The Geology of Southwestern Fidalgo Island Daryl Gusey Western Washington University
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Western Washington University Western CEDAR WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship Summer 1978 The Geology of Southwestern Fidalgo Island Daryl Gusey Western Washington University Follow this and additional works at: https://cedar.wwu.edu/wwuet Part of the Geology Commons Recommended Citation Gusey, Daryl, "The Geology of Southwestern Fidalgo Island" (1978). WWU Graduate School Collection. 831. https://cedar.wwu.edu/wwuet/831 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 by Daryl L. Gusey September 1978 THE GEOLOGY OF SOUTHWESTERN FIDALGO ISLAND by 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