Abstract
The St. Croix River valley (SCRV) is the product of dyssynchronous glacial advance and retreat of several glacial lobes during regional deglaciation of the Late Pleistocene-Early Holocene transition. Additionally, the SCRV has been dramatically shaped by multiple proglacial lake drainage episodes. Therefore, the SCRV is an important case study to understanding the impacts of proglacial lake spillover processes. However, the magnitude, frequency, timing, duration, and geomorphic impacts of these floods remain enigmatic. We focus on a specific reach of the SCRV, which we name the Lower SCRV for the purpose of this study. The lower SCRV contains several anomalous landforms which are in the immediate proximity of the Grantsburg sublobe (GSL) maximum extent and located south of a proglacial lake named glacial Lake Grantsburg (GLG). These landforms a bedrock gorge, an anomalously high and extensive terrace-like surface called the Osceola Bench (OB), terraces which step down from the OB to the modern river valley, a large paleochannel named the Horse Creek Channel (HCC), a high paleovalley located immediately south of the GLG margin identified as the Dresser Spillway, and fluvially sculpted forms like pendant bars. In this study, we provide a suite of datasets from six methodologies to help identify the nature and characteristics of GSL/GLG drainage and how it impacted the SCRV. Our methods include geomorphic mapping, hand augering, coring, optically stimulated luminescence (OSL) dating, and X-ray fluorescence (XRF).
Based on the results of our research, we suggest a variation of spillover process is responsible for GLG drainage. We name this process as transgressive ice-marginal outburst flooding (TIMOF), as we find GLG drainage was governed by the westward retreat of the GSL. This TIMOF process resulted in the creation of the anomalous landforms within the SCRV (e.g. HCC, OB, Dresser Spillway, pendant bars, terraces).
A quarry northwest of Dresser, WI, contains fine sands unconformably overlain by massively bedded gravel deposits, which we attribute to be Superior lobe outwash truncated by flood deposits of GLG. We find the Dresser Spillway is a strath-like surface capped by a thin veneer of sediments deposited during the waning stages of flow. The Dresser Spillway is topographically correlated with the HCC, suggesting that initial GLG drainage was routed out the spillway and into the HCC. However, the number and timing of drainage which occupied the HCC remain enigmatic. Following initial retreat of the GSL westward, a lower drainage route was opened: the OB. The OB is a strath-like surface with similar sedimentologic, geochemical, and stratigraphic characteristics as the Dresser Spillway. Furthermore, numerous pendant bars are superimposed on the bench surface. GPR data shows the pendant bar formed while drainage was shifting westward. Our data suggests the terrace equivalent in elevation to the OB is also a strath-like surface, likely the same surface as the OB. GPR and sediment data from two lower terraces immediately westward adjacent to the OB show these terraces are fill-cut terraces. When accounting for margin of error, all OSL data collected and presented in this study falls within the expected timeframe of GSL and GLG established by prior literature at 16.3-14.1 ka. Events of GSL and GLG were truncated by an erosional event that significantly cut into the valley. Subsequently, aggradation led to the formation of the Barrens fan and glacial Lake Anoka. Finally, high-magnitude meltwater discharges from the Superior basin incised the SCRV by ~60 m, forming the bedrock gorge visible at Interstate State Park, MN/WI.
Data presented in this study acts to constrain the events prior to and following those of the GSL/GLG. Additionally, our data contributes to ongoing deglacial mapping efforts and in reconstructing meltwater drainage routing from the continent to the oceans. We hope our newly proposed model will contribute to ongoing efforts to mitigate and prevent future hazards that spillover processes pose to mountainous communities globally.
Advisor
Phillip Larson
Committee Member
Douglas Faulkner
Committee Member
Andrew Wickert
Committee Member
Ronald Schirmer
Date of Degree
2024
Language
english
Document Type
Thesis
Degree
Master of Science (MS)
Program of Study
Geography
Department
Geography and Anthropology
College
Humanities and Social Sciences
Recommended Citation
Delikowski, H. (2024). Post-glacial Landscape Evolution of the Lower St. Croix River Valley: The Enigma of Glacial Lake Grantsburg, the Osceola Bench, and the St. Croix Dells [Master’s thesis, Minnesota State University, Mankato]. Cornerstone: A Collection of Scholarly and Creative Works for Minnesota State University, Mankato. https://cornerstone.lib.mnsu.edu/etds/1471/