DESCRIPTION OF MAP UNITS

Nonglacial deposits

Artificial fill (Holocene)—Mud, sand, and gravel of varying proportions, possibly including foreign

debris such as concrete, logs, timbers, or brick. Used for highway roadbeds and other construction.

Thickness generally greater than 2 m. Mapped where fill substantially obscures or has altered original

geologic deposit.

Beach deposits (Holocene)—Soft sand, silt, and mud deposited or reworked by wave or tidal action

at head of Port Discovery. Contains logs and timbers at or above high tide. Deposits lie within 2 m

of high tide.

Alluvial fan deposits (Holocene)—Boulders, cobbles, and soft sand deposited in lobate form where

streams emerge from confining valleys and reduced gradients cause sediment loads to be deposited.

Alluvium (Holocene)—Moderately sorted soft deposits of cobble gravel, pebbly sand, and sandy silt

along the floodplain of Snow and Salmon Creeks south of Port Discovery, and along Snow Creek

in the northwest corner of the map. Gradational with and including sediment equivalent to unit Qb

Wetland deposits (Holocene and late Pleistocene)—Peat, marsh, and bog deposits, with some

intermixed sand, silt, and clay. Unit is very soft, water saturated, and occupies shallow depressions

in till (Qvt) or low-lying areas of recessional outwash sands (Qvrs).

Landslide deposits (Holocene and late Pleistocene)—Diamicton of angular clasts of bedrock and

surficial deposits derived from upslope and commonly includes trees. Includes areas of irregular,

hummocky topography. Poorly consolidated. Landslide four kilometers south of Port Discovery

consists of house-sized blocks of Lyre Formation conglomerate.

Glacial deposits

DEPOSITS OF VASHON STADE OF FRASER GLACIATION OF ARMSTRONG AND OTHERS (1965)

(PLEISTOCENE)—Consists of:

Sand-dominated recessional outwash deposits (late Pleistocene)—Stratified unconsolidated sanddominated sand and gravel deposits. Occupies lower parts of lower Snow Creek-Crocker LakeLeland Lake valley. May include interbeds of gravel-dominated recessional outwash deposits (Qvrg)

Gravel-dominated recessional outwash deposits (late Pleistocene)—Stratified unconsolidated graveldominated sand and gravel deposits. Occupies lower margins of lower Snow Creek-Crocker LakeLeland Lake valley, but lies higher than sand-dominated recessional outwash deposits (Qvrs). May

include interbeds of sand-dominated recessional outwash deposits (Qvrs)

Fine-grained recessional glaciomarine or glaciolacustrine deposits (late Pleistocene)—Stratified

firm fine silt with rounded dropstones of pebbles and cobbles along Highway 20, southeast of Port

Discovery. Coarse silt and sand are interbedded. Beds are greater than 3-10 cm thick. Some contacts

between beds are convoluted, and sand dikes and blobs are present and cross bedding. Overlies

probable till

Till (late Pleistocene)—Compact and firm light to dark gray non-stratified diamict containing subangular

to well-rounded clasts, glacially transported and deposited. Clasts are commonly granitic and were

derived from the Cascade Range or Canada. Often overlies advance outwash deposits (units Qval,

Qvas, Qvag) in the eastern part of the quadrangle and bedrock in the western part of the quadrangle.

Generally forms an undulating surface a few meters to a few tens of meters thick. However, west

of Crocker Lake more than 75 m of till is exposed

Valley-fill glacial and glaciofluvial deposits (late Pleistocene)—Bedded sandy-matrix clast-rich

compact diamict with angular clasts and a few large boulders, interbedded with firm fine to coarse

sand and silt, and clast-poor clayey matrix diamict, which we consider as till. Clasts are commonly

basaltic with probable local provenance. Occupies the headwaters of the Little Quilcene River

drainage in the southwestern part of the map area

Sand-dominated advance outwash deposits (late Pleistocene)—Well-bedded, sand-dominated compact

sand and gravel deposits. May have interbeds of silt or clay. Deposited by streams and rivers issuing

from front of advancing ice sheet

Gravel-dominated advance outwash deposits (late Pleistocene)—Well-bedded gravel-dominated

compact sand and gravel deposits. Almost devoid of silt or clay, except near base as discontinuous

beds. Deposited by streams and rivers issuing from front of advancing ice sheet

Advance outwash lake deposits (late Pleistocene)—Laminated to massive silt, clayey silt, and silty

clay deposited in a lowland lake south of Port Discovery. Sediment is firm. Inferred to be proglacial

because it is overlain by sand-dominated advance outwash sediments

Indurated colluvium (Pleistocene?)—Indurated unsorted iron-stained angular colluvium on flanks

of the Olympic Mountains in the southwestern part of map area. Inferred to predate Vashon stade,

because sediment is indurated and lying on bedrock

Bedrock

Micaceous sediments (Eocene?)—Massive buff brown micaceous siltstone and fine sandstone on the

north side of Big Skidder Hill. Bedding commonly indistinct or not present. May be correlative to

