u.s. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

PRELIMINARY GEOLOGIC MAP OF THE

MOUNT HOOD 30- BY 60-MINUTE QUADRANGLE,

NORTHERN CASCADE RANGE, OREGON

By David R. Sherrod and William E. Scott1

OPEN-FILE REPORT 95-219

1995

This map is preliminary and has not been reviewed for confonnity with U.S. Geological Survey editorial standards or with

the North American Stratigraphic Code. Any use oftrade, firm, or product names is for descriptive purposes only and does

not imply endorsement by the U.S. Government.

1Both at Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661
PRELIMINARY GEOLOGIC MAP OF THE

MOUNT HOOD 30- BY 60-MINUTE QUADRANGLE,

NORTHERN CASCADE RANGE, OREGON

By David R. Sherrod and William E. Scott

ABOUT THIS MAP

This map shows the geology ofthe central and

eastern parts of the Cascade Range in northern

Oregon. The Quaternary andcsitic stratovolcano

of Mount Hood dominates the northwest quarter

ofthe quadrangle, but nearly the entire arca is underlain by arc-related volcanic and volcaniclastic

rocks of the Cascade Range. Most stratigraphic

units were emplaced since middle Miocene time,

and all arc Oligocene or younger. Despite tbe

proximity of the map area to the Portland

metropolitan area. large parts remained virtually

unstudied or known only from limited reconnaissance until the late 1970s. A nntable exception is

the area surrounding Mount Hood, where mapping and ebemical analyses by Wise (1969) provided a framework for geologic interpretation.

Mapping since 1975 was conducted first to understand tbe stratigraphy and structure of the

Columbia River Basalt Group (Anderson, 1978;

Vogt, 1981; J.L. Anderson, in Swanson and others, 1981; Vandiver-Powell, 1978; Burck, 1986)

and later to examine the gcotbcnnal potential of

Mount Hood (Priest and otbers, 1982). Additional mapping was completed in 1985 for a geologic map of the Cascade Range in Oregon

(Sberrod and Smith, 1989). From 198710 1990,

detailed mapping was conducted in three 15-

minute quadrangles on a limited basis (D.R. Sherrod, unpublisbed mapping) (sec fig. 1 for index

to mapping). An ongoing volcanic hazards study

of Mount Hood by the U.S. Geological Survey

(Scott and otbers, 1994) bas provided the catalyst

for completing the geologic map of the Mount

Hood 3Q-minute by 6Q-minute quadrangle.

As ofJune 1994, only two broad arcas still remain largely unmapped. One of these areas, labeled "unmapped" on tbe geologic map, lies in tbe

Salmon River valley soutb of Zigzag along the

west margin of tbe quadrangle. Altbough strata

of the Columbia River Basalt Group in the

Salmon River valley were mapped in detail by

Burck (1986), the overlying middle and upper(?)

Mioccnc lava flows. volcaniclastic strata. and

intrusions havc never been studied. The other

poorly known area, the Mutton Mountains in the

southeastern part of the map area, consists of

Oligocenc and lowcr Mioccnc volcanic and

volcaniclastic rocks. Overlying lava flows ofthe

Columbia River Basalt Group were mapped in

some detail by Anderson (in Swanson and others,

1981).

AGE DETERMINATIONS

Ages for many units were detennincd by isotopic methods, ebiefly potassium-argon (K-Ar).

For many samples. fusion and gas extraction was

conducted on sample splits, tbereby improving

the prccision of thc age detcnnination. In these

cases, a weigbted mean age is reported (table I),

weighted by the inverse of tbe variance of individual runs (a standard statistical method; for example, Taylor, 1982). The age of young Mount

Hood deposits is known from earbon-14 (14C)

dating of charcoal and wood.

Magnetic polarity was determined for many

samples to further cbaracterize stratigraphic units.

In some areas. a cbronostratigrapbic framework

was developed by combining isotopic ages and

sequential polarity cbanges; this framework allowed us to assign minimum ages for some undated stratigraphic units. We use 0.78 million

years as thc age of tbe most recent major ebron

boundary (base of Brunbes Normal-Polarity

Cbron) (Sbaekleton and others, 1990; Baksi and

others, 1992). The correlation chart shows ages

for other ebron and subcbron boundaries in

Brunhes, Matuyama. and Gauss time (from

Cande and Kent, 1992).
STRATIGRAPHIC SYNOPSIS

The Cascade Range volcanic arc has been active since about 40 million years (m.y.) ago

(Duncan and Kulm, 1989), altbougb the oldest

arc-related rocks. if present in northern Oregon.

are deeply buried. Only one canyon in tbe map

area (the Claekamas River canyon in the southwest comer) has cut deep enough to expose lower

Miocene volcaniclastic strata in the Cascade

Range (unit Tos). East ofthe range, mildly alkaline volcanic rocks of Oligocene and early

Miocene age are found in the Mutton Mountains.

By middle Miocene time, much oftbe map area

was flooded by tholeiitic basalt of the Columbia

River Basalt Group. The Columbia River Basalt

Group is Dot related to the volcanic arc but faIms

a useful stratigraphic marker. For infonnative

summaries of the emplacement. distribution, and

chemical evolution ofthe Columbia River Basalt

Group. the interested reader is referred to studies

by Beeson and others (1989), Nelson (1989),

Tolan and others (1989), Wright and others

(1989), or references cited therein.

Since the time of Columbia River Basalt, the

locus and composition of Cascade Range volcanism in the map area have shifted sporadically.

Andesitic eruptions were predominant in the

western part from about 14 to 10 m.y. ago

(Salmon and Sandy Rivers area), preducing the

Rhododendron Formation and overlying lava

flows. From about 8 to 6.5 m.y. ago, lithic pyroclastic debris oftbe Dalles Formation was shed

by chiefly andesitic volcanoes in the north-central

part of the map area (Hood River valley escarpment). Andesitic to dacitic volcanism was again

predominant about 5 to 3 m.y. ago, with known

eruptive centers located from Lookout Mountain

westward to Lolo Pass, probably including the

area now occupied by Mount Hood. A major

episode of mafic volcanisrn--basalt and basaltic

andesite-began about 3-4 m.y. ago and lasted

until about 2 m.y. ago. Since about 2 m.y. ago

(Quaternary according to Harland and others,

1982), volcanism has been concentrated along the

axis of the High Cascades. North and south of

Mount Hood, Quaternary volcaoic rocks are predominantly basaltic andesite lava flows; whereas

at Mount Hood itself, andesite is volumetrically

prooOOlinant, fonning pyroclastic and debris flow

deposits and lava flows.

2

7

8 8

... '-=__---L L---'-_-" --:---".J

1. Ancinon{l978) 1. O.R.Shtnod.~~.I985

2. r., SwanIio"IIIn:l 0Ihers, 19811 a DR 5tlwIod. U1llJb. ~enIy

3. 8Lwdl (19ll6) 1985-19lIl

4. Priesl n or-s 11982) 9. Vogt (1981)

5. W.E. Sc:ou. 1990-1993 10. wat.'s (19681)

6. C.R. Sherrod.~ 1990-1993 11. Wise (1969)

Figure 1. Index to mapping used on geologic map.

2
DESCRIPTION OF MAP UNITS

[Volcanic rocks are named according to chemical classification, in terms ofweight percent SiOz, as follows: basalt, <52 percent; basaltic andesite, ~52 and <57 percent; andesite, ~57 and <63 percent; dacite, ~63 and <68 percent; rhyodacite, ~68 and <72 percent; rhyolite, ~72 percent. This scheme follows the

recommendations ofthe ruGS Subcommission on the Systematics ofIgneous Rocks (Le Bas and Streckeisen, 1991) but is modified slightly to include a field for rhyodacite]

SURFICIAL DEPOSITS FOUND IN

ALL PARTS OF MAP AREA

Qal Alluvium (Holocene and Pleistocene)-Unconsolidated gravel,

sand, and silt. Includes sandy and

gravelly deposits along valley

floors and sandy and muddy to

peaty deposits of marshes and

meadows. Near Mount Hood,

this unit originates in several

ways: much in the upper forks of

Hood River and the Muddy Fork

of the Sandy River is glacial outwash and debris-flow deposits of

neoglacial age (past 3,000 yr), but

in the White, upper Salmon, and

Zigzag Rivers much consists of

debris-flow deposits and fluvial

sediments related to erosion and

reworking ofthe thick debris-flow

and pyroclastic-flow deposits that

were emplaced in these valleys

during Holocene time

Qca Colluvial and alluvial slope deposits (Holocene and Pleistocene)-Poorly sorted deposits

that form sheets and fans on lower

parts of valley walls; deposited by

small streams and various slope

processes. Grades downslope

into alluvium of valley floors (Oal)

and pinches out upslope against

bedrock. May include till and

outwash locally

Qt Talus (Holocene "'and Pleistocene)-Blocky to platy, coarse3

grained detritus; typically forms

aprons below steep cliffs. Characterized by open-work, unvegetated

talus deposits, especially near timberline or on south- or west-facing

slopes

Qls Landslide deposits (Holocene and

Pleistocene)---Poorly sorted deposits of slumps and earthflows.

Generally poorly exposed; mapped

on basis of morphology. Commonly found at the base of steep

slopes where incompetent rocks

underlie thick, more competent sequences of lava flows. Map-unit

symbol shown queried along West

Fork Hood River where a large

mass of material is of uncertain

origin

Q 9 Glacial deposits (Holocene and

Pleistocene)-Very poorly

sorted pebbles, cobbles, and boulders in fine-grained (silty sand)

matrix. Chiefly till; occurs as

ground and lateral moraines in

map area. Includes minor alluvium where reworked by streams.

Grades upslope into talus and

colluvium. Circles indicate

morainal crests. Divided in Mount

Hood area into following units:

Qgnt Till of neoglacial age

(Holocene)-Forms steep,

sharp-crested, unvegetated to

forested moraines that lie within

1.5 km of present glaciers. Lo-
cally includes small areas oftill of

Evans Creek age (Qget)

Qget Till of Evans Creek age

(Pleistocene)--Forms moraines

that, in many areas, mark the

maximwn extent of glaciers about

20,000 yr ago during the Fraser

glaciation. Also includes deposits

near neoglacial till (Qgnt) that were

deposited about 12,000 yr ago

near end of Fraser glaciation

Qo Outwash (Pleistocene)-Wellrounded pebbles and cobbles that

form thick terrace deposits along

Shellrock Creek and Oak Grove

Fork of Clackamas River

(southwestern part of map area).

Elsewhere, much outwash is included in alluvium (Qal). Divided

near Mount Hood into:

Qoe Outwash of Evans Creek

age--Only one locality, on northeast slope of basaltic andesite of

Cloud Cap (Qbac). Deposited

about 12,000 yr ago near end of

Fraser glaciation

Tv

Tiba

Tia

Cinder cone or small volcano

(Tertiary)-Cinders, scoria, and

agglutinate of vent deposits.

Composition chiefly basalt or

basaltic andesite

Intrusive basalt and basaltic andesite (Pliocene)-Very fine

grained to medium-grained massive lava. Plugs and thick dikes in

the Oak Grove Fork area commonly found surrounded by cinders, scoria, and agglutinate of

vent deposits (mapped separately)

Intrusive andesite (Pliocene and

Miocene)--Platy to blocky, very

fine grained to medium-grained,

aphyric to porphyritic lava. Forms

sills and plugs. Plug at Shellrock

Mountain in northeastern part of

map area contains 60.0 percent

Si02 (Wise, 1969, his No. 38).

Large mass on north side of Old

Maid Flat in northwestern part of

map area contains 58.4 percent

Si02 (Gannett, 1982, his sample

locality No. 7-2-9)

VENT DEPOSITS AND INTRUSIVE

ROCKS FOUND IN ALL PARTS OF

MAP AREA

Qv Cinder cone or small volcano

(Quaternary)-Cinders, scoria,

and agglutinate of vent deposits.

