Browsing by Author "Larson, Phillip C"
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Item Generation of a heavy-mineral glacial indicator dispersal train from a diabase sill, Nipigon region, northwestern Ontario(NRC Research Press, 2005) Larson, Phillip C; Mooers, Howard DThe heavy clinopyroxene mineral pigeonite forms a glacial indicator dispersal train originating from diabase intrusions in the Nipigon region of northwestern Ontario. Analysis and interpretation of the pigeonite dispersal pattern adjacent to the up-ice portion of the diabase provides a number of insights into the nature of glacial erosion of bedrock and the generation of heavy-mineral dispersal trains. Bedrock erosion and entrainment rates at the time of pigeonite dispersal train formation were high (3–14 mm·a–1), suggesting that bedrock erosion was rapid yet spatially and temporally restricted. Contrasting erosion rates between the diabase and surrounding greenstone lithologies suggests that modern shield topography is not an assemblage of equilibrium bedforms with respect to the ice sheet. This agrees with hypothesized low total erosion of shield bedrock during the Pleistocene. Pigeonite grain size coarsens over the diabase source, indicating that most of the pigeonite was quarried from outcrops as coarse diabase fragments. Down-ice of the diabase source the mean particle size of pigeonite recovered from till decreases, suggesting most of the pigeonite was liberated from bedrock by the comminution of coarse diabase clasts during glacial transport. While the conclusions drawn from this study may not necessarily apply to all heavy-mineral dispersal trains, the interpretive framework provides a foundation for comparative studies.Item Glacial indicator dispersal processes: a conceptual model(Taylor & Francis, 2004) Larson, Phillip C; Mooers, Howard DInterpretation of indicator dispersal trains preserved in till sheets is widely used to investigate past glacial processes and to explore for buried bedrock mineralization. We present a conceptual model of erosion and entrainment and transport of indicator material in a glacial system. Indicator concentration in an individual size fraction of till is controlled by dilution and comminution. Dilution is the result of incorporation of additional material to the glacier’s debris load down-ice of the indicator source, and is described in terms of erosivity and erosion length scale. Erosivity describes the amount of bed material eroded along a flowline, and is a function of both bed properties and the erosive power of the glacier. Erosion length scale describes the persistence of an indicator dispersal signal during transport, and controls both the maximum total indicator concentration and the eventual length of apparent dispersal. We adapt a modified batch grinding particle comminution model to describe breakdown of indicator material during transport and modification of the indicator particle size distribution. Indicator dispersal concentrations are the product of dilution and comminution processes.Item MASTODONS (Mammut americanum) AND THE LATE-GLACIAL VEGETATION OF THE EASTERN USA(Geological Society of America, 2018-11) Drazan, Jacqueline L; Mooers, Howard D; Moen, Ron; Pastor, John; Larson, Phillip C; Swartz, Jennifer A; David, Mady K; Bopray, Croix K; Jaksa, Michael P; Messer, Blake SNumerous studies of tooth plaques and remains of gut contents of have confirmed that mastodon diet was composed of woody browse species, forbs, nuts, and fruits. However, fossil gut contents also suggest that mastodon diet included significant amounts of spruce, even though spruce is a low-quality, chemically-defended food. Most extant large mammals only browse on spruce when all other food sources are exhausted, and mastodon tusk growth increments indicate that mastodons were not food limited as they moved toward extinction (Fisher, 2009). Here we review the vegetation associated with mastodon habitat from the Great Lakes to the Gulf Coast, USA, over the period 18-10 ka cal BP using pollen assemblage data from 29 sites located near proboscidean fossil remains. Pollen data were acquired from the Neotoma Database and pollen abundance was converted into species biomass abundance using the Landscape Reconstruction Algorithm (LRA) of Sugita (2004a, 2004b). Although spruce was the dominant conifer throughout the Great Lakes Region until ca. 10 ka cal BP, deciduous species such as ash, oak, and elm comprised 50% or more of the vegetation assemblages even at the earliest and northernmost sites, and remained at similar levels until mastodon extinction. Many of these species have been found in mastodon gut contents. These vegetation assemblage reconstructions support the suggestion that mastodons were not food limited as they neared extinction. Moreover, these analyses of landscapes surrounding mastodon sites strongly suggest that the contemporaneous forest, composed of large amounts of spruce intermixed with ash, elm, and oak, was unlike the forests found in much of eastern North America today.Item Regional Till Geochemical Survey of the Western Vermilion Greenstone Belt, Minnesota(University of Minnesota Duluth, 2004) Larson, Phillip CThis report presents results of a reconnaissance-scale fine-fraction (silt and clay; -250 mesh, <63μm) till geochemical survey of the western Vermilion Greenstone Belt (VGB). Survey results provide information on the background concentrations of a suite of precious, base metal, and pathfinder elements in till. The survey has identified a number of areas with elevated to anomalous precious and base metal concentrations in till associated with bedrock mineralization in the greenstone belt. The dataset provides a framework for interpretation of the significance of results of future regional and property-scale till geochemical surveys. The survey has identified a number of areas with anomalous precious and base metal concentrations. A prominent zinc anomaly (up to 266 ppm) is present down-ice of Fivemile Lake, underscoring the potential for significant undiscovered zinc mineralization in this area. Highly anomalous copper (314 ppm), gold (54 ppb), molybdenum (26 ppm), and zinc (368 ppm) are present in till in near Needleboy and Armstrong Lakes, suggesting the presence of significant mineralization in an essentially unexplored area. Anomalous gold concentrations suggest significant unidentified gold mineralization exists within the greenstone belt, most notably in the Bass Lake sequence between the Mud Creek shear zone and Vermilion Fault, where gold values up to 940 ppb were identified. Using the results of this survey as a baseline, higher density follow-up till sampling promises to more clearly define and determine the significance of anomalies, as well as determine the location of potential source rocks. Interpretation of mixing of exotic granitic with local greenstone lithologies indicate that over 50% of the fine-fraction material has been transported less than 3 km. Flow direction, transport distance, and probability sector data indicate most geochemical anomalies identified by this survey are correlative with previously identified areas with high potential for bedrock mineralization. This survey demonstrates that till composition in the VGB systematically reflects both bedrock composition and economic mineralization. Results suggest drift exploration methods in general and fine-fraction till geochemical surveys in particular, can be successfully incorporated into an integrated exploration program in the VGB. Higher density sampling is recommended to determine the significance of anomalies and more precisely define source rocks. Further work is recommended to test the applicability till geochemistry to property-scale investigations and of heavy mineral surveys.Item Sediment Transport Cycles of the Laurentide Ice Sheet(2008) Larson, Phillip C