Geology of Oregon Orr Orr.Pdf

Fifth Edition Geology of Oregon Elizabeth L. Orr William N. Orr University of Oregon Cover: Ripple-marked sand dunes on the Oregon Coast resemble a gigantic fingerprint (photo by Gary Tepfer). Copyright ® 1964 by Ewart M. Baldwin Copyright ® 1976, 1981, 1992, 2000 by Kendall/Hunt Publishing Company ISBN 0-7872-6608-6 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the copyright owner. Printed in the United States of America 10 98765432 Table of Contents Introduction 1 History of geologic study in Oregon 9 Blue Mountains 21 Klamath Mountains 51 Basin and Range 79 High Lava Plains 103 Deschutes-Columbia Plateau 121 Cascade Mountains 141 Coast Range 167 Willamette Valley 203 Bibliography 223 Glossary 245 Index 251 iii Dedicated to the graduates Acknowledgments Between the fourth and fifth editions of Geology of Oregon, the importance of global tectonics to the state has been ingrained even deeper. Riding on the leading edge of the moving North American plate, Oregon reflects the underlying mecha- nism of the plate collision boundary in virtually all aspects of its geology. In the seven years since the fourth edition was written, an irregular but continuing drumbeat of earthquakes reminds us of the forces beneath our feet and the need to prepare for catastrophic changes be they quakes, volcanoes, tsunamis, floods, or massive earth movements. Most profound is the discovery of irrefutable evidence that these catastrophes have been visited on the state quite regularly over the past few thousands of years as well as the absolute surety that similar disasters will occur in the near and distant future. A book of this nature, covering a state of such geologic diversity, could not have been written without the research efforts of everyone who has contributed to our understanding of the past. We would like to express our thanks to all of these un- named and named individuals: John Armentrout, Ewart Baldwin, Marvin Beeson, Howard Brooks, Bob Carson, Mike Cummings, Greg Harper, Paul Hammond, Klaus Neuendorf, George Priest, Tracy Vallier, Beverly Vogt, and Bob Yeats. Others provided literature, comments, and espresso. In this regard we thank Rich- ard Heinzkill, Gene Humphries, Allan Kays, Carol McKillip, and Cindy Shroba, as well as the helpful staff of the Map and Science libraries at the University of Oregon. Eugene, Oregon, 1999 V Introduction It is difficult to do justice to Oregon's unique and varied geologic past with the broad brush strokes of a summary. As the state experiences a variety of processes from erosion to volcanism and even glaciati- on, each leaves its unmistakable signature recorded in the rocks and strata. Plate Tectonics Because of Oregon's position on an active continental margin, tectonic processes have played a profound role in the state. With global or plate tecton ics, land masses are riding atop great plates of the earth's crust that are constantly in motion. As they move, the plates can pull apart, collide, or slide past each other. When a plate rifts or breaks up, molten rock pours up as lava from cracks that open in the stretched crust. As the lava cools in a rift zone under archipelago, both of these troughs derive volcanics the sea, a submarine mountain range or ridge is from the island chain, although the backarc basin constructed along the rupture. Currently an extensive receives sediments from erosion of the adjacent main- and growing system of undersea ridges exists along the land as well. floor of the world's oceans. When plates merge and Each moving tectonic plate has a leading and collide, the heavier one dives beneath the overriding trailing edge. Along the leading edge, where collision slab. Once this occurs, the veneer of sediments atop and subduction are taking place, there are large-scale the descending slab is scraped off and accreted to the earthquakes and widespread volcanic activity. The upper plate. At the same time, rocks carried down or leading edge, with its active continental margin, con- subducted beneath the overriding plate are melted to trasts sharply with the passive margin of the trailing be recycled. Some of the melted rock makes its way edge. Along passive margins of continents earthquakes through the overriding plate to emerge as volcanics. and volcanic activity are subdued to nonexistant. Here The remaining magma crystalizes beneath the surface the main geologic processes are erosion and deposition. forming a foundation to the volcanic chain. Much of Oregon's early geologic history Developed on moving crustal plates, volcanic concerns plate movements and the accretion of exotic island archipelagos are typically involved in accretion- terranes. Situated on the leading edge of the North ary tectonics. Carried on an oceanic plate, the island American plate, Oregon has repeatedly been the site of chain will eventually collide with a landmass on an multiple collisions with