UNITED STATES

DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

PRELIMINARY GEOLOGIC MAP OF THE CLEAR LAKE SE QUADRANGLE, 

SKAGIT COUNTY, WASHINGTON

By David P. Dethier, John T. Whetten and Paul R. Carrol1

U.S. Geological Survey 

Open-File Report 80-303

This report is preliminary and has not 

been edited or reviewed for conformity 

with Geological Survey standards and 

nomenclature

Seattle, Washington 

1980
INTRODUCTION

The U.S. Geological Survey is engaged in a program of regional geologic 

mapping at a scale of 1:100,000. In order to delineate map units and 

geologic structures adequately in areas of unusual geologic complexity, 

mapping is sometimes done at larger scales. This map is one of those larger 

scale maps, prepared as an intermediate step in compiling the geology of the 

Port Townsend quadrangle at a scale of 1:100,000 (see index map).

The use of the orthophoto base for this map is a compromise resulting 

from the need for a suitable large scale and the lack of base maps of 

adequate accuracy. The original plan for intermediate mapping in the 

northeastern part of the Port Townsend quadrangle area included using 

the Clear Lake 15-minute topographic quadrangle (scale 1:62,500) as a base. 

That map, derived from a 1941 plane-table survey, is not sufficiently 

accurate for the mapping objectives. We mapped on aerial photographs 

(photography in 1976; scale approximately 1:24,000) and compiled on the 

Clear Lake SE orthophotoquad, the southeast one-fourth of the Clear Lake 

15-minute topographic map. We supplemented our field mapping on the 1976 

photographs with aerial photographs taken in 1941 (scale approximately 

1:40,000); interpretation of these photographs aided us in mapping geologic 

structures and deposits now obscured by dense vegetation. This map is the 

third geologic map produced at a 1:24,000 scale from Clear Lake 

orthophotoquad maps (see Whetten, Dethier, and Carroll, 1979; 1980).

In addition to the geologic map on an orthophoto base, three other 

maps are included in this report: (a) an index map, (b) a geologic map 

showing only geologic units and structures, and (c) a copy of the southeast 

quarter of the Clear Lake 15-minute topographic map showing generalized 

bedrock units and structures, for the convenience of the reader in locating 

map features that are not apparent on the orthophoto base.

ACKNOWLEDGMENTS

Sally Safioles assisted in the field and Kathy Lombardo drafted the 

map.
REFERENCES

Bechtel, Inc., 1979, Report of geologic investigations in 1978-79: Report 

for Puget Sound Power and Light Co., v. 3, Appendix F.

Brown, E. H., Bradshaw, J. Y., and Mustoe, G. E., 1979, Plagiogranite and 

keratophyre in ophiolite on Fidalgo Island, Washington: Geological 

Society of America Bulletin, Part I, v. 90, p. 493-507.

Danner, W. R., 1957, A stratigraphic reconnaissance in the northwestern

Cascade Mountains and San Juan Islands of Washington State: Seattle, 

University of Washington, Ph.D. thesis, 562 p.

____1966, Limestone resources of western Washington: Washington Division of

Mines and Geology Bulletin 52, 474 p.

Lovseth, T. P., 1974, The Devils Mountain fault zone, northwestern Washington: 

Seattle, University of Washington, M.S. thesis, 29 p.

Marcus, K. L., 1980, An Eocene-Oligocene age basin in the northern Puget Sound 

of Washington: Geological Society of America Abstract with Programs, 

v. 12, p. 117.

Vance, J. A., 1977, The stratigraphy and structure of Orcas Island, San Juan 

Islands, _in_ Brown, E. H. , and Ellis, R. C., eds., Geological excursions 

in the Pacific Northwest: Bellingham, Western Washington University, 

p. 170-203.

Whetten, J. T., 1978, The Devils Mountain fault: A major Tertiary structure 

in northwest Washington: Geological Society of America Abstracts with 

Programs, v. 10, p. 153.

