Evolution of the Pacific Northwest. An Introduction to the Historical Geology of Washington State and Southern British Columbia

Transcription

1 Evolution of the Pacific Northwest An Introduction to the Historical Geology of Washington State and Southern British Columbia J. Figge 2009

2 This document is designed to be viewed in book format on the Adobe Reader platform. To format this properly, open the view category and select the page display option. From this, select the two page view option. This will display the document properly.

3 Except where specifically noted, all parts of this work including text, photographs and other images, are Protected under US and International copyright laws. John Figge 2009 The reproduction of this document for commercial distribution and sale, by any means, in whole or in part, is prohibited without the written permission of the author and the publisher. Similarly, it is prohibited to post this document on the Internet, in any form, in whole or in part, without the written permission of the author and the publisher. Teachers, students and other interested people are authorized by the author and the publisher to make a single electronic and/or print copy of this work, as private individuals, for their own educational purposes. Evolution of the Pacific Northwest An Introduction to the Historical Geology of Washington State and Southern British Columbia Edition 1.0 J. Figge 2009 Published by: The Northwest Geological Institute Seattle, WA www. northwestgeology.com

4 Evolution of the Pacific Northwest An Introduction to the Historical Geology of Washington State and Southern British Columbia John Figge North Seattle Community College 2009

5

6 Preface: Beyond Cascadia Some thirty-five years ago, Dr. Bates McKee at the University of Washington authored the original textbook on the geology of the Pacific Northwest. Cascadia, as he titled it, was written at the very dawn of the modern theories of plate tectonics, theories which literally revolutionized the course of modern geologic thought over the ensuing years. While McKee knew that this book would be obsolete in short order, he still went ahead and published it, with plans for an updated edition in due time. Tragically, he was killed in an airplane accident just a few years later. A second edition of Cascadia was never completed, and a replacement has never been offered. Over the past thirty-five years, our understanding of regional geology has grown dramatically. The new insights offered by our understanding of global tectonics have revealed this to be one of the most remarkable geologic provinces on Earth. As we have worked toward deciphering the geologic story behind this unique region, our models and theories have been continually revised and reconsidered, as new information is developed from ongoing study. In the midst of such constantly-evolving interpretations, few have felt comfortable with compiling a textbook of regional geology. Indeed some have suggested that such an endeavour would be premature at this date, given our current level of understanding. Such arguments will always be valid to a degree, today as they were some thirty-five years ago. McKee recognized that the context provided by the regional perspective ultimately, by the story of its geologic evolution, was the critical element in understanding the regional geology. Moreover, he saw that this context was the essential key to teaching this material. Unfortunately, it was not a context that was available in his time. He recognized that the key to that story would be found in the rapidly-emerging new theories of plate tectonics. He could have hardly imagined the fascinating tale of geologic evolution which has unfolded over the past thirty years. That story of regional geologic evolution, although not resolved to the point of scientific certainty, is by this date well-constrained to a handful of models. By adopting one such model, while acknowledging an appropriate degree of uncertainty, we gain the context which is essential for understanding and organizing this material. It is argued that the benefits of such a context outweigh the potential liabilities posed by the inevitable inaccuracies of such descriptions. That historical context allows us to venture beyond the limitations of McKee s Cascadia, and to provide a relevant structure for the description and organization of this material. Within that context, the Pacific Northwest is revealed as a truly world-class venue on the dynamics of plate tectonics, and as a spectacular showcase for some of the most fundamental Earth processes. It is a region which hosts a truly remarkable diversity of geologic features, dramatically illustrating the complex course of events which have produced the modern landscape. That diversity of features makes it an outstanding setting for an exploration into the science of geology. In the end, it offers the uncertainties surrounding its origins as an engaging, ongoing challenge to science and our imaginations. If students find themselves intrigued by these questions, then this endeavour has been entirely successful. This book is not intended as a conclusive description on the geology and geologic evolution of the Pacific Northwest. Instead it represents a way-point on the never-ending journey of science, as we continue to explore this fascinating subject. Just as it represents a major step beyond the limitations of McKee s Cascadia, it too will be superseded by future works of greater accuracy and detail. Indeed if we are not ready to re-write this story again a decade from now, it will be a sad commentary on the state of future research in regional geology. Toward that end, this work is dedicated. Image Note: Ruby Beach, Olympic Coast Decades ago, researchers theorized that an offshore island, now long vanished, was the source for the thick accumulations of Tertiary sediments in Washington. That island was given the name Cascadia. While that theory has long since been abandoned, we have come to find that Pacific island groups have played in important role in the evolution of the Pacific Northwest. i

7 Table of Contents Preface: Beyond Cascadia...i Forward: A Textbook of Pacific Northwest Geology...ii Introduction (I): The North American Pacific Northwest...iv Introduction (II): A New Model of Regional Geologic Evolution... vi Introduction (III): Historical Geology - Predicting the Course of Events Long Past...ix Introduction (IV) A Few Notes on Geologic Nomenclature...xii Introduction (V) A Brief Introduction to the Geography of Washington...xv Chapter 1: Chapter 2: Chapter 3: Evolution of the Ancestral North American Margin Introduction...1 Basement Rocks: Rocks of the North American Craton...2 The Ancient Supercontinent of Columbia: Rocks of the Belt-Purcell Supergroup...5 Stromatolites and the Length of the Precambrian Day...10 The Ancient Supercontinent of Rodinia: Rocks of the Buffalo Hump Formation...14 The Breakup of Rodinia: Rocks of the Windermere Group...16 Mining the Ancestral Margin...24 Deposition on a Passive Margin: Rocks of the Sauk and Tippecanoe Sequences...25 The Burgess Shale...31 Evolution of a Convergent Margin: Rocks of the Kootenai Arc...36 The Supercontinent of Pangea: The Dawn of the Last Cycle...38 Summary: The Evolution of the Ancestral North American Margin...39 The Omenica Episode: New Lands Along an Old Coast Introduction...43 The Dawn of the Pacific Northwest: North America Moves West...45 The Intermontane Belt: The First Megaterrane...46 The Southern Intermontane Belt: Rocks of the Quesnellian Archipelago...47 The Stikine Terrane: The Cache Creek Connection...53 The E. Jurassic Margin of North America: The Southern Boundary of the Pacific Northwest...54 Life on the Jurassic Intermontane Margin...56 The Accretion of the Intermontane Belt: The Dawn of the Omenica Episode...57 The Omenica Arc: The Heart of the Omenica Orogen...61 Deposition on the Leading Edge: Rocks of the Methow-Tyaughton Basin...64 Republic: A Town that Gold Built...75 The Arrival of the Insular Belt: The Evolution of a Japan-Type Margin...76 Summary: The Omenica Episode...78 The Coast Range Episode: New Lands Along an Evolving Margin Introduction...85 The Insular Belt: Anatomy of a Megaterrane...86 The Chilliwack Terrane...97 The Accretion of the Insular Belt: The Collapse of the Bridge River Basin...98 The Early Coast Range Arc: The Great Coast Range Batholith of North America The Accretion of the Melange Belt Terranes: Great Thrust Sheets of Oceanic Rock Ultramafic Rocks of the Pacific Northwest The Late Coast Range Arc: Magmatism in the Bridge River Belt The Coast Range Megalineament: Master Fault of the Coast Range Orogen Deposition Over the Coast Range Episode: Reflections on the Late K Paleogeography The Case for Large-Scale Lateral Displacements: The Baja British Columbia Hypothesis Summary: New Lands Along an Evolving Margin...138