smaller continental plates that oncoming plate where it is attached or accreted to the were swept up and added to North America. It is edge to become part of the mainland. As part of the estimated that two-thirds to three-fourths of the state accretion process, plate fragments making contact with is composed of rocks and sediments that originated the mainland rotate to obtain a "fit". The contact point elsewhere in the Pacific basin. Fragments of these between two colliding plates is often marked by a long terranes make up the geologic collage of Oregon. trough or trench. Between this trench and the island Throughout these collisions, volcanic events and chain a shallow forearc basin may develop. Behind the earthquakes mark the mileposts of changing land- archipelago, a backarc basin may subside between the scapes, climates, and flora and fauna. volcanic chain and the mainland. Parallel to the 1 7 Introduction Paleozoic Although the assembly of Oregon's component preserved. A few Mississippian brachiopods and corals fragments began in the lower Mesozoic over 200 as well as a diverse Pennsylvanian flora of fern and million years ago, pieces of the state were being fern-like plants that grew in an ancient upland are also fabricated in far-flung ancient oceans as far back as 400 found in rocks of central Oregon. Permian fossil million years before the present. The oldest known protozoa, corals, crinoids, molluscs, and brachiopods rocks in the state are Devonian, and during this period from the same region reflect both marine continental Oregon was a broad embayment of an ocean with a shelf as well as deep water conditions. Devonian rocks shoreline to the east across Idaho and Nevada. On the may occur in the terranes of the eastern Klamath Farallon oceanic plate sliding beneath the North Mountains, although the strata here has been so American continental plate, a line of volcanic islands thoroughly altered by heat and pressure that most of accumulated limestones and even a few terrestrial the evidence has been destroyed. deposits containing plant fossils in an offshore tropical latitude. Throughout the Paleozoic sediments contin- Mesozoic ued to develop in this distant oceanic environment. At At the beginning of the Triassic, events in some point in time the landmass collided with and was Oregon reflect a period of collision and accretion. It attached to the western margin of the North American was about this time that North America began break- plate, ing away from Europe and Africa to create the early Remnants of the terranes are found today in Atlantic Ocean. As the Atlantic grew progressively the center of the state in eastern Crook County and in larger, the ancient Pacific became smaller. During this parts of the Klamath Mountains to the southwest. In process, the continental shelf off the Atlantic Coast eastern Oregon a few scrappy exposures of Devonian expanded as sediments were dumped there from the limestone and a flint-like rock called chert contain intensive erosion of the mainland. Simultaneously the Oregon's oldest fossils. Within these rocks remains of West Coast grew by accreting large and small exotic microscopic creatures known as conodonts as well as blocks to its leading edge. In the Mesozoic of Oregon, specimens of brachiopods and tropical corals are the major areas of continental accretion were the Blue Mountains and Klamaths. Although today these regions As the continental plate moves west, it scrapes off and accretes smaller continental blocks and island archipelagos atop the oceanic plate. Transport Subduction zone between continental block and mainland Introduction 3 are across the state from each other, there are indica to this time, these are almost certainly exotic. The tions that during the Mesozoic they may have been side variety of cycads, conifers, and ginkgos in these floras by side near present day Idaho. Here they projected as indicate an uplands environment dotted with shallow low islands above waters of the deep ocean that lakes and marshes. Horsetails, quillworts, and the covered Oregon. abundance of large ferns point to a moist climate. Such Packets of imported rocks or terranes, bounded a diverse cosmopolitan flora, found across Oregon as by major faults, make up the bulk of the Blue Moun well as Alaska, Asia, Europe, and Antarctica, suggests tains and Klamaths. Evidence that the rocks in these widespread tropical and subtropical conditions. Alter areas are not native to Oregon or North America is nately it may mean that microcontinental blocks with confirmed by fossils found here. Triassic coral reefs in a similar flora were dispersed to all corners of the the Wallowas of eastern Oregon are particularly world. significant in that they represent a type only previously The skeletons of tiny glassy microfossils of known from the same period in the European Alps.

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