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Abstract
Soils on many of the outwash plains in southwestern Michigan have loamy upper profiles, despite being underlain by sand-textured outwash. The origin of this upper, loamy material has long been unknown. The purpose of this study is to analyze the spatio-textural characteristics of these loamy-textured sediments to ascertain their origin(s). The textural curves of this material have distinct bimodality, with clear silt and sand peaks. Because the sand peaks align with those in the outwash below, we conclude that the upper material is a mixture of an initially silty material with the sand from below, forming loamy textures. By applying a textural filtering operation to the data, we determined its original characteristics; nearly all of the soils originally had silt loam upper profiles, typical for loess. Field data showed that the loamy material is thickest east of a broad, north–south trending valley (the Niles-Thornapple Spillway) that once carried glacial meltwater. The material becomes thinner, generally better sorted, and finer in texture eastward, away from this channel. We conclude that the loamy mantle on many of the adjacent outwash plains is silt-rich loess, derived from the Niles-Thornapple Spillway and its tributary channels and transported on mainly westerly winds. The spillway was active between ca. 17.3 and 16.8 k cal. years ago. At this time, a large network of tunnel channels existed beneath the stagnant Saginaw lobe ice. Meltwater from the lobe funneled silt-rich sediment into the spillway, rendering it a prodigious silt source.
密西根西南方诸多冰川沉积平原的土壤, 儘管位于砂质冰川沉积平原下方, 但仍具有上层壤土剖面。此一上层壤土物质的来源, 长久以来不为人知。本研究的目的, 便在于分析这些壤土质地沉积物的空间—结构特徵, 以确认其来源。此一物质的结构曲线, 具有清晰的粉砂与砂两峰的特殊双模态。由于砂峰与位于下方冰川沉积平原之物一致, 我们因此断定, 上层物质是原本的粉砂物质和下方砂的混合物, 组成了壤土结构。我们对数据进行结构过滤, 确认其原本的特徵; 近乎所有的土壤原本皆具有粉砂壤土的上层剖面, 并以黄土为代表。田野数据显示, 壤土物质在曾经运送冰川融水的宽广南北向河谷 (耐士—曼陀罗溢洪道) 中的东边最为肥沃。该物质越往东远离此一渠道, 便愈益稀少、普遍更佳地区分, 且质地更为细緻。我们于结论中主张, 诸多邻近的冰川沉积平原的壤土覆盖物, 是粉砂丰富的黄土, 该物质起源于耐士—曼陀罗溢洪道及其支流水道, 并主要由西风带所运送。该溢洪道约在 17.3 和 16.8 千公曆年间活跃。于此期间, 大范围网络的地下河道, 存在于停滞的赛基诺冰舌之下。来自冰舌的冰川融水, 将富含粉砂的沉积物注入溢洪道中, 使其成为庞大的粉砂来源。
Los suelos de muchas de las planicies de acarreo fluvioglacial [outwash] en el sudoeste de Michigan tienen perfiles superiores arcillosos, a pesar de estar superpuestos sobre depósitos de textura arenosa. El origen de este material arcilloso superior se desconoce. El propósito de este estudio es analizar las características espacio-texturales de estos sedimentos de textura arcillosa para determinar su(s) origen(es). Las curvas de textura de este material tienen marcada bimodalidad, con claros picos de limo y arena. Debido a que los picos de arena se alinean con los del material outwash subyacente, concluimos que el material superior es una mezcla de un material inicialmente limoso con la arena de abajo, formando texturas limosas. Aplicando una operación de tamizado textural a los datos, determinamos sus características originales; casi todos los suelos tuvieron originalmente perfiles superiores de limo arcilloso, lo cual es típico del loess. Los datos de campo mostraron que el material arcilloso es del máximo espesor al este de un vasto valle de orientación norte-sur (el Desagüe Niles-Thornapple), que alguna vez fue recorrido por agua de deshielo glacial. El material se hace más delgado, generalmente mejor clasificado y de estructura más fina, hacia el este, lejos de aquel canal. Concluimos que el manto arcilloso en muchas de las planicies outwash adyacentes es de loess rico en limo, derivado del Desagüe Niles-Thornapple y sus canales tributarios, transportado hasta allí por los vientos oestes. El desagüe estuvo activo entre ca. 17.3 y 16.8 k cal. años atrás. En ese tiempo existió una gran red canales en forma de túneles por debajo del lóbulo de hielo estancado de Saginaw. El agua de deshielo del lóbulo hacía pasar sedimento rico en limo hasta el desagüe, convirtiendo así a este cauce en prodigiosa fuente de limo.
