Browsing by Subject "Geology"

Now showing 1 - 20 of 61
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    Anthropogenically-intensified erosion in incising river systems.
    (2012-07) Day, Stephanie S.
    Anthropogenic alterations to landscapes, such as agriculture, can lead to accelerated erosion in incising (or incised) rivers threatening infrastructure and property, and causing unnaturally high sediment loads, which threaten ecosystems. Steep bluffs and ravines are characteristic landforms in incising river systems, and by understanding erosion on these landforms we can begin to mitigate the impacts of the altered landscape. The Le Sueur River watershed, in southern Minnesota, provides an ideal location for studying the impacts of agricultural land-use on erosion in an incising river. Agriculture in this watershed is made possible through the use of tile drains, which remove water from the uplands and route it directly into ravines or the river, reducing the water that pools on the landscape and increasing flows in the river. Using terrestrial laser scanning (TLS) and aerial photographs, bluff erosion rates in the Le Sueur River were measured and used to develop a watershed scale sediment budget. In the Le Sueur River bluffs account for 56 ± 12% of the 2000-2010 average measured total suspended solids load. These data were also used to interpret how changes in land-use and climate have accelerated bluff retreat in this watershed. The data collected paired with field observations show that over-steepening at the bluff toe drives bluff retreat, yet weakening due to groundwater seeps and freeze-thaw also contributes to erosion. The increases in flow rates and volumes brought on by tile drainage in this watershed have resulted in increased bluff erosion. To study ravine response to changing overland flow hydrology, brought on by tile drainage, small physical experiments were used to measure how changing the delivery rate of a fixed volume of water impacts erosion. The results of these experiments showed that regardless of flow rate the volume of sediment removed remained the same, suggesting that the tile drains installed in the Le Sueur River watershed may have decreased ravine growth. Results of each of these projects independently improves our understanding of bluff erosion and ravine growth processes, yet combined they provide insight into how changing hydrology impacts erosion throughout an incising watershed. While agricultural landscape alterations, especially tile drains, have decreased ravine growth they have resulted in increased bluff erosion. Because bluffs in the Le Sueur River watershed account for more than half of the total sediment load, there is a net increase in sediment loads as a result of anthropogenic landscape alterations.
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    Application of the Soil and Water Assessment Tool (SWAT) to the Willow River Watershed, St. Croix County, Wisconsin.
    (2010-12) Murphy, Marylee Smith
    Identifying critical source areas of sediment and phosphorus nonpoint pollutant loads under alternative land use scenarios is aided by the use of hydrologic models. We applied the Soil and Water Assessment Tool (SWAT) to the Willow River watershed in St. Croix County, Wisconsin, to examine the effects of possible future scenarios and best management practices. The model was calibrated and validated to water year (WY) 1999 and WY2006 datasets, respectively, with land use configured for each year. The model performed well in calibration, but could not simulate conditions outside of the calibration conditions in the validation dataset. Sediment and phosphorus trapping was influenced by trapping between the landscape source and the watershed outlet in closed-drainage lakes, flow-through wetlands, and on-stream reservoirs. The relative contributions of pollutants were related to the landscape position of the source area and the number and trapping efficiency of the intermediate traps in the flow path. We simulated best management practices including lowered soil-test phosphorus, increased conservation tillage, lowered cattle dietary phosphorus, and changed agricultural crop rotations. Simulations demonstrated that conversion to mulch tillage and no-till from conventional tillage could reduce sediment yield on the converted lands by 3% to 27% and phosphorus yield by 5% to 21%. For the current mix of agricultural land management in the Willow Watershed, converting all cropland to mulch tillage would reduce watershed export of phosphorus by 1% and sediment export by 1%. Converting all of the agricultural land to no-till produced a modeled decrease in watershed export of sediment of 2% and a decrease in phosphorus of 7%. Simulations also demonstrated a 22% reduction watershed phosphorus export by reducing average agricultural soil-test phosphorus to 20 ppm. Converting all farm land from a mixture of cash grain rotations to a dairy rotation that included two years of corn and three years of alfalfa caused a modeled reduction in watershed phosphorus export of 15% and a modeled reduction in sediment export of iii 5%. Continued conversion of agricultural land to rural residential land uses produced lower modeled loads of watershed sediment export up to 13% and phosphorus export up to 27% depending on the area developed and the average lot size. Changes in point source phosphorus because of better wastewater treatment caused a decrease in modeled phosphorus delivery of 13% between the calibration and validation time periods. Alternative climate scenarios were also simulated, showing that evapotranspiration was the driver of the altered hydrologic cycle, and thus the driver of reduced sediment and phosphorus export.