Composition chiefly basalt or

basaltic andesite

Qiba Intrusive basalt and basaltic andesite (Pleistocene)-Ve ry

fine grained to medium-grained

massive lava. Forms plugs in

southwestern part of map area that

preswnably were (eeders (volcanic

necks) for surrounding lava flows

4

STRATIGRAPHIC UNITS EXPOSED

IN NORTHWESTERN PART

OF MAP AREA

Products of Old Maid eruptive

period (Holocene)-Rockavalanche and pyroclastic-flow

deposits originally containing hot

and cold dome rock, ash, and hot

gases. Deposits formed during

eruptions dominated by extrusion

of lava dome and repeated destruction of dome by gravitational

collapse and(or) explosive disruption. Melting of snow and ice by

such events generated debris flows

and floods. Also includes minor

avalanche deposits of hydrothermally altered debris from upper
(Stratigraphic units in northwestern part ofmap area, cont.)

flanks of Mount Hood volcano.

Valleys draining the west and

south flanks of the volcano accumulated thick sequences of volcaniclastic sediment. Subsequent

incision by streams resulted in

transport of sediment that is included in unit Qal. Radiocarbon

and tree-ring dating suggest the

Old Maid eruptive period occurred

between A.D. 1760 and 1810

(Cameron and Pringle, 1987).

Divided into:

Qhol Lava-Dacite dome forming Crater

Rock, near summit of Mount

Hood. Contains 62-64 percent

Si02 (Wise, 1969; White, 1980a;

Crandell, 1980)

Qhoc Pyroclastic-flow and debrisflow deposits-Poorly sorted

dacite pebbles, cobbles, and boulders in chiefly gray sandy matrix;

minor interbeds of silt and sand.

Forms thick fills in upper White

and Sandy river valleys. Pyroclastic-flow deposits are found as

far as 8 km from vent; debris-flow

and related alluvial deposits extend

down Sandy River to the

Columbia River and down White

River through Tygh Valley

(included in unit Qal) to the Deschutes River

Products of Timberline eruptive

period (Holocene}-Similar in

origin to products of Old Maid

eruptive period, but more extensive and voluminous. Forms thick

fills on southwest and west flanks

of Mpunt Hood. Radiocarbon

ages range from about 1,400 to

1,800 yr B.P. (Crandell, 1980;

Cameron and Pringle, 1987).

Consists of:

Qhtc Pyroclastic-fl~w._and debrisflow deposits-Poorly sorted

pebbles, cobbles, and boulders in

5

chiefly reddish-gray sandy matrix.

Juvenile blocks are andesite and

dacite containing 61-63 percent

Si02 (Wise, 1969; White, 1980a;

Crandell, 1980). Pyroclastic-flow

deposits extend as far as 12 km

from the vent down Zigzag River,

and debris-flow deposits extend

down Sandy River to the Columbia River

Qhdl Debris-avalanche deposit of Ladd

Creek (Holocene)-Poorly

sorted boulders to pebbles of

chiefly white to reddish-brown

hydrothermally altered lava in matrix of sand, silt, and clay; clasts

dominantly subangular. Forms

sheet-like deposit that originated

by debris avalanche from upper

north flank of Mount Hood. Surface of deposit marked by fractured megab locks and boulder

concentrations. Maximum-limiting radiocarbon age of about

4,000 yr B.P. (Meyer Rubin,

USGS, written commun., 1993),

but may date from start of Timberline eruptive period. (Sample

Nos. and ages are CAMS-6328,

3,880±80 14C yr B.P.; CAMS6329, 4,010±80 14C yr B.P.)

Related debris-flow deposit that

travelled down the West Fork of

Hood River is included in alluvium (Qal).

Qbp Basaltic andesite of Parkdale

(Holocene)--Small agglutinate

and cinder cone (about 2 km below Laurance Lake) and 6.5-kmlong lava flow. Contains 55 percent Si02 (Wise, 1969). Radiocarbon age of charcoal from below

lava flow is 6,890±130 yr B.P.

(Harris, 1973)
(Stratigraphic units in northwestern part ofmap area, cont.)

Qhpl

Qhpc

Products of Polallie eruptive period (Pleistocene)--Similar in

origin to deposits of Old Maid and

Timberline eruptive periods. Includes lava flows and fragmental

deposits from several vents on upper flanks; deposits occur widely

around volcano but are found

chiefly on south and east flanks.

Erupted episodically over a time

period of at least several thousand

years during the Fraser glaciation

(Crandell, 1980), which in the

Oregon Cascade Range probably

peaked about 20,000 yr ago and

persisted until about 12,000 yr

ago. Consists of:

Lava-Andesite and dacite flows

and domes (60-63 percent Si02;

Wise, 1969; White, 1980a; Crandell, 1980) that extend as far as 3

km from Mount Hood's summit

Pyroclastic-flow and debrisflow deposits-Poorly sorted

boulders, cobbles, and pebbles in

a chiefly gray sandy matrix. Includes minor debris-avalanche deposits of hydrothermally altered

material. Three sequentially

younger deposits occur adjacent to

Newton Creek; contact between

them labeled to show relative age:

Y, younger; 0, older. The youngest forms the valley floor, contains

no evidence of units emplaced at

high temperature, and probably

originated largely from stream reworking of older Polallie deposits.

Deposits preserved on ridgetops of

upper East Fork Hood River valley were probably emplaced close

to the time of maximum extent of

glaciers of Evans Creek advance

of Fraser glaciation (about 20,000

yr ago). Till of this age (unit

aget) that forms moraines in the

valley ofthe East Eork near Pocket

Creek contains abundant clasts

derived from Polallie deposits

6

Pre-Polallie andesitic and dacitic

rocks of Mount Hood (Pleistocene)-

Qhc Pyroclastic-flow and debrisflow deposits-Poorly sorted

boulders, cobbles, and pebbles in

a chiefly gray sandy matrix.

Forms thick dissected fill between

Muddy Fork of Sandy River and

McGee Creek, and below lava

flows of unit aha south of Muddy

Fork. Similar deposits are interbedded with lava flows of unit

Qha elsewhere, but are not mappable a~ the scale of this map.

Map-uDlt symbol shown queried 8

km west-northwest of Mount

Hood where age of unit is uncertain

Qha Andesite and dacite lavaCone-building flows and domes

(57-64 percent Si02; Wise, 1969;

White, 1980a). Forms much of

upper flanks of Mount Hood and

radial ridges that extend as far as

12 km from summit

Qbaa Basaltic andesite of Aschoff

Buttes (Pleistocene)-La v a

flows erupted from cinder cone

(Aschoff Buttes) at west edge of

~ap. Normal-polarity magnetization

Qbal Basaltic andesite of Lost Lake

Butte (Pleistocene)-L a va

flows that form small shield volcano at Lost Lake Butte at north

edge of map area. Olivine phenocrysts 1-3 rom (5-7 percent) and

clinopyroxene phenocrysts 1-3

rom (1-3 percent); thin-section descriptions by Wise (1969) also indicate microphenocrysts of orthopyr,?xene. Contains -54.8 percent SI02 (two analyses, Wise
(Stratigraphic units in northwestern part ofrnap area, cont.)

[1969], his Nos. 157 and 158).

Normal-polarity magnetization;

younger than 0.78 million years

(m.y.) on basis of well-preserved

landform. A small cirque with

floor at elevation of 3,800-4,000 ft

is carved in the north slope (north

ofmap area); thus unit is probably

older than about 20,000 yr

(maximum of most recent glaciation)

Qbas Basaltic andesite of Stump Creek

(Pleistocene)-Lava flows on

distal northwest flank of Mount

Hood; vent location unknown

Qbap Basaltic andesite of The Pinnacle

(Pleistocene}--Olivine-bearing,

slightly porphyritic lava flows.

Contains 55-57 percent Si02

(Wise, 1969, his No. 145; Bargar

and others, 1993, No. 41).

Erupted from The Pinnacle, a

glacially eroded scoria and agglutinate cone and feeder dikes on

north slope of Mount Hood.

Normal-polarity magnetization; KAr age of 0.14±0.02 m.y. (table

1, No.1)

Qbac Basaltic andesite of Cloud Cap

(Pleistocene)-Lava flows generally containing less olivine than

lava from The Pinnacle (Qbap).

Contains 55-58 percent Si02

(Wise, 1969, his Nos. 142, 143,

and 144 from two flows; and Bargar and others, 1993). Forms

small volcano whose lava overlies

pre-Polallie andesitic lava from

Mount Hood (Qha) and is surrounded by fragmental products of

the Polallie eruptive episode

(Qhpc). Normal-polarity magnetization. Potas.sium-argon age is

O.59±O.03 m.y. (table 1, No.6)

7

Qbc Basalt of Crutcher Bench (Pleistocene)-Porphyritic olivine

basalt that forms sequence of at

least four lava flows. Total

thickness at least 30 and perhaps

as much as 60 m. Normal-polarity

magnetization; Sandy River has incised 120 m to form Crutcher

Bench, indicating that unit was

probably emplaced in early part of

Brunhes Normal-Polarity Chron.

Erupted from cinder vent on valley

wall above Crutcher Bench (north

of Zigzag)

Qbdc Basaltic andesite of Devil Canyon

(Pleistocene)-Lava flows in

Devil Canyon and 2 km southeast

in Lady Creek. Devil Canyon

flow, erupted from vent at head of

canyon, is platy to blocky weathering, slightly porphyritic lava

with phenocrysts of olivine, -1

percent (~1 mm), and plagioclase,

1-3 percent (~1 mm). Contains

-54.6 percent Si02 (one analysis,

Wise, 1969, his No. 74). Normal-polarity magnetization; position in canyon floor indicates age

probably younger than 0.78 m.y.

Potassium-argon age is 0.71±0.04

m.y. (table 1, No.7). Flow in

Lady Creek, undated but also of

normal-polarity magnetization, is

lithologically similar and occupies

similar topographic setting in adjacent canyon floor. It was erupted

from a vent in the canyon of Lady

Creek

Qahr Andesite of Horseshoe Ridge

(Pleistocene)-Fine-grained,

nearly aphyric andesite lava. Vent

marked by domal mass at highest

elevation of outcrop area on northwest flank of Horseshoe Ridge,

and distal lava fonns topographic
(Stratigraphic units in northwestern part ofmap area, cant.)

bench on southeast side of Zigzag

River. Normal-polarity remanent

magnetization; Zigzag River has

incised 140 m below base of unit,

indicating emplacement occurred

in early part of Brunhes NormalPolarity Chron

78). Normal-polarity

magnetization; Zigzag River has

incised 200 m to form Enola Hill

bench; thus unit was probably emplaced in early part of Brunhes

Normal-Polarity Chron

Qap

Qbr

Qae

Andesite of Perry Lake (Pleistocene}--Fine- to medium-grained,

nearly aphyric andesite lava.

Contains 59.1 percent Si02 (one

analysis; Wise, 1969). Erupted

from vent of uncertain location

(but near Red Hill) and flowed

northeast down Bear Creek. Cut

by normal fault with about 20 m

offset. Normal-polarity magnetization; age younger than 0.78

m.y. because it overlies the

basaltic andesite of Blue Ridge

(Qbr). Perry Lake, which is labeled on the Mount Hood North

7.5-minute quadrangle, is a small,

shallow, mosquito-infested lake

on the flow's surface

Basaltic andesite of Blue Ridge

(Pleistocene}--Chiefly basaltic

andesite and lesser basalt. Erupted

from vents on Blue Ridge. Includes rocks with normal- and reversed-polarity magnetization;

youngest parts are probably

younger than 0.78 m.y. on basis

of cinder cone preservation in Red

Hill area. Potassium-argon ages

of 1.12±0.03 and 1.27±0.03 m.y.

(weighted mean ages, table 1,

Nos. 8 and 10) for samples near

base ofunit

Andesite of Enola Hill (pleistocene}--Moderately porphyritic

lava flow. Forms bench-capping

lava east of Rhododendron.

Contains -58.7 percent Si02 (one

analysis, Wise, 1969, his No.

