The Geology of Southwestern Fidalgo Island Daryl Gusey Western Washington University

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Summer 1978 The Geology of Southwestern Fidalgo Island Daryl Gusey Western Washington University

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This Masters Thesis is brought to you for free and open access by the WWU Graduate and Undergraduate Scholarship at Western CEDAR. It has been accepted for inclusion in WWU Graduate School Collection by an authorized administrator of Western CEDAR. For more information, please contact [email protected]. THE GEOLOGY OF SOUTHWESTERN FIDALGO ISLAND

* . L

A Thesis

' Presented to

" The Faculty of

Western Washington University

1 ) In Partial Fu^illment

Of the Requirements for the,,Degree '

Master of Science

Daryl L. Gusey

September 1978 THE GEOLOGY OF SOUTHWESTERN FIDALGO ISLAND

Daryl L. Gusey

Accepted in Partial Completion

Of the Requirements for the Degree

Master of Science

Dean of the Graduate School

Advisory Committee

Chairperson MASTER'S THESIS

In presenting this thesis in partial fulfillment of the requirements for a master's degree at Western Washington University, I grant to Western Washington University the non-exclusive royalty-free right to archive, reproduce, distribute, and display the thesis in any and all forms, including electronic format, via any digital library mechanisms maintained by WWU.

I represent and warrant this is my original work and does not infringe or violate any rights of others. I warrant that I have obtained written permissions from the owner of any third party copyrighted material included in these files.

I acknowledge that I retain ownership rights to the copyright of this work, including but not limited to the right to use all or part of this work in future works, such as articles or books.

Library users are granted permission for individual, research and non-commercial reproduction of this work for educational purposes only. Any further digital posting of this document requires specific permission from the author.

Any copying or publication of this thesis for commercial purposes, or for financial gain, is not allowed without my written permission. ABSTRACT

Detailed geologic mapping of rocks In the upper stratigraphic levels of the Fldalgo ophlollte Indicates that keratophyres and spllltes are Interbedded with tuffaceous sediments, sedimentary breccias, and graywacke. Sedimentary breccias containing plutonlc rock fragments as well as volcanic rock fragments are common. The graywackes were derived from a volcanic source area. Radlolarla In the tuffaceous sediments ,

Indicate a deposltlonal age of Lower Klmmerldglan to Upper Valanglnlan^

Compared to the stratigraphy and petrology of other ophlolltes and present-day tectonic environments, the Fldalgo Complex most closely resembles that of ancient and modem Island-arc sequences.

1 ACKNOWLEDGMENTS

I wish to thank the members of my thesis committee, Drs. E. H.

Brown, D. R. Pevear, C. A. Suczek, and Ada Swineford, for their help, guidance, and critical editing. Special thanks go bo Dr. E. A.

Pessagno of the University of Texas for dating the radiolarian samples.

Also, I thank Joan Roley for typing this thesis, the people of Fidalgo

Island for allowing access to their property and my wife, Marla, for support. TABLE OF CONTENTS

ABSTRACT...... i

ACKNOWLEDGEMENTS...... ii

LIST OF FIGURES...... vi

LIST OF TABLES. AND PLATES...... viii

INTRODUCTION...... 1

STRATIGRAPHY AND AGE...... 7

STRUCTURE...... 11

LITHOLOGIC DESCRIPTIONS ...... 13

Serpentinite ...... 13

Layered Gabbro...... 13

Plagiogranitic Dike Complex...... 13

Volcanic Rocks ...... 22

Graywacke...... 32

Sedimentary Breccia...... 43

Pelagic Argillite...... 49

Black Radiolarian Argillite...... 53

Green Radiolarian Argillite ...... 53

Chloritic Arenite Interbeds ...... 54

Green Argillite...... 64

Radiolarite...... 64

Tuffaceous Layers ...... 64

Other Lithologies...... 65

DISCUSSION...... 68

CONCLUSIONS...... 77

REFERENCES CITED...... 78

lii LIST OF FIGURES

Figure Page

1 Generalized geologic map of the San Juan 2 Islands. From Whetten and others, 1978.

2 Regional geologic setting of the Fidalgo 4 ophiolite. From Brown, 1977a.

3 Index map showing the field area of this 6 study. The triangle is the location of Mt. Erie. MAQ refers to the Marine Asphalt Company quarry. The dashed line is the boundary of Fig. 5.

4 Interpretive stratigraphic sections from 8 Fidalgo Island. After Brown, 1977a.

5 Index map showing the location of geo- 14 graphical place names used in the text. MAQ-Marine Asphalt quarry. RRQ-Red Rock quarry.

6 Autobrecciation in cross-cutting keratophyre 16 dike within the plagiogranitic dike complex. North of Rosario Head on the western shore line of Fidalgo Island.

7 Serpentinite body, 1000 m north of Rosario 17 Beach.

8 Flow-like structures at the contact between 18 serpentinite and dlorlte. The serpentinite is to the left. 1 km north of Rosario Head. The lens cap is 57 mm in diameter.

9 Photomicrograph showing pseudo-gneissic 19 texture in altered diorite (sample M187). A-amphibole, P-prehnite, Ar-aragonite. Crossed nicols.

10 A rodingite body in serpentinite about 1 km 20 north of Rosario Head. Note the dark rind and light core.

11 Photomicrograph showing relict Igneous 21 texture in rodingite (sample M187b).

12 Weathered surface of a brecciated volcanic 23 flow exposed along Ginnett Road.

iv Figure Page

13 Trachytic texture in keratophyre (sample 24 M92b). Crossed nicols.

14 Synneusis clump of plagioclase crystals in 25 a keratophyre (sample M88a). Crossed nicols.

15 Photomicrograph showing the rounding of 26 quartz and plagioclase by resorption in a quartz keratophyre (sample M28). Crossed nicols.

16 Photomicrograph showing epldote pseudomorphic 28 after amphibole, also present are plagioclase phenocrysts and synneusis, the groundmass is fine-grained epidote-chlorlte and plagioclase microlites. Quartz keratophyre (sample M143a). Crossed nicols.

17 Photomicrograph showing an amphibole phenocryst 29 in a quartz keratophyre (sample M28). Crossed nicols.

18 The red and green tuffaceous argillite unit. 30 Red Rock Quarry.

19 Photomicrograph showing radiolaria in tuffa- 31 ceous argillite (sample M78a). w/o crossed nicols.

20 Sample locations for radiolarian-bearing 34 tuffaceous argillites.

21 Sedimentary breccia, probably representing 35 debris flow. Mostly volcanic clasts in a tuffaceous matrix. Near the Red Rock quarry.

22 Graywacke exposed at the north end of the 37 Deception Pass Bridge.

23 Photomicrograph showing plagioclase (P) 38 crystals, volcanic rock fragments (V), quartz (Q), and chlorite (C) in a graywacke from Bowman Hill. Crossed nicols.

24 QFL diagram for graywackes on Fidalgo Island. 41 Q=quartz, F=feldspars, L=lithic fragments including chert).

25 Rock fragment diagram for graywackes on Fidalgo 42 Island. Rs=sedlmentary rock fragments (includ ing chert), Rv=volcanic rock fragments, Rm= metamorphic rock fragments.

V Page

Photomicrograph showing metamorphlc foliation 44 in graywacke from Capsante (sample M7).

Serpentinite clasts in a sedimentary breccia 46 near Lake Campbell.

Cumulate gabbro clasts in a sedimentary 47 breccia near Lake Campbell.

Pegmatitic diorlte clast in a sedimentary 48 breccia near Lake Campbell.

Near vertical dipping pelagic argillite 50 underlain by sedimentary breccia at the Marine Asphalt quarry. The hammer (encircled) lies near the contact between the two units.

Comparison of the chemistry of Fidalgo Island 51 sedimentary rocks with modern sediments from the Pacific. From Brown, 1977b.

Detailed stratigraphic section of the pelagic 52 argillite unit at the Marine Asphalt quarry.

X-ray diffractograms of oriented, 2 hm 55 fraction of green radiolarian argillite (sample PT12a) . Cu K 3i radiation. C= swelling chlorite (?), T=talc, A=amphibole. D spacings are in Angstroms.

Photomicrograph showing chlorite replacing 56 pyroxene in chloritic arenite (sample F105). w/o crossed nichols.

X-ray dif fractograms of oriented, Vim 57 fraction of chloritic arenite (sample 21-18). Cu K 3 radiation. D spacings are in Angstroms.

X-ray dif fractograms of oriented, Vim 53 fraction of chloritic arenite (sample G-2). Cu K 3 radiation. D spacings are in Angstroms, Analysis by D. R. Pevear.

Plot of chlorite, compositions. Mole % of A1 60 (IV) vs. Mole % fe (VI). Sample FI and F2 are from Fidalgo Island. Other numbers repre sent chlorite analyses reported in the liter ature. l-6=chlorltes associated with ultramafics. 7-14=chlorites associated with volcanics. 15-17=chlorltes associated with metamorphlc rocks. 18-20=chlorites from miscellaneous sources. See key for references.