8 Chapter 4: Chapter 5:.. Chapter 6: The Challis Episode: The Demise of the Kula Plate Introduction Overview: Modeling the Demise of the Kula Plate The Challis Arc: The Eastern Regime Metamorphic Core Complexes: The Dilemma of Detachment Faults Ultramylonites of the Okanogan and Kettle Complexes The Crescent Basalts: The Western Regime Deformation and Accretion in the Challis Episode: The Dynamics of a Changing Margin Other Faults and Features Regional Deposition in the Challis Episode: The Great Eocene Blanket of Sand The Stonerose Fossil Site The Regional Paleogeography: The Changing Landscape The End of the Challis Episode: The Return of the Farallon Plate Summary: The Demise of the Kula Plate Foliar Physiognamy The Cascade Episode: The Modern Regime Introduction The Dawn of the Cascade Episode: A New Pacific Northwest The Cascade Arc: Modern Arc Magmatism The Columbia River and Chilcotin Plateau Basalts: The Great Lava Floods Columnar Volcanic Rocks: Thermodynamic Geometry Deposition over the Cascade Episode: Reflections on an Evolving Landscape The Ginko Petrified Forest The Blue Lake Rhino Accretion in the Olympic Coast Belt: A Modern-Day Melange Belt Deformation over the Cascade Episode: Caught in a Bind Summary: The Modern Regime The Last Five Million Years: Evolution of the Modern Landscape Introduction: Evolution of the Modern Landscape DejaVu: The End of the Juan De Fuca Plate The Rise of the Cascade and Olympic Mountain Ranges: New Mtns along an Evolving Coast Alpine Glaciation of the Cascade and Olympic Mountains: Sculpting the Modern Ranges Continental Glaciation of the Puget Sound and Georgia Strait: Evolution of the Puget Lowland Continental Glaciation East of the Cascades: The Awesome Power of Water Unleashed Glacial Lake Columbia and the Grand Coulee: Diverting the Columbia The Moses Coulee Lake Bonneville and the Snake River Flood: Failure at Red Rock Pass Glacial Lake Missoula and the Spokane Floods: Cataclysm in the Columbia Basin J. Harlen Bretz and the Spokane Floods Recent Deposition in the Cascade Episode: Frosting on the Cake Touchet Beds Modern Volcanism of the Cascade Arc: Sentinals of the Pacific Coast Mima Mounds The Modern Tectonic Setting of the Pacific Northwest Summary: Evolution of the Modern Landscape Author Biography Acknowledgements References...338

9 Forward: A Textbook of Pacific Northwest Geology This book is intended primarily as an academic textbook for students in the geological sciences. Its purpose is to teach the physical and historical geology of this region, and to illustrate the scientific basis for those observations and interpretations. Coursework in this material is usually offered in the second or third year of instruction at most local colleges and universities. Such courses usually require introductory geology as a prerequisite course of study. There are strong arguments for making this an introductory course of instruction. Few places in the world better illustrate the fundamental processes and products of geology than here. At the same time however, the process of introductory instruction can easily become a distraction from the subject material at hand. Given the emphasis placed here on recognizing the continuity of events from a regional perspective, such distractions would present some major compromises in presenting this material. Accordingly, we resisted the temptation to make this a stand-alone introductory text, instead electing to refer readers to other sources for their introductory instruction. That course of instruction can be found in any modern textbook of geology, a course which will not be duplicated here. Accordingly, this book is written with an assumption that the reader has an introductory-level understanding of modern physical geology, and with the terminology which it uses. This does not need to be college coursework, just a familiarity with the materials presented in any introductory geology textbook. At the same time, we will take frequent opportunity to point out where the fundamental processes of geology are so spectacularly illustrated here. In doing so, we hope to reveal what a remarkable showcase this region represents for the geologic sciences as a whole. ii

10 This book is organized into six chapters, based on the historical model outlined in the following section. The reader is reminded that an understanding of that model is the fundamental basis for integrating this material. This model yields a broadly chronological treatment of the subject matter, organized in an episodic format. Beyond the four episodes which comprise the geologic evolution of the Pacific Northwest, it includes two supplemental chapters. The first of these deals with the evolution of the ancestral North American margin, upon which the Pacific Northwest was assembled; while a final chapter provides some additional details on events over the past five million years Finally, with all due respect for our very good neighbors to the north of the border, this textbook is largely intended for American students. While much of the geology here is fundamentally Canadian in context, most of the examples and discussions in this book center on American exposures of these rocks. This serves the American readers need for familiar and tangible examples, as well as their (rather embarrassing) lack of familiarity with Canadian geography. Considering that most of the (pre-tertiary) rocks are in fact better exposed to the north, readers are encouraged to extend their future studies into southern British Columbia. Image Notes: (Left) Mount Shuksan, North Cascades Mountains This is the most heavily glaciated range in the U.S., south of Alaska. (Above Right) Upper Palouse Canyon, south of Washtucna (Right) Lower Palouse Canyon, south of Washtucna The canyon of the Palouse River was cut by the great floods which swept this region over the recent ice ages. These floods were among the greatest ever recorded on the planet, and are responsible for much of the unique topography of Eastern Washington. iii

