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Document Type

Research

Keywords

Decatur County, pre-Illinoian till, clast pavements, Pleistocene stratigraphy

Abstract

The Pleistocene stratigraphy and sedimentology of two quarry exposures near Grand River and Decatur City in Decatur County, Iowa document a sequence of Pleistocene sediments overlying striated Pennsylvanian limestone which represent at least two pre-Illinoian glacial advances into the ancestral Grand River valley. Two pre-Illinoian diarnictons separated by a clast pavement were observed at the Decatur City quarry; a single diarnicton was present at the Grand River quarry. At both quarries, the diarnictons exhibit comparable lithologic properties and are genetically interpreted as basal tills. The pre-Illinoian tills are tentatively correlated with the Alburnett Formation in eastern Iowa, primarily on the basis of clay mineralogy data. Fluvial erosional and depositional processes succeeded till deposition at both quarry sites. The tills are overlain by a fining-upward fluvial sequence upon which a well developed Yarmouth-Sangamon paleosol is developed. Sangamon Soil developed upon a pebbly diarnicton overlies the fluvial sediments. The pebbly diarnicton probably originated as colluvium from pre-Illinoian tills at higher landscape positions during late Sangamonian pedimentation. Lastly, periglacial conditions during mid-to-late Wisconsinan time resulted in multiple episodes of loess deposition corresponding to, in ascending order, the Pisgah Formation, Farmdale Soil and Peoria Loess, all Wisconsinan stratigraphic units.

Publication Date

September-December 1994

Journal Title

Journal of the Iowa Academy of Science

Volume

100

Issue

3-4

First Page

65

Last Page

71

Copyright

© Copyright 1993 by the Iowa Academy of Science, Inc.

Language

EN

File Format

application/pdf