8

Qlb

Qa"

Basalt (Pleistocene or Pliocene?)-Remnants of olivine

basalt southwest of Lost Lake,

near Lolo Pass on northwest side

ofMount Hood, and north ofTilly

Jane Creek on northeast side of

Mount Hood. Reversed-polarity

magnetization. Potassium-argon

age of 1.37±0.10 m.y. from

sample in talus shed by unit near

Lolo Pass (table 1, No. II). Also

includes moderately porphryritic

basalt or basaltic andesite possessing normal-polarity polarization on

ridge east ofLost Lake Butte

Andesite of Laurance Lake

(Pleistocene}--One or two lava

flows of slightly porphyritic andesite that caps ridge north ofLaurance Lake. Unit commonly 60-

120 m thick; maximum thickness

ISO m at eastern limit of outcrop

where flows apparently filled an

ancestral canyon. Traceable upslope and west to Vista Ridge,

which may be the vent area.

Contains phenocrysts of olivine 1-

2 mm across (1 percent) and

clinopyroxene 1-2 mm (1-2 percent) and numerous microphenocrysts of opacitized amphibole

smaller than I mm (-I percent).

Reversed-polarity magnetization.

(Note: spelling oflake name varies

between Mount Hood North 7.5-

minute quadrangle (Laurance) and

Mount Hood 30- by 60-minute

quadrangle (Lawrance»
(Stratigraphic units in northwestern part ofmap area, cant.)

QTat Andesite of Tom Dick and Harry

Mountain (Pleistocene or

Pliocene)-Slightly to moderately porphyritic pyroxene andesite

lava flows. Mostly reversed-polarity magnetization, including

stratigraphically highest exposures

Qsg Rocks of Sandy Glacier volcano

(Pleistocene}--Thin flows and

interlayered tuff and breccia.

Composition ranges from basalt to

andesite (four analyses, Si02 from

52.5 to 60.0 percent) but interpreted by Wise (1969) as a single

magmatic sequence that evolved

during the building of a pre-Mount

Hood cone. Reversed-polarity

magnetization determined by laboratory analyses (C.A. Gardner,

USGS, oral commun., 1994); in

contrast, measurements using

portable fluxgate magnetometer

commonly exhibit nondipole behavior. Age controversial owing

to discordant K-Ar ages of

3.2±0.3 and 1.27±0.02 m.y.

(table 1, Nos. 18 and 9). We believe, however, that the younger

age is more likely correct because

rocks of Sandy Glacier overlie

basalt ofBald Mountain, which in

turn has similar remnant magnetism, petrography, and topographic elevation as dated basalt

(in OTb) northwest across Lolo

Pass (age 1.37±0.10 m.y.)

Qah Andesite of Hiyu Mountain

(Pleistocene}--Clinopyroxenebearing andesite with clots (to 1

cm) of olivine. Reversed-polarity

magnetization determined by laboratory analyses (C.A. Gardner,

USGS, oral commun., 1994); in

contrast, measurements using

portable fluxgate -magnetometer

commonly exhibit nondipole be9

havior. Overlies basalt flow (in

unit OTb) from which K-Ar age of

1.37±0.10 m.y. was obtained

QTbme Basaltic andesite of Mirror Lake

and Eureka Peak (Pliocene}--Lava flows on north side

of Tom Dick and Harry Mountain

and southeastward to Eureka

Peak. Correlation between exposures at these separate geographic

locations is tentatively based on

roughly similar elevations; however, mapping in the head of Still

Creek is limited only to generalized work by Wise (1969), who

did not subdivide rocks in this area

by composition

Tbbu Basalt of Bull Run Watershed

and other ridge-capping

basalt (Pliocene and Miocene?)-Lava flows chiefly

basaltic in composition but including minor basaltic andesite. Isotopic ages are mostly between

about 3 and 2 m.y., although ages

of 5.5±0.7 and 6.23±0.23 m.y.

(table 1, Nos. 22 and 24) are reported from strata that cap Horseshoe Ridge in the Mount Hood

Wilderness. The older ages are

difficult to interpret presently; for

example, the aforementioned age

of 6.23 m.y. is lithologically similar to a sample collected 0.5 kIn

distant that yielded an age of 2.48

m.y. (table 1, No. 14). Tentative

acceptance ofthe older ages is the

reason for extending the base of

the sequence into late Miocene

time

Lookout Mountain volcanic sequence (Pliocene}--Chiefly

lava flows that range in composition from basalt to dacite. Forms

most ofBluegrass Ridge and main
(Stratigraphic units in northwestern part ofmap area, cont.)

part of escarpment east of East

Fork Hood River in area of Lookout Mountain. Correlates in part

with the rocks of Barlow Ridge

and Gunsight Butte. Divided according to composition into:

Tlma Andesite--Moderately porphyritic

two-pyroxene andesite. Includes

interlayered olivine basalt flows in

lowest parts. Potassium-argon

ages of 2.74±0.03 and 3.1±0.2

m.y. (table 1, Nos. 16 and 17).

Andesite lava typically contains

59-61 percent SiOz (Wise, 1969,

his analyses 17, 63, 64, and 66)

Tlmb Basalt and basaltic andesite--

Lava flows and tuff breccia of

basalt and basaltic andesite. Exposure at southwest end of Bluegrass Ridge is olivine basalt and

may be near-vent material; it consists chiefly of tuff breccia containing subangular clasts 1-10 cm

across and fewer lava flows 1-3 m

thick that possess reversed-polarity magnetization. Exposure on

east side of Bluegrass Ridge is

vesicular, pyroxene-bearing basaltic andesite lava flows ofuncertain magnetic polarity

Tlmd Dacite--Slightly porphyritic, orthopyroxene-bearing dacite and

rhyodacite(?). Probably correlative with rhyodacite of Gordon

Butte (Trgx and Trgd), which is

exposed -4 km east of Lookout

Mountain in northeastern part of

map area. Mainly underlies the

andesite member. Map unit symbol shown queried for hornblende

dacite dome of uncertain age east

ofDog River

Trbg Rocks of Barlow Ridge and

Gunsight Butte (Pliocene

and Miocene>.-C~iefly andesite

lava flows and minor volcaniclastic strata; composition ranges from

10

basalt to dacite, but mapping is insufficient to subdivide by lithology. Includes slightly pyritized

rocks exposed locally in roadcuts

along State Highway 35 between

Barlow and Bennett Passes.

Potassium-argon age near top of

sequence at Gunsight Butte is

4.1±0.6 m.y. (table 1, No. 19).

Age of oldest parts unknown, but

rocks low in section at White

River are cut by dike with K-Ar

age of 5.26±0.44 m.y. (table 1,

No. 20). Pattern indicates tuff

breccia, probably block-and-ash

and laharic deposits

Tate Andesite of Tumble Creek (Pliocene)-81ightly to moderately

porphyritic lava flow (or perhaps

two flows) exposed on east

canyon wall of East Fork Hood

River near Polallie Creek. Contains 5-10 percent of blocky plagioclase phenocrysts 2-3 mm

across, scant opacitized amphibole

phenocrysts generally less than 2

mm, and trace amounts of fresh

olivine 1 mm across. Lacks pyroxene phenocrysts, which distinguishes it from overlying andesite

of Lookout Mountain volcanic sequence (Tlma). As much as 200 m

thick; columnar jointed in 60-mhigh cliffs east of State Highway

35 (near Sherwood campground)

Tb Basalt (Pliocene and (or)

Miocene)--Vesicular to massive,

slightly porphyritic olivine basalt

lava flows. Exposed in three

small separated areas, each occurrence probably unrelated to any

other. Basalt near Bear Creek

(north edge of map) contains -51

percent SiOZ (one analysis,Wise,

1969, No. 36); it presumably is

interbedded with volcaniclastic
(Stratigraphic units in northwestern part ofrnap area, cont.)

rocks of Middle Fork (Tvmf).

Basalt west ofButcherknife Ridge

contains 52 percent Si02 (one

analysis, Wise, 1969, No. 87),

possesses reversed-polarity magnetization, and is interbedded with

or overlies andesite of Lolo Pass.

Basalt near Barlow Crossing lacks

chemical analyses

Taop Andesite of Lolo Pass (Pliocene?

and Miocene}-Two-pyroxene

andesite lava flows and hornblende-bearing andesite lava and

tuff breccia. Unit is poorly exposed and known mainly from

roadcuts, although a few thick lava

flows and massive domes crop out

locally. Potassium-argon age of

5.8±0.8 m.y. from pyroxene andesite lava near Lolo Pass (table 1,

No. 23); hornblende andesite

dome yielded age of 6.25±0.11

m.y. (table 1, No. 25). Queried to

indicate rocks of uncertain assignment northwest ofVista Ridge

Tvrnf Volcaniclastic rocks of Middle

Fork (Miocene}-Tuff breccia,

conglomerate, and sandstone.

Generally poorly exposed in

northwestern part of Mount Hood

quadrangle. Nearly 100 m thick

where exposed in a gully northeast

of Lawrance Lake: coarse tuff

breccia in lower part has radially

jointed blocks as large as 4 m;

other primary pyroclastic beds

farther upsection contain bombs

80 cm across of frothed, darkbrown or olive-gray pyroxene andesite in a pink-weathering ashy

matrix; highest beds are conglomeratic. Mapped as Dalles Formation by Wise (1969), although unit

here may be as Ipuch as 3 m.y.

younger than 'the main mass of

Dalles Formation (Td) exposed in

11

the northeastern part ofthe Mount

Hood quadrangle (from the East

Fork Hood River eastward onto

the Columbia Plain; see discussion

of unit Td for northeastern part of

map area). Map-unit symbol

shown queried for poorly exposed

slope north ofThe Pinnacle

Taef Andesite of East Fork (Miocene)--81ightly to moderately

porphyritic two-pyroxene andesite

lava flows and minor breccia or

tuff breccia. Described in more

detail for northeastern part of map

area

Tilh Intrusive rocks of Laurel Hill

(Miocene}-The following description is taken from Wise

(1969): Porphyritic quartz diorite

to quartz monzonite, with scattered

patches of granophyre; exposures

at west end of Still Creek mass

show hornblende hornfels zone

about 60 m wide, and an epidote

hornfels zone as much as 1.5 kIn

wide is present in most ofthe wall

rocks surrounding the intrusions.

Rocks assigned to this unit may

represent several ages ofintrusion.

The main Laurel Hill mass has an

age between about 9 and 8 m.y.,

on basis ofthree different determinations (table 1, No. 29 and the

two preceding unnumbered samples that lack precise sample locations). A sill penetrated at 3,750-

ft depth in drill hole OMF-7a (on

Old Maid Flat) has an age of

9.30±0.87 (table 1, No. 31), and

the large standard deviation may

indicate that it, too, is about the

same age as the Laurel Hill mass.

An older age of intrusive activity

may be indicated by K-Ar age of

11.6±1.2 m.y. from the Still

Creek mass (table I, No. 37)
(Stratigraphic units in northwestern part ofmap area, cont.)

Ts Sedimentary rocks (Miocene)-

Fine- to medium-grained volcaniclastic sandstone. Caps ridges in

Zigzag Mountain area where interfingered with youngest parts of

andesite of Zigzag Mountain

(Tazm). Unit there is hornfelsed,

suggesting it may be fine-grained

upper part of Rhododendron Formation (Trh) and that intervening

andesite lava is intrusive. Overlain

by ridge-capping basalt (Tbbu) at

old Zigzag East fire lookout (site

of dating localities 14 and 24)

Tazm Andesite of Zigzag Mountain

(Miocene)-Very porphyritic

pyroxene andesite lava containing

plagioclase and hypersthene phenocrysts as large as 1 em. Widely

exposed throughout northwest

corner of map area. Originally

named Zigzag Mountain andesites

by Wise (1969), who recognized

the petrographic distinctiveness of

these rocks and mapped them as

far north as Lost Lake. Equivalent

to informally named Last Chance

andesite of Priest and others

(1982), but the name "Last Chance

Andesite" is herein abandoned because it was long ago preempted

for a formally designated unit in

New Mexico. Age is late Miocene; potassium-argon ages from

flows on Last Chance Mountain

are 10.7±0.5, 10.5±0.4, and

9.04±0.04 m.y. (table 1, Nos.

35b , 35a, and 30)

Trh Rhododendron Formation (Miocene)-Andesitic tuff breccia.

