Alsea Formation an Oligocene 

Marine Sedimentary Sequence 

In the Oregon Coast Range

GEOLOGICAL SURVEY BULLETIN 1395-F

Alsea Formation an Oligocene 

Marine Sedimentary Sequence 

In the Oregon Coast Range

By P. D. SNAVELY, JR., N. S. MAcLEOD, W. W. RAU, 

W. O. ADDICOTT, and J. E. PEARL

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1395-F

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1975
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Alsea Formation, an Oligocene marine sedimentary

sequence in the Oregon Coast Range. 

(Contributions to Stratigraphy) (Geological Survey

Bulletin 1395-F) 

Bibliography: p. 20-21. 

Supt. of Docs. No.: I 19.3:1395-F 

1. Geology, Stratigraphic-Oligocene. 2. Sediments (Geology)-Oregon-Coast

Range. I. Snavely, Parke Detweiler, 1919- II. Series. III. Series: United

States Geological Survey Bulletin 1395-F. 

QE75.B9 No. 1395-F [QE693] 557.3'08s [551.7*8] 74-34338

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington, D. C. 20402

Stock Number 024-001-02605
CONTENTS

Page

Abstract ____________________________________________ Fl

Introduction ___________________________________________ 1

Acknowledgment _____________________________________ 5

Geologic setting _____________________________________ 7

Physical characteristics ______________________________ 8

Age and depositional environment ____________________________ 10

Regional correlations and paleogeotogy _________________________-___ 18

References cited _____________________________________ 20

ILLUSTRATIONS

Page

FIGURE 1. Bedrock geologic map of the central Oregon coast ___________ F2

2. Geologic map showing distribution of the Alsea Formation at its type

locality on Alsea Bay _________________________ 4

3. Geologic map showing the distribution of the Alsea Formation at its

reference section along the Yaquina River ______________ 6

4. Photograph of tuffaceous siltstone and sandstone of the Alsea Formation

at its type section on the north side of Alsea Bay ___________ 9

5. Photomicrograph showing the abundant vesicular glass fragments and 

glass shards that characterize siltstone and sandstone of the Alsea 

Formation _________________________________ 11

6. Photograph showing Oligocene mollusks from the Alsea Formation,

Lincoln County __________________________________ 14

7. Paleogeologic map of western Oregon and Washington in Oligocene time 19

TABLE

Page

TABLE 1. Chemical analyses of selected sedimentary rocks from the Alsea

Formation _________________________________ Fll

CONTRIBUTIONS TO STRATIGRAPHY

ALSEA FORMATION 

AN OLIGOCENE MARINE SEDIMENTARY

SEQUENCE 

IN THE OREGON COAST RANGE

By P. D. SNAVELY, Jr., N. S. MacLEOD, W. W. RAU, 

W. O. ADDICOTT, and J. E. PEARL

ABSTRACT

The Alsea Formation, named in this report, crops out in a 65-km-long arcuate belt 

along the central Oregon coast, where it occurs in the upper part of a 6,000-m-thick 

Tertiary marine sequence that forms the Coast Range. The Alsea Formation consists 

principally of marine tuffaceous siltstone and very fine grained sandstone but also 

contains interbedded mudstone, glauconitic sandstone, fine-grained tuff, and reworked 

lapilli tuff. Abundant molluscan and foraminiferal faunas indicate an early (upper 

Refugian "Lincoln" Stage) to late (Zemorrian lower "Blakeley" stage) Oligocene age 

and an upper bathyal and sublittoral depositional environment. The Alsea Formation 

ranges in thickness from 50 to 1,100 m. It shows an inverse thickness relation to deltaic 

sandstone of the overlying upper Oligocene and lower Miocene Yaquina Formation that 

results partly from lateral interfingering of the upper part of the Alsea with the lower 

part of the Yaquina. The Alsea Formation and correlative units in western Oregon and 

Washington formed along a continental shelf that lay west of a volcanically active 

ancestral Cascade Range.

INTRODUCTION

Marine sedimentary rocks of Oligocene age form a westwarddipping arcuate outcrop belt open to the Pacific Ocean along the central Oregon coast (fig. 1). A revision of Oligocene stratigraphic 

nomenclature in this area has awaited detailed geologic mapping 

(Snavely and others, 1975a, b, c) in order to demonstrate the regional 

distribution of lithologic units. This report describes and names the 

sequence of tuffaceous siltstone and fine-grained sandstone that forms 

the lower part of the Oligocene sequence of coastal Oregon. These 

strata are here named the Alsea Formation for excellent exposures 

along the shores of Alsea Bay. The type section is designated as 

exposures along the north side of Alsea Bay between points A and B on 

the geologic map (fig. 2) and on the south side of the bay between C and 

D. A precise stratigraphic correlation is difficult to make between the

Fl
F2 CONTRIBUTIONS TO STRATIGRAPHY

124°00'