Scow Bay sandstone or the upper Eocene and lower Oligocene Makah Formation, the upper Oligocene

Pysht Formation, and the Miocene Clallam Formations at northwestern tip of Olympic Peninsula,

where these formations are described as micaceous by Snavely and others (1993) and Garver and

Brandon (1994)

Sandstone of Snow Creek—Sandstone and silty sandstone turbidites. Beds 5 cm to 1 m thick. Contains

channelized interbeds of pebble and granule conglomerate and siltstone up to 1 m thick. Clasts are

dominantly quartz and chert. Foraminifera indicate an upper Narizian age (Spencer, 1984)

LYRE FORMATION (MIDDLE EOCENE) divided into:

Upper conglomerate member—Pebble and cobble conglomerate in a matrix of medium to coarse

granule sand. Contains some sandstone interbeds. Dominantly chert, basalt, and conglomerate clasts

in decreasing order of abundance (Spencer, 1984). Clasts to 1 m

Andesite tuff and breccia—Andesite and hornblende andesite tuff and breccia, white to light gray.

Locally contains rare leaves and coalified wood. Commonly massive, but some tuffs are thin bedded

Lower conglomerate member—West of Cedar Flat unit is predominantly thick bedded to massive

conglomerate; to the south thin to thick bedded sandstone with minor conglomerate is most common.

Conglomerates are composed of well rounded pebbles and cobbles of chert with lesser amounts of

metasedimentary and igneous rocks, quartz, and graywacke. Lower part of unit south of Snow Creek

locally includes some angular to rounded basalt clasts. Sandstones are fine to coarse- grained and

commonly contain scattered pebbles. Siltstone is sandy and thinly to faintly bedded. Hornblende

separates from an andesite boulder in this unit collected just west of the quadrangle boundary, near

Mount Zion, yield K-Ar ages of 35.5 and 41.0 Ma (Yount and Gower, 1991). However, Spencer’s

(1984) biostratigraphic work indicates member is middle Eocene in age, and therefore it is likely

there was some argon loss in the samples analyzed

Lyre Formation, undifferentiated—Pebble and cobble conglomerate in a matrix of medium to coarse

granule sand. Contains some sandstone interbeds. Dominantly chert, basalt, and conglomerate clasts

in decreasing order of abundance. Clasts to 1 m. (data from Spencer, 1984). No direct fossil control

on age, but because it lies between the Narizian Aldwell Fm. and the upper Narizian sandstone of

Snow Creek, it is probably lower Narizian (Spencer, 1984)

Aldwell Formation (middle Eocene)—Massive to bedded slope mudstone and turbidite sandstone,

basaltic conglomerate, lithic sandstone, and siltstone. No fossils have been dated from within the

quadrangle, but along strike on the east side of Quilcene Bay foraminifera indicate an early Narizian

age (Spencer, 1984), where the section is almost 300-m thick. Coeval with Eocene basaltic sediments

(Tebs)

Basaltic sediments (middle Eocene)—Basaltic conglomerate and breccia, fine- to medium-grained,

dark gray to black massive sandstone, with thin local interbeds of dark siltstone. Lowermost few

meters is gray-green argillaceous limestone pods and basaltic sedimentary breccia. Total thickness

is approximately 230 m along Salmon Creek; benthic foraminifera and molluscs indicate age of

section ranges from upper Ulatisian to lower Narizian (Spencer, 1984)

CRESCENT FORMATION (LOWER AND MIDDLE EOCENE) divided into:

Basalt flows—Massive basaltic lava flows. May contain calcite or zeolite-filled amygdules. Lava may

contain pyroxene phenocrysts. Tops of flows may have red oxidized zones indicating subaerial

eruption. May contain minor basaltic breccia (Tcb)