Clast petrography varies but is

chiefly slightly porphyritic pyroxene andesite. Unit originated

mainly as pyroclastic flows and

lahars, but some parts also contain

12

fluvial volcaniclastic sandstone

and minor siltstone. Age is middle

and earliest late Miocene; overlies

Columbia River Basalt Group

(Tcgn2 and Tcwf) and underlies

andesite of Zigzag Mountain

(Tazm). Potassium-argon ages

from lava flows in upper part of

unit are 12.1±1.7, 11.26±0.08,

10.6±0.5, 10.58±0.08, and

9.5±2.4 m.y. (table 1, Nos. 38,

36, 34, 33, and 32)

Columbia River Basalt Group

(Miocene)-Lava flows of

tholeiitic flood basalt. Age in map

area is middle Miocene. Stipple in

Salmon River canyon indicates

wide zone of basaltic breccia in

hanging wall of reverse fault. Divided into:

Wanapum Basaltlewpr Priest Rapids member-Fineto medium-grained basalt. Reversed-polarity magnetization

(Vogt, 1981). Age about 14.5

m.y. (Rockwell Hanford Operations, in table 3 of Tolan and others, 1989; no error assigned to

age)

lewf Frenchman Springs member-Fine- to medium-grained

basalt, commonly containing scant

plagioclase phenocrysts as large as

1 em across

Grande Ronde Basalt-Finegrained to very fine grained basalt.

Subdivided on basis of magnetic

polarity into four magnetostratigraphic units, of which only two

are exposed in the northwestern

part of map area. Age between

about 17.2 and 15.4 m.y. on basis

of K-Ar age of 16.2±1.0 m.y.

from oldest known flow ofthe reversed-polarity unit 1 on Columbia

Plateau (Reidel and others, 1989,

p. 25) and 40Ar- 39Ar age of
15.6±0.2 m.y. from one of

youngest flows in the normal-polarity unit 2 (drill core sample from

central Columbia Plateau; Long

and Duncan, 1983)

Normal-polarity unit 2-

Chiefly 10w-MgO chemical type in

lower part and high-MgO chemical

type in upper part (Vogt, 1981)

Reversed-polarity unit 2-

Aphyric to slightly porphyritic lava

flow. Base not exposed; oldest

stratum exposed in northwestern

part ofmap area

STRATIGRAPHIC UNITS EXPOSED

IN SOUTHWESTERN PART

OF MAP AREA

Qob

in southwestern part of map area.

Polarity not well characterized because few ofthe domes form bold

outcrops suitable for sampling.

Probably entirely Pleistocene in

age, but duration therein uncertain

Olivine basalt (Pleistocene)-

Slightly porphyritic lava flows and

minor breccia. Lithologically

similar to older basaltic andesite

(Qoba) but chiefly basaltic in composition. Includes rocks with

normal- and reversed-polarity

magnetization; probably mostly

early Pleistocene (older than 0.78

m.y.), but youngest part may be

late Pleistocene. Exposed only in

area east and southeast of Linney

Creek campground

Qyba Younger basaltic andesite (Pleistocene)-Slightly porphyritic

lava flows and minor flow breccia.

Normal-polarity magnetization.

Erupted from shield volcanoes of

Summit Butte and North Wilson

and from a few small cinder cones

Qae Andesite of Cabin Creek (Pleistocene)-Nearly aphyric, mediumgray andesite with scant clinopyroxene less than 1 mm across and

trace amounts ofsmall opacitized

hornblende and olivine phenocrysts. Exposed only along

southwest edge of map; erupted

from vents 1 kIn to the south in

Breitenbush Hot Springs quadrangle. Normal-polarity magnetization; younger than 0.78 my.

Qas Andesite of Skyline Road (Pleistocene)-Light-gray, slightly

porphyritic lava, commonly with

scant, small opacitized amphibole

phenocrysts'. Forms scattered

domes along Cascade Range crest

13

Qoba Older basaltic andesite (Pleistocene)-Slightly porphyritic lava

flows and minor flow breccia.

Contained chiefly in shield volcanoes of Mount Wilson, Clear Lake

Butte, and Wests Butte. Reversed-polarity magnetization in

most exposures, although polarity

of Wests Butte remains unknown

Qaw Andesite of Wapinitia Pass

(Pleistocene)--Medium-gray,

moderately porphyritic lava. Plagioclase phenocrysts (1-3 mm) 5-

10 percent; clinopyroxene and orthopyroxene phenocrysts « 1

mm) 1-2 percent; and traces of

much-altered, fine-grained olivine.

Blocky- to platy-weathering. One

analysis, 62 percent Si02 (R.M.

Conrey, Wash. State Univ., unpub. data). Reversed-polarity

magnetization. Potassium-argon

age of 1.37±0.03 m.y. (weighted

mean oftwo determinations; table

1, No. 56)
(Stratigraphic units io southwestern part ofmap area, coot.)

Qdt Dacite of Frog Lake Buttes

(Pleistocene)-Thick, massive

lava, probably domes, at Frog

Lake Buttes and hill southwest of

Wapinitia Pass. Two analyses;

62.5-63.5 percent Si02 (our unpublished data). Reversed-polarity magnetization; early Pleistocene

in age. Unit may mark vent area

for 1.37-m.y. andesite of Wapinitia Pass (unit Qaw)

Qael Andesite west of Clear Lake

(Pleistocene)-Lava flows of

slightly porphyritic two-pyroxene

andesite. Reversed-polarity magnetization; probably similar in age

to andesite ofWapinitia Pass (unit

Qaw, -1.37 m.y. old) but somewhat less porphyritic. Queried for

exposure of uncertain correlation

on south valley wall of Salmon

Creek

Volcanic rocks in Trillium Lake

area (Pleistocene?, Pliocene,

and Miocene?)-Lava flows of

andesite and minor basalt. Unit

name is applied to inadequately

mapped and undated rocks with

uncertain stratigraphic relation to

strata in adjacent areas; could be

late Miocene and Pliocene in age

and represent deeply glaciated

parts of the stratigraphic sequence-or could be late Pliocene

and early Pleistocene and result

from emplacement after downcutting of Salmon River canyon.

Divided into:

aTtla2 Andesite (Pleistocene or Pliocene)-Lava flows of two-pyroxene andesite. Reversed-polarity magnetization. Overlies basalt

unit (QTtlb) (cotrelation chart

shows overlapping age in order

that this unit and the basalt unit

14

anlb

aTb

QTba

Tbg

(OTt/b) may both show possible

Pleistocene age assignment)

Basalt (Pleistocene or Pliocene)-Lava flows of olivine

basalt. Normally polarized

Andesite (Pliocene and (or)

Miocene)-Reversely polarized

lava flows. Magnetostratigraphy

of overlying flows indicates that

this chronozone is Pliocene or

older. Exposed only in area west

and east of Salmon River, about 4

kIn south ofTrillium Lake

Basalt (Pleistocene and Pliocene?)-Diktytaxitic (open-textured) to intergranular olivine

basalt and minor basaltic andesite.

Lithologically similar to underlying basaltic andesite of Oak Grove

Fork (Tbao) but predominantly

basaltic; contact between the two

in southwest comer of map area is

conjectural, and the purpose in

showing the unit there is to separate known Pliocene rocks from

younger strata. Elsewhere the unit

consists of isolated basalt lava

flows

Basaltic andesite (Pleistocene or

Pliocene?)-Slope-mantling lava

flow that forms bench at its northern limit along the Clackamas

River, south edge of map sheet.

Corresponds to a unit with K-Ar

age of 1.59±O.21 m.y. and chemical analysis containing -57 percent

Si02 (White, 1980b, his sample

No. 29)

Basalt near Ghost Creek (Pliocene or Miocene)-Very dark

gray to black, columnar-jointed

lava. Exposed only in roadcut

along U.S. Highway 26 between

Government Camp and Wapinitia
(Stratigraphic units in southwestern part of map area, cant.)

Pass. Contains scattered, finegrained olivine phenocrysts and

very sparse, yellow-weathering

plagioclase phenocrysts as large as

4 mm. Similar in hand specimen

and in manner of its columnar

jointing to some flows in the

Wanapum Basalt ofthe Columbia

River Basalt Group (Swanson and

others, 1979), but chemical analysis indicates otherwise (R.M.

Conrey, Wash. State Univ., unpub. data). Reversed-polarity

magnetization; magnetostratigraphy of overlying flows indicates

that this chronozone is Pliocene or

older

Tbao Basaltic andesite of the Oak

Grove Fork (Pliocene}-Lava

flows and minor breccia, chiefly

basaltic andesite in composition.

Slightly porphyritic, with most

rocks containing plagioclase and

olivine phenocrysts. Flows north

of High Rock commonly contain

3-5 percent clinopyroxene phenocrysts 2-3 mm across. Includes

minor basalt and andesite scattered

throughout stratigraphic sequence,

with diktytaxitic basalt found most

commonly at base of unit.

Erupted from cinder cones and

small shields, most ofwhich have

been destroyed by erosion or

buried by successive lava flows.

Fonns eastward-thickening wedge

nearly 1 kID thick along Oak

Grove Fork of Clackamas River.

Farther east, base of unit is unexposed and maximum thickness is

unknown; buried by younger

rocks east of there. Thins northward towards Salmon Butte,

probably owing to northward-diminishing eruptive activity. Farther north, howeyer, basalt and

lesser basaltic 'andesite of similar

age are widely exposed in the Bull

15

Tha

Tdb

Tba

Ta

Run River watershed. Potassium

argon ages from near base and top

of unit in Oak Grove Fork are

2.92±0.13 m.y. and 2.78±0.09

m.y., respectively (table 1, Nos.

42 and 41). Lower part increases

in age southward, however, because unit interfingers with -4-

m.y. andesite (unit Ta) in the

Clackamas River

Hornblende-bearing andesite

(Pliocene)-Pale- to mediumgray, slightly to moderately porphyritic, hornblende-bearing andesite lava flows. Unit near

Mount Mitchell is displaced approximately 30 m by a north-striking normal fault. Unit near Linney

Butte is small remnant oflava flow

exposed along ridge top

Dacite of Beaver Butte (Pliocene)-Moderately to very porphyritic, two-pyroxene dacite lava.

One analysis; contains 65 percent

Si02 (R.M. Conrey, Wash. State

Univ., unpub. data). Normally

polarized. Beaver Butte diverted

the reversely polarized 1.37-m.y.-

old andesite of Wapinitia Pass

(unit Qaw); thus the Beaver Butte

unit likely was erupted during the

Gauss Normal-Polarity chron or

earlier (prior to 2.60 m.y., time

scale ofCande and Kent, 1992)

Basaltic andesite (Pliocene)--Lava

flows south of Mount Wilson

(south edge of map). Nonnal-polarity magnetization; probably

about same age as dacite ofBeaver

Butte (Tdb) or andesite of Rocky

Point (Tar)

Andesite (Pliocene}-Lava flows of

slightly porphyritic, two-pyroxene
(Stratigraphic units in southwestern part of map area, cant.)

Tar

Tdl

Tys

andesite. Interbedded with lower

part of basaltic andesite of Oak

Grove Fork (Tbao) in the Clackamas River drainage. Continues

south of the map area in Breitenbush Hot Springs quadrangle,

where it correlates with andesite

and dacite ofthe Clackamas River

(Sherrod and Conrey, 1988); latter

unit has K-Ar ages of 4.20±0.10

m.y. (L.B. Gray, USGS, unpub.

data, 1990) and 4.04±0.17 m.y.