44°45' -

44°30' -

EXPLANATION

Cape Foulweather Basalt

[TWC I

Sandstone of Whale Cove

i- Depoe Bay Basalt

Astoria Formation

Nye Mudstone

JTyqj 

Yaquina Formation

Alsea Formation

Basaltic sandstone

Yachats Camptonitic 

Basalt volcanic rocks

5" _ Nestucca Formation

Yamhill Formation

Tyee Formation

Siletz River Volcanics

Contact, approximately located 

or inferred

Fault, approximately located 

Dashed where inferred

Intrusive rocks not shown 

0 5 10 MILES

5 10 KILOMETRES

L ~ 

Area of map

OREGON

FIGURE 1. Bedrock geology of central Oregon coast showing the relation of the Alsea 

Formation to other Tertiary units.

parts of the Alsea Formation that crop out on the north and south sides 

of Alsea Bay. Distinctive lithologic units that occur in the section 

along the north side of the bay, as the glauconitic sandstone bed at
ALSEA FORMATION, OREGON COAST RANGE F3

point G in figure 2, have not been recognized on the south side. Faulting in the bay further complicates correlation of strata across the bay. 

On the basis of lithologic and faunal similarities, the siltstone exposed 

at the top of the exposed section (A, fig. 2) on the north side of the bay is 

considered to correlate with the lower part of the siltstone unit (D, fig. 

2) that crops out on the south side of the bay.

The lower part of the Alsea Formation is covered by Quaternary 

deposits along the bay but is exposed along county and logging roads 

north and south of the bay. The upper part of the Alsea Formation 

crops out along the south side of the entrance to Alsea Bay (C, fig. 2), 

but its contact with the overlying Yaquina Formation is not exposed 

there. Although the formation is thickest at the type section, its contact relations are best seen along the Yaquina River, 17 km to the 

north. Therefore, a reference section is designated as those outcrops 

and road cuts along the north side of the Yaquina River from the base 

of the formation at B westward to its top at A on the map of the section 

(fig. 3).

On the basis of stratigraphic studies along the Yaquina River, Harrison and Eaton (1920) included tuffaceous siltstone and sandstone, 

the Alsea Formation of this report, in the upper part of their newly 

named Toledo division. The lower part of their Toledo division included thin-bedded to massive siltstone and interbedded glauconitic 

sandstone, later named the Moody Shale by Schenck (1927). Although 

Yokes, Norbisrath, and Snavely (1949) recognized the Eocene age of 

the Moody, they included it with the overlying tuffaceous sandstone 

and siltstone (Alsea Formation of this report) in their Toledo Formation. They stated, however, that future work would probably raise both 

lithologic units to formational rank. Mapping by Snavely, MacLeod, 

Wagner, and Rau (1975a, b, c) has shown that the lower part of the 

Toledo Formation, the Moody Shale Member, includes both the 

Yamhill Formation of late middle and late Eocene age and the Nestucca Formation of latest Eocene age and that the upper part of the 

Toledo Formation is the Alsea Formation of this report. The names 

Toledo Formation and Moody Shale Member are therefore considered 

obsolete.

Schenck (1928) described the stratigraphy and a few fossils of the 

well-exposed Oligocene sequence, the Alsea Formation of this report, 

at its type section on the north and south sides of Alsea Bay. He 

correctly assigned strata in the lower part of the sequence an early and 

middle Oligocene age, but assigned a Miocene age to the siltstone at 

the top of the sequence that is now known to be of late Oligocene age. 

On the basis of lithology, stratigraphic position, and selected fossils, he 

correlated his Oligocene strata on Alsea Bay with the type Toledo
F4 CONTRIBUTIONS TO STRATIGRAPHY

R. 12 W. R.11W.

1 MILE

i I

I I I I I I

0 1 KILOMETRE

FIGURE 2. Distribution of the Alsea Formation at its type locality on Alsea Bay. Lower 

part of Alsea Formation is exposed between A and B, upper part between C and D. 

Glauconitic sandstone bed (G) and massive sandstone (S) are referred to in text. 

Attitudes in the Alsea Formation near point C are from scattered outcrops that are 

intermittently exposed below beach sand.
ALSEA FORMATION, OREGON COAST RANGE F5

Formation on the Yaquina River. Vokes, Norbisrath, and Snavely 

(1949) included the lower part of the Alsea Formation of this report 

along Alsea Bay in the upper part of their Toledo Formation and the 

upper part of the Alsea in their Yaquina Formation.