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    C-29 Geologic Atlas of Clay County, Minnesota [Part A]
    (Minnesota Geological Survey, 2014) Bauer, Emily J.
    Surface and subsurface geology of Clay County, Mn., also includes data-base information, subsurface stratigraphy, bedrock topography, depth-to-bedrock and sand distribution models.
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    C-35, Geologic Atlas of Meeker County, Minnesota [Part A]
    (Minnesota Geological Survey, 2015) Meyer, Gary N.
    A County Geologic Atlas project is a study of a county's geology, and its mineral and ground-water resources. The information collected during the project is used to develop maps, data-base files, and reports. This same information is also produced as digital files for use with computers. The map information is formatted as geographic information system (GIS) files with associated data bases. The maps and reports are also reproduced as portable document files (PDFs) that can be opened on virtually any computer using the free Acrobat Reader from Adobe.com.
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    C-38, Geologic Atlas of Kanabec County, Minnesota
    (Minnesota Geological Survey, 2016) Boerboom, Terrence J.
    Surface and bedrock geology of Kanabec County, Mn., also includes data-base information, bedrock topography, depth to bedrock in GIS and image formats, as well subsurface interpretations of sand distribution between the land and bedrock surfaces.
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    Cambrian Sedimentation in the Bighorn Mountains
    (1920) Tieje, Arthur J.
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    Changes in seasonal precipitation of East Central North America with connections to global climate.
    (2010-10) Hardt, Benjamin Fulper
    Research on oxygen isotopes in stalagmites collected in West Virginia caves has yielded several new insights into regional climate. Oxygen isotopes most likely represent changes in the mean annual isotopic composition of precipitation, a parameter determined locally by the seasonal distribution of precipitation (Hardt et al., 2010). Holocene samples indicate that summer precipitation represented a greater proportion of annual totals, consistent with lake level results (Shuman and Donnelly, 2006) and climate models (Braconnot et al., 2007; Diffenbaugh et al., 2006). During the Pleistocene, seasonal precipitation varies on precessional timescales, although the phasing appears unusual in that it is in-phase with September insolation rather than June. This offset could be due to several processes, most likely in conjunction with each other. These processes include: changes in Gulf of Mexico sea surface temperatures, partially modulated by glacial meltwater routed through the Mississippi (Oglesby et al., 1989; H Wang et al., 2010); changes in the mean state of the tropical Pacific ocean between to El Nino- and La Nina-like conditions (Timmermann et al., 2007); and a seasonally-lagged sea surface temperature response in the subtropical North Atlantic, which would enhance the anticyclonic circulation of the Bermuda High. These same processes also appear to influence isotopic behavior over the last glacial Termination. During the last glacial maximum, oxygen isotopic composition is enriched, consistent with model results indicating wetter summers near the southern margin of the ice sheet (Bromwich et al., 2005), but inconsistent with a temperature control. Antiphasing in summer precipitation between West Virginia and Florida support a control on seasonal precipitation by changes in the position or size of the Bermuda High (Donders and de Boer et al., 2009). Comparison with speleothems from Northeast Brazil (Cruz et al, 2009) indicates a role for the El Nino Southern Oscillation due to its effect on the position of the Bermuda High (Seager et al., 2005).
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    Characterization of Asian monsoon variability since the penultimate interglacial on orbital and sub-orbital timescales, Dongge Cave, China.
