powell Buttes, subsurface(?) Powell Butte 44°11.63' 121°03.04' -- Whole 

--, no data reported

aRecalculated age varies more than 5 percent from published agewLatitude originally reported is too far north

* 40Ar/39Ar age determination

?Location accurate only to nearest one-quarter section; positional error may be as great as ±500 m†East of map area. Location measured after plotting sample location from photocopy of original field sheet (provided courtesy of G.R. Priest, Oregon Depar

Industries, 1995)

¥Recognized as intrusion on this map§May be from drill core. Records lost for sample description and material sampled (G.R. Priest, Oregon Department of Geology and Mineral Industries, writ§§Probably from drill core, corresponding to sample from Powell Buttes 1 geothermal well, 310-320 m depth, as described by Brown and others (1980a, p. 5

Table 2. Potassium-argon and 40Ar/ 39Ar isotopic ages from map area. See figure 3 for sample location map—Continued

Map

No.

Strat.

unit

Age

(Ma)

Quality Sample

No.

Geologic unit or 

geographic location

Quadrangle

(1:24,000 scale)

Lat. N

Long. W

Rock type Mat

da
45

Table 3. Chemical analyses from some Pleistocene tephra and rhyolite domes in map area

Bend Pumice

Location No. -- 9 -- -- -- -- -- -- --

Sample No. 3SXBP 3S001 BD16-5 BD16-6 BD16-7 BD16-8 BD16-9 BD23-2 BD23-1

References 1 1222222 2

Major-element analyses, normalized water-free (weight percent)

SiO2 74.72 72.52 74.73 74.66 74.89 74.39 74.28 73.80 72.88

Al2O3 14.30 16.54 13.66 13.74 13.54 13.79 14.35 14.74 15.59

FeO 1.83 2.35 1.82 1.83 1.80 1.90 1.89 1.92 2.15

MgO 0.08 0.12 0.12 0.17 0.13 0.27 0.16 0.28 0.26

CaO 0.82 0.83 0.89 0.87 0.94 0.93 0.87 0.89 0.90

Na2O 4.49 3.76 4.97 5.03 4.92 5.04 4.92 4.60 4.63

K2O 3.49 3.52 3.52 3.46 3.49 3.40 3.25 3.45 3.26

TiO2 0.16 0.21 0.13 0.14 0.13 0.14 0.15 0.17 0.18

P2O5 0.03 0.04 0.08 0.08 0.08 0.08 0.08 0.09 0.09

MnO 0.07 0.10 0.06 0.02 0.06 0.06 0.05 0.06 0.05

Original oxide

total

99.30 99.14 100.02 98.76 100.83 100.90 99.35 99.00 100.46

Trace-element analyses (parts per million)

Ni 11 18 -- -- -- -- -- -- --

Cr 1 4 -- -- -- -- -- -- --

Sc 4.3 6.3 -- -- -- -- -- -- --

V 0 0 -- -- -- -- -- -- --

Ba 794 986 -- -- -- -- -- -- --

Rb 84 77 -- -- -- -- -- -- --

Sr 65 68 -- -- -- -- -- -- --

Zr 231 264 -- -- -- -- -- -- --

Co 0.5 4.9 -- -- -- -- -- -- --

Zn 105 69 -- -- -- -- -- -- --

La 28.7 27.4 -- -- -- -- -- -- --

Ce 59.7 62.7 -- -- -- -- -- -- --

Nd 26.5 26.1 -- -- -- -- -- -- --

Sm 5.95 5.69 -- -- -- -- -- -- --

Eu 0.83 0.75 -- -- -- -- -- -- --

Tb 0.94 0.85 -- -- -- -- -- -- --

Yb 4.2 4 -- -- -- -- -- -- --

Lu 0.58 0.56 -- -- -- -- -- -- --

Cs 3.1 2.7 -- -- -- -- -- -- --

Hf 6.7 7.1 -- -- -- -- -- -- --

Ga 20 23 -- -- -- -- -- -- --

Y 36 37 -- -- -- -- -- -- --

Nb 16.9 20.8 -- -- -- -- -- -- --

Ta 1.2 1.16 -- -- -- -- -- -- --

Th 8.1 7.6 -- -- -- -- -- -- --

U 2.7 2.5 -- -- -- -- -- -- --

Pb -- -- -- -- -- -- -- -- --

References: 1, Hill (1992a); 2, Mimura (1992); 3, Taylor and Ferns (1995); 4, W.E. Scott, unpub. data; 5, R.M.