The tuffaceous siltstone and sandstone of Oligocene age that crops 

out in the west-central part of the Oregon Coast Range has been 

informally referred to as "siltstone of Alsea" in reports and on geologic 

maps by Snavely, MacLeod, and Rau (1969), Snavely, MacLeod, and 

Wagner (1972a, b, c), and MacLeod and Snavely (1973). These strata 

are here raised to formational rank, as they make up an important 

lithologic unit that has been mapped for more than 65 km along the 

west side of the central Oregon Coast Range (fig. 1).

ACKNOWLEDGMENT

The writers express their appreciation to Paula J. Quinterno for her 

assistance in the preparation of this report.

Nestucca Formation

Contact, approximately located

U_____ 

D 

Fault, approximately located

Dotted where concealed or inferred. U, 

upthrown side; D, downthrown side

Strike and dip of beds

FIGURE 2. Continued.
F6 CONTRIBUTIONS TO STRATIGRAPHY

R. 11 W.

U

Contact, approximately located. Dotted 

where concealed or inferred Fault, approximately located, showing 

dip. Dotted where concealed or inferred. 

U, upthrown side; D, downthrown side

Strike and dip of beds

FIGURE 3. Distribution of the Alsea Formation at its reference section A-B along 

the Yaquina River. Ml and M2 are molluscan localities referred to in text.
ALSEA FORMATION, OREGON COAST RANGE F7

GEOLOGIC SETTING

The Alsea Formation occurs in the upper part of a Tertiary sequence 

of marine sedimentary and volcanic rocks more than 6,000 m thick 

that crops out in the central part of the Oregon Coast Range. The Alsea 

Formation forms a narrow arcuate belt near the coast from near 

Yachats northward about 65 km to Siletz Bay (fig. 1).

Contacts between the Alsea Formation and underlying and overlying strata are well exposed in the reference section along the Yaquina 

River (fig. 3). Here, and throughout most of its area of outcrop, the 

massive to thick-bedded tuffaceous siltstone and sandstone of the 

Alsea Formation conformably overlies thin-bedded siltstone, the Nestucca Formation of late Eocene age. The contact is usually gradational 

over a few metres but in places is sharply defined. Glauconitic sandstone is locally present at the base of the Alsea Formation. In the 

northern part of its outcrop area, near Siletz Bay, the Alsea Formation 

rests on alkaline volcanic rocks that are interbedded in the upper part 

of the Nestucca Formation. In the southernmost area of outcrop, the 

Alsea Formation conformably overlies an unnamed basaltic sandstone 

unit of latest Eocene age that was derived from the underlying 

Yachats Basalt (Snavely and MacLeod, 1974).The Alsea Formation is 

overlain conformably by a predominantly sandstone sequence, the 

Yaquina Formation of late Oligocene and early Miocene age (fig. 1). In 

most places the tuffaceous siltstone of the Alsea Formation interbeds 

upward with coarse-grained sandstone of the Yaquina Formation; 

locally, however, a sharp contact is present between the two units.

The Alsea Formation ranges in thickness from 50 m to 1,100 m and 

shows an inverse thickness relation to the overlying Yaquina Formation. Where the broad, lenslike body of Yaquina sandstone is thickest 

near lat 44°45', the Alsea Formation is only 50-100 m thick; conversely, where the Yaquina Formation thins north and south of this 

area, the Alsea Formation increases in thickness to as much as 1,100 

m. This inverse thickness variation of the two formations results in 

part from a lateral gradation of the moderately deep water siltstone of 

the upper part of the Alsea Formation with coarse-grained clastic 

deltaic sedimentary rocks of the lower part of the Yaquina Formation. 

A thinning of the lower part of the Alsea Formation and of the underlying Nestucca Formation near lat 44°45' probably resulted from a slow 

rate of sedimentation over a broad bathymetric high that occupied this 

area in late Eocene to middle Oligocene time. Westward, on the Continental Shelf, the Yaquina Formation must also grade laterally into 

siltstone, as it was not encountered in the Standard-Union Nautilus 

test well, drilled about 18 km off Depoe Bay (Braislin and others, 1971, 

fig. 4). In this well more than 1,800 m of siltstone correlative with the
F8 CONTRIBUTIONS TO STRATIGRAPHY

Alsea Formation occupies the entire Oligocene sequence and is overlain by the lower Miocene Nye Mudstone.

PHYSICAL CHARACTERISTICS

The Alsea Formation typically consists of fossiliferous massive to 

medium-bedded tuffaceous siltstone and very fine grained sandstone 

(fig. 4). A few beds of massive mudstone are present in the sequence. 

The siltstone is ubiquitously tuffaceous, and beds of essentially pure 

fine-grained tuff 2 cm to 1 m thick are present. Interbeds of pumiceous 

lapilli tuff and glauconitic sandstone occur locally. Calcareous concretions and nodules, commonly alined parallel to bedding, occur in many 

outcrops. These concretions, typically 4-10 cm in diameter, commonly 

contain mollusks or fossil crabs in their cores. Clastic dikes were noted 

in only one outcrop near the east city limits of Waldport, whereas they 

are abundant in the underlying Nestucca Formation.

