Reconnaissance Geology and 

Geologic Hazards of the 

Offshore Coos Bay Basin, 

. Oregon 

Reconnaissance Geology and 

Geologic Hazards of the 

Offshore Coos Bay Basin, 

Oregon 

By SAMUEL H. CLARKE, Jr., MICHAEL E. FIELD, and 

CAROL A. HIROZAWA 

U.S. GEOLOGICAL SURVEY BULLETIN 1645 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE 1985 

For sale by the 

Distribution Branch, Text Products Section 

U.S. Geological Survey 

604 South Pickett St. 

Alexandria, VA 22304 

Library of Congress Cataloging-in-Publication Data 

Clarke, Samuel Harvey 

Reconnaissance geology and geologic hazards of the offshore 

Coos Bay basin, Oregon. 

(U.S. Geological Survey Bulletin 1645) 

Bibliography: p. 38-41 

Supt. of Docs. no.: I 19.3:1645 

1. Geology--Oregon--Coos Bay region (bay). 2. Natural disasters--Oregon--Coos Bay region (bay). 3. Rock deformation--Oregon--Coos Bay region (bay). I. field, Michael E. 

II. Hirozawa, Carol A. III. Title. IV. Series: United 

States. Geological Survey. Bulletin 1645. 

QE75.B9 no. 1645 557.3s 85-600266 

[QE156.C6) [551.46'08'09432) 
CONTENTS 

Abstract 1 

Introduction 1 

Geologic framework 2 

Background 2 

Geologic history 2 

Marine geology of the Coos Bay basin 5 

Structure 9 

Background 9 

Folds 13 

Faults 18 

Seismicity 22 

Sea-floor instability 25 

Sediment slides, flows, and creep 25 

Areas of uplift 27 

Shallow gas accumulation 27 

Possible gas hydrates 30 

Characteristics of surface sediment 30 

Sediment sources and lithology 30 

Sediment transport 34 

Geometry of Quaternary deposits 35 

Organic carbon and calcium carbonate content 35 

Summary of geologic hazards 36 

References cited 38 

FIGURES 

1. Map showing generalized plate-tectonic framework of east Pacific margin 2 

2. Map showing major geologic and physiographic features of the continental margin of the Pacific 

Northwest 3 

3. Stratigraphic correlation chart of major Tertiary units in western Oregon 4 

4. Map showing geophysical tracklines in Coos Bay basin for 1977 and 1978 cruises of research vessel 

Sea Sounder 6 

5. Map showing offshore area of Coos Bay basin 7 

6. Generalized geologic map of southern Oregon continental margin 8 

7. High-resolution seismic-reflection profile extending eastward across Heceta Bank and adjacent shelf 10 

8. High-resolution seismic-reflection profile extending eastward from Heceta Bank across the adjacent 

shelf 11 

9. High-resolution seismic-reflection profile across the inner shelf north of Cape Arago 12 

10. Deep-penetration seismic-reflection profile across shelf and slope between Heceta Bank and Cape 

Arago 13 

11. Deep-penetration and high-resolution seismic-reflection profiles across slope northwest of Cape 

Arago 14 

12. Structural geologic map of southern Oregon continental margin 15 

13. Deep-penetration seismic-reflection profile across inner shelf southeast of Heceta Bank 16 

14. Deep-penetration seismic-reflection profile across shelf and slope north of Coquille Bank 17 

15. Deep-penetration and high-resolution seismic-reflection profiles across outer shelf and slope west 

of Cape Arago 19 

16. Deep-penetration seismic-reflection profile across shelf and upper slope west of Cape Arago 20 

17. Deep-penetration and high-resolution seismic-reflection profiles across inner shelf north of Cape 

Arago 21 

18. Deep-penetration seismic-reflection profile across southeastern Heceta Bank and adjacent shelf 22 

19. High-resolution seismic-reflection profile across shelf southwest of Cape Arago 23 

20. Sketch map of southern Oregon continental margin, showing locations and magnitudes of seismic 

events 24 

21. Sketch map of Coos Bay basin, showing areas of sea-floor failures, hydrocarbon seepage, and shallow 

gas accumulation 26 

22. Sketch map of Coos Bay basin, showing locations of cores analyzed for hydrocarbon-gas content 28 

23. Deep-penetration seismic-reflection profile across Coquille Bank and adjacent shelf 31 

24. High-resolution seismic-reflection profile across Coquille Bank and adjacent shelf 32 

25. High-resolution seismic-reflection profile showing location of sampling stations west of Cape Arago 33 

III 
26. Map showing distribution of major sediment types on continental shelf and slope off central Oregon 36 

27. Isopach map showing thickness of Quaternary sediment in the Coos Bay basin 37 

TABLES 

1. Hydrocarbon gases in sediment, offshore Oregon 29 

2. Location, depth, length, type, and purpose of selected samples 34 

3. Textural parameters of selected sediment samples 35 

4. Carbon and calcium carbonate contents of selected samples 38 

IV 
Reconnaissance Geology and Geologic Hazards of 

the Offshore Coos Bay Basin, Oregon 

By Samuel H. Clarke, Jr., Michael E. Field, and Carol A. Hirozawa 

Abstract 

The offshore Coos Bay basin underlies the continental shelf and upper slope between Heceta Bank and 

Coquille Bank, Oregon, and comprises a succession of 

middle Eocene and younger principally marine sedimentary rocks 5,000+ m thick. Three acoustic units 

have been defined within this basin on the basis of 

seismic-reflection and sampling data: acoustic unit 1, 

inferred to be Quaternary in age and to be a maximum 

of about 500 m thick, is widespread on the continental 

shelf and slope; acoustic unit 2., inferred to be late 

Miocene to late Pliocene in age, is exposed on Heceta 

Bank, on the inner shelf between Cape Blanco and 

Cape Arago, and, locally, in breached anticlines; and 

acoustic unit 3, inferred to comprise strata of Eocene 

to middle Miocene age, is exposed locally in breached 

anticlines on the inner and middle continental shelf 

and on the middle of the continental slope. Recurrent 

major episodes of deformation are evidenced by 

regional unconformities of late Pliocene to early 

Pleistocene, early late Miocene, and middle to late 

Eocene age; these unconformities probably reflect 

significant changes in the history of plate convergence 

along this margin. 

Potential geologic hazards to offshore development include groups of short north- to northwesttrending faults that cut Quaternary (locally Holocene) 

deposits on the continental shelf between Cape Arago 

and Heceta Bank. Seismic activity appears to be low 

within the offshore basin area; however, a significant 

possibility exists of damage from tsunamis, sea-floor 

mass movement, and liquefaction resulting from 

earthquakes in adjacent regions, notably the northern 

California continental margin. Unstable sea-floor 

conditions resulting from recent subaqueous slides and 

flows appear to be uncommon, although the presence 

of locally steep slopes and thick accumulations of 

unconsolidated sediment with unknown engineering 

characteristics make site-specific studies of sea-flool 

stability advisable. Buried zones as large as 190 km 

in area and 2.00 to 2.50 m thick of disrupted and rotated 

acoustic reflectors appear to record episodes of seafloor failure during Pleistocene low stands of sea 

level. Several folds showing sea-floor relief, one of 

which appears to be diapiric, reflect tectonic instability of the modern upper continental slope along the 

west margin of the basin. Acoustic anomalies suggestive of shallow g~ accumulations cover areas of as 

much as 12.0 km , and combined geophysical and 

geochemical evidence indicate that gas is being vented 

at the sea floor in two localities. At one locality, 2.5 

km west of Cape Arago, samples contained high 

methane concentrations and equivocal indications of 

thermogenic hydrocarbon gas. Reduction of the 

bearing strength of the enclosing sediment by such gas 

accumulations enhances the possibility of failure; if 

these accumulations are of thermogenic origin, they 

may reflect an overpressured zone at depth. 

INTRODUC'nON 

This investigation summarizes aspects of the 

offshore and onshore geology of the Coos Bay basin, 

Oregon, that are significant in assessing potential 

geologic hazards to offshore resource exploration and 

development. Though focused on a specific offshore 

basin, this study is regional in scope and is designed to 

provide the geologic perspective necessary for sitespecific geohazard studies. The geologic phenomena 

investigated include faulting; seismicity; sea-floor 

instability resulting from active uplift, sediment slides 

and flows, and the presence of gas in sediment; and 

sediment transport and deposition. 

Much of the information presented here is based 

on seismic-reflection and sampling data collected 

during three cruises to the central Oregon offshore. 

The first cruise (S9-77NC), funded jointly by the U.s. 

Geological Survey and the U.S. Bureau of Land 

Management, was carried out aboard the research 

vessel Sea Sounder during November 1977. Objectives 

of that cruise were to provide a geologic 

reconnaissance of the offshore basin and to study the 

regional distribution of potentially hazardous geologic 

processes. The second and third cruises (Sl2-78NC, 

S13-79NC), funded by the U.S. Geological Survey and 

completed during 1978 and 1979, were carried out to 

augment previously collected seismic-reflection data, 

to collect samples for age control, and to verify the 

presence of shallow gas accumulations and gas seepage 

from the sea floor. 

The seismic-reflection data obtained during this 

study cover the spectrum from high-resolution profiles 

collected with 12-kHz, 3.5-kHz, and Uniboom systems 

to intermediate-penetration profiles obtained using an 

80- to 140-kJ sparker system. Subsurface features 

greater in vertical dimension than approximately 1.0 

to 1.5 m can be resolved on high-resolution data that 

range in subbottom penetration from 100 to more than 

lntroduetion 1 
300 m. Intermediate-penetration sparker data used to 

map structures at greater subbottom depths can 

resolve features larger than about 15 m in vertical 

dimension and commonly record information at depths 

of 1 to 2 km beneath the sea floor. Ship positioning 

was accomplished using satellite-navigation and Loran 

C systems that were integrated by means of a deadreckoning computer (DRC), and a shipborne electronic 

ranging system that employed shore-based 

transponders. Radar was used for positioning in areas 

of terrain masking and during periods of navigationalequipment malfunction. Estimated position accuracy 

ranges from .:!:_50 m where positions were determined 

principally by electronic ranging (approximately 90 

percent of the total trackline mileage) to an average 

of about +400 m where DRC estimations or radar 

positions were used exclusively (approximately 10 

percent of the total trackline mileage). Procedures of 

geophysical-data acquisition and navigation employed 

in this study were described in greater detail by Clarke 

and others (1981). 

Acknowledgments.--We thank many colleagues 

who contributed to the collection and analysis of data, 

in particular Robert Arnal, who made microfauna! age 

determinations, and Keith Kvenvolden and John Rapp, 

who performed geochemical analyses and 

interpretations. Teresa Hallinan, William R. 

Richmond, and Michael E. White made continuing and 

major contributions to our efforts in data collection 

and analysis, and to preparation of this report. Parke 

D. Snavely, Jr., and Holly C. Wagner shared their ideas 

and made significant contributions with their reviews. 

GEOLOGIC FRAMEWORK 

Background 

The continental margin of Oregon is situated 

near the boundarv between the North American, 

Pacific, and Gorda-Juan de Fuca (a remnant of the 

ancestral Farallon) crustal plates (fig. 1), and the 

Cenozoic geologic history of this region has been 

shaped foremost by interactions between these plates 

(Morgan, 1968; Atwater, 1970; Kulm and Fowler, 

1974b; Snavely and MacLeod, 1977; Snavely and others, 

1977, 1980; Drake, 1982). Sea-floor spreading at the 

Gorda and Juan de Fuca Ridges, and underthrusting of 

the Gorda-Juan de Fuca plate beneath the North 

American Continent have resulted in long-term 

compression of the margin. Simultaneously, the 

Pacific plate has moved northwestward relative to the 

North American plate at a rate of about 6 cm/yr along 

broad zones of right shear-the San Andreas and Queen 

Charlotte Islands fault systems. Folding and upwarp of 

the outer continental margin (shelf and slope) resulting 

from oblique plate convergence have been 

accompanied by the formation of a succession of 

predominantly north-trending marine basins to the 

east, in the present area of the Oregon continental 

shelf and the Oregon Coast and Cascade Ranges (fig. 

2); these basins were subsequently modified by 

episodes of strike-slip faulting (Snavely and Ma~Leod, 

1977; Snavely and others, 1980). 

West of the Cascade Range in central Oregon, 

terrigenous clastic sediment, volcanic rocks, and 

':II 

Figure 1. Generalized plate-tectonic framework of 

east Pacific plate margin, showing spreading ridges 

(double lines), major transform faults (single lilies), and 

zones of subduction (hatched lines). 

volcaniclastic rocks, with an aggregate thickness of 

about 8 km, have accumulated since early Tertiary 

time in an elongate, principally marine basin. The axis 

of this basin has shifted westward over time; during 

the middle Eocene, it probably lay about 20 to 40 km 

east of the present coastline in the area of the central 

Oregon Coast Range (Snavely and Wagner, 1963; 

Snavely and others, 1977, 1980). Since late Eocene 

time, however, deposition has been focused in a 

marginal basin occupying the present site of the 

Oregon continental shelf. The Pliocene and 

Pleistocene basin axis trends northward about 30 to 35 

km west of the present coastline. The greatest thickness of upper Paleogene and younger sedimentary 

rocks lies within 50 to 60 km of the coastline at water 

depths of from 50 to about 1,000 m. 