Vi Figure Page

38 Pillow basalt at Rosario Head. The lens 66 cap is 57 mm in diameter.

39 Ribbon chert and pillow basalt (lower left) 67 exposed at Rosario Head.

40 Stratigraphic sections of ophiolites. 1 after 69 Coleman, 1971; Robertson and Hudson, 1974. 2 after Hopson and others, 1975. 3 after Evarts, 1977. 4 after Almy, 1977. 5 modified after Brown, 1977a.

41 Idealized sedimentary section for oceanic sea 73 floor created at a mid-oceanic spreading center.

42 Cross section of an island-arc system based 74 on seismic profiles across the Marianas island- arc (after Karig, 1971a; and Garrison, 1974).

vii LIST OF TABLES AND PLATES

Table ^’age

1 Known dates for rocks of the Fidalgo Complex. 10

2 Biostratigraphic report for samples from 33 tuffaceous argillites. Determination by Dr. E. A. Pessagno, Jr.

3 Modal composition of graywackes on Fidalgo 40 Island. A minimum of 600 counts per sample.

Plate Page

1 The Geology of Part of Fidalgo Island Back Cover

viii INTRODUCTION

The rocks on Fidalgo Island, northwestern Washington, have been interpreted as an ophiolite suite (Hopson and Mattinson, 1973; Brown and Bradshaw, 1974; Brown, 1977a, b; Brown and others, 1978). The assemblage of rocks, named the Fidalgo Complex, has been shown to consist of serpentinite, layered gabbro, a plagiogranite dike complex, keratophyre

and spilite, pelagic sediments, and graywacke (Brown, 1977a). The

Fidalgo Complex is exposed elsewhere in the San Juan Islands and is mapped

as part of the Decatur terrane by Cowan and Whetten (1977). Whetten and

others (1978) suggest that the San Juan Islands consist of six thrust

bound terranes (Fig. 1), including the Decatur, which are defined on the

basis of age, lithology, and structure. The Decatur terrane is markedly

less deformed and less metamorphosed than some of the other terranes

(Whetten and others, 1978).

Nearby, in the North Cascades of Washington, rocks of similar age

and lithology, the Wells Creek Volcanics and the flysch-like Nooksack

Group sediments, are in thrust fault contact with the overlying Chilliwack

Group of Late Paleozoic age (Misch, 1966; Brown, 1977a; Fig. 2).

Peridotites of possible ophiolitic affinity are exposed in the North

Cascades at Twin Sisters and near Darrington (Fig. 2). Ophiolites of

similar age also have been found at the Point of Arches, Ingalls Peak, and

Tieton Reservoir (Brown, 1977a; Fig. 2). The relationship between the

Fidalgo Complex and these contemporary rocks of western Washington is

unknown.

Understanding the paleogeography of western Washington for the Jurassic

1 FIDALGO

2 Chuckanut Formation. Conglomerate and sandstone of Lopez terrane. Strongly foliated, highly deformed, fluviatile origin unconformably overlying older locally chaotic graywacke-argilllte flysch, rocks. Paleocene or Eocene. pillow lava, and ribbon chert (Upper Jurassic to middle Cretaceous) and blocks of metaplutonic X •H U o 3 3 o •o *d 4J d d 3 3 M W o •H CJ 3 3 d X 3 3 3 V< E d 00 O U o Vl 3 d E o 3 3 a o o d «k X W d3 U o 3 O 3 d 3 U 3 o> CV 3 M 3 3 3 • ^ CM u-i T> w > ■H 4-» •H CJ CtO U cO cO U 0) CO C 3 M M M u V o CO d u ^ o CO C U d 0 3 3 d •

v. w u ^ *-> -H J t rH •H rH « V4 u O 3 o o Qi u CO CO CO u 3 H 3 a o

•O •H * •H •U •O H ,. < 3 3 3 d ■U d 00 o a 3 U •rH C/) •3 P* Cl o •H CJ 4J 3 3 d d d o u a 3 O a 3 3

'o CJ X v> H •-) W U 3 3 O 3 O 3 o > *3 X o 3 d 3 3 M 3 VI 3 u a 3 d 3 3 3 d 3 o • 3 00 3 o 3 •>

•

*H f H •3 iH •H •H M • ■M o M 00 (0 CO M O V OO U 3 >> O M 4) 0> C 3 d U 00 3 3 • • • Tl •d iH •H • •H u T3 d 3 3 3 V 3 3 3 d < 4J OO o i •H 32 P4 d o y o Vi 3 3 • o d 3 u C 3 d o o a i •

•H •iH H P 3 3 o VI Vl 3 U a a 3 3 o d 3 • •o rH ^ •3 r^ M •H X X X X 4-» Boa Qi a o a a 0 X >s CO « d U V4 3 3 a t-l O 3 d 3 3 0 CJ ) • I

•H f-( T ^ *H »H d4 X u H CO c CO «U O N O M CO d M 4) o CO a a d. X O 3 U 3 3 o 3 3 3

c "o

Qu -H r-f •H H iH o C Cfl C CO 0 B M > «o d d 4) d d CO 3 3 > o d 3 d rH iH •H a 3 3 0 rX •H M *3 fH > (K4 •H Vl 00 (V » t U 3 O 3 3 CO •H •H 00 U 3 >s > 3 3

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>> 3 Cl 3 •> ^ *H »o •-) •H H

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T mh t ^ rH cjN m I vO v£) T3 < 1 o^ ^ w — U 1

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ec; •H 4J •U O' O 3 CO 4J 0) d CO d o 0) B CO u >> d cd Cd 0) d 0) •

49*

Figure 2. (From Brown, 1977a). Regional geologic setting of the Fidalgo ophiolite. PA-Point of the Arches ophiolite, Jurassic; TB=Turtleback Complex, Ordovician; B= Blakely Island ophiolite, Jurassic; CY=Cypress Island peridotite; TS=Twin Sisters dunite; YA“Yellow Aster Complex, Precambrian and Ordovician; D=Barrington ultramaflc bodies; I=Ingalls peridotite/ophiolite, Jurassic; T=Tieton ophiolite, Jurassic. The map is simplified and interpretive. It is based on data from the following sources; Huntting and others, 1961; Mlsch, 1966; Mattinson, 1972; Snavely and others, 1972; Carson, 1973; Hopson and Mattinson, 1973; Muller and others, 1974; Tabor, 1975; Vance and others, 1975; and R. Armstrong, 1976, personal communication re/age of blueschist metamorphism. Heavy contact lines represent high-angle faults.

4 time may be clearer when the tectonic setting of the Fidalgo Complex is known. The presence of abundant silicic rock suggests an island arc genesis for the Fidalgo Complex, but the pelagic sediments point to an origin at an oceanic spreading center (Brown, 1977a). Important to this problem is the stratigraphy of the volcanic and sedimentary units which, in spite of three previous studies (McLellan, 1927; Mulcahey, 1975; Artim, 1977), is not well known. The purpose of this research is to determine the stratigraphic relationships in the volcanic and sedimentary sections of the Fidalgo Complex, to compare the stratigraphy and petrology of the

Fidalgo Complex to those of other ophiolites, and to consider tectonic environments in which ophiolites may have formed (i.e., oceanic spreading centers, back-arc spreading centers, island arcs, and aseismic ridges).

This research perhaps will aid in determining the tectonic setting in which the Fidalgo Complex originated.

The studied area lies south of Mt. Erie, but also includes the outcrop

at the Marine Asphalt Company quarry (Fig. 3). Prior to this study these

areas, containing volcanic and sedimentary units, had not been mapped in

detail. The volcanic and sedimentary units elsewhere on Fidalgo Island had

been mapped in detail (Brown, 1977b). 6 STRATIGRAPHY AND AGE

Interpretive stratigraphic sections of the Fidalgo Complex are shown in Fig. 4. Lowermost in the section is serpentinized peridotite.

Contacts between the serpentinite and the layered gabbro have not been observed on Fidalgo Island, but close spatial relationships and graded bedding in the layered gabbro suggests that the serpentinite lies

stratigraphically below the layered gabbro (Brown, 1977a, b). Intrusive

into and stratigraphically above the layered gabbro is a dike complex of plagiogranitic rocks. The plagiogranite postdates the layered gabbro as

evidenced by cross-cutting dikes of plagiogranite in the layered gabbro.

Furthermore, the plagiogranite dikes contain xenollths of gabbro, and

have caused the contact metamorphism of the gabbro (Brown, 1977b). The

graded bedding in the gabbro also suggests that the plagiogranite is

stratigraphically above the layered gabbro. Chemical studies, supported

by these field relationships, have shown that the layered gabbro and the

plagiogranite are not co-magmatic (Brown and others, 1978). Within the

dike complex is a serpentinite pod that may be coeval with the plagio

granite, as evidenced by remnants of dikes that cut the serpentinite body.