Where freshly exposed, the tuffaceous siltstone and sandstone is 

dark to medium greenish gray and weathers spheroidally to a mottled 

light gray and light orange brown. The rock has a hackly to conchoidal 

fracture and is broken by numerous small irregular joints, commonly 

stained by hydrous iron oxides and sulfates. The tuffaceous siltstone 

contains scattered lithic and glauconite grains; Foraminifera and fish 

scales are present in many samples.

The sandstone commonly contains fine mica, carbonaceous material, and pumice fragments. Thin beds of pebble conglomerate composed of pumice and andesitic volcanic rocks occur locally. Although 

most of the sandstone is massive to thick bedded, a unit of thin-bedded 

tuffaceous sandstone and siltstone occurs in the lower part of the Alsea 

Formation on the north side of Alsea Bay (SW&, sec. 8, T. 13 S., R. 11 

W.). A lenslike unit of glauconitic and arkosic sandstone, as much as 

60 m thick, is interbedded in siltstone of the Alsea Formation along the 

east of Siletz Bay in the northernmost part of its area of outcrop (fig. 1). 

Fossiliferous micaceous arkosic sandstone occurs in the Alsea Formation near the southern limit of its area of outcrop, 5 km northeast of 

Yachats.

A distinctive siltstone unit occurs in the uppermost part of the 

sequence along the southwest side of Alsea Bay. It consists of more 

than 350 m of particularly well indurated massive dark-gray ironstained tuffaceous glauconitic siltstone containing scattered calcareous nodules and numerous resistant tuff interbeds 2-20 cm thick.

Beds of medium- to dark-greenish-gray medium-grained glauconitic 

sandstone Vfc-8 m thick are common in the Alsea Formation. They are 

generally massive and weather to a mottled bluish green and orange 

brown. In places they contain mollusks stained by iron oxide. The
ALSEA FORMATION, OREGON COAST RANGE F9

FIGURE 4. Tuffaceous siltstone and sandstone of the Alsea Formation in its type section 

on the north side of Alsea Bay.

glauconitic sandstone contains minor plagioclase, quartz, volcanic 

clasts, calcite, siderite(?), and opaque minerals. An 8-m-thick sandstone bed on the north side of Alsea Bay 100 m east of the U.S. 

Highway 101 bridge abutment (G, fig. 2) and several other beds contain as much as 90 percent glauconite. A chemical analysis of this 

glauconitic sandstone is shown in table 1, column 1. Scattered grains 

of glauconite commonly occur in the tuffaceous siltstone (in places 

replacing Foraminifera) and sandstone, and less commonly in tuff 

beds.
F10 CONTRIBUTIONS TO STRATIGRAPHY

Feldspathic sandstone in the Alsea Formation contains ubiquitous 

altered glass shards (fig. 5) and common andesitic and dacitic lithic 

fragments. Plagioclase, the most abundant clast type, commonly 

shows progressive and oscillatory zoning and is derived principally 

from volcanic rocks of intermediate composition. Other major constitutents include quartz, alkali feldspar, and glauconite and other 

clay minerals. Minor constituents include green hornblende, augite, 

hypersthene, opaque minerals, hydrous iron oxides, muscovite, 

biotite, chlorite, and apatite. Many sandstones are cemented by secondary carbonate. The mineral grains are angular to subangular. 

Near-euhedral crystals of plagioclase are common, and many delicate 

protrusions on glass shards are preserved. Siltstones are similar in 

lithology to feldspathic sandstones, but commonly the content of altered glass shards is much higher in the siltstones.

Lithic sandstone consists principally of andesitic to dacitic grains 

that show flow banding and contain plagioclase phenocrysts. Grains of 

altered glass with stretched or collapsed bubbles are abundant in some 

rocks. The matrix of the lithic sandstones is composed of finely comminuted volcanic grains, broken plagioclase crystals, and altered glass 

shards. The volcanic grains are texturally and mineralogically different from Eocene volcanic rocks in the Coast Range and probably had a 

Cascade provenance.

In addition to the high content of glass shards and fine tuff interbeds, 

the Alsea Formation contains beds of lapilli tuff such as along the east 

side of Lint Slough south of Alsea Bay (fig. 2). These beds consist of 

subrounded to subangular fragments of pumice and andesitic and 

dacitic volcanic rocks. A 10-m-thick lapilli tuff bed on the east side of 

Lint Slough grades upward from volcanic clasts 1-8 cm in diameter at 

the base into pumiceous lapilli and fine-grained tuff.

The chemical analyses for four typical samples of tuffaceous siltstone and fine-grained sandstone and one fine-grained tuff bed in the 

Alsea Formation are given in table 1. The chemical composition of the 

Alsea reflects the high percentage of glass shards and felsic volcanic 

glass derived from contemporaneous volcanism in an ancestral Cascade Range along the east margin of the basin (Snavely and Wagner, 

1963).

