294-8 Bedrock Geologic Map of the Peosta 7.5’ Quadrangle, Dubuque County, Iowa

Session: Geologic Mapping (Posters)

Presenting Author:

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Abstract:

The Bedrock Geologic Map of the Peosta 7.5’ Quadrangle is a continuation of a multi-year initiative by the Iowa Geological Survey under the USGS STATEMAP program to update the bedrock geology of Dubuque County, which is situated on the boundary between the East-Central Drift Plain and the Paleozoic Plateau landform regions. The East-Central Drift Plain dominates the mapping area and is bounded by the Silurian Escarpment, a prominent ridge of resistant Silurian dolostones, along the eastern and northern portions of the mapping area. Both landform regions are characterized by thin (<30 feet) glacial deposits draped over bedrock. The bedrock of the Peosta Quadrangle is dominated by strata of the Silurian and Ordovician systems. Geologic reconnaissance of 23 bedrock outcrops and one quarry within the mapping area were conducted during field activities. Additional subsurface information was derived from the analysis of over 150 borehole records, 91 of which have lithologic strip logs, and 45 passive seismic geophysical stations. Each borehole record was checked for locational accuracy using information from drillers logs, historic plat books, county accessor information, field verification, and direct communication with landowners. The depth to the surficial-bedrock contact was determined for each well and assigned an elevation value by subtracting it from the surface digital elevation model (DEM). Additional information was gained from an assessment of the Natural Resources Conservation Service (NRCS) County Soil Survey by identifying soil series that indicate shallow bedrock. A Bedrock Elevation map was created by manual digitization of elevation contours (drawn at a 25-foot contour interval) using Esri ArcGIS Pro 3.4 software, and a bedrock elevation raster was then generated using interpolations of the bedrock surface. A Quaternary Thickness map was created by subtracting the bedrock elevation raster from the surface DEM raster. A single rock core was drilled to assess local stratigraphic variability not captured in well cuttings, to verify thicknesses of select map units, and establish future subdivisions of mappable units in Dubuque County.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11032

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