Geologie History 

Aspects of the Cenozoic geologic history of the 

central Oregon continental margin were described by 

Snavely and Wagner (1963), Baldwin (1964, 1974), 

Braislin and others (1971), Baldwin and Beaulieu (1973), 

Kulm and Fowler (1974a, b), Snavely and others (1977, 

1980), and Newton (1980); the reader is directed to 

these studies for a comprehensive review. Parts of the 

following discussion, especially those pertaining to 

Miocene and older units, principally concern the 

onshore geologic record; they are intended to provide a 

2 Reeonnaissanee Geology and Geologie Hazards of the Offshore Coos Bay Basin, Oregon 
132° 

48° 

46° 

44° 

42° 

128° 

CASCADIA 

8 ASI N 

MENDOCINO FRACTURE ZONE 

100 

GORDA 

BAS IN 

200 

124" 

KLAMATH 

300 KILOMETERS 

122° 120° 

CALIFORNIA 

Figure 2. Major geologic and physiographic features of continental margin of the Pacific Northwest. 

Study area is shaded. 

geologic framework within which the offshore geology 

can be examined. 

A thick succession of Paleocene to middle 

Eocene tholeiitic to alkalic pillow basalt and breccia 

(Siletz River Volcanics, Roseburg Formation), 

interpreted as representing early Tertiary oceanic 

crust, underlies the Cenozoic sedimentary sequence in 

the central Oregon Coast Range and on the adjacent 

continental shelf (fig. 3; Snavely and Wagner, 1963; 

Snavely and others, 1968, 1977, 1980). The base of the 

volcanic succession is not exposed in the Oregon Coast 

Range, but the maximum thickness is estimated to be 

in excess of 3,000 m. Marine silt and volcanic sand 

(Umpqua Formation, Roseburg Formation) were deposited concurrently throughout the region in varied 

environments; gravel, sand, and silt (Lookingglass 

Formation, Flournoy Formation) were deposited in 

marginal marine and deltaic environments along the 

east and south margins of the marine basin. During 

middle Eocene time, an areally extensive sequence of 

submarine-fan and associated deposits (Tyee 

Formation), derived principally from uplift in the 

present area of the Klamath Mountains, prograded 

northward along the basin axis; these deposits have a 

maximum thickness of 3,000 m and consist mostly of 

massive rhythmically bedded lithic and arkosic sandGeologic Framework 3 
WESTERN OREGON 

WEST FLANK, COAST 

RANGE ANO OUTER 

CONTINENTAL SHElf 

z Unnamed marine 

o sedimentary rocks 

~ on outer continental 

., 

a. 