Basalt breccia—Rubbly, reddish weathering angular basaltic breccia. No silty or sandy matrix, indicating

subaerial eruption of lava. May contain minor basalt flows (Tcf)

Basalt flows and breccia—Rubbly, reddish weathering angular basaltic breccia, and massive basaltic

lava flows in roughly equal proportions. Breccia contains no silty or sandy matrix and lava flows

may have red oxidized zones

Basaltic submarine deposits—Pillow lava and pillow and lapilli breccia, amygdular lava flows, dark

gray calcareous mudstone, basaltic siltstone and sandstone. Intruded by basaltic sills. At head of

Quilcene River sandstone makes up approximately half of section. East of quadrangle at Olele Point,

Crescent Formation contains marine siltstone and sandstone with Ulatsian foraminifera (Yount and

Gower, 1991). At Bon Jon Pass, 1 km west of the southwest corner of the quadrangle, calcareous

nannoplankton from the top of this unit are referred to the CP 11 zone, or early Eocene (D. Bukry,

written communciation, 1998)

af

Qb

Qf

Qal

Qw

Qls

Qvrs

Qvrg

Qvt

Qvas

Qvag

Qval

Qci

Tss

Tluc

Tlav

Tllc

Tebs

Tcf

Tcb

Tcfb

D

U

EXPLANATION

Contact — Dashed where approximately located; dotted where concealed

Fault — Sense of offset uncertain; dashed where approximately located;

dotted where concealed

Fault — U, upthrown side; D, downthrown side

Reverse fault —Dashed where approximately located; dotted where concealed

Fault — Showing relative horizontal movement; dashed where approximately

located; dotted where concealed

Syncline — Dashed where approximately located, dotted where covered

Anticline — Dashed where approximately located

Tems

Tcs

T a

Tl

Tcf

Tcb

Tcf

Tcf

Qb

Qvrf

Qvt

Qval

Qval

Qval

Qal Tcf

Tcf

Tcfb

Qw

Tcb

Tebs

Tebs

Tcb af

Qvrg

Tcf

Tcfb

Tcf

Qvas

Tcb

Tluc

Qvrg

Tem

Tcf

Tcb

Tcf

Tcb

Tss

Qvt

Tcb

Qls

Qvt

Tss

Tl

Tss

Tl Qvt Tl

Qvt

Tss

Tss

Qf

Qw

Qw

Qvrg

Qvt

Qal

Qval

Qvrs

Tebs

Qvt

Tl

Qvt

Tcf

Tl

D

U

U D

D

U

D

U

U

D

U

D

U

D

?

?

?

?

?

?

?

?

?

?

?

?

?

?

15

24 45

41

31

23

18 10 13

15

16 18

15

11

26

30

21

19

12 30

19

43

30

25

28

41

7

14

24

47

32

28

34

32

15

24 28

22

22

56

67

35

68

58

47

25

18

56

36

60

18

11

16

65 68

44

17

28

3

14

12

49

19

48

28

28

39 23

22

8

22

25

10

20

12

48

18

46

32

31

15 21

11

30

29

12

9 17

22

21

12

9

22

38

5

25

30

20

25

50

25

Tss

Qw

Tss

Tss

Tss

Tcf

Qw

Tcf

Tcf

Tss

Tss

Tcf

Tss

Tss

Qw

af

Qvrs

Tebs

af

Qvv

Qvrf

Qls

Qls

Qls

Tluc

Qls

Qf

Qf

Tcf

Tluc

Tss

Tss

Tss

Qvt

Tluc

Tss

Tss

Tss

Qw

Tss

Qvt

Qw

Qvrg

Tss

Qf

Tss

Tlav

Qls

Tss Qw

Tluc

Qw

Qvrs

Tluc

Tss

Qvt

Qci

Tcs

Qvrs

Qvv

Qvv

Qw

Tcf

Qvt

Tluc Ta

Tcb

Tcf

Tcf

Tss

Qvt

Qvt

Qvt

Qvas

Tss

Tluc

Tluc

Tluc

Tluc

Qvt

Qvt

Tluc

Tllc

Tllc

Tcf

Qls

Tcf

?

?

D?

D

U

U D

U?

?

?

?

?

?

? ?

?

28

28

30

12

55

30

44

25

42

42

35

25

28

58

50

62

50

35 30

45

30

35

39

6

6

37

15

27

13

31

24

24

17

15

38

29

40

39

42

39

21

12

18

39

20

22

58 60 11

25

12

5

46

19 30 25

35

35 31 24 24 29

28

23 26

24

15

16

1715

15

17

21

12

22

39

45

20

45

55

40

45

45

Tcf?