Co-magmatlc with the plagiogranite, and stratigraphically above the

dike complex, are volcanic rocks consisting of keratophyre, spilite, and

quartz keratophyre (Brown and others, 1978). Evidence that these volcanic

rocks are co-magmatlc with the plagiogranite Includes identical chemical

compositions, gradations in texture, and overlapping ages of formation as

indicated by dikes of keratophyre in plagiogranite host and dikes of

plagiogranite in keratophyre host. Synchronous with this volcanism, as

evidenced by the interfingering and interbedding of the volcanic rocks

7 Southern Fidalgo

Northern Fidalgo*

' r ’• Siltstone&Graywacke kVv':-.-v.

Tuffaceous Argillite

Pelagic Argillite

^ 0 ^ - c ^ * Sedimentary Breccia

Keratophyre & Spilite vl Plagiogranite

>•••••••••« Gabbro

Serpentinite unexposed

, Interpretive stratigraphic sections from Fidalgo Island after Brown, 1977a,

8 with various sedimentary rocks, was the deposition of pelagic argillite, tuffaceous sediments, lithic graywacke, and sedimentary breccias.

Radiometric and paleontological dating of the rocks of the Fidalgo

Complex indicates an age of Middle Jurassic to Early Cretaceous (Table 1).

K/Ar dating of hornblende in diorite from the dike complex indicates an

age of about 155±5 m.y. B.P. (Brown, 1977a). U/Pb dates on zircons from

the plagiogranite on Fidalgo Island are 167±5 m.y. B.P. (Whetten and

others, 1978). Radiolaria from pelagic and tuffaceous argillites are

lower Kimmeridgian to upper Valanginian (Pessagno, 1977, 1978, written

communication).

9 t

TABLE 1, Known dates for rocks of the Fidalgo Cornplex § H o •< o M s Cd l-q o «*: E o O O E pH Cd Pi W CO 1-5 h h • • XA OO [ Os Os XA — XA w •ri p r-i T3 s CM pr-i rO •H p> t6 O CA (-1 ® P O • • 1 • vO \o \ XA c^ t- o\ pq r~i CA w XA CO X! CM pCi < H 1;* g i T3 cd P o ® p — a> G K cd ® o P ® P P • • 1 i

si XI m ® o w Cd O ct ” I I?* "S si U ■p -p r^ ■p -n xA CA f-i •H +1 •H p>

oo Os r^ E ^ •ri 0\ r-- rO oo •H P •H •H XA < -P ■H O 1-1 S CSI d ■ri o C s cd O & Cd o S H s Cd rt P ® Oi rt ClO O h 1 1

!a *ri •H | ■H r ■p CD o o o g E t — O ® — ® 3 cd P td 10 i i

’ -- •ri +3 -p r~i •H P^ rH Os C-- sA yG W •H •H 'd so CO fH P •H 'O < •H -4^ a ■H s (1> c H •H ® fl) cd O cd O Cd & cd E g Cd c ® Cd G hC p 3 P Pu 0) txb O cd 1 <> •s O

W r-i EH t ja p •H «H | *ri n> o P! O cd fn !>> O C — 3 C 3 ® O P p cd bO U S 1 ’ -- oo 1 •ri -P Os -P w •H •H SO sA XA I ■ri •ri •tJ ■ri >Si tP C CH — — S I -p

r-v -H Ti r-i Ti 1-1 -P C a O ® 3 bO P -- oo -P •P •ri •H 1 Os so sA •H *r{ •ri S=£l ■ri T( rq csj «p «H pp d CO ■p — u G cd 3 ( Cd o cd p ® ® m ro Q> P a P e e P bfl-H ® p cd TO P rt cd P 3 O ® O P h •t i i

r-v

•H H > •ri •H r-i 1-1 ■H ■P Cd O G G P ® cd 3 C 3 P o< u W) C ® 3 P bO i -- oo I •ri so _P Os r~- p p sA XA •H •H TJ Ut •ri •H ■ri T) Ip •P (P |P5 g ■p — $e 0} C h cd o cd cd ® P E S ® P 3 G ® cd m TO (D P P bfl-H p o ® O p 3 P P cd P 1 •» i

P ^

*H > rH •ri •H •H •P Cd o C ® Cd 3 C P P P P 3 G txO 3 P bO i i OO si 0\ r- •H P so XA •H P •H •ri H w •ri T) «P CP iSi •ri •ig g! -P s h

i i . argillite upper Tithonian STRUCTURE

Generally, bedding in the field area of this study dips to the north at angles ranging from sub-horizontal to vertical and overturned. Large scale structures, such as anticlines and synclines, could not be discerned, although locally small-scale folds were observed (e.g., north of the

Deception Pass Bridge). Numerous northwest and northeast trending faults cut the area, as evidenced by shear zones, slickensides, offset of bedding, juxtaposition of differing lithologies, anomalous bedding attitudes, and topographic expression.

Within the volcanic section, mapped contacts between the several

Interbedded units are approximate because of the generally poor exposure in the area. The determination of bedding attitudes in the tuffaceous units was difficult, unless the exposure was extremely good, because of its homogenous, fine-grained nature. On Plate 1, several of the tuffaceous argillite and siliceous tuff beds are shown to pinch out. They are interpreted to be pinchouts on the basis of the lack of evidence to indicate a fault. Conversely, some of the inferred faults may actually represent pinch outs or facies changes. Also, many small lenses and thin beds of tuffaceous argillite could not be shown at the scale of the geologic map (Plate 1).

Between Deception Pass and Rosario Head, along the northern shore line of Bowman Bay, rocks of the Lopez terrane (Cowan and Whetten, 1977) are separated from rocks of the Fidalgo Complex by a series of high-angle

faults. Also at this locality, Lopez terrane rocks (pillow basalt and

ribbon chert) appear to be tectonically mixed with rocks of the Fidalgo

11 Complex. The contact between the Fidalgo Complex and the Sinclair terrane (Whetten and others, 1978) at Capsante is probably a fault, although it cannot be observed in the field.

12 LITHOLOGIC DESCRIPTIONS

Serpentlnlte

The stratigraphically lowest member of the Fidalgo Complex, the serpentinized peridotite of McLellan's (1927) Fidalgo Formation, is exposed on Fidalgo Island at Washington Park and on nearby Burrows,

Allan, and Cypress Islands (Fig. 5; Plate 1). The serpentinites have a tectonite fabric and consist of serpentinized harzburgite with cross cutting veins of pyroxenite (Raleigh, 1965). A small pod of serpentinite is exposed along the western shoreline of Fidalgo Island within the plagiogranite dike complex.

Layered Gabbro

The layered gabbro is exposed on Fidalgo Island only at Alexander

Beach (Fig. 5; Plate 1). As described by Brown and others (1978), the gabbro shows cumulus layering that is graded upward to the northeast.

Slump structures, pyroxenite dikes and sills, and gabbroic pegmatite dikes are common. The primary mineral assemblage consists of plagioclase, orthopyroxene, and clinopyroxene, however, two types of alteration; regional greenschist facies metamorphism and contact metamorphism, have caused partial to complete replacement of the primary minerals.

Plagiogranite Dike Complex

A plagiogranitic dike complex occurs at several localities on Fidalgo

Island. In the field area of this study, the dike complex is well exposed along the western shore of Fidalgo Island. North of the field area, plagiogranite occurs at Mt. Erie, northeast of Alexander Beach, and north of Lake Erie (Fig. 5; Plate 1).

13 14 1

As described by Brovm and others (1978), the plaglogranite dikes are gradational in texture and mineralogy, and include quartz diorite, trondhjemite, albite granite, hornblende gabbro, and diorite. Diorite, with 56-58% Si02, predominates (90%). Plagioclase and hornblende are ubiquitous primary minerals. Potassium feldspar is absent and the K2O content is low. Texturally, the dikes range from pegmatitic to fine grained, and autobrecciation is observed in some (Fig. 6).

A small (30 x 50 m) lens of serpentlnized peridotite occurs in the plagiogranite dike complex on the western shoreline of Fidalgo Island about 1 km north of Rosario Head (Fig. 5; Plate 1). In outcrop, the well exposed, locally sheared and slickensided serpentinite body is silvery green (Fig. 7).

At the serpentinite-diorite contact, the diorite has fine-grained flow-like structures and rodingite mineralization (Fig. 8). Thin sections and polished slabs show that the flow-like structures consist of layers of prehnite-aragonite and cataclastic green amphibole with a pseudo-gneissic texture (Fig. 9). These structures probably were caused by the tectonic emplacement of the serpentinite body.

Present within the serpentinite body are rodingites (Fig. 10),

similar to those described by Coleman (1967) from various localities

including nearby Cypress Island. The rodingites are isolated bodies that

are sometimes aligned with respect to each other; they appear to have

been broken and pulled apart. The rodingites have cores of diopside and

chlorite, and rinds of chlorite and magnetite (blackwall). However, some

of the rodingites consist entirely of chlorite and magnetite. In thin

section they show cataclastic textures that are t)qjical of some other

15 the

the

within

Head

dike

Rosario

keratophyre

North Island,

'^idalgo complex.

cross-cutting

in dike

shoreline

ern st Autobrecciation we plagiogranitic

Figure

16 17 ’^i?;ure 8, Flovr-like structures at the contact between serpentinite and diorite. The sementinite is to the left, 1 km north o '' Rosario Head. The lens cap is ^7 ram in diameter.