Acknowledgments
This research was conducted as a part of a graduate seminar in Geography at Michigan State University (MSU). We thank the MSU Department of Geography for travel and logistical support. We also thank the several landowners and operators who kindly allowed us access to their land for soil excavation work: Tom Lutz, Dave and Stacey High, and Rob and Laurie Belson. Brad Miller assisted with data analysis and helped ensure that the filtering and QC algorithms were up to date. We also thank Alex Shackleton, who did much of the early, pioneering work on the loess of Berrien County. Reviews of this article by NRCS personnel working in Michigan, particularly Martin Rosek and Matt Bromley, helped strengthen the article and widen its application. We also thank Alan Kehew for reviewing a previous draft of the article and for providing helpful comments. Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement of the U.S. Government.
Funding
Alex Shackleton's work was supported by grant BCS-0850593 made to Randall Schaetzl by the National Science Foundation (NSF), GSS Program. Any opinions, findings, and conclusions or recommendations expressed are, however, those of the authors and do not necessarily reflect the views of the NSF.
Additional information
Notes on contributors
Michael D. Luehmann
MICHAEL D. LUEHMANN holds a Geography PhD from Michigan State and is currently a GIS analyst for Atwell, LLC, a land development and environmental consulting firm in Southfield, MI 48076. E-mail: [email protected]. His position at Atwell involves cartographic design and analyzing both geologic and cultural information within a GIS environment.
Brad G. Peter
BRAD G. PETER is a graduate student in the Department of Geography at Michigan State University, East Lansing, MI 48823. E-mail: [email protected]. His research interests include GIS, remote sensing, cartography, and spatiotemporal niche modeling.
Christopher B. Connallon
CHRISTOPHER B. CONNALLON is a recent graduate of the Master's program in the Department of Geography at Michigan State University, East Lansing, MI 48824. E-mail: [email protected]. His research interests include surficial geology, geomorphology, and soils.
Randall J. Schaetzl
RANDALL J. SCHAETZL is a Professor in the Department of Geography at Michigan State University, East Lansing, MI 48824. E-mail: [email protected]. His research interests include soils, landforms, and their coevolution throughout the Quaternary.
Samuel J. Smidt
SAMUEL J. SMIDT is a PhD student in the Department of Geological Sciences at Michigan State University, East Lansing, MI 48824. E-mail: [email protected]. His research interests include water transport, land use, and environmental policy.
Wei Liu
WEI LIU is a PhD student in the Department of Geography at Michigan State University, East Lansing, MI 48824. E-mail: [email protected]. Her research interests include spatial analysis, coupled human–environment modeling, and agent-based modeling.
Kevin A. Kincare
KEVIN A. KINCARE is a Research Geologist for the U.S. Geological Survey at Manistee, MI 49660. E-mail: [email protected]. His research interests include glacial geomorphology and sedimentology of the Great Lakes region.
Toni A. Walkowiak
TONI A. WALKOWIAK is a graduate student in the Department of Geography at Michigan State University, East Lansing, MI 48824. E-mail: [email protected]. Her research interests include spatial analysis with an emphasis on human–environmental biomimicry.
Elin Thorlund
ELIN THORLUND is a recent graduate in the Department of Geography at Michigan State University, East Lansing, MI 48824. E-mail: [email protected]. Her research interests include sustainability, human–environment interactions, and climate change.
Marie S. Holler
MARIE S. HOLLER is a graduate student in the Department of Urban Planning at the University of Sheffield, Sheffield, South Yorkshire S10 2TN, UK. E-mail: [email protected]. Her research interests include spatial analysis, GIS, and natural resource management.