(White, 1980b)

Andesite of Rocky Point (Pliocene)---Platy, two-pyroxene andesite. Slightly porphyritic; ferromagnesian minerals « 1 mm) 1-

2 percent in abundance and plagioclase «1 mm) 5 percent. Unit

probably fonned a broad andesitic

shield volcano as interpreted from

the vertical and lateral distribution

of flows. One vent area (shown

patterned) recognized at Rocky

Point (south of Beaver Butte) on

basis of abundant scoria and breccia. Nonnally polarized. Overlain

by dacite ofBeaver Butte (Tdb)

Dacite north of Long Ridge (pliocene)-Domes and lava flows

south of Beaver Butte. Moderately porphyritic; contains fresh to

variably oxidized hornblende phenocrysts ranging from 1 to 2 mm

in length

Younger sedimentary rocks (pliocene and Miocene?)---Sandstone and siltstone exposed beneath the basaltic andesite of Oak

Grove Fork (Tbao). Locally includes minor pebbly conglomerate. Thickness about 30 m. Wellknown for roadcuts in siltstone

rich in fossil flora. According to

J.A. Wolfe (USGS, written com16

Tr

Tma

mun., 1992), the fine-grained

beds contain abundant megafossils

ofAbies (true fir), Pinus (pine),

and Picea (spruce) accompanied

by broad-leaved plants such as

Quercus (oak), Castanopsis

(chinquapin), Acer (maple), and

Arcostaphylos (manzanita). The

fossil assemblage indicates a climate both wanner and drier than

presently found in the area; rainfall

during late Pliocene time may have

been one-halfthe 200 cm per year

that occurs presently (Wolfe,

1990)

Rhyolite (Pliocene or Miocene)---

Slightly porphyritic rhyolite in area

southeast ofBeaver Butte (head of

Butte Creek). Soft-weathering

owing to clay content; unit is hydrothennallyaltered. Forms angular, monolithologic rhyolite float in

roadcuts, all of which have

sloughed to some degree; fonns

light-colored soil elsewhere.

Probably correlative to equally

poorly exposed rhyolite at Hehe

and Sidwalter Buttes, 10 kIn south

of map area. Those buttes were

considered part of the Oligocene

and lower Miocene John Day

Fonnation by Waters (I 968b),

probably because rhyolitic rocks

farther east underlie the Columbia

River Basalt Group. It seems

more likely, however, that the

rhyolite in the Mount Hood quadrangle west of U.S. Highway 26

is younger than the Columbia

River Basalt Group and hence late

Miocene or Pliocene in age

Andesite of middle and late Miocene age (Miocene)---Porphyritic pyroxene andesite lava

exposed near Clackamas River and

between the Salmon River and
(Stratigraphic units in southwestern part ofmap area, cont.)

Still Creek in the western part of

map area. Younger than the

Columbia River Basalt Group, on

basis ofstratigraphic relations immediately west of map area;

potassium-argon age of 11.5±0.2

m.y. from sample along Clackamas River highway (table 1, No.

43)

Tfa Fine-grained andesite (Miocene)-Nearly aphyric mediumgray andesite that forms plugs and

sills(?) or lava flows. Only exposed near Salmon Butte and a

few kilometers to southwest

Tas Andesite of Salmon Butte (Miocene)-Moderately porphyritic

lava flows and volcaniclastic rocks

Tdp Dacite of Plaza Lake (Miocene)--

Light-bluish-gray to light-greenish-gray pyroxene dacite lava.

Plagioclase phenocrysts 3-4 mm

long, 10-15 percent; orthopyroxene phenocrysts only about 1 mm

long and less than 1 percent. Exposed only as small body west of

Salmon Butte (west edge of map).

The name "Plaza Lake" appears on

the High Rock 15-minute and 7.5-

minute quadrangles

Tbam Basaltic andesite of Mack Hall

Creek (Miocene)-Slightly

porphyritic lava flows and minor

near-vent(?) breccia. Olivine, 1-2

mm across and 1-3 percent in

abundance, is characteristically

oxidized to hematitic clays. Locally erupted unit exposed at head

of Mack Hall Creek, probably the

core of a small shield volcano.

Maximum thic~es~ approximately

250m

17

Trh Rhododendron Formation (Miocene)-Andesitic tuff breccia.

Clast petrography varies but

chiefly is slightly porphyritic pyroxene andesite. Unit originated

mainly as pyroclastic flows and

lahars but includes minor fluvial

volcaniclastic sandstone. Age is

middle Miocene; overlies

Columbia River Basalt Group

(units Teg and Tew) and underlies

basaltic andesite of Mack Hall

Creek (unit Tbam) and andesite of

Salmon Butte (unit Tas) in southwest corner of map area. As thick

as 500 m in canyon walls of

Salmon River. Unit can be traced

across canyon walls northward to

village of Rhododendron and beyond nearly to Lolo Pass in

northwest corner of map area.

Thins southward and pinches out

in Oak Grove Fork. Only partly

equivalent to Rhododendron Formation of Hammond and others

(1982), which was more broadly

defined to include andesitic rocks

ranging from middle Miocene to

Pliocene in age west and south of

the map area

Columbia River Basalt Group--

Divided into:

Tewf Frenchman Springs member of

the Wanapum Basalt-Finegrained lava flows with rare plagioclase phenocrysts as large as 2

cm. Normal-polarity magnetization (Anderson, 1978)

Tegn2 Reversed-polarity unit 2 of

the Grande Ronde BasaltVery fine grained lava flows

Tos Older sedimentary rocks (Miocene)-Chiefly cobble-to-boulder

volcaniclastic conglomerate and

sandstone. Commonly found beneath the Columbia River Basalt
Group or at about that stratigraphic

level

Toa Older andesite (Miocene and

Oligocene?}-Slightly to moderately porphyritic andesite and

basaltic andesite(?) lava flows.

Rocks exposed along Clackamas

River are hornblende bearing

STRATIGRAPHIC UNITS EXPOSED IN

NORTHEASTERN PART OF MAP AREA

abdr Basaltic andesite of Dog River

(Pleistocene}-Medium-gray

porphyritic lava that erupted from

cinder cones on Hood River escarpment near headwaters of Dog

River and flowed northwest toward Hood River valley and

northeast down South Fork ofMill

Creek. Distinctive rock owing to

its pale-gray plagioclase phenocrysts, which probably contain

numerous glass inclusions. One

analysis, 56.7 percent Si02 (R.M.

Conrey, Wash. State Univ., unpub. data). Younger than 0.78

m.y., on basis of normal polarity

magnetization, relatively well:~reserved cinder cones, and pOSItIOn

of lava on or near canyon floor.

Potassium-argon age of 3.7±0.2

m.y. (table 1, No. 46) was obtained from sample collected by

Anderson (1987) in South Fork of

Mill Creek north of map area and

seemingly part of the same intracanyon flow as mapped by Newcomb (1969) north of map area.

Anderson's sample JA85022 contains about 65 percent Si02, however, and therefore is an unlikely

equivalent to basaltic andesite of

Dog River a~ defined here

18

aTb Basaltic andesite and basalt

(Pleistocene and Pliocene?}-Lava flows erupted from

vents at Flag and Frailey Points

and in the headwaters of Dog

River along Hood River escarpment. Typically contains 1-2 percent of plagioclase phenocrysts (1-

2 mm) and -1 percent of olivine

phenocrysts (-1 mm). Chemical

analyses of five samples collected

along Hood River escarpment

range from 51.5 to 56 percent

Si02 (Wise, 1969, his numbers

108-112). Lava from Flag Point

vent shows reversed-polarity magnetization and thus is older than

0.78 m.y.

aTa Andesite (Pleistocene or Pliocene}-Lava flow exposed along

a 15-km stretch ofFivemile Creek.

Possesses geomorphic setting

characteristic of an intracanyon

lava flow and a K-Ar age of

1.7±OA m.y. (table 1, No. 44).

Vent area unknown

aTab Andesite of Badger Butte (Pleistocene or Pliocene}-Tw 0-

pyroxene, slightly porphyritic lava

flows. Reversed-polarity magnetization

aTs Sedimentary rocks and deposits

(Pleistocene and Pliocene?}-Poorly indurated or unconsolidated sand and gravel.

Forms broad fans east of ancestral

Badger Creek and White River;

elsewhere is restricted to narrow

lobes derived from smaller

drainages

Taft Andesite of Fifteenmile Creek

(Pliocene)-Medium-gray to

greenish-gray, porphyritic silicic
(Stratigraphic Wlits in northeastern part ofmap area, cant.)

andesite. Blocky plagioclase phenocrysts 2-5 mm across, 12-15

percent. Orthopyroxene much

more abundant than clinopyroxene, but both are less than 1 mm

across and less than 1 percent of

rock. Contains 62.6 percent Si02

(R.M. Conrey, Wash. State

Univ., unpub. analysis of dated

sample). Age is late Pliocene on

basis of K-Ar age of 2.86±0.04

m.y. (table 1, No. 45)

Rhyodacite of Gordon Butte (PIiocene}-Divided into:

Trgx Tuff breccia-Block-and-ash deposits that mantle slopes east of

Gordon Butte and Hootnany

Point. Unit probably originated as

debris from collapse of domes

(Trgd), on the basis of clast similarity, and was perhaps emplaced

well after eruptions ceased. Predates erosion that carved the modem canyons in map area

Trgd Domes and flows--Medium- to

light-gray, light-purplish-gray,

and white rhyodacite and rhyolite.

Massive to thickly flow jointed.

Blocky plagioclase phenocrysts

from 1 to 3 mm across, 10-30 percent in abundance, and commonly

slightly clay-altered and locally

stained with iron hydroxide minerals. Orthopyroxene and clinopyroxene visible locally, less than 1

mm across, and less than 1 percent

Tbah Basaltic andesite of Happy Ridge

(Pliocene}-Lava flow in lower

part of Happy Ridge at east end of

Tygh Valley. One analysis, 55.6

percent Si02 (R.M. Conrey,

Wash. State Univ., unpub. data).

Potassium-argon age of 7.6±0.8

m.y. reported' by· Farooqui and

others (1981b) (table 1, No. 50) is

probably too old; unit is younger

19

Talb

Ttv

than andesite ofJordan Butte (Taj),

from which they reported an age

of 5.1±0.5 m.y. (table 1, No. 48)

Andesite of Little Badger Creek

(Pliocene}-Mediurn-gray porphyritic lava. Varies from platy at

base to vesicular and massive in

upper part. Phenocrysts of plagioclase as large as 3 mm, 3-5

percent; clinopyroxene and orthopyroxene «1 mm) less than 1

percent; and trace amounts of

highly altered olivine as large as 1

mm. Reversed-polarity magnetization. Mainly younger than but

partly interfingered with Tygh

Valley Fonnation

Tygh Valley Formation (Pliocene

and Miocene?}-Volcaniclastic

rocks, chiefly sandstone and conglomerate in eastern part and ashflow and air-fall tuff in western

part. A tuff exposed in the unit

south of Tygh Valley yielded a KAI' age of 4.9±0.5 m.y. (table 1,

No. 47). Similar to the Tygh

Valley Formation of Farooqui and

others (1981 a) but lacks the

basaltic lava flows, which have

been mapped separately. Unit is

lithologically similar to and probably correlative with the Deschutes

Formation of Smith (1986), although it may include rocks

slightly younger than youngest

strata in the Deschutes Formation.

The Deschutes and Tygh Valley

Formations formed in structurally

separate basins, and their primary

volcanogenic detritus probably

was derived from discrete, noncontiguous volcanic fields in the

Cascade Range. Contains increasing amount of primary pyroclastic

material westward towards Cas-
(Stratigraphic units in northeastern part ofmap area. cont.)

cade Range, where pyroclasticrich beds are shown separately as:

Ttvp Pyroclastic and sedimentary

rocks-Partially welded ash-flow

tuff and unwelded air-fall deposits. Includes locally interbedded sandstone and conglomerate.

Deposit west of Friend townsite is

a unwelded rhyodacitic ash-flow

tuff (67.5 percent Si02, R.M.

Conrey, Wash. State Univ., unpub. data) with black pumiceous

lapilli and bombs as large as 30

em. Overlies the Dalles Formation

(Td) along chiefly conformable but

perhaps locally unconformable

contact. Distinguished from the

Dalles Formation by its dominantly pumiceous clastic component, in contrast to the lithic-rich,

pumice-poor Dalles Formation

Tb Basalt and basaltic andesite (Pliocene)-Diktytaxitic olivine

basalt and minor intergranular

basaltic andesite lava flows.