18 ----- 4

Figure 9, Photomicrograph showing pseudo—gneissic texture in altered diorite (sample M18?). A- amphibole, P- prehnite, Ar- aragonite. Crossed nicols. Figure 10, A rodingite body in serpentinite about 1 km north of Rosario Head. Note the dark rind and light core.

20 "-i , 5rnrn

Figure 11. Piotomicrogr?i5h showing relict igneous texture in rodingite (samnle Ml37b), rodlngites (Coleman, 1967). Locally, relict igneous textures can be observed (Fig. 11). The alignment of the rodingites and the relict igneous textures suggest that the rodingites are remnants of igneous dikes that cut the peridotite body.

Volcanic Rocks

Stratigraphically above the dike complex are interbedded submarine volcanic flows, flow breccias, tuffaceous argillites, pyroclastic breccias, and debris flow breccias that are distributed widely in the field area. The flows and flow breccias also crop out north of Mt. Erie.

Faulting and the lack of stratigraphic control make determination of the thickness of the unit difficult. Unfaulted partial sections indicate a probable thickness in excess of 1000 m. Radiolaria from the tuffaceous rocks yield an age of Late Jurassic-Early Cretaceous (Pessagno, 1978, written communication; Table 1).

The flows and flow breccias consist of keratophyre, spilite, and quartz keratophyre (Fig. 12). In outcrop they are light to dark brown, and on a freshly broken surface they are light grayish-green to dark green, depending on their composition. The brecciated flows commonly have a reddish, argillaceous (?) matrix. These flow rocks are generally massive, and they all lack pillow structures.

In thin section, flow textures range from felty to trachytic (Fig. 13), and sometimes hyalopilitic. Most of the flows are porphyritic, with phenocrysts of euhedral to subhedral albitic plagioclase and pyroxene.

Synneusis plagioclase and pyroxene are also common (Fig. 14). Also, round ing of quartz and plagioclase phenocrysts by resorption is observed (Fig.

15). Amygdules, filled with quartz, chlorite, or calclte, are generally

22 Figure 12, Weathered surface of a brecciated volcanic flow exposed along Ginnett Road.

23 I------1 . 5rv\vn Figure 13. Trschytic texture in a keratophyre (sample M92b), Crossed nicols.

24 •—------—I / m>r> Figure lU. Synneusis clump of plar^ioclase crystals in a keratophyre (sample M88a), Crossed nicols.

25 ♦ A / mm

Fip;ure 15. PhotoqraDh shovrinf^ the rounding of quartz and plagioclase by resorption in a quartz keratophyre (sample M28). Crossed nicols.

26 rare, although they are common in some of the more glassy rocks. No fresh

glass is seen; chloritization of glass, the groundmass, and pyroxenes is widespread. Epidote and calcite also occur in patches and as pseudomorphs of primary minerals. Epidote, in sample M143a, a quartz keratophyre,

is apparently pseudomorphic after amphibole (Fig. 16). Fresh amphibole was

observed in only one thin section (sample M28; Fig. 17).

The chemical composition of the volcanic rocks has been studied by

Brown and others (1978), who found that the silica content of the volcanic

flow rocks varies from 48% to greater than 70%. Compared to normal calc-

alkaline rocks, the K2O content is very low, possibly because of sea water-

magma exchange.

Widespread throughout the volcanic unit are interbeds of tuffaceous

argillite and pyroclastic breccias. The tuffaceous argillites vary from

red to green and black (Fig. 18). They typically are fine-grained and

siliceous, and generally contain well-preserved radiolaria. The interbeds

range in thickness from a few centimeters to about 50 m. Pinchouts and

lenses are common. Locally, the tuffaceous argillites are finely lami

nated, either a primary depositional feature or possibly as a result of

reworking and redeposition by currents.

Sand-sized and larger pyroclastic debris, including euhedral plagio-

clase, shards of altered volcanic glass, volcanic rock fragments (up to

5 cm in diameter), and angular quartz grains are common in the tuffaceous

argillite interbeds. Pyroclastic breccias consisting of volcanic detritus

in a red or green argillaceous matrix also have been observed.

In thin section, abundant, well-preserved radiolaria can be observed

in the tuffaceous argillite (Fig. 19). Several samples were processed

27 .... . "t . 5 »»im Figure l6. Photomicrograph showing epidote pseudomorphic after amphibole, also present are plagioclase phenocrysts and synneusis, the groundmass is fine-grained epidote-chlorite and plagio clase microlites. Quartz keratophyre (sample MlU3a), Crossed nicols.

28 ! f I

\------I nun Figure 17, Photomicrograph showing an amphibole phenocryst in a quartz keratophyre (sample M28), Crossed nicols.

29 'If ■ ■•f*

Fif^re 18, The red and green tuffaceous argillite unit. Red Rock quarry. I------( ,5 #»i»n

Figure 19. Photomicrograph showing radiolaria in tuffaceous argillite (sample M78a). w/o crossed nicols.

31 using the technique of Pessagno and Newport (1972). The radiolarian

concentrate was sent to Dr. E. A. Pessagno, Jr., University of Texas,

for biostratigraphic determination. Dates obtained range from lower

Kimmeridgian to upper Valanginian (Table 2). Sample locations are

shown in Fig. 20.

Sedimentary breccias, probably representing debris flows, typically

are associated with the interbeds of tuffaceous argillite and pyroclastic

breccias. These breccias consist of sub-angular to sub-rounded clasts

of volcanic detritus in a red or green argillaceous matrix (Fig. 21).

Clast size ranges from sand-size to about 10 cm. Plagiogranite and

tuffaceous argillite clasts are minor constituents. Rounded clasts of

red tuffaceous argillite can also be observed. Presumably, these

tuffaceous argillite fragments were at least partially lithified prior

to transport, or they were not transported far.

The tuffaceous argillite interbeds probably represent ponding of

sediment on an irregular volcanic terrane. The coarse nature of some

of the pyroclastic material within the tuffaceous argillite beds suggests

that the volcanic center was nearby. The debris flow material probably

was derived from erosion of crestal regions of the volcanic terrane.

Graywacke

Graywacke^ is exposed in the field area at Bowman Hill and at the

north end of Rodger Hill (Fig. 5; Plate 1). On the north-facing slope of

Bowman Hill the graywacke apparently is interbedded with the previously

described tuffaceous sediments. On Rodger Hill the graywacke unit

conformably overlies the tuffaceous sediments. At several outcrops

^Graywacke is used here to denote those sandstones that contain significant matrix (greater than 15% (similar to the usage by Pettijohn and others, 1972). 32 Table 2. Biostratigraphic report for sanqples from tuffaceous argillites. Determination by Dr, E, A, Pessagno, Jr.

M115

Acanthocircns variabilis (Squinabol) Archaeodictyomitra rigida Pessagno Archaeodictyomitra sp,

Biostratigraphic determination; Fauna coi»5 )rised mainly of abundant speci mens of Archaeodictyomitra. Lack of Parvicingulidae plus presence of A. variabilis suggests lower Kimmeridgian. Pre-zone 1,

M123a

Archaeodictyomitra sp. Parvicingula sp.

Biostratigraphic determination: Zone 1 to Zone 5: Upper Kimmeridgian to upper Valanginian.

Ml^U

Archaeodictyomitra sp, Parvicingula sp, Crucella sp. Paronaella sp,

Biostratigraphic determination; Zone 1 to Zone 5. Upper Kimmeridgian to upper Valanginian,

M157

Emlluvia sp, ^chaeodictyomitra sp. ^Parvicingula Hsuum maxwelll Pessagno ?

Biostratigraphic determination; Zone U or below, t^per Tithonian or lower. Possibly Zone 1 to Zone U: upper Kimmeridgian to upper Tlthonian.

33 34 Figure 21, Sedimentary breccia, probably representing debris flow. Mostly volcanic clasts in a tuffaceous matrix. Near the Red Rock ouarry.

35 graywacke is intercalated vtLth volcanic flows, tuffs, and debris flows.

Interbedding of the graywacke unit with the volcanic unit is good evidence that it is also Late Jurassic-Early Cretaceous. Therefore the graywacke unit does not predate the volcanic rocks in the area as Mulcahey (1975) and Vance (1975) suggested it might.

The best exposure of the graywacke unit is in the road cut at the north end of the Deception Pass Bridge (Fig. 22). At this locality, the graywackes are complexly folded and faulted, making measurement of the

thickness of the unit impossible. A cross section of an unfaulted (?)

sequence at Bowman Hill northeast of Deception Pass suggests that the

thickness of the unit exceeds 500 m (Plate 1).

Graywacke sands and siltstones, and shales are included in this unit.

Both massive and graded bedding are common. Beds vary in thickness from

a few centimeters to several meters. Shale rip-up clasts and soft

sediment deformational features were observed locally. No macrofossils

were found, but a few radiolaria were seen in thin section.