shelf 

~ Cape Foulweather Basalt 

z ., Sandstone of Whale Cove 

'g Deooe Bay Basalt 

EAST FLANK, 

COAST RANGE 

Terrace 

Deposits 

Absent 

~ ~ Astoria Formation 

~~

"' 

~~~~z=~~-P- Nye Mudstone Ian-t--be-a-- rin~ ~ 

basalt ~ 1-------------1 tuff and a 

~ associated volcanic 

., 

a. 

a. 

:::l 

~quina Formation 

Alsea Formation 

rocks 

Eugene 

Formation 

Beds of Kease0isher 

SOUTHERN 

COAST RANGE 

Port Orford Formation 

Empire Formation 

Sedimentary rocks 

of Miocene age 

' 

Tunnel Point Sandstone 

Bastendorff Formation 

l----+-1-------,~-,--------1age / Formation 1---------------1 

Nestucca 7 Y~chats ----------

z 

'-' 0 

Formation ~ Spencer Formation-------

~bmRB==%2=P~~~~====z=~ 

Coaledo Formation 

:::l 

Yamhill Formation Yamhill Formation !Sacchi Beach Member) 

r-~ ~ ~?-------

Tyee Formation 

Tyee Formation Tyee Formation 

(Tuffaecous 

- ---- Z siltstone 

-:~:::_"l_e!!_)Q_e!) _ 

(K1ngs Valley Flournoy Formation 

I) Siltstone Member) !=========! c_ _________ _ 

Lookingglass Formation 

Siletz River Volcanics Siletz River Volca nics 

Roseburg~ f---+-+---- ----+-----------1 

1--------?----1 

Format1on ~ 

z 

0 

< 

Base not exposed Base not exposed Base not exposed 

Figure 3. Correlation chart of major Tertiary units in 

western Oregon (from Snavely and others, 1977). 

stone and siltstone (Snavely and others, 1964; Lovell, 

1969). Hemipelagic mud and silt interbedded with thin 

sand (Yamhill Formation) were deposited elsewhere in 

the basin and were widespread by the beginning of late 

Eocene time, after deposition of the Tyee Formation. 

This region was subjected to intense deformation 

during middle and late Eocene time, and an 

unconformity at the base of upper Eocene strata is 

widespread in the central Oregon Coast Range and 

offshore. This tectonism probably denotes a major 

episode of plate convergence, possibly related to 

accretion onto the North American plate of Paleogene 

volcanic rocks in western Oregon and accompanied by 

seaward migration of the Eocene subduction zone 

(MacLeod and Snavely, 1971; Simpson and Cox, 1977; 

Snavely and others, 1980). The locus of underthrusting 

probably occupied the middle of the present 

continental shelf (Snavely and others, 1977, 1980). 

Late Eocene time was marked by widespread 

shoaling; the margin of the marine basin lay along the 

west edge of the present Cascade Range and the north 

edge of the present Klamath Mountains (Snavely and 

Wagner, 1963; Snavely and others, 1977). Shallow a~d 

marginal marine environments along the south margm 

of the basin are represented by a sequence of coalbearino- tuffaceous sandstone and siltstone (Coaledo 

Formation) as much as 1,000 m thick in the Coos Bay 

area. These deposits grade northward and westward 

into a thick (1,500+ m) tuffaceous siltstone, rich in 

organic materials, containing subordinate arkosic and 

volcanic sandstone (Nestucca Formation). Basalt flows 

and breccia (Yachats Basalt) that were erupted onto 

the basin floor locally near the present coastline 

formed islands where the accumulations were 

thickest. Late Eocene and Oligocene pyroclastic 

activity in the ancestral Cascade Range along the east 

margin of the basin resulted in deposition of as much 

as 1,500 m of tuffaceous silt and fine sand 

(Bastendorff Formation; Tunnel Point Sandstone; Alsea 

Formation). Uplift accompanied by emplacement of 

gabbroic to dioritic intrusive bodies during late Oligocene time limited marine deposition to areas west of 

the present continental shelf, the shelf itself, and the 

west flank of the Oregon Coast Range. Deposition of 

marine sediment in this area continued essentially 

uninterrupted into Miocene time. Along the east 

margin of the Coos Bay basin, which lay along the west 

flank of the Coast Range and near the present coastline in central Oregon (fig. 2), crustal instability and 

local volcanic activity are recorded by several 

unconformities, conspicuous upper Oligocene to middle 

Miocene shallow-marine clastic deposits (Yaquina 

Formation, Astoria Formation, and the sandstone of 

Whale Cove), and middle Miocene submarine and 

subaerial basaltic flows and breccia (Depoe Bay Basalt, 

Cape Foulweather Basalt). Deeper marine environments in open-margin basins are represented by 

deposits of clayey siltstone rich in organic material 

(Nye Mudstone). . . Uplift and erosion, accompanymg a maJor 

episode of underthrusting, occurred throughout the 

present area of the Oregon Coast Range and the 

adjacent continental shelf during late middle Miocene 

time, with folding principally along northeastward 

structural trends (Snavely and Wagner, 1963; Snavely 

and others, 1977, 1980). The unconformity created by 

this tectonism is recognizable in seismic-reflection 

records throughout most of the central Oregon continental shelf. Southward from Coos Bay, basin-margin 

deposition is represented by about 900 m of mass~ve 

poorly bedded fossiliferous sandstone (Empire 

Formation). Partly correlative deposits in the marine 

basin on the present continental shelf consist of more 

than 1,050 m of upper Miocene tuffaceous sandy 

siltstone and 1,500 m of Pliocene foraminiferal siltstone and sandstone; the Pliocene deposits are thickest 

in the major shelf synclines (Braislin and others, 1971). 

A Pliocene-Pleistocene unconformity can be 

traced throughout much of the central Oregon shelf. 

Strata beneath this unconformity are late Miocene to 

late Pliocene in age, whereas unconsolidated sediment 

overlying the unconformity at Deep Sea Drilling 

Project (DSDP) Site 176 is middle and late Pleistocene 

in age (Kulm and Fowler, 1974b). Pleistocene and 

Holocene deposits in this area are largely silt and sand, 

and are more than 500 m thick. 

4 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
Marine Geology of the Coos Bay Basin 

The geology of the central Oregon offshore has 

been mapped on a regional scale by members of the 

Oregon State University Department of Oceanography; 

their findings were presented in a series of theses and 

technical reports (notably Kulm, 1969; Kulm and 

Fowler, 1970; Fowler and Kulm, 1971) and were 

summarized by Kulm and Fowler (1974a, b). Mackay 

(1969) mapped the inner shelf between Cape Arago and 

Coquille Point, using a closely spaced network of 

single-channel seismic-reflection lines. Interpretive 

geologic cross sections were constructed across the 

offshore Coos Bay basin in the vicinity of Heceta and 

Coquille Banks by Couch and Braman (1980) and Couch 

and Pitts (1980), using combined gravity, magnetic, 

seismic, and well data. The geology of the adjacent 

area on land was mapped and described by Allen and 

Baldwin (1944), Wells and Peck (1961), Ramp (1972), 

Baldwin and Beaulieu (1973), Schlicker and Deacon 

(1974), and Beaulieu and Hughes (1975). Our study 

summarizes recent geologic and geophysical investigations conducted by the U.S. Geological Survey in the 

offshore Coos Bay basin (fig. 4). 

The Coos Bay basin occupies the shelf and upper 

slope of the central Oregon continental margin 

between Heceta and Coquille Banks (from about lat 

43° to 44°15'N.) (fig. 5). The continental shelf in this 

area ranges in breadth from 20 to 25 km off Cape 

Arago and Coquille Point, to a maximum of nearly 70 

km at Heceta Bank. The most prominent features "<>f 

this shelf segment are Heceta and Coquille Banks, 

which are exposed bedrock highs on the outer shelf 

with about 50 m of sea-floor relief. The shelf break 

occurs at a depth of about 140 m, estimated from 

bathymetric profiles and from the depth of deposits 

interpreted as late Pleistocene progradational 

sequences, and is best defined in the vicinity of the 

banks. The continental slope has a northward trend in 

this region and ranges in breadth from 35 km seaward 

of Heceta and Coquille Banks to 55-75 km in the area 

between the banks. It has an average declivity of 2°-

40, is steepest in the vicinity of the banks, and slopes 

westward to depths of 3,000 to 3,100 m in the 

Cascadia Basin. The lower part of the slope is 

conspicuously steep; declivities of 7°-14° are common 

between depths of 2,000 and 3,000 m. Large canyons 

do not cut the continental slope off central Oregon, 

although smaller submarine valleys and gullies are 

numerous. 

Three acoustic units separated by unconformities 

were defined in seismic-reflection profiles across the 

continental shelf between Heceta and Coquille Banks 

(figs. 5, 6). Age control for these units is provided by 

paleontologic analysis of samples collected during this 

study (fig. 6); by the age dates reported by Fowler and 

Kulm (1971), Kulm and Fowler (1974b), and Kulm 

(1980); and by the exploratory-well data reported by 

Snavely and others (1977, 1981). Acoustic unit 1 

overlies a conspicuous unconformity on the east flank 

of Heceta Bank (figs. 7, 8) and consists of sediment 

dated as Pleistocene in age (fig. 6; Kulm and Fowler, 

1974b). This unconformity marks a major tectonic 

event because it is traceable throughout much of the 

Oregon continental shelf and truncates strata of late 

Miocene to late Pliocene age. About 200 km north of 

the study area, at DSDP Site 176 on Nehalem Bank, 

the unconformity separates Pliocene siltstone from 

deposits dated as middle and late Pleistocene 

(maximum age 1.3 Ma) (Kulm and others, 1973). This 

unconformity can be identified elsewhere on the banks 

and inner shelf (see figs. 16, 23) but is indistinct in the 

centers of major synclines. Acoustic unit 1 is stratigraphically the uppermost (youngest) unit defined over 

most of the central Oregon shelf and slope. Locally on 

the inner shelf, however, it is overlain unconformably 

by a seaward-thinning sediment wedge as much as 15 m 

thick (fig. 9). This wedge of sediment is thought to 

represent Holocene deposits derived mostly from 

rivers draining the adjacent Coast Range and coastal 

plain. These deposits are included in acoustic unit 1 on 

figures 6 and 27 because they cannot be traced with 

confidence between adjacent seismic profiles. 

Acoustic unit 1 is widespread on the central 

Oregon continental shelf and upper slope, and is 

present locally on the midslope (fig. 6). Within the 

study area, it reaches a maximum thickness of about 

500 m in the axis of the broad, shallow syncline near 

the shelf edge between Cape Arago and Heceta Bank 

(fig. 10; see fig. 27). Substantial seaward progradation 

of the shelf edge apparently accompanied erosion of 

the inner shelf during Pleistocene low stands of sea 

level. Sediment of acoustic unit 1 overlaps older 

strata on the east flank of rleceta Bank, and on the 

upper slope and midslope is ponded landward of anticlinal folds in strata inferred to be of late Miocene and 

Pliocene age. Unconformities of limited extent that 

occur within acoustic unit 1 (fig. 7) reflect recurrent 

episodes of uplift and erosion coupled with downwarp 

of the adjacent basin during Pleistocene time. 

Pleistocene sediment of the central Oregon 

offshore area is described as sand and silt interbedded 

with minor gravel (Snavely and others, 1977, 1981). 

Deposits of this age penetrated at DSDP Site 176 

consist of greenish-gray clayey silt containing 

abundant coarse debris and shallow-water megafossils 

(Kulm and others, 1973). Pleistocene sediment cored 

during the present study is typically fine silt and clay. 

Acoustic unit 2 is widely exposed on Heceta 

Bank, on the inner continental shelf between Cape 

Blanco and Cape Arago, and in anticlinal axes 

elsewhere on the continental shelf and upper slope (fig. 

6). Acoustic unit 2 underwent widespread deformation 

by folding before the deposition of acoustic unit 1, 

which unconformably overlies it (fig. 7; see figs. 16, 

23). In turn, acoustic unit 2 unconformably overlies 

more intensely deformed strata of acoustic unit 3 (fig. 

10; see figs. 11, 15, 16). 

Strata of acoustic unit 2 on .tleceta Bank have 

been dated as late ivliocene and early and middle 

Pliocene (Fowler and Kulm, 1971; Kulm and Fowler, 

197 4b). Samples collected from acoustic unit 2 on 

Heceta Bank and from a youthful fold 45 km northwest 

of Cape Arago have been dated as late Pliocene (fig. 

6). Thus, acoustic unit 2 on Heceta Bank is correlative 

with upper Miocene and Pliocene marine sedimentary 

strata that crop out locally along the southern Oregon 

coast. Pleistocene sediment (acoustic unit 1) directly 

Geologie Framework 5 
25 50 KILOMETERS 

Figure 4. Geophysical tracklines in Coos Bay basin for 1977 and 1978 cruises of research vessel Sea 

Sounder. 

6 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
30' 124t 

River 

0Coos Bay 

25 50 KILOMETERS 

CONTOURS IN METERS 

Figure 5. Offshore area of Coos Bay basin. Bathymetry from U.S. Coast and Geodetic Survey 1308N-17 

(1968) and 1308N-22 (1968). 

Geologic Framework 7 
25 50 KILOMETERS 

Figure 6. Generalized geologic map of southern Oregon continental margin. The geologic mapunit symbols used on this figure may not conform to standards set by the U.S. Geological 

Survey. 

8 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
EXPLANATION 

ONSHORE GEOLOGY 

(From Beaulieu and Hughes, 1975, and 

Schlicker and Deacon, 1974) 

MODERN COASTAL SEDIMENTS-Principally deposits of 

active and stabilized dunes and inter-dune areas and tidal 

flats 

MODERN ALLUVIAL DEPOSITS 

PLEISTOCENE MARINE AND FLUVIAL TERRACE DEPOSITS 

MIOCENE A~D PLIOCENE MARINE SEDIMENTARY 

ROCKS 

EOCENE(?) TO MIOCENE INTRUSIVE ROCKS-Principally gabbro. diabase. and diorite r _ }PfTli 1 PALEOCENE. EOCENE. AND OLIGOCENE MARINE SEDIMENTARY ROCKS-Includes lesser nonmarine and voi-

~Tv 

~ 

canic rocks 

PALEOCENE AND EOCENE VOLCANIC ROCKS-Principally basalt and associated volcaniclastic rocks 

·:}~-~*' q MESOZOIC MELANGE-Fine- to coarse-grained marine 

sedimentary and volcanic rocks 

•,6 

Tpu-Qh 

CONTACT -Dashed where approximate: dotted where concealed 

FAULT -Dashed where approximate ; dotted where concealed 

OFFSHORE GEOLOGY 

ACOUSTIC UNIT 1-Inferred to comprise Quaternary shelf, 

slope, and basin deposits 

ACOUSTIC UNIT 2-Inferred to comprise strata of late Miocene and Pliocene age : principally siltstone and claystone 

ACOUSTIC UNIT 3-lnferred to comprise strata of Eocene 

through middle Miocene age: principally siltstone and sandstone with some interbedded volcanic and volcaniclastic 

rocks and lesser intrusive rocks 

SAMPLE LOCATION-Ages shown are based principally on 

benthic foraminiferal faunas; age ranges are shown where 

greater resolution was not obtained: ages in parentheses 

are probable ages within the range indicated; faunal identifications and age determinations by R. E. Arnal. Q. indicates Quaternary, undifferentiated; Qh . Holocene: Qp. 

Pleistocene: Tp. Pliocene. undifferentiated; Tpu , upper 

Pliocene: Tpl. lower Pliocene 

CONTACT -Dashed where approximate: queried where uncertain 

CONTACT BETWEEN ACOUSTIC UNITS 2 AND 3 WHERE 

COVERED BY VENEER OF QUATERNARY SEDIMENTS 

ON INNER SHELF-Queried where uncertain 

AXIS OF MAJOR ANTICLINE 

AXIS OF MAJOR SYNCLINE 

SLUMP 

Figure 6. Continued 

overlies upper Miocene rocks (acoustic unit 2) on the 

inner continental shelf between Cape Arago and 

Coquille Point (Kulm and Fowler, 1970, 1974b), and 