Youngest part of unit is normally

polarized and forms areally extensive exposures southeast of Gordon Butte. Older part, of undetermined magnetic polarity, is interfingered with Tygh Valley Formation

Taj Andesite of Jordan Butte (Pliocene or Miocene}-Platy to

massive, locally vesicular

medium-gray porphyritic andesite

lava flows. Potassium-argon age

of 5.1±0.5 m.y. (table 1, No. 48)

Tbas Basaltic andesite of Sunset

Spring (Miocene)-Fi n egrained, medium-gray lava, commonly with s~alL _clinopyroxene

phenocrysts: Outcrops restricted

to two small areas near Sunset

20

Tdm

Td

Spring in headwaters of Tygh

Creek, where unit underlies the

andesite ofJordan Butte (Taj) and

the rhyodacite of Gordon Butte

(Trgd)

Dacite of Mill Creek Buttes (Miocene}-Hornblende dacite domes

at headwaters of South Fork of

Mill Creek. Chemical analyses of

three samples range from 62 to 65

percent Si02 (Wise, 1969, his

numbers 99-101). Stipple indicates hornblende-bearing volcaniclastic deposits adjacent to main

domal mass. Potassium-argon age

of 6.2±1.3 m.y. (table 1, No. 49)

Dalles Formation (Miocene}-Andesitic to dacitic volcaniclastic

rocks ranging from block-and-ash

deposits in area east of Upper

Hood River Valley to sandstone

and conglomerate with subangular

clasts at east edge of map area.

Much of unit is lithic tuff breccia

that was probably deposited as debris flows. Pumiceous pyroclastic

flows are lacking or unexposed in

map area. Silica content of five

clasts ranges from about 60 to 64

percent (sites in map area and to

north; Gannett, 1982). Stratigraphic section exposed in canyon

walls of Fifteenmile Creek is

dominated by sandstone and conglomerate but contains as much as

30 percent angular-clast debrisflow deposits, some of which

were emplaced hot as suggested

by brownish-orange oxidized tops

and monolithologic clast assemblage. Sample collected from this

locality yielded K-Ar age of

7.73±0.16 m.y. (table 1, No. 52)

using crystals separated from an

andesite clast in one ofthe monolithologic lahar beds
(Stratigraphic units in northeastern part ofmap area, cont.)

Taef Andesite of East Fork Hood

River (Miocene)-Slightly to

moderately porphyritic two-pyroxene andesite lava flows and minor

breccia or tuff breccia. Rocks increasingly iron-stained and altered

southward towards Robinhood

Creek, where they overlie pyritized and moderately altered lava

of the Columbia River Basalt

Group. Rocks moderately to very

fresh north of Polallie Creek.

Potassium-argon ages of 7.0±0.8

m.y. from andesite lava east of

Hood River and 8.18±0.06 m.y.

from stratigraphically lower flow

exposed in new roadcuts along

State Highway 35 (table 1, Nos.

26 and 28, respectively). Interfingers northward with volcaniclastic

strata ofthe Dalles Formation

Tafv Andesite of Fivemile Butte (Miocene)-Slightly to moderately

porphyritic two-pyroxene andesite. Potassium-argon age of

7.71±O.17 m.y. (table 1, No. 51).

May also be unit from which

Bunker and others (1982) obtained

an age of 8.2±O.8 m.y. (see table

1, No. 53); if so, their reported

sample locality is incorrect. Normal-polarity magnetization

lowest beds dip about 40°,

whereas dips of about 5° characterize a capping conglomerate that

contains abundant clasts of the

Columbia River Basalt Group.

Studies elsewhere on Columbia

Plateau suggest that folding occurred mainly between 17 and 11

m.y. ago (for example, Reidel,

1984), and if applicable to timing

of defonnation at Tygh Ridge,

then unit is chiefly or entirely middle Miocene in age. Considered

correlative with lower parts ofEllensburg Fonnation and Simtustus

Fonnation of Smith (1986) (Smith

and others, 1989). Also probably

correlative with the Rhododendron

Formation (Trh, northwest comer

of map), a volcaniclastic unit west

and southwest ofMount Hood that

was emplaced between approximately 14 and 11 m.y. ago.

Queried for exposures ofuncertain

correlation northeast ofMaupin

Columbia River Basalt Group

(Miocene)-Lava flows of

tholeiitic flood basalt erupted from

dike swanns in Columbia Plateau

and Blue Mountains in eastern

Oregon. Only middle Miocene

formations of the group are present in map area. Divided into:

Tvs Volcaniclastic rocks (Miocene)-

Tuff breccia, pumiceous lapilli

tuff, and minor sandstone and

conglomerate. Unit confonnably

overlies Columbia River Basalt

Group and is unconformably

overlain by the Dalles Fonnation

(Td) and Tygh Valley Fonnation

(Ttv and Ttvp). Strata within unit

on south flank of Tygh Ridge anticline were .deposited during

growth ofthe anticline and consequently diminish in dip upsection;

21

Tcwpr

Tcwf

Wanapum Basalt-Divided into

Priest Rapids member-Fineto medium-grained basalt. Reversed-polarity magnetization.

Age about 14.5 m.y. (Rockwell

Hanford Operations, in table 3 of

Tolan and others, 1989; no error

assigned to age). On Columbia

Plateau at northeast comer ofmap

area

Frenchman Springs member-Slightly porphyritic lava,

typically with a few scattered
glomerophyric clots of plagioclase. In East Fork Hood River

valley and on Columbia Plateau.

Normal-polarity magnetization

(Swanson and others, 1981)

Grande Ronde Basalt-Divided into

Tcgn2 Normal-polarity unit 2-

Aphyric to very slightly porphyritic lava. In East Fork Hood

River valley and on Columbia

Plateau. Potassium-argon age of

15.0±0.3 m.y. (table 1, No. 54)

from normally polarized sample

collected by Watkins and Baksi

(1974) near Tygh Ridge

Tcgr2 Reversed-polarity unit 2-

Aphyric to very slightly porphyritic lava. Only on Columbia

Plain at east edge of map. Potassium-argon age of 16.8±OJ m.y.

(table 1, No. 55) from reversely

polarized sample collected by

Watkins and Baksi (1974) near

Tygh Ridge

STRATIGRAPHIC UNITS EXPOSED IN

SOUTHEASTERN PART OF MAP AREA

CTs Sedimentary rocks and deposits

(Pleistocene and Pliocene)-

Poorly indurated sand and gravel

in southeast corner of map area.

Weathers to form hillslopes mantled by pebbles and cobbles. Oldest part of unit near Tygh Valley

probably includes rhyodacite

breccia of Gordon Butte (Trgx) and

is partly of Pliocene age. Narrow

lobe of sedimentary deposits near

the junction of State Highway 216

and U.S. Highway 26 overlies

1.37-m.y. andesite of Wapinitia

Pass (unit Qaw) and terminates

abruptly against reversely polarized andesite of ~CubbinsGulch

(unit Qam); "it probably indicates

ancient drainage eastward towards

22

Juniper Flat through area now occupied by andesite of McCubbins

Gulch. Sedimentary deposits

south of Laughlin Hills are older

than 1.37-m.y.-old andesite of

Wapinitia Pass, because they extend from beneath an even older

basalt (unit QTb). These deposits

continue south ofmap area, where

they overlie prairie-forming basalt

with K-Ar age of 3.21±O.14 m.y.

(table 1, No. 58). Sedimentary

unit there could be outwash, but

its relatively old age would correspond to glacial episodes that are

undocumented from the Cascade

Range in Oregon

CTb Basalt (Pleistocene and Pliocene)-Slightly porphyritic

olivine basalt. Commonly shows

reversed-polarity magnetization

Cam Andesite of McCubbins Gulch

(Pleistocene)-Lava flow or

flows characterized by distinctive

plagioclase phenocrysts, which are

2-3 mm across and equant in outline (10 percent); waxy olivine

phenocrysts less than 1mm across

(1 percent), and traces of finegrained clinopyroxene. Chemical

analysis from sample at east edge

of flow near Pine Grove contains

59 percent Si02; unit probably

varies from andesite to basaltic andesite in composition. Reversedpolarity magnetization. Early

Pleistocene, approximately same

age as -1.37-m.y. andesite of

Wapinitia Pass (unit Qaw)

Cabl Andesite of Boulder Lake (Pleistocene)-8lightly porphyritic

lava flows. Normal-polarity

magnetization. Only exposed west

of Boulder Lake (head of Boulder
(Stratigraphic units in southeastern part ofmap area, cont.)

Creek), west of Grasshopper

Point

QTbg Basaltic andesite of Grasshopper

Point (Pleistocene or Pliocene)-Porphyritic, olivinebearing lava flows. Commonly

contains plagioclase phenocrysts

2-3 mm across, 10 percent; some

flows contain orthopyroxene and

clinopyroxene as microphenocrysts. Forms large shield volcano with summit at Grasshopper

Point. Silica content ranges from

54 to 58 percent on basis of two

analyses (R.M. Conrey, Wash.

State Univ., unpub. data). Reversed-polarity magnetization.

Age probably Pleistocene but only

known to be older than 0.78 m.y.,

and a late Pliocene age cannot be

ruled out

Tbj Basalt of Juniper Flat (Pliocene)-Diktytaxitic (open-textured) olivine basalt lava flows.

Normal-polarity magnetization.

Potassium-argon age is 2.77±0.36

m.y. (R.M. Conrey, Wash. State

Univ., unpub. data)

Tdgb Dacite of Graveyard Butte (Pliocene? or Miocene)--Domes of

intermediate to silicic, slightly

porphyritic lava. Includes two

domes 4 kIn east of Graveyard

Butte. Stipple indicates dacitic

cinder cone that underlies Graveyard Butte and adjacent area

Columbia River Basalt Group

(Miocene)--Divided into:

Tewf Wanapum Basalt, Frenchman

Springs member-Normal-polarity magnetizatio'n (Swanson and

others, 1981). May rest on thin

tuffaceous sandstone or siltstone

23

Tomt

Toma

Tomr

that locally overlies Grande Ronde

Basalt

Grande Ronde Basalt-Description from Swanson and others

(1981): Aphyric to very sparsely

plagioclase-phyric lava flows.

Generally fine-grained and petrographically nondistinctive. Flow

thickness commonly 15-25 m but

ranges from 1 m to more than 50

m. Divided into:

Normal-polarity unit 2

Reversed-polarity unit 2

Older volcanic rocks of the Mutton Mountains (Miocene and

Oligocene)-

Tuff and tuffaceous sedimentary rocks (Miocene)--Rhy-

~Wc~~Wlmdw~~~dtuff

and tuffaceous sandstone (Waters,

1968a)

Andesite (Miocene and

Oligocene)-Chiefly platy

aphyric lava flows. Contains minor pyroclastic and epiclastic

strata. Potassium-argon age from

interbedded sandine- and biotitebearing ash-flow tuff is 29.3±0.6

m.y. (table 1, No. 59)

Rhyolite (Miocene and

Oligocene)-Lava flows and

domes
EXPLANATION OF MAP SYMBOLS Lineament-Interpreted from aerial

photographs

II 111111111

• •

-tt t

t t

Contact-Approximately located.

Shown very ShOl1 dashed where

inferred; shown dotted to enclose

a zone of propylitic alteration between Salmon River and Still

Creek as mapped by Wise (1969)

Contact-8eparating sequentially

younger deposits of Polallie eruptive period in and adjacent to

Newton Creek east of Mount

Hood (unit Ohpc). Contact labeled

to show relative age of deposits: Y,

younger; 0, older

Contact-Bounding unmapped area in

Salmon Creek drainage and poorly

mapped area in Mutton Mountains

Fault-8howing dip locally; dashed

where approximately located or

inferred, dotted where concealed.

Ball and bar on downthrown side

Reverse or thrust fault-Dashed

where approximately located or

inferred; dotted where concealed.

Teeth on upper plate

Anticline-8howing crestline and locally showing plunge

Syncline-8howing troughline

Monocline-8hown only on Columbia

Plateau and on east wall of Hood

River escarpmentAbrupt decrease of dip in direction

of arrows

Abrupt increase of dip in.direction of

arrows

24

00000

19

-J-

{B'

-P-

*

x R

-5

Moraine crest-8hown in glacial deposits (unit Og)

Strike and dip of clastic strata or

layering in basalt flows

Inclined

Horizontal

Overturned--Found in lava flows of

Columbia River Basalt Group on

Tygh Ridge

Dike--Tick marks indicate composition; two ticks, basalt or basaltic

andesite; three ticks, andesite or

dacite

Intermediate-size volcan0-8howing

central buildup of basalt or

basaltic andesite shield volcanoes.