Whetten, J. T., Dethier, D. P., and Carroll, P. R., 1979, Preliminary geologic 

map of the Clear Lake NE quadrangle, Skagit County, Washington: 

U.S. Geological Survey Open-File Report 79-1468, scale 1:24,000.

Whetten, J. T., Dethier, D. P., and Carroll, P. R. , 1980, Preliminary geologic 

map of the Clear Lake NW quadrangle, Skagit County, Washington: 

U.S. Geological Survey Open-File Report 80-247, scale 1:24,000.

Whetten, J. T., Jones, D. L., Cowan, D. S., and Zartman, R. E., 1978, Ages of 

Mesozoic terranes in the San Juan Islands, Washington, in_Howell, D. G., 

and McDougall, K. A., eds., Mesozoic paleogeography of the Western United 

States: Society of Economic Paleontologists and Mineralogists, Pacific 

Section, p. 117-132.

Whetten, J. T., Zartman, R. E., Blakely, R. J., and Jones, D. L., Allochthonous 

Jurassic ophiolite in northwest Washington: Geological Society of America 

Bulletin, in press.
Port Townsend Quadrangl*

123«OO' 

48-30'

122-001

48"30'

48°OO'

SO 100 MILES

Index Map of Washington

Index map showing locations of Port Townsend (1:100,000), 

Clear Lake (1:62,500) and Clear Lake Orthophoto (1:24,000) 

quadrangles.
48' 22'30"

T 33 N

122*07'30*

Southeast quarter of the Clear Lake 15 minute topographic quadrangle (1:62,500) 

showing geographic names, faults, folds, contacts, and principal bedrock units. 

(Bulson Creek = Bulson Creek unit, Chuckanut = Chuckanut Formation and Tertiary 

rhyolite, Haystack = Haystack unit, Volcanic Sandstone = Volcanic SandstoneGreenstone unit, Trafton = Trafton unit, P.I. = Paleozoic Intrusive unit). 

Bulson Creek depositional contact indicated by dashed line.
CORRELATION OF MAP UNITS

QLs

— unconformity —

-fault-

-fault — 

probably also unconformity

Holocene

-Pleistocene

~| Oligocene- 

_j Eocene

— Eocene

-QUATERNARY

-TERTIARY

-JURASSIC

- fault-

*".^.-"lcl •"'••• J

a- .,;..:"• -'•• -MESOZOIC

-fault-

-MESOZOIC-PALEOZOIC

-faultPALEOZOIC
DESCRIPTION OF MAP UNITS

Qal ALLUVIUM - Holocene gravel and sand, and isolated silt deposits 

occurring along the North Fk. Stillaguamish River, its 

tributaries and Pilchuck Creek. Sediment is well-sorted 

and stratified, and clasts are subrounded to rounded. Alluvium 

mapped along Pilchuck Creek and the Stillaguamish tributaries 

contains less fine material than the Stillaguamish alluvium. 

Unit included low terraces 2-6 meters above the modern flood 

plain.

Qp PEAT - Fibrous to woody peat and muck deposits. Includes deposits 

in many shallow ponds and bogs near Lake Cavanaugh, and north 

and west of Mt. Washington. Tephra from Mount Mazama 

O6900 yrs BP) is present as a layer, 20 to 50 mm thick, in 

some bogs.

Qaf ALLUVIAL FAN - Latest Pleistocene and Holocene cobbles, sand, 

and boulders deposited on the north side of Frailey Mountain 

and in other areas where upland streams spill out onto valley 

floors. Deposits consist of poorly to moderately sorted and 

stratified alluvium, and sediment deposited by debris flows. 

Clasts are angular to subangular. Overlies recessional deposits,

Qoal OLDER ALLUVIUM - Latest Pleistocene and Holocene gravel, sand, 

and silt deposits occurring north of the North Fk. of the 

Stillaguamish River. Deposits are generally well-sorted and 

stratified, and clasts are subrounded and rounded. Includes 

minor (<1%) amounts of material derived from Glacier Peak. 

Forms terraces 5-15 meters above the present flood plain.