acoustic unit 2 is missing owing to pre-Pleistocene 

erosion on much of the inner shelf north of Cape 

Arago. Strata of late Miocene and Pliocene age also 

appear to be missing onshore, adjacent to the Coos Bay 

basin, except for a small area where Pliocene marine 

sediment is exposed east of Cape Arago (fig. 6; 

Beaulieu and Hughes; 1975). 

Braislin and others (1971) described upper 

Miocene (Mohnian and Delmontian) rocks of the 

central Oregon offshore as tuffaceous sandy siltstone; 

those of Pliocene age were described as a monotonous 

sequence of massive foraminifer-rich olive-gray siltstone and claystone that is similar biostratigraphically 

to Pliocene strata of the Ventura and Los Angeles 

basins of southern California. Kulm and Fowler 

(197 4a) reported that upper Miocene strata are 

principally fine-grained sandstone; MacKay (1969) 

speculated, on the basis of reflector characteristics 

and analogy to Pliocene rocks on land nearby, that the 

late Neogene strata on the inner shelf between Cape 

Arago and Coquille Point consist of massive sandstone 

of relatively uniform lithology. Indurated olive-gray 

mudstone of probable lat~ Pliocene age was cored 

during this study on Heceta Bank at a water depth of 

115 m and on the crest of a piercement structure 45 

km northwest of Cape Arago at a depth of 491 m (fig. 

11). 

Acoustic unit 3 is exposed in the cores of 

breached anticlines on the inner to middle continental 

shelf seaward of the mouth of the Umpqua River and 

Coquille Point, and on the middle of the continental 

slope seaward of Cape Arago (fig. 6). Acoustic unit 3 

is unconformably overlain by acoustic unit 2 (fig. 10; 

see figs. 11, 13, 15, 16) and is much more intensely 

deformed than are younger strata on the central 

Oregon continental shelf. The unconformity 

separating acoustic units 2 and 3 is thought to be 

correlative with the regional unconformity 

representing widespread late middle Miocene deformation on the continental margin off Oregon and 

Washington (Snavely and others, 1977, 1980). 'This 

unconformity separates upper Miocene and younger 

rocks from strata ranging in age from Eocene to 

middle Miocene (Beaulieu and Hughes, 1975; Snavely 

and others, 1977). Acoustic unit 3 was not sampled 

during this study, nor do published references show 

samples from these exposures; however, this unit is 

inferred to consist of upper Eocene to middle Miocene 

strata, on the basis of its stratigraphic position and 

presumed equivalence to upper Eocene to middle 

Miocene rocks that are bounded by regional 

unconformities onshore. Strata of this age were 

reported by Snavely and others (1977, 1980) to consist 

principally of siltstone interbedded with some turbidite 

sandstone, and volcanic rocks. 

Upper Eocene to middle Miocene rocks overlie 

older Tertiary strata along a regional unconformity in 

the central Oregon Coast Range and in the offshore 

well sections reported by Snavely and others 

(1977 ,1980). Beneath the inner shelf and Heceta Bank, 

these older strata appear to underlie acoustic unit 3 

unconformably. Because they do not crop out on the 

sea floor within the study area, however, and cannot 

be traced with confidence between adjacent lines, 

these older strata have not been mapped separately in 

this study. 

STRUCTURE 

Background 

Folds and faults in the offshore Coos Bay basin 

are shown on Figure 12. The vertical scale of the 

seismic profiles shown here as figures is greatly exaggerated, typically 4:1 to 5:1 for intermediateStructure 9 
CJ) 

Cl 

z 

0 

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0 10 15 

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0.2 

0.4 

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0.6-

17.5km 20 25 30 32km 

0.0 -

0.2-

0.4 

I I 

Figure 7. High-resolution seismic-reflection profile (77-05) extending eastward across Heceta 

Bank and adjacent shelf. Note contact between acoustic units 1 and Z (arrows), evidence of 

Quaternary deformation, and slump on west side of bank. See figure 4 for location of profile. 

EAST 

17.5 km 

I 

penetration sparker records and 12:1 to 14:1 for high- deposition of acoustic unit 2, and in acoustic unit 2. 

resolution records. Little acoustic enerBy is returned Faulting is indicated on seismic-reflection 

from reflectors dipping more than 15 -20°, and so records by the displacement of reflectors, the 

areas of steep dips and structural complexity, such as termination of conspicuous reflectors or an abrupt 

parts of Heceta Bank, are characterized by an absence change in the appearance of the seismic section, or by 

of coherent reflectors. Masking of subbottom an abrupt change in apparent dip. Faults that are 

structure by water-bottom multiples in shallow water, identified by using two or more of these criteria are 

such as on the inner continental shelf, makes interpre- considered to be well defined and are shown by solid 

tation difficult and interpretative results more subjec- lines on figure 12; those identified by using a single 

tive than elsewhere. criterion are inferred and are shown by dashed lines. 

'Three classes of folds are identified on Figure The orientation of faults was determined principally by 

12: (1) Folds that create sea-floor relief, typically in correlation between seismic-reflection lines. Faults 

the form of irregular or smoothly rounded, linear are queried where their existence, trend, or connection 

ridges; (2) folds that are breached at the sea floor; and between adjacent lines is questionable. 

{3) folds that are buried beneath younger, characteris- The ages of faults are based on the youngest 

tically less deformed deposits. Folds that create sea- acoustic unit they offset. The resolution of ages 

floor relief are generally the youngest folds present assigned to faults in the Coos Bay basin is limited by 

and show as much as 250 m of sea-floor relief (see the absence of geologic-age information in some parts 

figs. 11, 16); most reflect Quaternary deformation and of the basin and by the rather long time span 

some appear to be active. Breached folds, which represented by each acoustic unit. Faults extending to 

predominate on the inner shelf and banks at depths less the sea floor and those cutting sediment inferred to be 

than about 200 m, were truncated during Pleistocene Holocene in age on the inner shelf are significant in 

low stands of sea level. Sea-floor outcrops associated that they may be active. Faults that reach the sea 

with breached folds commonly involve strata of floor, however, also may be old faults exhumed by 

acoustic unit 2, inferred to be late Miocene and post-Tertiary erosion. Because Pleistocene and 

Pliocene in age, on the inner shelf and on Heceta Bank Holocene strata have not been differentiated beyond 

(fig. 7), and strata of acoustic unit 3, interpreted as the inner shelf, faults that reach the sea floor or 

middle Miocene and older, on the inner continental terminate within acoustic unit 1 are probably 

shelf (fig. 13). Some breached folds on the shelf and Quaternary in age. Faults terminating near the 

bank margins are overlapped along trend by strata of boundary between acoustic units 1 and 2 in basins 

acoustic unit 1. Breached folds probably reflect where the Pliocene-Pleistocene boundary is ill defined 

deformation principally during early late Miocene or are identified as Pliocene-Pleistocene in age. Faults 

Pliocene to Pleistocene time. Buried folds are seen mapped on land adjacent to the offshore Coos Bay 

both in acoustic unit 3, in which folds commonly basin were compiled from Schlicker and Deacon (1974) 

appear to have been truncated by erosion before the and Beaulieu and Hughes (1975). 

1 0 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
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Figure 8. High-resolution seismic-reflection profile (78-42.) extending eastward from Heceta Bank across adjacent shelf. Note 

contact between acoustic units 1 and 2. (arrows), deformation of acoustic unit 1, and enhanced reflector indicating possible shallow 

gas accumulation • 
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Figure 9. High-resolution seismic-reflection profile (78-3Z} across inner shelf north of Cape Arago. Note seaward-thinning wedge 

of Holocene sediment overlying acoustic unit 1; fault cuts unit 1 but not overlying Holocene sediment. 

..... ,,. ~·I . 
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10 2 7.5 km en I 0 0 .0 - . z 

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Figure 10. Deep-penetration seismic-reflection profile (78-38) across shelf and slope between 

Heceta Bank and Cape Arago. Note thickening of acoustic unit 1 in shallow syncline on upper slope; 

also note contacts between acoustic units 1 and 2, 2 and 3, and possible buried slide zone in acoustic 

unit 1. 

Folds 

The axial trend of the Coos Bay basin is approximately north (fig. 12). Fold axes in acoustic units 1 

and 2 have nearly the same trend, about N.10°W. 

Locally, as on the upper slope west of Cape Arago, 

folds trend north for most of their length but swing to 

a more northwesterly trend (about N.40°W .) near their 

south end. Folds in the Coos Bay basin are 2 to 12 km 

in breadth (avg 5 km), and several have a mappable 

length of 25 km (max 40 km). Folds involving acoustic 

units 1 and 2 are mainly symmetrical or asymmetrical 

with east-dipping axial planes that suggest a principal 

compressive stress from the west; some folds have 

west-dipping axial planes. Although most folds on the 

Oregon continental margin appear to be of "a normal 

compressional type" (Braislin and others, 1971; Snavely 

and others, 1980), reflecting underthrusting at the 

continental margin, some symmetrical folds on the 

upper slope (fig. 11) appear to be diapiric in origin. 

The outer continental shelf and slope off central 

Oregon have a history of Tertiary and Quaternary 

deformation that extends to the present time. Modern 

rates of uplift estimated for the Oregon continental 

margin range from 100 to 1,000 mm/m.y.; the most 

rapid rate is associated with uplift of the coastal 

terraces at Cape Blanco and with the uplift of abyssal 

deposits on the lower continental slope of central 

Oregon (Kulm and Fowler, 1974b; Snavely and others, 

1980). Assemblages of benthic foraminifers reflect 

depths greater than those from which they were 

collected and thus indicate uplift of the enclosing 

rocks subsequent to deposition. The upper Miocene 

and lower Pliocene rocks on Heceta Bank appear to 

have been uplifted 900 to 1,000 m, and the middle 

Pliocene strata on Coquille Bank as much as 500 to 600 

m (Kulm and Fowler, 1974b). 

Three discrete episodes of deformation are 

indicated by major unconformities apparent in seismicreflection profiles collected from the inner continental 

shelf (see figs. 10, 15, 16). The youngest 

unconformities separate acoustic units 1 and 2, and 

acoustic units 2 and 3; the oldest is within acoustic 

unit 3. These unconformities are inferred to be 

Pliocene-Pleistocene, early late Miocene, and middle 

to late Eocene in age, respectively. Angular 

discordance between acoustic units is greatest on the 

inner shelf and in the vicinity of Heceta and Coquille 

Banks. The area between the banks and shoreline 

consists of a series of generally north-trending synclines (fig. 12) filled with Quaternary sediment that 

onlaps folded Pliocene and older rocks on the adjacent 

banks and shelf (fig. 7). Some of these synclines may 

be superimposed on basins in older rocks. A thick 

sedimentary sequence beneath the outer shelf and 

inner continental slope is also suggested by the 

presence of a large negative free-air gravity anomaly 

(Dehlinger and others, 1968; Kulm and Fowler, 1974b). 

A multichannel seismic-reflection profile across 

the continental shelf and slope near the south end of 

Heceta Bank suggests that this bank was formed by a 

shoreward-steepening series of thrust faults associated 

with east-west compression of the continental margin 

(Seely and others, 1974). The average strike of upper 

Miocene to middle Pliocene strata on Heceta Bank is 

N.55°E. Pleistocene strata, however, strike N.30°E., 

perhaps reflecting a change in the direction of subduction during Pliocene to Pleistocene time. Heceta and 

Coquille Banks are characterized by positive free-air 

gravity anomalies (Dehlinger and others, 1968), consistent with the presence of Paleogene strata at shallow 

depths. 

Acoustic unit 2 on Heceta Bank is folded and 

truncated and is onlapped locally by acoustic unit 1. 

Deformation of the bank continued during deposition 

of acoustic unit 1, as this unit is folded and contains a 

local unconformity on the east side of the bank (fig. 

7). Seismic-reflection profiles suggest that acoustic 

unit 3 crops out locally on the bank along the axes of 

anticlinal folds, although this suggestion has not been 

verified by sampling. Thus, our data indicate that the 

most recent episode of deformation on Heceta Bank 

began during the Pliocene (probably late Pliocene) and 

continued into the Pleistocene. The bank was 

Structure 13 
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Figure 11. Deep-penetration (A) and high-resolution (_~) seismic-reflection profiles (77-14) across slope northwest 

of Cape Arago. Note contacts between acoustic units 1 and 2, 2 and 3, and apparent creep of surface sediment at 

east ernd of high-resolution record; also note young fold with as much as 250 m of sea-floor relief and showing 

evidence of piercement. Two course reversals appear in figure 11~ and so diapiric fold is crossed three times; one 

course reversal and two crossings of diapir appear in figure 11_!!. 

12.5 km 

I 

EAST 

EAST 
44' 

30' 

43" 

HECETA 

BANK 

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50 KILOMETERS 

Heceta 

Head 

Figure lZ. Structural geologic map of southern Oregon continental margin. Explanation is on next page. 

Structure 15 
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EXPLANATION 

FOLDS-Solid lines indicate folds that produce sea · floor relief: 

dashed where folds are breached at sea floor: dotted 

where folds are buried: queried where continuity is 

inferred 

FAULT TRACES - Solid where well located: dashed where 

inferred: queried where questionable 

FAULT SURFACE EXPRESSION AND AGE-Symbol appears on downthrown side of fault where relative move -

ment could be determined: symbo.l centered on fault 

trace where movement not determined 

Bars on downthrown side indicate sea-floor offset: bars 

centered on trace indicate fault reaches sea floor. but 

offset was not evident 

Fault cuts sediment interpreted to be Holocene in age 

Fault cuts acoustic unit 1. interpreted as Quaternary in age 

Fault cuts acoustic units 1 and 2 {undifferentiated). where 

contact could not be determined ; cuts inferred to be 

Pliocene and (or) Pleistocene in age 

Fault cuts acoustic unit 2 . inferred to be late Miocene to 

Pliocene in age 

Fault cuts acoustic unit 3. inferred to be Eocene to middle 

Miocene in age 

Figure lZ. Continued 

truncated during Pleistocene low stands of sea level 

and does not appear to have been uplifted substantially 

since that time, suggesting that the intensity of 

deformation diminished during the late Quaternary. 