AGE AND DEPOSITIONAL ENVIRONMENT

Molluscan assemblages of provincial Oligocene age occur throughout the Alsea Formation, but they are most common and are of greatest 

taxonomic diversity in the sandier strata. The Alsea Bay and Yaquina 

River sections of the Alsea Formation are of regional biostratigraphic 

significance because they contain the boundary between the provincial 

middle and late Oligocene. In the type section, the early and middle
ALSEA FORMATION, OREGON COAST RANGE Fll

FIGURE 5. Photomicrograph showing the abundant vesicular glass fragments and 

glass shards that characterize siltstone and sandstone of the Alsea Formation. Width 

of photomicrograph is 0.5 mm in thin section.

TABLE 1. Chemical analyses of selected sedimentary rocks from the Alsea Formation

SiO2 __________

AkOs _ ___

Fe2Oa

FeO _ ___

MgO __________

CaO ______

Na2O ___ _

K2O ___ _ _ _ H2OH2O+ ________

Ti02 __________

P2 O5 __________

MnO ______

CO2 ______

51.4

7.8

13.0

5.6

2.3

1.0

.87

4.9

5.0

4.5

.56

.19

.04

2.1

63.0

13.7

1.2

.44

1.2

3.3

.94

1.9

6.6

6.7

.82

.13

.04

<.05

1. Glauconitic sandstone, SE'/iSW1^, sec. 18, T. 13 S

2. Tuff, 1-m-thick bed,

3. Tuffaceous siltstone 

4. Tuffaceous siltstone

SWV4, sec. 19,

61.6

13.7

3.8

1.7

1.5

2.2

1.4

2.2

6.1

4.2

.75

.17

.05

<.05

60.0

13.8

4.4

1.6

1.8

3.1

1.3

1.8

5.0

5.4

.88

.18

.04

.57

58.5

14.9

4.0

1.7

1.5

2.2

1.1

1.2

9.1

4.1

.70

.12

.06

.12

56.5

14.0

5.8

1.5

1.7

4.1

2.0

1.6

5.0

5.4

1.1

.24

.15

1.1

., R. 11 W., Waldport quadrangle.

T. 13 S., R. 11 W., Waldport quadrangle.

, center sec. 10, T. 8 S., R. 11 W., Cape Foulweather quadrangle. 

, SEV4NW&, sec. 18, T. 11 S., R. 10 W., Toledo quadrangle.

5. Tuffaceous siltstone, center NE'A, :

6. Tuffaceous siltstone

sec. 35, T. US

, SWV4NWV4, sec. 7, T. 11 S.,

., R. 11 W.. Toledo quadrangle.

R. 10 W., Toledo quadrangle.

Oligocene Lincoln Stage of Weaver and others (1944) is represented by 

assemblages from the lower exposed part of the formation along the 

north shore of Alsea Bay (between G and B, fig. 2). These assemblages 

are also referable to the upper part of the Refugian Stage of Schenck 

andKleinpell (1936), a unit based on both molluscan and foraminiferal 

data. Molluscan assemblages from the upper part of the type section,
Fl2 CONTRIBUTIONS TO STRATIGRAPHY

along the north shore of Alsea Bay between A and G and south shore 

between C and D in figure 2, are referable to the lower part of the 

Blakeley Stage of Weaver and others (1944) of provincial late 

Oligocene age (Addicott, 1972).

At Alsea Bay, "Lincoln Stage" mollusks occur in an interval from 

about 430 to 760 m above the base of the exposed section on the north 

side of the bay (between G and B, fig. 2). One of the most characteristic 

species in these assemblages is the "Lincoln," and upper Refugian, 

index species Bruclarkia columbiana (Anderson and Martin) (fig. 

6A). Other mollusks indicative of assignment to the "Lincoln Stage" 

include Acila shumardi (Ball), Parvicardium lincolnensis (Weaver) 

(fig. 6D), Macrocallista pittsburgensis Ball (fig. 6F), Priscofusus 

chehalisensis (Weaver) (fig. 6G), and Turritella porterensis Weaver 

(fig. 6H). The uppermost 340 m of the Alsea Bay section (north shore 

between A and G, south shore between C and B, fig. 2) are referable to 

the lower upper Oligocene part of the "Blakeley Stage." The lowest 

stratigraphic occurrence of mollusks of the "Blakeley Stage" is in an 

8-m-thick glauconitic sandstone on the north side of the bay about 100 

m east of U.S. 101 highway bridge over Alsea Bay (G, fig. 2). A small, 

shallow-water assemblage from this bed includes Bruclarkia 

acuminata (Anderson and Martin) (fig. 6B, K) andPriscofusus stewarti 

(Tegland) (fig. 67), gastropods restricted to the "Blakeley Stage." 