Qvv

Qvrf

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

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

not imply endorsement by the U.S. Government.

This map was printed on an electronic plotter directly from digital files. Dimensional calibration may vary between electronic

plotters and between X and Y directions on the same plotter, and paper may change size due to atmospheric conditions;

therefore, scale and proportions may not be true on plots of this map.

For sale by U.S. Geological Survey, Map Distribution, Box 25286, Federal Center, Denver, CO 80225, 1-888-ASK-USGS

Available on the World Wide Web at http://geopubs.wr.usgs.gov/data.html

SCALE 1: 24 000

1 0 1 MILE 1/ 2

1 .5 0 1 KILOMETER

CONTOUR INTERVAL 40 FEET

NATIONAL GEODETIC DATUM OF 1929

QUADRANGLE LOCATION

WASHINGTON

TRUE NORTH

APPROXIMATE MEAN

DECLINATION, 1995

21°

MAGNETIC NORTH

OPEN-FILE REPORT 99-421

Version 1.0

Preliminary geologic map of the Uncas 7.5’ quadrangle

Clallam and Jefferson counties, Washington U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Base from U.S. Geological Survey, 1953 (rev. 1973)

Universal Transverse Mercator projection, zone 10

GEOLOGIC SUMMARY

INTRODUCTION

The Uncas quadrangle in the northeastern Olympic Peninsula covers the transition from the

accreted terranes of the Olympic Mountains on the west to the Tertiary and Quaternary basin fills of

the Puget Lowland to the east. The relief of the map area ranges from sea level at Port Discovery to

4116 feet on the flank of the Olympic Mountains to the southwest. Previous geologic mapping within

and marginal to the Uncas quadrangle includes Cady and others (1972), Brown and others (1960),

Tabor and Cady (1978), Yount and Gower (1991) and Yount and others (1993). Paleontologic and

stratigraphic investigations by University of Washington graduate students (Allison, 1959; Thoms,

1959; Sherman, 1960; Hamlin, 1962; Spencer, 1984) also encompass parts of the Uncas quadrangle.

Geologic mapping for this report was conducted in February 1998 by Haeussler and Wells following

preliminary mapping by Yount in 1976 and 1979. The description of surficial map units follows Yount

and others (1993) and Booth and Waldron (in press, 1998). Bedrock map units are modified from

Yount and Gower (1991) and Spencer (1984). Geologic time scale is that of Berggeren and others

(1995).

The Uncas quadrangle lies in the forearc of the Cascadia subduction zone, 10 km east of the

Cascadia accretionary complex exposed in the core of the Olympic Mountains (Tabor and Cady,

1978). Underthrusting of the accretionary complex beneath the forearc has uplifted and tilted to the

east the Coast Range basalt basement and overlying marginal basin strata, which comprise most of

the Uncas quadrangle. The Eocene submarine and subaerial tholeiitic basalt of the Crescent Formation

on the Olympic Peninsula is thought to be the exposed mafic basement of the Coast Range, an oceanic

terrane accreted to the margin in Eocene time (Snavely and others, 1968). The Coast Range basalt

terrane may have originated as an oceanic plateau or by oblique marginal rifting, but its subsequent

emplacement history is complex (Wells and others, 1984). In southern Oregon, onlapping strata

constrain the suturing to have occurred by 50 Ma; but on southern Vancouver Island where the terranebounding Leech River fault is exposed, Brandon and Vance (1992) concluded suturing to North

America occurred in the broad interval between 42 and 24 Ma. After emplacement of the Coast Range

basalt terrane, the Cascadia accretionary wedge developed by frontal accretion and underplating

(Clowes and others, 1987). Domal uplift of the part of the accretionary complex beneath the Olympic

Mountains occurred after ~18 million years ago (Brandon and others, 1998). Continental and alpine

glaciation during Quaternary time has reshaped the growing topography of the Olympic Mountains.

STRATIGRAPHY

The oldest rocks in the quadrangle belong to the lower and middle Eocene Crescent Formation,

which consists of submarine and subaerial basalt flows, breccia, and interbedded sedimentary rocks.