Coquille Bank is a north-trending asymmetric 

doubly plunging anticline. The west limb is inferred by 

Kulm and Fowler (1974b) to be downfaulted, on the 

basis of bathymetry and the absence of the thick 

Pliocene and Pleistocene section present on the east 

limb. The youngest pre-Quaternary rocks exposed in 

the axial parts of Coquille Bank are siltstone and 

claystone of middle Pliocene age (Kulm and Fowler, 

1970, 1974b; Kulm, 1980). Local thinning of the lower 

part of acoustic unit 1 from the base by onlap onto 

uparched strata of acoustic unit 2 suggests that the 

banks had positive sea-floor relief during the 

deposition of acoustic unit 1 (fig. 14). Acoustic unit 1 

WEST 

0 

0.0 I 

1.0 

2.0 

is truncated on Coquille Bank, and strata in the upper 

part of this unit within the structural saddle east of 

the bank thin toward the west and are upwarped along 

the bank margin (see figs. 23, 24), a relation suggesting 

that deformation continued well into the time of 

deposition of this unit. Additionally, Coquille Bank 

stands 60 to 90 m above the depth that would be 

expected if it were beveled concurrently with the 

wave-cut platform on the inner shelf east of the bank 

(see fig. 24). This platform presumably was cut during 

the late Wisconsin low stand of sea level. Either 

Coquille Bank stood above sea level as an island and 

resisted erosion during late Pleistocene timeseemingly unlikely because the core of the bank 

consists of poorly indurated and easily eroded siltstone 

and claystone-or it has been uplifted differentially 

since the last Pleistocene low stand. The amount of 

Holocene uplift (60 to 90 m) suggested for acoustic 

unit 1 is generally consistent with previous estimates 

(Kulm and Fowler, 1974b) of as much as 100 m uplift 

of Pleistocene deposits on the outer banks based on 

assemblages of benthic foraminifers. These observations suggest that the latest major episode of deformation on Coquille Bank commenced during late Pliocene 

time, increased in intensity during Pleistocene time, 

and continued into Holocene time. 

Piercement structures in coastal Washington 

with cores of Miocene siltstone were described by Rau 

and Grocock (1974). Snavely and others (1977) 

estimated that 50 to 100 such diapiric intrusions are 

present on the Washington and Oregon continental 

shelf. Several folds on the upper continental slope 

between Heceta and Coquille Banks appear to b~ 

recently active because they have sea-floor relief. 

One such fold, approximately 60 km northwest of 

Coquille Point, that was crossed by tracklines 77-14 

(fig. 11), 78-48, and 78-49 may be diapiric in origin. 

This fold shows evidence of piercement and about 250 

m of sea-floor relief. The lower part of acoustic unit 

1 is uplifted and truncated against the east flank of 

5 

I 

10 km 

I 

EAST 

Figure 13. Deep-penetration seismic-reflection profile {78-40) across inner shelf southeast of 

Heceta Bank. Note breached fold in which acoustic unit 3 is exposed at sea floor. 

16 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
f 

i 

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0 

z 

0 

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(/) 

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uJ 

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i= 

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fWEST 

0.0-

1.0 

2.0 

0 5 

• _v;. ~ .. - '' 

10 15 km 

. :; ! :_ ;.:~· . , .. 

.x:· · 

Figlue 14. Deep-penetration seismic-reflection profile (77-ZZ) across shelf and slope north of Coquille Bank. Note contacts 

between acoustic units 1 and Z, Z and 3, and onlap of lower part of acoustic unit 1 (below dotted line) onto acoustic unit z on 

east flank of Coquille Bank. 

EAST 

'' ~:-. 
the fold (fig. 11), and the upper part of acoustic unit 1, 

probably late Quaternary in age and the youngest 

sedimentary unit mapped here, thins depositionally as 

it approaches this feature. The core lacks coherent 

internal structure in seismic-reflection records, and 

samples indicate that it is composed of Pliocene 

(probably late Pliocene) mudstone (samples 14, 15, fig. 

6). Uplift apparently began during Quaternary 

(Pleistocene) time. The great sea-floor relief, thinning 

of young sediment against the flanks of the fold, and 

absence of sediment on the crest of the fold suggest 

that this structure may be undergoing uplift at 

present. 

Recent deformation without piercement or other 

evidence of diapirism is shown by other folds on the 

upper slope. For example, a fold crossed by tracklines 

77-18 and 78-30 (fig. 4) 20 to 25 km south of the 

piercement structure described above shows as much 

as about 150 m of sea-floor relief. Acoustic unit 1 

thins abruptly and is upturned along the east flank of 

this fold, and it is absent from the fold crest (figs. 15, 

16). This deformation appears to be wholly of 

Qua ternary age. 

Faults 

Although Quaternary faults are not so common 

as in the northern California continental margin, 

several have been mapped from seismic-reflection 

records in the vicinity of the offshore Coos Bay 

basin. Snavely and others (1977, 1980) noted that 

many north-trending faults on the Oregon coast offset 

geologically youthful strata and may be part of a 

north-trending group of faults mapped on seismic 

records from the inner shelf. Kulm and Fowler (197 4b) 

suggested that the steep linear escarpments west of 

Heceta and Coquille Banks are fault controlled. Most 

of the faults mapped during this study cannot be 

correlated with certainty between adjacent seismicreflection lines (fig. 12). A few faults, however, can 

be traced between two adjacent lines, a distance of 5 

to 8 km, and several can be traced for as much as 12 

to 15 km. Faults that terminate in acoustic unit 1, 

inferred to be Quaternary, and those that terminate in 

acoustic unit 2, inferred to be late Miocene and 

Pliocene, are about equal in number. Those faults 

cutting acoustic unit 2 are generally evenly distributed 

on the middle to inner continental shelf, upper slope, 

and banks, whereas those cutting acoustic unit 1 are 

most common on the shelf and bank tops. Faults on 

the inner shelf and bank tops commonly trend between 

N.30°W. and north; on the upper slope, faults 

terminatin~ in acoustic unit 2 have trends between 

about N.15 W. and N.5°E. 

A group of faults trending slightly west of north 

cuts acoustic unit 1 on the inner continental shelf 

about 15 to 25 km north of Cape Arago (figs. 9, 12, 

17). One 13-km-long fault of this group cuts deposits 

of presumed Holocene age, and several others may 

reach the sea floor. Further study of faults in this 

area is warranted as they appear to have been active 

during Qua ternary time and are near the Coos BayNorth Bend-Empire area. 

Acoustic unit 1 is cut by a fault (fig. 18) with 

possible major displacement on the southeast side of 

Heceta Bank (Kulm and Fowler, 197 4b). This fault 

trends N.30°W., is downthrown to the northeast, and is 

at least 15 km long (fig. 12). It and the cluster of 

faults north of Cape Arago have the same apparent 

age and trend; although those faults have not been 

traced between Cape Arago and Heceta Bank, the 

similarity in style of faulting suggests that they may 

be part of the same fault system. 

South-southwest of Cape Arago, acoustic unit 2 

is cut by a fault with a substantial vertical offset (fig. 

19). This fault trends about N.30°W ., is downthrown to 

the east, and has sea-floor expression that suggests 

recent movement. 

The Port Orford shear zone, a north-northwesttrending fault zone as much as 3 km wide that shows 

evidence of right slip, extends offshore from a point 

just northeast of Cape Blanco (Dott, 1962, 1965, 1971; 

Koch, 1966). This zone was active in post-middle 

Eocene and post-Miocene time. Pliocene and (or) 

Pleistocene activity on this fault zone is suggested by 

a possibly related north-trending fault at Cape Blanco 

that juxtaposes lower Pliocene marine sandstone with 

Mesozoic and Eocene strata and is capped by 

Pleistocene marine-terrace deposits (Dott, 1971). 

Recent tectonic activity in the Cape Blanco-Coos Bay 

area is also indica ted by uplifted and faulted marine 

terraces of Pliocene and Pleistocene age (Janda, 

1969). Dott (1979) proposed that the Port Orford shear 

zone and similar northwest-trending, approximately 

echelon shear zones that cut the Pacific margin of 

Oregon and northern California are part of the east 

boundary of the "Humboldt plate" (Herd, 1978). This 

plate, possibly more appropriately termed the 

"Humboldt block," is a narrow intracontinental 

structural block situated at the complex boundary 

between the North American, Pacific, and Gorda 

crustal plates; its proposed east boundary comprises a 

series of northwest-trending strike-slip faults. 

The offshore extension of the Port Orford shear 

zone was not identified in this study. Although 

Mackay (1969) may have crossed this zone about 10 km 

north of Cape Blanco, he speculated that it is covered 

by post-Miocene strata farther northwest on the 

shelf. However, shear associated with this zone may 

also be transferred to other northwest-trending faults 

in the offshore, such as those mapped during this study 

north of Cape Arago and on Heceta Bank that show 

evidence of Quaternary offset. Such a transfer of 

shear between echelon faults would accord with Herd's 

(1978, p. 725) observation that the line of faults 

forming the east boundary of the "Humboldt plate" is 

so young geologically "that there has been insufficient 

time to integrate the fault zones into a single surficial 

break." The possible existence in the Coos Bay basin 

of a zone of echelon strike-slip faults forming the 

boundary of a major structural block has obvious 

import to offshore development in the area. Tectonic 

activity associated with such a zone would probably be 

low in comparison with that farther south, judging 

from the present low level of seismicity and tectonic 

activity associated with the boundary between the 

Juan de Fuca and North American plates (Smith and 

Knapp, 1980). Nonetheless, additional study is 

required to determine the style and significance of 

Quaternary faults in the Coos Bay basin, their relation 

18 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
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depositional thinning and deformation of acoustic unit 1 along flanks of this fold. 

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Arago. Note series of faults extending to, or near, sea floor in acoustic units 1 and 2.. Acoustic unit 3 may crop out on sea 

floor in the area indicated by the brackets • 
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and adjacent shelf. Note faults extending to sea floor in acoustic unit 1. 

to the Port Orford and similar shear zones farther 

south, and the relation of these zones to the 

"Humboldt plate." " 

SEISMICITY 

Oregon is less active seismically than other 

Pacific coast States and has no record of major earthquake destruction. The record of seismic events, 

however, extends back only to the late 1800's and thus 

is too short to be an accurate predictor of future 

earthquake size or frequency. 

The major seismic zones in this region lie 200 to 

300 km or more off the coast of southern Oregon and 

northern California and are related to a series of 

growing oceanic ridges connected by transform 

faults. Plots of epicenters recorded conventionally 

from the Northeast Pacific for the period 1954-63 

show that earthquake activity off northern California 

and Oregon is located along the Gorda Ridge, the 

Mendocino Fracture Zone east of the Gorda Ridge, the 

Blanco Fracture Zone between the Gorda and Juan de 

Fuca Ridges, and within the Gorda Basin (fig. 2) (Bolt 

and others, 1968; Tobin and Sykes, 1968; Couch, 1980; 

Smith and Knapp, 1980). Relatively little seismic 

activity is associated with the Juan de Fuca Ridge or 

with the continental margin of Oregon north of Cape 

Blanco (Couch, 1980). A plot of 1,025 events recorded 

by hydroacoustic methods reveals a concentration of 

epicenters along the Gorda Ridge and the Blanco 

Fracture Zone (Northrup and others, 1968). Faultplane solutions for the Blanco Fracture Zone, the 

Gorda Basin, the Gorda Escarpment, and the Cascadia 

Basin (fig. 2) are consistent with right slip along southeast-trending faults; those for the Gorda Ridge are 

consistent with normal faulting along a plane parallel 

to the ridge (Couch and Pietrafesa, 1968; Tobin and 

Sykes, 1968). These solutions suggest that the regional 

seismicity is dominated by spreading at the Gorda 

Ridge, accompanied qy displacement along dextral 

transform faults (Blanco and Mendocino Fracture 

Zones) and by underthrusting at the northern 

California-southern Oregon continental margin. 

Right-lateral motions in the Gorda Basin may reflect 

the transfer of motion by transcurrent faulting from 

the San Andreas fault across the basin to the Blanco 

Fracture Zone (Bolt and others, 1968). The low level 

of seismic activity associated with the Juan de Fuca 

Ridge and the absence of an oceanic trench and 

Benioff zone along the continental margin of centralnorthern Oregon and Washington suggest that 

subduction is episodic. 

The locations of earthquakes that occurred 

during the period 1841-1975 in the area extendi~ from 

lat 42°30' to 44°10' N. and from long 124 00' to 

126°00' W. are plotted on figure 20. Location 

accuracy of earthquakes in this region before 1962 is 

estimated at 8 km. Since 1963, five recording stations 

in Oregon (Corvallis/OSU, Portland/OMS!, Klamath 

Falls/OTI, Blue Mountain/NOS, and Pine Mountain/DONASA) and improved traveltime curves have reduced 

the average uncertainty to about 4 km for earthquakes 

of M 3.5 (Couch and Lowell, 1971). Reported depths 

of earthquakes in Oregon indicate shallow foci, 

although data are scant. Several workers estimate an 

average focal depth of 5 to 15 km for earthquakes in 

that part of Oregon west of the Cascade Range (Couch 

and Lowell, 1971). Focal depths of earthquakes associated with the Gorda Ridge and the Blanco Fracture 

Zone are largely unknown but are considered to be 

shallow-EJ,pproximately 5 km (Brune, 1968). 

Only one earthquake has been recorded within 

the offshore Coos Bay basin and none has been 

recorded for more than 100 years within the adjacent 

coastal area (fig. 20; Schlicker and Deacon, 1974; 

Beaulieu and Hughes, 1975). Beaulieu and Hughes 

(1975), however, suggested that the earthquake 

potential for the area surrounding and south of Coos 

Bay is at least moderate and that earthquakes of 

modified Mercalli intensity (MMI) V to VII are to be 

expected. The total seismic-energy release in the 

western Oregon Coast Range province for the 100-year 

period from 1870 to 1970 has been computed as 

approximately equivalent to one M = 5.0 earthquake 

per decade (Couch and Lowell, 1971). The maximum 

MMI reported for an earthquake in this region is VI, 

2 2 Reconnaissance Geology and Geologie Hazards of the Offshore Coos Bay Basin, Oregon 
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Figure 19. High-resolution seismic-reflection profile (77-22) across shelf southwest of Cape Arago. Note fault {arrow) in acoustic 