In outcrop, the graywackes weather grayish tan to dark brown; on a

fresh surface, they are gray to light green. The graywackes are seen in

thin section to consist of sub-angular grains of volcanic rock fragments

and euhedral to subhedral plagioclase crystals (Fig. 23). Quartz is present

in amounts less than 5 percent. The argillaceous matrix comprises 20 to

25 percent of the rock. Some of the clasts are crushed and altered, which

made it difficult to distinguish between fine grained volcanic rock

fragments, altered plagioclase, and matrix.

The abundance of volcanic rock fragments and euhedral to subhedral

plagioclase and the paucity of quartz indicate a volcanic source area.

36 1

"fi Figure 22. Graywacke e:q)osed at the north end of the Deception Pass Dridge,

37 1

j,'>■

$ 1 "

H-... » > 5 mn\

Figure 23, Photomicrograph showin-^ plagioclase (P) crystals, vol canic rock fragments (V), quartz (Q), and chlorite (C) in a graywacke ^rom Bowman Hill, Crossed nicols.

38 Field relationships, such as the intercalation of the graywacke unit with the volcanic unit suggest that the source area was the adjacent volcanic terrane. Graded bedding and rip-up clasts suggest deposition by turbidity flow.

To aid in determining modal amounts, several thin sections were stained for the determination of plagioclase and potassium feldspar. For comparison, point counts were done on samples from three different gray wacke locations on Fidalgo Island: Bowman Hill, Capsante, and a ridge just west of Whistle Lake (Fig. 5). A minimum of 600 counts per sample was done. The modal data (Table 3) show that all samples are low in quartz and high in plagioclase and/or volcanic rock fragments. Potassium feldspar is absent from all samples stained. The data, plotted on a

QFL diagram (Fig. 24) show a scatter in the composition of the samples, though all samples are low in quartz. Chert is here considered to be a rock fragment. A few of the samples plot in the feldspathic graywacke field of Pettijohn and others (1972, Fig. 5-3, p. 158), but most of the samples fall in their lithic graywacke field. On a rock fragment diagram

(Fig. 25) samples from the three areas plot in distinctly different fields.

The lithic fragments in the Bowman Hill samples are almost entirely volcanic, and chert is absent. Samples from Whistle Lake are similar to those from

Bowman Hill, except that they contain chert clasts. The Capsante graywackes contain phyllite, as well as chert and volcanic rock fragments. Samples from all three areas also contain small amounts of plagiogranitic rock fragments.

Vance (1975) Included all the graywackes on Fidalgo Island and other nearby islands in his Lumml Formation. However, the relationship between

the Bowman Hill graywackes and the other graywackes on Fidalgo Island is

39 Table 3, Modal Composition of grayvackes on Fidalgo Island, A minimum of 600 counts per sample.

WHISTLE LAKE BOWMAN HILL1 CiPSANTE M13U M136 M168 £2 M7 F??a 121 1.6 Quartz 2,U 5.3 2.6 0.5 2.2 1.1 8.9 5.9

Plagioclase 32,1 2U.2 16.7 16.6 15.9 11.0 30.5 19.0 30.8

K-Feldspar ------■ - 2.1 0.7 3.1 Pyroxene l.U l.U - U.6 - 0.2

Chlorite 8.9 5.9 7.0 8.8 2.2 5.3 3.3 6.2 8.3 o.U 0.5 0.8 0.5 Epidote 1.2 - - -

Rock Fragments

Volcanic 27.U 33.5 U3.8 15.118.U 19.U 12.0 30.7 8.1

Plutonic 1.9 3.5 2.3 5.3 5.5 U.3 2.8 1.1 -

12.5 12.116.7 - Metamorph. - - - - Sedimentary 1.1 0.6 1.1 U.3 3.5 2.7 2.1 2.0 0.8

8.1 12.6 3.8 8.0 - Chert mm - 13.1 29. U U3.1 Matrix 20.2 23.0 23.3 21.6 25.1 2U.3 22.3

U.O Q 3.8 7.9 U.o 0.8 3.2 1.6 IU.8 10.0 32.U 7U.U F 50.6 35.9 25.1 26.2 22.U 16.2 50.7 21.6 R U5.6 56.2 70.9 73.0 7U.U 82.2 3U.5 57.6

RV 96.1 98.2 97.5 37.7 39.0 37.7 67.0 75.U 91.0

RS (+ chert) 3.9 1.8 2.5 31.0 35.2 29.8 33.0 2U.6 9.0

RM 31c 3 25.8 32.5 - - -

40 rnBowmanHiU

9 Whistle Lake

ACapsante

Figure 2U. QFL diagram for grayvackes on ^idalgo Island* Q» quartz, F-feldspars, L-lithic fragments (including chert).

41 m Bowman Hill

•Whistle Lake

ACapsante

Figure 25, Rock fragment diagram for graywackes on Fidalgo^ Island, Rs« sedimentary rock fragments (including chert), Rv* volcanic rock fragments, ,Rm* metamorphic rock fragments.

42 not clear. Artim (1977) reported a Late Jurassic to Cretaceous age for

fine-grained graywackes containing belemnites from quarries near Dean's

Corner and Howard's Corner (Fig. 5). Mineralogically, the Whistle Lake graywackes are similar to the Bowman Hill gra5nwackes. However, the

Whistle Lake Graywackes contain chert-pebble conglomerates (Brown, 1977 personal communication) that are absent from the Bowman Hill graywackes.

The Capsante graywackes also contain chert-pebble conglomerates, but they

are distinct from the other graywackes in that they contain phyllitlc

rock fragments and have a well-developed metamorphic foliation (Fig. 26).

Metamorphic actinolite is pervasive in the Capsante graywackes and is not

present in either the Bowman Hill graywackes or the Whistle Lake gra3n*rackes.

As suggested by Artim (1977), rocks grouped into the Lummi Formation by

Vance (1975) include several geologic units formed in apparently different

depositional environments and/or metamorphosed under different conditions.

Sedimentary Breccia

Sedimentary breccia has been found at three localities (Fig. 5):

near Lake Campbell, southwest of Heart Lake, and north of Cranberry Lake.

At the Lake Campbell and Heart Lake localities the breccia underlies

pelagic argillite as evidenced by graded interbeds within the pelagic

argillite, but at Cranberry Lake the top of the unit is not exposed (Brown,

1977a). The position of the breccias beneath pelagic argillite at these

two localities suggests that they may be correlative.

The breccia exposed along the west shore of Lake Campbell consists of

sub-rounded clasts of serpentinite and sub-angular clasts of pyroxenite.

layered gabbro, plaglogranite, and keratophyre. It is poorly sorted and

lacks discemable sedimentary structures. Clast size varies from less

43 I ■ 4 , 5/m m

Fissure 26, Photonicrop;raT3h shomng metanorphic foliation in ,^ra.'.TJacke fror. Capsante (samnle M7).

44 than 1 cm to 30 cm. The breccia seems to be overlain by pelagic argillite;

however, the exact contact with the pelagic argillite is not exposed, nor is

the base of the breccia. The thickness of the exposed breccia is about 100 m.

Near the base of the section, the breccia consists of clasts of serpen-

tinized peridotite (Fig. 27) and pyroxenite. The pyroxenite consists of

fresh clinopyroxene and orthopyroxene, with subordinate olivine. Nearby and

up-section (?) from the ultramafic detritus in the breccia, are clasts of

gabbro (Fig. 28). Cumulate textures can be seen in a few clasts. Ortho

pyroxene and clinopyroxene, with intercumulus plagioclase, are present.

Stratigraphically above the gabbroic breccia are diorite and keratophyre

clasts. The diorite detritus ranges in texture from medium-grained to

pegmatltic (Fig. 29).

Breccia southwest of Heart Lake and north of Cranberry Lake is deposi-

tlonal on plagiogranite. The well—exposed contact between the sedimentary

breccia and the underlying plagiogranite at the Marine Asphalt quarry is

irregular and appears to be an unconformity. Depositionally above the breccia

is pelagic argillite. The breccia at the quarry is approximately 75 m thick.

At the quarry, the sedimentary breccia consists of angular to sub-

angular clasts of plagiogranite and keratophyre. Clast size is typically

less than 10 cm, but some clasts are as large as 30 cm. The breccia commonly

lacks any discernable matrix; however, near the contact with the overlying

pelagic argillite and locally throughout the unit the breccia has an

argillaceous matrix.

The position of the sedimentary breccia beneath pelagic argillite

suggests a deep-marine genesis. Oceanographic studies have shown that sub marine faulting produces topographic relief and exposes various volcanic

45 46 Figure 28, Cumulate gabbro clasts in a sedimentary breccia near Lake Oamnbell.

47 Figure 29. Pegmatitic diorite clast in a sedimentary breccia near Lake Campbell,

48 and plutonic rocks on the sea floor. Serpentinite, serpentinite breccia, peridotite, gabbro, and plagiogranite, as well as volcanics have been dredged from submarine fault scarps (Bogdanov and Ploshko, 1967; Bonatti and others, 1971; Aumento and others, 1971; Melson and Thompson, 1971;

Bonatti and others, 1974; Murdmaa, 1976; Perfit, 1977). These breccias may represent talus accumulations at the base of a submarine fault scarp, or alternatively, they may represent slump deposits and/or debris flows.

