QUATERNARY GEOLOGIC MAPPING IN AN URBAN ENVIRONMENT

There is a need to update and upgrade 7½ minute Quaternary geologic maps for the highly urbanized area of northern Rhode Island to facilitate brownfield cleanup, new road alignments and major sewer system upgrades. Published USGS quadrangle maps exist, the result of a sustained cooperative program with the state of Rhode Island from 1950 to 1965. However, many maps are at 1:31,680 scale on pre-World War II topographic bases. We have completed an upgrade of the Providence 7½ by 15 minute quadrangle utilizing a 1987 digital 1:25,000 scale metric base with all data input into a GIS mapping system (MapInfo™). We utilized the Providence 1:31,680 surficial map (Smith, 1956), the digital topo base mentioned above, older 1:24,000 paper topo base maps (Providence and East Providence), 1:24,000 scale groundwater maps, and 1997 1:5,000 scale orthophotographs. We also used significant aquifer coverage and other cultural data from the Rhode Island Geographic Information System (RIGIS). New field mapping was carried out in Rhode Island portion of the East Providence quadrangle and field checks were made in the Providence quadrangle. Lack of field exposures in this urbanized area was partially compensated for by examining lithofacies and other data obtained from bore holes. Bore-hole data, the result of past road and bridge construction, ongoing road realignments and new sewer system construction is being organized into a comprehensive database by others in a cooperative project with the RI Department of Transportation. We found that migrating the digitized map units from the 1:31.680 topo base to the 1:25,000 scale metric base produced a mismatch of older units to newer topography, necessitating extensive movement of polygons with field checks. The topography on the older 1:24,000 topo bases was far superior to the newer 1:25,000 digital metric base and was used as a “patch” to aid in placement of polygons. Other problems were changes in fill, particularly along the estuarine shoreline and along the interstate and primary roads, the extensive filling of wetlands, and the complete removal of some ice-marginal stratified units (sand and gravel borrow). The upgraded map, now in digital format with new explanation and correlation diagram, will be available as a 1:24,000 scale, publish-on-demand paper map and the digital coverage, with FGDC metadata, available as downloadable files.