Many occurrences interpreted on

basis of landfonn, owing to lack

of dissection

Thermal remanent magnetization

(TRM) measurement-N, normal

polarity; R, reversed polarity

Potassium-argon sample locality-8howing map number (table

I). Samples from bedrock exposures except No. 31, which is from

drill core. Nos. 46, 57, and 58 are

from outside map area and not

plotted on map. Thermal remanent magnetization indicated

by R (reversed polarity) or N

(nonnal polarity) where known

Drill hole locality-8howing name or

number for holes deeper than 1
km. Two holes west of Mount

Hood in upper reach of Old Maid

Flat (OMF-I and OMF-7A) and

one hole on south flank of Mount

Hood (Pucci). Locations from

Priest and others (1982) and Hook

(1982). Symbol for OMF-7A

nearly filled by dot showing K-Ar

sample locality No. 31

25

Snow and ice--Showing extent of

major glaciers. Limited to higher

elevations of Mount Hood.

Names of separate glaciers shown

where space allows
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Geology and geothermal resources of the BreitenbushAustin Hot Springs area, Clackamas and Marion Counties, Oregon: Oregon Department ofGeology and Mineral Industries Open-File Report 0-88-5, p. 1-14.

Sherrod, D.R, and Pickthorn, L.G., 1989, A note on the

origin of Bull Run and Lost Lakes, Western Cascades,

Oregon: Oregon Geology, v. 51, no. 3, p. 60.

Sherrod, D.R., and Smith, J.G., 1989, Preliminary map of

upper Eocene to Holocene volcanic and related rocks in

the Cascade Range, Oregon: U.S. Geological Survey

Open-File Report 89-14, scale 1:500,000.

Smith, G.A., 1986, Simtustus Formation: Paleogeographic and stratigraphic significance ofa newly defmed

Miocene unit in the Deschutes basin, central Oregon:

Oregon Geology, v. 48, no. 6, p. 63-72.

Smith, G.A., Bjornstad, B.N., and Fecht, K.R., 1989,

Neogene terrestrial sedimentation on and adjacent to the

Columbia Plateau; Washington, Oregon, and IdallO, ill

Reidel, S.P., and Hooper, P.R., eds., Volcanism and

tectonism in the Columbia River flood-basalt province:

Geological Society of America Special Paper 239, p.

187-198.

Steiger, R.H., and Jager, E., 1977, Subcommission on

geochronology: Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, no. 3, p. 359-362.

Swanson, D.A., Anderson, J.L., Camp, V.E., Hooper,

P.R., Taubeneck, W.H., and Wright, T.L., 1981, Reconnaissance geologic map of the Columbia River

Basalt Group, northern Oregon and western Idaho: U.S.

Geological Survey Open-File Report 81-797, scale

1:250,000.
Swanson, D.A., Wright, T.L., Hooper, P.R, and Bentley,

RD., 1979, Revisions in stratigraphic nomenclature of

the Columbia River Basalt Group: U.S. Geological

Survey Bulletin l457-G, 59 p.

Taylor, J.R., 1982, An introduction to error analysis:

Mill Valley, Calif., University Science Books, 270 p.

Tolan, T.L., Reidel, S.P., Beeson, M.H., Anderson, J.L.,

Fecht, K.R., and Swanson, D.A., 1989, Revisions to

the estimates of the areal extent and volume of the

Columbia River Basalt Group, in Reidel, S.P., and

Hooper, P.R, eds., Volcanism and tectonism in the

Columbia River flood-basalt province: Geological Society of America Special Paper 239, p. 1-20.

Vandiver-Powell, Lorraine, 1978, The structure, stratigraphy, and correlation of the Grande Ronde basalts on

Tygh Ridge, Wasco County, Oregon: Moscow, University of Idaho, M.S. thesis, 57 p.

Vogt, B.F., 1981, The stratigraphy and structure of the

Columbia River Basalt Group in the Bull Run watershed, Multnornah and Clackamas Counties, Oregon:

Portland. Oreg., Portland State University. M.S. thesis,

151 p.

Waters, A.C., 1968a, Reconnaissance geologic map ofthe

Dufur quadrangle, Hood River, Sherman, and Wasco

Counties, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-556, scale

1:125,000.

---1968b, Reconnaissance geologic map ofthe Madras

quadrangle, Jefferson and Wasco Counties, Oregon:

28

U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-555, scale 1:125,000.

Watkins, N.D., and Baksi, A.K., 1974, Magnetostratigraphy and oroclinal folding of the Columbia River,

Steens, and Owyhee Basalts in Oregon, Washington,

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p. 148-189.

White, Craig, 1980a, Geology and geochemistry of Mt.

Hood volcano: Oregon Department of Geology and

Mineral Industries Special Paper 8, 26 p.

---1980b, Geology of the Breitenbush Hot Springs

quadrangle, Oregon: Oregon Department of Geology

and Mineral Industries Special Paper 9, 26 p.

Wise, W.S., 1969, Geology and petrology of the Mt.

Hood area: A study of High Cascade volcanism: Geological Society of America Bulletin, v. 80, no. 6, p.

969-1,006.

Wright, T.L., Mangan, Margaret, Swanson, D.A., 1989,

Chemical data for flows and feeder dikes ofthe Yakima

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71 p.

Wolfe, J.A., 1990, Estimates of Pliocene precipitation and

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Geological Survey Open-File Report 90-64, p. 39-42.
iu ount Hood 30 b 60' quadra

[Samples gro ped ccording to m p se tor nd arranged in orde of incre sing age within ea h gr up. Ages calculate usi g c nstants for radioact ve d ca

recomme ded by e International Uni n of Geological Science Sub 0 ission on Geochron logy (Steiger and Jager, 197 ); th refore, ages may iffer sli

referenc. F r m Itiple K20 det rmin tion , value in parenthe s is arit metic mean used i age calculation. Assi_ ed a e is the calculated ag"e from ea

multiple extra tion are listed; in thos cas s the assigned age is th we ghted mean age (w ighti g proportional to e in erse of the variances 0 ind vid

1982). ges lack ng map numbe s ar of ncertain location. Man sa pie locations were erifi d and remeasured y us usin original air photo (sa p

location maps (fie d sheets; Keith and oth s, 1985), or publi ed ap ( riest and others, I 82, fig. 5). Sample 10 ations ar reported to neares 0.0 m

minutes) exce t fo less precisely loca d s mples, which are s own to n arest 0.1 minute (e or ay be several tenth of minu es in some cases). Fo mo

sample-s ecifi rmation. The age of tratigraphic units a e fu ther elucidated in the escri tion of Map Units

Lncorim 40Arraj Calculated Map Latitude Long tude General geographic I cale Materi I K20 (10- 11 Ipe+nt age No. (N) (V ) and map unit 1< ock type dated (f,t percent) molesf ) 40A rad (Ma) ( a) 

Ages lor 1 Il art of map area I 8 MHI 45°27.20' I 1·40 25' The Pinnacle, Qbap Basaltic Whole rock 0.95 0.01994 1.9

T4±0102

andesite

2 45°20.58' °44l58' 1 Moum Hood, Qha 1 1 1 Andesite Whole rock 1.45 0.38±0 02 

0.07485 9.0 0.36±0.03

0.08857 7.4 0.42±0.04

N 3 I 45°23.25' 1ft 043183' I Mount Hood, Qha1 I I I Andesite Whole Irock I 1.03 01.45±01.02 

I.D 0.05831 48 0.39±0.05

0.06323 72 0.43±0.04

0.07618 19.4 0.51±0.04

4 7JZZIOk8 45·20.58' 161.44l58' I Mount Hood, Qha 1 1 1 Andesite Whole 1rock 1 1.56 Q.49±01.04 

0.1017 49 0.45±0.060.1155 7.9 0.51±0.05

5 8lMH2$ 45·20.63' °441.48' 1 Mount Hood, Qha 1 1 1 Andesite Whole 1rock 1 0.96 Q.53±01.04 

0.03959 56 0.29±0.08

0.07909 115 0.57±0.05

0.07840 32 0.57±0.10

0.1139 93 0.82±0.13

6 8lMHIO 45°24.30' 10 391.23' 1 Cloud Cap, Qbac 1 1 1 Basaltic Whole I rock I 1.09 q.59±01·03 

andesite 0.07701 48 0.49±0.06

0.09822 88 0.63±O.07

0.1015 158 0.65±0.05

Footnotes1at ebd ofl tabular data

I I I I I I I I
....,

o

T"c.ti,," 40Arrad Calculated Assi ne

Map Sa nple Latitude Longitude ( eneral geographic I cale Materia K10 (10- 11 Perc nt age age

No. no. (N) (V. ) and map unit R ck type dated ("t percent) moles/g 40Ar ad (Ma) (M) 

7 79 MOO 8 45°20.62' I 1°47. 3' Devil Canyon, Qbdc Basaltic Whole ock 0.78 o71±0 04 

andesite 0.07042 4. 0.63±0.06

0.08838 8. 0.79±0.06

8 82 I1H2 45°26.10' I 1°43. 5' Vista Ridge, Qbr Basaltic Wbole ock 1.05 1 12±0 03 

andesite 0.1547 9. 1.02±0.04

0.1839 7. 1.22±0.04

9 79 GVl IIA 45°22.97' 1 ]°43. 3' Sandy Glacier volca 0, Andesite Whole ock (1.08) 127±0 02 

Qsg 1.08 0.1769 7. 1.14±0.05

1.08 0.1944 10. 1.25±0.05

1.07 0.2015 23. 1.30±0.03

0.2046 18. 1.32±0.05

10 79 llil0 2 45°26.50' 1 1°43. 0' Blue Ridge, Qbr Basaltic Whole ock 1.07 127±0 03 

andesite 0.1679 19. 1.09±0.06

0.1750 16. 1.14±0.06

0.2273 21. 1.48±0.05

11 79 Pll09 45°25.30' 1 1°49. 0' West side of Lola Pa s, Basalt Whole ock 0.81 137±0 10 

QTb 0.1517 6. 1.30±0.13

0.1717 13. 1.47±0.15

12 79 FOO(4 45'23.95' I 1'35. 0' Tamanawas Falls, Andesite Whole ock 1.54 180±0 04 

Qha? 0.3730 6. 1.68±0.06

0.4165 9. 1.88±0.05

13 79 MOO 6 45°20.78' 1 1°51. 7' Horseshoe Ridge, TI bu Basaltic Whole ock 1.32 239±0 07 

andesite 0.4263 34. 2.24±0.1O

0.4891 36. 2.57±0.11

14 79 ZLI( 85 45°21.05' 1 1°48. 5' Horseshoe Ridge, T bu Basaltic Whole ock 0.78 248±0 04 

andesite 0.2695 14. 2.40±0.05

0.2874 27. 2.56±0.07 

0.3684 16. 3.28±0.21

15 S5 47 45°25.62' I 1°55. 4' Thimble Mountain, bbu Basalt Whole ock (1.619) 254±0 05 

1.612 0.5998 41. 2.57±0.07 

1.608 0.5860 46. 2.51±0.07

1.633

1.624
w

Tne.';, 40Arr d Calculated Ass gn

Map S mpl Latitude Lon itud General geographic ocale Materi I Kp (10- 1 Per ent age age

No. no. (N) (N) and map unit ock type datec (""t percent) moles/g) 40Arrod (Ma) a) 

16 7 SWC 0010 45°23.72' 21 °3 .55' East Fork Hood Riv"r Andesite Whole rock 1.69 0.6668 44.3 .74± .03 

(escarpment), Tl

17 70 45°20.5' 21°3 .6' West end of Looko t Andesite Whole rock 2.13 0.95 .1±0.2 

Mountain. Tlma 

18 97 45°23.0' 21 °4 .9' Sandy Glacier vole Andesite Whole rock 1.62 0.775 .2±0.3 

Qsg 

19 49 45°18.9' 21°3 .6' Gunsight Bune, Tr g Andesite Whole rock 1.53 0.925 .1±0.6 