    (2010-01) Kelly, Megan Jean
    Dongge Cave, located in southern China, is ideally suited for studies of past climate. When calcite is deposited under equilibrium conditions, oxygen isotope ratios recorded by Dongge Cave stalagmites may be considered a proxy for the intensity of Asian Monsoon precipitation. Ages of five stalagmites from Dongge Cave were obtained using uranium-thorium dating methods, providing a continuous record of Asian Monsoon variability between 50 and 250 ky BP, covering much of the last three glacial-interglacial cycles. Confirming previous indications, the Asian Monsoon record closely follows Northern Hemisphere summer insolation over orbital timescales, and is punctuated by millennial-scale strong monsoon events that correspond with abrupt Greenland warming events during the last glacial period. Conventional theory suggests that glacial-interglacial climate shifts are most directly related to variations in high northern latitude summer insolation. A critical relationship between Asian Monsoon intensity and atmospheric methane concentration at the last three glacial terminations was established that enables the correlation of the Asian Monsoon oxygen isotope record to ice core records in both hemispheres. This correlation strategy sheds light on the timing and sequence of events surrounding last three glacial terminations, demonstrating that these climate shifts occurred in two phases, with much of the marine termination, the full rise in atmospheric carbon dioxide and Antarctic temperature, and initial rise in atmospheric methane occurring prior to monsoon strengthening and presumed Greenland temperature rise. The two-phased pattern indicates that Northern Hemisphere summer insolation plays a key role initiating climate changes at major deglaciations, but feedbacks internal to the climate system must also be involved. Similar correlation strategies were utilized to study abrupt, millennial-scale climate variability during the last two glacial periods. A link between speleothem records of Asian Monsoon intensity and Greenland temperature is clearly demonstrated across the last glacial period. The North Atlantic/Greenland climate signal is propagated to the low-latitude Asian Monsoon region through oceanic and atmospheric processes related to the position of the Intertropical Convergence Zone. While Greenland ice core records are not available during the penultimate glacial, the larger, longer duriation Asian Monsoon events can be linked to Antarctic temperature maximum, continuing the trend seen during the last glacial cycle and suggesting that the thermal bipolar seesaw mechanism persisted into the older climate cycle. Thus, mechanisms of millennial climate variability appear to be similar during the last two glacial periods.
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    Climatic and anthropogenic influences on aquatic ecosystems in the valley of the Great Lakes, Mongolia.
    (2008-12) Shinneman, Avery Lynn Cook
    Climate warming and major land use changes have profoundly affected the Mongolian landscape in the past several decades. As in many arid and semi-arid regions, water resources are critically important for ecological, social, and economic viability. In Mongolia, traditional semi-nomadic pastoralism contributes substantially to the national economy as well as to individual subsistence and depends on limited freshwater resources to provide for grazing herds and human needs. Yet, because of substantial variability across this immense region, its remoteness, and recent political transitions, little work has been done to monitor water quality or to set baseline standards against which to measure future changes. Compounding the problem is a lack of well-resolved paleo-ecological and paleo-climatological work. These data are necessary to provide a foundation for understanding the natural variability in the aquatic systems of the region, especially with recent changes in climate and land use. This work is a contribution to developing these records by first, developing diatom-based inference models for total phosphorus and salinity, and second, applying the models to investigate lacustrine sediment records of past changes. The diatom-based inference models were based on a survey of the water chemistry, physical characteristics, and diatom flora of 64 lakes in western Mongolia. The region had a diverse diatom flora with over 300 species, nearly 100 of which had not been previously reported from Mongolia, from lakes ranging from fresh to hypersaline. The many isolated lake basins provided unique ecosystems where multiple unique communities, novel species distributions, and new and endemic flora were found. Three of these new species are described here in a careful examination of the genus Cyclotella in western Mongolia. Canonical correspondence analysis (CCA) was used to identify four variables (specific conductivity, total phosphorus, bicarbonate, and lake morphology) that were significantly related to the distribution of diatoms; predictive models were developed for specific conductivity and total phosphorus using weighted averaging regression and calibration methods. The application of these models to dated lake sediment cores, along with interpretations of other geochemical and sediment characteristics, was then used to develop records of variability in lake salinity and nutrient flux. The interpretation of diatom and sedimentary records demonstrated increases in nutrient fluxes to the lakes related to climate warming and major changes in land-use over the last 20 years. Diatom-inferred lake salinity was correlated with changes in temperature over the past 2000 years, as inferred from tree-ring records, demonstrating a positive relationship between increased warming and increased lake salinity in recent geologic history. Changes in warm-season temperature, as inferred from tree-rings, in the most recent decades were less-well correlated with inferred changes in salinity than over most of the 2000 year record. However, instrumental records of winter temperature were well correlated with recent shifts in inferred salinity, perhaps suggesting recent changes in climate that are unique from those over the past several thousand years.
