University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 2005 Sedimentary record of glacial dynamics lake level fluctuations and tectonics: Late Pleistocene-Holocene structural and stratigraphic analysis of the Flathead Lake basin and the Mission Valley Montana United States America Michael H. Hofmann The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Hofmann, Michael H., "Sedimentary record of glacial dynamics lake level fluctuations and tectonics: Late Pleistocene-Holocene structural and stratigraphic analysis of the Flathead Lake basin and the Mission Valley Montana United States America" (2005). Graduate Student Theses, Dissertations, & Professional Papers. 9583. https://scholarworks.umt.edu/etd/9583 This Dissertation is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. NOTE TO USERS The CD is not included in this original manuscript. This reproduction is the best copy available. __ ® UMI Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Maureen and Mike MANSFIELD LIBRARY The University of Montana Permission is granted by the author to reproduce this material in its entirety, provided that this material is used for scholarly purposes and is properly cited in published works and reports. **Please check "Yes" or "No" and provide signature** Yes, I grant permission No, I do not grant permission Author's Signature: Date: Yj j0 'S Any copying for commercial purposes or financial gain may be undertaken only with the author's explicit consent. 8/98 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SEDIMENTARY RECORD OF GLACIAL DYNAMICS, LAKE LEVEL FLUCTUATIONS, AND TECTONICS: LATE PLEISTOCENE - HOLOCENE STRUCTURAL AND STRATIGRAPHIC ANALYSIS OF THE FLATHEAD LAKE BASIN AND THE MISSION VALLEY, MONTANA, USA by Michael H. Hofmann Dipl.Geol. Universitat Erlangen-Niirnberg, Germany, 2001 presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy The University of Montana December 2005 Approved by: C. •erson Dean, Graduate School -3o-os Date Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 3205772 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. ® UMI UMI Microform 3205772 Copyright 2006 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hofmann, Michael H., Ph.D., December 2005 Geology Sedimentary Record of glacial dynamics, lake level fluctuations, and tectonics: Late Pleistocene - Holocene structural and stratigraphic analysis of the Flathead Lake basin and the Mission Valley, Montana, USA Chairperson: Marc H. Hendrix /n srt Integrated analysis of high-resolution seismic reflection profiles, sediment cores and onshore geologic mapping from Flathead Lake and adjacent areas in northwest Montana has revealed considerable information about late Pleistocene and Holocene geologic and tectonic history of the region. During the last glacial maximum, the Flathead Lobe of the Cordilleran Ice sheet advanced twice to its terminal position just south of present day Flathead Lake. During retreat of the lobe, a pro-glacial lake developed with a surface elevation higher than that of modern Flathead Lake. Fault scarps offsetting glacial deposits but not younger stratigraphic units indicate that a tectonic event along the Mission Fault system also occurred during this early post-glacial time. Just after the deposition of the Glacier Peak tephra (13,180 ±120 cal yr BP) the surface level of Flathead Lake dropped to an initial Holocene lowstand at ~10,000 cal yr BP. This lowstand coincided with another period of tectonic activity along the Mission Fault system. After 10,000 cal yr BP, the lake level stabilized for about 2,000 years before dropping to its lowest Holocene level at -7,600 cal yr BP. Seismic and core data from shallow parts of the lake indicate the presence of a fundamental erosional unconformity at this time. This unconformity coincides with the deposition of the Mount Mazama tephra (7,630±80 cal yr BP) and provides information about a post-glacial lake-level lowstand during which the surface of the lake dropped about 15m below the modern lake level and Flathead Lake lost about 25% of its present volume. Although no increase in CaCQ? content was observed along this unconformity, simple hydrologic and geochemical models suggest that the lake level lowering was related to a significant decrease in precipitation, likely triggered by the Mount Mazama volcanic eruption. After this short extreme drought, the lake refilled roughly to its modern day level and fluctuated slightly throughout the reminder of the Holocene. Tectonic activity in the study area was low throughout the Holocene, although several vertically offset seismic reflections in Holocene seismic stratigraphic units that may be seismically-related were recognized. ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PREFACE FOREWORD The Flathead and Mission Valleys in northwestern Montana have a colorful late Pleistocene and Holocene history that can be interpreted from the wide variety of sediments and landforms of this age that occur in the area. The most obvious landforms are the Poison moraine, a terminal moraine from the Flathead lobe of the late Pleistocene Cordilleran Ice Sheet, and different sets of alpine moraines that reflect the advance of glaciers from the Mission Range into the Flathead Valley. Another relict of the late Pleistocene glaciation and the neotectonic history of the area is Flathead Lake, with a surface area of 496km2 the biggest naturally occurring freshwater lake west of the Mississippi River. Sediments in this lake captured the late Pleistocene and Holocene neotectonic, glacial, and climate history in great detail and are the cornerstones of many of the interpretations published in this dissertation. By making use of this tremendous geologic natural laboratory this dissertation focuses on the detailed interpretation of the latest Pleistocene and Holocene history of the Flathead and Mission Valleys. Its goal is to establish the spatial and temporal linkages between the sediment fill history of these valleys and its relationship to advances of late iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Pleistocene glaciers, fluctuations in lake levels and drought frequency, and movements on major faults. It is especially important to better understand the late Pleistocene and Holocene history in the study area because northwestern Montana is a region of steady population growth. Detailed documentation of the depositional history and the transport mechanism of the Quaternary sediments will help to elucidate the distribution of the sediments and other geologic features such as major faults. This will be of immediate use to community planners concerned about new groundwater resources and seismic hazards in the Flathead and Mission Valleys. This dissertation comprises four papers that describe the late Pleistocene and Holocene sediment fill history of the Flathead and Mission Valleys in northwestern Montana and the Flathead Lake region in particular. The first paper (Chapter 1), entitled "Results of geologic mapping of the East Bay 7.5' Quadrangle, Northwest Montana" is published in large parts in the Montana Bureau of Mines and Geology Open File Report 496. Chapter 1 summarizes the geology of parts of the northern Mission Valley in order to match the offshore data from the Flathead Lake sub-bottom (described in Chapters 2-4) to deposits exposed onshore around the margins of the lake. This chapter contains an explanation of new geologic mapping and a complete stratigraphic overview of the Flathead Lake vicinity. Particular attention is given to the Ethology and facies iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the late Pleistocene and Holocene deposits to establish their correlation to the sediments and facies mapped offshore, within the