A Geologic and Hydrogeologic Investigation of the Smithfield 7.5 Minute Quadrangle, Fulton County, Illinois, USA

The Smithfield 7.5 minute Quadrangle lies approximately 46 miles southwest of Peoria, Illinois in Fulton County. The area is primarily agricultural with gallery forest along the Spoon River and its tributaries. Three major unconformities are evident within the quadrangle: the Later Mississippian-Early Pennsylvanian Unconformity with more than 25 meters of local relief, the Pennsylvanian-Quaternary Unconformity with more than 40 meters of relief on the bedrock surface and the unconformity between the bedrock units and the slackwater deposits of the Last Glacial Maximum. Silt-rich slackwater terrace deposits (Carmi Member of the Equality Formation) accumulated during major flooding events as glacial melt water in the Illinois River backed up the Spoon River. The slackwater sediment occurs as depositional terraces. Subsequent down cutting by the Spoon River provides opportunities to examine and map Quaternary geology as well as the Mississippian Salem and St. Louis Formations and the Pennsylvanian Tradewater and Carbondale Formations.

No significant scientific work has been done in this area in more than 70 years. Existing stratigraphic nomenclature has become outdated and the Quaternary deposits have never been mapped. Mapping the Smithfield Quadrangle provides an opportunity to bring existing stratigraphic nomenclature up to date. Mapping will also allow a better understanding of the complex geology exposed in the area as well as provide understanding of post-glacial environments.