unit 2 that offsets sea floor. Also note apparent onlap of acoustic unit 1 onto unit 2, and deformation of unit 1 on shelf edge at 

west side of record. 

EAST 
25 50 KILOMETERS 

Figure ZO. Sketch map of southern Oregon continental margin, showing locations and magnitudes of seismic 

events. Dots from NOAA Earthquake Data File, 1841-1975 (NGSDC); circles from Couch and others (1974). 

2 4 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
and the corresponding e~pected maximum ground 

acceleration is 31.6 cm/s . Although the offshore 

Coos Bay basin is devoid of epicenters for the period 

1841-1974, the Port Orford earthquake of 1973, 

centered near Cape Blanco, had a magnitude of about 

6.3 (computed from observed intensity). An earthquake of M = 6.0, centered at sea about 90 km west of 

Cape Blanco, was recorded in 1938 (fig. 20; Couch and 

others, 1974). 

The possibility of damage resulting from an 

earthquake in adjacent areas, notably the northern 

California continental margin, should be considered in 

an assessment of the geologic hazards to development 

in the Coos Bay basin. The effects, other than ground 

motion, of such earthquakes may include sea-floor 

mass movement, liquefaction, differential subsidence, 

and tsunamis. Couch and Deacon (1972) indicated that 

the recurrence interval of an M = 8.0 earthquake in the 

Gorda Basin is 130 years, andearthquakes as large as 

M = 7.3 have been recorded along the Mendocino 

Escarpment. A 1922 earthquake of M = 7.3 (calculated 

maximum intensity X) centered ifl the Gorda Basin 

about 90 km northwest of Cape Mendocino (Couch and 

others, 197 4) had an MMI of V in Coos Bay; Beaulieu 

and Hughes (1975), assuming similar attenuation of 

seismic waves, estimated that the "largest possible" 

earthquake along the Mendocino Escarpment would 

generate an MMI of VII in the Coos Bay area and that 

the corresponding gr~nd acceleration would be less 

than 0.10 g_ (48.8 cm/s ). 

SEA-FLOOR INSTABILITY 

Sea-floor instability here refers to conditions 

that result from sea-floor failure due to mass movement, liquefaction, or tectonic deformation of seafloor sediment. Potentially unstable sea-floor 

conditions identified in the offshore Coos Bay basin 

are associated with subaqueous sediment slides (both 

surficial and buried), sediment creep, active uplift and 

possible diapirism, accumulations of shallow gas, and 

possible gas hydrates. Additional information 

concerning instability in the coastal area adjacent to 

the Coos Bay basin, and the engineering properties of 

geologic units exposed in this region, is contained in 

the reports by Schlicker and Deacon (197 4) and 

Beaulieu and Hughes (1975). 

Sediment Slides, Flows, and Creep 

Subaqueous slides are mass movements of rigid 

or semiconsolidated masses along discrete shear 

surfaces, with relatively little internal deformation 

(Dott, 1963). Slides are commonly identifiable on 

seismic-reflection records by the presence of some or 

all of the following major characteristics: (1) A headscarp where the slip surface extends upward to the sea 

floor, (2) compressional ridges and folded and 

contorted subbottom reflectors resulting from smallscale thrusting and folding at the toe of the slide, (3) 

transverse (tensional) cracks in the body of the slide, 

(4) evidence of rotation with limited internal deformation of reflectors, and (5) the presence of a slip 

surface, which may be concave upward or planar, 

represented by a discrete failure plane or by an 

intensely deformed zone beneath the slide mass. The 

term "slump" is commonly applied to a slide that shows 

evidence of rotational movement along a curved slip 

surface. Subaqueous slides have been documented on 

slopes of less than 1° and range upward in size from 

simple failures covering tens of square meters to 

composite failure zones thousands of square kilometers 

in area and hundreds of meters thick (Moore, 1961; 

Heezen and Drake, 1964; Lewis, 1971; Hampton and 

Bouma, 1977). Subaqueous mass flows involve the 

gravitational downslope movement of water-saturated 

unconsolidated sediment; the moving mass may behave 

plastically or as a highly yiscous fluid, and movement 

may be slow or rapid (Dott, 1963). The velocity and 

displacement of flow characteristically decrease with 

depth below the sea floor, so that the deposit lacks a 

distinct slip surface. Subaqueous-flow deposits are 

identifiable on seismic-reflection records by (1) the 

presence of anomalously thick sediment masses apparently detached from underlying strata, (2) the absence 

of an identifiable slip plane, and (3) acoustic transparency or chaotic internal structure. Sediment creep in 

the marine environment is a poorly understood and 

poorly documented phenomenon. As used here, this 

term refers to the slow more or less continuous 

downslope movement of the upper layers of unconsolidated sediment. The occurrence of creep is inferred 

from seismic-reflection records by the presence of 

hummocky sea-floor topography, deformed but identifiable acoustic bedding in the upper sediment layers, a 

downward decrease in the degree of deformation, and 

the apparent absence of a slip surface. Creep may 

affect large areas and is commonly associated with 

other types of failure. Zones of past sea-floor failure 

should be viewed as having an unknown potential for 

renewed movement. Some of these zones may have 

become more stable as a consequence of having failed, 

whereas others may have unchanged or even reduced 

stability. Consequently, the geologic and engineering 

properties of zones of past failure and of adjacent 

deposits require study on a case-by-case basis. 

Few areas of modern sea-floor mass movement 

were identified in seismic reflection records from the 

offshore Coos Bay basin (fig. 21). This is surprising 

considering the large areas of sea-floor failure present 

about 100 km south in the Eel River basin (Field and 

others, 1980), a geologic setting that is similar in many 

respects to the Coos Bay basin, and also considering 

the locally thick accumulations of Quaternary 

sediment (see fig. 27) in the basin on relatively steep 

(2°-4°) slopes. The very high rate of sediment influx 

from the Klamath and Eel Rivers and the more active 

seismic environment of the northern California region 

may be responsible for the abundance of modern slope 

failures in that area. Failures may also be more 

prevalent on the middle to lower continental slope 

west of the study area, where thick sections of faulted 

upper Tertiary and Quaternary sediment are present 

and slopes of 7°-14° are common. In addition, peat 

and devitrified volcanic ash, both of which are present 

in Cenozoic deposits onshore and on the continental 

shelf, may contribute to failure by reducing shear 

resistance. It is noteworthy that ground failure is 

common in water-saturated upper Tertiary and 

Qua ternary deposits along the Oregon coast and on 

Sea-Floor Instability 2 5 
30' 

EXPLANATION 

0:;) ACTIVE OR RECENT ROTATIONAL SLUMPS AND DEBRIS SLIDES-Characterized by hummocky sea floor. rotation of reflectors. and evidence of separation and move -

0 ment 

CHAOTIC SUBSURFACE REFLECTORS-Interpreted as 

buried slumps and slides: dashed where boundaries are 

uncertain 

UNSTABLE SURFACE SEDIMENTS-Bulging and separation of reflectors suggest incipient failure : dashed where 

boundaries are uncertain 

ACOUSTIC ANOMALIES-Interpreted from seismic-reflection data as possible shallow gas accumulations 

POSSIBLE GAS SEEPS-Interpreted from seismic-reflection 

records based on presence of plumes in water column. apparent cratering of sea floor. and acoustic anomalies in shallow subbottom 

ll' 

25 50 KILOMETERS 

Fig\ue 2.1. Sketch map of Coos Bay basin, showing areas of sea-floor failure, hydrocarbon seepage, and 

shallow gas accumulation. 

2 6 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
steep slopes elsewhere in the Oregon and Washington 

offshore region. 

Slides were identified in the Coos Bay basin on 

the west flanks of Heceta Bank and Coquille Bank (fig. 

21), areas having relatively steep slopes but generally 

lacking thick deposits of Quaternary sediment. These 

3lides occur in water depths of 150 to 450 m and 700 to 

900 m, respectively, and each is about 15 m thick. The 

deposit west of Heceta Bank appears to be a rotational 

slump about 4 km long (fig. 7), and the deposit west of 

Coquille Bank is a slide of undetermined type about 2 

km across. 

Two areas of possible sediment creep were 

mapped on slopes at depths of 500 to 600 m. The 

southern area appears to be about 3.5 km long and has 

a zone of disturbance extending about 10 m below the 

sea floor. This area of creep may be superimposed on 

an older buried slide, and so the unstable zone could 

extend considerably deeper. The northern area, 40 km 

northwest of Cape Arago, is crossed by two seismic 

lines. The disturbed zone is about 2.8 by 4 km, and 

extends to a depth of 5 to 8 m below the sea floor (fig. 

11). 

Buried zones of chaotically disrupted (broken and 

rotated) reflectors stratigraphically low in acoustic 

unit 1 were identified in the central part of the basin 

south of Heceta Bank (fig. 21). Similar zones have 

been mapped in the Eel River basin (Field and others, 

1980), where they are associated with extensive 

modern slump deposits which they resemble in acoustic 

character. These buried failure zones are interpreted 

as slides in Quaternary sediment, possibly associated 

with rapid sediment influx during a low stand of sea 

level in Pleistocene time. The largest zone (fig. 10) is 

in central Coos Bay basin below the edge of the 

modern shelf at water depths of 250 to 600 m; it is 

nearly 20 km long and 13 km wide at its2

widest point, 

and covers an estimated area of 190 km . Reflectors 

in the lower part of this zone are backrotated, and the 

degree of disruption of reflectors increases downslope. The upper surface of the zone ranges from 150 

to 300 ms in acoustic depth below the sea floor 

(equivalent to a subbottom depth of about 125-250 m, 

at an assumed velocity of 1,650 m/s), and the zone is 

as much as 250 to 300 ms (about 200 to 250 m) thick. 

Smaller zones of disrupted subsurface reflectors occur 

south of Heceta Bank and east of the buried slump 

area described above. These areas, which are crossed 

by only one seismic line each, are 5 and 2 km across, 

respectively. 

Areas of Uplift 

Several folds that show sea-floor relief on the 

upper continental slope between Heceta and Coquille 

Banks appear to be active; one of these folds, 60 km 

northwest of Coquille Point, shows evidence of piercement (figs. 11, 12; see subsection above entitled 

"Folds"). The uppermost (youngest) strata of acoustic 

unit 1 thin depositionally and are deformed against the 

flanks of these features and are absent from their 

crests. Subjacent strata are uparched over the folds, 

and sea-floor relief of as much as 250 m is ~pparent on 

seismic-reflection profiles (figs. 11, 15, 16). Piercement occurs locally along the axis of a fold about 45 

km northwest of Cape Arago (fig. 11). The core of this 

fold is acoustically incoherent, and the irregularity of 

topography on the ridge crest, where core material is 

exposed, is consistent with recent flowage. Samples of 

sheared olive-gray mudstone of Pliocene (probably late 

Pliocene) age were obtained from the ridge crest 

(samples 14, 15, fig. 6). Uplift here apparently 

commenced in Quaternary time and may be presently 

active. 

Diapirism has been documented from the 

northern California continental margin (Field and 

others, 1980; Field and Gardner, 1980), and diapiric 

intrusions are thought to be abundant on the 

Washington and Oregon continental shelf (Snavely and 

others, 1977) and along the Washington coast. In the 

latter area, Pliocene strata are intruded locally by a 

pervasively sheared melange of volcanic and 

sedimentary rocks in a mudlike matrix of clay and 

siltstone fragments which contain Miocene 

foraminifers (Rau and Grocock, 1974). The association 

of gas seeps and abnormally high subsurface gas 

pressures with the coastal Washington intrusions 

suggests that diapirism there is associated with 

overpressured zones in the subsurface. Geochemical 

analyses for hydrocarbon gases were performed on four 

sediment samples collected on and adjacent to the 

piercement structure mapped in the offshore Coos Bay 

basin; these analyses did not reveal evidence of gas 

leakage into near-surface sediment (fig. 22; samples 

15G, 16G, 17G, and 18G, table 1; Kvenvolden and 

Rapp, 1981). However, a sample from Quaternary 

sediment ponded on the crest of a piercement 

structure in the offshore Eel River basin of northern 

California contained anomalously high concentrations 

of high-molecular-weight hydrocarbon gases and 

gasoline-range hydrocarbons (Field and others, 1979; 

Kvenvolden and others, 1980). These gases are 

interpreted to have had a thermogenic origin within 

the underlying Neogene sedimentary section and to 

have leaked through fractures into unconsolidated 

sediment ponded locally on the ridge crest. This 

occurrence of thermogenic hydrocarbons in the Eel 

River basin and the association of gas seepage with 

diapirism in coastal Washington suggest that further 

sampling of diapiric structures might also reveal the 

presence of thermogenic hydrocarbons in the offshore 

Coos Bay basin. 

These youthful folds clearly reflect recent 

tectonic instability and thus merit careful study. 

Their potential as hazards to offshore development, 

however, cannot be assessed without additional 

information concerning their distribution, manner of 

emplacement, rate and style of uplift, relation to 

overpressured zones, and association with other forms 

of sea-floor instability. 

Shallow Gas Accumulation 

Natural gas of biogenic and thermogenic origin 

may be present in marine sediment. Biogenic gas, 

principally methane, is derived from bacterial alteration of organic rna terial in the sediment (Claypool and 

Kaplan, 1974). Thermogenic gas, containing methane 

as well as heavier hydrocarbon gases, is a byproduct of 

petroleum formation (Bernard and others, 1976). The 

Sea-Floor Instability 2 7 
125° 

43° 

30' 

43° 

10 

124°30' 

Umpqua River 

20 KILOMETERS 

Figure zz. Sketch map of Coos Bay basin, showing locations of cores analyzed for hydrocarbon-gas content. 

presence of thermogenic gas in sediment can reflect the pore space of unconsolidated sediment can reduce 

an overpressured (that is, in excess of hydrostatic the shear strength of the enclosing sediment, thereby 

pressure) zone that is discharging gas into the increasing the likelihood of failure; under some 

overlying strata either directly or by way of a conduit circumstances gas-charged sediment can liquefy 

such as a fault or bedding plane. Inadvertent spontaneously when subjected to cyclic loading, as 

penetration of an overpressured zone or gas-escape from earthquake shaking (Hall and Ensiminger, 1979). 

conduit accompanied by sudden venting of gas at the Consequently, the identification of areas of possible 

surface can pose a hazard to drilling operations. shallow gas accumulation is an important adjunct to 

