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Geomorphic and Chronologic Assessment of Glacial Lake Agassiz Strandlines in Polk County, NW Minnesota

Casaus, Maureen Patricia Redmond
http://orcid.org/0009-0004-0861-491X
http://rave.ohiolink.edu/etdc/view?acc_num=toledo168315538984155

Abstract Details

2023, Master of Science, University of Toledo, Geology.
Glacial Lake Agassiz is a paleolake that formed over central North America at the end of the last Ice Age at ~14.3 ka, and its drainage into the North Atlantic likely contributed to the Younger Dryas cooling at ~12,900–11,700 cal yr BP. Drainage events are identified through the chronologic and geomorphic analyses of strandlines, including beach ridges, escarpments, and spits, which record past lake levels. Strandlines in a study area in the southern basin of Lake Agassiz in NW Minnesota warp upward toward the north as a result of glacioisostatic adjustment (GIA). Older strandlines have a steeper north-south gradient than younger strandlines due to differential rates of rebound. The study area contains a series of strandlines that are classified into 11 strandline groups (SGs) rather than named beach ridges previously identified along other sections of the Lake Agassiz coastline. SGs are composed of one or two well-developed beach ridges, or sets of ridges, that branch out and increase in number toward the south, representing many minor drops in lake level up to ~3 m due to a combination of incision of the southern outlet and uplift caused by GIA. A beach ridge formation model developed from littoral sediment sample analyses and GPR data at 100–500 MHz frequencies suggest that strandlines in the study area formed in an overall depositional regressive system with intermittent storm events. The estimated beach ridge rate of formation is ~8.5-17 yrs/ridge based on ~70 Lockhart Phase beach ridges in the study area and the corresponding optically stimulated luminescence (OSL) dates for the Herman through Tintah beaches and meltwater signatures during drainage events. Strandline chronology is also assessed based on eight OSL dating samples. Most of the dates fell within the error margin for published dates from associated strandlines at the southern outlet. However, samples from the Campbell and Tintah beaches are approximately 3,000 years older than published dates. Two OSL samples currently undergoing laboratory analyses may bring clarity to the differences in ages. Strandlines were mapped in detail on a surficial geology map at a scale of 1:24,000 as a USGS EDMAP project. A new lithologic unit (Qlsg) composed of silt, sand, and gravel that make up strandlines have aided in the analysis of strandline formation in the southern basin of Lake Agassiz. Data analyses reveal that: 1) lake level fall in the study area was overall continuous and gradual through time with many low-magnitude drops in lake level, rather than few major episodic events, and 2) strandlines become progressively younger with decreasing elevation lakeward.
Timothy Fisher (Committee Chair)
Richard Becker (Committee Member)
Harry Jol (Committee Member)
Geology; Geomorphology
Glacioisostatic Adjustment; Ground Penetrating Radar; ArcGIS; Coastal Geomorphology; Lake Level

Recommended Citations

Citations

  • Casaus, M. P. R. (2023). Geomorphic and Chronologic Assessment of Glacial Lake Agassiz Strandlines in Polk County, NW Minnesota [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo168315538984155

    APA Style (7th edition)

  • Casaus, Maureen. Geomorphic and Chronologic Assessment of Glacial Lake Agassiz Strandlines in Polk County, NW Minnesota. 2023. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo168315538984155.

    MLA Style (8th edition)

  • Casaus, Maureen. "Geomorphic and Chronologic Assessment of Glacial Lake Agassiz Strandlines in Polk County, NW Minnesota." Master's thesis, University of Toledo, 2023. http://rave.ohiolink.edu/etdc/view?acc_num=toledo168315538984155

    Chicago Manual of Style (17th edition)

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This open access ETD is published by University of Toledo and OhioLINK.