11 Introduction (I) The North American Pacific Northwest The Pacific Northwest is home to some of the most varied, complex and spectacular geology to be found anywhere in the world. It is a patchwork, stitched together of the remains of ancient island groups and ocean-floor rocks, rafted to our shore by the processes of plate tectonics, and overprinted by a succession of volcanic chains that have been erupting along this coast for over 200 million years. It is a region that has been sliced, folded and sheared by the forces of plate tectonics, intruded by generations of granitic batholiths and capped by a long history of volcanism. On its southern end, it is a region blanketed in voluminous sedimentary packages, a region buried in immense lava flows, a region carved by generations of glaciers and scoured by the most colossal floods known in the history of the planet. No region on Earth can claim to a greater variety of geologic features, to a greater complexity in their assembly, or as a more spectacular showpiece on the planetary-scale forces which have been responsible for their development. The Pacific Northwest is home to some of the most dramatic geology in the world. It is, by virtue of that character, certainly one of the best places imaginable to explore the science of geology. With just a basic understanding of that subject, this region is revealed as the product of a truly fascinating course of geologic evolution, a story that is reflected in every mountain, outcrop, boulder and grain of sand which makes up this unique province. iv

12 In the end, that course of geologic evolution, as reflected in the modern physical geology, is what defines the Pacific Northwest as a distinct geologic province. That province, it should be emphasized, is different from that which most American students would identify as the Pacific Northwest. The geologic Pacific Northwest includes almost all of Alaska, most of British Columbia, and Washington State west of roughly Spokane. Its southern end broadly follows the Washington Oregon border. It does not include the southern and eastern components which are part of the geopolitical Pacific Northwest of the United States. The geologic Pacific Northwest is the North American Pacific Northwest, a distinct geologic province extending from Washington to Alaska. The unique characteristic of this region is that it did not exist prior to Mid-Jurassic time. It consists of volcanic island belts and scraps of ocean floor rocks which have been added to the edge of the continent over the past 200 million years, and overprinted by several episodes of volcanism and mountain-building. While its southern extent is largely Alaska Accreted Terranes of the Pacific Northwest Washington Ancestral North America Canada United States obscured by recent volcanic flows, the large-scale geologic trends suggest a southern boundary somewhere along the Washington Oregon border. The North American Pacific Northwest is a vast region extending some 3500 km (2200 mi) from end to end. At this scale, it absolutely dwarfs he other geologic provinces of the North American Cordillera. It is of unwieldy proportions for the purposes of regional geology, and extends well beyond most people s familiarity with the regional geography. Accordingly, we will focus on only the southern third of the province, comprised essentially of Washington State and southern British Columbia. This is a province of manageable size for the purposes of regional geology, and is the most familiar part of the region for most students. In the end, the purpose of this work is to share with students the truly spectacular and illustrative geology which characterizes the region, and to reveal the fascinating story of geologic evolution which is written in these rocks. If you avail yourselves of this information, we can guarantee that you will never look upon this landscape in the same way again. Image Notes: (Left) Olympic Mountains, Hurricane Ridge. These are among the youngest mountains on the planet. (Above) The accreted terranes of the Pacific Northwest, added to the continent over the last 200 million years. v

13 Introduction (II) A New Model of Regional Geologic Evolution Geologists have long viewed the world with an understanding that the course of geologic evolution is fundamentally episodic in character. We see this on a small scale, as illustrated in the individual bedding planes of a sedimentary rock; and we see it on a large scale, as illustrated by the supercontinent cycle itself. Far from being an ongoing set of steady-state conditions, the geologic world works in cycles of measurable duration. Recognizing those cycles is one of the major challenges of historical geology, as they are the benchmarks by which the course of geologic evolution is mapped. Since the dawn of the modern theories of plate tectonics, researchers have been trying to discern the fundamental episodic structure to the geologic evolution of the Pacific Northwest. That structure is the essential context for understanding the more local geologic descriptions, and is the logical basis for teaching the subject as a whole. In an attempt to discern that structure, a variety of models have been proposed. These include models structured around accretionary events, stratigraphic sequences, magmatic-arc regimes and orogenic episodes, to name a few of the criteria suggested. Some of these models arrive at similar conclusions from different perspectives, while in other respects find themselves in fundamental disagreement. While all have encouraged a vigorous debate, none has proven a wholly satisfactory approach. vi

14 This work advances a new model for regional geologic evolution. This model suggests that the shortcomings of the previously-suggested criteria lie in the fact that they are all secondary phenomena. If one is looking for the fundamental episodic criteria, it almost certainly lies in the plate-tectonic relationships themselves. All of the other criteria which have been suggested are consequent and subordinate to those relationships. After an extensive consideration of all the subordinate criteria, this model concludes that The basic episodic character of regional geologic evolution in the Pacific Northwest is defined by fundamental changes in the location and/or character of the interaction between the continental and oceanic plates along this part of the continental margin. By fundamental changes it includes only major changes in the location of the margin (as in large-scale accretionary events), or changes between divergent, convergent and transform margins. Implicitly, it relegates accretionary, magmatic, stratigraphic and orogenic events and/or episodes to a secondary status. In application, this model defines four distinct episodes in the geologic history of the Pacific Northwest. The first of these was initiated in Mid-Jurassic time, with a major accretionary event (the Intermontane Belt). The second was initiated in mid-cretaceous time, with another major accretionary event (the Insular Belt). The third episode was initiated as the oceanic (Kula) plate achieved a transform relationship with the western margin in Early Eocene time, while the fourth episode commenced as the (oceanic) Juan De Fuca (Farallon) plate was reinstated in a convergent regime in latest Eocene time. By no coincidence, these episodes are broadly synchronous with the four major magmatic arc regimes which have developed here over the last 200 million years. While these regimes do not define these episodes, they do to a significant degree characterize them. In an effort to avoid introducing new nomenclature, this model adopts the names of those regimes for these larger regional episodes. By name, they include the Jura-Cretaceous Omenica Episode, the Cretaceous Eocene (Canadian) Coast Range Episode, the Eocene Challis Episode, and the recent Cascade Episode. The Challis and Cascade episodes are broadly recognized in the United States, having been previously defined along magmatic-arc, orogenic and stratigraphic criteria. The Omenica and Coast Range episodes are broadly recognized in Canada, having been defined along similar criteria. In decades past, the Omenica Orogeny was known in Washington as the Columbian Orogeny. vii Image Notes: (Far Left) Upper Methow Valley, near Mazama. A classic glacial valley (Left) Palouse Falls, south of Washtucna

