Sediment Distribution and Sedimentation Rates in the Western Arm of Lake Superior Using 3.5 Khz Seismic Reflection Profiles and 210Pb Geochronology
1985-05
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Sediment Distribution and Sedimentation Rates in the Western Arm of Lake Superior Using 3.5 Khz Seismic Reflection Profiles and 210Pb Geochronology
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1985-05
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About 700 km of high-resolution (3.5 kHz) seismic reflection profiles provided data for a comprehensive scrutiny of the late-glacial and postglacial sediment accumulation in the western arm of Lake Superior. This sub-basin of Lake Superior deserves detailed study because, while it embodies only about 5% of the lake's surface area, it receives over 60% of the lake's annual sediment load. Most of this material is derived from the "red clay area" of Wisconsin. Five gravity cores were analysed for 210Pb geochronolgy. These data were used to calculate modern sedimentation rates and were incorporated into the calculation of a modern sediment budget for the sub-basin. A seismic facies map was constructed from data acquired on two University of Minnesota cruises. Based on acoustic signature, profiles were denoted as (I) having a simple prolonged bottom reflector; (II) having a thin (< 10 meter) transparent horizon corresponding to lacustrine sediment over glacial till (the acoustic basement); (III) displaying a thicker (> 10 meter) transparent horizon with numerous internal reflectors. Type (IV) profiles are slumped sediment sections located on the flat basin plain, and are interpreted as zones of sediment failure induced by isostatic tectonism. Type (V) profiles are slumped zones located on the steeper slopes of the north shore trough and are the result of failure of unstable saturated sediment and/or tectonism during postglacial isostatic rebound. A fine-grained lacustrine sediment isopach map reveals maximum sediment thicknesses of about 27 meters in the deeper parts of the north shore trough. Relict morpho-sedimentary features identified as a series of submerged wave-cut terraces along the Wisconsin shore and as a buried bay-head bar near Duluth are the first tangible evidence for the low stage Glacial Lake Minong in the western Superior basin. Sedimentation rates from 210Pb geochronology utilizing the constant initial concentration method vary between 0.26 and 0.026 cm/year in the western arm. Pollen analysis of one core yields a rate comparable to the 210Pb-derived sedimentation rate. Using published sources and sedimentation rate data from this study, a sediment budget for the western arm estimates that 2.9 x 106 metric tons of sediment is input to the basin annually. Outputs through sedimentation amount to about 0.7 x 106 metric tons per year with the remaining 75% of the material being transported out of the arm. Most of this transfer takes place in the spring and fall when the lake and shoreline are ice-free, and when the water column is isothermal.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Christopher Alfred Scholz in partial fulfillment of the requirements for the degree of Master of Science, May 1985.
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