Additionally, gas of either type present as bubbles in studies of sediment instability in the offshore. 

2 8 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
Table 1. Hydrocarbon gases in sediment, offshore Oregon 

[From Kvenvolden and Rapp ( 19 81 ) J 

water 

Interval depth 

Station (em) {m) c, c2 

and sample 

15G 0-10 543 2,060 95 

15G 50-60 543 397 

15G 100-110 543 80 

16G 0-10 679 42 

16G 50-60 679 3,930 70 

16G 100-110 679 418 

16G 150-160 679 28 

16G 200-210 679 5,650 32 

17G 0-10 675 1,740 45 

17G 100-110 675 3,341 35 

17G 200-210 675 251 

18G 0-10 610 1,040 22 

18G 100-110 610 306 

18G 200-210 610 271 

2 3G1 0-10 15 7 2,900 56 

2 3G1 65-75 157 11,000 59 

2 3G3 0-10 15 7 5,010 106 

2 3G3 18-28 157 4,480 106 

3 3G1 44-54 15 0 12,500 15 3 

33G3 76-86 14 9 1,950 26 

34G1 65-75 15 0 3,610 96 

34G2 65-75 15 0 8 5, 7 00 808 

35Gl 42-52 15 0 1,980 43 

36Gl 77-87 150 8,810 82 

53G1 100-110 217 5,830 37 

53G2 100-110 213 6,480 26 

5 4Gl 140-150 259 36,200,000 4,270 1S4G1 225-235 259 31,000,000 3,180 

54G2 140-150 260 17,800 1,860 

54G2 219-229 260 25,300,000 2,520 

5 4G3 140-150 259 4,510,000 3,870 1s4G3 255-265 259 35,000,000 2,990 

SSG 113-123 237 119,000 1,280 

1 rsotope sample taken. 

Gas in sediment is inferred from medium- and 

high-resolution seismic-reflection records on the basis 

of one or more of the following criteria: (1) Amplitude 

anomalies, apparent as enhanced or "bright" subbottom 

reflectors, that denote significant acoustic velocity 

contrasts (fig. 8); (2) the sharp termination or apparent 

displacement of reflectors, commonly associated with 

acoustically turbid zones; (3) the absence of surface 

multiples, indicating absorption of the acoustic signal 

(Nelson and others, 1978) and (4) the presence in 

reflectors of "pulldowns," apparent depressions 

re~ulting from the decreased velocity of sound in 

gaseous sediment and consequent delayed arrivals of 

acfuustic returns. Side-scan sonographs and underwater 

video or photographic coverage can show seep mounds 

or era ters on the sea floor and gas bubbles in the water 

column. In all cases, sampling and geochemical 

Concentrations in wet sediment (nL/L) 

c1 

c2:1 c3 c3: 1 i-c4 ...!!_-C4 

c2 + c3 

1977 

82 67 126 7 18 1 3 

88 49 98 7 11 33 

22 15 42 12 0 

82 43 84 20 

28 21 59 59 

35 18 66 26 

53 36 120 10 10 31 

31 24 82 7 3 132 

43 54 12 0 17 17 31 

50 48 113 14 14 29 

1979 

40 57 75 11 15 60 

37 14 59 0 0 49 

49 42 79 11 16 26 

35 50 63 10 12 100 

31 13 72 0 0 35 

44 49 94 9 14 67 

21 19 48 0 0 100 

24 12 57 0 0 17 0 

34 130 37 66 14 8,200 

53 142 60 80 12 9,300 

31 16 5 56 43 12 9 

46 134 0 59 13 9,500 

18 194 57 59 12 1,100 

18 98 50 75 15 11,000 

77 62 102 0 0 89 

analyses are needed to verify the presence of gas and 

identify its origin. 

Areas of inferred shallow gas accumulation in 

the Coos Bay basin range in size from about 0.5 km 

across ~rossed by a single line) to as much as about 

120 km (fig. 21). The largest zones, 20 to 25 km west 

of Cape Arago and Coquille Point, lie in areas of 

Quaternary sedimentation on the lower shelf and upper 

slope, at water depths ranging from 135 m to aDout 

40.0 m (fig. 21). Sampling was conducted in these two 

large areas and on and adjacent to the piercement 

structure northwest of Cape Arago (see fig. 22) to 

determine if gas is present in sea-floor sediment and, 

if so, to identify its origin. The analytical procedures 

and results of this sampling program were reported by 

Kvenvolden and Rapp (1981); table 1 summarizes the 

results. 

Sea-Floor Instability 2 9 
Several mapped areas of gas-charged sediment 

are associated with faults that cut Quaternary 

deposits. One such area, 22 to 23 km southwest of the 

mouth of the Umpqua River (fig. 21), appears to be 

bounded on the east and west by faults (fig. 12). 

Seepage in an area west of Coquille Point appears to 

reach the surface along a fault that cuts the 

Quaternary section (figs. 23, 24). Depth below the sea 

bottom of the upper surface of the gassy zones ranges 

from 10 to 40 ms (approx 8 to 33 m) and averages about 

24 ms (approx 20 m). The sea floor 20 to 25 km southwest of Coquille Point appears to be era tered where 

the apparent gassy zone extends to the surface, and 

high-resolution seismic-reflection records suggest 

bubbles in the water column above the crater (fig. 

24). A side-scan sonograp)l taken in this area appears 

to show a northwest-trending linear crater field, which 

may represent seepage along a fault or faults. The 

inability to obtain long cores hampered attempts to 

confirm the presence of gas in this area. Short cores 

taken from the vicinity of this apparent gas seep area 

and subjected to chromatographic analysis at sea to 

determine their gas content did not contain unusually 

high concentrations of hydrocarbon gases (fig. 22; 

samples 23G, 33G-36G, table 1). There is evidence in 

one sample (34G2), however, that gas concentrations 

may increase with depth, and so gas-charging, though 

not confirmed, is a possibility. Longer cores at:e 

required for further testing at this site. In addition, 

sample 34G2 contains nearly 10 times more methane 

(C ) than do other samples collected in this vicinity, 

altbough the significance of such an indication from a 

single core is unclear. 

Samples 54G1, 54G2, and 54G3, from a possible 

seep area 25 km west of Cape Arago, contain anomalously high concentrations of hydrocarbon gases (fig. 

22; table 1). These samples were collected within and 

adjacent to a crater that may be associated with 

geologically youthful faults near the shelf edge (fig. 

25). The high methane (C1) concentrations in these 

samples suggest that they may be gas charged. The 

methane (C1) to ethane (C2.) ratio, about 10,000, 

suggests the methane is biogemc in origin (Bernard and 

others, 1976). However, carbon-isotopic compositions 

of methane in samples 54G1 and 54G3 are -56.4 and 

-53.3 permil, respectively, at the heavy end of the 

range of values (-89 to -50 permil) observed for 

bacterially generated methane from natural sources 

(Fuex, 1977), but within the range of values (-58 to -40 

permil) observed for gas associated with petroleum 

generation (Kvenvolden and Rapp, 1981). The large 

ethane (C2) to ethene (C2.

1> ratios (apprx 60 to 220) 

also reflect a possible ttiermogenic source for the 

ethane (Kvenvolden and others, 1981). Thus, although 

the major source of methane resides in bacterial 

alteration of organic matter, the gas mixture may also 

have a thermogenic component (Kvenvolden and Rapp, 

1981). In addition, we note that core penetration at 

sample site 54 was nearly twice that at sites 53 and 

55, located outside the crater, approximately 900 m 

east of site 54 and containing relatively little 

methane. The greater core penetration at site 54 may 

directly reflect the effect of gas in reducing the 

bearing strength of sediment at this site. 

Possible Gas Hydrates 

Bottom-simulating reflectors (BSR's) (Scholl and 

Creager, 1973) are conspicuous reflecting horizons 

that cross coherent reflections from bedding planes 

and are approximately concordant with the overlying 

sea bottom. BSR's are present on sparker records 

across the middle and lower continental slope (depth of 

water approx 650 - 2,000 m) seaward of the Coos Bay 

basin, where they characteristically occur at depths 

ranging from 250 to 400 ms below the sea floor. 

Although their origin is uncertain, the BSR's may 

indicate the presence of gas hydrates composed of 

solid, icelike mixtures of natural gas and water 

(Milton, 1976; Kvenvolden and McMenamin, 1980). If 

gas is present in quantity and if extraction proves 

feasible, such zones could become economically 

important. Gas hydrates also may form impenetrable 

barriers to the upward migration of free natural gas. 

Inadvertent penetration of such a barrier presumably 

could result in the sudden, uncontrolled venting of gas 

to the surface, a condition that would be hazardous to 

drilling operations. 

CHARACTERISTICS OF SURF ACE SEDIMENT 

Sampling in the Coos Bay basin was directed 

primarily toward obtaining geochemical data and 

geologic-age information to facilitate interpretation 

of seismic records; table 2 lists the locations, water 

depths, and lengths of cores, and table 3 gives textural 

data for selected core samples. Procedures employed 

in sampling and textural analyses are described by 

Clarke and others (1981, app. A). 

The reader is directed to the report by Kulm and 

others (1975) for the type and distribution of sediment 

facies and the factors affecting sediment transport on 

the Oregon shelf. Scheidegger and others (1971) 

mapped sediment provenance and dispersal patterns, 

and Byrne and Panshin (1977) showed the distribution 

of shelf-sediment types. Spigai (1971) and Kulm and 

others (1968a, b) reported on the heavy minerals of 

Oregon shelf and beach sands, and Komar and others 

(1972) described the wave climate on the Oregon 

shelf. The following section summarizes of the 

characteristics of surface sediment on the central 

Oregon shelf, incorporating data from these reports 

and from samples collected during the present study. 

Sediment Sourees and Lithology 

The principal sources of sediment along the 

southern and central Oregon shelf are the Rogue and 

Klamath Rivers. These and eight other rivers draining 

the Klamath Mountains as far north as the Millecoma 

River of the CCfs Bay area have a combined annual 

flow of 38.6 km (Scheidegger and others, 1971). Th~ 

Umpqua River, which has an annual flow of 8.4 km 

and drains parts of the Cascade Range, Klamath 

Mountains, and the southern Oregon Coast Range, 

supplies sediment to the inner shelf of central 

Oregon. In addition, erosion of uplifted Pleistocene 

marine-terrace deposits along the central and southern 

Oregon coast provides a locally important source of 

3 0 Reconnaissance Geology and Geologie Hazards of the Offshore Coos Bay Basin, Oregon 
S' 

~ ~· .... i' 

0 

""" 

~ S' D 

m 

.... 

~ a 

s 

.... 

~ 

...... 

(f) 

c 

z 

0 

u 

w 

(f) 

~ 

u..i 

:2: 

i= 

_. 

w 

> <( 

a: 

1-

u 

~ (f) 

jjj 

(f) 

>-

<( 

~ 0 

~ 1-

WEST 

0 

I 

0.0-

2.0 

COQUILLE BANK 5 

I 

,· -•.. : ·. ,_ 

::r·,.-:. 

'l' 

i":' 

~::; :: 

10 

I ~ 

15 km 

I 

Figure 2.3. Deep-penetration seismic-reflection profile (77-26) across Coquille Bank and adjacent shelf. Note contacts 

between acoustic units 1 and 2, and 2 and 3; also note fault and deformation of reflectors thought to be associated with 

shallow gas in acoustic unit 1 (area indicated by bracket). Compare with figure 24. 

EAST 
c,.) 

t.:> 

r 

I· 

~ 

~ e. 

a 

i 

! 

~· 

I 

0 

"""' f 

g 

;-

8' 

CJ 

~ 

! 

r ,p· 

' 

(/) 

c 

z 

0 

u 

w 

(/) 

~ 

w 

:2 

i= 

....1 

w 

> 

<( 

a: 

~ 

u 

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jjj 

(/) 

>-

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WEST 

0 

0.0 

WEST 

0 

0.0 

5 

I 

0.2- ~ .l. ,__ -~ ~-~:' ciofV.'. ~ .il' ·_' ~·- j~t,,·~~·-" :'11111 

10 15 

I 

EAST 

17 km 

I 

0·

4-

~ I "'-.'I -.· I I ..•.. I . = A::.:;~.;;;~;..~§"! \• I .~-. r•tMultlpie~- .. ~ .~··· ·•···· ;: ... !> •. t>'\l , . ··;·.T· :"·l;}Y;'····i· •• ·.,wi(JI.~ 

B 

Figure 24. High-resolution seismic-reflection profile (77-2.6) across Coquille Bank and adjacent shelf. !!J 1.2.-kJ 

"boomer" record. ~ 3.5-kHz profiler record. Note contact between acoustic units 1 and Z, depositional thinning of 

strata in upper part of acoustic unit 1 east of Coquille Bank, truncation of acoustic unit 1 on bank margin, and seafloor relief of bank above adjacent shelf, indicating recent uplift. Also note enhanced or "bright" reflector above 

acoustically turbid zone in acoustic unit 1, apparent cratering of sea floor above this zone, and possible gas bubbles 

in water column above craters-all features suggesting the presence of shallow gas in acoustic unit 1. Sampling 

station 2.3 is within cratered zone; stations 33-36 are nearby. 

EAST 

18.5 km 

I 
EAST WEST 

0 4.4 km 

I I 

(/) 0.0- 0 

z 

0 

u 

UJ 

(/) 0.1-

~ 

w 

::!: 

i= 

....J 0.2 UJ 

> <( 

a: 

1-

u 0.3 

~ (/) 

jjj 

(/) 

>- 0.4 <( 

~ 0 

~ 1-

FiguN ZS. High-resolution seismic-reflection profile (77-ZO), showing location of sampling 

station 54 and projected locations of stations 53 and 55, approximately 24 km west of Cape 

Arago. Note sea-floor crater (station 54) and underlying acoustically turbid zone. 

sand. Byrne (1963) estimated that erosion rates on the 

north coast of Oregon range from 0.6 m/yr for 

sedimentary rocks overlain by terrace gravel to 16 

m/yr for unconsolidated gravel and sand. 

Sand predominates along the inner continental 

shelf seaward to a depth of 50 to 100 m within the 

study area (fig. 26; Byrne and Panshin, 1977). These 

deposits are in large part modern and are of characteristically clean and well-sorted fine to very fine sand 

(approx 2. 75 - 3.259), arkosic in composition 

(Scheidegger and others, 1971; Kulm and others, 1975). 

Sediment coarser than very fine sand, as well as some 

silt and clay from fluvial sources, tends to be trapped 

in the numerous coastal estuaries, and so the coarse 

sediment on the continental shelf is mostly relict 

(Kulm and others, 1975). Relict fine to coarse sand 

and gravel are characteristically iron stained, solution 

pitted, and altered, and are probably late Pleistocene 

in age. Deposits of relict sand are most common off 

erosionally resistant headlands and along parts of the 

coast where small rivers provide a relatively minor 

supply of sediment to the marine environment (Kulm 

and others, 1975). 

Sediment flanking submarine outcrops appears to 

consist of mixtures of reworked residual material from 

the outcrops, relict sand and gravel, and possibly fine 

modern sediment as well (Kulm and others, 1975). 

Relatively coarse relict sand is also found in isolated 

patches near the shelf edge. Submerged deposits of 

heavy minerals, probably formed during Pleistocene 

low stands of sea level, are present on the shelf 

between Coos Bay and the Oregon-California State 

line (Kulm and others, 1968a). Biogenic components, 

principally sponge spicules and microfossils, make up a 

minor part of the shelf sand but locally are important 

constituents of fine mud. Authigenic glauconite 

occurs locally in concentrations exceeding 50 percent 

of the coarse fraction along the outer edge of the shelf 

and on the flanks of topographic highs (Kulm and 

others, 1975). Clays in modern sediment north of 

Coquille Bank consist of approximately equal proportions of chlorite, illite, and montmorillonite. 

However, clays in samples from Coquille Bank southward to northern California consist principally of 

chlorite (reflecting derivation from the Klamath 

Mountains), lesser amounts of illite, and minor 

amounts of montmorillonite (Kulm and Fowler, 1970). 

A mud facies composed of materials with mean 

sizes finer than 49 is present on the middle shelf off 

the Umpqua River and is irregularly distributed on 

parts of the outer shelf and inner continental slope 

(fig. 26; Byrne and Panshin, 1977; Kulm and others, 

1975). The net rate of mud accumulation on the 