Qls LANDSLIDE - Pleistocene and Holocene landslide deposits,

generally occurring downslope from source-area scars which 

resemble small glacial cirques. Deposits are compact, poorly 

sorted, nonstratified, and consist of angular boulders and 

cobbles in a matrix of finer material. The boundaries of the 

landslides with other bedrock and surficial units are drawn 

principally from interpretation of aerial photographs. Most 

landslides include transported unconsolidated material and 

blocks of locally derived bedrock. Unit includes isolated 

areas of bedrock, recessional fan deposits, and undisturbed 

glacial till too small to be shown at a scale of 1:24,000.

RECESSIONAL DEPOSITS

Qvr ' RECESSIONAL OUTWASH - Late Pleistocene sand and gravel

containing fine sand and silt in some areas. Deposits 

are poorly to well-sorted, stratified, and consist of 

subangular to rounded clasts. Forms terraces as much 

as 80 m above modern channels near the North Fk. 

Stillaguamish River. Includes extensive areas of 

stratified silt and fine sand exposed near Pilchuck 

Creek.

i
Qvrf ALLUVIAL FAN - Extensive alluvial fans are exposed on

the slopes of Frailey Mountain and Mt. Washington. 

Deposits consist of poorly to moderately sorted and 

stratified alluvium, and sediment deposited by 

debris flows. Clasts are angular to subangular. Fan 

deposits overlie and interfinger with recessional 

outwash on the south side of Frailey Mountain and on 

the north side of Mt. Washington.

Qvrl LAHARS - Terraces composed of lahars and alluvium rich in

volcanic gravel and sand lie 20-45 m above and one or two 

km north of the North Fk. Stillaguamish River. The volcanic 

debris originated on the upper slopes of Glacier Peak and 

flowed down the North Fk. Stillaguamish River more 

than 11,700 YBP (J. Beget, U.S. Geological Survey, personal 

communication, 1979).

Qvt TILL - Late Pleistocene nonsorted, nonstratified, compact till 

consisting of angular to subrounded cobbles and boulders in 

a matrix of sand, silt and clay. Till mantles much of the 

upland area. The prominent northwest-trending ridges and 

troughs north of Pilchuck Creek and west of Mt. Washington 

are formed largely of bedrock which is discontinuously 

mantled with till; unit includes isolated bedrock outcrops.

Tb BULSON CREEK UNIT OF LOVSETH (1975) - Conglomerate, lithic 

sandstone, and siltstone; outcrops are predominately 

poorly-bedded conglomerate with minor sandstone lenses. 

Bulson Creek sediment was derived from underlying Mzu and 

PzMzu units and deposited in fluvial and alluvial fan 

environments. Lovseth (1975) and Marcus (1980) report 

Late Eocene to Early Oligocene shallow-water marine fossils 

in the unit west of the map area. Bulson Creek unconformably 

overlies Mzu and PzMzu on the north side of Frailey Mountain 

and is truncated by the Devils Mountain fault.

Tr RHYOLITE - Ash-flow tuffs and flows, locally brecciated, with 

minor andesite flows (Lovseth, 1975). Lovseth (1975) 

obtained a fission-track age of 41.5 ± 3.4 m.y. (Late 

Eocene) on zircon from rhyolite from the Hendricks quarry 

south of Big Lake. The unit appears to be interbedded with 

unit Tc on the northwest edge of the map.

Td BASALT DIKE - Dark, homogeneous fine-grained rock occurring as 

thick, well-jointed bodies. K-Ar whole rock ages obtained 

by Bechtel (1979) are 41.2 ± 1.8 m.y., 49.9 ± 2.2 m.y., and 

46.4 ± 2.2 m.y.

