Browsing by Subject "Lake Agassiz"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    The geologic history of Lake of the Woods, Minnesota, reconstructed using seismic-reflection imaging and sediment core analysis
    (2014-07) Hougardy, Devin D.
    The history of glacial Lake Agassiz is complex and has intrigued researchers for over a century. Over the course of its ~5,000 year existence, the size, shape, and location of Lake Agassiz changed dramatically depending on the location of the southern margin of the Laurentide Ice Sheet (LIS), the location and elevation of outflow channels, and differential isostatic rebound. Some of the best-preserved sequences of Lake Agassiz sediments are found in remnant lake basins where erosional processes are less pronounced than in adjacent higher-elevation regions. Lake of the Woods (LOTW), Minnesota, is among the largest of the Lake Agassiz remnant lakes and is an ideal location for Lake Agassiz sediment accumulation.High-resolution seismic-reflection (CHIRP) data collected from the southern basin of LOTW reveal up to 28 m of stratified lacustrine sediment deposited on top of glacial diamicton and bedrock. Five seismic units (SU A-E) were identified and described based on their reflection character, reflection configuration, and external geometries. Three prominent erosional unconformities (UNCF 1-3) underlie the upper three seismic units and indicate that deposition at LOTW was interrupted by a series of relatively large fluctuations in lake level. The lowermost unconformity (UNCF-1) truncates uniformly draped reflections within SU-B at the margins of the basin, where as much as four meters of sediment were eroded. The drop in lake level is interpreted to be contemporaneous with the onset of the low-stand Moorhead phase of Lake Agassiz identified from subaerial deposits in the Red River Valley, Rainy River basin, and Lake Winnipeg. A rise in lake level, indicated by onlapping reflections within SU-C onto UNCF-1, shifted the wave base outwards and as much as 11 m of sediment were deposited (SU-C) in the middle of the basin before a second drop, and subsequent rise, in lake level resulted in the formation of UNCF-2. Reflections in the lower part of SU-D onlap onto UNCF-2 only near the margins of the basin, suggesting that water occupied much of the middle of the southern basin after lake level drawdown. The reflection character and configuration of SU-C and SU-D are genetically different indicating that the depositional environment had changed following the formation of UNCF-2. Piston-type sediment cores collected from the southern basin of LOTW at depths that correspond to the middle of SU-D contain high amounts of organic material and charcoal fragments and sediment that are probably not related to Lake Agassiz. Instead, they were likely deposited during a transitional phase between when Lake Agassiz left the LOTW basin (UNCF-2) and inundation of LOTW from the northern basin due to differential isostatic rebound (UNCF-3). All sediment cores collected from the southern basin of LOTW record the uppermost unconformity, analogous in depth to UNCF-3 in the seismic images, which separates modern sediments from mid to late-Holocene sediments. The lithology of sediments below this unconformity varies across the basin from gray clay to laminated silt and clay. Radiocarbon ages from two peat layers immediately below the unconformity indicate that subaerial conditions had existed prior to the formation of UNCF-1, at about 7.75 ka cal BP. The timing correlates well with other lakes in the upper Midwest that record a prolonged dry climate during the mid-Holocene. UNCF-3 is planar and erosional across the entire survey area but erosion is greatest in the northern part of the basin as the result of a southward transgressing wave base driven by differential isostatic rebound. Deposition in the southern basin probably resumed around 3.3 ka cal BP, though no radiocarbon dates were collected directly above UNCF-3. The lithology of sediment above UNCF-3 is highly uniform across the basin and represents modern sedimentation. Late-Holocene sedimentation rates were calculated at about 0.9 mm year-1 and are roughly double the sedimentation rates in the NW Angle basin, suggesting that erosion of the southern shoreline contributes significantly to deposition in the southern basin.
  • Thumbnail Image
    Item
    Long-Term Hydrologic Sustainability of Calcareous Fens Along Glacial Lake Agassiz Beach Ridges, Northwestern Mn
    (2020-06) Budde, Nicholas
    Calcareous fens are peat-accumulating wetlands fed by calcium-rich groundwater that support several threatened species of plants that evolved to thrive in these geochemical conditions. Fifty-three of Minnesota’s nearly 300 identified calcareous fens are located in the Glacial Lake Agassiz beach ridge complex in northwestern Minnesota. Each of these fens is located immediately downslope of large sand/gravel beach ridges, where peat aprons have accumulated on the seepage face. This investigation characterizes the hydrology and landscape setting of two calcareous fens that are typical of the larger groups. Three potential sources of water to the fens are considered: groundwater from the surficial beach ridge aquifers, underlying confined aquifers, or a combination of the two influenced by seasonal hydrology. Water levels in wells in the confined aquifers, surficial beach ridge aquifers, and in and below the fens were compared with rainfall hydrographs to identify hydrologic connections. Hydrologic responses to rainfall events and associated hydraulic gradients suggest the calcareous fens are well-connected to the beach-ridge aquifer. Wells in the beach-ridge aquifers and wells in and below the fens respond synchronously to rainfall events. Water chemistry and stable isotopes are similar within the beach ridge aquifer and calcareous fens and differ significantly from water in confined aquifers. Beach ridge aquifer complexes are relatively thin (<8-10 m) and overly thick clay/clay loam till. These shallow aquifers exhibit high seasonal recharge and have permanent saturated zones, providing a continual source of water for the fens. Electrical resistivity profiles and 3D aquifer models characterized the glacial stratigraphy and highlight the well-developed physical connection between beach ridge aquifers and calcareous fens.
  • Thumbnail Image
    Item
    A Possible Late Pleistocene Impact Crater in Central North America and Its Relation to the Younger Dryas Stadial
    (2020-08-04) Tovar Rodriguez, David
    The causes that started the Younger Dryas (YD) event remain hotly debated. Studies indicate that the drainage of Lake Agassiz into the North Atlantic Ocean and south through the Mississippi River caused a considerable change in oceanic thermal currents, thus producing a decrease in global temperature. Other studies indicate that perhaps the impact of an extraterrestrial body (asteroid fragment) could have impacted the Earth 12.9 ky BP ago, triggering a series of events that caused global temperature drop. The presence of high concentrations of iridium, charcoal, fullerenes, and molten glass, considered by-products of extraterrestrial impacts, have been reported in sediments of the same age; however, there is no impact structure identified so far. In this work, the Roseau structure's geomorphological features are analyzed in detail to determine if impacted layers with plastic deformation located between hard rocks and a thin layer of water might explain the particular shape of the studied structure. Geophysical data of the study area do not show gravimetric anomalies related to a possible impact structure. One hypothesis developed on this works is related to the structure's shape might be explained by atmospheric explosions dynamics due to the disintegration of material when it comes into contact with the atmosphere. Relationship between structure's diameter (D) and deformed strata thickness (h) as well as the relationship between the diameter of the projectile (d) and the depth of the water column (H ), which is considered in this study due to the geographical considerations of the area 12.9 ky ago and BP, are consistent with an extraterrestrial event. Other hypotheses, such as lake processes and glacial processes, are difficult to reconcile with the reported observations, so the impact hypothesis and its relationship with the formation of the Roseau structure are viable.