15 The critical difference that distinguishes this model is that it resolves to a summary statement which reflects the most basic observation derived over the last thirty years of plate-tectonic reinterpretation: that the fundamental character of regional geologic evolution here is vested in the relationships between the continental and oceanic plates along this common margin. It is a summary statement that most modern regional geologists would probably accept, at least in the broadest of principles. This is in itself a somewhat remarkable attribute. Millions of Years Ago Era Period Epoch Tertiary Cretaceous Miocene Oligocene Eocene Local Episodes Cascade Episode Challis Episode Coast Range Episode The body of this work is organized around this model, where the course of regional geologic evolution is described as a succession of four episodes, the Omenica, the Coast Range, the Challis and the Cascade. As we consider these in order, this model will be described in further detail. These four episodes cover the last 180 million years, the period over which the modern Pacific Northwest has been assembled and developed. In the interests of context, consideration is also given to the pre-northwest evolution of the original continental margin, while Cenozoic Meso- Zoic Paleo- Zoic recognizing that its development is more accurately described by a different model. Finally, additional consideration is given to the events of the last five million years, events which are so graphically evidenced on the modern landscape. In the end, this produces a succession of 6 chapters, as follows: Jurassic Triassic Permian Chapter 1 The Ancestral Margin of North America Chapter 2: The Omenica Episode Chapter 3: The Coast Range Episode Chapter 4: The Challis Episode Chapter 5: The Cascade Episode Chapter 6: The Last Five Million Years Omenica Episode Pre-Pacific Northwest North America Image Notes: (Above) Geologic time scale, showing local episodes in the evolution of the Pacific Northwest (Above Right) Selkirk Range, northeastern Washington. Old rocks of ancestral North America. viii

16 Introduction (III) Historical Geology: Predicting the Course of Events Long Past Geology is a science at the third order of description, beyond that of physics and chemistry. As a scientific discipline, its primary goal is to describe how the Earth works, in terms of processes and products. With this information, we gain the ability to predict the course of future events. This is a field of considerable practical value if you live in a world of earthquakes, volcanoes, floods, tsunamis, or other geologic phenomena worth predicting. Unlike in physics and chemistry however, there is an intimate relationship between products and processes in geology. The physical geology of our planet is intimately imbedded in the dynamic processes which have produced it over the course of geologic time. Those processes are reflected in the very character of the rocks they have produced, affording us an abundance of information on the past course of geologic events. For many of us, this is the truly magical part about this science the ability to look back in time over the course of events which have produced the modern world. This capacity the ability to predict the course of events long past, is a truly fascinating realm unique to the field of geology. ix

17 This ability provides much more than just the necessary context for understanding the current physical geology. It is an eminently worthy line of inquiry in itself. As we come to understand the course of the Earth s evolution over the vastness of geologic time, we gain a spatial and temporal context for the modern human experience. To understand the course of events which have produced the modern geologic and physiographic environment, and to understand our own position relative to that history, is a unique perspective which no other scientific discipline affords. It is a perspective which changes the way you perceive time, the way you perceive the world around you, and the way you perceive your relationship with the universe. It is a truly remarkable state of awareness to be gained for the simple price of contemplating rocks. Whether or not it affords you a new personal cosmologic perspective, that historical context is the fundamental basis for understanding the regional geology here. As McKee recognized thirty years ago, you are unlikely to learn the complex physical geology of this province without a historical context which gives it meaning, and along which you can organize your knowledge. With that context however, comes an important caveat. The normal procedure for science is to use our knowledge of the present to predict the future. This prediction is tested as the future unfolds, either supporting or refuting our hypothesis. This process of empirical validation is the very foundation of science, and is at the heart of the scientific method. The observations of pure science are verifiable, testable and repeatable. This is the realm of modern physical geology, which seeks to describe how the Earth works, and to predict the course of future events. By contrast, historical geology is an applied science. It takes our knowledge about how the Earth works, combines it with our knowledge of the modern physical geology, and predicts the most likely course of events which have prox

18 duced the modern setting. The caveat is that unlike in pure science, there are no opportunities to empirically verify our predictions. The past will never happen again, affording us the opportunity of being proven right or wrong. By this constraint, models of historical geology never reach the status of scientific fact, no matter how refined they may be. This does not mean that they can t be accurate. The vast amount of information preserved in the regional rock record, particularly in a region as complex and varied as the Pacific Northwest, places enormous constraints on the possible course of geologic evolution. The more information we acquire on the physical geology, the more constrained and refined our models of history become. At the same time, our growing understanding of Earth processes continues to define the range of possibilities which our models can encompass. Eventually, those parameters will constrain us to just one model, which is the most accurate prediction possible. While such a prediction will never achieve the status of scientific fact, we will be able to hold it in similar confidence. After a mere thirty years of consideration, we are certainly not yet at that point. Our current understanding of the physical geology and the process of plate tectonics still accommodate a number of possible historical models. At this point however, at least on a regional level, that number is limited to a handful of theories. Most of those models differ principally in details of timing or extent, rather than the truly fundamental aspects. While those details remain some of the more contentious issues of debate, they do not necessarily represent unreasonable constraints on a truly regional model. As we will hope to illustrate, the model which is advanced here is robust enough to accommodate most of these remaining differences in interpretation. Image Notes: (Left) Border Peaks, North Cascades Mountains. North of Glacier, WA (Below) Frenchman Coulee, Central Washington. North of George, WA xi

19 Introduction (IV) A Few Notes on Geologic Nomenclature The nomenclature of regional geology includes the names of various members, formations, groups and other stratigraphic designations, along with the different terranes, structural features, and provinces which describe broader regions. There are over a hundred designated formations within the state of Washington alone, along with at least a dozen major terrane units and nearly as many geographic provinces. While this plethora of names may seem bewildering to the uninitiated, there is a certain element of logic to its development, and a good measure of utility in its application. Most geologic features are named in the exploration and reconnaissance phase of geologic research. In this phase, geologists are notoriously disposed toward affixing names (new names) to every mappable feature in their domain. Convention dictates that new features (e.g. formations, plutons, faults, etc) be named for the nearest town or major geographic feature to the area of discovery. By this system, most geologic features are named for towns, rivers, lakes or mountains. Accordingly, the various names all reference the geographic locale where they were first studied. This xii