central Oregon shelf is slow; fines tend to be 

Characteristics of Surface Sediment 3 3 
Table z. Location, depth, length, type, and purpose of selected samples 

sample Sample Water Sample 

Station number tyEe Pur2ose Latitude Lon9:itude deEth(m) len9:th (m) 

1977 

1 lG Gravity core Stratigraphic and +44.0500 -124.6621 12 5 2.00 

sedimentologic 

information 

2 2G do •••••••••• do •••••••••••••••• +44.0594 -124.7032 115 1.62 

3 3G do •••••••••• do •••••••••••••••• +44.0577 -124.7358 140 .38 

4 4G do •••••••••• do •••••••••••••••• +44.0561 -124.7342 14 0 .4 5 

5 5G do •••••••••• do •••••••••••••••• +44.0599 -124.7551 13 5 core catcher 

6 6G do •••••••••• do •••••••••••••••• +44.0595 -124.7655 13 0 Do 

7 7Gl do •••••••••• do •••••••••••••••• +44.0536 -124.7824 12 0 Do 

7 7G2 do •••••••••• do •••••••••••••••• +44.0551 -124.7802 120 Do 

8 8Gl do •••••••••• do •••••••••••••••• +44.0574 -124.7902 115 Do 

8 8G2 do •••••••••• do •••••••••••••••• +44.0598 -124.7903 115 Do 

9G do •••••••••• do •••••••••••••••• +44.0534 -124.8018 105 .27 

10 lOG do •••••••••• do •••••••••••••••• +44.0542 -124.8132 080 .19 

11 11G do •••••••••• do •••••••••••••••• +44.0508 -124.8913 133 .36 

12 12G do •••••••••• do •••••••••••••••• +44.0564 -124.9246 182 .37 

13 13G do •••••••••• do •••••••••••••••• +44.0604 -124.9284 212 1. 96 

14 14G do •••••••••• do •••••••••••••••• +43.6480 -124.8523 491 Core catcher 

15 15G do •••••••• do •••••••••••••••• +43.6205 -124.8196 543 1. 4 5 

16 16Gl do •••••••• do •••••••••••••••• +43.5677 -124.8259 670 2.65 

16 16G2 do •••••••• Gas analysis +43.5568 -124.8272 679 2.62 

1 7 17G1 do •••••••• do •••••••••••••••• +43.5792 -124.8294 675 2.84 

17 1_7G2 do •••••••• Stratigraphic and +43.5775 -124.8122 685 3.00 

sedimentologic 

information 

18 18G do •••••••• Gas analysis +4 3. 5 721 -124.7577 610 2.24 

19 19G do •••••••• stratigraphic and +43.5676 -124.737f) 540 2. 2 7 

sedimentologic 

information 

21 21G do •••••••• do •••••••••••••••• +43.5778 -124.3803 115 .o 5 

22 2 2G do •••••••• do •••••••••••••••• +43.5815 -124.3398 100 2.00 

23 23Gl do •••••••• Gas analysis +43.0525 -124.7042 157 .a 5 

23 2 3G2 do •••••••• stratigraphic and +43.0528 -124.6975 155 Core catcher 

sedimentologic 

information 

23 23G3 do •••••••• Gas analysis +43.0537 -124.69f)7 157 .30 

1979 

33 33Gl Gravity core Gas analysis +43.0493 -124.6953 150 .54 

33 33G2 do •••••••••• do •••••••••••••••• +43.0492 -124.6963 155 .25 

33 33G3 do •••••••••• do •••••••••••••••• +43.0497 -124.6952 149 .86 

34 34Gl do •••••••••• do •••••••••••••••• +43.0523 -124.f)967 150 .75 

34 34G2 Gravity core Gas Analysis +43.0523 -124.6972 151 .75 

35 35G do •••••••••• do •••••••••••••••• +43.0533 -124.7018 15 0 .52 

36 36G do •••••••••• do •••••••••••••••• +43.0540 -124.6963 150 .87 

53 53Gl do •••••••••• do •••••••••••••••• +43.3095 -124.7013 217 l. 3 3 

53 5 3G2 do •••••••••• do •••••••••••••••• +43.3132 -124.6963 213 1.59 

54 54Gl do •••••••••• do •••••••••••••••• +4 3. 312 3 -124.7118 259 2.35 

54 54G2 do •••••••••• do •••••••••••••••• +43.3122 -124.7123 260 2.29 

54 54G3 do •••••••••• do •••••••••••••••• +43.3120 -124.7123 259 2.65 

55 55G do •••••••••• do •••••••••••••••• +43.3130 -124.7025 237 1.23 

resuspended by wave action and then transported Sediment Transport 

seaward into progressively deeper water. This wave 

stirring controls the landward extent of the mud Modern littoral transport of sand-size material is 

facies, and shelf-edge turbulence and sediment northward, and this has been the dominant transport 

bypassing limit its .seaward growth (Kulm and others, direction since late Wisconsin time (approx 18,000 yr 

1975). Mixing by benthic organisms of this river- B.P.) (Scheidegger and others, 1971; Kulm and others, 

supplied mud with relict and modern sand has created 1975). However, littoral drift along the outer edge of 

a mixed mud-and-sand facies that covers much of the the modern continental shelf during the late Wisconsin 

middle and outer shelf and parts of the upper slope off regression apparently was more effective than at 

central Oregon (fig. 26). present; as much as 315 km of northward transport 

3 4 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
Table 3. Textural parameters of selected sediment samples 

[See table 2 for 

Station Sample Sand Mean 

(gravel} Mud (Inman} 

(pet) (pet) (f,t} 

1 1G 52 36 3.22 

(12) 

3 3G 84 16 2.65 

4 4G 74 26 3.35 

11 11 G 66 11 .96 

( 2 3) 

16 16G 1 99 7.29 

17 17G2 2 98 6.52 

19 19G 1 99 4.57 

22 2 2G 32 68 5.48 

occurred during the Wisconsin low stand of sea level 

(Kulm and others, 1975). The modern strandline has 

been relatively stationary since the Holocene transgression, and littoral transport has been limited by 

numerous coastal estuaries that tend to trap material 

coarser than very fine sand and by erosionally resistant 

headlands that hinder the northward movement of sand 

along the coast (Kulm and others, 1975). 

The wave climate off the Oregon coast, 

especially during the winter months, is vigorous. Fine 

terrigenous and biogenic sediment deposited from 

suspension on the Oregon shelf is resuspended by wave 

and current action and is moved seaward as a lowdensity bottom flow concentrated in swales and small 

valleys on the middle to outer shelf as it is transported 

toward the continental slope and abyssal plain (Kulm 

and others, 1975). Studies by Komar and others (1972) 

showed that surface waves associated with winter 

storms off the Oregon coast should generate bottom 

orbital velocities sufficient to form ripples in very fine 

sand at water depths as great as 150 to 200 m. Photographic observation confirms that oscillatory ripple 

marks are present across the shelf to depths as great 

as 200 m and that ripples formed by summer waves 

extend to depths of 50 to 100 m. In addition, nearbottom currents associated with intense winter storms, 

with velocities as high as 75 cm/s, sufficient to erode 

and transport sand, have been measured off the 

Columbia River at a depth of about 50 m (Smith and 

Hopkins, 1972). The origin of these currents and their 

influence on sediment dispersal are unclear. Currentvelocity measurements 50 to 210 em above the bottom 

at middle- and outer-shelf depths (90 to 165 m) 

indicate velocities as high as about 25 cm/s in currents 

that partly are tidally induced (Harlett, 1972; Komar 

and others, 1972). Near-bottom (1-2 m above the 

seabed) current velocities have been measured as high 

as 40 cm/s on the upper continental slope (500 to 

1,000 m), and as high as 10 cm/s on the lower continental slope (1,000 to 2,800 m) off central Oregon 

(Korgen and others, 1970). 

Fine sediment accumulates on the lower continental slope in this region at rates estimated at as 

sample locations] 

Skewness 

Median Sorting 16/84 Kurtosis 

(Inman} (Inman} (Inman) (Inman) 

on (f,t) 

2.16 2.76 0.39 0.88 

2.01 .96 .68 1.78 

2.57 1.61 .49 1.07 

1.49 2.03 -.26 1.12 

6.62 2.07 .32 .83 

6.00 1.94 .27 • 9 0 

4.30 .48 .56 2.67 

4.73 1.99 .38 • 9 0 

much as 70 cm/1,000 yr, and a sedimentation rate of 

about 10 cm/1,000 yr has been calculated for the upper 

continental slope (Carlson, 1968; Spigai, 1971). Relict 

sand, probably late Pleistocene in age and exposed 

locally at depths as great as 500 m on benches on the 

continental slope, indicates that little or no 

sedimentation has occurred on some parts of the 

Oregon slope since the Holocene transgression (Spigai, 

1971). 

Geometry of Quaternary Deposits 

The distribution and thickness of acoustic unit 1, 

interpreted to be of Quaternary age and the youngest 

sedimentary sequence that could be traced throughout 

the study area, are shown as Figure 27. An 

unconformity at the base of this unit can be traced 

with fair confidence along the margins of banks and on 

the inner shelf. This unconformity is indistinct in the 

centers of major synclines, and so the thickness of 

acoustic unit 1 in shelf-and-slope basins should be 

considered a minimum. The age, lithology and stratigraphic relations of acoustic unit 1 were discussed 

above in the subsection entitled "Marine Geology of 

the Coos Bay Basin." 

Organic-carbon and Calcium Carbonate Content 

A total of ten samples from 8 stations on the 

central Oregon shelf were analyzed for organic carbon 

and CaC03 content; analytical procedures were 

described by Clarke and others (1981, app. A), and 

table 4 lists the results. The trends apparent in the 

values obtained are questionable owing to the small 

number and wide geographic distribution of samples. 

Organic-carbon content averages 1.5 weight 

percent but ranges from about 0.4 to 3.0 weight 

percent; only one sample (with 3.04 weight percent) 

contains more than 2 weight percent. Generally, high 

organic-carbon content appears to be associated with 

deeper water and with decreasing grain size. Calcium 

carbonate content is characteristically low; eight of 

Characteristics of Surface Sediment 3 5 
20 40 KILOMETERS 

EXPLANATION 

~ROCK 

0 SAND 

bf\)] MUDDY SAND 

1m MUD 

Figure Z6. Map showing distribution of major sediment 

types on continental shelf and slope off central Oregon 

(from Byrne and Panshin, 1977). Heavy line denotes 

approximate location of shelf edge. Sand consists of 

sediment with mean sizes coarser than 4~; mud 

consists of silt and day with mean sizes finer than 4~. 

the samples (from seven stations) contain less than 1 

weight percent Caco3• Two samples have 

anomalously high calcium carbonate contents (11.3 and 

18.6 weight percent), probably reflecting the presence 

of small fragments of shelly rna terial. The calcium 

carbonate appears to be derived principally from 

calcareous nannofossils and tests of foraminifers, with 

a secondary contribution from mollusk and echinoderm 

fragments. 

SUMMARY OF GEOLOGIC HAZARDS 

Active faulting in the offshore Coos Bay basin 

poses a potential hazard to sea-floor installations. 

Groups of geologically youthful north- to northwesttrending faults are present on the inner shelf north of 

Cape Arago and on Heceta Bank. Individual faults can 

be traced for at least 15 km, and these groups of faults 

may reflect a broad zone of active shear (fig. 12). In 

addition, a northwest-trending fault 10 km long that 

has sea-floor expression cuts the inner shelf southsouthwest of Cape Arago. Additional detailed study of 

these faults is recommended to determine the type and 

history of offset and their relation to faults onshore, 

and to assess their potential hazard to structures 

offshore. 

Little seismic activity has been recorded in the 

immediate vicinity of the Coos Bay basin during the 

period 1841-1975 (fig. 20). Seismic hazards are 

probably associated principally with ground shaking, 

earthquake-induced sea-floor mass movement and 

liquefaction, and tsunamis resulting from earthquakes 

in adjacent regions, notably along the Gorda Ridge, the 

Mendocino Escarpment, the Blanco Fracture Zone, and 

in the Gorda Basin. Beaulieu and Hughes (1975) 

estimated that a major earthquake in the vicinity of 

the Mendocino Escarpment could generate an MMI of 

VII in the Coos Bay area, with a corresponding ground 

acceleration of 0.10 g_. The tsunami hazard associated 

with such an earthquake would be minimized by the 

predominately horizontal motions accompanying 

faulting in the northern California-southern Oregon 

continental margin. The Alaskan earthquake of 1964, 

however, generated wave heights along the Oregon 

coast of 1.2 and 4.2 m above the prevailing mean high 

water, and wave heights greater than 6 m were 

recorded at Crescent City, Calif. Substantial damage 

to coastal structures was done in Oregon, and 

catastrophic losses of property occurred locally in 

northern California. Thus, the tsunami hazard to 

facilities constructed in shallow areas on Heceta and 

Coquille Banks and on the inner shelf adjoining the 

Coos Bay basin could be significant (Beaulieu and 

Hughes, 1975). 

Evidence of modern sea-floor mass movement 

(fig. 21) is uncommon on the seismic-reflection records 

collected during this study. However, Quaternary 

sediment of largely unknown engineering properties is 

locally thick (fig. 27) and in some areas contains peat 

and devitrified volcanic ash, which can reduce shear 

resistance. Slopes sufficiently steep to promote 

failure in unconsolidated sediment are prevalent below 

the shelf edge in the Coos Bay basin. Evidence of 

older, buried slides is present in areas of apparently 

high sediment influx during Pleistocene time. In 

3 6 Reconnaissance Geology and Geologic Hazards of the Offshore Coos Bay Basin, Oregon 
125°30' 125° 

30' 

OREGON 

43"' 

CALIF Cape Blanco· 

25 50 KILOMETERS 

I 

I 

f 

~L.t. l Head 

J.. 

I 

I 

t 

I 

i 

I 

I 

7 

I 

I 

I 

I 

·j 

! 

I 

l 

I 

Figure Z7 • .!:sopach map showing thickness of Quaternary sediment in the Coos Bay basin. Explanation 

is on next page. 

Summary of Geologic Hazards 3 7 
EXPLANATION 

(?---1 AREAS DEVOID OF QUATERNARY SEDIMENT -Deter-

....__7_y mined from high-resolution seismic-reflection profiles. Con-

. tacts dashed where inferred: queried where control lim-

~ ited ISOPACHS SHOWING THICKNESS OF QUATERNARY 

""~ __ ) SEDIMENT -In seconds of penetration (2-way travel time). r,{j,4' -\' Approximate thickness in meters, assuming an acoustic 

O; velocity of 1650 m/s. is shown in parentheses. Contours 

dashed where inferred: queried where control limited (~'- \~, SLUMP ...._ __ ) 

Figure Z7. Continued 

Table 4. Carbon and calcium carbonate contents of 

selected samples 

Sample 

OlG 

OlG 

0 3G 

04G 

llG 

16G 

17G2 

17G2 

19G 

22G 

[All analyses in weight percent, according to the procedure of 

Clarke and others ( 1981, app. A). See table 2 for sample 

locations] 

Organic Inorganic Total 

Interval Depth carbon carbon caco3 carbon 

(em) (m) 

2-4 125 1. 73 2. 2 3 18.60 3. 9 6 

12-14 12 5 • 9 5 • 0 2 .13 • 9 7 

1-3 140 • 4 3 .02 .18 • 4 5 

1-3 140 • 7 0 • 0 7 • 60 • 7 7 

1-3 133 3. 04 1. 35 11.27 4. 4 0 

1-3 6 7 0 1.69 .11 • 88 1. 80 

2-4 685 1.97 • 07 • 59 2. 0 4 

19-21 6 8 5 1. 79 • 0 7 • 59 1. 86 

1-3 540 1. 57 .10 • 80 1.66 

1-3 10 0 1.14 • 0 9 • 7 3 1. 23 

suggestive of shallow gas zones are extensive in the 

offshore Coos Bay basin (fig. 21). Side-scan 

sonographs, high-resolution seismic-reflection profiles, 

and samples from the area of one suspected shallow 

gas zone indicate that gas is being vented at the sea 

floor along a linear northwest-trending zone that may 

be associated with youthful faults. High 

concentrations of biogenic methane were confirmed in 

a second area, which also bore evidence suggestive of 

gas venting and faulting. The degree of hazard posed 

by shallow gas in this area cannot be assessed without 

confirmation of the presence of gas in other areas and 

without determination of the type of gas present, the 

occurrence of associated faulting, and the effect of 

gas in the bearing properties of the enclosing 

sediment. Such assessment would entail additional 

seismic-reflection profiling-preferably with records 

processed to preserve true relative amplitudes-and 

sampling of sediment above suspected gas zones to 

confirm the presence of gas, determine its origin, and 

measure its concentration. 

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