Stratigraphically higher assemblages from southwest of Waldport 

include Liracassis rex (Tegland) (fig. 6C, E) and Acila gettysburgensis 

(Reagan) (fig. 6J). The biozone of L. rex is restricted to the lower part of 

the "Blakeley Stage" (Burham, 1944).

The much thinner reference section exposed along the Yaquina 

River, about 17 km to the north, provides additional biostratigraphic 

control. A diverse assemblage referable to the "Lincoln Stage" occurs 

within 80 m Stratigraphically of the contact with the underlying Nestucca Formation (M2, fig. 3), and mollusks referable to the lower part 

of the "Blakeley Stage" occur within 35 m of the contact with the 

overlying Yaquina Formation (Ml, fig. 3).

The "Lincoln" or Refugian molluscan assemblages from the lower 

part of the Alsea Formation along the Yaquina River and from the 

middle part of the formation at Alsea Bay represent middle sublittoral, 

or neritic, depths. Molluscan assemblages in the 160 m of section below 

the glauconitic sandstone (G, fig. 2) on the north side of Alsea Bay 

suggest shallowing to depths of about 20-30 m near the top of the 

middle part of the formation. The foraminiferal fauna from this interval also indicates shallow-water depths. This fossiliferous sandstone 

appears to be a very shallow-water, inner sublittoral deposit with 

abundant specimens of Crepidula ungana Ball that form stacks and 

coils and a Mytilus similar to the modern bay mussel M. edulis Linne.
ALSEA FORMATION, OREGON COAST RANGE F13

Stratigraphically higher moliuscan assemblages along the southwest 

shore of Alsea Bay (between C and D, fig. 2) are characterized by very 

low diversity and by genera indicative of marked deepening to outer 

sublittoral or upper bathyal depths. Mollusks from within 35 m of the 

contact with the overlying Yaquina Formation at Yaquina River indicate shallowing again to inner sublittoral depths, possibly 10-15 m. 

The foraminiferal faunas also suggest shallow-water depths near the 

upper contact.

Foraminifera are abundant and varied throughout much of the 

Alsea Formation and therefore particularly useful in placing the formation in the Pacific coast biostratigraphic framework. Foraminifera 

from the lower part of the Alsea Formation are referable to the Refugian Stage of Schenckand Kleinpell (1936) and those from the upper 

part of the formation to the Zemorrian Stage of Kleinpell (1938).

Some of the more characteristic and commonly occurring forms of 

the lower part of the Alsea Formation assigned to the Refugian Stage 

are as follows:

Cassidulina galvinensis Cushman and Frizzell

Ceratobulimina washburnei Cushman and Schenck

Cibicides elmaensis Rau

Epistomina eocenica (Cushman and Hanna)

Guttulina spp.

Plectofrondicularia packardi packardi Cushman and Schenck

Robulus spp. {large}

Sigmomorphina aff, S. schencki Cushman and Ozawa

Spiroloculina texana Cushman and Ellison

Uvigerina cocoaensis Cushman

The Zemorrian Stage of Kleinpell (1938) is well represented in the 

upper part of the formation. Some of the more common and characteristic species from that part of the formation are as follows:

Bolivina marginata adelaidana Cushman and Kleinpell

Buccella mansfieldi oregonensis (Cushman and R.E. and K.C. 

Stewart)

Buliminella bassendorfensis Cushman and Parker

Cassidulinoides cf. C. californiensis Bramlette

Cibicides elmaensis Rau var. A.

Entosolenia sp.

Nonion cf. N. incisum kernensis Kleinpell

Robulus brevispinosus (Nuttall)

Uvigerina obesa impolita Cushman and Lairning

Virgulina cf. V. bramlettei Galloway and Morrey 

Although the Refugian Zemorrian boundary is gradational in 

places, it is well defined in the Alsea's type section at Alsea Bay. 

Cassidulina crassipunctata, Nonion cf. N. incisum kernensis, and
F14 CONTRIBUTIONS TO STRATIGRAPHY
ALSEA FORMATION, OREGON COAST RANGE F15

oq P
F16 CONTRIBUTIONS TO STRATIGRAPHY

Buccella mansfieldi oregonensis are rarely found below the Zemorrian 

Stage in Pacific coast Tertiary sequences. In the Alsea Bay section, the 

lowest occurrence of these species (S, fig. 2) is approximately 15 m 

below the base of the 25-m-thick sandstone bed exposed on the north 

side of the bay. Most of the foraminiferal species known to be characteristic of the Refugian Stage are not found above this horizon. A 

molluscan fauna characteristic of the Refugian Stage, however, occurs 

approximately 90 m stratigraphically higher than the position of the 

Refugian- Zemorrian boundary indicated by Foraminifera.