Pelagic Argillite

The pelagic argillite unit is exposed northwest of Mount Erie and southwest of Heart Lake, and near the western shore of Lake Campbell

(Fig. 5; Plate 1). It is poorly exposed near Lake Campbell, but it is believed to overlie sedimentary breccia as indicated by graded bedding in a tuffaceous interbed in the pelagic argillite. The best exposure of pelagic argillite is at the Marine Asphalt quarry. Here, the steeply dipping argillite overlies a sedimentary breccia of plagiogranite and keratophyre clasts (Fig. 30).

These argillite beds are believed to be pelagic based on the follow ing evidence: (1) the abundance of radiolaria; (2) the fine grain size;

(3) their metal content; and (4) the lack of terrigenous detritus.

Generally, the pelagic argillite unit is a radiolarian-bearing, metal- enriched argillite with minor tuffaceous debris. The concentration of metals (Ba, Cu, Ni, Co, and Mh) is similar to that in present-day Pacific pelagic sediments (Brown, 1977b; Fig. 31).

A detailed geologic map and stratigraphic section (Fig. 32) of the

Marine Asphalt quarry was made using plane-tables methods. Combined

49 Figure 30, Near vertical dipping pelagic argillite underlain by sedimentary breccia at the Marine Asphalt quarry. The hammer (encircled) lies near the contact between the two units.

50 1

0) E •H Ho

C cd iH CO

o P a c ific txD I—1 cd T) the

«H

O from

-P m •H B(D O sediments

0} p> «H

o modem

c d o 0} 1977b. ■H with

o Brow o rocks

c. ID - 2 + < 5! o> U. o o pleistocene unconformity deposits

radiolarite

green argillite

chloritic arenite 15 □ m green rad. a argillite black rad. argillite

75 sedimentary m ^ 1 breccia

plagiogranite unconformity h( ,N N. » \ kV 'x />'> ~

Figure 32. Detailed stratigraphic section of the pelagic argillite unit at the Marine Asphalt quarry.

52 plane-table and petrographic analysis has made possible the subdivision of the pelagic argillite unit. The various sub-units are discussed below in stratigraphic order.

Black Radiolarian Argillite

Lowermost in the section is a black radiolarian argillite composed of radiolarian debris in a dark, dense matrix of clay material and pyrite.

The pyrite is typically framboldal and sometimes forms layers that are up to a centimeter thick. Some of the framboids may actually be pseudomorphs of radiolarla. Radiolaria from sample PT18, from a black radiolarian argillite bed approximately 55 m from the base of the pelagic clay-size unit, were dated by Dr. E. A. Pessagno, Jr., as zone 1 to zone 2, subzone

2A, Late Klmmeridgian to Early Tithonian. Radiolaria present in sample

PT18 include Mirifusus sp., Acanthocircus variabilis, Archaeodictymitra rlgida, Hsuum Maxwell!, Parvicingula sp. (?), and Praeconocaryomma magnimamma

(Pessagno, 1977, vrritten communication).

X-ray diffraction data from the packed powder samples of the black

radiolarian argillite indicate the presence of quartz, pyrite, albite, and

chlorite. Present in the clay-size fraction are chlorite, illlte, quartz,

and a mixed-layer clay. The weak 001 and 002 chlorite peaks suggest that

the chlorite is iron-rich (Brindley and Cillery, 1956).

Green Radiolarian Argillite

The green radiolarian argillite beds consist of poorly preserved

radiolaria, clay minerals, and minor tuffaceous debris. All samples

analyzed from this sub-unit contain biogenic and/or detrital quartz, and

normal or expandable chlorite, also present are minor amounts of plagio-

clase, illite, calcite, talc, and amphibole. One sample, PT12a, contains

53 0 a chlorite that is unusual in that the 14 A peak shifts with glyceration

0 0 to a double peak, one at 15.7 A, and one at 18.0 A (Fig. 33). The expan-

0 sion to 18.0 A would normally mean the presence of a smectite. However,

0 when the sample was heat-treated, the 10 A peak that would be expected with

a smectite did not appear. This behavior is similar to that of the swelling

chlorite reported by Honeybome (1951) from the Keuper marl.

Chloritic Arenite Interbeds

Present within the green radiolarian argillite are interbeds of

chloritic arenite that are generally less than 5 cm thick but range up to

40 cm. Some of these beds are massive, and others are clearly graded.

These interbeds consist mostly of sand-size chlorite grains in a micritic

matrix, though locally the matrix is sparry. The interference colors of

the chlorite are variable, ranging from lower first order to upper second

order, and anomalous blue interference colors are rare. Mesh textures,

and cleavage traces are common in the chlorites. Also, chlorite is observed

replacing pyroxene (Fig. 34), and calcite commonly replaces chlorite.

Accessory minerals include chromite, quartz, plagioclase, cllnopyroxene,

garnet, amphibole, and volcanic and sedimentary rock fragments.

Diffractograms of the clay-size fraction of the chloritic arenite

indicate the presence of a well-crystallized, slightly expandable chlorite

(Fig. 35). Sample G-2 clearly shows a superlattice reflection that shifts

0 0 with glycolation from 29.4 'A to 33.9 A (Fig. 36). In this sample, a regularly interstratified chlorite-smectite (corrensite) may be present.

A diffractogram of the unoriented, less than 2 yni fraction indicates that

the chlorite is trioctahedral. The chlorite present approximates the Ilb

polytype of Hayes (1970). The lib chlorite polytype is typically detrital.

54 Degrees 29

Figure 33, X-ray diffractograms of oriented, <2/«in fraction of green radiolarian argillite (sample PT12a), Cu K** radiation, G* swelling chlorite (?), T*» talc, A«.amphibole, D spacings are in Angstroms,

55 t'—...... m

Fis^re Photomicrograph showing chlorite replacing pyroxene in 3h. chloritic arenite (sample F105). w/o crossed nicols.

56 Figure 35. X-ray dif'’ractofrrams of oriented, <2>^jn fraction of chloritic arenite (sample 21-18), Cu K»c radiation, D spacings are in Angstroms,

57 1

Figure 36 , X-ray diffractograns of oriented- 2 /Ajn fraction of chloritic arenite (sample G--2), .Cu Kv radiation, D spacings are in Angstroms, Analysis by D, R, Pevear,

58 An approximate structural formula for the chlorite was derived, from the less than 2 pm diffractogram, using the methods of Brindley (1961), and Shirozu

(1958). The derived formula is (Mgs.i4FeQ.48AI0.33)(Sis.62^10.38) Olo(OM)8* I The chemical composition of sand-size chlorite grains, obtained by electron microprobe (by E.H. Brown and J.Y. Bradshaw) is (Mg4,88^®0.35^10.7^ (^^3.1iAIq.89)

Oio(OH)8- The calculated formulas are compared to published chlorite analyses

on Fig. 37, which shows mole percent octahedral iron versus mole percent tetra- (VI) hedral aluminum. The Fidalgo chlorites are Mg-rich and plot in the low Fe ’ (IV) low A1 field, as do chlorites associated with serpentinized peridotites.

As shown in Fig. 37, chlorites associated with volcanic rocks are typically

higher in octahedral iron, although points 9 and 10 are notable exceptions.

The association of the chlorite with chromite, the mesh textures seen in

some of the chlorites, the chlorite composition, and chlorite seen replacing

pyroxenes collectively suggest an ultramafic source for the chloritic arenite

interbeds. Further, the mineralogical immaturity suggests a local source.

Sedimentary serpentinites from the Mid-Atlantic Ridge (Bonatti and others,

1973) are mineralogically and texturally similar to the Fidalgo chloritic

arenites. The main difference between the Fidalgo rocks and the MAR sedimentary

serpentinites is that chlorite is the main constituent of the Fidalgo rocks

(though the detection of serpentine in the presence of chlorite is difficult),

whereas serpentine is the major component of the MAR rocks. However, chlorite

is common in sedimentary serpentinites of alpine orogenic belts (Lockwood, 1971).

The chlorite in sedimentary serpentinites of orogenic belts may have formed by

the transformation of serpentine to chlorite on the sea floor (Bonatti and

others, 1973). Thus, a serpentinite source seems possible for the chloritic

arenites on Fidalgo Island.

59 Figure

M ol % Fe 37.

miscellaneous numbers associated ature. Fe Plot chlorites

(VI).

represent 1-6 chlorite

associated Sample

with =

chlorites sources.

metamorphic

FI compositions. 60 chlorite

and with

associated F2 See

volcanics.

analyses are key rocks.

Mole

from for

with

reported references. Fidalgo % 18-20

A1 15-17 ultramafics.