20 nomenclature gives a strong regional flavor to the geology here, and a more tangible lexicon than is afforded by most scientific systems of description. Clearly this is a system which benefits students who already have a good familiarity with the geography of the region. Knowing where the towns of Darrington, Addy, Entiat, and Winthrop are is of inestimable value in locating the Darrington Phyllite, Addy Quartzite, the Entiat Pluton and the Winthrop Sandstone. Toward this end, it would be difficult to overstate the value of a good familiarity with the regional geography as a prerequisite to considering the regional geology. As our knowledge of regional geology has moved out of the exploration and reconnaissance phase, it has become apparent that many features are equivalent between adjacent areas. In these cases, convention dictates that the name of the first-named feature be retained, and that more recent appellations be abandoned. At this point in time, there is enough debate about the equivalence of certain units that a contraction of terms is often used, as in the Addy- Gypsy quartzite, or the Napeequa Hozameen terrane. In these cases, the oldest title is given precedence. At this level, it depends largely on whether one is a lumper or a splitter with regards to such features. The author is decidedly a Image Notes: (Left) Shankers Bend, Similkameen River, near Oroville, WA (Above Right) Patterson Lake, Methow Region. Near Winthrop,WA (Right) Moses Coulee, Waterville Plateau. xiii

21 lumper in this respect, a disposition which serves to simply the regional geology considerably. This would be the end of the story were it not for the peculiar qualities which attend the 49th parallel of latitude through this region. This line separates American from Canadian Geology, two remarkably independent lines of scientific research which have produced two completely independent sets of regional nomenclature. North of the border, the Addy-Gypsy formation is known as the Reno-Quartzite Ranges Formation, and the Napeequa-Hozameen Terrane is known as the Bridge River Group. In virtually every case, geologic features have been given different names on different sides of the border. Because most research is governmentally-funded, and therefore limited by political boundaries, this has evolved (incredibly enough) as a workable arrangement for those purposes. Complimenting this arrangement, most American authors and researchers have historically attempted to frame the geologic evolution of Washington State in an American geologic context. This compliments the largely American perspective on cordilleran geology which is taught in most American schools, and effectively avoids most of the nomenclatural issues across the border. The increasing recognition that this region largely represents the southern end of the Canadian system puts American researchers in an awkward position. It is difficult to argue for the primacy of some American names when they are major elements of the Canadian system, and when they are best illustrated in Canadian exposures. At the same time, there is a certain reluctance among American researchers to abandon their nomenclature, which in some cases is recognized by a majority of the scientific community. While these issues are not going to be resolved here, we are going to take a step in that direction. This does not represent a proposal for a revision of the nomenclature, but is simply a selective consolidation of terms for the benefit of the reader. In each case, all the interpreted affinities of a feature are noted in its introduction (e.g. the Addy, Gypsy, Reno and Quartzite Ranges Formations), but are subsequently identified by a single name (e.g. the Addy Formation). In general, that name is chosen by accepted conventions, with some exceptions taken for common usage and familiarity to American students. In the end, much of the resulting nomenclature has a distinctively Canadian flavour. The same advice offered earlier applies here again. A familiarity with the geography of southern British Columbia (and southeastern Alaska) is of invaluable aid in giving context to these names. Knowing the locations of the towns of Wrangell, Alaska and Quesnel, British Columbia, for example, means that you will always know the relative positions of the Wrangell and Quesnel Terranes two major geologic features of the Cordillera. Most American students are woefully ignorant of the geography north of their border. A few hours spent reviewing this subject will provide this important aspect of context to the nomenclature. Image: Penn Cove, Whidbey Island xiv

22 Introduction (V) A Brief Introduction to the Geography of Washington State Because the physical geology is expressed on a geographic landscape, a familiarity with the regional geography is an absolute prerequisite for study in this area. Those of us who were born and raised here take this knowledge for granted, but in an increasingly mobile society, few people still cultivate such an appreciation for the land they live on. Add in all the distractions of modern urban life, and few people are particularly well-informed on matters of regional geography. Accordingly, it seems only appropriate that we consider at least a brief review of this aspect before embarking on the geologic story of the region. Washington State and southernmost British Columbia are comprised of a number of distinct geographic provinces, most of which are geologically distinctive in some respects. From the Cascade Crest to the west, all of these provinces are aligned along north-south axes, paralleling the dominant structural features of the region. Starting from the west, both Vancouver Island and the Olympic Peninsula are distinct provinces, separated by the Strait of Juan de Fuca. To the south, the Willapa Hills of southwestern Washington completes this set of elevated maritime and insular regions. To the east of this uplifted coastal belt lies the Puget Georgia Basin, a long trough between the coastal belt and the Cascade Mountains to the east. The northern end is the Georgia Basin, extending south as the Puget Basin, and ultimately, the Willamette Basin of Oregon. East of this long basin rises the Cascade Range, extending as far north as the Fraser River. Image: Methow-Pasayten region, looking southeast from Harts Pass xv

23 Methow - Pasayten Okanogan Highlands Rocky Mountains Olympic Mountains Cascade Mountains Puget - Georgia Basin Willapa Hills Columbia Plateau Image: Map showing the generalized geographic provinces of Washington State. East of the Cascade Mountains, this normal north-south alignment of geographic provinces is in large part buried beneath massive basalt flows which form the Columbia Plateau. These extend to the southern margins of the state, and as far north as the Columbia River in North-Central Washington. This plateau province is locally incised by rivers, and drains down into the Pasco Basin. From there, the Columbia River turns west and flows to the Pacific. East of the plateau, the rich farmlands of the Palouse overlie these basalt flows, draining south to the Snake River system. South of the Snake River, the unique Blue Mountains province anchors the southeastern corner of the state. The geographic provinces north of the Columbia Plateau retain the north-south alignment which coincides with regional structural patterns. On the western end, these merge with the Cascade Range in the Pasayten region, a mountainous province which extends east from the hydrologic crest almost to the Okanogan River. This river is a major geographic boundary. To the east lie the Okanogan Highlands, merging on their eastern end with the Kettle Range. The Kettle Range is the westernmost range in the foothills of the Rocky Mountains. Its eastern slope descends to the Columbia River, where it flows south out of Canada. East of the Columbia River, foothills rise into the Selkirk Mountains, part of the Rocky Mountains belt. Each of these geographic regions is a distinct province in many ways, and within them are included numerous subprovinces of equal distinction. Collectively, they paint an amazingly diverse picture of the landscape of Washington State. Few places on Earth display such an astonishing variety of landscapes over such a limited geographic area. As we will explore in our studies on the geology of the region, this diversity of landscapes reflects the overall geologic character of the province. Most of the central portion of Olympic Peninsula is occupied by the Olympic Mountains, a modest range of peaks rising to a maximum elevation of 7963 feet (2427 m). This is a compact range of peaks with a broadly radial drainage pattern, rising to its highest point at Mount Olympus. These mountains rise abruptly from the western shores of Hood Canal, and slope more gently toward the Pacific to the west. xvi

