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Timeline Reconstruction of Holocene Jökulhlaups Along the Jökulsá Á

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Timeline Reconstruction of Holocene Jökulhlaups Along the Jökulsá Á Copyright by Greta Hoe Wells 2016 The Thesis Committee for Greta Hoe Wells Certifies that this is the approved version of the following thesis: Timeline Reconstruction of Holocene Jökulhlaups along the Jökulsá á Fjöllum Channel, Iceland APPROVED BY SUPERVISING COMMITTEE: Supervisor: Sheryl Luzzadder-Beach Timothy P. Beach Daene C. McKinney Timeline Reconstruction of Holocene Jökulhlaups along the Jökulsá á Fjöllum Channel, Iceland by Greta Hoe Wells, B.A. Thesis Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Master of Arts The University of Texas at Austin May 2016 Abstract Timeline Reconstruction of Holocene Jökulhlaups along the Jökulsá á Fjöllum Channel, Iceland Greta Hoe Wells, M.A. The University of Texas at Austin, 2016 Supervisor: Sheryl Luzzadder-Beach The Jökulsá á Fjöllum is Iceland’s second longest river, draining from the Vatnajökull ice cap and winding for over 200 km through the eastern highlands before emptying into the Arctic Ocean. Hydrothermal activity and subglacial eruptions beneath Vatnajökull generate enormous quantities of meltwater, which can drain catastrophically in outburst floods, known as jökulhlaups. Jökulhlaups have flowed through the Jökulsá á Fjöllum channel throughout the Holocene, but intense debate exists over their timing and magnitude. While previous studies report a peak flood discharge of 0.9 x 106 m3 s-1, Howard et al. (2012) found evidence of a peak discharge of 2.2 x 107 m3 s-1, which would make this the largest known flood on Earth. This project seeks to test Howard et al.’s (2012) hypothesis and, more broadly, to reconstruct a timeline of Holocene jökulhlaups iv along the Jökulsá á Fjöllum. First, it reviews current literature and research on Icelandic geology and megafloods; second, it reviews and critiques previous research methods and evidence of Holocene jökulhlaups along the channel, while also presenting new geomorphologic and geochronological evidence from field work in August 2015; and finally, it sets up a framework for future research and explores unanswered questions regarding the history of Holocene jökulhlaups along the Jökulsá á Fjöllum. v Table of Contents List of Figures ..........................................................................................................x INTRODUCTION..................................................................................................1 PART I: LITERATURE REVIEW ......................................................................4 CHAPTER 1: THE GREAT MEGAFLOOD DEBATE .........................................4 CHAPTER 2: GEOLOGY OF ICELAND ..............................................................8 General Topography .......................................................................................8 Tectonic Setting ..............................................................................................9 Volcanism .....................................................................................................11 Geologic Units ..............................................................................................12 Tectonic Uplift, Subsidence, and Glacial Isostatic Adjustment ...................14 CHAPTER 3: GLACIATION, CLIMATE, AND SOILS IN ICELAND .............19 Glaciation History .........................................................................................19 Climate ..........................................................................................................22 Soil Types and Erosion .................................................................................22 Soil Types ............................................................................................22 Soil Erosion ..........................................................................................25 CHAPTER 4: JÖKULHLAUP TRIGGERS AND DRAINAGE DYNAMICS ....28 Subglacial Lake Formation ...........................................................................28 Subglacial Lake Drainage Triggers ..............................................................30 Subglacial Volcanic Eruptions: Direct Drainage ..........................................30 Subglacial Drainage Dynamics .....................................................................31 CHAPTER 5: MEGAFLOODS .............................................................................33 Megafloods on Earth .....................................................................................33 Megafloods on Mars .....................................................................................35 CHAPTER 6: JÖKULHLAUP GEOMORPHOLOGIC FEATURES ..................36 Jökulhlaup Erosional Features ......................................................................37 vi Erosional Processes ..............................................................................37 Gorges ..................................................................................................40 Cataracts ...............................................................................................42 Grooves, Flutes, and Potholes ..............................................................43 Streamlined Hills .................................................................................44 Spillways ..............................................................................................46 Trimlines ..............................................................................................47 Obstacle Marks ....................................................................................47 Jökulhlaup Depositional Features .................................................................49 Boulder Clusters, Bars, and Erratics ....................................................49 Megaripples..........................................................................................53 Bars…………………………………………………………………...55 Longitudinal Bars........................................................................56 Expansion Bars ...........................................................................56 Pendant Bars ...............................................................................56 Eddy Bars and Slackwater Deposits ...........................................57 Point Bars ....................................................................................59 Wash limits ..........................................................................................59 Ice blocks and Kettle Holes .................................................................60 Rip-up Clasts ........................................................................................63 Jökulhlaup Sedimentology ...................................................................65 CHAPTER 7: JÖKULHLAUP HISTORY ALONG THE JÖKULSÁ Á FJÖLLUM .......................................................................................................................67 PART II: REVIEW AND CRITIQUE OF CURRENT EVIDENCE ...............70 CHAPTER 8: STUDY AREA ...............................................................................70 Field Site .......................................................................................................70 General Topography ............................................................................70 Geology ................................................................................................76 Vatnajökull ....................................................................................................78 Tectonic Setting and Volcanism ..........................................................79 vii Jökulhlaup Generation .........................................................................84 Glacial Fluctuations .............................................................................86 Jökulsá á Fjöllum ..........................................................................................87 CHAPTER 9: EVIDENCE OF HOLOCENE JÖKULHLAUPS ALONG THE JÖKULSÁ Á FJÖLLUM: FLOOD MAGNITUDE AND EXTENT ...........93 Geomorphologic Evidence............................................................................93 Kverkfjallarani .....................................................................................94 Vaðalda ................................................................................................95 Upptyppingar .......................................................................................95 New Evidence: Cataract and Spillway ........................................96 New Evidence: Boulder Field, Cataract, and Moat ..................101 Herðubreið .........................................................................................109 New Evidence: Boulder Field and Cataract ..............................113 Möðrudalur ........................................................................................117 Jökulsárgljúfur ...................................................................................117 Ásbyrgi ...............................................................................................121 Critiques of Geomorphologic Interpretation ...............................................124 Hydraulic Modeling ....................................................................................127 Critiques of Hydraulic Modeling ................................................................135 CHAPTER 10: EVIDENCE OF HOLOCENE JÖKULHLAUPS ALONG THE JÖKULSÁ
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