AN ABSTRACT OF THE THESIS OF James William McDougall for the degree of Master of Science in Geology presented on Lune, icnct Title: GEOLOGY AND STRUCTURALEVOLUTION OF THE FOSS RIVER-DECEPTION CREEK AREA,CASCADE MOUNTAINS, WASHINGTOV, Redacted for Privacy Abstract approved: Robert S. Yekis Southwest of Stevens Pass, Washington,immediately west of the crest of the Cascade Range, pre-Tertiaryrocks include the Chiwaukum Schist, dominantly biotite-quartzschist characterized by a polyphase metamorphic history,that correlates with schistose basement east of the area of study.Pre-Tertiary Easton Schist, dominated by graphitic phyllite, is principallyexposed in a horst on Tonga Ridge, however, it also occurs eastof the horst.Altered peridotite correlated to Late Jurassic IngallsComplex crops out on the western margin of the Mount Stuart uplift nearDeception Pass. The Mount Stuart batholith of Late Cretaceous age,dominantly granodiorite to tonalite, and its satellite, the Beck lerPeak stock, intrude Chiwaukum Schist, Easton Schist, andIngalls Complex. Tertiary rocks include early Eocene Swauk Formation, a thick sequence of fluviatile polymictic conglomerateand arkosic sandstone that contains clasts resembling metamorphic and plutonic basement rocks in the northwestern part of the thesis area.The Swauk Formation lacks clasts of Chiwaukum Schist that would be ex- pected from source areas to the east and northeast.The Oligocene (?) Mount Daniel volcanics, dominated by altered pyroclastic rocks, in- trude and unconformably overlie the Swauk Formation.The middle Miocene Snoqualmie batholith intrudes the Swauk Formation, Mount Daniel volcanic body, and Evergreen fault. South of the Stevens Pass highway the Easton Schist and associ- ated Beck ler Peak stock are bounded on the east and west by high angle faults defining a horst on Tonga Ridge.The S 05°E-trending Evergreen fault forms the eastern boundary of the horst and extends southward to the Snoqualmie batholith intrusive contact.The A#5 30° E-trending Tonga Ridge fault, marked by serpentinite pods and highly sheared rocks, is the western boundary of the horst and coalesces with the Evergreen fault south of Tonga Ridge. Contrasting metamorphic terranes, the Chiwaukum Schist and Easton Schist, were juxtaposed in the vicinity of the Evergreen fault prior to Mount Stuart intrusion; rocks of Mount Stuart lithology occur on both sides of the Evergreen fault zone.The Evergreen fault did not undergo large scale dextral displacement subsequent to Mount Stuart intrusion postulated for the Straight Creek fault. Dip-slip displacement characterized by reduction in magnitude of offset to the south elevated the TongaRidge horst; this displace- ment continued after deposition ofthe Swauk Formation.Isostatic uplift of the Mount Stuart block accompaniedSwauk deposition and deformation, faulting along the Evergreen andTonga Ridge faults, and normal faulting along the western marginof the Mount Stuart uplift. Geology and Structural Evolution of the Foss River-Deception Creek Area, Cascade Mountains, Washington by James William McDougall A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Commencement June 1980 APPROVED: Redacted for Privacy Chairman of Department oYGeology and Professor of Geology in charge of major Redacted for Privacy Dean of Graduate School 0 Date thesis is presented --..\urn e "-I I CI f--) CI Typed by Opal Grossnicklaus for James William McDougall TABLE OF CONTENTS I.INTRODUCTION 1 Geographic Description and Access 1 Geologic Overview 3 Previous Work 5 Purpose of the Study 8 Methods of Investigation 9 Acknowledgements 10 II.PRE-TERTIARY METAMORPHIC BASEMENT ROCKS 11 Chiwaukum Schist 11 Correlation and Areal Extent 11 Biotite-Quartz Schist 14 Epidote Amphibolite 17 Greenstone 17 Easton Schist 18 General Statement 18 Graphitic Phyllite 18 Greenschist and Greenstone 22 Discussion: C,;,/nparison of Pre-Tertiary Metamorphic Basement Lithologies 24 III.INTRUSIVE ROCKS OF THE MOUNT STUART BLOCK AND TONGA RIDGE 26 Ingalls Complex 26 Mount Stuart Batholith and Beck ler Peak Stock 27 General Statement 27 Litho logy 29 Altered diorite (hornfels) or altered ultramafic rock 29 G ranodio rite - tonalite - mafic tonalite 33 IV.SWAUK FORMATION 35 Distribution, Age, and Thickness 35 Petrology 39 Provenance 42 Depositional Model of the Swauk Formation 46 Conclusion 51 V,MOUNT DANIEL VOLCANICS 53 Areal Distribution 53 Litho logy 54 General Statement 54 Porphyritic Rocks 55 Basal Breccia 55 Pyroclastic Rocks 56 Contact Relationships and Structure 57 VI,SNOQUALMIE BA THOLITH 59 General Statement 59 VII.STRUCTURAL GEOLOGY 61 Evergreen-Straight Creek Fault System 61 General Statement 61 Fault Descriptions in the Tonga Ridge Vicinity 62 Models of Fault Offset 68 