Pillow lavas, pillow and lapilli breccia, amygdaloidal lava flows, dark gray calcareous mudstone,

basaltic siltstone, and sandstone make up the lower submarine unit (unit Tcs) exposed in the southwestern

corner of the quadrangle. Calcareous nannoplankton from interbedded turbidite siltstone and sandstone

at the top of the submarine unit at Bon Jon Pass, 1 km west of the quadrangle, are referred to the CP

11 zone, or early Eocene (about 50 Ma), by D. Bukry (written communication, 1998). Most of the

upper part of the Crescent Formation consists of amygdaloidal basaltic flows and breccia (units Tcf,

Tcb, Tcfb). The breccia are monolithologic, and the flows sometimes have rubbly reddish weathering

surfaces indicative of subaerial eruption. However, Spencer (1984) noted that south of the study area,

on the west shore of Quilcene Bay, there are massive basaltic flows interbedded with pillow lavas,

indicating complex interfingering of environments typical of oceanic islands. No ages have been

determined from the subaerial part of the Crescent Formation within the quadrangle. In the Bremerton

area, 40 km to the south, Duncan (1982) reported an 40Ar/39Ar age of 55 Ma, and Babcock and others

(1992) report an 40Ar/39Ar age of 50.3 ± 1.5 Ma. East of the quadrangle, Yount and Gower (1991)

report foraminifera referable to the Ulatisian stage (early middle Eocene) within marine siltstones

and sandstones of the Crescent Formation.

Marine basaltic conglomerate, sandstone, and mudstone (unit Tebs) overlie the basaltic flows and

breccia of the Crescent Formation. These basaltic sediments are found in the northeastern part of the

quadrangle along the Salmon Creek drainage and along Highway 101 on the west side of Port

Discovery. Spencer (1984) measured the section in Salmon Creek as more than 232-m thick and

argued that they are sourced from the Crescent Formation. Spencer (1984) found the contact between

the underlying basalts and the upper sediments was irregular and undulatory, with limestone and

basaltic breccia at the base of the section, suggesting a period of erosion between deposition of the

two units. Subaerial basalt flows and breccia stratigraphically beneath the marine basaltic sandstones

indicates subsidence following the end of basaltic volcanism. The basaltic sediments contain upper

Ulatisian to lower Narizian benthic foraminifera and molluscs indicative of middle Eocene age. Macro-

, micro-, and trace fossils are indicative of subtropical to warm temperate water conditions in a neritic

depositional environment with water depths <200 m (Spencer, 1984).

The middle Eocene Aldwell Formation is found in one small exposure in the southeastern corner

of the quadrangle along Highway 101. Better exposures can be found just south of the quadrangle

along the highway. Regionally, the Aldwell Formation consists of thin-bedded slope mudstone and

thin-bedded turbidite sandstone (Tabor and Cady, 1978). Massive to well-bedded basaltic conglomerate

and lithic sandstone occur locally. Along Quilcene Bay, basal Aldwell Formation interfingers with

basaltic sediments, and the contact is placed where coarse, massive to thick-bedded volcaniclastic

sediments give way to the more thinly bedded sandstone and siltstone of the Aldwell (Spencer, 1984,

p. 24-25). The upper contact of the Aldwell Formation is not exposed, but it may be unconformable

based on relations with the overlying Lyre Formation elsewhere on the Olympic Peninsula. Foraminifera

from the Aldwell Formation are referable to the lower Narizian or upper Ulatisian stages. Calcareous

nannoplankton from the Aldwell are referable to the CP 14a zone (upper middle Eocene, about 44-

41 Ma) at Quilcene Bay (Bukry, written communication). This fossil data indicates the Aldwell

Formation is coeval with the middle Eocene basaltic sediments described above. The paleoecology

of faunas collected from the Aldwell Formation are indicative of cool to cold water in the bathayal

zone (Spencer, 1984), which is quite different from the neritic water depths and warm temperatures

during deposition of the upper Crescent Formation sediments.

The late middle Eocene Lyre Formation is about 300-m thick in the study area (Spencer, 1984)

and consists mostly of conglomerate, with some interbedded andesite tuff and breccia. There is no

direct fossil control on the age of this Formation in the study area, but to the west, it contains

foraminifera referable to the upper Narizian stage (Snavely and others, 1993). The conglomerates are

both clast and matrix supported. Commonly there is no preferred orientation for the clasts, which

indicates deposition by debris flows in a submarine fan environment. The interbedded andesitic

volcanic rocks on Mount Zion (unit Tlav) may represent post-Crescent rejuvenation of Coast Range

volcanism or may represent distal Cascade-derived lavas. Silicic volcanic rocks presumably part of

the Crescent Formation have been described in the Sequim quadrangle to the north (H. Schasse,

personal communication, 1998). In the southern part of the Uncas quadrangle, the Lyre Formation

lies on the Aldwell Formation. Along the western edge of the quadrangle, and northeast of the

quadrangle, the Lyre Formation lies directly on the Crescent Formation. Spencer (1984) observed the

contact in the upper part of the Snow Creek drainage, and described it as irregular and undulatory.

