Abstract

Megaflood (or catastrophic flood) landscapes resulting from overflow or outburst flooding processes have been a focus of geoscientific inquiry dating back nearly 150 years. In these landscapes, the geomorphic forms, both depositional and erosional, have been used to elucidate on the magnitude, frequency, chronology, and landscape evolutionary processes during these high-magnitude events. In addition, recent research has pointed to the landscape altering consequences of these events in creating and/or reorganizing drainage basins downstream of where these events originate – creating new hydrogeomorphic systems. Two significant questions remain in this research: 1) what is the genesis of megaflood depositional dune-like landforms that resemble smaller bedforms in fluvial systems, and 2) because much of the attention of this work has focused downstream of the overflow or outburst flood, how do basins evolve as they catastrophically drain? Given the numerous landscapes that contain these dune-like landforms and abundance of landscapes that have experienced overflow and outburst flood events, further research is necessary. This study attempts to add to our understanding of megaflood geomorphic processes by investigating a newly discovered site that contains two trains of dune-like landforms, just south of the shore of Lake Superior, near Christmas, MI, USA. Herein, we name this study site the Christmas Dunes. Preliminary observations, in the field and in LiDAR-derived imagery, reveal the presence of large, and often imbricated boulders, on the surface of these dune-like landforms and two 19–25.5 m deep bedrock canyons in close proximity to the dune-like landforms. The presence of these landforms at the Christmas Dunes study site suggests a previously unrecognized episode of high-magnitude meltwater discharge across this landscape predating modern Lake Superior. Therefore, these landforms represent a gap in our understanding of the meltwater routing history and proglacial lake-level fluctuations within the Lake Superior basin. In addition, given their location and elevation, they represent an important site to constrain the location of ice margins in the Lake Superior basin during deglaciation. To address the paucity of data at the Christmas Dunes site, we collected 20 ground penetrating radar (GPR) lines, measurements of 814 boulders (dimensions, strike and dip), 7 cosmogenic isotope samples for 10Be surface exposure ages, and morphologic analysis of the landforms using newly available LiDAR-derived DEMs. The GPR data from a landform most proximal to a spillway contained inclined reflections that dip up-flow. It is possible dipping reflections are imbricated boulders buried but the ~19-31° reflection angle is smaller than the typical imbrication angles of surface boulders (21-59°). In addition, no down-flow reflections indicating a potential buried boulder could be positively identified. Thus, we hypothesize these reflections resemble the structure of antidune-like landforms. The presence of antidunes suggests that the flow(s) stopped abruptly because antidunes are commonly obliterated once a flow transitions from supercritical to subcritical. We hypothesize rapid lake level draw-down caused abrupt spillway abandonment allowing the antidune forms to be preserved – this may be reinforced by the boulders armoring the surface of the landforms. Dune-like landforms further from the spillways contain subhorizontal to inclined GPR reflections, the latter interpreted as primarily down-flow dipping sedimentary structures. The transition from antidunes to normal-dune like sedimentary structure suggests a transition in flow regime beyond the most spillway-proximal landforms. Boulder B-axis diameters, dune height, and wavelength decrease with distance from the spillways, supporting the interpretation of a flow and transport-regime shift. Preliminary estimates of paleodischarge suggest flows may have been 0.22 Sv (Fischer et al. 2024), similar to estimates of 0.106 – 0.33 Sv by Breckenridge and Fisher (2021), but future estimates can be made with the new data presented in this thesis. 10Be surface exposure ages (~10.56 ± 657 ka) fit the limited, but well established geochronological constraints, with deposition of the dune-like landforms occurring after the Marquette Readvance to the Grand Marais Moraine (11,541 ± 298 cal BP; Lowell et al. 1999) and before the ice margin had retreated to the Caribou Subbasin, in the interior of modern Lake Superior (10,329 ± 97 cal BP; Breckenridge 2007; Hydro and Longstaffe 2011). The enigmatic issue of ice margin position during the deposition of Christmas Dunes, between the Grand Marais Moraine and Caribou Subbasin, precludes a conclusive hypothesis for the genesis of these landforms. However, given the available evidence and observed similarities with megaflood literature in other landscapes (e.g. Camas Prairie – Missoula Floods), we favor a hypothesis that the Christmas Dune landforms formed during rapid proglacial lake draw-down across the sandstone bedrock ridge into which the spillways are incised. Flow velocities increased as they were confined into the sandstone spillways. This allowed for plucking and entrainment of boulders that were transported and subsequently deposited subaqueously. Subaqueous deposition would be necessary to reduce settling velocity given the friable nature of the sandstone boulders. The Christmas Dune landforms also represent a well-preserved location indicative of internal basin evolution dynamics during the rapid draining of a proglacial lake basin – a concept inadequately understood in overflow and outburst flood literature – and has striking similarities to the Camas Prairie of the Missoula Floods. Future detailed comparisons between these landscapes may help to understand how draining basins evolve. We hypothesize that this event likely occurred when an ice-dam failed east of the Christmas Dunes study site and allowed meltwater from Glacial Lake Duluth to access meltwater in Glacial Lake Minong, which flowed out the eastern outlet (St. Mary’s River) of the Lake Superior basin. This event abruptly rerouted southward-flowing meltwater from the Au Train-Whitefish spillway across the Christmas Dunes study site. Paleo-lake levels at the time of this event are not well constrained given the uncertainty of inputs to Glacial Lake Duluth from Glacial Lake Agassiz. The catastrophic nature of the flows necessary to create the Christmas Dunes suggests a unique event in the Lake Superior basin. Nearby landscapes west of the Christmas Dunes, and at correlative elevations, appear to also indicate catastrophic drainage. We speculate that Lake Agassiz may have contributed to a rapid rise in lake level resulting in ice-dam failure east of the Christmas Dunes site and ultimately resulting in these megaflood erosional and depositional features. Future research on these landscapes may serve to test our hypothesis.

Advisor

Phillip Larson

Committee Member

Karen Gran

Committee Member

Andy Breckenridge

Date of Degree

2024

Language

english

Document Type

Thesis

Degree

Master of Science (MS)

Program of Study

Geography

Department

Geography and Anthropology

College

Social and Behavioral Sciences

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