Conrey and D.R. Sherrod, unpub. data; 6, Hill and Taylor (1989); 7, Hughes (1983).

Sample locations:

3SXBP, from quarry 1.2 km south of Overturf Butte in Bend city limits.

3S001, deposit mantling hillslopes at 4,340 ft elevation, 3 km west of Bull Spring.

BD16-5, 6, 7, 8, 9, exposure 2.4 km west of Overturf Butte; sampled downsection from -5 (near top) to -9 (near

base).

BD23-2, 1, quarry along O.B. Riley Road, 450 m southeast of Tumalo State Park; sampled near base (23-2

overlies 23-1).

[Major element analyses normalized to 100 percent with all iron as Fe2+. Also shown are oxide totals from original 

analysis. Dashes indicate elements not analyzed. Location numbers are keyed to appendix 1 and figure 8; dashes 

indicate sample localities not shown on figure 8]
46

Table 3. Chemical analyses from some Pleistocene tephra and rhyolite domes in map area—Continued

Rhyolite domes north of Triangle Hill Three Creek Butte Melvin Butte

Location No. -- -- -- -- -- -- -- --

Sample No. 3S012 S95-B160 3S075 S95-B163 3S032 S-3-CRBT 3S034 S-MELVIN

References 1 5 1 5 1, 3 4 1, 3 4

Major-element analyses, normalized water-free (weight percent)

SiO2 74.57 74.59 75.34 75.11 74.81 75.06 74.95 75.12

Al2O3 14.11 13.74 13.78 13.51 13.76 13.52 13.75 13.51

FeO 1.68 1.44 1.57 1.52 1.71 1.73 1.58 1.72

MgO 0.17 0.23 0.08 0.00 0.00 0.14 0.02 0.13

CaO 0.71 0.70 0.66 0.66 0.71 0.72 0.72 0.71

Na2O 5.06 5.57 4.81 5.44 5.28 5.21 5.27 5.16

K2O 3.46 3.50 3.55 3.56 3.49 3.43 3.47 3.45

TiO2 0.15 0.14 0.14 0.13 0.15 0.14 0.16 0.14

P2O5 0.02 0.02 0.02 0.02 0.02 0.05 0.02 0.05

MnO 0.06 0.06 0.05 0.05 0.06 <0.5 0.06 <0.5

Original

oxide total

99.94 101.15 99.95 101.24 99.58 99.12 99.66 99.17

Trace-element analyses (parts per million)

Ni 12 7 12 6 10 <2 10 <2

Cr 0 0 <4 0 <3 <1 <3 2

Sc 4.1 7 4.3 4 4.3 4 4.3 4

V 010 92 0 <2 2<2

Ba 797 766 801 783 796 790 818 780

Rb 82 87 87 87 86 -- 82 --

Sr 54 48 42 41 52 49 50 47

Zr 218 224 201 210 222 -- 219 --

Co 0.5 -- 0.7 -- 0.5 1 0.5 1

Zn 88 52 71 46 84 49 64 34

La 23.0 -- 19.4 -- 25.3 30 24.2 30

Ce 48.1 -- 43.4 -- 52.6 51 53 49

Nd 20.8 -- 17.5 -- 20.4 28 21.8 27

Sm 4.26 -- 3.84 -- 4.77 -- 4.71 --

Eu 0.59 -- 0.54 -- 0.64 -- 0.63 --

Tb 0.70 -- 0.64 -- 0.78 -- 0.78 --

Yb 3.1 -- 3.1 -- 3.7 4 3.5 3

Lu 0.47 -- 0.46 -- 0.53 -- 0.54 --

Cs 0.8 -- 1.6 -- 1.5 -- 1.6 --

Hf 6.1 -- 6.1 -- 6.5 -- 6.5 --

Ga 22 19 22 18 20 18 18 19

Y 29 39 26 28 31 34 32 32

Nb 16.5 16.4 17.4 15.2 17.8 8 18 9

Ta 1.01 -- 1.04 -- 1 -- 1.04 --

Th 7.7 8 8.0 8 7.9 9 7.8 8

U 2.2 -- 2.3 -- 2.7 -- 2.5 --

Pb -- 9 -- 12 -- 8 -- 9

References: 1, Hill (1992a); 2, Mimura (1992); 3, Taylor and Ferns (1995); 4, W.E. Scott, unpub. data; 5, R.M.

Conrey and D.R. Sherrod, unpub. data; 6, Hill and Taylor (1989); 7, Hughes (1983).