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    Determination of small melt fraction peridotite partial melts using re foil melt traps
    (2011-03) Davis, Frederick Arthur
    The global record of trace element and isotopic compositions of ocean island basalts (OIB) suggest that they are derived from several distinct reservoirs in the mantle. Major and minor element compositions of OIB also span a relatively large range of compositions when compared with the more nearly uniform basaltic compositions erupted at mid-ocean ridges. This may be due to distinct melting lithologies or conditions of melting beneath different oceanic islands. The upper mantle is widely believed to be composed primarily of peridotite, which likely contributes to the formation of OIB. Melting at most OIB localities is confined to depths beneath the rigid oceanic lithosphere (~100 km) at pressures ~3 GPa where garnet is the stable aluminous phase, and the high incompatible element concentrations in these basalts suggest that melting is limited to low melt fractions. Thus, identifying the composition of low melt fraction partial melts of garnet peridotite at 3 GPa is key to understanding the genesis of OIB. Low melt fraction partial melts of garnet peridotite were produced experimentally at 3 GPa in a piston cylinder apparatus using a new Re-foil melt trap. Melt traps allow partial melts to be segregated from crystalline residues, preventing changes in the compositions of small melt pools that can result from crystallization onto nearby mineral grains while the experiment is quenching. Re-foil melt traps are similar to diamond aggregate and vitreous carbon sphere traps, but rely upon the wetting properties of Re as well as pressure gradients to attract the melt phase. Re traps are easier to polish than diamond traps and, unlike vitreous carbon spheres, are capable of withstanding high temperatures required for peridotite partial melting at 3 GPa Partial melting experiments were performed in Pt/C capsules containing natural peridotite powder, KLB-1, or an oxide mixture approaching the composition of KLB-1. Experiments were performed from 1470-1530 °C and produced melt fractions between 0.059 and 0.235. In contrast with previous peridotite melting studies at 3 GPa, melts in the lowest melt fraction experiments coexisted with garnet. Resulting melt compositions are similar to those found by other peridotite partial melting studies for many components, including: TiO2, Cr2O3, MnO, MgO, and Na2O; however, there are relatively large differences in FeO* and CaO from the melt compositions of Walter (1998). FeO* in Re trap experiments varies from 6.6-7.9%, considerably lower than the Walter (1998) melts which are never lower than 8.8% FeO* at 3 GPa, and CaO varies from 11.4-14.3%, significantly higher than melts in Walter (1998) that vary from 7.7- 10.9%. The Re trap method successfully produces large melt pools that can be analyzed by electron microprobe at melt fractions as low as 0.05, although this may be about the lower limit of its effectiveness. The disagreements with previous experiments in CaO and FeO* contents are likely the result of the heterogeneous character of the melt pools and cast doubt on whether the measured compositions of melts produced in these experiments are true low-degree peridotite melt compositions.
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    The dynamics of migmatite domes in extending orogens.