24 Image: (Above) The Olympic Mountains, from Hurricane Ridge The central portion of the Peninsula is preserved in the Olympic National Park, or reserved in adjacent Forest Service lands. The mountains are cloaked in heavy stands of fir and hemlock, and the west (Pacific) side of the range is a rainforest setting - where precipitation approaches 200 inches per year. The northern coastal strip is part of the National Park, and other coastal sections are reserved as Indian reservations (Makah, Hoh, Quillayute and Quinault Reservations). The peninsula is ringed by Highway 101, but the western coastal strip remains only sparsely settled. There are no major towns between Hoquiam and Forks, most of the western length of the coast. Forks Pacific Ocean Neah Bay Clallam Bay Port Angeles Olympic Mountains Strait of Juan De Fuca Sequim Kitsap Peninsula Numerous small towns dot the eastern shoreline of the Peninsula along the route of highway 101, from Shelton on Image (Right) Map of the Olympic Peninsula Hoquiam Aberdeen Shelton 101 xvii

25 Image: Neah Bay, on the northwestern tip of the Olympic Peninsula. Vancouver Island in the distance. Neah Bay is the cultural center for the Makah Tribe. the south to Sequim on the north end. Shelton is a venerable old logging town, while most of the settlements along the highway are centers for vacation homes along the Hood Canal. Sequim is a rapidly-growing metropolitan center, a popular locale for retired folks from the east side of the Sound. It sits in the rain shadow of the Olympics, and boasts a mild and dry climate which many find appealing. The major town on the north end of the peninsula is Port Angeles, home to about 19,000 people. The main entrance to the Olympic National Park lies at its back door, making it is a popular recreation area. From P.A. the highway heads west to Lake Crescent, Clallam Bay and ultimately Neah Bay at the western tip of the peninsula. Before heading to Clallam Bay, the main road heads south to Forks. Forks is a town of about 3,000 people, with a strong heritage of logging as its foundation. A somewhat reclusive and isolated community, it serves a broad rural area across the northwestern end of the Peninsula. The rolling landscape of the Willapa Hills marks the southeastern corner of Washington State. Historically, this is timber country, feeding mills in Aberdeen, Hoquiam, Kelso and Longview, the major port cities of the region. These towns also host fishing fleets, which troll the coastline for salmon and other species. Increasingly, tourism is playing a major economic role in these areas. In the end, however, timber still reigns as king in this region. The area is sparsely populated, and the low rolling hills are well-suited to harvesting the forestlands. The region is served by SR 12, which follows the Chehalis River Valley, by Highway 6, which follows the Willapa River, and Highway 4 which follows the Columbia River. U.S. 101 follows the western coastline, linking the coastal cities and towns. On the south end, the hills drop down to the Columbia River. Highway 4 extends west from Longview to meet US 101 on the coast. The small town of Ilwaco sits at the mouth of the Columbia. Along the river, numerous small com- xviii

26 munities are sited, including Skamokawa and Cathlamet. Once again, timber remains the economic mainstay of this region, feeding mills in Kelso and Longview to the west. Longview and Kelso are a major population center of about 50,000 people, located along the Columbia River. These are blue-collar towns which were built by the timber trade, the home to large lumber and paper mills and supporting industries. There are significant port facilities here, located on the Columbia River waterway. Increasingly, tourism is playing a significant economic role in this area. The coastline between Ilwaco and Aberdeen doesn t feature the expansive beaches that are seen to the north, but Ilwaco Pacific Ocean Hoquiam Aberdeen Willapa Bay Raymond South Bend 4 Columbia River 6 Skomakawa 12 Chehalis Centralia I-5 Kelso, Longview Cathlamet Image Notes: (Above) Map of the Willapa Hills area of southwestern Washington. Highway 12 follows the Chehalis River. (Below) Willapa Bay Sunset. Image by Iawatha xix

27 Image: (Above) The Willapa Hills, from near Skomakawa. Image by Timov the bays and estuaries along the shoreline here are pristine settings which draw visitors from around the world. Willapa Bay is a major way-point for birds migrating along the coast, and is a popular locale for watching wildlife. Similarly, the sleepy towns of Skamokawa and Cathlamet find themselves hosting vacationers looking for rural relaxation away from the urban population centers. A low trough between the Olympic and Cascade Mountains, the Puget Basin is the most populated region of the northwest. Some three and a half million people live here, largely along the shores of the Puget Sound. Geographically, the basin extends south to the prairie-lands of Centralia and Chehalis, and north as far as Fidalgo Island west of Mount Vernon. North of here, the Georgia Basin extends up to the Fraser Lowlands of British Columbia. Along the east side of the basin, towns and cities are located about 30 miles apart, a day s journey by horse or boat. From south to north, these include Centralia, Olympia, Tacoma, Seattle, Everett, Mount Vernon and Bellingham. Centralia and Chehalis have their origins as a coal-mining region, while Olympia claims the title of the State Capitol. Tacoma, Seattle and Everett are the major cities of the urban corridor along the east side of the Puget Sound, a rapidly-growing region scheduled to add an additional 1.5 million people over the next twenty years. The town of Mt. Vernon sits above the mouth of the Skagit River, at the northern end of the basin. To the north, Bellingham (population ~70,000) lies beyond the Chuckanut Range, a modest uplift which separates the Puget and Georgia Basins. Bellingham lies on the coastal plain of the Strait Georgia of Georgia Strait Puget Sound Bremerton Gig Harbor Olympia 5 Bellingham Tacoma Mount Vernon 20 Everett 2 Seattle Bellevue 90 Kitsap Peninsula Image: (Right) Map of the Puget Sound region, showing major cities and highways. xx 5 12