The contact northeast of the quadrangle contains basaltic cobble and boulder conglomerate. From

these observations, Spencer (1984) inferred there was considerable relief on the surface beneath the

Lyre Formation. Spencer (1984) measured 418 paleocurrent indicators in the Lyre Formation and

found a consistent south-southwest direction of transport. The base of the section has a higher

percentage of basaltic clasts indicating erosion of the underlying Crescent Formation. Chert and

phyllite clasts are also common in the conglomerates, and Snavely and others (1993) and Garver and

Brandon (1994) concluded similar clasts in the Lyre Formation at the northwestern tip of the Olympic

Peninsula were derived from Mesozoic sediments on southern Vancouver Island.

We informally refer to silty sandstone and sandstone turbidites in the Snow Creek drainage as the

sandstone of Snow Creek. The section in the upper Snow Creek drainage is 567-m thick; foraminifera

are referable to the upper Narizian stage and are, “... indicative of cool to cold water temperatures

and water depths in the lower bathayal zone” (Spencer, 1984, p. 135). Petrography of the sandstones

indicates chert and quartz predominate, with chert being a significant proportion of the clasts (Spencer,

1984). Spencer (1984) notes the Snow Creek unit and the rest of the Lyre Formation are related

because of a conformable contact and because there is an abundance of chert and metamorphic clasts

in both units, which indicates a similar source area. Deposition of the sandstone of Snow Creek

appears to be localized because it is not found northeast of the quadrangle, where the Townsend Shale,

a unit found outside of the study area, lies unconformably on the Lyre Formation (Thoms, 1959), and

the Snow Creek member is missing.

Yount and Gower (1991) and Tabor and Cady (1978) map the Snow Creek unit as the Twin River

Formation, and Spencer (1984) considers it to be a member of the Lyre Formation. We prefer to map

it as a separate unit similar to the Hoko River Formation of the western Olympic Peninsula (Snavely

and others, 1993). We thus avoid expanding the Lyre Formation, and we avoid using the term “Twin

River,” which has been used as a Group name to refer to several formations that range in age from

upper Narizian (uppermost middle Eocene) to Miocene time (e.g. Garver and Brandon, 1994). The

lowermost of the formations in the Twin River Group is the Hoko River Formation, which is similar

in composition and age to the sandstone of Snow Creek, as used in this study.

Eocene(?) micaceous sediments on the north side of Big Skidder Hill (unit Tems) are problematic.

There are no ages from this unit, and contacts with the adjacent Lyre and Crescent Formations are

not clear. Massive micaceous siltstone and fine sandstone predominate. They could represent one of

three micaceous sequences in the forearc: early middle Eocene Tyee Formation equivalents (Scow

Bay sandstone?), Armentrout and Berti, 1977; late middle Eocene Puget Group; or upper Oligocene

Pysht Formation and the Miocene Clallam Formation (Garver and Brandon, 1994). Because the

micaceous sediments apparently rest on Crescent Formation and are in an upthrown block against

Lyre Formation on Big Skidder Hill, it appears likely it may be one of the pre-Lyre units.

Overlying all of the Tertiary units are surficial deposits related to the Vashon stade of the Fraser

glaciation of Armstrong and others (1965). The only surficial deposits we found that may predate this

stade are indurated colluvial deposits on the flank of the Olympic Mountains in the southwestern part

of the study area (unit Qci). Sediments predating the Vashon advance are usually indurated, due in

part to the great weight of the ice that overlay the region. The colluvial deposits are also indurated

and lie directly on bedrock. They contain locally derived clasts and thus may be correlative with the

Evans Creek stade of the Fraser glaciation (e.g. Booth, 1990), a time of widespread alpine ice advance

in the Cascade Range and Olympic Mountains that predated the culmination of the Vashon-age ice

sheet by about 5,000 years.