Sample locations:

3S012, 5,400-ft elevation, 460 m east from summit of hill with spot elevation 5,009 ft (Tumalo Falls quadrangle),

1.3 km north-northeast of Triangle Hill.

S95-B160, similar location as 3S012, but at 5,540-ft elevation, 300 m east-northeast from summit of hill.

3S075, 5,400-ft elevation, upslope from abandoned spur road, 122 m northwest from summit of hill with spot

elevation 5,875 ft (Tumalo Falls quadrangle), 1,920 m north-northwest of Triangle Hill.

S95-B163, similar location as 3S075, but at 5,800-ft elevation, 60 m north-northwest from summit of hill.

3S032, near summit of Three Creek Butte.

S-3-CRBT, near summit of Three Creek Butte.

3S034, 4,960-ft elevation, south summit area of Melvin Butte.

S-MELVIN, 4,860-ft elevation, south-southwest flank of Melvin Butte.
47

Table 3. Chemical analyses from some Pleistocene tephra and rhyolite domes in map area—Continued

Pumice of Columbia Canal Pumice of Bottle Creek

along Bottle Creek 5 km northeast

Location No. 10 3 4 2 11 11 8

Sample No. 3S079 RC95-178 830920-6 830920-5 3S024 S830920-1 3S018

References 1, 6 5 4 4 1 4 1

Major-element analyses, normalized water-free (weight percent)

SiO2 67.66 67.60 67.70 67.85 71.91 70.08 72.08

Al2O3 17.27 16.53 17.25 16.19 16.32 17.44 16.41

FeO 4.16 3.87 4.04 3.89 2.53 2.81 2.38

MgO 0.84 0.91 0.94 1.03 0.22 0.39 0.22

CaO 2.41 2.35 2.52 2.55 1.17 1.23 1.03

Na2O 4.37 5.43 4.47 5.09 4.33 4.72 4.47

K2O 2.18 2.20 1.99 2.26 3.14 2.90 3.07

TiO2 0.82 0.81 0.81 0.81 0.26 0.28 0.23

P2O5 0.16 0.18 0.11 0.12 0.04 0.09 0.04

MnO 0.13 0.12 0.16 0.21 0.08 0.06 0.07

Original

oxide total

99.03 99.03 92.17 93.88 99.29 93.46 99.33

Trace-element analyses (parts per million)

Ni 7 6 3 <2 11 <2 9

Cr 3 6 4 2 2 3 0

Sc 11.1 6 13 12 5.0 6 4.9

V 36 23 12 14 1 3 17

Ba 616 701 620 690 830 900 859

Rb 44 42 -- -- 74 -- 73

Sr 278 288 280 290 113 140 125

Zr 231 261 -- -- 270 -- 281

Co 2.1 2 3 1.1 2 0.9

Zn 87 81 77 80 84 68 87

La 19.8 -- 27 25 24.3 30 27.9

Ce 39.9 -- 60 51 48.2 56 48.2

Nd 22.1 -- 37 31 23 32 25.7

Sm 6.06 -- -- -- 5.09 -- 5.50

Eu 1.37 -- -- -- 0.84 -- 0.83

Tb 0.87 -- -- -- 0.78 -- 0.86

Yb 3.4 -- 4 4 3.4 4 3.8

Lu 0.5 -- -- -- 0.54 -- 0.56

Cs 1.6 -- -- -- 2.1 -- 2.0

Hf 5.5 -- -- -- 6.7 -- 7.0

Ga 21 20 22 18 23 19 21

Y 36 43 39 36 34 38 38

Nb 14.3 16 <4 15 15.7 18 15.6

Ta 0.66 -- -- -- 0.86 -- 0.89

Th 3.8 4 <4 <4 6.2 6 6.4

U 1.4 -- -- -- 2.2 -- 2.3

Pb -- 9 <4 12 -- 10 --

References: 1, Hill (1992a); 2, Mimura (1992); 3, Taylor and Ferns (1995); 4, W.E. Scott, unpub. data; 5, R.M.

Conrey and D.R. Sherrod, unpub. data; 6, Hill and Taylor (1989); 7, Hughes (1983).

Sample locations:

3S079, exposure in Columbia Canal along its narrow canyon reach, 2.1 km southeast of Bull Spring

RC95-178, roadcut, U.S. Forest Service Road (USFS) 1514, 1.7 km west of junction with Road 16, 10 km

south-southwest of Sisters.