28 Image Notes: (Right) The City of Seattle, Elliott Bay to the left, Looking north. Image courtesy of the Washington State Department of Transportation. Seattle is the economic and cultural center of the Puget Sound region, and has long been recognized as one of the best places in the country to live. (Below) Puget Sound and the Olympic Mountains. From the beach at Discovery Park, west-central Seattle. Fraser Nooksack lowlands, the southern end of the Georgia Basin. To the west, the San Juan Islands occupy the southern end of that basin, a very popular vacation retreat in the rain shadow of the Olympic Mountains. The central portion of the Puget Basin is occupied by a myriad of peninsulas, islands and inlets which comprise the coastlines of the Puget Sound. The major feature is the Kitsap Peninsula, which lies between the Hood Canal and the Puget Sound. The major settlement on the Peninsula is the city of Bremerton, home to the US Naval Shipyards. Nearby is Port Bangor, home to the North Pacific Trident submarine fleet. The military is a strong presence on the Peninsula. The other major community on the peninsula is Gig Harbor, located where Highway 18 crosses the Tacoma Narrows portion of the Sound. This is a popular bedroom community for those working in Tacoma. The rest of the peninsula is relatively sparsely populated, a region of farmlands and timber. Coastal roads provide access to waterfront properties and vacation homes which occupy much of the shorelines. Less-accessible are the numerous islands which dot the Puget Sound. Some of these are served by the State Ferry System, including Anderson, Vashon, Bainbridge and Whidbey. All the islands in the Sound are relatively low-lying features, rising no more than 500 feet above the tidewaters. Most of these are timbered landscapes, and many of the smaller ones are preserved as state parklands. xxi

29 The Cascade Mountains separate western and eastern Washington. They rise from a modest uplift of about 1,000 feet at the Columbia River to an elevation of about 9,000 feet at the Canadian Border, and broaden considerably on their northern end. The range is breached by only five major roadways: White Pass, to the east of Chehalis, Chinook Pass east of Mt. Rainier, Snoqualmie Pass east of Seattle, Stevens Pass east of Everett, and via the North Cascades Highway, east of Mt Vernon. Only three of these are all-season roads. Both geographically and geologically, the range is broken down into the southern Cascades south of Snoqualmie Pass, and the northern Cascades north of that point. The North Cascades are the most rugged and heavily-glaciated range in the country outside of Alaska. South of Snoqualmie Pass, the topography is more subdued consisting of more modest peaks of less rugged character. Along the entire range are situated a succession of large stratovolcanoes (Mt. Garibaldi, Mt. Baker, Glacier Peak, Mt. Rainier, Mt. St. Helens, Mt. Adams) which rise above the regional crestline. Significant portions of the Cascades, particularly on the northern end, are preserved in National Parks and wilderness areas. Broad areas are reserved as National Forest lands or other Federal lands. South of Snoqualmie Pass, large sections are owned by timber companies as commercial forestlands, along with National Forest lands. Along the west slopes, numerous small towns dot the foothills. Most of these have their origins as timber communities. Lower down, along the major rivers, communities have developed as local service centers for timber, agricultural and development interests. Along the dry east slopes of the range, towns are much less numerous. The lower slopes in the southern portions of the range are used as open rangeland, and a large portion is reserved as part of the Yakama Indian Reservation. Mt. St. Helens Mt. Baker Mt. Vernon Everett Seattle 410 Mt. Rainier Glacier Peak 90 Chinook Pass Mt. Adams Stevens Pass Snoqualmie Pass White Pass Columbia River Ross Lake Cle Elum Washington Pass 97 Lake Chelan Wenatchee Ellensburg Yakima Image: Map of the Cascade Range in Washington, showing major cities and highways. Highways 410 and 20 are closed in the winter. xxii

30 Images: (Above) Summit of Eldorado Peak, North Cascades Mountains. This is the most glaciated region in the U.S., outside of Alaska. Image courtesy of Erik Andersen. (Right) The North Cascades, from north of Rainy Pass. The Cascade Range broadens appreciably north of Snoqualmie Pass. On the west side, it extends to tidewater north of Mt. Vernon, it s western outposts being the San Juan Islands. On the east side, it broadens to take in the Stuart Range north of Snoqualmie Pass, and broadens in its northern end to merge with the rugged Pasayten Region north of the Methow River. While a distinct province in itself, the Methow- Pasayten region extends this mountainous province from tidewater to the Okanogan River, a distance of some 150 miles. North of the border, the range extends to Hope, B.C., along the Fraser River. xxiii

31 Images: (Right) Map of the Columbia Plateau, showing major features, towns and highways. (Below) Columbia River Basalt Flows, in the Moses Coulee on the Waterville Plateau. This is a rarely-visited part of the state. c a s c a d e m o u n t a i 2 Snoqualmie Pass 12 Cle Elum Methow - Pasayten Yakima Lake Chelan Wenatchee Ellensburg Okanogan River Waterville Waterville Plateau Columbia R. Columbia Plateau Ephrata Columbia R.. Okanogan Highlands 2 Moses Lake 395 Pasco Basin Tri-Cities Davenport Ritzville Snake River Palouse River Rocky Mountains Walla Walla Spokane Blue Mtns n s Columbia R. Oregon The Columbia Plateau covers much of eastern Washington, all except the regions north of the Columbia River, and along the eastern margin of the state. This region has its origins as a series of voluminous basalt flows about 17 million years ago, when this became the dominant character of the landscape here. It is a vast area, but it all shares in this common bedrock and structural style. This high plateau dips south into the Pasco Basin, which is the structural low-point to the drainage here. It is bordered along its north-central and northwestern portions by the Columbia River, which then cuts through the province down to the Pasco Basin. There it is joined by the Snake River, which drains the Palouse region to the east, and the Blue Mountains province to the southeast. xxiv

32 Images: (Above) The Snake River, below its confluence with the Palouse (Right) Farmlands of the Palouse. This area is renoun for its highyield wheat crops, grown in the rich soils of the Palouse Loess. Land-use in the Columbia Basin is largely two-fold. On those lands where there is sufficient soil, farming is the dominant activity. The rich Palouse soils and abundant irrigation water make this the most productive agricultural land in the world. On those lands where the soil has been eroded away (scoured by floods during the last ice age), ranching in the dominant use. Open rangelands here are dominated by bunchgrass and sagebrush, and require considerable acreage to support livestock. The rich soils of the Palouse region support large-scale wheat farming, while the well-drained gravels of the Wahluke Slope support fruit crops, and extensive vineyards. The Yakima Valley is known for its fruit and hop crops, the region around Othello is managed extensively for seed production. Where soil is present, agriculture is an intensive endeavour here. xxv