Neither the base of the Refugian Stage nor the top of the Zemorrian 

Stage is represented in the Alsea Formation in either the Alsea Bay 

section or the Yaquina River section. Locally, however, the formation 

may extend down into the lower part of the Refugian Stage, for a few 

species commonly known in that part of the stage, such as Uvigerina 

cocoaensis and Vaginulinopsis saundersi, do occur in the lower part of 

the formation at Alsea Bay. Only a few poorly preserved and nondiagnostic foraminiferal assemblages have been found in the lower 400 m 

of the formation exposed in the Alsea Bay section.

The lower 650 m of the Alsea Formation in the Alsea Bay section is 

referred to the Refugian Stage, whereas only about the lower 60 m of 

the formation exposed in the Yaquina River section is placed in the 

Refugian Stage. Thinning of the lower (Refugian) part of the Alsea 

Formation in the Yaquina River section is interpreted as resulting 

from a slower sedimentation rate across a broad high that existed in 

this area in late Eocene to middle Oligocene time (Snavely and others, 

1972b). The Nestucca Formation, which conformably underlies the 

Alsea Formation, also is thinner in the Yaquina River area than in 

adjacent areas.

The uppermost part of the Zemorrian Stage is represented in the 

superjacent Yaquina Formation. The occurrence of such forms as 

Virgulina cf. V. bramlettei, Cassidulinoides cf. C. californiensis, and 

Nonion cf. N. incisum kernensis in the Alsea Formation at Alsea Bay 

suggests that the Alsea may also extend up into the upper part of the 

Zemorrian Stage.

Most workers now place the lower part of the Refugian Stage in the 

uppermost part of the Pacific coast Eocene sequence and the upper part 

of the Refugian Stage and all of the Zemorrian Stage in the Oligocene 

(Kleinpell and Weaver, 1963). On this basis, the Alsea Formation is 

Oligocene. The lowest part of the type Alsea, however, contains a few 

foraminiferal species that suggest a latest Eocene age.

Foraminiferal assemblages of the Alsea Formation compare very 

well with, and therefore suggest correlation with, those known in a 

number of other areas of the Pacific Northwest. Faunas of the Lincoln 

Creek Formation of southwest Washington, particularly those of the
ALSEA FORMATION, OREGON COAST RANGE F17

local Cassidulina galvinensis zone and Pseudoglandulina aff. P. 

inflata zone (Beikman and others, 1967), are very similar to the 

Foraminifera of the Alsea Formation. Foraminiferal faunas from the 

lower part of Weaver's (1912) type Blakeley Formation of the Puget 

Sound area, the middle and upper parts of the Twin River Formation of 

the northern Olympic Peninsula (Rau, 1964), and an Oligocene sequence exposed on the southwestern part of Vancouver Island between 

Port San Juan and Carmanah Point are very similar to the faunas of 

the Alsea Formation.

Moderate water depths are suggested by the faunas of the lower 

(Refugian) part of the formation at Alsea Bay (S to B, fig. 2), where 

abundant Gyroidina orbicularis planata, Cassidulina galvinensis, 

Ceratobulimina washburnei, costate Uuigerina, and other relatively 

deepwater forms occur. Although upper bathyal depths may have 

existed for a short time, faunas of the lower part of the formation 

mostly represent an outer-shelf environment. The characteristic 

robust size of the Foraminifera in this part of the formation suggest 

optimum conditions for their development and an abundance of nutrients. Unusually large Foraminifera are typical of many of the 

faunas of the Refugian Stage in the Pacific Northwest from westcentral Oregon to as far north as southern Vancouver Island, suggesting that a similar shelf environment extended throughout these 

areas.

Water depths became progressively shallower in the early part of 

the Zemorrian Stage, as inner sublittoral conditions are indicated by 

the occurrence of Elphidum aff. E. californicum and Nonion cf. N. 

incisum kernensis in the middle part of the formation (A to S, fig. 2). 

The upper part of the Alsea Formation on Alsea Bay (C to D, fig. 2) was 

deposited in an outer sublittoral or possibly an upper bathyal environment, as indicated by the occurrence of Gyroidina orbicularis 

planata and Cassidulina crassipunctata; in contrast, the uppermost 

part of the Alsea Formation on the Yaquina River (fig. 3) contains 

Foraminifera typical of an inner sublittoral environment. This difference suggests that the basin deepened southward from the Yaquina 

River at the time the sediments of the upper part of the Alsea Formation were being deposited. Deepening in this direction is also consistent with the interpretation that shallow water deltaic sands in the 

lower part of the Yaquina Formation are equivalent to the deeper 

water siltstone of the upper part of the type Alsea Formation.