    (2009-06) Kruckenberg, Seth Caley
    Two migmatite domes, the Okanogan dome (Washington state) and the Naxos dome (Cyclades, Greece) record the dynamics of migmatite dome formation in extending orogens. Both domes are exposed in cordilleran-style metamorphic core complexes that developed during extension of previously thickened crust and contain a core of anatectic migmatite mantled by metasedimentary rocks and exposed below crustal detachments. Structural observations in both domes indicate that deformation in dome migmatites took place in the presence of melt and flow within the dome migmatites was coeval with detachment faulting, upper crustal extension, and migmatite dome formation. In the Okanogan dome, the timing of partial melting is constrained by U-Pb sensitive high-resolution ion microprobe (SHRIMP) dating of zircon, monazite, and titanite. Results of the U-Pb analysis document Paleocene-Eocene ages of partial melting and migmatite crystallization in the Okanogan dome based on a range of 206Pb/238U ages from ca. 61-49 Ma for zircon and monazite in migmatite leucosomes of various structural settings, interpreted as the timing of migmatite crystallization. Similar ages of migmatite crystallization are reported in other domes of the northern Cordillera, suggesting that partial melting was widespread in this region during Paleocene to Eocene time. The conditions of metamorphism in the Okanogan dome associated with migmatization are constrained by mineral assemblages and reaction textures preserved in sapphirine- and corundum-bearing orthoamphibole-cordierite gneiss layers that structurally overlie the Okanogan migmatite domain. Petrologic and pseudosection analyses of these Mg-Al-rich layers suggest conditions of metamorphism reached ~ 720-750 ºC at moderate pressure (ca. 4 kbar). An earlier, high-pressure metamorphic history is recorded by relict kyanite pseudomorphed by symplectite and coronal reaction textures. A minimum of 4 kbar of decompression is suggested by the calculated conditions of metamorphism at relatively shallow crustal levels (~13 km) calculated conditions of metamorphism at relatively shallow crustal levels (~13 km) simpler pattern of deformation developed during viscous flow. At the scale of the Naxos dome, lineations obtained by AMS are highly variable in plunge with steeply plunging lineations commonly associated with the cores of subdomes or adjacent synforms and more shallowly plunging lineations elsewhere. The combination of structural and magnetic observations in the Naxos dome suggest that the combined effects of upwelling of low-viscosity migmatites under dominantly NNE-SSW extension, E-W compression, and top-to-the-north shearing likely contributed to the structural pattern recorded in the Naxos dome migmatites. The presence of subdomes within the migmatitic core of the Naxos dome, and discordant structural relationships between the migmatitic foliation and that in the mantling units at some localities in the dome, suggest that the internal dynamics of the migmatites (i.e., buoyancy, convection) were likely significant in the formation of the Naxos dome and subdomes. Multidisciplinary study of the Okanogan and Naxos domes emphasize the role of partial melting and flow of migmatites in the formation of domes developed during extension of previously thickened crust. Observations in both domes suggest that partial melting and flow within the migmatites was kinematically consistent with regional patterns of extension reflecting a component of lateral flow during their formation. The results of metamorphic petrology in the Okanogan dome, and the combined structural and magnetic analysis of the Naxos dome, further suggest that vertical flow of partially molten crust is also an important process in the formation of migmatite domes (and associated subdomes) in extending orogens.
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    The effect of water on partial melting in the upper mantle.
