A NEW DIGITAL BEDROCK GEOLOGIC MAP FOR NORTHEAST IOWA:   PHASE 2 (OF 4) IN THE PRODUCTION OF A NEW STATEWIDE MAP

by
R.R. Anderson, B.J. Witzke, B.J. Bunker, G.A. Ludvigson, R.M. McKay, and J.D. Giglierano

The Geological Society of America
33rd Annual North-Central Section Meeting
Clarion Hotel and Convention Center, Champaign, IL, April 22-23, 1999
1999 Abstracts with Programs, v. 31, no. 5, p. A-1

ABSTRACT


The new bedrock geologic map of northeast Iowa, the second phase of a four-part program to produce a new, digital bedrock geologic map of Iowa, was compiled with assistance from the USGS STATEMAP Program.  The mapping area incorporates a significant portion of the classic Upper Mississippi Valley Paleozoic outcrop belt in Iowa.  The region is covered by a variably thick mantle of Quaternary material.  The southern and western portions of the region include a substantial cover of Pleistocene glacial deposits, burying much of the bedrock surface.  Our understanding of the bedrock geology in this area is derived from study of numerous well records and scattered bedrock exposures found primarily along stream courses and in quarries.  By contrast, glacial deposits are largely absent across much of the eastern and northern portions of the map area, which displays numerous exposures of Cambrian and Ordovician strata.  Neogene erosional episodes of bedrock incision across much of NE Iowa have formed a complex bedrock topographic surface now largely covered by Quaternary deposits.  Some of these buried bedrock valleys are filled by up to 400 ft (120 m) of alluvium and glacial till.  The location of these buried bedrock valleys has been interpreted from available well penetrations, and their form and depth is strongly reflected in the new bedrock geologic map.  Data to produce the map were derived from a number of sources including field studies and bedrock exposure mapping programs; study of extensive subsurface well records archived at the Geological Survey Bureau; and location of bedrock exposure and shallow bedrock provided by the detailed county-scale soils maps.  A multi-faceted mapping strategy was utilized, including the mapping of all exposed bedrock; identifying and mapping all surface well data; re-evaluating the stratigraphy of all bedrock exposures and available well data; and mapping of bedrock topography.  Well and outcrop data were then used to construct local to regional structure contour maps for various stratigraphic datums.  These structure maps were intercepted with the bedrock topography to construct contacts in areas of Quaternary cover and limited data coverage.  All geologic contacts of mapping units were drawn digitally using ESRI's ArcView 3.0 software and prepared as coverages for the Iowa Department of Natural Resources -- Geographic Information System (GIS) library.  The boundaries separating the various map units were selected to reflect prominent lithologic changes and/or major regional unconformities or disconformities.  The map units are considered the most practical and recognizable packages of strata for mapping purposes at a 1:100,000 scale.