Age and Origin of Fluvial Terraces in the Central Coast Range, Western Oregon By STEPHEN F. PERSONIUS U.S. GEOLOGICAL SURVEY BULLETIN 2038 Radiocarbon and thermoluminescence dating methods are used to determine the ages and probable origins offluvial terraces UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Robert M Hirsch, Acting Director For sale by USGS Map Distribution Box 25286, Building 810 Denver Federal Center Denver, CO 80225 Any use of trade, product, or finn names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government Library of Congress Cataloging-in-Publication Data Personius, Stephen F. Age and origin of fluvial terraces in the central Coast Range, western Oregon I by Stephen F. Personius. p. em. - (U.S. Geological Survey bulletin; 2038) Includes bibliographical references. Supt. of Docs. no.: I 19.3:B2038 1. Terraces (Geology)-Oregon. 2. Terraces (Geology)-Coast Ranges. I. Title. II. Series. QE75.B9 no. 2038 [GB595.07] 557.3 s-dc20 92-23352 [551.4'42] CIP CONTENTS Abstract ................................................................................................................................ 1 Introduction .......................................................................................................................... 1 Scope and Purpose ..................................................................................................... 1 Geomorphic Setting ................................................................................................... 4 Climate ...................................................................................................................... .. 4 Bedrock Geology ....................................................................................................... 4 Methods ............................................................................................................................... 4 Terrace Dating Methods .................. ........................................................................... 4 Radiocarbon Dating.............................................................................................. 4 Thermoluminescence Dating................................................................................ 5 Other Dating Methods .................... ...................................................................... 8 Terrace Heights and Profile Construction .................................................................. 10 General Characteristics of Coast Range Terraces ................................................................ 11 Terrace Stratigraphy ................................................................................................... 11 Controls On Terrace Formation.................................................................................. 11 Sediment Accumulation Rates.................................................................................... 12 Terraces Along the Umpqua River ...................................................................................... 14 Latest Pleistocene and Holocene Terraces ................................................................ 14 Pleistocene-Holocene Transition (PHT) Terrace Exposures Near McGee Creek 14 PHT Terrace near Elkton ..................................................................................... 16 PHT Terraces Near the Coast ............................................................................. .. 16 Low Terraces Near the Coast ............................................................................. .. 19 Other Holocene Terraces ..................................................................................... 19 Older Terraces ........................................................................................................... 20 Marine Terraces ................................................................................................... 20 Intermediate Terraces Near Reedsport ............................................................... .. 21 High Terrace at Reedsport ................................................................................... 23 High Terrace Along Crestview Drive .................................................................. 25 High Terraces Along Henderer Road ................................................................. .. 26 Terraces Along the Smith River .......................................................................................... 27 Terraces Upstream from the Island ............................................................................ 27 Terraces in the Island ................................................................................................ .. 30 Terraces Between the Island and Brainard Creek. .................................................... .. 33 Terraces Near Brainard Creek ................................................................................... 33 Terraces Downstream from Brainard Creek .............................................................. 34 Terraces Along the Siuslaw River ...................................................................................... .. 36 Holocene Terrace ....................................................................................................... 36 Older Terraces ........................................................................................................... 36 Marine Terraces ................................................................................................... 36 Older Fluvial Terraces ......................................................................................... 39 Terraces Along the Siletz River ........................................................................................... 41 Latest Pleistocene and Holocene Terraces ................................................................ 43 Late Pleistocene Terraces .......................................................................................... 43 Older Terraces ........................................................................................................... 46 Marine Terraces ................................................................................................... 46 Older Fluvial Terraces ......................................................................................... 46 Terraces Along the CO<Juille River ............................................................................ 48 Discussion ............................................................................................................................ 51 Origin of Latest Pleistocene/Early Holocene Aggradation ....................................... 51 III IV CONTENTS Discussion-Continued Relations Between the PHT and Other Coast Range Terraces ................................... 52 Conclusion............................................................................................................................. 52 Acknowledgments................................................................................................................. 53 References Cited .. ............... .. .............. .......... ............. ........................................................... 53 PLATES [Plates are in pocket] 1. Generalized map of terraces along the lower Umpqua River, central Coast Range, western Oregon. 2. Generalized map of terraces along the lower Smith River, central Coast Range, western Oregon. 3. Generalized map of terraces along the lower Siuslaw River, central Coast Range, western Oregon. 4. Generalized map of terraces along the lower Siletz River, central Coast Range, western Oregon. FIGURES 1. Map showing regional tectonic setting of the Pacific Northwest................................................................................ 2 2. Map showing the location of major rivers in western Oregon and Washington......................................................... 3 3. Stratigraphic columns of three fluvial terrace exposures along the Umpqua River where both thermoluminescence (1L) and radiocarbon samples were collected ............................................................................................................ 9 4. Stratigraphic columns of radiocarbon-dated fluvial sediments used to calculate sedimentation rates........................ 13 5. Longitudinal profile of terraces along the lower reach of the Umpqua River............................................................. 15 6. Photograph ofPHT (Pleistocene-Holocene transition) terrace on the Umpqua River near McGee Creek ................. 16 7. Rod-and-level profile of stripped bedrock strath and PHT (Pleistocene-Holocene transition) terrace near McGee Creek on the Umpqua River........................................................................................................................... 17 8. Stratigraphic column from a PHT (Pleistocene-Holocene transition) terrace exposure near McGee Creek at station 88-U2 ........... .. ....... ... .. .. ... ... ............ ............. .............................. ..................................................... .. .... ... ..... 18 9. Stratigraphic column from a terrace exposure about 2 km downstream from the highway bridge near Scottsburg at station 87-Ul4........................................................................................................................................................