•3

Tc CHUCKANUT FORMATION - Fine - to coarse-grained feldspathic

sandstone, siltstone, and coal, massively bedded to finely 

laminated, with abundant plant debris suggestive of quietwater to fluvial deposition. Chuckanut-type rocks in map 

area are Eocene or younger on the basis of Early to Middle 

Eocene fission-track ages of detrital zircons from sandstone 

from Coal Mountain (NE of map area) (C. W. Naeser, oral 

communication, 1979), and Early to Middle Eocene (Bechtel, 1979) 

dikes that intrude the unit.
Ju UNDIFFERENTIATED ROCKS OF THE HAYSTACK UNIT - A tectonic mixture 

without discernable stratigraphic order including greenstone 

and metaplutonic rock, sedimentary rock, serpentinite, and 

silica-carbonate rock, and assigned by Lovseth (1975) to the 

"Rocks of Table Mountain." Nearly concordant Jurassic U-Pb 

ages (160-170 m.y.) were obtained from two quartz diorite 

bodies (Whetten, Zartman, Blakely, and Jones, in press). One 

dated locality occurs within the map area (SW% sec. 20, T. 33 N., 

R. 6 E%); the other locality is in the Clear Lake NE quadrangle. 

Whetten and others (in press) correlated rocks of Haystack unit 

with the Fidalgo ophiolite (Brown, Bradshaw and Mustoe, 1979) 

in the San Juan Islands on the basis of aeromagnetic properties, 

age, and lithology. Rocks of the Haystack Unit are more 

strongly metamorphosed in the map area that in the San Juan 

Islands; aragoniJLEL, epidote, and actinolite are present in 

greenstone, and most sedimentary rocks are moderately to 

strongly foliated. Rocks of the Haystack unit are in fault 

contact with Tertiary and pre-Tertiary units along the 

Devils Mountain fault zone and the Table Mountain fault. 

Unit Ju designates bedrock areas of the Haystack plate that 

are a) composed of several lithologies, b) poorly exposed, or 

c) not examined during the mapping. The Haystack unit is 

divided into:

g/(g) GREENSTONE - Generally massive and non-foliated but locally 

weakly to moderately foliated. Pillow and pillow breccia 

structures commonly preserved. Unit includes metaplutonic 

rocks ranging from gabbro to quartz diorite shown on map 

as (g).

s METASEDIMENTARY ROCK - Slate, argillite, and metagraywacke, 

moderately to strongly foliated, with relict bedding 

locally preserved.

sp SERPENTINITE - Serpentinite commonly separates blocks of 

different lithologies. Green to black outcrops vary 

from strongly foliated to massive.

sc SILICA - CARBONATE ROCK - Brownish weathering, commonly occurs 

along faults.

Mzu UNDIFFERENTIATED ROCKS OF THE VOLCANIC SANDSTONE-GREENSTONE UNIT - 

A tectonic mixture consisting predominately of medium-grained 

sandstone and greenstone with minor amounts of fine-grained 

sandstone, siltstone, argillite, and chert. The unit, assigned 

by Danner (1957) to the Olo Mountain unit and, in part, by 

Lovseth (1975) to the "Rocks of Frailey Mountain," lithologically 

resembles the Upper Jurassic to Lower Cretaceous Constitution 

unit in the San Juan Islands (Vance, 1977; Whetten and others, 

1978) and is considered Mesozoic on the basis of this 

correlation. The lithologies suggest derivation from a 

volcanic arc terrane. The lithologies listed below are 

indicated for each outcrop in order of decreasing abundance 

of each lithology.
(v) VOLCANIC SANDSTONE - Massive, light bluish-gray mediumgrained sandstone, with shale chips and nearly euhedral 

feldspar phenoclasts commonly visible. Unit is highly 

fractured and extensively veined with thin veinlets of 

prehnite, but is generally non-foliated.

[g] GREENSTONE - Ranges from massive, non-foliated flows and 

pillow lava to thin tuffs interbedded with (vj , (sj , and 

(cj . Unit comprises about 40% of the rocks exposed on 

the Frailey Mountain road. By itself, this unit 

cannot be distinguished from other greenstone units in 

the map area.

[s] SILTSTONE, ARGILLITE, FINE-GRAINED SANDSTONE - Brown to black 

fine-grained sedimentary rocks, commonly associated with 

fc] ; may or may not show incipient foliation.