20 7 IC2039A 45°17.22' 21°3 .50' Upper White River cany n, Diorite Plagiof:lase 0.51 0.3869 .9 .26± .44 

dike cuning unit Trbg

21 8 MHI 45°25.50' 21°4 .20' Vista Ridge, Qbr Basaltic Whole rock 0.55 0.4314 .5 .44± .19

andesite

22 72 45°21.03' 21°51 .97' Horseshoe Ridge, bbu Basaltic Whole rock 1.45 1.175 .5±0.7 

andesite 

23 33 45°25.64' 21°41 .16' Lolo Pass, Taop Andesite Whole rock 1.20 1.025 .8±0 8 

24 7 ZZL 089 45°21.05' 21°4! .20' Horseshoe Ridge, bbu Basaltic Whole rock 0.26 .23± .23 

andesite 0.2315 I .2 6.17±0.29

0.2376 .7 6.34±0.39

25 7 LPI 08 45°25.13' 21°4 .77' Lolo Pass, Taop Andesite Whole rock 1.22 .25± .11 

1.072 4 .0 6.09±0.22

1.108 5 .2 6.30±0.12

26 40 45°25.4' 21°3 .0' Above Polallie ca nd, Andesite Whole rock 1.33 1.35 .O±O 8 

Taef 

27 H ornb ende 45°22.5'3 21 °51 .8,3 Zigzag Mountain a ea, Andesite Horn lend 0.35 0.350 ±23 

Trh clast 

28 7 EFO! 16A 45°26.52' 21°3 .47' East Fork Hood Rh er, T ef Andesite Whole rock 1.3 1.535 6(,2 .18± .0

- 10 509 Roadcut on .S. Hwy 26 West of Govt. CamJ, Quartz Horn lend 0.29 0.3379 I .3 .I±O 6 

Tilh diorite 
W

N

Tn"hn. 40Arra Calculated Assi ne

Map Sa mple Latitude Long tude geographic I cale Material K20 (10- 11 Pen ent age aj e

No. o. (N) ('i ) and map unit Rpck type dated ( t percent) 40A rad (Ma) (Ma) 

.- Ite 512 Roadcut on L .s.H f'y 26 West of Govt. Camp, Quartz Hornb ende 0.31 0.3885 13.6 8.6±0. 

Tilh diorite 

29 79 HIO 8A 45°18.35' I 1°48 87' Laurel Hill, Tilh Quartz Whole rock 0.67 8.66±0 II 

diorite 0.8316 27.3 8.60±0.14

0.8458 27.0 8.75±0.18

30 80r FOO 2A 45°24.03' I 1°49 55' Last Chance Mount in. Andesite Whole rock 0.90 9.04±0 04 

Tazm 0.9961 47.6 7.67±0.10

1.128 43.5 8.69±0.09

1.255 48.5 9.66±0.07

1.256 67.8 9.67±0.11

31 aT F.7A -375( 1 45°23.74' I 1°48 42' Old Maid Flat, Tilh Quartz Hornb ende 0.230 0.309 15 4 9.30±0 87 

Irr-22 (3,750·ft depth in a ) diorite 

32 M ·191 H-47 45°23.74' I 1°49 56' Last Chance Mount Andesite Whole rock 0.945 1.300 54 9.5±2. 

lava flow in unit rh 

33 81MHI4 45°20.66' I 1°54 32' W. end Zigzag Mtn Andesite Whole rock 0.95 10 58±0 08 

lava flow in unit rh 1.449 55.8 10.56±0.12

(type section) 1.152 60.3 10.59±0.11

34 M ·101 T-22 45°23.81' I 1°48 70' Last Chance Mount in, Basaltic Whole rock 0.956 1.469 364 106±O. 

lava flow in unit rh andesite 

35a LCM/AI ·48 45°23.84' I 1°49 87' Last Chance Mount in. Andesite Whole rock 0.965 1.460 484 105±O. 

Tazm 

35b L( MID -240 45°23.84' I 1°49 87' Last Chance Mount in, Andesite Whole rock 0.965 1.495 32 4 107±0. 

Tazm 

36 45°23.03' I 1°52 50' Lolo Pass road, Andesite Whole rock 0.98 II 26±0 08 

lava flow in unit rh 1.581 66.8 11.I7±0.12 

1.605 53.6 11.34±0.12

37 42 45°16.19' I 1°47 55' Still Creek phase of Laur I Quartz Whole rock 2.14 3.65 116±1. 

Hill intrusion, Tilh diorite 

Footnotes at en of tabular data
Map S4mplel

No.

Latitude

(N) ("VI)

General geographic locale

and map unit ock type

Materi41

dated

K20

({vt percent)

40Arrad

\ 10- 1

moles/e)

Perfent

Calculated

age

(Ma)

a e

( a) 

38 MH·20/UT·2l!8 45·23.70' Last Chance Mounjain,

lava flow in unit

Dacite Whole Irock 2.28 3.983 1041 1i.1±1. 

Ages for I dart I of

39 Grww-21-85 I 45·01.73' &b ·521.57' map area NW of Peavine Mtn'l

QTb

Andesite Whole Irock 0.916 0.2029 2 .54±O.07 

40

41

S88-22

S$-26

45·19.65'45·09.44' 21·5(1.39'

Hunchback Mounta,n,

Tbbu

Near Frazier Forks,

Tbao

Basalt

Basaltic

andesite

Whole Irock I (0.715)

0.723

0.7160.712

0.708

Whole Irock I (0.848)

0.8430.843

0.846

0.858

0.2671

0.2722

0.34680.3324

221. 8

18.2

32

1.

8

19.0

2.59±0.08

2.64±0.08

2.84tO.08

2.72±0.08

.62±O.06

.78tO.06

\N\N 42 GrwW-II-85 45·03.95' Kink Creek quarry,

Tbao

Basaltic Whole I rock

andesite

1.012 4.2637 III .92±O.13 

43 S88-26 45·01.60' Clackamas River, Tfna Andesite Plagioflase I (0.249)

0.250

0.2510.251

0.244

0.409

0.421

25

1.

1

32.6

11.4±0.34

11.7±0.35

lU±O. 

Ages

44

ror I dart I of

JA85023 I 45·29.52' "121·ld.98'

map area

Fivemile Creek cartyon5 Basaltic

andesite

Whole Irock 1.078 0.265 .7±0. 

45 S88-25 45·20.45' Cold Point,

Taft

Andesite Plagioflase I (0.991)

0.9840.986

0.9960.996

0.41760.3920

0.4122

6 .7

38.7

2.93±0.07

2.75tO.07

2.89tO.07

.86±O.06 

Footnotesl at efid o( tabular data
w

.j:-

I 40Arra Calculated Ass gn

Map Sample Laiitude Long itude General geographic Ipcale Materi I K20 (10-11 Per ent age age

No. o. (N) ( II) and map unit Fock type dated percent) moles! ) 40A rad (Ma) a) 

46 J 8502 45'31.35' I 1'19.67' South Fork Mill ere ek, Dacite Whole rock 3.043 1.620 .7tO. 

(east of ma are ) Qbmc?6

47 H6-WB 45'14.7' I 1'10.8' Tygh Valley. Tuff Plagio lase 1.592 .0±0.57 

Ttv 1.048 914 4.6 

1.260 1604 5.5

48 H9-WB 45'20.5' I 1'20.5' West of Friend Andesite Whole rock 1.715 .3±0.5 7 

Taj 1.280 91 4 5.2 

1.328 6 34 5.4

49 P -MCI-I 45'27.00' I 1'31 .92,8 Mill Creek Bunes, Dacite Homb end. 0.271 0.1384 37 .2±1.3 

Tdm 

50 ICI-WB 45'16.5' I 1'15 6' North of Happy Rid e, Basaltic --9 __9 __ 9 __ 9 __9 .6±0.810 

Tbah andesite 

51 S 8-23 45'24.39' I 1'27 78' Fivemile Butte. Dacite Plagio lase (0.224) .71± .17 

Tafv 0.221 0.2525 285 7.81±0.24

0.220 0.2459 337 7.61±0.23

0.226

0.229

52 S 8-24 45'23.26' I 1'20 05' Fifteenmile Creek, Dacite Plagio lase (0.250) .73± .16 

Td (lahar clast) 0.249 0.2780 346 7.70tO.23

0.249 0.2797 28 I 7.75±0.23

0.255

0.248

53 IU3-WB 45'24.0,11 I 1'25.5'11 Eightmile Crossing, Andesite Whole rock 1.028 .2±0 87. 

Tafv 1.435 1344 9.7 

.995 23 54 6.7

wl2 

54 I th fl 45'17.4' I 1'10 8' Butler Canyon (Tyg Ri, ge Basalt Whole rock 1.84 (3.990) (67 9) I .O±O. 

anticline), Tcgn2 3.972 660 14.9

4.007 698 15.0

Footnotes at e d of tabular data
Tnc.';m 40Arra Calculated Assi ne

Map Latitude Long tude General geographic Ipcale Materi I KzO (10. 11 Perc ent age a e

No. (N) (V ) and map unit Fock type dated ( t percent) ) 40A rad (Ma) (M a) 

55 18th flcl>w lZ 45°16.4' I 1°11 3' Butler Canyon (Tyg Ri ge Basalt Whole rock 1.89 (4.596)1 (82. 8)13 16.8±0. 

I anticline), Tcgr2 4.596 80.5 16.8

4.596 85.0 16.8

5.176 34.1 18.9

Ages for 1southeastern ptrt 1of Imap area 56 S -44 45°09.43' I 1°39 15' U.S. Highway 26. I Andesite Whole Irock I (1.427)

I

11·37±01·04 

Qaw 1.417 0.2741 I 30.17 1.33±0.04

1.439 0.2905 51.2 1.41±0.04

1.419

1.433

57 twt" tt Schoolie Pasture, Basalt Whole rock 1.44±0.09 

(south of pare) QTb 

58 S -13 44°52.3' I 1°25 I' Mill Creek bridge, Basalt Whole rock 

(southeast of map area) Tb 

59 S,-28

I I

45°03.91' Ir027190' I Mutton Mountains'1 I Rhyolitic SOda,tnidite(7.08) 82.18 

'-" interbedded in u it To b ash-flow 7.14 30.11

V1 tuff 7.15

7.02

7.02

I(No. d others (1985», but our work indicates it is od Iy JOInted andesite lava fl!W tt 

Z(No.

3(No. 27) atitu e and longitude from Fieb laces the sample in ast reek drainage. In c ntras, ori inal location data pi ce t is 

bed in Lo t Cr ek" (Wise (1969, foot ote 0 table 2). We use he C st Cr ek site but, regardless age is too young.

4(Nos. 31, 2, 3 , 35a, 35b, 38, 47, 4 ,53 An earlier compilati n (F ebelk m and others (1983» inco ectly listed the perc almO pheric argon as perc nt ra 'o

S(No. 44) Irati aphic unit unce ain. Age discussed further in f Map Units for and site f Fivemile Butte (u )

6(No. 46) ee D scription of Ma Uni for discussion of this s pie' age and stratigraphic assi omen.

7(Nos. 47, 8, 5 ) Age is arithm tic ean a d, as calculated here is s ightl older (0.2 mooY.) th that reported by Bunker d ot ers

8(No. 49) ampl location corrob rated by . E. Hammond (writt n co un, 1986) and modified sligh ly from that reported by iebelll.orn and others (1983".

9(No. 50) ata ever published.

W(No. 0) A e to old. Unit over! es st ata w th K-Ar ages of 5.1± .5 M and .9±0.5 Ma.

II(No. 53) S pie is mislocated or age s t old. Location is in Plioc ne or Pleistocene basalt butrWithlf 600 m of Miocene£deSile.

IZ(Nos.. 54, 5)

°

geologic map, samp e loc tion is plotted to ma ch g ology Published sample I catio map (Watkins and aks; I974)b their fig. 10) is at Ivery 

loca IDg e s mp les.

B(No. 54) alue' parentheses is aver ge fr m two extractions. he 18.9-M age for No. 54 app entl was discarded by W tkins and aksi (1974).