Deception Pass Fault 73 General Statement 73 Structural Synthesis 75 REFERENCES CITED 82 APPENDIX A.Geologic map of the Foss River- Deception Creek area 87 APPENDIX B.Cross Section B-B' 88 LIST OF ILLUSTRATIONS Figure Page 1 Index map 2 2 Regional isograd map of Barrovian metamorphic zones in schistose basement rocks 13 3 Photomicrographs of isoclinal folds in banded c'chist, crossed nicols and plane light 16 4 Drawing of corrugated appearance of Easton phyllite in thin section 20 5 Diagrammatic sketch of fractures in banded schist 23 6 Photomicrographs of altered mafic or ultramafic rock near Deception Pass, crossed nicols and planelight 30 7 Syncline in Swauk sedimentary rocks 37 8 Regional southward dip of Swauk strata 37 9 Diagrammatic cross section (A-A') of the composite thickness of the Swauk section in the Tonga Ridge vicinity 38 10 Polymictic Swauk conglomerate in the northern part of the thesis area 40 11 Diagram showing distribution and relative quartz, feldspar, and lithic component compositions of se- lected Swauk arkose to lithic feldspathic sandstone 44 12 Diagrammatic sketch: channel fill in Swauk Formation with truncation of underlying beds 50 13 Diagrammatic sketch: channel fill in Swauk Formation without truncation of underlying beds 50 14 Mount Daniel 54 15 Diagram of segments of the Evergreen and Tonga Ridge faults 65 Figure page. 16 Photomicrograph, serpentinized ultramafic rock from a pod along the Tonga Ridge fault 67 17 Photomicrograph, cataclastically deformed rock of the Swauk Formation along the Evergreen fault 69 GEOLOGY AND STRUCTURAL EVOLUTION OF THE FOSS RIVER-DECEPTION CREEK AREA, CASCADE MOUNTAINS, WASHINGTON I.INTRODUCTION Geographic Description and Access The Foss River-Deception Creek area lies south of the Stevens Pass highway (U. S. Highway 2) near Skyhomish, Washington, immedi- ately to the west of and including a small part of the crest of the Cascade range (fig.1).The area of study comprises more than 100km.2(40 sq. mi. ) of the Skykomish, Scenic, Mount Daniel, and Big Snow Mountain 7 1/2 minute 1:24, 000 quadrangles. Two major drainages, the East Fork of the Foss River and Deception Creek, dominate the western and eastern parts, respec- tively, of the area.Tonga Ridge, Blue Ridge, Terrace Mountain, and the northern slopes of Mount Daniel comprise the higher eleva- tions.The landscape is characterized by rugged, mountainous relief with elevations that vary from less than 1, 400 feet ('430 meters) to 6, 361 feet ("1940 meters) at the summit of Terrace Mountain.Vege- tation is dense below snowline resulting from an average annual pre- cipitation of over 100 inches, and exposures are commonly limited. The snowpack covers some high areas into late July and August. A logging road that originates along the Foss River provides 2 Fig. 1.Location of the Foss River-Deception Creek area, Cascade Mountains, Washington. 3 access to the northern and eastern sides ofTonga Ridge.Most of the area of study is in the Alpine Lakes Wilderness Area and is therefore restricted to access by hiking trails.The major trails are on Tonga Ridge, along the East Fork ofthe Foss River, and along Deception Creek to Deception Pass and Marmot Lake. Geologic Overview The Foss River-Deception Creek area is transitional between two contrasting provinces of the north-south-trending Cascade Range. To the north, exposures of pre-Tertiary metamorphic and plutonic rocks predominate.South of the region, particularly south of Snoqualmie Pass and into Oregon, Tertiary volcanic rocks mask older rocks and structures.The area of study includes pre-Tertiary metamorphic rocks, the western edge of the Late Cretaceous age Mount Stuart intrusion, early Tertiary continental sedimentary rocks, Tertiary volcanic rocks, and parts of the edge of the middle Miocene Snoqualmie batholith. Pre-Tertiary metamorphic rocks of the area are divided into two groups, the Easton Schist and the Chiwaukum Schist based on mineralogy and apparent metamorphic history.Both units are largely metasedimentary rocks derived from pelitic sediments, with subordinate metavolcanic rocks.The ages of both units are unresolved.Rocks of the Easton Schist and Chiwaukum Schist are 4 loosely correlated with rocks of the Shuksan Metamorphic Suite and Skagit Metamorphic Suite, respectively, from work in the North Cascades by Peter Misch and Joseph Vance and their students at the University of Washington. The N 05°W-trending Evergreen fault of Yeats extends through the western part of my area and provides evidence for at least two periods of displacement.Pre-Tertiary offset produced contrast in metamorphic grade of basement rocks across the Evergreen fault zone prior to intrusion of the Mount Stuart batholith.Sense of displacement