[^ CHERT - Generally very thin-bedded, black to gray lenticular 

beds associated with [D . Thick-bedded red radiolarian 

chert occurs on the NW side of Mount Washington; similar 

chert N of Lake Cavanaugh is also assigned to this unit.

PzMzu UNDIFFERENTIATED ROCKS OF THE TRAFTON UNIT OF DANNER (1957, 1966) - 

A chaotically deformed tectonic mixture of chert, argillite, 

greenstone, and limestone. Danner (1957, 1966) has identified 

Permian Tethyan fusulinids in limestone pods, and D. L. Jones 

(oral commun., 1977-79) has identified radiolaria in chert 

ranging from mid-Paleozoic to Jurassic. No fossils have been 

identified from within the map area. The lithologies listed 

below are indicated for each outcrop in order of decreasing 

abundance of each lithology:

(c) CHERT - Black to gray ribbon chert commonly associated

with(s) , intensely crumpled and broken by folding and 

faulting.

0 ARGILLITE AND SILTSTONE - Black, non-foliated to incipiently

foliated, occurs in association with (c)and (g). Unit includes 

minor fine-grained sandstone.

(J) GREENSTONE - Predominantly green tuff associated withg)

and (s), but also including pillow lava, and massive Tlows. 

By itself, unit is undistinguishable from other 

greenstone units in map area.

@ LIMESTONE - Fault-bounded blocks of gray limestone. Some 

areas have been extensively quarried (Danner, 1966).

Pzi COARSE-GRAINED INTRUSIVE ROCK - Mapped at two localities: the 

outcrop in the SW corner of the map is pyroxene gabbro; 

the hill NW of Lake Cavanaugh (sec". 18, T. 33 N. , R. 6 E) 

ranges from weakly foliated hornblende-biotite quartz diorite 

to fine-grained quartz-amphibole gneiss with plagioclase 

porphyroblasts. The origin of the more foliated rock is not 

known. Similar quartz diorite associated with PzMzu 

outside of map area was dated as mid-Paleozoic 

(R. E. Zartman, written commun., 1978).
DESCRIPTION OF FAULTS

The northwest-trending fault cutting across the northeast corner of the 

map is inferred to be the extension of the Table Mountain fault mapped in the 

Clear Lake NE and NW quadrangles.

The Devils Mountain fault is an east-trending fault zone bounded by two 

relatively well-defined parallel faults. The pre-Tertiary rocks in the fault 

zone are a mixture of megablocks and breccias derived from the terranes on either 

side of the fault zone. Numerous concealed faults are inferred to bound the 

blocks, but the faults are not shown on the map because there is little to 

constrain their location or orientation. The Chuckanut Formation occurs as a 

relatively coherent unit within the fault zone on the west side of the map. It 

is bounded on the north and south by faults, but the contact on the east could 

be a fault or an unconformity.

An apparently sinuous contact separates the Trafton unit from the VolcanicSandstone-Greenstone unit in the southern part of the map. If the lithologic 

correlations suggested in this report are correct (see "Description of Map 

Units") this contact may be one of the east-dipping Cretaceous thrust faults 

mapped in the San Juan Islands (Whetten and others, 1978) offset to the east 

by movement on the Devils Mountain fault. Alternatively, the contact could 

be a high angle fault. The contact is not exposed in the map area.

10
MAP SYMBOLS

Outcrop visited by us but consisting of non-foliated 

rock. Symbol occasionally used for other rocks where 

foliation or bedding could not be measured. Symbol 

denotes outcrops within large areas mapped as bedrock, 

and isolated outcrops in areas of Quaternary sediments

Contact - dotted where concealed

Fault - dashed where inferred, dotted where concealed,

Strike and dip of beds. Bedding tops generally not 

known.

*^ Strike and dip of foliation.

i Vertical foliation.

_ .-- Inferred axis of anticline, dotted where concealed.

!,

T

Inferred axis of syncline, dotted where concealed.

Sample locality for U-Pb date on zircon from quartz 

diorite.

11