During the Vashon advance, the Vashon ice sheet bifurcated northeast of the study area into the

Puget lobe, which occupied the Puget Lowland, and the Juan de Fuca lobe, which occupied the Strait

of Juan de Fuca (i.e. Waitt and Thorson, 1983). The Puget lobe blocked northward drainage from the

Puget Sound region and formed proglacial lakes that drained southward along the Chehalis River

valley to the Pacific Ocean at the southwestern extent of Puget Sound. The Juan de Fuca lobe

presumably blocked drainage of Port Discovery before occupying it. Pebbly and stoney silts (unit

Qval) near Discovery Bay are interpreted as these proglacial lake deposits. These sediments are

overlain by both sand- and gravel-dominated advance outwash deposits (units Qvas and Qvag) that

were deposited by streams and rivers issuing from the front of the advancing ice sheet. The ice sheet

first covered the study area around 18,000-17,500 cal yr B.P., reached its maximum extent in southern

Puget Sound around 16,950 cal yr B.P. (Porter and Swanson, 1998), and covered the region with up

to 1500 m of ice (Booth, 1987). Till (unit Qvt) marks the presence of the ice sheet in the study area,

and mantles most surfaces. Granitic clasts within the till attest to a source at least 100-km away in

the Coast Mountains of British Columbia.

The Puget lobe rapidly retreated from its maximum extent and a series of proglacial lakes formed.

Retreat of the Juan de Fuca lobe is not as well documented, but it probably also retreated rapidly.

When the ice front retreated to the latitude of the study area around 16,500 cal yr B.P. (Porter and

Swanson, 1998), the lakes no longer drained to the south through the Chehalis River valley, but began

to drain northward through the Leland Creek spillway (Thorson 1979, 1980; Waitt and Thorson,

1983), located along the Port Discovery-Crocker Lake-Leland Lake valley on the east side of the

study area. This northward-draining lake is named Lake Bretz (Waitt and Thorson, 1983), although

it was initially named ‘Lake Leland’ by Thorson (1979, 1980). The Leland Creek spillway is 10-15

km long, has a fairly constant width of 200-400 m, and is constrained by the Tertiary sediments on

the valley walls. The current high point of the Leland Creek spillway lies at an elevation of 68.6 m,

between Crocker Lake and Leland Lake (Thorson, 1979). Thorson (1989) reports silty lake sediments

extending to an elevation 60-m to the south of the spillway crest, but these are generally absent to

the north. There are some stratified fine silts with rounded dropstones of pebbles and cobbles along

Highway 20, southeast of Port Discovery. Higher energy recessional outwash deposits (units Qvrs

and Qvrg) overlie, or are incised into, till and were probably deposited while the spillway was active.

Recessional gravels lie on point bars exclusively north of the high point of the Leland Creek spillway,

possibly due to the higher velocity of streamflow. Following deglaciation there was isostatic rebound

as well as a rise in sea level. Details of this history are elusive within the study area, but isostatic

rebound explains the presence of glaciomarine or glaciolacustrine deposits at elevations above sea

level. Thorson (1989) estimated the magnitude of rebound at Gardiner as 140 m, a few kilometers

north of the study area on the west shore of Discovery Bay.

Following the Vashon glaciation, landslides developed on some steep hillsides (unit Qls). Perhaps

the most interesting of these is a landslide about 4 kilometers south of the head of Port Discovery,

where a massive jumble of Lyre Formation conglomerate, with blocks as large as houses, slumped off

the east-facing hillside. As a result, the landslide deflected Snow Creek eastward. The modern depositional

environment includes alluvium (unit Qal) and alluvial fan deposits (unit Qf) along modern streams,

and beach deposits (unit Qb) along the shores of Port Discovery. Human modification of the

geomorphology is mostly seen as artificial fill (unit af) along Highway 101.

STRUCTURAL HISTORY

The structure of the Uncas quadrangle appears difficult to interpret, with faults striking north, east,

northeast and northwest. Overall, the area has been tilted eastward on the flank of the growing Olympic

uplift, which was accompanied by folding and faulting of the bedrock strata into an eastward plunging

synclinal structure. Although attitudes in the Crescent Formation are similar to those in the Lyre