In summary, the Refugian and Zemorrian foraminiferal assemblages of the Alsea Formation generally suggest an offshore shelf-type 

environment which became nearly littoral at times, especially during 

the deposition of the middle part of the formation on Alsea Bay and 

uppermost part of the formation on the Yaquina River. Water temper-
F18 CONTRIBUTIONS TO STRATIGRAPHY

atures were cool, considering the shallow depths represented by parts 

of the formation.

REGIONAL CORRELATIONS AND PALEOGEOLOGY

The Alsea Formation represents part of a continental shelf facies of 

Oligocene age that was widespread in western Oregon and Washington (fig. 7). Marine sequences that correlate with the Alsea Formation 

in the Oregon Coast Range include the Eugene Formation (Smith, 

1924) on the southeast flank and the Pittsburgh Bluff Formation 

(Schenck, 1927) and the upper part of the Keasey Formation (Schenck, 

1927; Moore and Yokes, 1953) in the northern part. In Washington, 

correlative units include the Lincoln Creek Formation (Weaver, 1937; 

Beikman and others, 1967) in the southwestern part, the lower part of 

the Blakeley Formation (Weaver, 1912) in northern Puget Sound, and 

the middle and upper members of the Twin River Formation (Arnold 

and Hannibal, 1913; Brown and Gower, 1958) of the northeastern 

Olympic Peninsula. Neritic sedimentary rocks of Oligocene age also 

crop out along the south side of Vancouver Island, British Columbia, 

between Port San Juan and Carmanah Point.

Deepwater Oligocene strata are exposed in the northwestern Olympic Peninsula, across the Straits of Juan de Fuca from correlative 

shallow-water marine sedimentary rocks of Vancouver Island. These 

include the middle and upper members of the Twin River Formation 

between Port Angeles and Clallam Bay (Brown and others, 1960; 

Gower, 1960) and unnamed strata exposed farther west between ClallamBay and Cape Flattery, where about 3,000 m of Oligocene siltstone 

with interbedded units of turbidite sandstone contain a foraminiferal 

fauna indicative of bathyal depths. A steep Oligocene continental 

slope that separated deep marginal basin deposits of the northwestern 

Olympic Peninsula from shelf deposits of Vancouver Island probably 

lay along the western part of the Straits of Juan de Fuca and swung 

southward near Port Angeles (fig. 7) Sparse subsurface information 

suggests that deepwater deposits of Oligocene age probably formed in 

the area of the present Continental Shelf of Oregon and Washington as 

shown in the paleogeologic map, figure 7.

The abundant volcanic detritus in the Alsea Formation differs 

chemically and petrographically from Eocene volcanic rocks of the 

Coast Range but is similar to some of the Oligocene volcanic rocks from 

the western Cascades described by Peck, Griggs, Schlicker, Wells, and 

Dole (1964). It is therefore likely that vigorous pyroclastic volcanism 

occurred in an ancestral Cascade Range along the east margin of the 

shelf in Oligocene time. Great quantities of volcanic ash fell directly 

into the sea or was carried by streams to the marine environment. 

Some volcanic debris carried by streams to the margin of the basin was
ALSEA FORMATION, OREGON COAST RANGE

125° 123° CANADA

F19

43°-

EXPLANATION 

MARINE DEPOSITS

Silt and turbidite 

sands; deep marginal 

basin facies

CONTINENTAL DEPOSITS

Pre-Oligocene rocks

Approximate contact

Volcanic vent

50 MILES

50 KILOMETRES

FIGURE 7. Inferred distribution of Oligocene shelf and deepwater marginal basin facies 

in western Oregon and Washington and the adjacent continental margin.

transported into deeper water by submarine mudflows (Snavely and 

Wagner, 1963). Only a small amount of volcanic material reached the 

present site of the northwestern part of the Olympic Peninsula, as only 

a few thin tuff beds are present in the deepwater Oligocene sequence 

exposed there.

Deposition of the shelf sediments of the Alsea Formation and correlative stratigraphic units in western Oregon was terminated in late 

Oligocene time by regional uplift in the present site of the Oregon 

Coast Range. This uplift essentially restricted marine deposition to 

the west flank of the Coast Range and to the present Continental Shelf 

except for a restricted embayment that extended into the northern 

part of the Willamette trough. Several large westward flowing streams
F20 CONTRIBUTIONS TO STRATIGRAPHY

breached the ancestral Coast Range uplift and carried sand and gravel 

to form thick deltaic deposits near the present coast. One such deposit 

is represented by the upper Oligocene and lower Miocene Yaquina 

Formation. In western Washington only minor changes occurred in 

the early and middle Oligocene depositional environment, and shelftype sedimentation continued into late Oligocene time. Beyond the 

shelf, the deposition of sediments in the deep marginal basin off both 

Oregon and Washington was uninterrupted.

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ALSEA FORMATION, OREGON COAST RANGE F21

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