    (2010-06) Tenner, Travis Jay
    This thesis presents experimental constraints on incipient melting of mantle peridotite under hydrated conditions. High P-T experiments were performed at pressures of 3 to 13 GPa, and at temperatures of 1200-1450°C. These experiments measure mineral/melt H 2 O partitioning and storage capacity of peridotite components, as well as determine melting phase relations and the compositions of partial melts and residues of hydrated peridotite. Incipient melt H 2 O concentrations are estimated by peridotite/melt H 2 O partitioning ([Special characters omitted.] ). To parameterize [Special characters omitted.] , mineral/melt H 2 O partition coefficients were determined for all crystalline phases of the peridotite solidus assemblage (Chapter 2). Combining these [Special characters omitted.] values with corresponding modal abundances along the solidus yields a [Special characters omitted.] of 0.005-0.010 from 1 to 5 GPa, which is dependent on pressure due to varying garnet and pyroxene modal abundances, and to variable pyroxene Al content. This [Special characters omitted.] range predicts that incipient melts of MORB source (50-200 ppm H 2 O bulk ) and OIB source (300-1000 ppm H 2 O bulk ) upper mantle contain 0.5-3.8 wt.% and 3-20 wt.% dissolved H 2 O, respectively. The amount of dissolved H 2 O in incipient melt dictates hydrous solidus depression, Δ T , which ultimately controls the stability of hydrous melts at P and T . This Δ T -H 2 O melt relationship was investigated at 3.5 GPa by partially melting hydrated peridotite from 1200-1450°C (Chapter 3). Mass balance of phases allows for determination of melt fractions ( F ) from experiments, as well as estimation of H 2 O melt . Δ T values are quantified as the difference in melting temperature between dry and wet peridotite at a particular F . Parameterization of Δ T as a function of H 2 O melt predicts that solidus melts with 1.5, 5, 10, and 15 wt.% dissolved H 2 O generate Δ T values of 50, 150, 250, and 300°C, respectively. Combination of this paramterization with [Special characters omitted.] (Chapter 2) insinuates that 500 ppm H 2 O bulk is necessary to stabilize melt across the observed seismic low velocity zone (LVZ) beneath oceanic lithosphere, which is significantly greater than the MORB source upper mantle H 2 O bulk of 50-200 ppm. This observation argues against suggestions that hydrous melting is solely responsible for the LVZ. At higher pressures the aforementioned parameterizations are difficult to constrain experimentally, but the onset of hydrous melting can be determined by the peridotite H 2 O storage capacity, defined as the maximum H 2 O concentration that peridotite can store without stabilizing a hydrous fluid or melt. A new method of determining a minerals H 2 O storage capacity is employed, in which a hydrated monomineralic layer is equilibrated with a layer of hydrated peridotite and a small amount of melt (Chapter 4). Experiments were carried out at conditions near the 410 km transition zone (TZ) depth to investigate hydrous melting due to the H 2 O storage capacity contrast between the TZ and upper mantle. Measured olivine and orthopyroxene H 2 O storage capacities, combined with estimates of garnet H 2 O storage capacity, and P -dependent lherzolite modes, yields a peridotite H 2 O storage capacity of 700-1100 ppm directly above 410 km. This is not consistent with pervasive melting above 410 km, as this range is several times greater than MORB source upper mantle H 2 O bulk . However, regional melting in areas such as H 2 O-rich OIB source, or areas of recent subduction may likely occur, leaving residues with ∼1000 ppm H 2 O bulk .
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    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.
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    The Geology of Minnesota, Vol. 2 of the Final Report
    (Minnesota Geological Survey, 1888) Winchell, N.H.; Upham, Warren
    Summary of the geological and natural history studies carried out in Minnesota between 1872 and 1901. Published in 6 volumes. Vol 2 summarizes geologic and natural history information by county between 1882-1885.
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    Geology of the St. Croix Dalles
    (1897) Berkey, Charles P.
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    GEOPHYSICAL AND PETROLOGIC INVESTIGATIONS OF INVERSELY CORRELATED AEROMAGNETIC AND BOUGUER GRAVITY ANOMALIES WITHIN PRECAMBRIAN GNEISS TERRANE NEAR GARVIN, SOUTHWESTERN MINNESOTA
    (Minnesota Geological Survey, 2023) Southwick, David L; Chandler, V.W.; McSwiggen, Peter L
    Geophysical models and petrologic inferences presented here are consistent with a buried gabbroic to noritic intrusion as the principal source of two geographically coincident, kilometer-scale, inversely correlated gravity and magnetic anomalies (the Garvin anomalies) located within Archean continental crust near the present-day southern margin of the Superior Craton in Minnesota. Two-dimensional profiles modeled from the total magnetic anomaly, reduced to pole, and the Bouguer gravity anomaly, upward-continued to 2 kilometers, are fit to geologically reasonable distributions of source rocks that have density and magnetic susceptibility values within the ranges reported for gabbro or norite intrusions.