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The Quaternary of Minnesota

Chapter in book
Authors Carrie Jennings
Mark D. Johnson
Published in In J. Ehlers, P.L. Gibbard and P.D. Hughes, editors: Developments in Quaternary Science, Vol. 15, Amsterdam, The Netherlands, 2011, © Copyright 2011 Elsevier B.V.
Pages 499-511
ISBN 978-0-444-53447-7
Publisher Elsevier
Place of publication Amsterdam
Publication year 2011
Published at Department of Earth Sciences
Pages 499-511
Language en
Keywords Quaternaru glacial geology stratigraphy Minnesota USA
Subject categories Earth and Related Environmental Sciences, History of geology and palaeontology, Quaternary geology

Abstract

The rich and complex sedimentary record of glaciation in Minnesota includes deposits of glaciers, associated rivers and lakes, as well as windblown and other periglacial deposits that span the Quaternary Period and are at the surface in nearly every part of the state. The thickness of these deposits is commonly more than 100 m. One of the thickest, and perhaps most stratigraphically complete, records of the Quaternary in Minnesota lies beneath the Coteau des Prairies, an inter-ice stream sediment highland that spans the Minnesota–South Dakota border where over 300-m of fine-grained diamictons are preserved (Fig. 38.1). Though the glacial deposits of the state are dominated by a complex Late Wisconsinan history (marine isotope stage (MIS) 2), Minnesota has many lithostratigraphical units from the Middle and likely Early Pleistocene. For example, Minnesota has units older than the Sangamon Geosol (>125 ka), units older than volcanic ashes derived from Yellowstone (>610 ka) (Boellstorff, 1978), as well as magnetically reversed units from prior to the Brunhes– Matayuma boundary (>788 ka). Recent cosmogenic burial dating of glacigenic sediment (Balco et al., 2005) indicates that numerous glacial stratigraphical units were deposited prior to MIS 14. A rare bedrock exposure in the southern part of the Coteau des Prairie highland was striated as early as 640–740 ka (MIS 16 or 14) based on cosmogenic exposure dating of the quartzite using a paired-isotope system (Bierman et al., 1999). A stack of 12 tills surrounding this isolated bedrock high is therefore most likely a record of glaciation prior to and including MIS 14–16. Ice streams that were active primarily during MIS 2 focused erosion on either side of the Coteau des Prairie leaving it as a remnant between broad erosional unconformities (Fig. 38.1). The southeastern corner of Minnesota was also glaciated many times early in the Quaternary Period but remained ice-free duringMIS2–4, during which time it was affected by strong, northwesterly, periglacial winds and permafrost (Zanner, 1999). Thus, the earlier record of glaciation of this part of Minnesota is obscured and in places, confined to sinkholes and caves (e.g. Milske et al., 1983). The southern margin of the Late Wisconsinan (MIS 2) Laurentide ice sheet produced many dynamic ice protuberances or lobes that emanated from discrete ice-source areas (Fig. 38.2). Some of the tributary ice sheds had distinctive bedrock geology allowing the provenance of the ice as well as the evolution of ice sheds to be discerned. This condition has produced distinct lithologic compositions for the tills derived from different ice centres and has provided a basis for differentiating and formalising lithostratigraphical units (Johnson et al., in preparation). Four broad source regions have been identified and their characteristics are shown in Table 38.1 and Fig. 38.3. Minnesota’s pre-MIS 2 till units share the same broadly defined provenance regions indicating that older glaciations had similar sources areas. However, the shape of the former ice margins is much more difficult to determine from the scattered subsurface information and therefore ice dynamics more difficult to infer. Where the pre-MIS 2 ice limits are at the surface, their breadth and more southerly extent suggest that at the very least, the ice lobes were broader. It remains possible that the ice dynamics were substantially different at times in the past and did not lead to the creation of ice streams and lobes, for example, during the ice-sheet-build-up phase of each glaciation and prior to the Middle Pleistocene transition (when the frequency of glaciation and volume of changed from 41,000-year periodicity to

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