Formation, an unconformable contact is inferred by the absence of the Aldwell Formation and the

deposition of Lyre conglomerate directly on the Crescent Formation. This suggests deformation began

in late middle Eocene time. In the Cape Flattery area, at the northwestern tip of the Olympic Peninsula,

the Lyre overlies the main thrust at the base of the Crescent Formation and suggests that it postdates

the beginning of accretionary wedge formation (Snavely and others, 1993). On Big Skidder Hill, the

attitudes in the Lyre are variable. The attitudes of the overlying Snow Creek member of the Lyre

generally strike north or north-northeasterly, which suggests the younger member saw a different

structural history than the rest of the Lyre Formation. However, Spencer (1984) argues for a conformable

contact at the base of the Snow Creek member, and outcrops of the Snow Creek member along Snow

Creek can be interpreted as being positioned near the nose of an eastward-plunging syncline. The

eastward plunge of the fold along upper Snow Creek, the eastward dips of most units, and the

northwest-striking depositional contacts between the Lyre and the Crescent Formations in the

southwestern part of the quadrangle are caused by eastward tilting of the entire area during uplift of

the Olympic Mountains, since about 18 Ma (Brandon and others, 1998).

There are three major fault zones in the map area: east-west faults flanking Big Skidder Hill, a

major northwest-striking dextral fault zone along Deadfall Creek, and north-striking faults along the

Discovery Bay-Crocker Lake zone.

The east-west striking faults on the north and south flanks of Big Skidder Hill may be reverse(?)

faults in which Crescent Formation is faulted southward over the Lyre Formation and the Lyre is

likewise faulted over sandstone of Snow Creek. Fault scarp breccias indicative of topography and

faulting during deposition were not observed and suggest the faults formed later, due to north-south

compression of the Olympic Peninsula. Another east-west striking fault with south-side up motion

may exist about a mile north of Big Skidder Hill. A fault is needed to explain the presence of the

Eocene(?) micaceous sediments on the north side of Big Skidder Hill topographically above the

Aldwell-age basaltic sediments in the Salmon Creek drainage.

In the southwestern corner of the quadrangle, the right-lateral Bon Jon Pass fault heads northwest

along Deadfall Creek and the little Quilcene River. This fault is exposed in a quarry near the summit

of the pass where a 10-m wide shear zone with strong horizontal slickensides juxtaposes Crescent

Formation submarine deposits on the southwest with Crescent Formation basaltic flows on the

northeast. Locally, these slickensides indicate right-lateral offset. Northeast-trending faults cutting

through Cedar Flat juxtapose Lyre Formation with the Crescent Formation and may be normal or leftlateral faults kinematically related to the Bon Jon Pass fault.

A north-northwest trending linear trough in the center of the quadrangle extends along Ripley

Creek, the north fork of Andrews Creek, and through the valley west of Big Skidder Hill. West of

Little Skidder Hill it corresponds with the contact between the sandstone of Snow Creek and the Lyre

Formation. Outcrops along the walls of the Snow Creek drainage reveal several near vertical fractures,

some of which indicate right-lateral offset. No major fault was observed, but areas of no exposure

in the creek could obscure a fault of moderate displacement.

Along the Discovery Bay-Crocker Lake trend the Lyre Formation and sandstone of Snow Creek

on the east side of the valley is folded with northeast strikes, suggesting sinistral drag along the northnortheast fault zone. Total displacement across this fault zone seems limited, given that the stratigraphy

is similar on both sides and that a number of structural elements (east-west striking faults, small

Crescent uplifts) also appear to correlate across the Leland Creek Valley if small sinistral slip is

allowed.

Overall, the deformation and fault pattern generally suggests north-south compression in postSnow Creek time, a conclusion consistent with underthrusting of the Crescent beneath Vancouver

Island and continued northward migration of the Coast Range in response to oblique subduction

(Clowes and others, 1987; Wells and others, 1998).

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Strike and dip of beds

Inclined

Horizontal

Dip of foreset beds

15

22f

 PRELIMINARY GEOLOGIC MAP OF THE UNCAS 7.5’ QUADRANGLE

CLALLAM AND JEFFERSON COUNTIES, WASHINGTON

By

Peter J. Haeussler, Jim Yount, and Ray Wells

1999

CORRELATION OF MAP UNITS

Unconformity

Qci

Qval

Qvas Qvag

Qvt

af Qb Qf Qal Qw Qls

Qvrs Qvrg

Holocene

Pleistocene QUATERNARY

TERTIARY

Tems

T a Tebs

Tss

Tluc

Tlav

Tllc

Tl

Unconformity

Unconformity

Tcs

Tcb Tcf Tcfb

middle Eocene

lower Eocene

upper Eocene