THESIS APPROVAL The abstract and thesis of Darren L. Beckstrand for the Master of Science in Geology presented November 22, 2000, and accepted by the thesis committee and the Department. COMMITTEE APPROVALS: _______________________________________ Curt D. Peterson, Chair _______________________________________ Errol C. Stock _______________________________________ Georg H. Grathoff _______________________________________ Virginia L. Butler Representative of the Office of Graduate Studies DEPARTMENTAL APPROVAL: _______________________________________ Ansel G. Johnson, Chair Department of Geology ABSTRACT An abstract of the thesis of Darren L. Beckstrand for the Master of Science in Geology presented November 22, 2000 Title: Origin of the Coos Bay and Florence Dune Sheets, South Central Coast, Oregon The Coos Bay and Florence dune sheets are situated on Oregon’s south central coast, between Florence to the north and Coos Bay to the south. These two dune sheets comprise approximately 70 km of coastline; the largest coastal dune fields of Oregon. Heavy mineral analysis indicates a primary sand source from the Umpqua River, as evidenced by high hypersthene:augite ratios and a secondary source from streams to the south, yielding metamorphic amphiboles. Complex directional emplacement trends indicate onshore transport of sand; however, different data suggest either a southern or a northern sand source. Dune lee- slope azimuth analyses suggest a northern sand source, whereas particle-size analysis indicates a southern source. Dune thickness and depth increase to the north. Accommodation space also increases to the north due to possible tectonic uplift to the south and incision by the paleo-Siuslaw River channel to the north. Previously, these dune sheets were assumed to be Holocene features forced onshore during the latest de-glacial sea level rise. A Holocene age would require a high sea level stand emplacement model. Thermoluminescent (TL) dating of sand and C14 dating of organics suggest at least three pulses of sand supply. The oldest TL date 2 obtained is 70.2 ± 3.0 ka. Most of the eastern edge dune TL dates are between 30 and 40 ka. The westernmost sands are chiefly 1-3 ka. The earliest ages (70 and 30-40 ka) prohibit a high stand emplacement model. Rather, these ages suggest a sea level low stand model with sand transported across the exposed continental shelf to present dunal positions. Variable sea levels during the Pleistocene could have alternately encouraged sand transport alongshore (high-stand) and left the inner-continental shelf sands exposed to onshore eolian transport (low-stand), thereby episodically supplying the coastal dunes with the freshly available sand. Sand supply in the latest Holocene (1-3 ka) prograded a portion of the shoreline and covered some westernmost stabilized Pleistocene dune soils. However, late Holocene sand advances (hundreds of meters) are small compared to late Pleistocene advances (several kilometers). These results will impact previous interpretations of neotectonics, archaeology, and coastal sand supply in the south-central Oregon coastal zone. ORIGIN OF THE COOS BAY AND FLORENCE DUNE SHEETS, SOUTH CENTRAL COAST, OREGON by DARREN L. BECKSTRAND A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in GEOLOGY Portland State University 2001 Dedication For Tiffany and Natalie Acknowledgments This thesis would not have been possible without the complete support of my wife, Tiffany. Her support and patience during the writing process, along with the love of my beautiful daughter Natalie, made this daunting task more worthwhile and fulfilling. My encouraging parents, Blaine and Marilyn Beckstrand, and in-laws Warren and Lynn Bevan, also provided the financial and emotional support that was needed. I lovingly thank my family for everything they have done for my education and, by extension, me. Many thanks are given to Dr. Curt Peterson who, with much patience and after hours, has edited and revised this manuscript and provided insight and understanding when needed. Gratitude is in order for Dr. Georg Grathoff, who visited the coast with me and freely provided knowledge and interpretation to soil clay mineralogy. I also gratefully thank geologist Dr. Errol Stock, of Griffith University, Brisbane, Australia, for his support and much needed help throughout the steep learning curve of Oregon’s coastal dunes. Geologist Courtney Cloyd of the Siuslaw National Forest provided the field expertise and immediate knowledge of the surrounding landscape, as well as the impetus for funding through the Forest Service. ODNRA Manager Ed Becker provided for access, ground logistics, and lodging for numerous researchers through the duration of this study. The primary financial support for this thesis has been through U.S.D.A., U.S. Forest Service, Siuslaw National Forest Purchase Order numbers 43-04T0-9-0079 and 43-04T0-0009. Oregon Sea Grant also provided funding through grant number NSD-04. ii Table of Contents Dedication..............................................................................................................................i Acknowledgments................................................................................................................ii Table of Contents ............................................................................................................... iii List of Tables.......................................................................................................................vi List of Figures................................................................................................................... viii Introduction ..........................................................................................................................1 Background...........................................................................................................................7 Study Area......................................................................................................................7 Geography................................................................................................................7 Local Geology..............................................................................................................11 Sediment transport considerations ........................................................................11 Origins of coastal dunes ........................................................................................12 Oregon’s dune systems..........................................................................................14 Dune/sediment mapping and descriptions ......................................................18 Previous interpretations of sediment source ...................................................19 Local Paleoclimate ..........................................................................................19 Eustatic sea level changes ...............................................................................20 Continental shelf sediment composition.........................................................21 Soils..................................................................................................................23 Anthropology...................................................................................................25 Methodology ......................................................................................................................27 Sand source ..................................................................................................................27 Heavy mineralogy..................................................................................................27 Grain size analysis .................................................................................................28 Laboratory procedures.....................................................................................28 Data processing................................................................................................29 Surficial and subsurface dune morphology.................................................................29 Dune azimuth analysis (GIS based) ......................................................................29 Dune azimuth analysis.....................................................................................30 Subsurface investigation........................................................................................32 Vibracoring......................................................................................................32 Drilling.............................................................................................................34 Sand thickness and depth.......................................................................................34 Well logs..........................................................................................................35 Age dating ....................................................................................................................38 Soil characteristics .................................................................................................38 Munsell soil colors ..........................................................................................39 Harden’s soil maturity index...........................................................................39 Buntley-Westin color index