(IV) =

= chlorites Island.

in vs. chlorites

the

Mole

liter

Other

from 7-14 %

= Key to Figure 37

1. Chlorite from serpentinite. Deer, Howie, and Zussman, 1962.

2. Chlorite associated with serpentine in chromite ore. Brown and Bailey, 1963.

3. Chlorite from the weathering of a serpentinite. Ducloux, Neunier, and Velde, 1976.

4. Chlorite associated with chromite in serpentinized peridotites. Halaam, Harding, and Tresham, 1976.

5. Chlorite from metaperidotite. Frost, 1975.

6. Chlorite from metamorphosed chloritic blackwall. Frost, 1975.

7. Chlorite from the weathering of basic pumice and tuffs. Ball, 1966.

8. Chlorite from spilite. Battey, 1956.

9. Chlorite from altered submarine acidic tuffs and lavas. Shirozu, Sakasegawa, Katsvunoto, and Ozaki, 1975.

10. Same as 9.

11. Chlorite from amygdules in andesitic tuff breccia. Kimbara, Shimoda, and Sudo, 1973.

12. Chlorite pseudomorph of plagioclase phenocryst in hydrothermally altered basalt from the Mid-Atlantic Ridge. Humphris and Thompson, 1978.

13. Chlorite from the alteration of glassy matrix of a hydrothermally altered basalt from the Mid—Atlantic Ridge. Humphris and Thompson, 1978.

14. Chlorite pseudomorph of olivine phenocryst from hydrothermally altered basalt from the Mid—Atlantic Ridge. Humphris and Thompson, 1978.

15. Chlorite from actinolite-chlorite schist. Deer, Howie, and Zussman, 1962.

16. Chlorite from metamorphosed limestone. Deer, Howie, and Zussman, 1962.

17. Chlorite from albite-epidote-chlorite-actinolite-calcite schist. Hutton, 1938.

61 Key to Figure 37 (cont.)

18. Chlorite from a mudstone; presumed source is the weathering of a granitic rock. Evans and Adams, 1975.

19. Chlorite from the alteration of biotite. Dodge, 1973.

20. Chlorite from a hydrothermal quartz-copper vein. Shirozu and Bailey, 1965.

62 Volcanic glass typically alters to smectite, and in a Mg-rich environment, the smectite may diagenetically alter to chlorite through an intermediate regularly interstratified smectite-chlorite (Dunoyer de Segonzac, 1970). This intermediate stage in the development of chlorite from smectite may be what is seen in the diffractogram of sample G-2 (Fig. 36). Thus, volcanic glass may be a possible source for the chlorite in the chloritic arenite. However, this volcanic source does not explain the presence of chromite, unless the volcanic source is ultramafic. At the Marine Asphalt quarry, debris produced by the mining activity was found that consists of a high-Mg volcanic rock. Prelimi nary chemical analyses of several samples indicate a MgO content in excess of

12%, and an SiO content of less than 48%.

Carbonate in the pelagic argillite unit is generally restricted to the chloritic arenite beds. This association of the carbonate with the chlorite

suggests that they may have a common source. The absence of carbonate in the

surrounding sediments indicates that the pelagic argillite may have been below

the carbonate compensation depth (CCD). Chemical alteration, by sea water,

of ultramafic rocks exposed on the sea floor has been suggested for the pro

duction of carbonate in the deep sea environment (Bonattl and others, 1973).

According to this model, the ultramafic rock may have been serpentlnized in

situ and the carbonate was produced l^s a product of serpentinization), and

then serpentinite detritus and the carbonate were deposited together. Alter

natively, ultramafic detritus may have been deposited, and then serpentlnized

or chloritized. The carbonate matrix may have been produced at the same time

as the serpentinization or chloritization.

The graded bedding observed in some of the chloritic arenite beds suggest

that deposition was by turbidity currents. Another possible mode of deposition.

63 which is especially viable if the volcanic glass source for the chloritic

arenites is considered, is by ash fall.

Green Argillite

The green argillite sub-unit is similar to the green radiolarian

argillite unit, except that it lacks radiolaria. Mineralogically, it con

sists of quartz, chlorite or swelling chlorite (or both), and talc (as

determined by X-ray diffraction). The quartz present may have been derived

from the dissolution of radiolaria, as detrital quartz was not observed in

thin section. However, the quartz crystals may have been too small to detect

with the microscope, even if they were detrital. Fine-grained quartz, an

end product of continental weathering and transported by global winds,

has been reported to be present in pelagic sediments (Griffin and other,

1968).

Radiolarite

The radiolarite sub-unit contains abundant radiolaria, but they are

too poorly preserved for biostratigraphic determination. The radiolaria in

some samples appear to be bedded, suggesting "blooms", or redeposition by

currents. As in the other sub-units, quartz and chlorite are present (as

determined by X-ray diffraction). In some samples the radiolaria have been

replaced by chlorite. Illite and plagioclase are accessory minerals. The

quartz present in the diffractograms probably represents recrystallized

radiolarian debris.

Tuffaceous Layers

Throughout the pelagic argillite unit are thin (generally less than

5 cm) layers of tuffaceous debris. These layers consist of angular sand-

size grains of quartz, subhedral to euhedral plagioclase, and volcanic rock

64 fragments. These layers are generally non-graded and discontinuous.

Other Lithologies

Other lithologies present in the field area of this study, but not included in the Fidalgo Complex, are pillow basalt (Fig. 38), "ribbon" chert (Fig. 39), and graywacke. These rocks crop out on Fidalgo Island in the vicinity of Rosario Head (Fig. 5; Plate 1), but are widespread throughout the San Juan Islands. There is no evidence to suggest that these rocks are related to the Fidalgo Complex. Cowan and Whetten (1977) include

them in their Lopez terrane because of the tectonic mixing that is observed near Rosario Head.

65 66 Figure 39. Ribbon chert and pillovr basalt (lower left) exposed at Rosario Head.

67 DISCUSSION

The key to the tectonic setting in which the Fidalgo Complex formed may lie in the volcanic and sedimentary rocks, since, as shown in stratigraphic sections of other ophiolites (Fig. 40), lowermost in every ophiolite is serpentinized peridotite and, usually, gabbro. The "classic" ophiolite

(e.g., Troodos) typically has a sheeted diabasic dike complex stratigraphically above the gabbroic rocks. Above the sheeted dike complex are pillow basalts. The lower pillow basalts of the Troodos Complex are chemically similar to mid-oceanic ridge tholeiites, but they show a subordinate calc- alkaline trend (Miyashiro, 1973), and some are highly silicic (Moores and

Vine, 1971). Locally interbedded with but generally above the pillow basalt at Troodos are radiolarian-bearing pelagic sediments (Moores and

Vine, 1971; Robertson and Hudson, 1974). The Troodos ophiolite has been variously Interpreted to have formed in a primitive island arc (Ewart and

Bryan, 1972; Miyashiro, 1973), at a mid-oceanic spreading center (Moores and Vine, 1971; Gass and Smewing, 1973; Robertson and Hudson, 1974;

Hynes, 1975; Moores, 1975), or in a marginal basin (Smewing and others, 1975).

However, it seems to be generally accepted that the presence of a sheeted dike complex implies an extenslonal environment, i.e., a spreading center.

But, if Karig's (1971b; Karig and Moore, 1975) model for the development of marginal basins is correct, as Miyashiro (1973) points out, an Island arc may also be an extenslonal environment, and thus a sheeted dike complex would be expected to form there also.

The Fidalgo Complex differs from the Troodos Complex in that (1) it lacks a true, sheeted dike complex; (2) it contains more abundant silicic volcanic rocks (Brown and others, 1978); (3) the volcanic rocks lack pillow

68 1

• r> • 1s rH U~l U I ON O' 4J rH M-tCO SC M T3(U

0 (1) co in 4-> 4-4 c -l CO 1-1 - 0) 0) 4-> -tJ B •H >+-l iH rH cd o t3 rH cd C3 ExD o> M o 1—1 oo CO M •H 4J (U •» 4J 14 4J 6 Cd

69 structures; and (4) the volcanic rocks are interbedded with abundant pyro clastic and volcaniclastlc sediments.

Ophiolites of an age similar to that of the Fidalgo Complex are wide spread through the California Coast Ranges and, like the Fidalgo Complex, the California ophiolites lack sheeted diabasic dike complexes (Bailey and others, 1970). As described by Hopson and others (1975), and Hopson and Frano (1977), the volcanic rocks of the Point Sal ophiolite (Fig. 40), unlike those of the Fidalgo Complex, consist mostly of spillte of mid- oceanic tholelitic affinity. They are both pillowed and massive, and commonly contain interpillow pelagic sediments. Bedded tuffs are not found within the volcanic sequence at Point Sal, as they are in the Fidalgo

Complex. Overlying the volcanic rocks are tuffaceous cherts, which are overlain by continentally derived mudstones and sandstones. The Point

Sal ophiolite is interpreted to have formed in a spreading ridge setting on the basis of chemical trends of the volcanic rocks that suggest a ridge setting, and the oceanic to terrigenous stratigraphy in the sedimentary

section (Hopson and Frano, 1977).