33 Image: (Above) Looking north from the top of Steptoe Butte, across the farmlands of the Palouse. (Below) Peaks of the Methow region, looking southwest from Slate Mountain. The plateau extends west into the Yakima Valley, where it is bordered by a northwest-southeast range of hills known as the Yakima Mountains, known to the south as the Horse Heaven Hills. It extends west to the Kittitas Valley between Ellensburg and Cle Elum, and north to the Columbia River city of Wenatchee, then as far as Bridgeport. In the northwest corner it rises to form the Waterville Plateau, cut along its eastern margin by the Grand Coulee. The Columbia River forms the northern boundary of the province as far east as the Spokane River. Here it angles southeast, excluding the eastern margin of the state. It then turns west to include the Snake River region of southeastern Washington. The region is dotted with a myriad of small towns, but the major population centers include the Tri- Cities in the Pasco Basin, Yakima and Ellensburg along the western margin, and Moses Lake in the northern center of the region. The largest city is Spokane, although it technically lies outside of this province. Pullman and Colfax serve the southeastern corner of the region, while Walla Walla is the major town in the Snake River region. xxvi

34 Image: (Above) The Methow Valley, from the high country around Goat Mountain. (Below) Map showing general features of the Methow-Pasayten region, including major towns and highways. Northwest of the confluence of the Okanogan and Columbia Rivers lies a mountainous region which merges with the North Cascades to the west. This is a region drained by the Methow and Chewuch Rivers, a province known as the Methow-Pasayten region. This area lies east of the hydrologic crest of the Casacades, and effectively extends east to the Okanogan River. This is a region of mountain peaks extending as high as 8000 feet (2600m), cut by deep river valleys. The upper portions of the Methow Valley have become increasingly popular locales for summer homes and retirement estates, while the lower portions are managed for agriculture particularly for fruit production. The relatively dry mountain ranges here are popular for recreational use, and are served by an extensive network of mountain roads. The main population center is in Twisp, home to about a thousand people. The Ross Lake Cascade Mountains Methow - Pasayten Region Washington Pass Lake Chelan Twisp Winthrop 20 Methow River Methow Chelan Pateros 97 Columbia River Omak Oroville Loomis Okanogan River Tonasket Conconully Bridgeport Waterville Plateau Okanogan xxvii

35 Image Notes: (Above) Nespelem Valley, northeast of Nespelem (Below) Map showing the Okanogan Highlands region region is sparsely populated, and many businesses are dependent on seasonal traffic along SR 20. Things get much quieter here in the winter, when that highway closes for the season. The Okanogan Valley is a major regional boundary in north-central Washington. It is a major north-south corridor through to British Columbia, it is a major fruitproducing region, and the towns of Omak and Oroville are major population centers along its length. West of the Okanogan River lies the Okanogan Highlands, stretching east to the Kettle Range and the Columbia River. The southern half of this region, which is more mountainous and timbered, is reserved as the Colville Indian Reservation. The northern half is a more open country, a mixture of grasslands and forest. Much of it is high open rolling country of a dry climate and harsh winters. Along its northern end, the Kettle River flows south into Washington out of Canada, then east, only to return north again to British Columbia. The eastern Okanogan River Oroville Tonasket Omak Bridgeport 97 Waterville Plateau 17 Chesaw Okanogan Highlands 20 Republic Colville Indian Reservation 155 Columbia River Nespelem Kettle River 21 Sanpoil River Keller Danville K E T T L E Sherman Pass R A N G E Inchelium Hunters Laurier Kettle Falls Kettle River Spokane 25 River xxviii

36 Image: Kettle River Valley, looking northeast from the Franson Peak Lookout, north of Republic margin of the province is the Sanpoil River and Curlew Creek, beyond which it climbs into the Kettle Range. At their juncture is the town of Republic, the major town on the Highlands, founded on the highly productive gold mines in this area. Agriculture, mining and timber have long been the economic mainstays of this region. It is a remote and little-known province with a sparse population and limited resources. Increasingly, it is becoming a popular locale for tourism, drawn by the spectacular natural setting and the pleasant summer climate. To the east, the highlands rise to the Kettle Range, which stands above the Columbia River. Highway 20 crosses this range at 5575-foot Sherman Pass, the highest all-season road in the state. The north-south trending Kettle Range represents the westernmost range of the Rocky Mountain foothills. Image: West of Chesaw, on the Oroville - Toroda Road. These are beautiful high - country grasslands, just south of the international border. xxix

37 East of the Columbia River, the landscape rises into the foothills of the Rocky Mountains. The Huckleberry Range, which lies between the Columbia and the wide Colville Valley, is an outlier of this system. The Colville Valley extends north from the Spokane area, hosting towns like Chewelah, Colville and Kettle Falls. Northeast of the Colville Valley, a modest range of mountains intervenes before reaching the Pend Oreille Valley, a major north-south trending feature on the landscape here. At the northern end of this valley are the towns of Metaline and Metaline Falls, the historic headquarters for mining operations in this area. To the east of the Pend Oreille Valley rise the Rocky Mountains. Much of this region, particularly to the north and east, is mountainous country. Mining has long been an economic mainstay in this region, rising and falling with world mineral prices. Lead and silver have been the principal metals, supporting communities like Metaline on the Pend Oreille River and Northport on the Columbia River. Timber harvesting has always been a contributing enterprise, in an area with limited agricultural lands. It is a popular recreation area for those living south in Spokane, and includes important routes north into British Columbia. Kettle River Columbia River Hunters Kettle Falls Colville Northport Columbia River H u c k l e b e r r y M t n Addy Chewelah Colville River Rocky Mountains Springdale Ione Metaline Pend Oreille River Newport Spokane River image notes: (Above) Map of the Rocky Mountains Region in northeastern Washington (Below) The Columbia River (Lake Roosevelt), looking north, from south of Hunters. xxx

38 Image Notes: (Above) Highlands along Alladin Creek, south of Northport. View is looking southwest. (Right) Southwest from Salmo Mountain, in the far northeastern corner of the state. These are peaks of the Selkirk Range in Washington. xxxi