830920-6, surface-mantling deposit along USFS Road 16 near benchmark 3823, 7.5 km south-southwest of Sisters

830920-5, cone-mantling deposit in cinder quarry 3.3 km south of Trout Creek Butte.

3S024, creek-bank exposure, south wall of Bottle Creek, 6,600-ft elevation, east side of USFS Road 370, 7.7 km

east of Broken Top.

S830920-1, same location as 3S024.

3S018, cone mantling deposit exposed in bulldozer scraping at 6,140-ft elevation along USFS spur road 4602-350.

This locality is 5.2 km northeast of 3S024 and S830920-1, which are correlative but in a substantially thicker

part of tephra-fall deposit.
48

Table 3. Chemical analyses from some Pleistocene tephra and rhyolite domes in map area—Continued

Pumice SSW. of Three

Creek Butte

Lava of Obsidian Cliffs Pumice on Pilot Butte

at Pilot Butte East Tumalo quarry

5 5 -- -- 13 12

Sample No. 830920-4 3S086 TS-690 3S139 3S007 830803-6

References 4 1 7 1 1 4

Major-element analyses, normalized water-free (weight percent)

SiO2 76.78 75.81 76.02 76.23 72.63 72.13

Al2O3 13.52 14.24 12.84 13.42 15.45 15.41

FeO 1.01 1.07 1.10 1.16 2.40 2.43

MgO 0.18 0.09 0.50 0.11 0.47 0.33

CaO 0.79 0.93 0.90 0.97 1.42 1.23

Na2O 4.07 4.23 4.71 4.47 4.06 4.90

K2O 3.54 3.41 3.83 3.42 3.17 3.20

TiO2 0.08 0.13 0.09 0.14 0.25 0.22

P2O5 0.03 0.04 -- 0.04 0.05 0.07

MnO <0.5 0.04 -- 0.04 0.08 0.06

Original

oxide total

94.68 99.72 99.71 99.83 99.00 93.44

Trace-element analyses (parts per million)

Ni <2 11 9 <10 13 <2

Cr 2 4 2 1 2 2

Sc <2 2.0 3.1 1.6 4.9

0

5

V <2 0-- 9 <2

Ba 930 861 880 876 776 780

Rb -- 74 115 79 67 --

Sr 100 115 160 119 130 110

Zr -- 97 130 96 253 --

Co <1 0.9 0.9 0.6 1.5 1

Zn 29 58 -- 64 69 57

La 22 20.0 21 20 21.8 27

Ce 40 43.5 71.5 44.6 44.9 50

Nd 15 16.3 18.0 14.9 20.7 27

Sm -- 2.77 2.83 2.63 4.71 --

Eu -- 0.44 0.85 0.42 0.78 --

Tb -- 0.41 0.46 0.39 0.71 --

Yb 2 1.5 1.7 1.5 3.1 4

Lu -- 0.26 0.29 0.24 0.48 --

Cs -- 1.8 4.16 1.94 1.7 --

Hf -- 3.3 5.1 3.2 6.1 --

Ga 16 17 -- 15 19 18

Y 14 16 -- 15 32 33

Nb <4 10.9 -- 10 14.8 15

Ta -- 0.69 1.54 0.74 0.75 --

Th 5 7.3 12.2 7.3 5.5 8

U -- 4.5 2.9 2.6 1.8 --

Pb <4 -- -- -- -- 14

References: 1, Hill (1992a); 2, Mimura (1992); 3, Taylor and Ferns (1995); 4, W.E. Scott, unpub. data; 5, R.M.

Conrey and D.R. Sherrod, unpub. data; 6, Hill and Taylor (1989); 7, Hughes (1983).

Sample locations:

830920-4, roadcut at 5,400-ft elevation along USFS spur road 1628-600, 2.3 km south-southwest of Three Creek Butte

3S086, same location as 830920-4.

TS-690, summit of small hill about 150 m northwest of Sister Spring, 3.6 km west of North Sister.

3S139, near Prouty memorial plaque, about 975 m north of Sister Spring, 3.7 km west of North Sister.

3S007, cone-mantling deposit on west side of Pilot Butte at 3,720-ft elevation upslope from foot of Pilot Butte

access road.

830803-6, cone-mantling deposit exposed at west edge of cinder quarry in hill with spot elevation 3,670 ft (Tumalo

Falls quadrangle) or 1,728 m (on Bend 30- x 60-minute quadrangle), 2.1 km east of Tumalo Lake.