The Del Puerto ophiolite (Fig. 40), also in the California Coast

Ranges, contains relatively abundant intermediate to silicic volcanic rocks, pillowed and massive, that are apparently differentiates of a hydrous low-K subalkaline basaltic magma (Evarts, 1977). As Evarts (1977) points out, these silicic volcanics are extremely rare in samples obtained from the

rf> ocean floor; however, rocks similar to these have been described from various

arc settings. Above the volcanic rocks, and apparently not interbedded with

them, are interlayered tuffaceous sediments and andesitic volcaniclastlc

rocks. On the basis of this evidence, and the lack of a sheeted diabasic

70 dike complex, Evarts (1977) concluded that the Del Puerto ophiolite formed in a marginal basin or island arc setting.

The ophiolite at Del Puerto has features that are similar to certain aspects of the Fidalgo Complex. Both have an abundance of silicic volcanic rocks and an absence of a sheeted diabasic dike complex. They differ in that the tuffaceous sediments and volcaniclastic sands and debris are interbedded with the volcanic flows in the Fidalgo Complex but not at Del

Puerto.

The Fidalgo Complex is strikingly similar to the Triassic complex near Sparta, Oregon (Fig. 40). In both complexes there is an abundance of plagiogranite and co-genetic silicic volcanic rock (Brown and others,

1978; Almy, 1977). Like the volcanic rocks of the Fidalgo Complex, those in the Sparta area (the Clover Creek Greenstones) contain an abundance of interbedded volcaniclastic sediments. The complex at Sparta has been interpreted to represent an island arc sequence on the basis of the abundance of volcanogenic sediments and the overlying limestone (Martin Bridge For mation) which is of reef origin (Protska, 1963; Brooks, 1976). The lower part of the section, the serpentinite and gabbro, may be the oceanic crust on which the arc was built, or alternatively, it may be related to arc magmatism (Almy, 1977).

Theoretical models of ocean floor genesis at spreading centers suggest

that pillow basalts are first covered by calcareous pelagic sediments (if

the ridge is above the CCD). However, no Investigation has suggested that

the pelagic sediments or tuffaceous sediments are widely interbedded with

the pillow basalts. As the newly formed sea floor moves laterally away

from the ridge crest, it subsides to depths below the CCD. Below that

71 depth, the pelagic sediments are generally siliceous, argillaceous, or both.

As the sea floor approaches a continental margin, terrigenous sediments are deposited on top of the pelagic sediments. Thus, the derived strati graphic section for ridge-derived oceanic crust would appear as in Fig. 41.

This model is obviously oversimplified, but it should serve as a first

approximation. The thickness of the pelagic unit in Fig. 41 is related

to the distance the spreading center is from a source of continental

sediment. Assuming constant sedimentation and spreading rates, the further

the spreading center is from the continental slope, the thicker the pelagic

section will become prior to subduction or emplacement on the continent.

If the spreading center was near a continental slope and within the range

of turbidites, then turbidites may rest directly on newly formed oceanic

basalt. This possibility may present a problem in the interpretation of

ophiolltes, since turbidites resting on oceanic basalts is thought to

represent marginal basin stratigraphy.

The Fidalgo Complex differs from mid-oceanic spreading center derived

oceanic crust in that (1) it lacks pillow basalt; (2) it has a volcanic

section that contains an abundance of tuffaceous sediments and volcani-

clastic debris; (3) it apparently does not have an oceanic to terrigenous

sedimentary section (as in Fig. 41); and (4) it contains an abundance of

Intermediate to silicic plutonlc and volcanic rocks.

It has been generally accepted that back arc or marginal basins form

by crustal extension and the formation of new "oceanic crust" in the back

arc region of an island arc system (Fig. 42) (Karig, 1971b ; Karig and

Moore, 1975). The tectonic setting of this new oceanic crust created in a

marginal basin differs from the tectonic setting at a mid-oceanic spreading

72 r f r.i I

i' L t COARSE t i TURBIDITES

FINE

TURBIDITES

SILICEOUS PELAGIC SEOIMENT CALCAREOUS PELAGIC SEDIMENT

PILLOW

BASALT

Fipure hi • Idealized sedimentary section for oceanic sea floor created at a mid-oceanic spreading center.

73 I f

ISLAND ARC SYSTEMS 74

Figure 42. (After Karig, 1971a; Garrison, 1974.) Cross section of an island arc system based on seismic profiles across the Marianas island arc. center. Thus, a different suite of sediments may be expected. Klein

(1975, fig. 9, p. 1017) related sedimentation within Southwest Pacific marginal basins to a sequence of tectonic events. According to his model, during initial rifting in the marginal basin, sediments are slumped onto

the basin floor. As spreading continues (as new crust is being formed), coarse-grained turbidites and debris flow sediments are deposited. In

the late stages of spreading, normal graded turbidites are deposited as

slope gradients are decreased. When volcanism and spreading stops, pelagic

sediments are deposited. If tectonic resurgence occurs, a clastic wedge will cover the pelagic sediments.

Similarly, Karig and Moore (1975), discussed a sequence of marginal basin development and sedimentation. Crustal extension begins near the

volcanic arc (i.e., the arc splits). Volcanic activity supplies volcani-

clastic debris to the newly forming basin floor. As the basin widens,

volcaniclastlc rocks from the active arc can no longer reach the distal

portions of the basin. The remnant arc side of the marginal basin may be

isolated as the axial high forms a sediment dam (Fig. 42). Beyond the

distal end of the volcaniclastlc apron, and on the remnant arc side of the

basin, pelagic sediments accumulate. The non-biogenous fractions of the

pelagic sediment contain high concentrations of volcanic glass and pheno-

crysts. Thus, volcaniclastlc rocks or pelagic sediments may rest on newly

formed marginal basin "oceanic" crust. Turbidites resting directly on an

ophlolitic slab may be diagnostic of marginal basin sea floor (Karig, 1972).

If a marginal basin becomes isolated from turbidites, pelagic sediments

containing tuffaceous debris may be deposited directly on marginal basin

75 crust. But, as Karig and Moore (1975) point out, marginal basins are seldom isolated from turbiditic sediments.

Sedimentation in island arcs is contemporaneous with the volcanic processes within the arc-trench system (Dickinson, 1974). The volcanic rocks of the arc, of andesitic compositional range, are the source of the volcaniclastic and epiclastic sediments that characterize island arc sedimentation (Monger and Ross, 1971). In the early stages of arc develop ment, volcanic rocks are interbedded with thick sections of tuffaceous and marine sediments (Scholl and other, 1975). Pelagic sedimentation in an arc setting would be restricted to regions removed from active volcanism and would contain an abundance of volcanically derived material (Karig,

1971a; Karig and Moore, 1975).

The Fidalgo Complex has many features that can be interpreted to have formed in an island arc setting. First, the volcanic rocks of the Fidalgo

Complex are of andesitic compositional range (based on Si02 content).

Also, these volcanic rocks are interbedded with tuffaceous sedimentary rocks and volcaniclastic debris, which probably represents erosion of the arc contemporaneous with the volcanism. The pelagic argillite unit probably

formed away from the active arc, possibly near a remnant arc. Faulting may have caused considerable relief and may have acted as a sediment dam,

shielding the site of pelagic sedimentation from the sediments of the

active arc. At the base of fault scarps, talus breccias were deposited

and were overlain by pelagic sediments. The tuffaceous layers in the

pelagic argillite unit suggest that it was within the range of air-borne

pyroclastic debris originating from the active arc.

Another tectonic setting, which generally has not been considered as

76 a possible environment in which an ophiolite forms, is the seamount or aseismic ridge. As perceived by Garrison (1974), during the early stages of the building of the volcanic structure on the sea floor, episodes of volcanism alternate with pelagic sedimentation. The result is the inter bedding of volcanic rocks with pelagic sediments. The volcanic rocks of aseismic ridges are typically basaltic, but they are generally more silicic than mid-oceanic ridge basalts (Hekinian and Thompson, 1976). The pre viously deposited sediments may be intruded by the basaltic magma (Garrison,

1974). On top of the volcanic rocks of the seamount calcareous sediments are first deposited, as the crestal region may lie above the CCD. As the sea floor, on which the seamount was built, is generally subsiding as it moves away from the ridge crest, the calcareous sediments may be capped by deeper water sediments (Garrison, 1974).

CONSLUSIONS

The Fidalgo Complex is unlike the Troodos ophiolite in several respects, but it is similar to the California Coast Range ophiolites.

However, the greatest similarities can be drawn with the Triassic island arc sequence near Sparta, Oregon. Of the modern tectonic environments, the Fidalgo Complex approximates most closely the conditions that are observed and expected in an island arc setting. Because of the abundance of interbedded and interfingered volcanic rocks, tuffaceous sediments, lithic graywackes and debris breccias, and the silicic nature of the upper plutonic and volcanic rocks, at least the upper portion of the Fidalgo

Complex is best interpreted to have formed in an island arc setting. The serpentinlte and layered gabbro may be related to the arc magmatism, or alternatively, they may represent oceanic crust on which the arc developed.

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